Applied Science, Faculty of
Engineering, School of (Okanagan)
DSpace
UBCO
Ghaffarkhah, Ahmadreza
Doctor of Philosophy - PhD
University of British Columbia
This thesis explores how structural design influences the performance of advanced materials in electromagnetic interference (EMI) shielding and strain/pressure sensing. Four material categories—laminated, 3D printed, polymer nanocomposites, and aerogels—are explored using various fabrication methods. The journey begins with the synthesis and characterization of foundational nanomaterials, such as Ti₃C₂Tₓ, graphene oxide (GO), and magnetic GO (mGO). These materials form the basis of our structures. Laminated structures are then created through versatile drop-casting, yielding highly conductive Ti₃C₂Tₓ /PEDOT:PSS films. This approach ensures uniformity, cost-effectiveness, and ease of production. A wet-transfer technique is introduced to adapt these films to intricate shapes, ensuring scalability for EMI shielding applications. The development continues with laminated 3D-printed structures tailored for EMI shielding and strain sensing. Challenges associated with extrusion printing, including resolution and the need for high electrical conductivity and mechanical flexibility, are addressed. This phase results in additive-free inks for high-resolution extrusion printing and composite structures with outstanding electrical conductivity and mechanical flexibility. Notably, the material consumption for these 3D-printed structures is nearly 75% lower than that of traditional laminated structures, resulting in significantly reduced fabrication costs essential for large-scale applications. Next, the thesis delves into aerogel-based conductive materials and polymer nanocomposites. A novel "liquid streaming" approach is introduced for aerogel fabrication, using nanoparticle surfactants to stabilize aqueous suspensions of Ti₃C₂Tₓ/GO in a nonpolar medium. These aerogels offer simplicity, reduced pre-processing, exceptional EMI shielding effectiveness, specific EMI shielding effectiveness, and low density, enhanced by micro- and macro-scale porosities that significantly improve absorption characteristics. The final chapter presents Janus liquids, enabling the creation of responsive aerogels customized for piezoresistive sensing, human motion monitoring, and EMI shielding with absorption-dominant characteristics. This research deepens our understanding of structural design's influence on EMI shielding and pressure sensing and showcases the potential of multifunctional materials for future electronic and sensing applications.
Structural Design and Fabrication of Electrically Conductive Materials for Electromagnetic Interference Shielding and Strain/Pressure Sensing by Ahmadreza Ghaffarkhah B.S., Petroleum University of Technology, 2015 M.S., Amirkabir University, 2018 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE COLLEGE OF GRADUATE STUDIES (Mechanical Engineering) THE UNIVERSITY OF BRITISH COLUMBIA Okanagan December 2023 © Ahmadreza Ghaffarkhah, 2023 ii The following individuals certify that they have read, and recommend to the College of Graduate Studies for acceptance, a dissertation entitled: Structural Design and Fabrication of Electrically Conductive Materials for Electromagnetic Interference Shielding and Strain/Pressure Sensing submitted by Ahmadreza Ghaffarkhah in partial fulfillment of the requirements of the degree of Doctor of Philosophy. Dr. Mohammad Arjmand, School of Engineering Supervisor Dr. Ian Foulds, School of Engineering Supervisory Committee Member Dr. Abbas Milani, School of Engineering Supervisory Committee Member Dr. Sumi Siddiqua, School of Engineering University Examiner Dr. Patrick C. Lee, University of Toronto External Examiner iii Abstract This thesis explores how structural design influences the performance of advanced materials in electromagnetic interference (EMI) shielding and strain/pressure sensing. Four material categories—laminated, 3D printed, polymer nanocomposites, and aerogels—are explored using various fabrication methods. The journey begins with the synthesis and characterization of foundational nanomaterials, such as Ti3C2Tx, graphene oxide (GO), and magnetic GO (mGO). These materials form the basis of our structures. Laminated structures are then created through versatile drop-casting, yielding highly conductive Ti3C2Tx /PEDOT:PSS films. This approach ensures uniformity, cost-effectiveness, and ease of production. A wet-transfer technique is introduced to adapt these films to intricate shapes, ensuring scalability for EMI shielding applications. The development continues with laminated 3D-printed structures tailored for EMI shielding and strain sensing. Challenges associated with extrusion printing, including resolution and the need for high electrical conductivity and mechanical flexibility, are addressed. This phase results in additive-free inks for high-resolution extrusion printing and composite structures with outstanding electrical conductivity and mechanical flexibility. Notably, the material consumption for these 3D-printed structures is nearly 75% lower than that of traditional laminated structures, resulting in significantly reduced fabrication costs essential for large-scale applications. Next, the thesis delves into aerogel-based conductive materials and polymer nanocomposites. A novel "liquid streaming" approach is introduced for aerogel fabrication, using nanoparticle surfactants to stabilize aqueous suspensions of Ti3C2Tx/GO in a nonpolar medium. These aerogels offer simplicity, reduced pre-processing, exceptional EMI shielding effectiveness, specific EMI shielding effectiveness, and low density, enhanced by micro- and macro-scale porosities that iv significantly improve absorption characteristics. The final chapter presents Janus liquids, enabling the creation of responsive aerogels customized for piezoresistive sensing, human motion monitoring, and EMI shielding with absorption-dominant characteristics. This research deepens our understanding of structural design's influence on EMI shielding and pressure sensing and showcases the potential of multifunctional materials for future electronic and sensing applications. v Lay summary This study focuses on creating synthetic conductive materials as alternatives to traditional metals for tasks such as protecting electronic devices from electromagnetic interference (EMI) and sensing pressure or strain. We're exploring various structural designs and processing methods to produce these materials. Our aim is to design synthetic EMI shields that can efficiently absorb electromagnetic disturbances, safeguarding crucial electronics in medical, military, and communication devices, and even benefiting human health. Simultaneously, we're using these synthetic materials as strain/pressure sensors—devices that convert physical changes into measurable signals. This has many applications, such as monitoring human motion or aiding in medical diagnostics. Our research is designed with a step-by-step approach, starting from the simplest methods and structures and gradually progressing to more advanced techniques. The goal is to develop structures that effectively address the current challenges of EMI shielding and strain/pressure sensing, opening up numerous possibilities for large-scale production of these modern synthetic materials. vi Preface Ahmadreza Ghaffarkhah authored this doctoral dissertation under the guidance of Dr. Mohammad Arjmand at the School of Engineering, The University of British Columbia. Various sections of this dissertation have resulted in the creation of journal articles. The following sections provide a summary of the publications derived from this dissertation and highlight the author's contributions. Journal Publications: • A version of Chapter 4 has been published in the following article. Ghaffarkhah, A., Kamkar, M., Riazi, H., Hosseini, E., Dijvejin, Z.A., Golovin, K., Soroush, M. and Arjmand, M., 2021. Scalable manufacturing of flexible and highly conductive Ti3C2Tx/PEDOT: PSS thin films for electromagnetic interference shielding. New Journal of Chemistry, 45(44), pp.20787-20799. Contribution: I was responsible for conducting experiments, data analysis, and article composition. • A version of Chapter 5 has been published in the following article. Ghaffarkhah, A., Kamkar, M., Dijvejin, Z.A., Riazi, H., Ghaderi, S., Golovin, K., Soroush, M. and Arjmand, M., 2022. High-resolution extrusion printing of Ti3C2-based inks for wearable human motion monitoring and electromagnetic interference shielding. Carbon, 191, pp.277-289. Contribution: I was responsible for conducting experiments, data analysis, and article composition. • A version of Chapter 6 has been published in the following article. Ghaffarkhah, A., Hashemi, S.A., Rostami, S., Amini, M., Ahmadijokani, F., Pournaghshband Isfahani, A., Mhatre, S.E., Rojas, O.J., Kamkar, M., Wuttke, S. and vii Soroush, M., 2023. Ultra‐Flyweight cryogels of MXene/graphene oxide for electromagnetic interference shielding. Advanced Functional Materials, pp.2304748. Contribution: I was responsible for conducting experiments, data analysis, and article composition. • A version of Chapter 7 has been published in the following article. The supplementary information of this article was also utilized in the preparation of Chapter 3 and a portion of Chapter 2. Ghaffarkhah, A., Hashemi, S.A., Ahmadijokani, F., Goodarzi, M., Riazi, H., Mhatre, S.E., Zaremba, O., Rojas, O., Soroush, M., Russell, T. and Wuttke, S., 2023. Functional Janus liquids and Aerogels. In press. Nature Communications. Contribution: I was responsible for conducting experiments, data analysis, and article composition. viii Table of contents Abstract .......................................................................................................................................... iii Lay summary ...................................................................................................................................v Preface............................................................................................................................................ vi Table of contents .......................................................................................................................... viii List of tables ................................................................................................................................. xiii List of figures .................................................................................................................................xv List of symbols ......................................................................................................................... xxxiv List of abbreviations ..................................................................................................................xxxv Acknowledgements .................................................................................................................. xxxvi Dedication .............................................................................................................................. xxxviii Chapter 1: Background and thesis organization ..............................................................................1 1.1 Motivation ....................................................................................................................... 1 1.2 Research gaps and objectives.......................................................................................... 2 1.3 Thesis organization and novelties ................................................................................... 5 1.4 Research journey overview ............................................................................................. 8 Chapter 2: Background and literature review ................................................................................11 2.1 Structural design of conductive architectures ............................................................... 11 2.1.1 Conductive polymer nanocomposites ....................................................................... 11 2.1.2 Laminated thin films ................................................................................................. 12 2.1.3 Aerogels and foams................................................................................................... 17 2.1.4 3D printed structures ................................................................................................. 20 2.2 Piezoresistive sensors.................................................................................................... 22 ix 2.2.1 Laminated-fracture-based strain sensors................................................................... 24 2.2.2 Aerogels and foam pressure sensors ......................................................................... 25 2.2.3 Hydrogel-based stain/pressure sensors ..................................................................... 27 2.3 Shielding setup and background concept ...................................................................... 29 2.3.1 Conceptual mechanism of EMI shielding ................................................................. 29 2.3.2 EMI shielding parameters and formulations ............................................................. 32 2.3.3 Shielding effectiveness and logarithmic parameters................................................. 33 2.3.4 Shielding mechanism evaluation criteria and common mistakes ............................. 36 Chapter 3: Synthesis and characterization of graphene oxide and MXene nanoflakes .................39 3.1 Materials ....................................................................................................................... 39 3.2 Synthesis recipes ........................................................................................................... 40 3.2.1 Synthesis of GO ........................................................................................................ 40 3.2.2 Synthesis of mGO ..................................................................................................... 40 3.2.3 Synthesis of Ti3C2Tx ................................................................................................. 41 3.3 Characterization ............................................................................................................ 42 3.3.1 Characterizations of synthesized GO nanoflakes ..................................................... 42 3.3.2 Characterizations of synthesized mGO nanoflakes .................................................. 45 3.3.3 Characterizations of synthesized Ti3C2Tx nanoflakes ............................................... 47 Chapter 4: Scalable manufacturing of flexible and highly conductive Ti3C2Tx/PEDOT:PSS thin films for EMI Shielding .................................................................................................................52 4.1 Introduction ................................................................................................................... 52 4.2 Experimental section ..................................................................................................... 55 4.2.1 Materials ................................................................................................................... 55 x 4.2.2 Fabrication of Ti3C2Tx and Ti3C2Tx/PEDOT:PSS films ........................................... 55 4.2.3 Treatment of Ti3C2Tx/PEDOT:PSS films ................................................................. 56 4.2.4 Characterization ........................................................................................................ 57 4.3 Results and discussion .................................................................................................. 58 4.3.1 Drop-casting of Ti3C2Tx/PEDOT:PSS ...................................................................... 58 4.3.2 EMI shielding characteristics of Ti3C2Tx/PEDOT:PSS films ................................... 67 4.3.3 Post-treatment and wet-transferring of Ti3C2Tx/PEDOT:PSS films ......................... 71 4.4 Conclusion .................................................................................................................... 78 Chapter 5: High-resolution extrusion printing of Ti3C2Tx-based inks for wearable human motion monitoring and EMI Shielding ......................................................................................................79 5.1 Introduction ................................................................................................................... 79 5.2 Experimental section ..................................................................................................... 84 5.2.1 Materials ................................................................................................................... 84 5.2.2 Preparation of Ti3C2Tx and Ti3C2Tx/PEDOT:PSS inks............................................. 84 5.2.3 Printing procedure ..................................................................................................... 85 5.2.4 Developing EMI shields ........................................................................................... 85 5.2.5 Developing wearable sensors .................................................................................... 87 5.2.6 Characterization ........................................................................................................ 88 5.3 Results and discussion .................................................................................................. 89 5.3.1 Beyond printing of Ti3C2Tx/PEDOT:PSS ................................................................. 89 5.3.2 Printed EMI shields................................................................................................... 98 5.3.3 Printed piezo-resistive sensors ................................................................................ 106 5.4 Conclusion .................................................................................................................. 113 xi Chapter 6: Ultra‐Flyweight aerogels of Ti3C2Tx/GO for EMI shielding .....................................115 6.1 Introduction ................................................................................................................. 115 6.2 Experimental section ................................................................................................... 119 6.2.1 Materials ................................................................................................................. 119 6.2.2 Fabrication of Ti3C2Tx bead-shaped structured liquids .......................................... 119 6.2.3 Fabrication of worm-like GO structured liquids ..................................................... 120 6.2.4 Fabrication of hybrid Ti3C2Tx/GO worm-like structured liquids and aerogels ...... 120 6.2.5 Fabrication of Ti3C2Tx/GO/epoxy nanocomposites ................................................ 121 6.2.6 Characterization ...................................................................................................... 121 6.3 Results and discussion ................................................................................................ 122 6.3.1 From liquid templates to ultra-lightweight aerogels ............................................... 122 6.3.2 EMI shielding of Ti3C2Tx/GO worm-like aerogels ................................................. 152 6.4 Conclusion .................................................................................................................. 167 Chapter 7: Functional Janus structured liquids and aerogels .......................................................168 7.1 Introduction ................................................................................................................. 168 7.2 Experimental section ................................................................................................... 170 7.2.1 Materials ................................................................................................................. 170 7.2.2 Fabrication of structured liquids and aerogels ........................................................ 171 7.2.2.1 Ti3C2Tx beads .................................................................................................. 171 7.2.2.2 GO worm-like structures................................................................................. 171 7.2.2.3 Ti3C2Tx/GO worm-like structures ................................................................... 171 7.2.2.4 Janus liquids and aerogels ............................................................................... 172 7.2.3 Characterization ...................................................................................................... 174 xii 7.3 Results and discussion ................................................................................................ 175 7.3.1 Shaping GO non-responsive liquid threads ............................................................ 175 7.3.2 Shaping mGO/GO magnetic liquid threads ............................................................ 177 7.3.3 Shaping Ti3C2Tx/GO conductive liquid threads ..................................................... 180 7.3.4 Shaping Ti3C2Tx/GO conductive liquid threads ..................................................... 182 7.3.5 Fabrication of Janus liquids and aerogels ............................................................... 187 7.3.6 Janus EMI shields with absorption-dominant characteristics ................................. 195 7.3.7 Janus piezoresistive sensors for human motion monitoring. .................................. 206 7.4 Conclusion .................................................................................................................. 216 Chapter 8: Conclusion and future work .......................................................................................217 8.1 Summary of main findings.......................................................................................... 217 8.2 Analyzing the pros and cons of the developed synthetic structures in this thesis ...... 222 8.3 Future work recommendations ................................................................................... 223 Bibliography ................................................................................................................................228 xiii List of tables Table 1-1 A concise overview of the concepts and research presented in this thesis. .................. 10 Table 4-1 Compositions of the Ti3C2Tx MXene and Ti3C2Tx MXene/PEDOT:PSS solutions used for drop-casting. 1 mL of each of the solutions was used for covering 9 cm2 of polycarbonate sheet........................................................................................................................................................ 56 Table 4-2 Previous studies used drop-casting or inkjet printing approach for PEDOT:PSS and its composites..................................................................................................................................... 61 Table 4-3 Average static, advancing, and receding contact angles of water on the polycarbonate sheet. ............................................................................................................................................. 62 Table 4-4 Average static, advancing, and receding contact angles of the aqueous solution of Ti3C2Tx or Ti3C2Tx/PEDOT:PSS (25 wt%) on activated polycarbonate sheet. ............................ 62 Table 4-5 Previous studies of thickness, conductivity, SET, and specific EMI shielding effectiveness of various nanocomposites. ..................................................................................... 76 Table 5-1 Composition of prepared inks. ..................................................................................... 85 Table 5-2 The EMI shielding characteristics of various structures developed based on MXene...................................................................................................................................................... 106 Table 5-3 Normalized changes in resistance and gauge factor of the printed sensor in various strains. ......................................................................................................................................... 112 Table 5-4 A literature review on Ti3C2Tx-based piezo-resistive strain sensors .......................... 113 Table 6-1 A comparison of the SSE/t vs. density of the Ti3C2Tx/GO worm-like aerogels with the previous best practices in the literature. ...................................................................................... 158 Table 6-2 A comparison of the SET vs. filler loading of the Ti3C2Tx/GO/epoxy nanocomposites with the previous best practices in the literature. ........................................................................ 165 xiv Table 7-1 A comparison of the SET vs. A of the magnetic/conductive Janus structures with the previous best practices in the literature. In the case that R and A coefficients were not directly reported in the papers, these coefficients are calculated based on the formulations explained in section three of this file. .............................................................................................................. 204 Table 7-2 A comparison between the main characteristics of our sensor and those structures reported in the previous best practices in the literature. ............................................................. 211 xv List of figures Figure 1-1 Addressing the main challenges of EMI shielding and sensing: key factors and the significance of structural design. .................................................................................................... 4 Figure 1-2 Experimental framework of this thesis. ........................................................................ 9 Figure 2-1 A variety of MXene thin films, each several micrometers in thickness, have been created using the VAF technique for the purpose of EMI shielding. Reproduced with permission [5]. Copyright 2020, American Chemical Society. ........................................................................ 15 Figure 2-2 Schematic representation of freeze casting for MXene/Graphene suspension reveals the process in which ice crystals serve as templates, aligning the 2D nanomaterials. Reproduced with permission [19]. Copyright 2018, American Chemical Society. .................................................... 19 Figure 2-3 (a) FDM is employed in the production of PLA/GNPs/CNTs nanocomposites, forming intricate honeycomb structures tailored for EMI shielding applications. Reproduced with permission [20]. Copyright 2020, American Chemical Society. (b-c) Extrusion printing of MXene/PEDOT:PSS inks is harnessed to create versatile 3D printed structures, including laminated and aerogel-based designs. Reproduced with permission [21]. Copyright 2022, Wiley-VCH. ............................................................................................................................................. 22 Figure 2-4 (a) Schematic illustration showcasing the sensing mechanisms of Laminated-fracture-based strain sensors developed based on Ti3C2Tx. SEM images of various fracture-based strain sensors developed based on Ti3C2Tx at their maximum tolerable strains. The formation of microcracks is evident in (c-d). Reproduced with permission [24]. Copyright 2019, Wiley-VCH........................................................................................................................................................ 25 xvi Figure 2-5 (a) Synthesis procedure and (b) sensing mechanism of a piezoresistive-based pressure sensor employing a composite aerogel made from GO/Ti3C2Tx. Reproduced with permission [23]. Copyright 2018, American Chemical Society. ............................................................................. 27 Figure 3-1 (a) X-ray diffractogram, (b) Micro-Raman spectra, (c) TGA, (d) FTIR spectra, and (e-f) TEM images of GO. (f) FESEM images of the cross-section of a GO film after vacuum filtration. The specifics of these characterizations are outlined in Section 3.3.1. ......................................... 44 Figure 3-2 (a) X-ray diffractogram, (b) Micro-Raman spectra, (c) TGA, (d-e) FESEM, and (f-g) EDX analysis of mGO. The EDX map presented in part (d) has been taken from the section completely covered by Fe3O4. The specifics of these characterizations are outlined in Section 3.3.2........................................................................................................................................................ 47 Figure 3-3 X-ray diffractogram of Ti3C2Tx laminated films that annealed at (a) 60 °C and (b) 250 °C. (c) Micro-Raman spectroscopy and (d) FTIR analyses of Ti3C2Tx. The specifics of these characterizations are outlined in Section 3.3.3. ............................................................................ 50 Figure 3-4 HRTEM analysis of Ti3C2Tx flakes. (a-b) illustrates the two-dimensional morphology of Ti3C2Tx flakes, while (c-d) presents high-resolution images showcasing the atomic arrangement of this material. ............................................................................................................................. 51 Figure 4-1 (a) The drying process of drop-casted films. Contact angles, photographs, 3D intensity profiles, and height profiles of the dried droplets of (b) the pristine Ti3C2Tx on untreated polycarbonate, (c) Ti3C2Tx/PEDOT:PSS (25 wt% of polymer) on untreated polycarbonate, (d) the pristine Ti3C2Tx on treated polycarbonate, and (e) Ti3C2Tx/PEDOT:PSS (25 wt% of polymer) on treated polycarbonate. ................................................................................................................... 60 xvii Figure 4-2 Cross-section FESEM images of (a) the pristine Ti3C2Tx and Ti3C2Tx/PEDOT:PSS composite films containing (b) 10, (c) 25, (d) 35, and (e) 50 wt% of the polymer. All scale bars correspond to 2 µm. ...................................................................................................................... 64 Figure 4-3 Picture and height profile of the dried droplets of (a) pure PEDOT:PSS on untreated polycarbonate, (b) pure PEDOT:PSS on treated polycarbonate, and (c) Ti3C2Tx/PEDOT:PSS (50 wt% of polymer) on treated polycarbonate. (d) The thickness variation of drop-casted Ti3C2Tx/PEDOT:PSS (25 wt% of polymer) onto medium glass and polycarbonate substrates (30mm x 30mm). The pictures show the prepared film on a polycarbonate and glass substrate. 65 Figure 4-4 The thickness variation and thickness error compared to the average reported value for the Ti3C2Tx/PEDOT:PSS (25 wt% of polymer) film prepared by (a) drop-casting on an activated polycarbonate sheet and (b) VAF method. (c) The variation of the total EMI shielding in various locations of a large film of Ti3C2Tx/PEDOT:PSS (25 wt% of polymer) that was prepared by drop-casting on an activated polycarbonate. (d) The thickness variation and thickness error compared to the average reported value for the Ti3C2Tx/PEDOT:PSS (25 wt% of polymer) film that was prepared by drop-casting on a Teflon sheet. ................................................................................. 67 Figure 4-5 (a) The EMI shielding characteristics, electrical conductivity, and thickness of the pristine Ti3C2Tx and the nanocomposite films that contained various concentrations of the polymer and (b) the structure and EMI shielding behavior of the Ti3C2Tx/PEDOT:PSS nanocomposite containing 25 wt% of polymer. ..................................................................................................... 69 Figure 4-6 Reflectance and absorbance of pristine Ti3C2Tx and the composite films that contained various concentrations of polymer. ............................................................................................... 69 Figure 4-7 (a) Total EMI shielding of pristine Ti3C2Tx and nanocomposite films that contained 10 and 25 wt% of the polymer after 500 bending cycles and (b) height profiles of pristine Ti3C2Tx and xviii nanocomposite films that contained 10 and 25 wt% of the polymer after 500 bending cycles (scale is 600 µm). .................................................................................................................................... 71 Figure 4-8 (a) EMI shielding characteristics, electrical conductivities, and thicknesses of Ti3C2Tx/PEDOT:PSS nanocomposite films (25 wt% of polymer) after treatment with methanol, sulfuric acid, and co-treatment with both of them and (b) the structure and EMI shielding behavior of the Ti3C2Tx/PEDOT:PSS nanocomposite containing 25 wt% of the polymer after co-treatment with sulfuric acid and methanol. ................................................................................................... 72 Figure 4-9 XPS results for the composite film containing 25 wt% of PEDOT:PSS before and after post-treatment with sulfuric acid and methanol. ........................................................................... 74 Figure 4-10 (a) Schematic description of wet-transferring of the Ti3C2Tx/PEDOT:PSS nanocomposite film to a new substrate, (b) optical image of the Ti3C2Tx/PEDOT:PSS nanocomposite films transferred to various substrates with different geometries, (c) the total EMI shielding of the co-treated Ti3C2Tx/PEDOT:PSS films on the polycarbonate and double-sided scotch tape before and after 500 bending cycles, and (d) a comparison of the SSE/t of the co-treated Ti3C2Tx/PEDOT:PSS composite film (25 wt% of polymer) with the reported nanocomposites films. ............................................................................................................................................. 78 Figure 5-1 Color picture, height map, and geometrical characteristics of the printed grids. (a) Grid G1, (b) Grid G2, and (c) Grid G3. All samples were printed with pure Ti3C2Tx inks. For (a-c), the average thickness at the cross-section, line thickness, and line width is equal to 14.1, 10.6, and 250 µm, respectively. ........................................................................................................................... 86 Figure 5-2 The flexible double-sided tape used as the substrate for the sensor. .......................... 88 Figure 5-3 (a, b) Ink rheological characterization. (a) The viscosity of the prepared samples as a function of shear rate (s–1) and (b) elastic modulus G′ and loss modulus G′′ of the samples as a xix function of shear stress (Pa). (c) Solid-like behavior of extruded filaments. Filaments of the prepared inks can sustain their own weight and remain suspended from the needle even after 30 min. ............................................................................................................................................... 90 Figure 5-4 (a-b) The viscosity of the prepared samples as a function of shear rate (s-1) and (d-f) elastic modulus G′ and loss modulus G′′ of the samples as a function of shear stress (Pa). (a, d) pure Ti3C2Tx inks, (b, e) Ti3C2Tx/PEDOT:PSS composite inks containing 20 wt% polymer, and (c, f) Ti3C2Tx/PEDOT:PSS composite inks containing 50 wt% polymer. .................................... 91 Figure 5-5 (a) Color picture and height map of a grid that is printed by a pure Ti3C2Tx ink with a solid content of 50 mg/ml. The width of the printed lines after drying is almost equal to the inner diameter of the needle, and no filament layout is observed for printed tracks. (b) The physical state of pure Ti3C2Tx inks with various solid contents. When the inks become less concentrated (from left to right), the prepared suspensions lose their mechanical integrity and solid dominant behavior. (c) Color picture and height map of a grid that is printed by a pure Ti3C2Tx ink with a solid content of 40 mg/ml. The width of printed lines after drying is much larger than the nozzle inner diameter due to the filament layout. (d) Photograph of pure Ti3C2Tx ink with a solid content of 20 mg/ml. In this case, the elastic modulus is so low that the ink cannot produce any filament and forms droplets during the printing. Scale bars in (a, c) correspond to 3 mm. ......................................... 93 Figure 5-6 (a) Photograph of the fabricated microstructures on various commercially available substrates. From left to right: Pure Ti3C2Tx on glass (using 410 µm nozzle), Ti3C2Tx/PEDOT:PSS composite containing 80 wt% MXene on PET and CA (using 200 µm nozzle), and Ti3C2Tx/PEDOT:PSS composite containing 50 wt% MXene on PC (using 200 µm nozzle) and double-sided tape (using 410 µm nozzle). (b) Free-standing and micrometer-thick Ti3C2Tx xx structure. (c) 3D-printed architectures of Ti3C2Tx and Ti3C2Tx/PEDOT:PSS. All scale bars correspond to 1 cm. ....................................................................................................................... 95 Figure 5-7 (a, b) The process of extrusion printing of Ti3C2Tx/PEDOT:PSS composite inks. (a) Schematic illustration of printing. Ti3C2Tx flakes are getting aligned upon extrusion printing because of the shear alignment of flakes inside the needle. Besides, polymer complexes can be readily spread between Ti3C2Tx flakes due to the electrostatic interactions of PSS chains and Ti3C2Tx sheets (b) Photograph of the printing process of Ti3C2Tx/PEDOT:PSS composite ink containing 50 wt% polymer. The thickness and width spatial uniformity, average thickness and width, and color pictures of the extrusion-printed lines of (c) pure Ti3C2Tx, and Ti3C2Tx /PEDOT:PSS composites containing (d) 20 wt% and (e) 50 wt% polymer. Scale bars in (c-e) correspond to 1 cm. ....................................................................................................................... 97 Figure 5-8 (a) The electrical conductivity and thickness of Ti3C2Tx and Ti3C2Tx/PEDOT:PSS structures prepared via the VAF method. These laminated structures denoted as “full” were also used as a benchmark to evaluate the EMI shielding characteristics of the printed structures. (b) The average EMI SE (SER, SEA, and SET) of the fabricated structures. (c) Reflectance, absorbance, and transmittance coefficients of the fabricated structures. (d) The G1 grid of pure Ti3C2Tx printed onto double-sided tape. This structure can be easily applied to unconventional geometries with curves, angles, and corners. ..................................................................................................................... 102 Figure 5-9 (a) The EMI shielding and electrical characteristics of the G1 grid of pure Ti3C2Tx and Ti3C2Tx/PEDOT:PSS composites before and after 5,000 bending cycles. (b) A comparison of the SSE/t of the G1 grids of the prepared samples with the MXene-based shields reported previously in the literature. ........................................................................................................................... 105 xxi Figure 5-10 (a) The fabricated piezo-resistive sensor based on the printed grids of pure Ti3C2Tx with the inter-filament spacing of 1 mm. A visual illustration of the crack gap opening in various strains of (b) 0%, (c) 1%, (d) 2%, (e) 4%, and (f) 8%. (b–f) have a same magnification. The scale bar in (f) corresponds to 400 µm. (g) The corresponding normalized change in resistance and current for the printed Ti3C2Tx sensor at various strains. (h) The response time and (i) recovery time of the fabricated sensors are calculated at a strain of 1%. .................................................. 109 Figure 5-11 Stability of the electrical characteristics of the fabricated sensor. I and I0 were measured under a no lateral strain (0%). .................................................................................... 110 Figure 5-12 The recorded response for (a) normal and (b) harsh human breathing. (c) The recorded response for the multiple cycles of raising and relaxing of human eyebrows. (d) The recorded response of the sensor for the word “MXene” and a humming sound. (e) The capability of the fabricated sensor for recording the contraction of the biceps. The fabricated structure was able to keep its response after 2,000 bending cycles (40 runs) and multiple cycles of loading and unloading of the sensor to the subject's arm. ............................................................................................... 112 Figure 6-1 Digital image demonstrating tubule formation upon streaming GO aqueous suspension (10 mg/ml) into hexane-POSS. All scale bars correspond to 2 cm. ........................................... 122 Figure 6-2 Expansion/contraction test of a 1 mg/ml aqueous dispersion of GO in the hexane-POSS domain. As the droplet contracts, its volume diminishes, resulting in decreased interfacial area and applying compressive forces. Elastic interfacial assemblies respond by forming distinctive wrinkles on the interface. ............................................................................................................ 123 Figure 6-3 (a-b) Digital images demonstrating bead formation upon streaming Ti3C2Tx aqueous suspension (10 mg/ml) into hexane-POSS. The co-assembly of Ti3C2Tx and POSS is not efficient enough to prevent Plateau-Rayleigh instabilities, and the liquid jet breaks into smaller packets to xxii reduce surface energy. (c) Interfacial characterizing of NPs aqueous suspension in hexane domain with and without POSS. In part (c), the terms "80wt%" and "50wt%" were assigned to the hybrid systems, referring to suspensions containing 80wt% and 50wt% of Ti3C2Tx, respectively. All scale bars in (a and d) correspond to 2 cm. .......................................................................................... 124 Figure 6-4 Dynamic interfacial characterizing of NPs aqueous suspension (10 mg/ml) in hexane domain (a) with and (b) without POSS. Particularly in the case of a hexane-POSS medium, measuring the dynamic interfacial tension becomes highly complex due to the rapid assembly of NPs, forming a jammed layer at the interface that resembles skin. This skin exhibits solid-like behavior and quickly distorts the pendant droplet. When the two sides of the droplet do not precisely mirror each other, the tensiometer cannot accurately measure the interfacial tension, resulting in the termination of the experiment. Consequently, we refer to the recorded values for samples in the hexane-POSS medium as "approximate equilibrium interfacial tension." ......... 125 Figure 6-5 Schematic illustration of Ti3C2Tx beads formation. .................................................. 126 Figure 6-6 Microscope images of Ti3C2Tx beads (a) before and (b) after removal of hexane. (c) Digital image of Ti3C2Tx beads 120 minutes after removal of hexane. (d-f) FESEM images of freeze-dried Ti3C2Tx aerogel beads. Scale bars in (a-b), (c), (d-e), and (f) correspond to 1 mm, 1 cm, 200 µm, and 100 µm, respectively. ...................................................................................... 127 Figure 6-7 The conducted expansion/contraction experiments using a 1 mg/ml Ti3C2Tx aqueous suspension in a hexane-POSS medium. (a) We observed that when using approximately a 5 ml droplet, it frequently detached from the injection head with ease a few seconds following the expansion phase. (b) Even after reducing the droplet size to 3 ml, we did not observe a substantial interfacial skin formation for the pure Ti3C2Tx aqueous suspension. Some might argue that the MXene suspension's dark hue hinders skin detection. However, it's important to note that we xxiii observed such skin formation in the Ti3C2Tx/GO aqueous suspension, which has the same dark color, as highlighted later in this article. Additionally, during the pendant drop test, we noticed that either pure GO or Ti3C2Tx/GO droplets remained firmly attached to the setup needles, even when the hexane-POSS bath underwent intense agitation. Yet, for the pure Ti3C2Tx aqueous suspension, the droplet readily detached from the needle with the slightest vibration. This finding underscores that MXene alone fails to form robust interfacial skin. .............................................................. 128 Figure 6-8 Digital image demonstrating tubule formation upon streaming 10 mg/ml Ti3C2Tx/GO aqueous containing 80 wt% Ti3C2Tx. The scale bar corresponds to 2 cm. ................................. 130 Figure 6-9 (a) Schematic illustration of Ti3C2Tx/GO liquid threads formation. (b) Schematic illustration of NPs-ligand interactions during Ti3C2Tx/GO liquid thread formation. ................. 131 Figure 6-10 Contraction test of a 1 mg/ml aqueous dispersion of Ti3C2Tx/GO in the hexane-POSS domain. As the droplet undergoes contraction, its volume decreases, leading to a reduction in interfacial area and applying compressive forces. In response, the elastic interfacial assemblies exhibit a unique pattern of wrinkles along the interface. ............................................................ 132 Figure 6-11 Microscopic images of Ti3C2Tx/GO liquid threads 30 minutes after removal of hexane. The scale bar corresponds to 1 mm. ............................................................................................ 133 Figure 6-12 The liquid-like behavior (flowability) of Ti3C2Tx/GO (a) 80wt%- 10 mg/ml and (b) 50wt%- 10 mg/ml suspensions. (c) Ti3C2Tx/GO 80wt%- 10 mg/ml suspension is unable to produce tubular filaments upon extrusion at a high pressure (330 μm nozzle at 30 psi extrusion pressure). 2 mL suspension of Ti3C2Tx /GO (d) 80wt%- 10 mg/ml and (e) 50wt%- 10 mg/ml suspensions and their sculpted emulsion counterpart. Upon extrusion, the volume occupied by the liquid threads exceeds that of the original suspensions, which is attributable to the presence of voids between the entangled tubes. Please note that all scale bars in figures (a-e) represent 2 cm. ........................ 135 xxiv Figure 6-13 (a) Electrical conductivity and (b) density of worm-like Ti3C2Tx/GO aerogels. .... 137 Figure 6-14 The density of the fabricated aerogels, even in the case of 10 mg/ml suspensions, is low enough that an electrostatically charged plastic rod can lift and move the fabricated structures without any physical contact. I to VI delineate the chronological sequence of image capture, with I representing the period when the rod is distant from the sample, and VI signifying the moment when the rod elevates the sample. The scale bar corresponds to 2 cm. ...................................... 137 Figure 6-15 (a-e) The FESEM images of Ti3C2Tx/GO worm-like aerogels. (f) Digital images of Ti3C2Tx/GO liquid threads and (g) the corresponding microscope image showcasing the entangled morphology of these structures. (h-i) Digital images of Ti3C2Tx/GO worm-like aerogels and (j) the corresponding microscope image showcasing the entangled morphology of these structures. Scale bars in (h-i), (f), (a, g), (b, j), (c-d), and (e) correspond to 2 cm, 1 cm, 2 mm, 1 mm, 500 µm, and 200 µm, respectively. .................................................................................................................. 138 Figure 6-16 (a-e) The FESEM images of Ti3C2Tx/GO worm-like aerogels. (f) Digital images of Ti3C2Tx/GO liquid threads and (g) the corresponding microscope image showcasing the entangled morphology of these structures. (h-i) Digital images of Ti3C2Tx/GO worm-like aerogels and (j) the corresponding microscope image showcasing the entangled morphology of these structures. (k) X-ray diffractogram and (l) micro-Raman spectroscopy analyses of Ti3C2Tx, GO, and their resulting aerogels prepared by Ti3C2Tx/GO 80wt%- 10 mg/ml and 50wt%- 10 mg/ml suspensions. Scale bars in (h-i), (f), (a, g), (b, j), (c-d), and (e) correspond to 2 cm, 1 cm, 2 mm, 1 mm, 500 µm, and 200 µm, respectively. .................................................................................................................. 139 Figure 6-17 (a) X-ray diffractogram and (b) micro-Raman spectroscopy analyses of Ti3C2Tx, GO, and their resulting aerogels prepared by Ti3C2Tx/GO 80wt%- 10 mg/ml and 50wt%- 10 mg/ml suspensions. ................................................................................................................................ 142 xxv Figure 6-18 The FTIR spectrum of (a) GO, (b) Ti3C2Tx, (c) GO aerogel, and (d) Ti3C2Tx/GO aerogel 80wt%- 10 mg/ml. .......................................................................................................... 144 Figure 6-19 XPS analysis of (a) GO, (b) Ti3C2Tx, (c) GO aerogel, and (d) Ti3C2Tx/ GO aerogel...................................................................................................................................................... 147 Figure 6-20 High-resolution deconvoluted spectra of (a) C1s, (b) O1s, and (c) N1s of GO. ..... 148 Figure 6-21 High-resolution deconvoluted spectra of (a) C1s, (b) O1s, (c) Ti 2P, and (d) N1s of Ti3C2Tx. ....................................................................................................................................... 149 Figure 6-22 High-resolution deconvoluted spectra of (a) C1s, (b) O1s, (c) Ti 2P, and (d) N1s of GO aerogel. ................................................................................................................................. 150 Figure 6-23 High-resolution deconvoluted spectra of (a) C1s, (b) O1s, (c) Ti 2P, and (d) N1s of hybrid Ti3C2Tx/GO aerogel. ........................................................................................................ 151 Figure 6-24 EMI shielding characteristics of Ti3C2Tx/GO worm-like aerogels prepared by 80wt%- 10 mg/ml and 80wt%- 5 mg/ml suspensions. ............................................................................. 153 Figure 6-25 EMI shielding characteristics of Ti3C2Tx/GO aerogels prepared by 5 mg/ml Ti3C2Tx /GO aqueous containing 20 wt% GO, printed with a 200 μm nozzle. I and II refer to the data obtained from two different sides of the samples. The thickness of these samples was 3 mm. Three different samples were tested: (a, b) sample one, (c, d) sample two, and (e, f) sample three. ... 154 Figure 6-26 EMI shielding characteristics of Ti3C2Tx/GO aerogels prepared by 10 mg/ml Ti3C2Tx /GO aqueous containing 20 wt% GO, printed with a 200 μm nozzle. I and II refer to the data obtained from two different sides of the samples. The thickness of these samples was 3 mm. Three different samples were tested: (a, b) sample one, (c, d) sample two, and (e, f) sample three. ... 155 Figure 6-27 Schematic illustration of internal scattering in Ti3C2Tx/GO aerogels from macro-to micro-scale. ................................................................................................................................. 156 xxvi Figure 6-28 A comparison of the SSE/t vs. density of the Ti3C2Tx/GO worm-like aerogels with the previous best practices in the literature. ...................................................................................... 157 Figure 6-29 EMI shielding characteristics of Ti3C2Tx/GO/epoxy nanocomposites prepared by infusing epoxy precursor into worm-like aerogels. The parent aerogels were made by 80wt%- 10 mg/ml and 80wt%- 5 mg/ml suspensions. .................................................................................. 160 Figure 6-30 EMI shielding characteristics of epoxy/ Ti3C2Tx /GO composites prepared by 5 mg/ml Ti3C2Tx /GO aqueous containing 20 wt% GO, printed with a 200 μm nozzle. I and II refer to the data obtained from two different sides of the samples. The thickness of these samples was 3 mm. Three different samples were tested: (a, b) sample one, (c, d) sample two, and (e, f) sample three...................................................................................................................................................... 161 Figure 6-31 EMI shielding characteristics of epoxy/Ti3C2Tx/GO composites prepared by 10 mg/ml Ti3C2Tx /GO aqueous containing 20 wt% GO, printed with a 200 μm nozzle. I and II refer to the data obtained from two different sides of the samples. The thickness of these samples was 3 mm. Three different samples were tested: (a, b) sample one, (c, d) sample two, and (e, f) sample three...................................................................................................................................................... 162 Figure 6-32 EMI shielding characteristics of pure epoxy. I and II refer to the data obtained from two different sides of the samples. The thickness of these samples was 3 mm. Three different samples were tested: (a, b) sample one, (c, d) sample two, and (e, f) sample three. .................. 163 Figure 6-33 A comparison of the SET vs. filler loading of the Ti3C2Tx/GO/epoxy nanocomposites with the previous best practices in the literature. ........................................................................ 164 Figure 6-34 A comparison of the normalized SET vs. SET of the Ti3C2Tx/GO/epoxy nanocomposites with the previous best practices in the literature. ............................................. 166 xxvii Figure 7-1 (a) Digital images demonstrating tubule formation upon streaming GO aqueous suspension (10 mg/ml) into hexane-POSS. (b-e) FESEM images of worm-like aerogels of GO. Scale bars in (a), (b), (c), (d), and (e) correspond to 2 cm, 2000, 1000, 500, and 100 µm, respectively. ................................................................................................................................ 176 Figure 7-2 (a) Digital image demonstrating tubule formation upon streaming GO aqueous suspension (5 mg/ml) into hexane-POSS. (b) GO-liquid threads with a worm-like morphology after three months of aging. All scale bars in (a-b) correspond to 2 cm. .................................... 177 Figure 7-3 Interfacial characterization of NPs aqueous suspension in hexane domain (a) without and (b, c) with POSS. The aqueous suspension of Ti3C2Tx (10 mg/ml), GO (10 mg/ml), and hybrid systems containing 20 and 50 wt% GO, i.e., 2 and 5 mg/ml of GO in 10 mg/ml Ti3C2Tx/GO suspensions, were tested in this experiment. (d) The average equilibrium IFT of the aqueous suspensions/hexane with and without POSS. (e) The schematic illustration of NPSs assemblies in the presence of POSS results in lower equilibrium IFT of the aqueous suspensions/hexane. Contraction of a pendent drop of (f) GO (10 mg/ml); (g) Ti3C2Tx (10 mg/ml); (h) Ti3C2Tx/GO 80 wt%; and (i) Ti3C2Tx/GO 50 wt% in hexane with and without 1 mg/mL POSS. ....................... 178 Figure 7-4 (a) The process of streaming mGO/GO 50 wt% and their response to an external magnetic field. (b) The prepared mGO/GO showed enhanced structural integrity and minimum shrinkage upon freeze-drying. (c) Digital image demonstrating tubule formation upon streaming 10 mg/ml mGO aqueous suspension. The mGO aqueous suspension is capable of fabricating liquid threads. (d) However, upon freeze-drying, relatively low structural stability and high-volume shrinkage for mGO aerogels were observed. All scale bars in (a-d) correspond to 2 cm. ......... 179 xxviii Figure 7-5 (a) Digital and (b) confocal images of Ti3C2Tx beads. (c-d) FESEM images of Ti3C2Tx aerogel beads. The red arrows in (d-e) highlight the laminated skin that covers the bulk of aerogels. Scale bars in (a), (b), (c, d), and (e) correspond to 2 cm, 1000, 500, and 100 µm, respectively. 181 Figure 7-6 (a) Digital image demonstrating bead formation upon streaming Ti3C2Tx aqueous suspension (10 mg/ml) into hexane-POSS. Ti3C2Tx beads (b) before and (c) after three months of aging. (d-h) FESEM images of Ti3C2Tx aerogel beads. These structures showcase laminated skin covering the bulk of aerogels. Scale bars in (a-c), (e), (d, f), (g), and (h) corresponded to 2cm, 400, 200, 50, and 10 µm, respectively. ............................................................................................... 181 Figure 7-7 (a) Digital images demonstrating tubule formation upon streaming 10 mg/ml Ti3C2Tx /GO aqueous containing 20 wt% GO. The rapid assembly of NPSs suppresses the Plateau-Rayleigh instabilities and stabilizes the aqueous jet of Ti3C2Tx/GO. (b-e) FESEM images of Ti3C2Tx/GO worm-like aerogels. The red arrow in (d) highlights the laminated skin that covers the bulk of aerogels. Scale bars in (a), (b), (c), (d), and (e) correspond to 2 cm, 2000, 500, 200, and 100 µm, respectively. ................................................................................................................................ 184 Figure 7-8 FESEM images of bulk and filaments of Ti3C2Tx/GO (20 wt% GO) aerogels generated via liquid streaming approach using needles with different gauge numbers of (a) 21, (b) 18, and (c) 15. In the case of these samples, the extrusion pressure was fixed at 15 psi. Scale bars equal (I) 2 mm and (II-III) 1 mm. .............................................................................................................. 185 Figure 7-9 Filament diameter of Ti3C2Tx/GO (20 wt% GO) aerogels generated via liquid streaming approach using needles with different gauge numbers of (a) 21, (b) 18, and (c) 15. Average filament diameter of Ti3C2Tx/GO (20 wt% GO) aerogels prepared at different needle gauges. In the case of these samples, the extrusion pressure was fixed at 15 psi. ....................................... 186 xxix Figure 7-10 Cross-sectional micro-CT images of Ti3C2Tx/GO (20 wt% GO) aerogels prepared via gauge 15 needles. In the case of these samples, the extrusion pressure was fixed at 15 psi. ..... 186 Figure 7-11 Cross-sectional micro-CT images of Ti3C2Tx/GO (20 wt% GO) aerogels prepared via gauge 21 needles. In the case of these samples, the extrusion pressure was fixed at 15 psi. It's worth emphasizing that, when employing smaller needles for extruding filaments, the section of the sample situated nearer to the bottom of the holding vials demonstrates a denser composition with diminished macro-scale voids, as evident in images I to VI....................................................... 187 Figure 7-12 (a) Schematic illustration of Janus liquid threads formation. In this graphical representation, GO and Ti3C2Tx nanosheets are represented by gold and black colors, respectively. (b) Magnetic/conductive Janus structures were fabricated by merging streams of Ti3C2Tx/GO (20 wt% GO) and mGO/GO (50 wt% GO). (c-d) Digital images of Ti3C2Tx/GO:mGO/GO magnetic/conductive aerogels. All scale bares correspond to 2cm. ............................................ 189 Figure 7-13 (a-d) Confocal images of the Janus liquid threads prepared by merging streams of Ti3C2Tx/GO-10 mg/ml (20 wt% GO) and mGO/GO-10 mg/ml (50 wt% GO) inks. In Figures a-b, 200 µm nozzles were used for both inks. However, in Figures c-d, we used different needles for each ink, i.e., 200 µm for Ti3C2Tx/GO and 400 µm for mGO/GO, and fabricated Janus structures in which the magnetic parts' volume is larger than that containing Ti3C2Tx. All scale bars correspond to 500 µm. ................................................................................................................ 190 Figure 7-14 FESEM and EDX analysis of (a-e) Ti3C2Tx/GO:mGO/GO and (f-j) Ti3C2Tx/GO:GO Janus aerogels. All scale bars correspond to 1000 µm. .............................................................. 191 Figure 7-15 FESEM images of Ti3C2Tx/GO (20 wt% GO):GO Janus aerogels generated using needles with different gauge numbers of (a) 21 and (b) 18. Scale bars correspond to (I) 2 mm and (II-III) 1 mm. ............................................................................................................................... 192 xxx Figure 7-16 Filament diameter distribution of Ti3C2Tx/GO (20 wt% GO):GO Janus aerogels generated using needles with different gauge numbers of (a) 21 and (b) 18. (c) Average filament diameter of Ti3C2Tx/GO (20 wt% GO):GO Janus aerogels prepared by needles with different gauge numbers. ...................................................................................................................................... 192 Figure 7-17 Micro-scale porosity of the hierarchical aerogels made of (a) 10 mg/ml GO, (b) 10 mg/ml mGO/GO (50 wt% GO), and (c)10 mg/ml Ti3C2Tx/GO (20 wt% GO) suspensions at (I-II) 500 µm, and (III) 200 µm scale bars. .......................................................................................... 193 Figure 7-18 Micro-scale pore size distribution of aerogels made of (a) 10 mg/ml GO, (b) 10 mg/ml mGO/GO (50 wt% GO), and (c)10 mg/ml Ti3C2Tx/GO (20 wt% GO) suspensions. Average micro-scale porosities of generated aerogels out of 10 mg/ml GO, mGO/GO, and Ti3C2Tx/GO suspensions. ................................................................................................................................ 194 Figure 7-19 A comparison between the compressibility of various fabricated aerogels. (a) Worm-like aerogels of pure GO showcased elastic behavior, (b) while Ti3C2Tx/GO worm-like aerogels cannot elastically recover after compression. The Janus aerogels of (c, e) Ti3C2Tx/GO:mGO/GO and (d) Ti3C2Tx/GO:GO also showcased exceptional compressibility. (e) The Janus aerogel of Ti3C2Tx/GO:mGO/GO was capable of pushing a ~20 g sample upward after compression. All scale bars in (a-e) correspond to 2 cm. ................................................................................................ 194 Figure 7-20 (a, b) EMI shielding characteristics of the worm-like structures composed of Ti3C2Tx/GO (20 wt% GO), mGO/GO (50 wt% GO), and pure GO. (c) Electrical conductivity of Ti3C2Tx/GO (20 wt% GO) worm-like aerogels. The thickness of these aerogels is 3mm. ........ 198 Figure 7-21 EMI shielding characteristics of pure GO aerogels, printed with a 200 μm nozzles. I and II refer to the data obtained from two different sides of the samples. The thickness of these xxxi samples was 3 mm. Three different samples were tested: (a, b) sample one, (c, d) sample two, and (e, f) sample three. ...................................................................................................................... 199 Figure 7-22 EMI shielding characteristics of mGO/GO (50 wt% GO) aerogels, printed with a 200 μm nozzles. I and II refer to the data obtained from two different sides of the samples. The thickness of these samples was 3 mm. Three different samples were tested: (a, b) sample one, (c, d) sample two, and (e, f) sample three. ....................................................................................... 200 Figure 7-23 (a) EMI shielding characteristics of Ti3C2Tx/GO (20 wt% GO) and mGO/GO (50 wt% GO) Janus aerogels prepared by two 200 µm nozzles. (b) Schematic illustration of internal scattering in Ti3C2Tx/GO (20 wt% GO) and mGO/GO (50 wt% GO) Janus aerogels from macro-to micro-scale. (c, d) EMI shielding characteristics of copper coated magnetic/conductive Janus aerogels. This shield consists of a 3 mm Janus aerogel covered with 0.05 mm conductive copper tape from one side. The Janus aerogel of this shield is designed in a way that the magnetic parts' volume is larger than the conductive domain. (c) summarizes the EMI shielding characteristics of the aerogel sides, while (d) showcases those characteristics that belong to the copper side of the shield. (e) A comparison of the SET vs. A of the magnetic/conductive Janus structures with the previous best practices in the literature. For more information, see Table 7-1. ......................... 201 Figure 7-24 EMI shielding characteristics of Ti3C2Tx/GO (20 wt% GO) and mGO/GO (50 wt% GO) Janus aerogels, printed with two 200 μm nozzles. I and II refer to the data obtained from two different sides of the samples. The thickness of these samples was 5 mm. Three different samples were tested: (a, b) sample one, (c, d) sample two, and (e, f) sample three. ................................ 202 Figure 7-25 EMI shielding characteristics of copper coated magnetic/conductive Janus aerogels. This shield consists of a 3 mm Janus aerogel covered with 0.05 mm conductive copper tape from one side. The Janus aerogel of this shield is designed in a way that the magnetic parts' volume is xxxii larger than the conductive domain. I and II refer to the data obtained from two different sides of the samples: (I) Janus aerogel side and (II) copper side. Three different samples were tested: (a, b) sample one, (c, d) sample two, and (e, f) sample three. .............................................................. 203 Figure 7-26 (a) The stress-strain curves of the Janus aerogels of Ti3C2Tx/GO (20 wt% GO):GO at different sets of strains. (b) Digital images showcasing the compressibility of Ti3C2Tx/GO (20 wt% GO):GO aerogels. (c) The cyclic piezoresistive sensing performance of the Janus aerogel under different compression strains. The scale bar in (b) corresponds to 2 cm. ................................... 207 Figure 7-27 Fatigue hysteresis tests of Ti3C2Tx/GO (20 wt% GO) and GO Janus aerogels. The samples were compressed up to (a) 40% and (b) 50% strain for 100 cycles. The results showed minor relaxation at those strain levels, where the aerogels maintained ~80% of their initial Young's moduli and maximum stresses. ................................................................................................... 208 Figure 7-28 SEM images of Janus aerogels of Ti3C2Tx/GO (20 wt% GO) and GO after 100 compression cycles. The sample shown in (c) was compressed during SEM imaging. All scale bars in (a-c) correspond to 1 mm. ....................................................................................................... 208 Figure 7-29 The (a) sensitivity and (b-c) response/recovery times of the fabricated sensor. The recorded signal for (d) soft and (e) hard press of the sensor. ...................................................... 210 Figure 7-30 The durability of the sensor after 2,000 compression cycles at real conditions. To assess the durability of these sensors, five different participants were asked to touch the sensor, and its response was recorded over the course of the experiment. ............................................. 213 Figure 7-31 The recorded response and digital images of the sensing performance of the fabricated structures (a-b) before and (c-d) after three months. All scale bars in (b, d) correspond to 2 cm...................................................................................................................................................... 214 xxxiii Figure 7-32 The capability of the sensor for monitoring (a) finger movements and (b) contraction-relaxation biceps muscles, both as large-scale body movements. The recorded response for various subtle human motions, including (c) talking, (d) coughing, and (e) facial muscle movements. 215 Figure 7-33 The capability of the fabricated device to distinguish different alphabets and even polysyllabic words when attached to the skin outside of the larynx of a human subject. .......... 215 xxxiv List of symbols GF Gauge factor s Sensitivity R Reflectance A Absorbance T Transmittance PR Reflectance wave power PT Transmitted wave power PI Incident wave power dB Decibels SET Total shielding effectiveness SER Reflection loss SEA Absorption loss SEM Multiple reflections loss ɳ Impedances ɳ𝟎 Impedances of air σ Conductivity μ Permeability δ Skin depth SSE/t Specific shielding effectiveness ρ Density RL Reflection loss 𝛼 Attenuation constant 𝛆 Dielectric permittivity 𝛚 Angular frequency xxxv List of abbreviations CA Cellulose acetate EDS Energy dispersive spectrometer EMI Electromagnetic interference FESEM Field emission scanning electron microscope FTIR Fourier-transform infrared spectroscopy GO Graphene oxide HRTEM High-resolution transmission electron microscope mGO Magnetic graphene oxide PAM Polyacrylamide PEDOT:PSS Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate PEI Polyethylenimine PET Polyethylene terephthalate POSS-NH2 PSS-[3-(2-aminoethyl)amino]propyl-heptaisobutyl substituted POSS SE Shielding effectiveness SSE Specific shielding effectiveness TGA Thermogravimetric analyzer VAF Vacuum-assisted filtration VNA Vector network analyzer XPS X-ray photoelectron spectroscopy XRD X-ray diffraction xxxvi Acknowledgements I wish to seize this moment to express my heartfelt gratitude for the invaluable assistance and unwavering support I have been fortunate to receive over the past four years. My esteemed supervisor, Dr. Mohammad Arjmand, has not only guided me through my academic journey but has also served as an enduring wellspring of inspiration. His relentless dedication and indefatigable work ethic have made working alongside him as a PhD student a profound privilege. I must also extend my deepest appreciation to Dr. Milad Kamkar, a former postdoctoral researcher within our research group, who has now ascended to the position of Assistant Professor at the University of Waterloo. His guidance and insightful advice during the initial stages of my PhD laid a strong foundation and ensured a smooth trajectory through this arduous academic voyage. The completion of this thesis has only been made possible through collaborative efforts with several international universities and institutes. I am immensely grateful for the support and mentorship provided by luminaries such as Professor Orlando J. Rojas from the Bioproducts Institute at the University of British Columbia, Professor Masoud Soroush from Drexel University, and Professor Thomas P. Russell from the Lawrence Berkeley National Laboratory. These collaborations have not only enriched my academic knowledge but have also instilled within me the essence of effective teamwork, which I believe will resonate throughout my entire academic career. I cannot overstate my appreciation for Professor Stefan Wuttke from the Basque Centre for Materials, who has not only imparted profound scientific insights but has also been a pillar of motivation and unwavering support throughout these transformative years. Beyond the realm of research, I have had the privilege of learning from him how to be a compassionate and positive presence within the academic environment. xxxvii Lastly, and most profoundly, I wish to convey my deepest gratitude to my wife, whose unwavering support has been the cornerstone of my journey through the labyrinth of my PhD. Without her unwavering assistance, none of this would have been conceivable. xxxviii Dedication To my wife, Sonia 1 Chapter 1: Background and thesis organization The primary goal of this thesis is to explore how structural design influences the development of advanced conductive structures. Our focus lies on four specific types of structural design: laminated, 3D printed, polymer nanocomposites, and aerogel-based conductive structures, each created through various processing methods. We then delve into assessing the performance of these structures in the context of two prominent applications: electromagnetic interference (EMI) shielding and strain/pressure sensing. In the following sections, we will elaborate on the motivations and the step-by-step progression of our research as presented in this thesis. 1.1 Motivation In the contemporary landscape, research groups across the world are engaged in the development of synthetic conductive structures. The motivation is quite clear: traditional conductive metals, commonly used in electronic devices, no longer meet the requisite properties for certain components of modern electronics. For example, in applications like EMI shielding, which is pivotal for the performance of smart electronics, military equipment, and high-frequency devices, we have moved away from traditional metals due to their weight, bulkiness, and susceptibility to corrosion. This shift is particularly significant when considering the development of compact and lightweight electronic devices. Furthermore, traditional conductors can no longer support innovative applications, including sensitive human motion monitoring systems. The new generation of sensors based on synthetic conductive structures such as aerogels offers numerous novel perspectives, including real-time monitoring, accurate conversion of movement into signals, and rapid response times, making them indispensable in the emerging electronic revolution. However, many material scientists working in the field of synthetic conductive materials tend to focus primarily on material selection, occasionally overlooking the crucial role of processing in 2 literature. In this thesis, our motivation is to explore various processing strategies for designing distinct categories of conductive structures and to evaluate their performance in EMI shielding and sensing applications. We begin with straightforward processing methods, such as drop casting, allowing us to gain insights into material selection, setup preparation, and characterization. Our work then progresses to advanced processing techniques, highlighting the essential role that structural design plays in the development of multifunctional materials. 1.2 Research gaps and objectives The primary objectives of this thesis are intricately linked to the existing limitations of synthetic conductive structures in the realms of EMI shielding and strain/pressure sensing applications. In the context of synthetic conductive materials for EMI shielding, a critical concern is the prevalence of reflection-dominant EMI shielding mechanism evident in much of the existing literature. Notably, these synthetic EMI shields inadvertently generate secondary EMI shielding pollution, akin to the issues associated with conventional metals they aim to replace. Aside from this challenge, we need to reduce materials consumption, fabrication costs, and overall size of synthetic conductive structures to enhance their viability in compact electronics. Improving the mechanical characteristics of various types of synthetic conductive structures is also imperative for their integration into the next generation of flexible electronics. Moreover, these conductive structures must be easily adaptable to diverse and complex geometries found in equipment across fields such as medical, military, and communication devices. Similar challenges have also been noted with the synthetic strain/pressure sensors. To scale production and meet market demands for human motion monitoring and medical sensors, there is a pressing need to reduce fabrication costs and material consumption. Simultaneously, the development of strain/pressure sensors requires a delicate balance between achieving a high 3 detection range and optimal sensitivity. It is worth noting that many existing studies showcase high sensitivity but fall short of providing an expansive detection range, limiting their applicability. Additionally, the stability of these sensors must be rigorously examined in real-life applications, considering forces acting in different directions, as well as environmental parameters such as temperature and humidity. Addressing these multifaceted challenges demands focused attention on four key parameters: the selection of materials, processing methods employed for conductive structure fabrication, structural design considerations, and the ultimate properties achieved by each structure (refer to Figure 1-1). Careful material selection is imperative, focusing on easily processable materials capable of generating the requisite electrical conductivity for both EMI shielding and strain/pressure sensing. Prime examples include 2D nanomaterials such as MXene and graphene, along with intrinsically conductive polymers like poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). However, material selection alone is insufficient. The processing methods must yield structures with predefined properties that effectively address the challenges of EMI shielding and strain/pressure sensing. Unfortunately, current studies predominantly concentrate on material types, often neglecting structural design considerations. The central objective of this thesis is to underscore the pivotal role of structural design and processing in overcoming key challenges associated with EMI shielding and strain/pressure sensing. Herein, our research journey commenced with utilizing the drop-casting method to fabricate cost-effective and scalable laminated thin films suitable for EMI shielding applications. The overarching goal was to devise a strategy requiring no specialized equipment, yielding lightweight and flexible films adaptable to compact and flexible electronics. Subsequently, a transfer printing 4 approach was employed to conform these laminated films to complex geometries, addressing challenges inherent in EMI shielding. Transitioning to an extrusion printing approach facilitated the fabrication of 3D structures with minimal material requirements, customizable to various geometries inherent in diverse electronic devices. Additionally, highly porous, lightweight, and mechanically robust aerogels were developed using a novel fabrication technique termed liquid streaming, catering to both EMI shielding and pressure sensing applications. These soft materials proved particularly effective for EMI shielding, leveraging the presence of numerous interfaces within the structures to absorb electromagnetic waves, thereby overcoming the reflection-dominant shielding mechanisms prevalent in traditional EMI shields. Moreover, the developed aerogels served as a backbone for the fabrication of cost-effective polymer nanocomposites, incorporating a minimal amount of filler content and a unique absorption-dominant EMI shielding mechanism. Figure 1-1 Addressing the main challenges of EMI shielding and sensing: key factors and the significance of structural design. 5 1.3 Thesis organization and novelties Our journey in this thesis begins with a comprehensive literature review in Chapter 2, where we delve into the fundamentals of structural design, provide background on EMI shielding, and offer a brief overview of pressure/strain sensing. Chapter 3 takes us through the synthesis recipes and characterization of three essential nanomaterials: Ti3C2Tx, graphene oxide (GO), and magnetic GO (mGO). These nanomaterials serve as the fundamental building blocks for the structural designs in this thesis, and our results highlight their successful synthesis, each possessing unique characteristics. In Chapter 4, we embark on our exploration of conductive structural design using a straightforward yet versatile drop-casting method. In essence, we craft highly conductive laminated films of Ti3C2Tx/PEDOT:PSS through drop-casting. While our approach ensures the uniformity of our nanocomposite films, on par with conventional techniques like vacuum-assisted filtration (VAF), it offers significant cost savings and a simplified preparation process. Furthermore, we introduce a wet-transfer technique that allows us to adapt these drop-casted films onto intricate geometries with curves, angles, and corners. This seemingly simple project not only presents a scalable processing method for EMI shields but also acquaints us with the foundational aspects of EMI shielding measurements and calibration, material handling, serving as a cornerstone for showcasing our advancements in EMI shielding absorbers. Chapter 5 marks a significant milestone as we introduce one of the pioneering instances of laminated 3D printed structures featuring Ti3C2Tx/PEDOT:PSS. These structures find applications in the realms of EMI shielding and strain sensing. We grapple with two key challenges inherent to extrusion printing: its constrained printing resolution and the difficulty of fabricating structures that possess both high electrical conductivity and mechanical flexibility. Our journey commences 6 with the development of aqueous, additive-free inks, housing Ti3C2Tx and Ti3C2Tx/PEDOT:PSS, complete with ideal rheological properties for achieving high-resolution extrusion printing. This endeavor also led to the genesis of composite structures of MXenes with both high electrical conductivity and mechanical flexibility. This knowledge paves the way for the creation of ultrathin, flexible EMI shields, applied onto double-sided tapes. These printed shields prove adept at adapting to complex geometries and exhibit impressive EMI shielding characteristics. Furthermore, we employ printed grids of pure Ti3C2Tx to fashion sensitive strain sensors, delivering an extraordinary gauge factor of 11,300 at a 4% strain. These sensors exhibit the capability to monitor a broad spectrum of subtle human movements, encompassing actions like breathing, facial muscle movements, speech, and the contraction and relaxation of bicep muscles. Throughout this project, we not only expand the horizon of cost-effective printed structures but also deepen our understanding of EMI shielding within laminated structures, where even in the context of printed grids, the mechanism relies on the reflection of electromagnetic waves. Additionally, we familiarized ourselves with the principles of strain sensing and the secure packaging of delicate sensors, a seemingly simple yet intricate task that required multiple iterations to achieve success. Chapter 6 introduces a fresh perspective on EMI shielding structures, focusing on aerogels and polymer nanocomposites. In our pursuit of aerogel fabrication, we devised an entirely novel approach known as "liquid streaming." This innovative method capitalizes on the interfacial jamming of nanoparticle surfactants (NPSs) to stabilize aqueous suspensions of Ti3C2Tx/GO within a nonpolar medium. This unique process results in the creation of filamentous liquids that act as templates for crafting ultralight aerogels, characterized by micro- and macro-scale porosities. The outcomes are nothing short of remarkable, as the performance of these structures 7 either matches or surpasses that of some of the finest available porous EMI shields. What sets these aerogels apart is their significant advantages: they require no chemical or thermal reduction or extensive pre-processing. Notably, these aerogels exhibit outstanding EMI shielding effectiveness, specific EMI shielding effectiveness, and exceptionally low density. Furthermore, these aerogels serve as the backbone for highly efficient EMI shields, even at a low filler loading range of 0.3-0.7 wt% when integrated into conductive epoxy nanocomposites. In terms of EMI shielding attributes, the presence of micro- and macro-scale porosities substantially enhances the absorption characteristics of these aerogels. Intriguingly, when incorporated into epoxy nanocomposites, these structures venture into the realm of absorption-dominance, which marks a notable departure from the laminated structures explored in the previous section. This pivotal observation underscores the pivotal role of structural design in shaping the EMI shielding capabilities of conductive materials, which serves as the core theme of this thesis. Chapter 7 continues our exploration of aerogel-based conductive materials, introducing a novel concept we refer to as Janus liquids. Within this section, we unveil the creation of Janus liquids with customizable nanoparticle (NP) distributions by merging two aqueous NP dispersions within an apolar liquid. Through this anisotropic integration platform, various types of NPs, including magnetic, conductive, or non-responsive ones, can be selectively confined to opposite sides of the original interface using suspensions like magnetic graphene oxide (mGO)/GO, Ti3C2Tx/GO, or GO. These resulting Janus liquids serve as templates for versatile, responsive, and mechanically robust aerogels, suitable for applications in piezoresistive sensing, human motion monitoring, and EMI shielding, with a tuned absorption mechanism. In particular, the EMI shields created with this 8 approach outperform current counterparts in terms of wave absorption owing to their high porosity spanning from micro- to macro-scales and the non-interfering magnetic and conductive networks introduced by the Janus architecture. Furthermore, our research addresses long-standing issues pertaining to the mechanical characteristics of MXene-based aerogels. The development of Janus MXene/GO materials led to the creation of Janus pressure sensors for piezoresistive sensing and human motion monitoring, boasting remarkable sensitivity and an exceptional operating range. In summary, our findings emphasize that the Janus liquid concept provides a distinctive alternative to traditional composite, blend, and grafted system fabrication methods, enabling the simultaneous attainment of diverse functionalities within a single structure without interference. The key insights and innovations from each chapter are summarized in Table 1-1, and Figure 1-2 provides a visual roadmap, elucidating the significance of each chapter in understanding the broader context of structural design for EMI shielding and strain sensing. 1.4 Research journey overview In summary, this thesis unveils a coherent journey, guiding the exploration through a spectrum of processing methodologies—from fundamental to advanced—to confront essential challenges in EMI shielding and strain sensing. Beginning with the application of basic processing techniques, such as drop-casting, for crafting synthetic conductive structures, we successfully navigate issues related to fabrication costs and scalability. However, persistent challenges, especially concerning the reflection-dominant shielding mechanisms crucial for effective EMI shielding, propel our progression. Advancing further, we make notable strides in both cost reduction and the accommodation of intricate geometries by embracing 3D printing. Despite these advancements, 9 challenges persist, particularly in mitigating reflection-dominant shielding mechanisms and expanding the detection range for strain sensors within the realm of 3D printed structures. In response to these ongoing challenges, we adopted an innovative processing technology known as liquid streaming. This distinctive approach facilitates the cost-effective fabrication of highly porous EMI shields with a worm-like morphology, representing a departure from conventional methods. Subsequently, we emphasize the significance of these soft functional structures by introducing functional Janus structures. This development showcases distinctive absorption-dominant characteristics and highly sensitive pressure sensors with an expanded detection range. Figure 1-2 Experimental framework of this thesis. anomaterial synthesis: Ti3C2Tx/P OT:P filmsFabricated via drop casting approach Ti3C2Tx /P OT:P gridsAnother class of structures orm li e structures of Ti 3C2Tx/GO and based constructs Janus liquids and their transformation into Janus 10 Table 1-1 A concise overview of the concepts and research presented in this thesis. Chapter # Short concept Remarks 2 Literature review • Background of Structural Design • Understanding EMI Shielding: Concepts and Formulations • A Brief Overview of Strain and Pressure Sensing 3 Nanomaterial synthesis • Synthesis recipes and characterization of Ti3C2Tx, graphene oxide (GO), and Magnetic graphene oxide (mGO) 4 Laminated and conductive structures via drop-casting of Ti3C2Tx/PEDOT:PSS • Showcasing a straightforward yet versatile drop-casting method for fabricating EMI shields • Understanding the foundational aspects of EMI shielding measurements and calibration, • Grasping the concept of reflection-dominant EMI shields and impedance mismatch • Gaining valuable insights into material handling 5 3D printed structures featuring Ti3C2Tx/PEDOT:PSS • Introducing 3D printed structures of Ti3C2Tx /PEDOT:PSS as a novel and cost-effective generation of EMI shields and strain sensors. • Addressing two key challenges inherent to extrusion printing: its constrained printing resolution and the difficulty of fabricating structures that possess both high electrical conductivity and mechanical flexibility. • Expanding our knowledge of laminated EMI shields. • Familiarizing ourselves with the principles of strain sensing and the secure packaging of delicate sensors. 6 Worm-like structures of Ti3C2Tx/GO for EMI shielding • Introducing an entirely innovative approach called liquid streaming for the fabrication of aerogels. • Highlighting the impact of porosity in enhancing the EMI absorption characteristics of materials. • Developing cost-effective polymer nanocomposites by infusing epoxy into worm-like structures and their application in EMI shielding. • Progress toward the criteria for absorption-dominant EMI shielding through aerogel structural design. 7 Creating Janus liquids and their transformation into Janus aerogels • Presenting the production of Janus liquids featuring customizable nanoparticle (NP) distributions, achieved by combining two aqueous NP dispersions within a nonpolar liquid. • This anisotropic integration approach enables the selective placement of different types of NPs, such as magnetic, conductive, or non-responsive, on opposing sides of the initial interface. • Unveiling a groundbreaking generation of EMI shields dominated by absorption, showcasing exceptional performance. • Designing Janus pressure sensors for piezoresistive sensing and human motion monitoring, characterized by remarkable sensitivity and an extensive operating range 8 Conclusion and future work • A concise overview of accomplishments, accompanied by suggestions aligned with the current project, for prospective research endeavors. Chapter 2: Background and literature review The core focus of this thesis revolves around the engineering of synthetic conductive structures designed for EMI shielding and strain/pressure sensing applications. Recognizing the pivotal role of structural design in our research, we have dedicated an entire section to exploring various structural possibilities based on conductive materials. This includes discussions on laminated structures, porous compositions, traditional polymer nanocomposites, and aerogel/foams. Additionally, we have devoted a section to elucidate the advantages and significance of 3D printing in the context of structural design. In the subsequent literature review, we also delve into the fundamental principles, mechanisms, and formulations that underpin our EMI shielding calculations and the development and characterization of strain/pressure sensors. This section aims to offer a concise overview of the concepts that will be frequently referenced in the subsequent chapters of this thesis. 2.1 Structural design of conductive architectures 2.1.1 Conductive polymer nanocomposites Conductive polymer nanocomposites represent the vanguard of synthetic materials in the realm of EMI shielding and electronic packaging, poised to replace conventional metallic shields. Their popularity has soared, thanks to the inherent processability of base polymers, opening up vast potential in numerous industries and marking a transformative shift in EMI shielding solutions. Given this ease of processability, it is of paramount importance to underscore the creation of cost-effective, high-performance polymer nanocomposite shields, with a special focus on manufacturing techniques like extrusion and injection molding [1,2]. Irrespective of the chosen processing technique, the establishment of continuous conductive networks within insulating polymers is critical to ensure adequate conductivity in these structures. 12 This entails reaching a nanofiller content threshold that surpasses the electrical percolation threshold—a model that defines long-range connectivity in random media. If the filler content falls short of this threshold, it impedes the formation of a robust conductive network. Conversely, exceeding the threshold significantly enhances the nanocomposite's conductivity. It's worth noting that the percolation threshold is closely tied to the dispersion and aspect ratios of the nanofillers. Creating a robust conductive network within these nanocomposites often requires a substantial inclusion of nanofillers, which may not be optimal for large-scale applications. Consequently, there's a strong emphasis on developing polymer nanocomposites that incorporate low filler content while ensuring an even distribution [1,3]. It's worth noting that various studies have explored the possibility of encapsulating conductive aerogels or foams within polymers like polydimethylsiloxane (PDMS) and epoxy [4]. However, this strategy has yet to fully unlock its industrial potential, as large-scale fabrication of these conductive porous structures remains a subject of debate. Therefore, one of the central objectives of this thesis is to develop scalable polymer nanocomposites based on aerogel/foam encapsulation, with the potential to bridge the gap between promising research findings and practical industrial applications. 2.1.2 Laminated thin films Laminated thin films have emerged as a captivating realm of scientific exploration, with a specific focus on highly conductive materials such as MXene, PEDOT:PSS, and graphene. These films, typically only a few micrometers thick, have garnered immense interest from materials scientists for their versatile applications spanning electromagnetic interference (EMI) shielding, sensing technologies, thermal heating solutions, and more. Their appeal is deeply rooted in their compact 13 and lightweight nature, which renders them exceptionally well-suited for scenarios where the thickness of a shielding material is a pivotal consideration [5,6]. One of the standout advantages of these ultra-thin films is their ability to function effectively as protective barriers without adding significant bulk or weight. When we venture into the realm of nanometer-scale thickness, these laminated structures unveil an extraordinary facet – optical transparency. This optical property opens the gateway to an array of novel and groundbreaking applications, including advanced window heaters and transparent EMI shields [7,8]. Various processing technologies have been employed to date for the development of micrometer thick thin films made from conductive materials such as MXene, graphene, and PEDOT:PSS. These methods encompass drop-casting, vacuum-assisted filtration (VAF), blade coating, and even spray coating [9,10]. Among these techniques, vacuum filtration entails the preparation of a stable suspension, which is subsequently filtered using a suction filtration device. In this process, particles accumulate firmly on the surface of a porous membrane, culminating in the formation of a thick layer with the desired thickness. The inherent self-limiting nature of this filtration process yields a uniform, self-leveling film on the filter membrane [5]. However, vacuum filtration comes with its own set of drawbacks. The entrapment of air bubbles during the filtration process can lead to the creation of minor defects or imperfections in the resulting film. Additionally, vacuum filtration can be resource-intensive, potentially resulting in substantial material wastage during large-scale applications [11]. Nevertheless, despite these limitations, VAF remains a popular choice in experimental-scale research and design. As an illustrative example, Figure 2-1 showcases recent studies featuring various MXene thin films developed through the VAF method, with applications geared towards EMI shielding [5]. This enduring popularity of vacuum-assisted filtration underscores its unique capacity to harness the 14 potential of micrometer films, offering a valuable tool for researchers and innovators to explore the frontiers of materials science and embark on new horizons in technological applications. Spray coating, as a deposition method, often finds its practicality within structures featuring a thickness of approximately 1µm or less [9]. However, it frequently encounters challenges related to ensuring uniformity, particularly when the structural thickness exceeds the 1µm threshold. Conversely, casting or drop casting represents another noteworthy technique employed in the fabrication of micrometer-thick structures. Casting, being a versatile approach, demands minimal specialized equipment and capital investment, rendering it of paramount importance for large-scale industrial thin film production [9,12]. However, achieving uniformity in structures through the casting approach necessitates a deep understanding of the chemistry governing both the substrate and the suspension being cast. In essence, the synergy of functional groups present in the substrate and the nanomaterials within the suspension significantly influences the uniformity of the resulting thin films. This interplay of chemical attributes is a key determinant of the quality and consistency achievable in the final thin films. 15 Figure 2-1 A variety of MXene thin films, each several micrometers in thickness, have been created using the VAF technique for the purpose of EMI shielding. Reproduced with permission [5]. Copyright 2020, American Chemical Society. In contrast, blade coating, a scalable technology widely adopted in industrial settings for the production of free-standing films and coatings, emerges as a well-suited approach to address these specific requirements [10,13]. To illustrate this point, consider the production of scalable, self-supporting Ti3C2Tx films featuring well-aligned MXene flakes through the blade coating method. 16 The alignment of Ti3C2Tx flakes was achieved by applying shear stress through the adjustment of the blade speed, which dictates the rate at which the blade is moved from start to end and the blade height, which represents the distance between the substrate and the blade edge [10,13]. Significantly, it has been observed that films created through the blade coating process exhibit superior electrical and mechanical properties when compared to those generated via vacuum-assisted filtration [13]. This observation underscores the pivotal importance of maintaining meticulous control over the orientation of flakes within the Ti3C2Tx film, serving as a critical factor in achieving a harmonious synthesis of elevated strength and enhanced conductivity. These findings present valuable insights, particularly in the context of academic research and its potential implications. Highly conductive transparent thin films represent a prominent category of laminated structures extensively integrated into a wide range of electronic devices, photovoltaics (including solar cells), mirrors, and the optical industry. In the past, conventional composite materials based on indium tin oxide (ITO) served as the primary choice for the development of conductive transparent thin films. However, the utilization of ITO has faced constraints stemming from indium scarcity and its inherent brittleness. Consequently, the scientific community has shifted its focus towards the exploration of a new generation of synthetic materials, including metallic grids with micron-scale line widths and high aspect ratios, such as AgNWs, MXene, and graphene, with the aim of supplanting ITO [7,14,15]. Moreover, the development of transparent thin films has primarily centered on methodologies such as spin coating, dip coating, and spray deposition, all capable of crafting submicron transparent structures [7]. However, it is worth noting that these processing technologies exhibit a substantial propensity for material wastage, which significantly hampers the feasibility of large-scale production of highly conductive transparent thin films. 17 2.1.3 Aerogels and foams Porous structures, such as foams, sponges, and aerogels, have emerged as a noteworthy category of conductive multifunctional frameworks. In the context of EMI shielding, porous structures have proven to be effective in dissipating incident electromagnetic waves while minimizing secondary reflections, provided their electrical characteristics are precisely engineered. These structures hold the promise of achieving exceptional shielding effectiveness, augmented by their key physicochemical attributes, specifically low density (0.003-0.500 g/cm³) and high porosities (80-99.8%). Porous structures also find applications in the development of advanced pressure sensors. Within this class of sensing platforms, highly porous materials housing a complex three-dimensional network of interconnected nanomaterials can convert minute changes in applied pressure into variations in electrical signals, such as changes in resistivity or conductance. In this intricate structure, when external pressure is applied to the aerogel or foam, it undergoes mechanical deformation, thereby altering the spacing between these conductive pathways. Consequently, the electrical resistance or conductance of the aerogel/foam experiences modification that we can record [16]. Our primary focus in this thesis revolves around the development of conductive aerogels and their utilization in EMI shielding and sensing applications. Therefore, it is imperative to introduce fabrication strategies that ensure the creation of highly conductive and mechanically robust aerogels. Notably, the drying process is of paramount importance in aerogel fabrication. However, during the typical drying process, capillary forces resulting from the vaporization of solvent molecules can lead to the collapse of pore structures, affecting the textural properties of aerogels. To circumvent this issue, the freeze-drying method is usually employed to minimize stresses on 18 the pores. In this method, the solvent, typically water, is frozen and then sublimated under the solvent's triple point conditions, resulting in the evaporation of frozen solvent molecules without significant stress or damage to the pore structures [17]. It's worth noting that the fabrication of aerogels often requires techniques that align nanomaterials before the freeze-drying approach to enhance the mechanical characteristics and electrical conductivity of the final structures. These methods include chemical-induced gelation, freeze casting, and interfacial assembly, which are routinely employed for 2D nanomaterial-based aerogels. The chemical-induced gelation process typically creates stable media with a uniform distribution of nanoparticles before the drying process. Gelation occurs when colloidal particles in a sol or solution come together to form a spatial framework under specific temperature and pressure conditions, with the structure's spaces filled by a proper liquid as a dispersing phase. During the gelation process, when the concentration of nanoparticle suspensions exceeds the critical gel concentration, the attractive and repulsive forces between the sheets become unbalanced. As a result, the dispersion phase becomes viscous, transforming into an elastic colloid capable of withstanding greater pressure, which is beneficial for the structure's stability after freeze-drying [17]. On the other hand, the freeze-casting method focuses on the influence of ice crystal formation and growth on the mechanical properties of porous ceramics. This approach utilizes ice crystals as templates during water solidification, leading to effective phase separation. Subsequent extraction of the frozen solvent through sublimation results in the creation of a 3D hierarchical network with aligned porosities (refer to Figure 2-2) [17,18]. Freeze casting is also valuable for engineering the architecture of 2D nanomaterial aerogels, where nanosheets are stacked between ice crystals and form continuous 3D networks after sublimation. The freeze-casting approach is particularly 19 intriguing for EMI shielding and sensing applications, as the resulting aerogels exhibit exceptional mechanical compressibility and electrical conductivity [17]. Figure 2-2 Schematic representation of freeze casting for MXene/Graphene suspension reveals the process in which ice crystals serve as templates, aligning the 2D nanomaterials. Reproduced with permission [19]. Copyright 2018, American Chemical Society. The assembly of nanomaterials at the interface of two immiscible liquids represents an emerging platform for the fabrication of soft-functional materials. In this technique, a reduction in Helmholtz free energy drives solid particles toward the liquid-liquid interface. However, the energy of the liquid interface for nanomaterials is much lower than for micrometer-sized colloidal particles. Consequently, nanoparticle assemblies at liquid-liquid interfaces tend to be unstable due to their dynamic nature, size dependence, and association with the constant adsorption and desorption of nanomaterials. To overcome these instabilities, the addition of oligomeric/polymeric ligands at the oil-water interface has been recognized as an efficient way to enhance nanoparticle interfacial 20 performance. In this strategy, nanoparticles and ligands interact at the interface, creating nanoparticle surfactants that allow the nanoparticles to jam at the interface [17]. The interfacial assembly and jamming of nanoparticles at the liquid-liquid interface result in non-equilibrium forms of one liquid phase within another immiscible liquid. It's important to note that these structured liquids are programmable, as they can be un-jammed and re-shaped. Such a programmable fabrication platform serves as an innovative pre-treatment for large-scale fabrication of robust aerogels with well-defined micro- to macro-scale characteristics. In this thesis, we have employed interfacial assembly as a novel technique to develop a new class of worm-like structures for EMI shielding and sensing applications, achieving exceptional performance that cannot be attained through other aerogel fabrication methods [17]. 2.1.4 3D printed structures 3D printing is, in reality, an evolving approach, and it doesn't directly fit into a distinct category of structural design. In fact, 3D printing can be employed to create polymer nanocomposites through Fused Deposition Modeling (FDM) [20] or to fabricate laminated 3D printed conductive structures through a process known as Direct Ink Writing (DIW), which is also referred to as extrusion printing (Refer to Figure 2-3) [21]. Moreover, several studies have ventured into developing conductive 3D printed hydrogels using various conductive materials, including MXene, graphene, and PEDOT:PSS, and subsequently transforming these hydrogels into 3D printed conductive aerogels. The merits of 3D printing are so significant that we have chosen to dedicate a specific section to 3D printed conductive structures in our work [20,21]. Since its inception with Charles Hull's pioneering stereolithography in 1980, a multitude of innovative additive manufacturing techniques, including FDM, electron beam melting (EBM), laser sintering (LS), DIW, and laminated object manufacturing (LOM), have emerged [20-22]. Each 21 of these techniques contributes to the fabrication of complex 3D structures. These printing methods enable the creation of designs with intricate features such as spiral channels, chevrons, and custom porosities throughout the structures. The flexibility inherent in 3D printing allows users to fine-tune both micro and macro designs of shielding structures, optimizing their electromagnetic wave (EMW) dissipation capabilities through strategies like internal reflections. Thanks to its remarkable efficiency, cost-effectiveness, and versatility in design, as well as its capacity to craft intricate geometries, 3D printing has left an indelible mark on technological advancements. Its proficiency in manipulating textural attributes and printing multi-material composites has positioned additive manufacturing as a central pillar of the third industrial revolution, particularly in applications related to EMI shielding and sensing [22]. Due to these advantages, we have given special attention to 3D printing in this thesis. Specifically, we have developed various 3D printed laminated structures made of MXenes and PEDOT:PSS and investigated their capabilities in EMI shielding and strain sensing. 22 Figure 2-3 (a) FDM is employed in the production of PLA/GNPs/CNTs nanocomposites, forming intricate honeycomb structures tailored for EMI shielding applications. Reproduced with permission [20]. Copyright 2020, American Chemical Society. (b-c) Extrusion printing of MXene/PEDOT:PSS inks is harnessed to create versatile 3D printed structures, including laminated and aerogel-based designs. Reproduced with permission [21]. Copyright 2022, Wiley-VCH. 2.2 Piezoresistive sensors The conversion principle of piezoresistive sensors lies in their ability to deform under external pressure/strain, altering the distribution density and contact state of the conductive filler inside. This results in predictable changes in the sensor's resistance. Piezoresistive sensors are valued for their wide pressure and strain sensing capabilities, and their straightforward operation and fabrication have made them a fascinating subject in material science. Researchers primarily 23 enhance piezoresistive pressure/strain sensor performance through two avenues: optimizing active materials and refining device structures [23-27]. In the domain of material design, carbon-based materials, like carbon black, carbon nanotubes (CNT), graphene, and polymers, exhibit remarkable piezoresistive properties, especially when combined with metal nanowires or metal nanoparticles (NPs) [25,26]. However, the recent focus on strain sensor development has shifted towards an emerging class of nanomaterials referred to as MXenes. MXenes, especially Ti3C2TX, provide a compelling foundation for the design of high-performance piezoresistive strain and pressure sensors owing to their exceptional electrical conductivity. This conductivity allows for efficient charge transport in response to mechanical deformation, facilitating precise measurements of changes in strain and pressure. Moreover, MXenes can be tailored to specific requirements through composition and surface termination adjustments, allowing for customization of sensitivity and response characteristics [26]. These materials also display remarkable mechanical robustness, especially when forming composites with polymers or undergoing processing techniques like blade coating or printing that align their flakes. This resilience renders them highly suitable for sensors subject to mechanical stress, such as those used in industrial or automotive settings. Furthermore, MXenes exhibit low-temperature sensitivity, ensuring the stability of their electrical properties across a wide temperature range, a crucial aspect for sensors deployed in diverse environmental conditions [25,26]. All in all, the exceptional electrical conductivity, tunability, and mechanical resilience of MXenes composites position them as a highly promising material for the development of piezoresistive strain and pressure sensors. These sensors find applications across a wide spectrum of industries, ranging from electronics to healthcare, underscoring the pivotal role of MXenes in advancing sensor technology. However, it's important to integrate the inherent sensing characteristics of 24 MXenes into an in-depth structural design to achieve optimal sensing performance across various conditions. In the subsequent sections, we delve into strain sensors under various structural designs, including aerogels and foams, hydrogels, and laminated-fracture-based sensors. 2.2.1 Laminated-fracture-based strain sensors Laminated thin films of highly conductive materials, such as MXenes, have emerged as a prevalent choice for crafting exceptionally sensitive crack-based strain sensors. For instance, let's consider pure Ti3C2Tx. The fabrication of Ti3C2Tx-based thin films often involves techniques like vacuum-assisted filtration (VAF), spray coating, casting, and more. Typically, these films are encapsulated within a stretchable polymer matrix and subsequently subjected to a crack propagation process. In these strain sensors, applying mechanical strain leads to the expansion of gaps between pre-existing cracks, resulting in a significant reduction in the conductance of the sensing layer (refer to Figure 2-4). With increasing strain, the edges of these cracks eventually become disconnected, causing the conductance to drop to zero [11,24]. An intriguing analogy can be drawn from the realm of nature, akin to the intriguing slit organs found in spiders. Much like fracture-based sensors, these spider organs also feature an array of cracks within their structures. When external stimuli are applied, these crack-shaped organs exhibit ultra-sensitive displacements, enabling the detection of minuscule vibrations in their surroundings. This sensitivity to small deformations opens the door to numerous applications in human health and motion monitoring. However, it's worth noting that the presence of these intrinsic cracks does impose certain limitations on the working range of these sensors. Consequently, laminated-fracture-based sensors are not ideal for monitoring large-scale strains. In conventional resistive strain sensors, the fundamental sensing mechanism involves the conversion of tensile strain into changes in resistance or currents. The sensitivity of such sensors 25 is quantified by the gauge factor (GF), defined as the ratio of the relative change in resistance (ΔR) to the initial resistance (R0) multiplied by the tensile strain (ε): 𝑮𝑭 =(𝑹−𝑹𝟎)𝜺𝑹𝟎 Equation 2-1 This gauge factor is a fundamental parameter that quantifies the sensor's sensitivity to mechanical strain and is central to understanding and characterizing its performance. Figure 2-4 (a) Schematic illustration showcasing the sensing mechanisms of Laminated-fracture-based strain sensors developed based on Ti3C2Tx. SEM images of various fracture-based strain sensors developed based on Ti3C2Tx at their maximum tolerable strains. The formation of microcracks is evident in (c-d). Reproduced with permission [24]. Copyright 2019, Wiley-VCH. 2.2.2 Aerogels and foam pressure sensors Conductive and compressible aerogels, crafted from synthetic materials, have garnered significant attention as a cornerstone in the development of pressure sensors capable of converting applied stress into variations in current or resistivity. This class of piezoresistive sensors boasts a versatile 26 array of applications, ranging from detecting human physiological conditions to enabling facial expression recognition, voice recognition, human motion monitoring, robotic tactile perception, and facilitating remote human-computer interaction [25,26]. The sensing mechanism of these innovative sensors revolves around the inherent characteristics of conductive aerogels and foam structures, which consist of highly porous materials housing a complex three-dimensional network of interconnected NPs or nanowires. Within this intricate structure, conductive pathways are established. When external pressure is applied to the aerogel or foam, it undergoes mechanical deformation, thereby altering the spacing between these conductive pathways. Consequently, the electrical resistance or conductance of the aerogel/foam experiences modification (refer to Figure 2-5). This innate sensitivity to pressure-induced deformation is pivotal, allowing even minute variations in pressure to yield substantial changes in electrical resistance or conductance. The shift in resistance or conductance is meticulously measured and subsequently converted into a precise and dependable pressure reading, rendering these piezoresistive pressure sensors, driven by conductive aerogels, highly sensitive and invaluable in diverse fields for precise pressure monitoring [23]. To gauge the performance of these sensors, they are routinely assessed based on sensitivity (s), as expressed by the equation: 𝑺 =∆𝑰𝑰𝟎∆𝑷 Equation 2-2 Where P signifies the pressure, typically in kPa-1, ΔI represents the relative change in current, and I0 denotes the initial current. It is noteworthy that Ti3C2Tx stands out as an ideal candidate for the development of aerogel/foam-based sensors due to its exceptional attributes, including high electrical conductivity, ease of processability, and, notably, water processability, which aligns seamlessly with the synthesis 27 methods employed in aerogel/foam fabrication. However, the inherent lack of mechanical compressibility in pure Ti3C2Tx aerogels often necessitates the development of hybrid aerogels, combining these materials with a wide range of polymers and even other 2D nanomaterials including GO to enhance the mechanical characteristics of the final structures. Additionally, novel fabrication strategies, such as freeze-casting, chemical-induced gelation, and interfacial assembly, are among the techniques commonly employed to improve the mechanical compressibility of porous structures based on Ti3C2Tx. Figure 2-5 (a) Synthesis procedure and (b) sensing mechanism of a piezoresistive-based pressure sensor employing a composite aerogel made from GO/Ti3C2Tx. Reproduced with permission [23]. Copyright 2018, American Chemical Society. 2.2.3 Hydrogel-based stain/pressure sensors Hydrogels are three-dimensional crosslinked polymer networks formed from either natural or synthetic materials. They possess high water-holding capacity, thanks to the hydrophilic groups in 28 the polymers and their crosslinked structure. Hydrogel-based strain and pressure sensors show tremendous potential in various fields. Their chemistry, network structure, and mechanical properties can be easily tailored to resemble those of biological tissues. Besides, the elastic modulus of hydrogels closely approximates that of human skin, making hydrogel-based strain and pressure sensors more compatible with biological tissues. This compatibility lays the foundation for designing intelligent skins and implantable devices [25-28]. Conductive hydrogel-based piezoresistive sensors leverage piezoresistivity, where mechanical deformation induces changes in electrical resistance. These sensors incorporate conductive elements, such as intrinsically conductive polymers or NPs, into the hydrogel matrix to establish a conductive network. Upon the application of an external force, the hydrogel undergoes deformation, thereby altering the conductive pathways and, as a result, the electrical resistance. Changes in resistance directly correlate with the applied force or pressure, making these sensors ideal for applications in wearable health monitors, soft robotics, and medical devices requiring precise pressure sensing [25-28]. It's worth noting that the performance of hydrogel-based sensors should be evaluated based on the gauge factor or sensitivity, as previously explained for their use as strain or pressure sensors, respectively. Despite the advantages of this class of sensors, we did not adopt them for the studies conducted in this thesis due to significant limitations arising from the conductive nanofillers and polymers in use. Specifically, while several studies have used hydrogels of MXenes as their sensing platforms, we observed rapid oxidation of MXenes in hydrogel-based mediums, limiting their applications and durability. Additionally, we found that fracture-based sensors and aerogel-based structures exhibited superior gauge factors and sensitivities compared to hydrogel-based MXene sensors. 29 Therefore, our focus in the remainder of this thesis excludes hydrogel-based systems in the development of piezoresistive sensors. 2.3 Shielding setup and background concept This section delves into the foundational concept of shielding and the methodologies employed for assessing diverse specimens. It is noteworthy that EMI shielding evaluations are routinely carried out in the X-band frequency range (8.2-12.4 GHz). This preference is influenced by the widespread application of this frequency range in radar systems, satellite communication, and other critical domains such as military and aerospace, where ensuring effective shielding is paramount. Moreover, the X-band is frequently employed in testing due to the ready availability and standardization of test equipment tailored for this specific range [29]. In this thesis, the EMI shielding properties of materials within the X-band frequency range are evaluated using a Vector Network Analyzer (VNA), with the Keysight model P9374A and WR-90 rectangular waveguide. Samples are affixed to an offset and positioned between WR-90 waveguide adaptors connected to the VNA through microwave cables. Further elaboration on the background concepts of EMI shielding and the formulations utilized is expounded upon in subsequent sections. 2.3.1 Conceptual mechanism of EMI shielding Electromagnetic (EM) waves, known as synchronized oscillations of electric and magnetic fields, interact with a surface or interior of an electromagnetic shield through three different pathways: reflection, absorption, and multiple reflections. Owing to some morphological design and material selection of electromagnetic shields, another shielding phenomenon, namely multiple internal reflections or internal scattering, could occur that demonstrates the EMI shielding characteristic of an electromagnetic shield. The initial interactive mechanism between an electromagnetic wave and a shield is the reflection that occurs due to the impedance mismatch between the conductive shield 30 or the conductive components within the shield and the free space [29-32]. The impedance of an electromagnetic wave is known as the ratio of the transverse components of the electric and magnetic fields. In the conductive shields, the impedance becomes much lower compared to the free space (377 Ω) because of their conductive additives, leading to the inability of the incident electromagnetic waves to propagate within the conductive components. This process leads to the transfer of energy from the incident electromagnetic waves to the free-charged particles and electrons in the conductive shield, forcing them to oscillate. The enforced oscillating charges of the shield create an induced or scattered field nown as “reflection.” Importantly, the reflection mechanism mainly happens at the surface of an electromagnetic shield and chiefly depends on the electrical conductivity of the as-developed shielding structures [29,33]. Along with the reflection mechanism, a portion of electromagnetic waves dissipates in the form of heat upon interacting with the EMI shielding materials/structures. This electromagnetic wave dissipating mechanism is nown as “absorption” and occurs in a structure/material with finite electrical conductivity and electrical/magnetic dipoles [29,34]; this mechanism is the ultimate goal of an EMI shielding system that should be achieved via smart structural design and component selection. Additionally, a portion of the incident electromagnetic waves also passes through the shielding structures/materials nown as “transmission.” There are also other important mechanisms that could significantly affect the total shielding characteristic of an EMI shielding configuration, namely multiple reflections and internal scattering. In previous studies, these concepts were wrongly confused with each other [35]. The multiple reflections mechanism occurs within thin EMI shielding films. In this regard, when incident electromagnetic waves pass through the front interface of a thin EMI shield, they might reflect back to the front interface upon interaction with the back interface, in which a portion of 31 the reflected electromagnetic waves leaves the shield and the trapped ones re-reflects within the shielding structure to contribute within the second transmission. This process repeatedly happens upon the incident electromagnetic wave completely loses its energy and becomes totally dissipated. The multiple reflection process is chiefly dependent on the thickness of the shield and becomes negligible at thicknesses close to or larger than the skin depth, viz., a distance beneath the shield's front surface where the intensity of the electric field declines to 1/e of the incident wave’s intensity. On the other hand, if the thic ness of the thin film electromagnetic shield becomes smaller than the skin depth, multiple reflections between the front and back interfaces of the shield will decline the shielding effectiveness of the final structure [29,36,37]. Importantly, internal scattering or internal multiple reflections occurs within materials with numerous internal interfaces or designed porosities such as aerogels, foams, segregated structures, and multi-layered shields. Contrary to the multiple reflections, the internal scattering can promote the shielding performance of a shield and boost its shielding effectiveness by means of absorption via boosting the dissipation of infiltrated electromagnetic waves into the structure of shields with numerous interfaces or porosities. In this matter, the numerous interfaces within a porous or smartly designed configuration with mismatching impedances contribute to multiple back-and-forth reflections within the shield, prolonging the propagation pathway of electromagnetic waves prior to transmission, and improving the effective interaction of the shield with electromagnetic waves till dissipating their energy. The numerous back-and-forth reflections between interfaces of a shield lead to a decline in the energy of the electromagnetic wave in the form of heat and increase the absorption rate of the shield, viz., contributing to the absorption mechanism. Hence, the term internal scattering must be distinguished from the multiple reflections. The internal scattering refers to numerous internal interfaces, e.g., porosities, within the shield that trap 32 the intruding electromagnetic waves and dissipate their energy in the form of heat via back-and-forth reflections between impedance mismatching interfaces. In contrast, multiple reflections refer to reflection between the front and back interfaces of a thin film shield that reduces the shielding effectiveness by letting the infiltrated electromagnetic waves escape the shield [29,36,38,39]. It is important to highlight that the evaluation of internal scattering and the impact of multiple reflections on EMI shielding parameters is commonly conducted from theoretical perspectives. However, it is generally considered acceptable to disregard multiple reflections when the thickness of a shield is higher than the skin depth or if the total shielding effectiveness of a sample exceeds 15 dB. Additionally, the influence of internal scattering can be assessed by examining the total shielding effectiveness and absorption index of a sample. In essence, incorporating numerous internal interfaces in the design of EMI shields facilitates the dissipation of electromagnetic waves through internal scattering, enhancing the overall shielding effectiveness of a sample. Moreover, by minimizing reflections at the interface of the shielding material, there is potential to observe the impact of internal scattering on the absorption coefficient [29,36,38,39]. 2.3.2 EMI shielding parameters and formulations In this thesis, a two-port VNA is employed to characterize the shielding performance of the as-developed structures. In this approach, the system sends a signal from port one (S1) with a single defined frequency to the sample. The receiver detects the transmitted (S21) and reflected (S11) waves and subsequently measures the magnitude and the phase data of each received signal. The same operation occurs from port two (S2), and the devised system sends the same signal at the same frequency to the sample, measuring the transmitted (S12) and reflected (S22) waves by the receiver on the opposite side. When the device shifts to the next frequency, the measurements occur again. At the end of the assessment, the transmission and reflection measurements are 33 provided in terms of complex scattering, i.e., S- parameters. Importantly, the homogeneity of the sample will result in equal values of S11 and S22, as well as S21 and S12 [11]. The obtained scattering parameters are employed to measure the EMI shielding coefficients, including reflectance (R), absorbance (A), and transmittance (T). The responsive EMI shielding mechanism of the developed structures can be precisely assessed according to the obtained EMI shielding coefficients. The R coefficient (reflection) is defined as the power of the reflected wave (PR) divided by the incident wave (PI). This coefficient can be calculated from the complex S11 parameter via the following formula: 𝐑 =𝐏𝐑𝐏𝐈= 𝐒𝟏𝟏𝟐 Equation 2-3 in which the S11 is the complex S- parameter calculated according to the reflected voltage magnitude divided by the incident voltage magnitude in port S1. Additionally, the T coefficient (transmittance) is known as the power of the transmitted wave (PT) divided by the incident wave that can be calculated via the complex S21 parameter: 𝐓 =𝐏𝐓𝐏𝐈= 𝐒𝟐𝟏𝟐 Equation 2-4 where S21 is the transmitted voltage magnitude from port 1 to port 2 divided by the incident voltage magnitude in port 1. The A coefficient (absorbance), which is the dominant goal of an effective shield, can be measured via the following equation: 𝐀 = 𝟏 − 𝐑 − 𝐓 Equation 2-5 2.3.3 Shielding effectiveness and logarithmic parameters The shielding effectiveness, known as the EMI shielding effectiveness (SET), is another essential parameter that is widely used for judging the total shielding performance of an EMI shield. The SET parameter is the safeguarding performance of a shield against incident electromagnetic waves, defined as the total power of transmitted electromagnetic waves divided by the total power of the 34 incident electromagnetic waves on a logarithmic scale, which can be calculated via the following equation: 𝐒𝐄𝐓(𝐝𝐁) = 𝟏𝟎𝐥𝐨𝐠𝐏𝐓𝐏𝐈= 𝟐𝟎𝐥𝐨𝐠𝐄𝐓𝐄𝐈 Equation 2-6 in which the P and E parameters correspond to the power intensity and intensity of the electric field, respectively. The subscripts T and I are attributed to the transmitted and incident electromagnetic waves, respectively. According to the chel unoff’s theory, the EMI shielding effectiveness, i.e., SET, is defined as the sum of shielding performance resulting from the reflection loss (SER), absorption loss (SEA), and multiple reflections loss (SEM) [40]: 𝐒𝐄𝐓 = 𝐒𝐄𝐑 + 𝐒𝐄𝐀 + 𝐒𝐄𝐌 Equation 2-7 The magnitude of the reflection loss or SER for both front and back interfaces of the shield can be precisely measured by using the simplified version of Fresnel’s equation for a monolithic conductive shield that could be defined as following [36]: 𝐒𝐄𝐑(𝐝𝐁) = 𝟐𝟎𝐥𝐨𝐠(ɳ+ɳ𝟎)𝟐𝟒ɳɳ𝟎= 𝟑𝟗. 𝟓 + 𝟏𝟎𝐥𝐨𝐠𝛔𝟐𝛑𝐟𝛍= 𝟏𝟎 𝐥𝐨𝐠 ( 𝟏𝟏−𝐑 ) Equation 2-8 in which parameters ɳ and ɳ0 are attributed to the impedance of the shield and air in Ω, respectively. Other parameters such as µ, σ, and f correspond to the magnetic permeability, electrical conductivity, and the incident electromagnetic wave’s frequency in Hz, respectively. As evidenced in equation 2-8, the SER has a direct relationship with the electrical conductivity, and upon an increase in the shield's electrical conductivity, the reflection loss will increase accordingly. Likewise, the magnetic permeability and the frequency of the incident electromagnetic waves also play a crucial role in the total magnitude of the reflection loss [36]. Upon propagation of an electromagnetic wave into a lossy medium with finite electrical conductivity and electrical/magnetic dipoles, the infiltrated electromagnetic wave will be attenuated upon energy dissipation. The dissipated energy will be transformed into the form of 35 heat that can be expressed as the attenuation constant (α) and be calculated via the following formulation [37,41]: 𝛂 = √𝛍𝛆𝟐[√𝟏 + (𝛔𝛚𝛆)𝟐− 𝟏] Equation 2-9 where ε and ω correspond to the dielectric permittivity and angular frequency (2πf). When an electromagnetic wave enters a lossy medium, the strength or amplitude (E) of the electromagnetic wave abates exponentially to E = E0e−αd within an EMI shield with a thickness of d. Effective attenuation of the incident electromagnetic waves has several requirements, such as I) the finite electrical conductivity for effectual ohmic losses that contribute to the interaction of available electrons in the shield with the incident electromagnetic wave [42], high magnetic permeability to improve the magnetic losses that are accompanied with the magnetic hysteresis loss and eddy current loss [43], and high dielectric permittivity for efficacious dielectric losses that occurs due to the generation of multiple nano and/or micro capacitors [44]. The total absorption loss (SEA) of the shield can be measured via the following equation: 𝐒𝐄𝐀(𝐝𝐁) = 𝟐𝟎𝐥𝐨𝐠𝐞𝛂𝐝 = 𝟐𝟎 (𝐝𝛅) 𝐥𝐨𝐠𝟏𝟎 𝐞 = 𝟖. 𝟔𝟖 (𝐝𝛅) = 𝟖. 𝟕𝐝√𝛑𝐟𝛍𝛔 = 𝟏𝟎 𝐥𝐨𝐠 ( 𝟏 ― 𝐑 𝐓 ) Equation 2-10 in which δ is attributed to the s in depth or electromagnetic wave’s penetration depth in µm. The skin depth is a highly important parameter in the shielding measurement because it expresses the distance beneath the shield's top interface in which the electric field's intensity declines to 1/e of the initial intensity of the incident electromagnetic wave. For an electrically conductive shield, the skin depth can be calculated via the following formula: 𝛅 =𝟏𝛂= (√𝛑𝐟𝛔𝛍)−𝟏 Equation 2-11 According to the formulation of the absorption loss (equation 2-10), a soar in the electrical conductivity and thickness of a shield can improve the absorption loss, whereas permittivity and 36 permeability are employed to measure the absorption loss. Additionally, in a thin film shield or a shield with a small thickness, the reflection from the bottom interface of the shield affects the final transmission owing to the multiple reflections, where the reflected electromagnetic wave from the bottom interface re-reflect after interaction with the top interface and lead to a second transmission. Continuing in this process causes multiple reflections between a shield's top and bottom interfaces, leading to a lower value of SET for the shield. The multiple reflection loss (SEM) can be measured via the following equation: 𝐒𝐄𝐌(𝐝𝐁) = 𝟐𝟎 𝐥𝐨𝐠𝟏𝟎(𝟏 − 𝐞−𝛂𝐝) = 𝟐𝟎 𝐥𝐨𝐠𝟏𝟎 [𝟏 − 𝐞−𝟐𝐝𝛅 ] Equation 2-12 Another important parameter for lightweight shields, especially aerogels, is the specific shielding effectiveness (SSE) (dB cm2 g-1) that considers both a shield's density and thickness. This parameter can be measured via the following equation [42]: 𝑺𝑺𝑬 =𝑺𝑬𝑻𝛒∗𝐭 Equation 2-13 in which the ρ is the density (g cm-3), t is the thickness (cm), and SET is the EMI shielding effectiveness (dB). SSE shows the shield's effectiveness for the application that the weight is a crucial parameter such as lightweight shields. 2.3.4 Shielding mechanism evaluation criteria and common mistakes The dominant shielding mechanism of an EMI shielding structure must be assessed via shielding coefficients, i.e., R and A values. In fact, by considering the ratio of the incident electromagnetic waves’ power to the transmitted and reflected powers of electromagnetic waves, it is possible to accurately determine if the shielding mechanism of an EMI shield is based on either absorption or reflection mechanism. Correspondingly, if the magnitude of the R coefficient becomes higher than the A coefficient, the dominant mechanism of the shield is based on the reflection mechanism. 37 This matter happens for highly conductive shields owing to the impendence mismatch between the free space and the conductive shield, which leads to the generation of secondary reflections in response to the incident electromagnetic waves. In contrast, a higher A value than R demonstrates the dominant absorption mechanism, viz., A˃0.5: absorption mechanism and A˂0.5 or R˃0.5: reflection mechanism, showing a higher rate of electromagnetic waves dissipation and their subsequent transformation to heat [30]. However, some recent studies mistakenly used the logarithmic values of SEA and SER to judge the shielding mechanism of EMI shielding structures [35]. Based on their logic, if SEA>SER or SEA/SER>1, the dominant shielding mechanism is absorption. This statement is entirely wrong, as the shielding mechanism must be assessed based on the shielding coefficients as indicators of the total power of electromagnetic waves rather than the SE parameters that illuminate the logarithmic ratios of power for electromagnetic waves [30]. As a clarification, by considering the equation of the absorption loss (SEA=10 log ( 1 ― R T )), it can be seen that this parameter is defined as the ratio of the penetrated waves to the shield (after deduction of the reflectance from the incident waves) over the transmitted waves; hence, it is not the indicator of the ratio of the incident electromagnetic waves power over the reflected or transmitted electromagnetic waves power. Thereby, the shielding mechanism must be judged based on the shielding coefficient instead of the logarithmic values of SE parameters [45]. For instance, in a developed shield by Ghaffarkhah et al. [11], they reported shielding A, R, and T coefficients of 8.54  10–3, 0.9914561, and 2.35  10–7, respectively. Accordingly, to the obtained outcomes, this shield reflects about 99.14 % of incident electromagnetic waves (R˃0.5) and only absorb or dissipate about 0.85 % of the electromagnetic waves. Hence, the dominant shielding mechanism of this shield is based on the reflection mechanism. On the contrary, they reported SEA and SER values of 47.095 and 21.043 (much higher SEA than SER or SEA/SER=2.238˃1). Herein, 38 if the mechanism is judged based on SE parameters, it becomes absorption dominant, while the shielding coefficients demonstrate an opposite outcome. Therefore, the shielding coefficient must be used instead of logarithmic SE parameters to judge the shielding mechanism of an EMI shielding structure. 39 Chapter 3: Synthesis and characterization of graphene oxide and MXene nanoflakes In several chapters within this thesis, 2D nanomaterials such as Ti3C2Tx, graphene oxide (GO), and magnetic graphene oxide (mGO) have served as the fundamental components for various structures utilized in EMI shielding and sensing applications. Recognizing the pivotal role of these nanomaterials, this chapter is dedicated to elucidating their synthesis methodologies, the characterization tools employed to assess their properties, and the chemical analysis validating the successful fabrication of these 2D nanomaterials. 3.1 Materials For the synthesis of GO, highly concentrated H2SO4 (95-98%), H3PO4 (85%), KMnO4, H2O2 (30%), HCl (37%), and anhydrous ethanol (≤ 0.003% H2O) were provided by Sigma-Aldrich, and the large graphite flakes were provided by Asbury Carbons. The MAX phase, Titanium Aluminum Carbide (Ti3AlC2), used for synthesizing Ti3C2Tx, was provided by Anasori's lab at Indiana University Purdue University Indianapolis. The reagents used, such as HCl (41-58%) and hydrofluoric acid (HF), were provided by Fisher Scientific Co. LiCl (99.98%), which was used for delaminating multilayer Ti3C2Tx flakes, was supplied by Alfa Aser. For the synthesis of magnetic nanoparticles (NPs), i.e., Fe3O4 NPs that are used for synthesizing hybrid Fe3O4/GO nanoflakes, analytical grade iron (II) chloride tetrahydrate (FeCl2.4H2O), iron (III) chloride hexahydrate (FeCl3.7H2O), and ammonium hydroxide (NH4OH 28-30%) were used (supplied by Sigma-Aldrich). In all stages of the process, ultra-pure deionized water (18.2 MΩ) was used, which was generated via ELGA model MEDICA EDI 15/30. 40 3.2 Synthesis recipes 3.2.1 Synthesis of GO The GO was synthesized through the exfoliation of graphite flakes using a modified version of the Hummers' Method, as reported by Marcano et al.[46], with some modifications based on our previous reports [32,33]. To achieve this, a mixture of H2SO4:H3PO4 in a ratio of 9:1 (360 mL:40 mL) was prepared and stirred at 300 rpm for 10 minutes. Next, 3 g of high-quality large graphite flakes were added to the mixture and stirred for 30 minutes. The mixture's temperature was then lowered using an ice bath, followed by the slow addition of 18 g of KMnO4. The acidic complex resulting from this process was then stirred for an additional 30 minutes under the same conditions, and the temperature was slightly raised to 50 °C. The reaction was maintained at this temperature for 18 hours and then terminated using a mixture of 800 mL of ultrapure deionized (DI) water (with a resistivity of 18.2 MΩ) and 5 mL of H2O2. The obtained exfoliated GO flakes were then purified with ultrapure DI water, 30 vol% HCl, and ethyl alcohol. The purified mixture was diluted to approximately 2 mg/ml and sonicated for 30 minutes at moderate power. The few-layer graphite oxide flakes were separated from the main product through centrifugation at 3,500 rpm for 1 hour. The isolated supernatant was then centrifuged at 11,000 rpm for 1 hour, yielding a concentrated GO paste (~10 mg/ml). The stock solution was stored in a sealed container until further use. 3.2.2 Synthesis of mGO For the decoration of GO with magnetic NPs, first, Fe3O4 NPs were synthesized and thence immobilized on the surface of GO through the ultrasonic assembly. Briefly, the magnetic Fe3O4 NPs were synthesized via co-precipitation of iron (II) chloride tetrahydrate (FeCl2.4H2O) and iron (III) chloride hexahydrate (FeCl3.7H2O). Accordingly, 834.5 mg FeCl2.4H2O and 394.2 mg 41 FeCl3.7H2O were added to 100 mL ultrapure deionized water and vigorously stirred at ambient temperature under a nitrogen (N2) atmosphere. Afterward, ammonia solution was dropwise added to the resulting suspension till reaching pH 9. The suspension was stirred for 1h at 60 °C until forming Fe3O4 NPs. Next, the precipitated magnetic NPs were washed several times with ultrapure deionized water and separated via a strong magnet. The resulting Fe3O4 NPs were dried at 60 °C overnight. For the immobilization of Fe3O4 on the GO, 10 mL of the stock solution of GO (concentration of 10 mg/ml) was dispersed in 25 mL of deionized water; meanwhile, 100 mg of the magnetite NPs were dispersed in 15 mL of deionized water using ultrasonication and subsequent stirring. The GO solution was placed under a probe sonicator, and the stable Fe3O4 suspension was dropwise added to it. The resulting mixture was stirred at 50 °C overnight and thence concentrated using centrifugation at 11000 rpm for further experiments. 3.2.3 Synthesis of Ti3C2Tx The well-exfoliated Ti3C2Tx flakes were synthesized based on our previous report [11]. To synthesize the multilayer Ti3C2Tx, the MAX phase, i.e., Ti3AlC2, with an average size of ˂75 µm, was etched with HF. For this purpose, 2 g of MAX phase was slowly added to a mixture composed of 12 mL DI water, 6 mL HCl (12 M), and 2 mL HF (29 M) over the course of 5 min. The reaction was completed after 24 h stirring at room temperature. The resulting mixture was separated through centrifugation at 3500 rpm for 5 min, and the remnant of acids was removed through washing cycles with DI water until the pH of the suspension reached 6. The as-synthesized few-layer Ti3C2Tx flakes were then delaminated by adding 2 g LiCl to 40 mL of the stock solution of Ti3C2Tx, followed by overnight stirring at 350 rpm. The obtained suspension was then shaken severely to delaminate Ti3C2Tx flakes, and the multilayer flakes or unreacted product was 42 separated from the well-exfoliated ones through centrifugation at 3,500 rpm for 5 min. The purified product was redispersed in DI water and concentrated until further use. 3.3 Characterization The nanomaterials were characterized with different techniques to confirm their successful formation and desired quality. In this regard, the initial characterizations were performed by using Fourier-transform infrared spectroscopy (FTIR) (NicoletTM iS20) in the spectrum range of 600 to 4000 cm-1, micro-Raman spectroscopy (Bruker Senterra II) equipped with an Olympus U-TV1X-2 microscope, and X-ray diffraction (XRD) (Bruker model D8 Advance). The morphology of 2D GO and Ti3C2Tx flakes were analyzed using a Field Emission Scanning Electron Microscope (FESEM) (Tescan Mira 3 XMU) coupled with an Oxford Instruments X-Max energy dispersive spectrometer (EDS) detector and a High-Resolution Transmission Electron Microscope (HRTEM) (80-300 Titan LB) equipped with an aberration-corrector (CEOS); this HRTEM operates at 300 kV, and images were obtained via a First Light CCD, Gatan. 3.3.1 Characterizations of synthesized GO nanoflakes GO flakes were analyzed via diverse techniques to confirm their successful formation. Figure 3-1 (a) shows the X-ray diffractogram of GO. As shown, GO presents a well-defined peak at 2θ of 11.15, corresponding to the (001) crystalline plane of GO and confirms the successful exfoliation of the graphite flakes [47]. The Raman spectrum of GO shows two well-defined peaks at 1351.5 cm-1 and 1602.5 cm-1, attributed to the D- and G-bands of GO flakes, respectively (Figure 3-1(b)) [48]. The ratio of ID/IG for the GO was measured to be 0.962, indicating the controlled rate of defects in the structure of exfoliated GO flakes. Figure 3-1(c) illustrates GO flakes' thermogravimetric analysis (TGA). As depicted, the GO showed a small weight loss at about 160 °C, corresponding to the water's volatilization. The major weight loss between 160 to 225 °C was caused owing to 43 the decomposition of oxygen-based functional groups (-OH) along with the release of CO and CO2. The other slight mass losses between 300 to 900 °C correspond to the detachment of more stable oxygen-based functional groups from GO flakes. The slight weight loss at more than 300 °C mainly originated from CO release [49]. The FTIR spectrum of GO also depicted the main fingerprint peaks of GO flakes, produced via the improved modified Hummers method (Figure 3-1(d)) [46]. Accordingly, the appeared peaks correspond to the stretching vibration of hydroxy functional groups (-OH) (3420 cm-1), C=O in carbonyl moieties, and carboxylic acid groups (1740 cm-1), C=C double bond carbon atoms (1640 cm-1), C-O-C (1215 cm-1), C-O (1090 cm-1), and C-OH (1410 cm-1) [49,50]. The TEM images of the as-developed GO flakes (Figure 3-1(e-f)) depict the well-exfoliation of the GO and its 2D morphology. FESEM image of the GO (Figure 3-1(g)) also shows the successful exfoliation of graphite flakes and the formation of GO with a large active surface area, ideal for its subsequent decoration with magnetic NPs, i.e., Fe3O4. 44 Figure 3-1 (a) X-ray diffractogram, (b) Micro-Raman spectra, (c) TGA, (d) FTIR spectra, and (e-f) TEM images of GO. (f) FESEM images of the cross-section of a GO film after vacuum filtration. The specifics of these characterizations are outlined in Section 3.3.1. 45 3.3.2 Characterizations of synthesized mGO nanoflakes Figure 3-2 illustrates the outcome of characterization for the decorated GO with Fe3O4 NPs (mGO). Figure 3-2 (a) shows the X-ray diffractogram of mGO; as depicted, the ultrasonic process and subsequent overnight stirring at 50 °C led to the reduction of the GO and shifted its 2θ peak from 11.15 to 18.50, indicating the decline in the d-spacing of the GO and reduction of GO flakes. Other appeared peaks in the X-ray diffractogram of mGO are generated owing to the decoration of Fe3O4 NPs on the surface of GO. Correspondingly, 2θ peaks of 30.40, 35.60, 43.40, 53.60, 57.30, and 62.90 are attributed to the (220), (311), (400), (422), (511), (400) crystalline plane of magnetite NPs; these outcomes are well matched with the standard card of JCPDS 19-0629, indicating the successful synthesis and high purity of magnetite NPs [51]. The micro-Raman spectrum of the mGO (Figure 3-2 (b)) shows the D-band and G-band peaks of GO at about 1347.5 and 1593.5 cm-1 with an ID/IG ratio of 0.886, indicating the formation of magnetic GO flakes with controlled defects. The outcome of Zeta potential for the mGO also showed that the as-developed compounds are negatively charged (-15 eV), leading to their stability and uniform distribution within the water [1]. The TGA graph of mGO reveals three main weight losses for the developed magnetic flakes (Figure 3-2 (c)). In this regard, the first weight loss at about 120 °C could be attributed to water desorption, which includes 6% of the total weight of the compound. The second part starts from 150 °C up to 250 °C, corresponding to the degradation of oxygen-based functional groups. This weight loss is slightly continued till 750 °C, leading to the detachment of more stable functional groups from the surface of mGO. The third part is assigned to the degradation of GO flakes at high temperatures [52]. 46 Furthermore, the FESEM images of mGO indicate the homogeneous decoration of Fe3O4 NPs on the surface of GO and the formation of 2D magnetic flakes (Figures 3-2 (d-e)). Figures 3-2 (f-g) show the zoom area and the corresponding energy-dispersive X-ray spectroscopy (EDX) map of iron elements on the surface of GO, respectively, confirming their homogeneous distribution on the surface of GO. In this regard, the EDX analysis revealed that the mGO is comprised of 52.3 wt% iron, 31.0 wt% oxygen, 13.2 wt% carbon, 3.2 wt% nitrogen, and 0.3 wt% sulfur (Figure 3-2 (h)). 47 Figure 3-2 (a) X-ray diffractogram, (b) Micro-Raman spectra, (c) TGA, (d-e) FESEM, and (f-g) EDX analysis of mGO. The EDX map presented in part (d) has been taken from the section completely covered by Fe3O4. The specifics of these characterizations are outlined in Section 3.3.2. 3.3.3 Characterizations of synthesized Ti3C2Tx nanoflakes The synthesized Ti3C2Tx flakes were assessed using different characterization techniques. In Figures 3-3 (a-b), the X-ray diffractogram of Ti3C2Tx flakes heated at 60 °C and 250 °C can be seen, respectively. As depicted in part (a), the well-defined 2θ peaks at 7.91, 16.94, 26.84, 35.09, and 43.76 are attributed to the (002), (004), (006), (008), and (0010) crystalline planes of Ti3C2Tx, 48 respectively [53]. Heating the Ti3C2Tx flakes at 250 °C affected the interlayer spacing of these flakes and led to a slight shift of 2θ peaks to 8.36, 17.36, 26.33, 35.21, and 44.33, respectively. The micro-Raman spectroscopy provides essential information regarding the quality and functionality of Ti3C2Tx flakes. Figure 3-3 (c) illustrates the Raman spectrum of Ti3C2Tx flakes in the range of 100 to 800 cm-1, obtained via the 785 nm laser. The vibration of the Ti3C2Tx could be described by the mean of Mulliken symbols as 4Eg+2A1g+4Eu+2A2u, where Eg and A1g belong to the active modes of Raman, while Eu and A2u are attributed to the active modes of IR. The Eg and A1g correspond to the in-plane and out-of-plane vibrations of carbon and titanium atoms, respectively. On the other hand, active modes of IR, i.e., Eu and A2u, originated from the in-plane and out-of-plane vibrations of carbon atoms [54]. The Raman spectrum of Ti3C2Tx can be divided into three main regions, as illustrated in Figure 3-3 (c). These regions include the (I) flake region (including a sharp peak at 203 cm-1), which is attributed to the vibration of titanium layers, carbon, and surface functional groups, (II) the Tx region (230 to 470 cm-1), corresponding to the vibration of surface functional groups, i.e., O(OH), O2, and (OH)2, attached to the Ti atoms, and (III) the carbon region (580-730 cm-1), attributed to the in-plane and out-of-plane vibrations of carbon atoms within the structure of Ti3C2Tx [54]. Out of these regions, there is a strong and sharp peak at about 120.5 cm-1, attributed to the resonant peak and responsible for the plasmonic properties of the Ti3C2Tx [55]. The flake region mainly consisted of A1g (Ti, C, O) and Eg (Ti, C, O) modes, attributed to the out-of-plane and in-plane vibrations of surface functional groups and Ti atoms within the outer layer along with carbon atoms. This region shows the vibration of flake as the vibration of surface functional groups, titanium and carbon. The Flake region includes the maximum number of atoms from the unit cell of Ti3C2Tx. Moreover, the presence of water molecules within the structure of 49 Ti3C2Tx improves the stiffness of out-of-plane vibrations and enlarges the interlayer spacing. This matter shifts the A1g (C) peak to about 720 cm-1. Such a shift in the A1g (C) peak reveals that the vibrations of carbon layers within the Ti3C2Tx flake do not depend on the flake's orientation [54]. These outcomes depict the successful synthesis of Ti3C2Tx with high quality and essential functionalities for external applications. Figure 3-3 (d) shows the FTIR spectrum of Ti3C2Tx flakes. As depicted, the delaminated Ti3C2Tx flakes revealed a well-defined FTIR spectrum with the desired functional groups for homogeneous dispersion within the aquatic media. As illustrated, the broad peak at 3349.26 cm-1 corresponds to the stretching vibration of hydroxyl (-OH) functional groups. Other appeared peaks at about 2900.41 and 1636.78 cm-1 are attributed to the -CH2 and -OH functional groups, respectively. Numerous appeared peaks in area A of Figure 3-3 (d) correspond to the vibration of different functional groups on the surface of Ti3C2Tx flakes, including C-F (1099.70 cm-1), O-H (1395.24 cm-1), C-H (1456.95 cm-1), aromatic ring (1507.09 cm-1), and C=C (1559.16 cm-1), respectively [56]. The outcome of the XRD, micro-Raman spectroscopy, and FTIR analyses clearly confirm the successful exfoliation and formation of high-quality Ti3C2Tx flakes with minimized defects. 50 Figure 3-3 X-ray diffractogram of Ti3C2Tx laminated films that annealed at (a) 60 °C and (b) 250 °C. (c) Micro-Raman spectroscopy and (d) FTIR analyses of Ti3C2Tx. The specifics of these characterizations are outlined in Section 3.3.3. Likewise, the HRTEM analysis of Ti3C2Tx flakes confirmed the previously obtained data. For obtaining a high conductivity rate and boosted EMI shielding, it is essential to maintain the size of Ti3C2Tx flakes as large as possible while minimizing the generation of defects [57]. As depicted in Figures 3-4 (a-b), the Ti3C2Tx flakes are perfectly exfoliated and present a well-defined 2D morphology with a large active surface area. The atomic morphology of the Ti3C2Tx flakes (Figures 3-4 (c-d)) further confirms this matter, indicating the formation of high-quality flakes with a well-arranged atomic morphology and the least vacancies. These outcomes confirm the 51 successful synthesis of Ti3C2Tx out of the primary MAX phase source with the desired quality, size, morphology, and surface functionalities. Figure 3-4 HRTEM analysis of Ti3C2Tx flakes. (a-b) illustrates the two-dimensional morphology of Ti3C2Tx flakes, while (c-d) presents high-resolution images showcasing the atomic arrangement of this material. 52 Chapter 4: Scalable manufacturing of flexible and highly conductive Ti3C2Tx/PEDOT:PSS thin films for EMI Shielding The ever-increasing proliferation of miniaturized and wearable electronics demands lightweight and easy-to-process electromagnetic interference (EMI) shielding materials. Herein, we develop flexible micrometer-thick Ti3C2Tx/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) nanocomposite films with exceptional electrical conductivity and shielding effectiveness, and with sufficiently uniform thickness. In particular, a ~7 𝜇𝑚-thick nanocomposite film containing 25 wt% PEDOT:PSS has an exceptional electrical conductivity of 2,900±400 S/cm and an EMI shielding effectiveness of 55.4 dB after a co-treatment with sulfuric acid and methanol. To the best of our knowledge, this is the highest electrical conductivity ever reported for MXene-based polymer nanocomposites. Besides, the thin film possesses a specific EMI shielding effectiveness of 38,079 dB cm2 g-1, which is among the highest values for conductive polymer nanocomposites. The inclusion of PEDOT:PSS endows great film-forming ability to Ti3C2Tx suspensions and hampers unwanted phenomena such as the coffee-ring effect. We also implement a wet-transfer approach to form Ti3C2Tx/PEDOT:PSS nanocomposite films on complex geometries with curves, angles, and corners, which not only opens up new opportunities for developing EMI shields on complex geometries but also offers a feasible pathway for developing flexible and wearable electronics. 4.1 Introduction The proliferation of smart, compact, and highly integrated electronics calls for high-performance electromagnetic interference (EMI) shielding materials to safeguard electronics and human health against electromagnetic waves [7,58]. More importantly, the rapid development and growing 53 popularity of portable and wearable smart electronics necessitate more stringent requirements (e.g., scalable, lightweight, minimal thickness, and flexibility for EMI shielding materials) [59]. As such, efforts have been made to satisfy these demanding requirements [6,36,60]. Metals such as Ag, Cu, and Al are excellent barriers against electromagnetic noises owing to their exceptional conductivity. However, metal-based EMI shields suffer a number of major drawbacks, including heavy weight, susceptibility to corrosion, and rigidity, which limit their application in wearable, portable, and miniaturized electronics [5,61]. Thus, an increasing number of studies have focused on flexible and lightweight shielding solutions that can meet the current demands of the electronics market. Carbon-based materials (e.g., graphene [62], multi-walled carbon nanotube (MWCNT) [63], and reduced graphene oxide [64]) are being studied as conductive fillers for lightweight and flexible EMI shields. Indeed, the family of carbon nano-allotropes offers promising advantages, including high strength, chemical inertness, and low density compared with metals [65]. However, carbon-based materials do not provide sufficient EMI shielding capacity at small thicknesses [61,66]. In 2016, Shahzad et al.[42] reported an outstanding EMI shielding effectiveness (SE) of 92 dB for a 45 𝜇𝑚 thick Ti3C2Tx film. This striking EMI shielding performance of Ti3C2Tx originates from its high metallic conductivity conjugated with wave scattering inside the layered structure of Ti3C2Tx films. More recently, Gogotsi and coworkers [5] performed a systematic study on EMI shielding properties of 16 different MXenes, including single metal MXenes, ordered double-metal carbide MXenes, and random solid solution MXenes of M and X elements, and reported an exceptional EMI SE of 21 dB for a 40 nm thick Ti3C2Tx films. Despite the encouraging advances, forming large-area MXene thin films with high EMI shielding is still challenging. The drawbacks in the fabrication of micrometer-thick MXene films stem from 54 the commonly used processing methods or the nature of MXene suspensions that are unable to form uniform films. For instance, the vacuum-assisted filtration (VAF) method, which is employed by a major body of the literature to develop MXene films, is a tedious and expensive process and cannot render large-area films [67,68]. Other well-known technologies such as spin-coating, dip-coating, and spray coating are mostly suitable for making submicron thin films of MXene and MXene composites (<1µm), failing to render high EMI shielding effectiveness [5,67-69]. Recently, roll-to-roll processing, blade coating, and drop-casting have been used to produce large-area films of several micrometers in thickness [13,67,70]. Among these approaches, drop-casting requires the lowest capital equipment costs and processing temperature, and therefore, it has a significant potential for bulk processing of MXenes [67,68]. However, drop-casted MXene films suffered from poor mechanical flexibility and uneven thickness across the surface [71]. In an attempt to address these limitations, drop-casted composite films of MXenes with various polymers, including polyacrylamide (PAM) and polyethylenimine (PEI), have been suggested [72]. Although this strategy significantly enhances the flexibility of the films, the use of non-conductive polymers considerably reduces the electrical conductivity, and thus, the total EMI shielding performance of the final composite films [71]. To tackle this problem, some recent studies have proposed utilization of inherently conductive polymers, e.g., poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), to design nanocomposite EMI shields [66,71]. However, most of these studies only considered PEDOT:PSS as a conductive binder that sustains the electrical conductivity of the incorporated nanofillers and provides acceptable mechanical properties. Herein, we show that PEDOT:PSS is capable of offering more to the field of multifunctional nanocomposites. In brief, the current chapter reveals the unique film-forming properties of PEDOT:PSS, arising from its chemical structure, which can be transferred to its nanocomposites. 55 Our results show that drop-casting of the Ti3C2Tx and PEDOT:PSS suspensions allow for the production of small (micrometer-scale), medium (30mm x 30mm), and large (120mm x 80mm) films with a relatively uniform thickness. We also explore the reliability and the mechanisms behind the casting of PEDOT:PSS. Our results justify the popularity of drop-casting and inkjet printing of PEDOT:PSS and its composites in the literature (refer to Table 4-1). The quality of films generated with our drop-casting technique is comparable with common techniques, such as VAF. However, our technique is significantly less expensive and is a more facile processing approach. In this chapter, we also employ a wet-transfer approach to transfer the drop-casted Ti3C2Tx/PEDOT:PSS nanocomposite films onto complex geometries with curves, angles, and corners. Such a flexible, transferable, and highly conductive EMI shield has promising applications in flexible electronics and EMI shields with complex geometries. 4.2 Experimental section 4.2.1 Materials The synthesis recipe and characterization of Ti3C2Tx flakes were discussed in chapter 3. Sulfuric acid (H2SO4, 95-98%) and methanol (CH3OH, super gradient grade) were purchased from Sigma-Aldrich. PEDOT:PSS (Clevios, PH1000) aqueous dispersion (1–1.3 wt%) was purchased from Heraeus. All chemicals were used as they were received without further purification. 4.2.2 Fabrication of Ti3C2Tx and Ti3C2Tx/PEDOT:PSS films Large Ti3C2Tx and Ti3C2Tx/PEDOT:PSS films (120 mm x 80 mm) were fabricated via drop-casting on activated polycarbonate sheets. The polycarbonate substrates were thoroughly washed with detergent, water, and ethanol and sonicated for 30 min to remove contaminants. The polymer sheets were then dried at 60 ˚ C under vacuum condition for 2 h and treated via a UV–ozone cleaner (PSDP Pro, Novascan Technologies Inc., USA) for 30 min. For preparing the 56 Ti3C2Tx/PEDOT:PSS suspensions, PEDOT:PSS aqueous suspension was stirred vigorously for 1 h, and then added to Ti3C2Tx suspensions. Upon further stirring for 30 min, the desired aqueous suspensions of Ti3C2Tx/PEDOT:PSS were slowly pipetted onto the activated polycarbonate sheets on a completely leveled surface. Pristine Ti3C2Tx was also naturally cast onto the activated polycarbonate sheet. It is worth noting that a specific amount of suspension per area was cast onto the substrate (Table 4-1). For example, for the composite film containing 25 wt% of polymer, one ml of Ti3C2Tx/PEDOT:PSS suspension containing 11.25 mg of Ti3C2Tx and 3.75 mg of PEDOT:PSS was cast onto every 9 cm2 of the substrate. The samples were then left to dry at 60 ˚C for 4 h under a stream of dry nitrogen. Finally, the films were dried under vacuum conditions for 12 h at 90˚C for the complete removal of any remaining moisture. For comparison, freestanding Ti3C2Tx/polymer films were also fabricated by the VAF method and drop-casting on a hydrophobic Teflon sheet, as described elsewhere [66,67,73]. Table 4-1 Compositions of the Ti3C2Tx MXene and Ti3C2Tx MXene/PEDOT:PSS solutions used for drop-casting. 1 mL of each of the solutions was used for covering 9 cm2 of polycarbonate sheet. Sample name PEDOT:PSS content (mg) Ti3C2Tx MXene content (mg) Water content (mL) Pure Ti3C2Tx - 15 1 10 wt% 1.5 13.5 25 wt% 3.75 11.25 35 wt% 5.25 9.75 50 wt% 7.5 7.5 4.2.3 Treatment of Ti3C2Tx/PEDOT:PSS films The Ti3C2Tx/PEDOT:PSS composite films (25 wt% of polymer) were treated by submerging into sulfuric acid (80 wt %) or methanol for 6 h and then washing with DI water several times. For the case of co-treatment, the composite film was immersed into sulfuric acid for 6 h and then washed with methanol. We found that after the treatment, the composite films tended to detach from the polycarbonate substrate. This might be due to the reduction of the polymer chains' adhesion to the 57 substrate after the removal of excess PSS [74]. A strategy was designed in order to keep the composite films on the substrate. First, we slowly submerged the composite film into the acid or methanol at an angle of 45˚ compared to the liquid meniscus. In this step, almost one centimeter from the top side of the composite film remained outside the treatment solution. After 6 h, the film was slowly removed and washed with methanol or DI water. The wrinkles on the surface of the treated film were then removed with the help of a very soft brush soaked in methanol or water. The brush was slowly moved on the surface of the wet film, and then the film was dried at room temperature for 2 h. Finally, the film was dried under vacuum at 90 ˚C for 12h. After that, one centimeter from each side of the prepared film was cut away, and the rest of the film was used for further characterization. In this study, composite films undergone the co-treatment process were also transferred to various substrates with different geometries. For this purpose, Ti3C2Tx/PEDOT:PSS composite films (25 wt% of polymer) were completely submerged into sulfuric acid for 6 h and then washed with methanol several times. Next, a gentle agitation was used to detach the composite film from the polycarbonate substrate. The free-floating films were solid enough to be transferred to various substrates in a methanol bath. After transferring, a very soft brush (soaked in methanol) was used to remove the wrinkles of the wet film and produce a smooth surface. The transferred film was dried at room temperature for 2 h. Finally, the prepared samples were transferred to a vacuum oven at 90 ˚C for 12h. This method, called the wet transfer approach, can be adapted to various geometries (e.g., curves, angles, and corners). 4.2.4 Characterization The FESEM was performed by a Tescan Mira 3 XMU equipped with a Field Emission Gun. The surface morphology and thickness of Ti3C2Tx/PEDOT:PSS composite films and pristine Ti3C2Tx 58 films were evaluated using a laser interferometer (Olympus LEXT OLS5000). The electrical conductivity of the prepared films was measured using a Loresta GP resistivity meter (MCP-T700 model, Mitsubishi Chemical Co.) connected with a four-pin probe. The EMI shielding properties are derived from the guidelines and methods described in Chapter 2 of this thesis. 4.3 Results and discussion 4.3.1 Drop-casting of Ti3C2Tx/PEDOT:PSS Drop-casting of PEDOT:PSS and its composites has been used widely to develop micrometer-thick nanocomposite films (Table 4-2). To investigate the reliability of this method, we studied the uniformity of very small (micrometer-scale), medium (30mm x 30mm), and large (120mm x 80mm) drop-casted films. The drying process of the drop-casted films is depicted in Figure 4-1 (a). First, the polycarbonate was treated with UV-ozone to anchor polar groups onto the surface [75,76]. As previously reported [75,77], the UV-ozone treatment of polycarbonate results in the degradation of the C-C bonds and the formation of polar groups on the surface of the substrate. The presence of these polar groups reduces the contact angle and enhances the wetting envelope of polycarbonate, as shown in Table 4-3. After the surface treatment, the Ti3C2Tx and Ti3C2Tx/PEDOT:PSS suspensions can be easily spread on the polycarbonate due to their increased surface energies (Figures 4-1 (b-e)). The receding contact angle of Ti3C2Tx/PEDOT:PSS (25 wt% of polymer) was found to be zero and lower than that of the pristine Ti3C2Tx (Table 4-4), which indicates that adding PEDOT:PSS to Ti3C2Tx suspensions increases the interaction of liquid with the substrate. The uniformity of the Ti3C2Tx/PEDOT:PSS films is due to: (i) the higher interaction of the suspensions containing PEDOT:PSS with the polycarbonate substrate, which can mitigate the transport of particles to the 59 contact line and therefore enhances the homogeneity of the drop-casted films [78,79]; and (ii) the gel transition of the colloids during solvent evaporation [78,80]; as the water evaporates, the concentration of PEDOT:PSS and Ti3C2Tx increases, and a gel-like colloid forms. The resulting highly viscous fluid resists capillary stresses and unwanted flow of particles to the contact line. This improves the uniformity of the drop-casted films. 60 Figure 4-1 (a) The drying process of drop-casted films. Contact angles, photographs, 3D intensity profiles, and height profiles of the dried droplets of (b) the pristine Ti3C2Tx on untreated polycarbonate, (c) Ti3C2Tx/PEDOT:PSS (25 wt% of polymer) on untreated polycarbonate, (d) the pristine Ti3C2Tx on treated polycarbonate, and (e) Ti3C2Tx/PEDOT:PSS (25 wt% of polymer) on treated polycarbonate. 61 Table 4-2 Previous studies used drop-casting or inkjet printing approach for PEDOT:PSS and its composites. Material Substrate Method Reference Cu-Bi0.5Sb1.5Te3 /PEDOT:PSS Silicon dioxide (Activated by plasma) Drop-casting [81] PEDOT:PSS Glass (Activated by UV/ozone) Drop-casting [82] PEDOT:PSS/ethanol, and Graphene dots (GDs)/ PEDOT:PSS/ethanol Fluorine-doped SnO2 Drop-casting [83] SnSe/PEDOT:PSS/ dimethyl sulfoxide (DMSO) Glass Drop-casting [84] PEDOT:PSS coated Te nanorod/ PEDOT:PSS Glass Drop-casting [85] Bi2Te3 nanosheet/PEDOT: PSS Glass Drop-casting [86] PEDOT: PSS Silicon Drop-casting [87] PEDOT: PSS Glass wafer Drop-casting [88] PEDOT: PSS Silicon dioxide (Activated by plasma) Drop-casting [89] PEDOT: PSS Glass Drop-casting [90] DMSO/PEDOT:PSS Polydimethylsiloxane (PDMS) Drop-casting [91] PEDOT:PSS)/waterborne polyurethane (WPU)/ Carbon nanocoils (CNCs) Glass or elastomeric substrates (Activated by plasma) Drop-casting [92] PEDOT:PSS/ single-walled carbon nanotubes (SWCNTs) Polyimide Drop-casting [93] DMSO/PEDOT:PSS and ethylene glycol (EG)/ PEDOT:PSS Polypropylene, glass, and poly(ethyleneterephthalate) (PET) Drop-casting [94] PEDOT:PSS/SWCNTs/ DMSO Glass Drop-casting [95] Bi2Te3 nanosheet/PEDOT: PSS Glass Drop-casting [96] Cellulose/PEDOT:PSS/DMSO/glycerol Glass Drop-casting [97] PEDOT: PSS Glass Drop-casting [98] PEDOT: PSS/DMSO Glass (Activated by aqua regia) Drop-casting [99] Graphene oxide (GO)/PEDOT:PSS/DMSO Glass (Activated with a mixture of concentrated sulfuric acid and hydrogen peroxide) Drop-casting [100] PEDOT:PSS/ CNTs Polyimide Drop-casting [101] PEDOT: PSS/DMSO Glass Drop-casting [102] PEDOT: PSS Glass Drop-casting [103] Ag2Se NW/PEDOT:PSS Glass Drop-casting [104] PEDOT:PSS/ GDs Fluorine-doped SnO2 Drop-casting [105] PEDOT-PSS/SWCNTs PET Inkjet printing [106] PEDOT-PSS/vegetal glycerol Polyimide Inkjet printing [107] PEDOT:PSS/DMSO Pre-treated polyethylene naphthalate Inkjet printing [108] PEDOT:PSS Polyimide (Activated by plasma) Inkjet printing [109] PEDOT:PSS PET Inkjet printing [110] PEDOT-PSS/ ethanol/ diethylene glycol Polyimide (Activated by plasma) Inkjet printing [111] 62 Table 4-3 Average static, advancing, and receding contact angles of water on the polycarbonate sheet. Average static contact angle Average advancing contact angle Average receding contact angle Before treatment 81.6 89.2 69.6 After treatment for 30 min 24.2 28.6 19.9 Table 4-4 Average static, advancing, and receding contact angles of the aqueous solution of Ti3C2Tx or Ti3C2Tx/PEDOT:PSS (25 wt%) on activated polycarbonate sheet. Average static contact angle Average advancing contact angle Average receding contact angle Ti3C2Tx 17.3 21.1 14.2 Ti3C2Tx /PEDOT:PSS (25 wt%) 15.9 19.3 0 Figures 4-1 (b-e) show pictures and height profiles of Ti3C2Tx and Ti3C2Tx/PEDOT:PSS droplets that were dried on the surface of the polycarbonate sheet before and after activation. In the case of drop-casting onto the untreated substrate, a ring-like stain with drastically larger thickness near the perimeter of the dried droplet was observed. This phenomenon, which is called the coffee-ring effect, might be explained by the transport of non-volatile solutes to the contact line due to capillary flow [112]. Upon surface activation of the polycarbonate sheet, no drastic increase in the thickness near the perimeter of the drop-casted films was observed. This suppression of the coffee-ring effect might be explained by the interaction of the non-volatile solutes with the substrate, as described before. Besides, upon surface activation of the polycarbonate substrate, the contact angle is much lower, and the droplets can spread on the substrate. The heightmaps presented in Figures 4-1 (b-e) show that the diameter on the untreated polycarbonate is around 3 mm, whereas the diameter on the activated polycarbonate is about 9 mm. Due to the larger surface area and less curvature of droplets on the activated substrate, the heat transfer at the solid surface and liquid-vapor interface increases. In this case, evaporation is much faster, and the coffee ring effect is 63 suppressed [112]. For the case of the pristine Ti3C2Tx, a very rough surface was observed for the dried droplet (Figure 4-1 (d)). In fact, many spots with very high height and even some holes or vacancies were observed on the surface of the dried Ti3C2Tx droplets. The irregularity of the drop-casted Ti3C2Tx droplets might be explained by the dense stacking of Ti3C2Tx during the solvent evaporation [66]. The stacking of Ti3C2Tx flakes leads to the formation of large agglomerates in the solvent, resulting in small bumps on the surface of the dried droplets. The deposition of Ti3C2Tx agglomerates next to each other also creates large vacancies, which is evident in the FESEM image of pristine Ti3C2Tx and even PEDOT:PSS/Ti3C2Tx composite that contained only 10 wt% of the polymer (Figures 4-2 (a-b)). However, by increasing the loading of the polymer, these vacancies disappear, and more uniform polymer nanocomposites were observed (Figures 4-2 (c-e)). In fact, due to the electrostatic interactions between Ti3C2Tx and PEDOT:PSS, the polymer chains can spread between the Ti3C2Tx flakes [66]. This effectively suppresses the dense stacking of the Ti3C2Tx sheets. On the basis of this interaction and the removal of the large agglomerates, the droplet containing Ti3C2Tx and PEDOT:PSS (25 wt% of polymer) showed a more uniform surface upon drying (Figure 4-1 (e)). Aside from the ability to form a uniform film, these results highlight the great potential of the fabricated Ti3C2Tx/PEDOT:PSS composite (25 wt% of polymer) as a unique conductive ink for the inkjet printing applications due to its uniform deposition upon solvent evaporation [78,113]. 64 Figure 4-2 Cross-section FESEM images of (a) the pristine Ti3C2Tx and Ti3C2Tx/PEDOT:PSS composite films containing (b) 10, (c) 25, (d) 35, and (e) 50 wt% of the polymer. All scale bars correspond to 2 µm. The height profile of the pure PEDOT:PSS droplets that dried on the surface of polycarbonate before and after activation was also investigated. Figure 4-3 (a) shows a noticeable coffee ring effect when pure PEDOT:PSS was drop-cast onto the inactive polycarbonate sheet. However, in the case of drop-casting on an activated sheet, a concentrated stain with a relatively higher height at the center, compared to the perimeter of the droplet, was observed (Figure 4-3 (b)). A similar concentrated stain was also observed for the Ti3C2Tx/PEDOT:PSS droplets that contained 50 wt% of the polymer (Figure 4-3 (c)). The results presented here show that controlling the uniformity of the drop-casted or inkjet-printed Ti3C2Tx/PEDOT:PSS conductive ink is rather complex. Many parameters, including the surface activity of the substrate, the concentration of the non-volatile solutes, and the rheological properties of the ink, should be tuned to obtain uniformly dried residues after solvent evaporation. 65 Figure 4-3 Picture and height profile of the dried droplets of (a) pure PEDOT:PSS on untreated polycarbonate, (b) pure PEDOT:PSS on treated polycarbonate, and (c) Ti3C2Tx/PEDOT:PSS (50 wt% of polymer) on treated polycarbonate. (d) The thickness variation of drop-casted Ti3C2Tx/PEDOT:PSS (25 wt% of polymer) onto medium glass and polycarbonate substrates (30mm x 30mm). The pictures show the prepared film on a polycarbonate and glass substrate. Aside from the microliter casting, we also investigated the uniformity of medium and large films prepared through drop-casting under a constant nitrogen purge. Figure 4-3 (d) shows that drop-casting of Ti3C2Tx/PEDOT:PSS (25 wt% of polymer) onto medium-sized glass or polycarbonate substrates (30mm x 30mm) can produce uniform films. These results are in agreement with previous studies [81,82,89]. More interestingly, drop-casting onto a large polycarbonate substrate (120mm x 80mm) also produced an acceptably uniform film with an average value of ~ 8 μm with a lower and upper bound 7.2 to 9.2 μm, respectively (Figure 4-4 (a)). The thickness uniformity of this drop-casted sample is comparable to that of a film produced using VAF (Figure 4-4 (b)) while its manufacturing costs is significantly less. Moreover, the variation of the total EMI shielding at 66 various locations across this film was minimal (Figure 4-4 (c)). This uniformity of thickness and EMI shielding characteristics of large drop-casted films can be explained by the high interaction of the suspensions containing PEDOT:PSS with the polycarbonate substrate, the gel transition of the colloids during solvent evaporation, and suppression of the dense stacking of the Ti3C2Tx sheets as described previously. However, a large film of Ti3C2Tx/PEDOT:PSS produced by drop-casting on a Teflon sheet exhibited significantly high thickness variation (Figure 4-4 (d)). In this case, upon solvent evaporation, the film's edge dried, detached from the Teflon substrate, and moved toward the center of the film. Due to this behavior, the undried solvent moved toward the center of the drop-casted film and caused a huge thickness variation. 67 Figure 4-4 The thickness variation and thickness error compared to the average reported value for the Ti3C2Tx/PEDOT:PSS (25 wt% of polymer) film prepared by (a) drop-casting on an activated polycarbonate sheet and (b) VAF method. (c) The variation of the total EMI shielding in various locations of a large film of Ti3C2Tx/PEDOT:PSS (25 wt% of polymer) that was prepared by drop-casting on an activated polycarbonate. (d) The thickness variation and thickness error compared to the average reported value for the Ti3C2Tx/PEDOT:PSS (25 wt% of polymer) film that was prepared by drop-casting on a Teflon sheet. 4.3.2 EMI shielding characteristics of Ti3C2Tx/PEDOT:PSS films The EMI shielding characteristics, electrical conductivity, and average thickness of the prepared films that contained various percentages of PEDOT:PSS are shown in Figure 4-5 (a). In the case of pristine Ti3C2Tx, the electrical conductivity and SET were reported to be 4500± 300 S/cm and 68 63.9 dB, respectively. These values are in the same order of magnitude as the results reported previously in the literature [5,114]. However, by increasing the loading of PEDOT:PSS in the composite films, both the conductivity and EMI shielding of the prepared films were reduced. This reduction of the EMI shielding characteristics can be explained by the drastically lower electrical conductivity of the untreated PEDOT:PSS compared to Ti3C2Tx. The structure and the EMI shielding behavior of the Ti3C2Tx/PEDOT:PSS nanocomposite that contained 25 wt% of polymer is schematically shown in Figure 4-5 (b). Due to the electrostatic interactions between Ti3C2Tx and PEDOT:PSS, polymer chains can be spread between Ti3C2Tx flakes [66]. Generally, the PEDOT:PSS complex tends to form PEDOT-rich cores surrounded by the PSS-rich shells due to the ionic interactions between PEDOT and PSS chains [115]. When the electromagnetic waves penetrate through the composite films, their energy is transferred to free charge carriers. In this case, the Ti3C2Tx/PEDOT:PSS nanocomposite films might re-emit the energy (reflection), dissipate the energy in the form of heat (absorption) or allow a portion of the electromagnetic wave to pass through (transmission) [7]. In the case of the nanocomposite film that contained 25 wt% of PEDOT:PSS, more than 92.38% of the incident wave was reflected. This shows that the shielding mechanism of Ti3C2Tx/PEDOT:PSS (25 wt% of polymer) was mainly due to the reflection of electromagnetic waves. Such an EMI shielding mechanism that mainly relies on the reflection of electromagnetic waves has also been observed for the other cases of this study, as most of the incident waves were reflected upon reaching pristine Ti3C2Tx or Ti3C2Tx/PEDOT:PSS (Figure 4-6). This reflection-based mechanism can be explained by the conductive nature of Ti3C2Tx and PEDOT:PSS containing a large number of mobile charge carriers. These free charge carriers respond to the incident wave by creating a scattered or induced field and reflect most of the incoming waves. Indeed, increasing PEDOT:PSS's weight percentage 69 reduces the conductivity and number of available mobile charge careers. Therefore, the reflectance coefficient and even the total EMI shielding effectiveness of composited films are lower than pure Ti3C2Tx. However, even in the case of composite films containing 50 wt% of PEDOT:PSS, the reflectance coefficient is higher than the absorbance coefficient since we still have lots of moveable charge carriers inside the structures that can reflect up to 72.82% of incoming waves (refer to Figure 4-6). Figure 4-5 (a) The EMI shielding characteristics, electrical conductivity, and thickness of the pristine Ti3C2Tx and the nanocomposite films that contained various concentrations of the polymer and (b) the structure and EMI shielding behavior of the Ti3C2Tx/PEDOT:PSS nanocomposite containing 25 wt% of polymer. Figure 4-6 Reflectance and absorbance of pristine Ti3C2Tx and the composite films that contained various concentrations of polymer. 70 Despite the higher electrical conductivity and SET, the pristine Ti3C2Tx and the nanocomposite films that contained only 10 wt% of the polymer showed inferior mechanical properties. After 500 bending cycles, the total EMI shielding of the abovementioned films reduced drastically. While, for the nanocomposite film containing 25 wt% of PEDOT:PSS, it remained unchanged (Figure 4-7 (a)). This reduction in EMI shielding characteristics can be explained by the formation of micro-cracks inside pristine Ti3C2Tx or the nanocomposite film that contained 10 wt% of polymer (Figure 4-7 (b)). This significantly reduces their electrical conductivity and their ability to block electromagnetic waves via the reflection mechanism. As a result of these micro-cracks, the prepared films delaminated from the polycarbonate substrates and degraded. These results are in line with the characteristics of pristine Ti3C2Tx and the nanocomposite film that contained 10 wt% of polymer, which showed large vacancies inside their structure (as described in Figures 4-2 (a-b) and 4-1 (d)). These vacancies suppress the dissipation of fracture energy and promote fracture propagation [66]. However, for the case of the nanocomposite film that contained 25 wt% of PEDOT:PSS, no micro-cracks were observed (Figure 4-7 (b)). Such promising mechanical properties might be explained by the absence of large vacancies and holes inside the prepared nanocomposite film (Figure 4-2 (c-e) and 4-1 (e)). 71 Figure 4-7 (a) Total EMI shielding of pristine Ti3C2Tx and nanocomposite films that contained 10 and 25 wt% of the polymer after 500 bending cycles and (b) height profiles of pristine Ti3C2Tx and nanocomposite films that contained 10 and 25 wt% of the polymer after 500 bending cycles (scale is 600 µm). 4.3.3 Post-treatment and wet-transferring of Ti3C2Tx/PEDOT:PSS films As discussed before, the addition of PEDOT:PSS significantly enhances the film-forming properties of Ti3C2Tx suspensions along with the mechanical characteristics of the prepared EMI shields. However, the role of this conductive polymer is beyond these points for EMI shielding. In fact, compared to non-conductive polymers, the reduction of electrical conductivity, and thus, the total EMI shielding performance of the final composite films of Ti3C2Tx is much lower for the case of the composite structures containing PEDOT:PSS. The electrical conductivity of the Ti3C2Tx/PEDOT:PSS composites can also be further improved to the level of pure Ti3C2Tx with a series of chemical treatments. In this case, the number of free charge carriers increases in the composite films, resulting in higher EMI shielding effectiveness. For instance, Figures 4-8 (a-b) show the enhanced electrical conductivity and the total EMI shielding of the Ti3C2Tx/PEDOT:PSS film (25 wt% of polymer) after the post-treatment with sulfuric acid or/and methanol. Typically, PEDOT:PSS tends to form PEDOT-rich cores surrounded by insulating PSS segments. Upon treatment with methanol or sulfuric acid, a portion of insulating PSS segments was removed from 72 the film, and the electrical conductivity of the composite film was enhanced. This was confirmed by the reduction of film thickness after both of these treatments. As shown in Figure 4-8 (a), the conductivity and EMI shielding of the sample treated with sulfuric acid was higher than those of the sample treated with methanol. This is due to the fact that sulfuric acid is capable of removing more PSS and making a more oriented PEDOT structure compared to organic solvents [115,116]. Figure 4-8 (a) EMI shielding characteristics, electrical conductivities, and thicknesses of Ti3C2Tx/PEDOT:PSS nanocomposite films (25 wt% of polymer) after treatment with methanol, sulfuric acid, and co-treatment with both of them and (b) the structure and EMI shielding behavior of the Ti3C2Tx/PEDOT:PSS nanocomposite containing 25 wt% of the polymer after co-treatment with sulfuric acid and methanol. To confirm the removal of PSS upon the above-mentioned treatments, X-ray photoelectron spectroscopy (XPS) was performed on the nanocomposite film containing 25 wt% of PEDOT:PSS before and after post-treatment with sulfuric acid and methanol (Figure 4-9). PEDOT:PSS demonstrates S2P peaks, denoting two different types of sulfur atoms with distinct binding energies. The broad peak between 166 and 170 eV arises from the PSS sulfur atoms, whereas the peak between 162 and 166 eV arises from the sulfur atoms of PEDOT thiophene ring [7,117]. As can be seen, upon treatment with methanol or sulfuric acid, the intensity of the PSS S2p peak reduces, 73 indicating the removal of the excess PSS from the nanocomposite film. However, the reduction of the PSS S2p peak was much more significant in the case of sulfuric acid post-treatment, confirming the unique capability of sulfuric acid to remove insulating PSS and make a more oriented PEDOT structure compared to methanol. Upon treatment with sulfuric acid, the concentrated acid undergoes autoprotolysis and creates H3SO4+ and HSO4− ions [118]. These ions stabilize the segregated states of PEDOT:PSS chains and promote the removal of excess PSS. In the case of sulfuric acid treatment, the removal of PSS is so strong that the PEDOT chains can form a dense network of nanofibrils due to their strong π-π stacking nature and the rigidity of their backbone [118]. This oriented structure of PEDOT promotes efficient intra- and inter-chain charge transport, thereby resulting in a highly conducting state [116,119]. On the other hand, the mechanism of methanol treatment relies on screening the ionic interaction between PEDOT and PSS that facilitates the phase separation and removal of PSS [115,119,120]. The screening effect of methanol also forms linearly oriented PEDOT chains. However, the crystallinity and ordered structure of the methanol-treated samples are lower than those properties of sulfuric acid-treated films [119]. The conductivity of PEDOT:PSS films treated with sulfuric acid is known to decline upon washing with water [121]. To address this problem, we employed a co-treatment of sulfuric acid and methanol to further improve the electrical conductivity and total EMI shielding of Ti3C2Tx/PEDOT:PSS films (25 wt% of polymer). In brief, the nanocomposite sheet was submerged into sulfuric acid and then washed with methanol successively. The wrinkles on the surface of the film were removed with the help of a soft brush, and the film was dried under vacuum. A very high electrical conductivity of 2,900±400 S/cm and a high total EMI shielding of 55.4 dB were observed for the co-treated sample (Figure 4-8 (a)). The structure and EMI shielding behavior of the co-treated 74 Ti3C2Tx/PEDOT:PSS nanocomposite film are shown in Figure 4-8 (b). In this case, a highly conductive network of PEDOT:PSS nanofibrils spread between the Ti3C2Tx sheets. Compared to the untreated sample, both the Ti3C2Tx flakes and polymer chains, which act as glue to hold the Ti3C2Tx flakes together, are highly conductive. Thus, the polymer chains added to Ti3C2Tx to improve the flexibility did not drastically reduce the composite film's electrical conductivity. In terms of EMI shielding behavior, more than 96.93% of the incident wave was reflected, and only 0.00017 % of the electromagnetic wave was transmitted through the nanocomposite film. Such a superb performance of the co-treated Ti3C2Tx/PEDOT:PSS nanocomposite film might be explained by the high conductivity of the prepared samples, which was in the same order of magnitude as pristine Ti3C2Tx samples. Figure 4-9 XPS results for the composite film containing 25 wt% of PEDOT:PSS before and after post-treatment with sulfuric acid and methanol. The co-treated Ti3C2Tx/PEDOT:PSS film (25 wt% of polymer) was transferred to various substrates without any obvious decrease in its EMI shielding properties. The procedure of wet-transferring of the nanocomposite films to various substrates is shown in Figure 4-10 (a). First, 75 the nanocomposite film was completely submerged into sulfuric acid for 6 h and then washed with methanol. After this treatment, a gentle agitation detached the composite film from the polycarbonate substrate. The free-floating nanocomposite film was then transferred to a new substrate in a methanol bath. Finally, the wrinkles on the surface of the film were removed by a soft brush soaked in methanol, and the film was dried under vacuum. Figure 4-10 (b) shows the co-treated Ti3C2Tx/PEDOT:PSS film (25 wt% of polymer) transferred to a glass sheet, pyrex glass cylinder, polyethylene terephthalate (PET) sheet, and double-sided scotch tape. This simple and non-destructive transferring approach can be used to deposit micrometer-thick EMI shields on complex substrates with curves, angles, and corners. The technology may be adapted to many geometries for developing novel flexible electronics, energy harvesting devices, or sensing applications. The total EMI shielding of the Ti3C2Tx/PEDOT:PSS film on the polycarbonate substrate was compared to the nanocomposite film that was transferred to the double-sided scotch tape before and after 500 bending cycles (Figure 4-10 (c)). The EMI shielding of the prepared film was unchanged after transferring to the new substrate. The developed nanocomposite film on both of these substrates showed excellent mechanical properties without any change in total EMI shielding after the bending test. Furthermore, the electrical conductivity, SET, and specific EMI shielding effectiveness (SSE/t = SET / (density * thickness)) of the co-treated Ti3C2Tx/PEDOT:PSS films were compared to those properties of nanocomposites reported in the literature (Figure 4-10 (d) and Table 4-5). Due to the exceptionally high electrical conductivity of Ti3C2Tx and co-treated PEDOT:PSS, the nanocomposite films developed in this study exhibit the highest conductivity reported for micrometer-thick polymer nanocomposites. The exceptional electrical conductivity and the internal EMI characteristics of Ti3C2Tx justify the very high EMI shielding properties of 76 the co-treated Ti3C2Tx/PEDOT:PSS compared to the other polymer nanocomposite shields. These results, along with the wet-transferring approach introduced here, highlight the potential of the developed EMI shields. Table 4-5 Previous studies of thickness, conductivity, SET, and specific EMI shielding effectiveness of various nanocomposites. Material Thickness (mm) Conductivity (s/cm) SET (dB) SSE/t (dB cm2 g-1) Reference Ti3C2Tx/PEDOT:PSS 0.011 340.5 42.1 19498 [66] Ti2CTx/Polyvinyl alcohol (PVA) 0.1 2.0 × 10–4 26 5136 [122] Ti3C2Tx/Cellulose nanofiber (CNF) 0.035 1.43 39.6 7029 [123] Ti3C2Tx/Silver nanowires (AgNW)/ Nanocellulose (NC) 0.0169 300 42.74 16724 [124] Ti3C2Tx/Aramid nanofibers (ANF) 0.012 - 34.71 21971 [125] Ti3C2Tx/PVA 0.027 7.16 44.4 9343 [126] Polystyrene/Graphene 2.5 1.25 29 322 [127] Poly(3-hydrobutyrate-co-3-hydroxyvalerate) (PHBV)/ AgNW 0.018 - 45.9 19678 [128] Ti3C2Tx/Polyacrylonitrile (PAN)/TiO2 0.045 92.68 32 4085 [129] Ti3C2Tx/rGO/Epoxy 2 6.95 56.4 9400 [19] Graphene/PEDOT:PSS 0.8 6.84 70 841 [130] V2O5/Polyaniline (PANI) 6 0.016 34.7 2770.3 [131] Ti3C2Tx/AgNW/CNF 0.035 373.78 59.7 10647 [132] Ti3C2Tx/Xanthan 0.00684 115.29 34.1 14,490 [133] Poly vinylidene fluoride (PVDF)/MWCNT 0.9 0.074 32 259 [134] Graphene/ PMMA 0.24 3.11 19 1042 [135] Ti3C2Tx/PANI 0.376 3.25 35.3 1700 [136] Poly(oxymethylene) (POM)/ MWCNT 2 3.33 58.6 344.4 [137] Ti3C2Tx/CNF 0.047 7.39 24 2647 [138] MWCNT/Polycarbonate 2.16 3 39 164 [139] Ti3C2Tx/PEDOT:PSS 0.007 2900 ± 400 55.42 38079 This work 77 78 Figure 4-10 (a) Schematic description of wet-transferring of the Ti3C2Tx/PEDOT:PSS nanocomposite film to a new substrate, (b) optical image of the Ti3C2Tx/PEDOT:PSS nanocomposite films transferred to various substrates with different geometries, (c) the total EMI shielding of the co-treated Ti3C2Tx/PEDOT:PSS films on the polycarbonate and double-sided scotch tape before and after 500 bending cycles, and (d) a comparison of the SSE/t of the co-treated Ti3C2Tx/PEDOT:PSS composite film (25 wt% of polymer) with the reported nanocomposites films. 4.4 Conclusion We developed a simple, yet versatile, drop-casting approach along with a co-treatment with sulfuric acid and methanol to fabricate highly conductive Ti3C2Tx/PEDOT:PSS nanocomposite films. While the uniformity of our drop-casted nanocomposite films is comparable with that of the composite films prepared via common techniques, such as VAF, their manufacturing costs are significantly less and their preparation method is more facile. The ~7 µm thick nanocomposite film containing 25 wt% of PEDOT:PSS showed an electrical conductivity of 2,900±400 S/cm and a total EMI shielding of 55.4 dB. It also exhibited an SSE/t of 38,079 dB cm2 g-1, higher than most existing conductive polymer nanocomposites. The developed nanocomposite film also showed excellent mechanical properties without any change in total EMI shielding after 500 bending cycles. We also introduced a wet-transfer approach to transfer the drop-casted Ti3C2Tx/PEDOT:PSS nanocomposite films onto complex geometries with curves, angles, and corners. Such a flexible, transferable, and highly conductive EMI shield has promising applications in flexible electronics and EMI shields with complex geometries. 79 Chapter 5: High-resolution extrusion printing of Ti3C2Tx-based inks for wearable human motion monitoring and EMI Shielding This work addresses two major challenges of MXene-based printing: its low printing resolution (i.e., filament spreading > 120%) and its inability to create structures with simultaneously high electrical conductivity and mechanical flexibility. We first report high-resolution extrusion printing of Ti3C2Tx and composite of Ti3C2Tx/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). The printing has negligible filament spreading (< 25%) and low thickness and width variations (< 20%) compared to their average values. We then fabricate Ti3C2Tx/PEDOT:PSS composite structures that possess simultaneously exceptional electrical conductivity and flexibility (conductivity of 1,600±400 S/cm for 80 wt% Ti3C2Tx, which can withstand up to 3,000 bending cycles). Addressing the two challenges expands the applications of MXene-based printing. For instance, micrometer-thick grids printed with our method show excellent EMI shielding effectiveness and superior specific EMI shielding effectiveness, reaching 38.4 dB and 43,000 dB cm2 g–1 in the case of pure Ti3C2Tx and 28.1 dB and 32,000 dB cm2 g-1 for the composite containing 80 wt% Ti3C2Tx. The printed structures of pure Ti3C2Tx also perform as highly-sensitive strain sensors with a gauge factor of 11,300 at a strain of 4%. The prepared sensors are capable of monitoring various human activities, including breathing, facial muscles movements, and talking. 5.1 Introduction Over the past few decades, mechanically flexible and electrically conductive materials have piqued the interest of physicists, materials scientists, and engineers. Up to now, a range of promising and versatile applications, including wearable sensors and actuators [140,141], energy storage devices 80 [142,143], health-monitoring systems [144], and flexible EMI shields [13,31,32,145,146], have been proposed based on materials with suitable electrical and mechanical properties. Nevertheless, there is still much room for further advances in manufacturing technologies and the choice of materials for developing cost-effective and highly flexible electronics. From a materials design perspective, carbon-based nanomaterials (e.g., carbon nanotube (CNT) and graphene) [147], intrinsically conductive polymers (e.g., poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and polyaniline) [7,148], MXenes [42,149], and their composites are considered as promising candidates for developing flexible conductive structures. Among these, intrinsically conductive polymers usually demonstrate relatively low electrical conductivity or require harsh acidic and solvent treatments to gain an acceptable level of electrical conductivity [116,119]. However, these chemical treatments, which are mainly employed for PEDOT:PSS, are not compatible with most substrates used for flexible electronics [90,150]. Furthermore, pure graphene and CNT are not attractive due to their poor dispersion in polar media and the formation of non-uniform films [65,151]. MXenes, a growing family of two-dimensional (2D) materials that consist of few-atom-thick layers of transition metal carbides, nitrides, or carbonitrides, are ideal candidates for developing flexible conductive structures. Ti3C2Tx MXene, in particular, is the most studied type of MXene that possesses all fundamental characteristics required for flexible electronics, including outstanding electrical conductivity, ease of processability, and lightweight [36,42,128,152]. Pure Ti3C2Tx containing highly aligned flakes [13,153] or composites of Ti3C2Tx with various materials (e.g., poly(vinyl alcohol) [154], cellulose nanofibers [138,155], aramid nanofibers [156], and PEDOT:PSS [66]) have been demonstrated to have outstanding mechanical properties. However, the fabrication of Ti3C2Tx or its composites structures mainly relies on conventional manufacturing 81 technologies such as vacuum-assisted filtration (VAF) [66], spin-coating [157], dip-coating [158], spray coating [159], interfacial assembly [160], and blade coating [13] with various limitations and challenges. For example, the VAF method, which is employed by the majority of studies to develop free-standing Ti3C2Tx films, is a time-consuming and expensive method that is unable to produce large-area films [67]. Other well-known technologies such as spin-coating, dip-coating, interfacial assembly, and spray coating are mostly suitable for making submicron thin films of MXenes and MXene composites (<1µm) [67]. More importantly, the above-mentioned manufacturing techniques are mostly limited to laminated and compact structures with no pattern or architecture flexibility. Extrusion printing of conductive materials is a promising strategy for macro-scale additive patterning with outstanding architecture flexibility [9,161]. Compared to conventional manufacturing technologies, extrusion printing offers customization, reduction in materials waste, and rapid production, opening up numerous opportunities for wearable and smart electronics [9,142]. However, only limited studies have reported extrusion printing of MXenes [142,162,163]. Besides, printing of MXenes still requires further advances in printing resolution, as even those few studies on this topic barely discussed the filament spreading and relative width and thickness variation of the printed structures. The pioneering study of Zhang et al. [142] on the extrusion printing of MXene aqueous inks is among the few examples that focused on the resolution of printing. However, even in that work, a relatively high filament spreading of 120% was observed for the prepared ink (~36 mg/ml of Ti3C2Tx), where the inner diameter of the printing nozzle was 200 µm, and the average width of the printed lines was around 440 µm. Furthermore, to enhance the viscoelastic characteristics of MXene inks and thus reduce filament spreading, adding secondary fillers such as activated carbon, CNT, and graphene oxide was practiced. Still, none completely solved the problem, as they yet showed 40 to 100% filament spreading [164]. One possible way to evaluate 82 and then optimize the printing quality is establishing a quantitative correlation between printing resolution (e.g., filament spreading and the variation of width and thickness of the printed tracks) and the rheological characteristics of the inks. Such a correlation is not dealt with in previous studies. Extrusion printing of MXenes also requires further advances in ink formulation to achieve structures with simultaneously high mechanical flexibility and electrical conductivity. Along this line, composite inks of MXenes and non-conductive polymeric binders (e.g., cellulose nanofibrils (CNF)) were reported to enhance the mechanical flexibility of the printed structures [165]. However, this strategy dramatically reduced the electrical conductivity of structures and significantly limited their applications in wearable electronics. One way to deal with this problem is to develop composite inks of MXenes and conductive polymers such as PEDOT:PSS, which can enhance the flexibility of the printed structures while sustaining the electrical conductivity of the incorporated MXenes [12,66]. Addressing these challenges can potentially widen the applications of MXene-based extrusion printing. Up to now, extrusion printing of MXene-based structures (e.g., pure Ti3C2Tx [142,163], NiCoP/Ti3C2Tx [166], CNT/Ti3C2Tx [167], CNT/ GO/ activated carbon/ Ti3C2Tx composites [164], and divalent cation-gelated Ti3C2Tx [168]) has been mainly employed for energy storage applications, including batteries and supercapacitors. However, other well-known applications of MXenes, including wearable motion sensors, health-monitoring systems, and flexible EMI shields, have been barely realized through the extrusion printing approach. To the best of our knowledge, the pioneering work of Cao et al. [165] on the extrusion printing of CNF/ Ti3C2Tx inks is one of the only few studies that focused on developing wearable strain sensors. However, this study involved the printing of CNF/Ti3C2Tx inks into an ethanol coagulation bath that not only added an extra step to 83 the printing process but also significantly reduced the printing quality. Furthermore, despite the advantageous exceptional electrical conductivity of MXene, there are no studies on the extrusion printing of flexible and micrometer-thick MXene-based EMI shields. Nevertheless, we believe such a processing technology can open up numerous opportunities in the field of EMI shielding, including lightweight, customization, and fast production, especially if some novel composite inks of MXenes are developed to obtain flexible yet highly conductive shields. Herein, we develop aqueous and additive-free inks of pure Ti3C2Tx and Ti3C2Tx/PEDOT:PSS composites that are suitable for high-resolution extrusion printing. In order to achieve outstanding printing quality, the rheological characteristics of the inks are quantitatively related to the filament spreading and the width and thickness variation of the printed tracks. Along this line, we achieve a negligible filament spreading of 25% and thickness and width variations of only 20% compared to the average values for pure Ti3C2Tx and the composite containing 80 wt% Ti3C2Tx. Furthermore, the composite structures of Ti3C2Tx/PEDOT:PSS show simultaneously high electrical conductivity and mechanical flexibility (e.g., a structure containing 80 wt% Ti3C2Tx withstands up to 3,000 bending cycles and shows a conductivity of 1,600±400 S/cm). Our processing approach is further employed to develop flexible, lightweight, and micrometer-thick EMI shields on double-sided tapes that can be adapted to complex geometries. The resulting printed EMI shields with a grid-like structure possess excellent EMI shielding characteristics (e.g., 38.4 dB and 43,000 dB cm2 g–1 in the case of pure Ti3C2Tx and 28.1 dB and 32,000 dB cm2 g–1 for the composite containing 80 wt% Ti3C2Tx) and sustain their performance after numerous bending cycles. This exceptional specific EMI shielding effectiveness outperforms all laminated structures of MXenes reported in the literature, introducing these printed structures as a novel class of EMI shields for advanced applications. Ultrasensitive wearable motion sensors with a gauge factor (GF) of up to 11,300 are 84 also developed based on the free-standing printed structures of pure Ti3C2Tx. The prepared sensors are able to monitor various human activities, including breathing, facial muscles movements, talking, as well as contraction and the relaxation of the biceps muscle. 5.2 Experimental section 5.2.1 Materials The synthesis recipe and characterization of Ti3C2Tx flakes were discussed in Chapter 3. PEDOT:PSS (Clevios, PH1000) aqueous dispersion (1–1.3 wt%) was purchased from Heraeus. Polydimethylsiloxane (PDMS) (Silgard 184) was obtained from Sigma-Aldrich. All chemicals were used as they were received without further purification. 5.2.2 Preparation of Ti3C2Tx and Ti3C2Tx/PEDOT:PSS inks The Ti3C2Tx paste was obtained by centrifuging the Ti3C2Tx suspension at 11,000 rpm for 1 h. The concentration of the prepared pastes was calculated to be between 60 to 90 mg/ml. In order to adjust the concentration of the Ti3C2Tx inks between 20 to 50 mg/ml, a required amount of water was added to the prepared pastes. The obtained inks were then homogenized using a mechanical mixer at 2,000 rpm for 10 min. For preparing the Ti3C2Tx/PEDOT:PSS inks, the commercially available PEDOT:PSS aqueous solution (Clevios, PH1000) was first cryogenically frozen and then lyophilized for 72 h to isolate PEDOT:PSS nanofibrils. The isolated PEDOT:PSS nanofibrils with varying concentrations were then re-dispersed in DI water using a mechanical mixer at 2,500 rpm for 6 h to prepare concentrated suspensions (55 and 60 mg/ml). The desired amount of concentrated suspension of PEDOT:PSS was then added to the required amount of Ti3C2Tx pastes to prepare the composite inks containing 20 and 50 wt% polymers (Refer to Table 5-1). The prepared inks were finally homogenized at 2,000 rpm for 20 min. 85 Table 5-1 Composition of prepared inks. Ink name Preparation Content Pure Ti3C2Tx Ti3C2Tx suspension was centrifuged at 11,000 rpm for 1h. A required amount of water was added to adjust the concentration of the paste. 50 mg/ml of Ti3C2Tx 80 wt% of Ti3C2Tx 1 ml of Ti3C2Tx paste (55 mg/ml) was added to 0.25 ml of PEDOT:PSS paste (55 mg/ml) 44 mg/ml of Ti3C2Tx + 11 mg/ml of PEDOT:PSS 50 wt% of Ti3C2Tx 1 ml of Ti3C2Tx paste (60 mg/ml) was added to 1 ml of PEDOT:PSS paste (60 mg/ml) 30 mg/ml of Ti3C2Tx + 30 mg/ml of PEDOT:PSS 5.2.3 Printing procedure Printable Ti3C2Tx and Ti3C2Tx/PEDOT:PSS inks were loaded to 10 mL syringes attached to a nozzle (200 to 410, gauge 27 to 22) and deposited using a 3D Bioprinter (Allevi, USA). The desired printing paths were designed by CAD drawings (SolidWorks 2016, Dassault Systèmes) and converted to G-code by commercial software (Allevi Bioprint online software) to command the x-y-z motion of the printer head. Based on the type of ink, the applied pressure and the print head speed were set to be 60-80 psi and 5-8 mm s-1, respectively. The prepared inks were deposited onto various substrates (e.g., glass, PC, PET, CA, and double-sided tape) and then dried overnight at 60 ˚C under vacuum conditions. Aside from double-sided tape, all substrates were treated with UV-ozone (PSDP Pro, Novascan Technologies Inc., USA) for 30 min. 5.2.4 Developing EMI shields Flexible and lightweight micrometer-thick grids of Ti3C2Tx and Ti3C2Tx/PEDOT:PSS were developed as a novel class of EMI shields. The detailed geometries of the printed EMI shields are provided in Figure 5-1. For comparison, the laminated structures of Ti3C2Tx and Ti3C2Tx/PEDOT:PSS were also developed using the conventional VAF method, as described elsewhere [66,71]. The printed grids of Ti3C2Tx and Ti3C2Tx/PEDOT:PSS were applied to double-sided tapes (with a thickness of ~50 µm) and adapted to various geometries with curves, angles, 86 and corners. These grids can also be covered with a layer of scotch tape to preserve Ti3C2Tx from humidity and subsequent oxidation. Figure 5-1 Color picture, height map, and geometrical characteristics of the printed grids. (a) Grid G1, (b) Grid G2, and (c) Grid G3. All samples were printed with pure Ti3C2Tx inks. For (a-c), the average thickness at the cross-section, line thickness, and line width is equal to 14.1, 10.6, and 250 µm, respectively. 87 5.2.5 Developing wearable sensors Cost-effective wearable sensors were developed by extrusion printing of pure Ti3C2Tx inks. In the first step, a free-standing grid of pure MXene with an inter-filament spacing of 1 mm was developed by the extrusion printing of Ti3C2Tx inks onto a PDMS substrate. The printed grid was then dried overnight at 60 ˚C under vacuum conditions and then detached from the P M substrate. Two copper wires were then attached to the two ends of Ti3C2Tx using silver paste as the connection site. In order to improve the durability of contact points, a small piece of aluminum foil was also attached to the top of the silver paste. In the next step, a thin layer of PDMS was spin-coated at 1,000 rpm for 30 s onto a glass slide. The free-standing Ti3C2Tx structure was directly transferred onto the uncured P M film and dried at 90 ˚C for 3 h. In the last step, the fabricated device was encapsulated by a PDMS top layer (spin-coating the PDMS at 1,000 rpm for 30s). After drying, the prepared sensor was carefully detached from the glass slide and transferred to a flexible double-sided tape (Figure 5-2). To generate and propagate cracks uniformly, the fabricated sensor was bent over a rod with a 1 mm radius of curvature and stretched up to 5% of its initial length at a constant speed of 50 μm s-1 using a tensile apparatus that included a force gauge with a resolution of 0.1 N (NEXTECH, DFS500), mounted on a step motor with a motion controller. Afterward, a multichannel potentiostat (Metrohm Autolab, the Netherlands) was employed to investigate the current vs. time behavior of the printed sensors at a constant voltage of 1 V. The behavior of the sensor at various strain conditions was evaluated with the tensile apparatus. To obtain the gauge factor of the fabricated sensors, a constant speed of 50 μm s-1 was used. Finally, the printed Ti3C2Tx sensors were employed for real-time human motion and health monitoring. A written consent from all participants was obtained prior to the research. 88 Figure 5-2 The flexible double-sided tape used as the substrate for the sensor. 5.2.6 Characterization The FESEM images of the printed structures were taken by a Tescan Mira 3 XMU equipped with a field emission gun. The 3D profile and thickness of the prepared structures were evaluated using a 3D laser scanning microscope (Olympus LEXT OLS5000). The rheological properties of Ti3C2Tx and Ti3C2Tx/PEDOT:PSS inks were evaluated using an Anton-Paar MCR 102 rheometer equipped with a 25 mm parallel-plate geometry at a gap size of 0.3 mm and temperature of 25 °C. To reach the desired condition (stable morphology), a rest time of 1 min in the rheometer was used after loading the sample. Apparent viscosity 𝜂 vs. shear rate ?̇? was monitored via flow curve tests from a shear rate of 0.01 to 300 s-1. hear storage modulus G′ and loss modulus G″ were measured as a function of shear strain by oscillatory tests in an amplitude range of 𝛾0 = 0.01–1000 %, corresponding to the shear stress of 𝜏0 ~1–500 Pa, and at a frequency of 1 Hz. The electrical conductivity of the prepared samples was measured using a Loresta GP resistivity meter (MCP-T700 model, Mitsubishi Chemical Co.) connected with a four-pin probe. For the electrical conductivity measurements, Ti3C2Tx and Ti3C2Tx/PEDOT:PSS laminated structures were 89 fabricated using the VAF method. The EMI shielding properties are derived from the guidelines and methods described in Chapter 2 of this thesis. 5.3 Results and discussion 5.3.1 Beyond printing of Ti3C2Tx/PEDOT:PSS The first step in extrusion printing of wearable and lightweight electronics is developing a printable ink that allows a smooth flow through the narrow needles while still enabling the extruded filaments to retain their original shape after printing. A printable ink of pure Ti3C2Tx was developed by centrifuging Ti3C2Tx suspension at 11,000 rpm for 1 h. Water was then added to the collected paste to prepare a Ti3C2Tx ink with a concentration of 50 mg/ml. In order to prepare the Ti3C2Tx/PEDOT:PSS inks, a commercially available PEDOT:PSS suspension was cryogenically frozen and then lyophilized for 72 h to isolate P OT:P nanofibrils. The isolated P OT:P nanofibrils were then re-dispersed in water to fabricate pure PEDOT:PSS paste. Then, a required amount of Ti3C2Tx and PEDOT:PSS pastes were mixed to obtain composite inks containing 80 wt% (44 mg/ml Ti3C2Tx and 11 mg/ml PEDOT:PSS) and 50 wt% (30 mg/ml Ti3C2Tx and PEDOT:PSS) of MXene. The composition of the prepared inks is summarized in Table 5-1. A rheological study was carried out to examine the printability of Ti3C2Tx and Ti3C2Tx /PEDOT:PSS inks (Figures 5-3 and 5-4). All samples exhibited shear-thinning behavior, which is favorable for extrusion printing (Figure 5-3 (a)). For these samples, viscosity decreases at high shear rates, allowing a smooth extrusion of prepared inks through a narrow nozzle. At lower shear rates, the viscosity is much higher and this is favorable for the post-printing of extruded inks. The viscosity values of the prepared Ti3C2Tx inks are in the same order of magnitude as those of concentrated Ti3C2Tx inks (290 mg/ml) previously reported in the literature [162]. This indicates 90 that the ink preparation and synthesis of Ti3C2Tx with no sonication and low defects are effective in tuning rheology at low concentrations. The viscoelastic fingerprints of printable Ti3C2Tx and Ti3C2Tx/PEDOT:PSS inks show a solid-like behavior (i.e., storage modulus G′ > loss modulus G″) at the plateau region, where G′ is independent of stress or input strain amplitude (Figure 5-3 (b)). Such a solid-like behavior can be clearly observed in Figure 5-3 (c), where filaments of the prepared inks can sustain their own weight and remain suspended from the needle even after 30 min. However, the viscoelastic network demolishes at the yield stress, resulting in a liquid-li e behavior with G″ > G′. This viscoelastic transition promotes the smooth flow of the inks through the printing nozzle while still allowing the printed tracks to retain their shape after printing. These results are in line with the topography results of the printed structures, where the width of printed lines after drying is almost equal to the inner diameter of the needle, and negligible filament spreading is observed for printed tracks (Figure 5-5 (a)). Figure 5-3 (a, b) Ink rheological characterization. (a) The viscosity of the prepared samples as a function of shear rate (s–1) and (b) elastic modulus G′ and loss modulus G′′ of the samples as a function of shear stress (Pa). (c) olid-like behavior of extruded filaments. Filaments of the prepared inks can sustain their own weight and remain suspended from the needle even after 30 min. 91 Figure 5-4 (a-b) The viscosity of the prepared samples as a function of shear rate (s-1) and (d-f) elastic modulus G′ and loss modulus G′′ of the samples as a function of shear stress (Pa). (a, d) pure Ti3C2Tx inks, (b, e) Ti3C2Tx/PEDOT:PSS composite inks containing 20 wt% polymer, and (c, f) Ti3C2Tx/PEDOT:PSS composite inks containing 50 wt% polymer. 92 The in ’s viscoelastic properties depend on the concentration of Ti3C2Tx and PEDOT:PSS. Figures 5-3 (b) and 5-4 (d-f) clearly show the elastic modulus and yield stress reduce as the inks become less concentrated. These results confirm the occurrence of a sol-gel transition upon increasing the solid content, leading to the formation of hydrogels with required viscoelastic properties for extrusion printing. The gel formation can be attributed to the interaction of Ti3C2Tx flakes with each other in the case of the pure MXene hydrogel or to the hybrid interactions of solid materials (e.g., Ti3C2Tx flakes-polymer, polymer-polymer, and Ti3C2Tx-Ti3C2Tx) in the composite hydrogels. Therefore, when the inks become less concentrated, the prepared suspensions lose their mechanical integrity and their predominantly solid-like behavior (as can be visually observed in Figure 5-5 (b) for pure Ti3C2Tx inks with various concentrations). In this case, the extruded filaments might lose their original shape on the substrate due to filament spreading. For example, for the pure Ti3C2Tx ink with a concentration of 40 mg/ml, the width of the printed lines after drying is much larger than the nozzle inner diameter (i.e., around 500 µm for a 200 µm nozzle), as shown in Figure 5-5 (c). When the concentration of the Ti3C2Tx ink was further reduced to 20 mg/ml, the elastic modulus was so low that the ink could not produce any filament and instead formed droplets during printing (Figure 5-5 (d)). 93 Figure 5-5 (a) Color picture and height map of a grid that is printed by a pure Ti3C2Tx ink with a solid content of 50 mg/ml. The width of the printed lines after drying is almost equal to the inner diameter of the needle, and no filament layout is observed for printed tracks. (b) The physical state of pure Ti3C2Tx inks with various solid contents. When the inks become less concentrated (from left to right), the prepared suspensions lose their mechanical integrity and solid dominant behavior. (c) Color picture and height map of a grid that is printed by a pure Ti3C2Tx ink with a solid content of 40 mg/ml. The width of printed lines after drying is much larger than the nozzle inner diameter due to the filament layout. (d) Photograph of pure Ti3C2Tx ink with a solid content of 20 mg/ml. In this case, the elastic modulus is so low that the ink cannot produce any filament and forms droplets during the printing. Scale bars in (a, c) correspond to 3 mm. Extrusion printing of Ti3C2Tx and its composite inks is particularly attractive for the fabrication of micro-structures on various commercially available substrates (Figure 5-6 (a)). The adhesion between the printed structures and several substrates (e.g., glass, polycarbonate (PC), polyethylene terephthalate (PET), and cellulose acetate (CA)) can be substantially improved by UV-ozone 94 treatment, as previously reported by many studies [76,169]. More interestingly, extrusion printing provides an opportunity to design complex structures on unconventional substrates such as double-sided tape. In this case, the adhesive of commercially available and low-cost tapes can ensure the adhesion of the printed structures to the substrate. The prepared structure can be readily applied to various geometries with different shapes, thereby generating numerous opportunities for wearable electronics and EMI shields. Free-standing micrometer-thick structures of Ti3C2Tx with an average thickness of ~15 µm were also fabricated (Figure 5-6 (a)). To this aim, Ti3C2Tx inks were deposited onto a hydrophobic PDMS sheet that was prepared via spin-coating of PDMS on a glass substrate at 1,000 rpm for 30 s. Then, the printed structure was easily removed from the substrate upon drying due to the low adhesion of the hydrophilic Ti3C2Tx to the PDMS substrate. Aside from these micrometer-thick layers, 3D structures of up to several millimeters in height were also achieved by stacking multiple layers of Ti3C2Tx or its composite inks onto each other (Figure 5-6 (c)). Due to the strong viscoelastic properties of the prepared inks, the extruded filaments can retain their shape while remaining sufficiently fluid for interlayer coalescence [162]. The prepared 3D structures have various applications for energy storage devices, as reported by many studies [162,163]. 95 Figure 5-6 (a) Photograph of the fabricated microstructures on various commercially available substrates. From left to right: Pure Ti3C2Tx on glass (using 410 µm nozzle), Ti3C2Tx/PEDOT:PSS composite containing 80 wt% MXene on PET and CA (using 200 µm nozzle), and Ti3C2Tx/PEDOT:PSS composite containing 50 wt% MXene on PC (using 200 µm nozzle) and double-sided tape (using 410 µm nozzle). (b) Free-standing and micrometer-thick Ti3C2Tx structure. (c) 3D-printed architectures of Ti3C2Tx and Ti3C2Tx/PEDOT:PSS. All scale bars correspond to 1 cm. The process of extrusion printing of Ti3C2Tx/PEDOT:PSS composite inks is shown in Figure 5-7 (a-b). Generally, PEDOT:PSS tends to form a complex with a PEDOT-rich core surrounded by the PSS-rich shells due to the ionic interactions between PEDOT and PSS chains [115]. Due to the electrostatic interactions of PSS chains and Ti3C2Tx, polymer complexes can be readily spread between MXene flakes [66]. This is mainly because of the shear alignment of Ti3C2Tx inside the needle during printing, as previously reported in the literature [162]. In printed electronics, printing homogeneous lines is of utmost importance to ensure reproducible electrical characteristics [142,170]. Here, the variation of width and thickness, the difference between the line thickness and the inner diameter of the needle, and topographic analysis of the printed structures were employed to evaluate the reliability of the extrusion printing of Ti3C2Tx and Ti3C2Tx/PEDOT:PSS inks (Figure 5-7 (c-e)). The width and thickness variations of the printed 96 lines are in an acceptable range compared to previous studies on extrusion printing of 2D materials [142]. Specifically, for the ink containing pure Ti3C2Tx, the thickness and width variation compared to average values are below 15 and 20%, respectively. Further, for the case of pure Ti3C2Tx and Ti3C2Tx/PEDOT:PSS inks, the average width of the printed lines is only ~ 25% larger than the inner diameter of the nozzle (line width of ~250 µm). This value for the composite ink holding 50 wt% PEDOT:PSS is slightly higher (around 35%, line width of ~ 270 µm). This can be related to the inferior viscoelastic properties of this composite ink compared to pure Ti3C2Tx or the composite ink containing 20 wt% PEDOT:PSS (Refer to Figure 5-3 (b)). These results further highlight the ability of the extruded filaments of the prepared inks to retain their original shapes with negligible filament spreading. It is worth noting that previous best practices in the extrusion printing of Ti3C2Tx that used the same 200 µm nozzle reported printed lines with a width of 440 µm (120% larger compared to the inner diameter of the needle) [142,162]. More interestingly, even those MXene inks containing various materials (e.g., activated carbon, CNT, and graphene oxide) for enhancing their viscoelastic characteristics showed 40 to 100% filament spreading higher than the values reported for the additive-free inks of this study [164]. Therefore, the Ti3C2Tx and Ti3C2Tx /PEDOT:PSS inks prepared in this study offer unique advantages for high-resolution printing of conductive structures, where the width of printed tracks are almost similar to the inner diameter of the used nozzle, and the digitally designed patterns can be achieved with a high degree of accuracy. 97 Figure 5-7 (a, b) The process of extrusion printing of Ti3C2Tx/PEDOT:PSS composite inks. (a) Schematic illustration of printing. Ti3C2Tx flakes are getting aligned upon extrusion printing because of the shear alignment of flakes inside the needle. Besides, polymer complexes can be readily spread between Ti3C2Tx flakes due to the electrostatic interactions of PSS chains and Ti3C2Tx sheets (b) Photograph of the printing process of Ti3C2Tx/PEDOT:PSS composite ink containing 50 wt% polymer. The thickness and width spatial uniformity, average thickness and width, 98 and color pictures of the extrusion-printed lines of (c) pure Ti3C2Tx, and Ti3C2Tx /PEDOT:PSS composites containing (d) 20 wt% and (e) 50 wt% polymer. Scale bars in (c-e) correspond to 1 cm. 5.3.2 Printed EMI shields Despite the unique advantages of extrusion printing, its potential in the fabrication of wearable electronics has yet to be investigated. Herein, we report lightweight, flexible, and cost-effective EMI shields based on micrometer-thick printed grids of Ti3C2Tx or Ti3C2Tx/PEDOT:PSS composites. The geometry of the printed EMI shields (grids G1, G2, and G3) is summarized in Figure 5-1. Compared to the laminated and compact EMI shields previously reported in the literature, these printed EMI shields require less material and benefit from macro-scale design control, making them more tunable than conventional shields. As discussed above, these structures can also be applied to the various substrates and, in the case of double-sided tape, can be adapted to various geometries with curves, angles, and corners. Before discussing the EMI shielding characteristics of the printed structures, it is important to comprehend the mechanisms of EMI shielding and the parameters affecting the shielding effectiveness of Ti3C2Tx and Ti3C2Tx/PEDOT:PSS grids. The primary mechanism of EMI shielding is the reflection that occurs due to the impedance mismatch between free space and the electrically conductive material [7,32,171,172]. The wave impedance of an electromagnetic wave is the ratio of the transverse components of the electric and magnetic fields. The characteristic impedance of free space is around 377 Ω. However, in highly conductive materials, the impedance is much lower, showing the inability of electromagnetic waves to propagate through conductive materials. In this case, the energy of electromagnetic waves transfers to the electrons and other charged particles inside the shield and forces them to oscillate. The forced oscillating charges are known to interact with incident waves and reflect them in the form of a scattered field (reflection) 99 [3,171,173]. As the charge carriers possess high kinetic energy and vibrate at high frequencies, the energy of the incident wave also dissipates in the form of heat (absorption) [3,171]. Beyond this point, a portion of electromagnetic waves might pass through the designed EMI shields (transmission). In the case of printed grids, the mobile charge carriers can freely move in the printed paths across the structure and respond to the incident wave by creating a scattered or induced field [3,174]. When the distance between the printed lines is smaller than the wavelength of the incident wave, the conductive paths can easily interact with the incident wave and reduce the strength of the electromagnetic field [3,174]. Here, we investigated the EMI shielding characteristics of the printed grids of Ti3C2Tx or Ti3C2Tx/PEDOT:PSS composites and introduced them as a novel class of micrometer thick EMI shields. In order to investigate the electrical conductivity of Ti3C2Tx and Ti3C2Tx/PEDOT:PSS composites, free-standing films of these materials were fabricated using the conventional VAF method (Figure 5-8 (a)). By adding or increasing the loading of PEDOT:PSS, the electrical conductivity of the structures reduces. The electrical conductivity of pure Ti3C2Tx was 3,800 ±200 S/cm. This value is in the same order of magnitude as the results reported previously in the literature for micrometer-thick Ti3C2Tx films [5,36]. However, the electrical conductivity of the prepared films reduced to 1,600 ±400 S/cm and 100 ±30 S/cm upon adding 20 and 50 wt% PEDOT:PSS. This reduction of electrical conductivity can be explained by the intercalation of PEDOT:PSS into Ti3C2Tx nanosheets and reducing the contact density of Ti3C2Tx flakes. Indeed, PEDOT:PSS's electrical conductivity is substantially lower than Ti3C2Tx. Therefore, charge transfer between MXene flakes is drastically reduced upon adding PEDOT:PSS. 100 The EMI shielding characteristics of the printed structures are summarized in Figure 5-8 (b-c). The shielding effectiveness, also known as total EMI shielding effectiveness (SET), is routinely used for investigating the performance of a shield. When the value of SET increases, an EMI shield can effectively block electromagnetic waves. As shown in Chapter 2, SET is calculated by the summation of EMI reflection loss (SER) and EMI absorption loss (SEA). Besides, the amount of electromagnetic waves that gets reflected, absorbed, or transmitted from a shield is shown by reflectance (R), absorbance (A), and transmittance (T) coefficients, respectively. The geometry (Refer to Figure 5-1) and conductivity of the printed structures are the parameters affecting the shieling characteristics of the prepared EMI shields. As expected, by increasing the loading of PEDOT:PSS, the total EMI shielding effectiveness of the prepared shields reduces due to a decline in the electrical conductivity of the shield and the number of mobile charge carriers that can interact with the incident waves [175]. The total EMI shielding effectiveness of the printed grids is relatively lower compared to the laminated structures prepared via the VAF method. Such a reduction in EMI shielding effectiveness is even more obvious when the inter-filament spacing increases from pattern G1 to G3; this can be attributed to a lower amount of conductive paths and mobile charges in the case of larger inter-filament spacing [63]. The results presented in Figure 5-8 (b) demonstrate the EMI shielding reliability, and show that the G1 grids of pure Ti3C2Tx and Ti3C2Tx/PEDOT:PSS composite containing 80 wt% MXene have EMI shielding effectiveness of 38.4 and 28.1 dB, respectively. This means that more than 99.98% (G1, pure Ti3C2Tx) and 99.83% (G1, 80 wt% Ti3C2Tx) of the incoming electromagnetic waves can be blocked using these grids. Therefore, these grids are more than sufficient for most commercial and even military applications of EMI shields, as previously discussed in the literature [71,176]. Such an exceptional performance of the printed 101 grids was obtained using a relatively low amount of materials to fabricate these grids. For example, for developing the G1 grid of pure Ti3C2Tx (16 mm×28 mm), only 4 mg of MXene was used. This value is almost 75% lower than the amount of the material used for developing Ti3C2Tx films with similar dimensions using the VAF method. Further, as shown in Figure 5-8 (d), these grids can be applied to double-sided tapes and adapted to unconventional geometries such as curves. As can be seen in Figure 5-8 (c), the reflectance of the printed structures is reduced by increasing the inter-filament spacing. This can be due to the existence of less conductive paths and mobile charges that can interact with the incident waves and reflect them in the form of a scattered field. These results are in line with the previous studies that explained the EMI shielding characteristics of the conductive grids by their equivalent impedance [171,177]. Increasing the grid's interlayer spacing increases impedance, resulting in lower reflectance and even shielding effectiveness. Nevertheless, for all of the printed structures, the value of reflectance was higher than absorbance. Therefore, the main mechanism of EMI shielding for the printed structures is based on reflecting electromagnetic waves. This might be explained by the conductive nature of Ti3C2Tx and its composites with PEDOT:PSS, providing abundant mobile charge carriers that can reflect electromagnetic waves. It is worth noting that some of the previous studies mistakenly used SER and SEA to interpret the shielding mechanism of various materials [35]. However, SER and SEA are relative quantities and have nothing to do with absolute power values and shielding mechanisms. Comparing the results presented in Figures 5-8 (b-c), we can see the SEA is higher than SER for all structures. However, as mentioned before, the dominant mechanism of EMI shielding is based on the reflection of incident waves for these samples. For instance, for the G1 grid of Ti3C2Tx /PEDOT:PSS (80 wt% MXene), SEA and SER are equal to 19.6 and 8.5 dB, respectively. However, 102 the mechanism of EMI shielding is based on reflection, as more than 85 % of the incident waves was reflected by this structure. Figure 5-8 (a) The electrical conductivity and thickness of Ti3C2Tx and Ti3C2Tx/PEDOT:PSS structures prepared via the VAF method. These laminated structures denoted as “full” were also used as a benchmar to evaluate the MI shielding characteristics of the printed structures. (b) The average EMI SE (SER, SEA, and SET) of the fabricated structures. (c) Reflectance, absorbance, and transmittance coefficients of the fabricated structures. (d) The G1 grid of pure Ti3C2Tx printed onto double-sided tape. This structure can be easily applied to unconventional geometries with curves, angles, and corners. A designed EMI shield might experience multiple bending cycles throughout its lifetime and should sustain its EMI shielding characteristics even under these severe conditions. The EMI shielding and electrical characteristics of the G1 grid of pure Ti3C2Tx and Ti3C2Tx/PEDOT:PSS composites were investigated after repeated bending cycles using a custom device (Figure 5-9 (a)). The printed structures of pure Ti3C2Tx and the composites containing 80 and 50 wt% MXene retained their electrical conductivity up to 1,000, 3,000, and 5,000 bending cycles, respectively, and no changes in their electrical current at a constant voltage were observed. Of note, upon 103 increasing the polymer content, PEDOT:PSS can spread between the Ti3C2Tx sheets, suppressing the crack propagation and improving the mechanical properties, e.g., flexibility, of the samples [66]. This is why the structure containing pure Ti3C2Tx or the composite containing 80 wt% Ti3C2Tx (small amount of polymer) showed no or very low conductance after 5,000 bending cycles, while the conductivity of the composite containing 50 wt% MXene remained constant under similar bending conditions. The shielding effectiveness of the printed structures after 5,000 bending cycles is shown in Figure 5-9 (a). For the case of pure Ti3C2Tx, the EMI shielding effectiveness was reduced from 38.4 to 19.5 dB after 5,000 bending cycles. One might ask why the printed structure can shield electromagnetic waves even after 5,000 bending cycles when the sample showed no conduction as a fixed voltage was applied to its ends. When this sample was bent up to 5,000 cycles, several cracks were formed across the printed structure that did not allow the electrons to move from one side to the other side of the grid. However, there are still several domains of conductive materials that are filled with mobile charges. The electrons of these domains can still interact with the incident wave and block the incoming electromagnetic field. Indeed, the freedom of mobile charges reduces in this case of the segregated structure, and therefore, the total EMI shielding effectiveness of the sample is substantially lower. These data unfold that conduction is not the absolute criterion for EMI shielding, as conduction needs an interconnected network of conductive paths, whereas this is not necessary for EMI shielding [3]. However, when we have an inter-connected network of conductive materials, the mobile charges have an elongated conductive path to go through in each half cycle of alternating field, and thus shielding effectiveness increases, as confirmed by previous studies [3,178]. 104 Among all the prepared structures, the composite of Ti3C2Tx/PEDOT:PSS containing 20 wt% polymer demonstrated the most interesting EMI shielding characteristics after repeated bending cycles. First, the prepared structures sustain their electrical conductivity up to 3,000 bending cycles. Further, even after 5,000 bending cycles, the EMI shielding effectiveness of the prepared film was 21.6 dB. In this case, the prepared film can block more than 99% of incoming electromagnetic waves and meet the requirements for most of the commercial applications of EMI shields [71,176]. The specific EMI shielding effectiveness (SSE/t = SET / density * thickness) of the G1 grids of the prepared samples was compared with the MXene-based shields reported previously in the literature (Figure 5-9 (b) and Table 5-2). In our comparison, we included all types of shields, such as laminated and compact structures and porous structures of MXene. For the pure c and the composite containing 80 wt% MXene, the SSE/t of the printed G1 grids was equal to 43,000 and 32,000 dB cm2 g-1, respectively. Compared to the compact and laminated structures of MXene, the printed structures developed in this work showed the highest SSE/t in micrometer thicknesses, while having the smallest amount of materials and weight. The printed structures in this study also presented higher SSE/t compared to most of the developed porous composite structures of MXene. However, the thickness of the printed grids is much smaller than the porous composite shields. The only reported EMI shields that have higher SSE/t compared to our samples are porous structures of pure MXene. However, this class of EMI shields cannot withstand multiple bending cycles [36]. Therefore, the cost-effective, flexible, lightweight, and high-performance printed structures reported in this study are an effective approach for addressing the current demand for EMI shields. 105 Figure 5-9 (a) The EMI shielding and electrical characteristics of the G1 grid of pure Ti3C2Tx and Ti3C2Tx/PEDOT:PSS composites before and after 5,000 bending cycles. (b) A comparison of the SSE/t of the G1 grids of the prepared samples with the MXene-based shields reported previously in the literature. 106 Table 5-2 The EMI shielding characteristics of various structures developed based on MXene. Type of EMI shield Material Thickness (mm) SE (dB) SSE/t (dB cm2 g−1) Ref. Compact and laminates structures Ti3C2Tx 0.045 92 8554 [42] Ti3C2Tx /SA 0.009 57 30015 [42] Ti3C2Tx /Wax 0.8 70 1776 [179] Ti3C2Tx /CNF 0.047 24 2647 [138] Ti3C2Tx /CNF/CNT 0.038 38.4 8020 [180] Ti3C2Tx /HEC 0.1 24 7000 [181] Ti3C2Tx /ANF 0.017 33 15529 [182] Ti3C2Tx /PEDOT:PSS 0.011 42.1 19497 [66] Ti3C2Tx /TiO2/rGO 0.009 28 30293 [183] Ti3C2Tx /rGO/PVDF 0.35 54 1944 [184] Ti3C2Tx /Ni/PVDF 0.10 19.5 1177 [185] Ti3C2Tx /PVA 0.027 44.4 9343 [126] This work (pure Ti3C2Tx) 0.014 38.33 42932 This work (80 wt% Ti3C2Tx) 0.021 28.06 32240 This work (50 wt% Ti3C2Tx) 0.024 16.26 20246 Porous (pure MXene) Ti3C2Tx 1 70.6 64182 [38] Ti3CN 1 69.2 62909 [38] Ti2C 1 54.1 49182 [38] Ti3C2Tx foam 0.006 32 136752 [186] Ti3C2Tx foam 0.018 50 69444 [186] Ti3C2Tx foam 0.06 70 53030 [186] Ti3C2Tx 2 75 18116 [187] Porous (MXene based composites) Ti3C2Tx /rGO 3 50.7 36737 [188] Ti3C2Tx /AgNW 2 52.6 5313 [189] Ti3C2Tx /SA/PDMS 2 72 18000 [190] Ti3C2Tx /PVA 5 33 5136 [122] Ti3C2Tx /CNT 3 104 8253 [191] Ti3C2Tx /rGO 2 52.7 5988 [19] Ti3C2Tx /CA 0.026 54.3 17586 [192] 5.3.3 Printed piezo-resistive sensors We also fabricated a piezo-resistive sensor using the printed grids of Ti3C2Tx with an inter-filament spacing of 1 mm (Figure 5-10 and 5-12). To this end, a free-standing grid of Ti3C2Tx was transferred to a thin layer of uncured PDMS (prepared via spin-coating of PDMS on a glass substrate at 1,000 rpm for 30 s). Finally, in order to increase the durability of the sensor and reduce 107 the influence of humidity and moisture caused by sweat, the fabricated structure was encapsulated by a PDMS top layer with another round of spin-coating. Figure 5-11 shows that the encapsulated sensor can effectively sustain its original electrical characteristics even several months after its fabrication. This exceptional performance can be rationalized by the fabrication process, which isolates the Ti3C2Tx grids from humidity and the high-quality Ti3C2Tx flakes with low defects that are reasonably resistant to oxidation [193,194]. The sensing mechanism of the developed piezo-resistive sensors is related to the formation of microcracks throughout the grid. As reported previously, the stress developed at the Ti3C2Tx-PDMS interface during the PDMS curing creates cracks inside the sensing layer of the fabricated structures [195]. However, in this work, the crack intensity was further increased by bending and stretching the developed structure over a rod with a 1 mm radius. In this case, the printed structures were bent over the rod and attached to a tensile apparatus that stretched the sensor up to 5% at a constant speed of 50 μm s-1. A visual illustration of the crack gap opening under external stress is shown in Figures 5-10 (b-f). By increasing the strain, the gap between the pre-generated cracks increases, and the sensing layer's conductance reduces significantly. By further increasing the strain, the edges of the cracks become disconnected, leading to zero conductance. An analogy may be drawn to what has been reported for the slit organs of spiders. Similar to our sensor, the slit organs consist of a number of cracks in their structures. Any external stimuli result in an ultrasensitive displacement of these crack-shaped organs and, therefore, enables the detection of minute vibrations in their surroundings [195,196]. Our crack-based sensor was also capable of detecting small deformations, which have several applications in human health and motion monitoring. 108 The strain sensing performance of the printed sensors is shown in Figure 5-10 (g) and Table 5-3. Typically, the strain was introduced via a tensile apparatus, and the corresponding normalized changes in resistance and current were evaluated. Upon applying the strain, the cracks in the printed structures become separated, and the resistance increases. For instance, at 1% strain, the normalized change in the resistance of the printed structure was around 4,900%, confirming the printed sensor’s sensitivity. Upon further stretching up to 4% strain, the adjacent cracks moved farther apart, and the resistance of the sensing film almost linearly increased with the strain. In order to evaluate the sensitivity, the GF of the printed Ti3C2Tx sensors was evaluated (GF = (∆𝑅/𝑅0)/∆𝜀, where ∆𝑅 is the change in resistance, 𝑅0 is the initial resistance and ∆𝜀 is the change in strain). Routinely, sensors with GF>100 are adequate for high-sensitivity detection of physiological activities, including swallowing, speaking, and facial expression [197]. Herein, the printed sensor showed a variable GF, with a higher GF observed as the tensile strain increased (Table 5-3). The GF of the printed sensors reached 11,300 at 4% strain, which is higher than most values reported for the MXene-based sensors (Table 5-4). Furthermore, at a strain of 1%, the response and recovery time of the fabricated sensor were 90 and 100 ms, respectively. Compared to previous studies, these values are outstanding for wearable sensors [195,198]. 109 Figure 5-10 (a) The fabricated piezo-resistive sensor based on the printed grids of pure Ti3C2Tx with the inter-filament spacing of 1 mm. A visual illustration of the crack gap opening in various strains of (b) 0%, (c) 1%, (d) 2%, (e) 4%, and (f) 8%. (b–f) have a same magnification. The scale bar in (f) corresponds to 400 µm. (g) The corresponding normalized change in resistance and current for the printed Ti3C2Tx sensor at various strains. (h) The response time and (i) recovery time of the fabricated sensors are calculated at a strain of 1%. 110 Figure 5-11 Stability of the electrical characteristics of the fabricated sensor. I and I0 were measured under a no lateral strain (0%). The printed Ti3C2Tx sensors with outstanding sensitivity were also employed for real-time human motion and health monitoring. For instance, human breathing can be easily identified by recording the changes in the sensor’s current in a repeatable manner. The recorded response for harsh and normal human breathing is shown in Figures 5-12 (a-b). In the case of harsh breathing, the strain applied onto the sensor was higher due to larger chest movements. Therefore, the induced cracks of the sensors moved farther away during harsh breathing, resulting in a larger reduction of current compared to the baseline. The printed sensors also showed great sensitivity to facial muscles movements. Figure 5-12 (c) shows the response of the sensor to multiple cycles of the raising and relaxing of human eyebrows. This response can be potentially used for human emotion recognition. The Ti3C2Tx strain sensor was also capable of distinguishing different sounds and even polysyllabic words when attached to the skin outside of the larynx of a human subject. For instance, the recorded response of the sensor for the word “MXene” and a humming sound is shown in Figure 5-12 (d). 111 The durability of the fabricated sensors is also of utmost importance for human motion and health monitoring. To assess durability, the sensor was attached to a human arm and undergone multiple runs of biceps contraction and relaxation (each run had 50 bending cycles). After each run, the sensor was detached from the subject arm and stored for the next runs to simulate the exact situation that a wearable sensor is supposed to be used in real-life applications. Figure 5-12 (e) demonstrates the capability of the fabricated sensor for recording the contraction of the biceps. Such a response can be used in designing real-time monitoring systems that evaluate the performance of athletes and provide significant guidelines regarding muscle fatigue and preventing injuries. More importantly, after 2,000 bending cycles (40 runs) and multiple cycles of loading and unloading of the sensor to the subject's arm, the fabricated structure was able to sustain its response. In this instance, even following 2,000 bending cycles, the response effectively discerns the contraction and relaxation of biceps muscles, with the baseline of the sensors approximately decreasing by only 10% compared to its initial value. 112 Figure 5-12 The recorded response for (a) normal and (b) harsh human breathing. (c) The recorded response for the multiple cycles of raising and relaxing of human eyebrows. (d) The recorded response of the sensor for the word “MXene” and a humming sound. (e) The capability of the fabricated sensor for recording the contraction of the biceps. The fabricated structure was able to keep its response after 2,000 bending cycles (40 runs) and multiple cycles of loading and unloading of the sensor to the subject's arm. Table 5-3 Normalized changes in resistance and gauge factor of the printed sensor in various strains. Strain (%) Average (∆𝑅/𝑅0) Average gauge factor (GF) 1 49 4,900 2 137.8 6,894 3 311.5 10,383 4 453.5 11,339 113 Table 5-4 A literature review on Ti3C2Tx-based piezo-resistive strain sensors Material Method of fabrication Sensitivity/Gauge factor Ref. Ti3C2Tx Ink-jet printing GF=180 [199] Ti3C2Tx Screen printing of electrode and dip coating of sensing layer 3.81 (kPa−1) [200] Ti3C2Tx/RGO Freeze drying 22.56 (kPa−1) [23] Ti3C2Tx/PVA Composite hydrogel preparation GF=80 [201] Ti3C2Tx/RGO Vacuum assisted filtration GF=1148 [202] Ti3C2Tx Dip coating 442 (kPa−1) [203] Ti3C2Tx Laser patterning GF=7,400 [195] Ti3C2Tx/CNTs/TPU Fabrication of TPU foam containing Ti3C2Tx and CNTs GF=363 [204] Ti3C2Tx/CNC Dip coating GF=17.4 [205] Ti3C2Tx Extrusion printing GF=11,339 This work 5.4 Conclusion This chapter focused on two major challenges of extrusion printing, including its low printing resolution and its inability to print structures with simultaneously high electrical conductivity and mechanical flexibility. We demonstrated aqueous and additive-free inks of Ti3C2Tx and Ti3C2Tx/PEDOT:PSS with rheological properties that are suitable for high-resolution extrusion printing. The width of printed tracks of the pure Ti3C2Tx and the composite ink containing 80 wt% MXene was almost similar to the inner diameter of the extrusion nozzle with only 25% of filament spreading. Furthermore, the thickness and width variation of the printed tracks was around 20% compared to average values, highlighting the outstanding printing resolution. We also achieved composite structures of MXenes with simultaneously high electrical conductivity and mechanical flexibility (e.g., a structure containing 80 wt% Ti3C2Tx showed a conductivity of 1,600 ±400 S/cm up to 3,000 bending cycles). We further employed this knowledge to develop flexible, lightweight, and micrometer-thick EMI shields on double-sided tapes. The printed shields were able to be adapted into complex geometries and showcased remarkable EMI shielding characteristics (e.g., 114 SET=38.4 dB and SSE/t=43,000 dB cm2 g–1 for Ti3C2Tx and SET =28.1 dB and SSE/t=32,000 dB cm2 g–1 for the composite containing 80 wt% Ti3C2Tx). The printed grids of pure Ti3C2Tx were also employed for developing sensitive strain sensors with a gauge factor of 11,300 at a strain of 4%. These strain sensors were capable of monitoring a wide range of subtle human movements, including breathing, facial muscles movements, talking, and contraction and relaxation of the biceps muscle. Considering the facile fabrication, outstanding flexibility, and significant printing resolution, the developed inks can open up numerous opportunities for wearable electronics and beyond. 115 Chapter 6: U ‐F w aerogels of Ti3C2Tx/GO for EMI shielding MXene and graphene aerogels have demonstrated excellent EMI shielding effectiveness due to their exceptional electrical conductivity, low density, and ability to dissipate electromagnetic waves through numerous internal interfaces. However, their synthesis demands costly reduction techniques and/or pre-processing methods such as freeze-casting to achieve high EMI shielding and mechanical performance. Furthermore, limited research has been conducted on optimizing the aerogel microstructures and porosity to enhance EMI shielding effectiveness while reducing materials consumption. In this chapter, we present a novel approach to produce ultra-lightweight aerogels composed of Ti3C2Tx/GO displaying multiscale porosity to enable high-performance EMI shielding. Our method uses controllable templating through the interfacial assembly of filamentous-structured liquids that are readily converted into EMI aerogels. The obtained ultra‐flyweight aerogels (3-7 mg/cm3) exhibit outstanding specific EMI shielding effectiveness (33,000-50,000 dB cm2/g) while eliminating the need for chemical or thermal reduction. Furthermore, exceptional shielding is achieved when the Ti3C2Tx/GO aerogels are used as the backbone of conductive epoxy nanocomposites, yielding EMI shielding effectiveness of 31.7-51.4 dB at a low filler loading (0.3-0.7 wt%). Overall, we introduce a one-of-a-kind EMI shielding system that is readily processed while affording scalability and performance. 6.1 Introduction The rapid adoption of digital technologies has transformed our ways of life, enabling us to achieve a myriad of goals, from education and entertainment to communication and beyond. However, the ubiquitous use of wireless platforms like 5G or 6G, miniaturized electronics, and high-frequency devices has spawned a new type of pollution known as electromagnetic interference (EMI) 116 [11,31,206,207]. EMI is a silent menace, capable of wreaking havoc on sensitive electronics and biological systems, posing severe threats to our health, the environment, and our economies. As our dependence on digital technologies continues to increase, the detrimental impact of EMI becomes a concern we simply cannot afford to overlook. Hence, it is of utmost importance to develop effective EMI shielding solutions. This not only mitigates these unwanted emissions but also safeguards against the harmful effects of this emerging form of pollution [32,206,208]. Thus far, the quest to develop materials that meet the critical requirements for efficient EMI shielding remains in progress. Historically, metals such as silver (Ag), copper (Cu), aluminum (Al), and stainless steel have been favored due to their superior electrical conductivity, which renders them effective defenses against electromagnetic waves [11,12,31]. Nevertheless, factors such as high density, challenging processability, susceptibility to corrosion, and a shielding mechanism primarily based on reflection, render these metals ill-suited for today's electronics - particularly in the realm of wearable, portable, and miniaturized devices. Consequently, researchers have shifted their focus towards lightweight, easily processable materials capable of meeting the exacting demands of the modern electronics market. Among all, carbonaceous nanomaterials, such as carbon nanotubes, graphene, reduced graphene oxide, and MXenes, have demonstrated exceptional EMI shielding capabilities [11,12,33,209,210]. In addition, intrinsically conductive polymers, such as poly(3,4-ethylene dioxythiophene:polystyrene sulfonate) (PEDOT:PSS), polypyrrole, and polyaniline, as well as highly conductive metallic nanowires, have been used for EMI shielding [7,211]. However, the high electrical conductivity of most of these materials leads to shielding by reflection, which results in secondary pollution [31,206]. Moreover, the considerable fabrication costs and the extensive amount of nanomaterials required for these EMI shields must be substantially reduced before they can be viable for wide-scale applications [206,212]. 117 Recent initiatives aimed at addressing the complexities of EMI shielding have primarily concentrated on porous composite structures fabricated using conductive nanomaterials [213,214]. These porous EMI shields contain numerous interfaces that cause multiple internal reflections and lengthen the wave propagation path, which increases interactions between the shield and the propagating waves [206,214,215]. As a result, these shields can achieve high EMI absorption while using minimal amounts of nanomaterials [206]. Additionally, porous conductive structures can serve as a framework for developing conductive nanocomposites with exceptional EMI shielding capabilities. This is achieved through conductive porous structures filled with polymers such as epoxy and polydimethylsiloxane (PDMS), creating stable EMI shields [33,216]. Given their unique characteristics, porous EMI shields, particularly those crafted from 2D nanomaterials, emerge as promising contenders for broad-based industry applications. They offer a host of advantages, such as ultra-low density, structural integrity, high electrical conductivity-to-density ratio, and substantial internal surface area—all essential properties for effective wave attenuation [33,209]. These factors position 2D nanomaterial aerogels as well-suited for large-scale EMI shielding deployment. Nevertheless, there are still several core issues that limit these structures as far as their adoption in modern EMI shielding. Firstly, producing aerogels based on 2D nanomaterials usually requires costly reduction (chemical/thermal) or pre-processing techniques such as freeze-casting to achieve high EMI shielding characteristics and mechanical performance. Furthermore, engineering the microstructure and porosity of aerogels to enhance EMI shielding proves to be a complex task. This complexity extends to programming pathways to create internal interfaces that boost the scattering and absorption of electromagnetic waves. 118 To address the limitations of porous EMI shields, structured liquids based on functional nanomaterials have been proposed to template aerogels with multiscale and tunable porosities [33,217,218]. These structured liquids can be achieved through the self-assembly and jamming of nanoparticles (NPs) at the liquid-liquid interface [219,220,221]. In practice, interfacial jamming occurs when functionalized NPs dispersed in one liquid interact with polymeric/oligomeric ligands dissolved in a second immiscible liquid, forming nanoparticle surfactants (NPSs) [33,219,222,223]. Jamming of NPSs creates a densely packed particle layer at the liquid/liquid interface, which maintains the integrity of structured liquids and prevents the otherwise tendency to minimize the interfacial area [224]. When employing structured liquids as a template for aerogels production, the NPS interface, which preserves the integrity of nonequilibrium shapes, also bolsters the mechanical strength of the resulting porous structures [33]. Besides, the porosity, density, pore size distribution, and even the final structural arrangement of the aerogels can be optimized by simply adjusting the general characteristics of the liquid templates [33,217,222]. However, this potential of liquid templates in the fabrication of aerogels still requires further advances in terms of functionality. Developing conductive aerogels with enhanced mechanical stability, ultra-low density, and well-defined micro- and macro-scale pore structure based on liquid templating remains daunting. With the benefit of tunability, these types of liquid templates can readily surpass some of the routine strategies used to tailor the structure of aerogels, e.g., freeze casting [225], chemically-induced assemblies [19,226], and interfacial electrochemical self-assembly [227]. In this chapter, we employed interfacial jamming of NPSs to stabilize a stream of aqueous suspensions of Ti3C2Tx/GO falling in a nonpolar medium. This strategy yields filamentous Ti3C2Tx/GO structured liquids that were used as a liquid template for fabricating ultra-flyweight and worm-like aerogels with multiscale porosities (micro- and macro-scaled). These worm-like 119 Ti3C2Tx/GO aerogels demonstrate exceptional EMI shielding effectiveness, specific EMI shielding effectiveness, and ultra-low density of 44.7-69.2 dB, 33,000-50,000 dB cm2/g, and 3-7 mg/cm3, respectively. Notably, no chemical or thermal reduction was required to achieve these impressive features. Furthermore, when these aerogels were employed as a backbone for conductive epoxy nanocomposites, they resulted in highly effective EMI shields, demonstrating a specific EMI shielding effectiveness of 31.7 to 51.4 dB at a low filler loading of just 0.3 to 0.7 wt%. This achievement paves the way for the development of scalable EMI shielding solutions. 6.2 Experimental section 6.2.1 Materials Synthesis of recipes and characterization of GO and Ti3C2Tx flakes were discussed in Chapter 3. The extrusion medium, hexane, and the ligand, PSS-[3-(2-aminoethyl)amino]propyl-heptaisobutyl substituted POSS (POSS-NH2), were supplied by Sigma-Aldrich and used during the process, respectively. In all stages of the process, ultra-pure deionized water (18.2 MΩ) was used, which was generated via ELGA model MEDICA EDI 15/30. 6.2.2 Fabrication of Ti3C2Tx bead-shaped structured liquids For the generation of Ti3C2Tx bead-shaped structured liquids, an aqueous suspension of Ti3C2Tx with concertation of 5-10 mg/ml was loaded into a plastic syringe and extruded into the hexane containing 1 mg/ml POSS via a 3D Bioprinter (Allevi, USA). In all steps, 200 µm (ID) dispensing needles were used, and extrusion pressure was set at 10-15 psi. Upon injection of the aqueous suspension of Ti3C2Tx into the hexane-POSS domain, the stream of Ti3C2Tx suspension breaks down into discrete droplets forming liquid beads. 120 6.2.3 Fabrication of worm-like GO structured liquids Worm-like GO structured liquids were generated by extruding 5-10 mg/ml of GO aqueous suspension into the hexane-POSS domain. To achieve this, a dispensing needle with an inner diameter of 200 µm was utilized, and the extrusion pressure was adjusted to 15-30 psi. Upon injection of the GO suspension into the hexane-POSS domain, an interfacial skin was formed at the liquid-liquid interface of two immiscible liquids due to a tight electrostatic bond between the ligand (POSS) and GO. This robust skin prevented the collapse of the aqueous jet into discrete droplets by suppressing Plateau-Rayleigh instabilities and the applied compressive forces to the liquid threads. It is important to note that the rigid structure of POSS molecules, which consist of a cubic inorganic silica-cage core with side chains, greatly enhances the mechanical strength of the GO/POSS assemblies. This feature makes them highly suitable for liquid streaming applications [33,217]. The choice of POSS as a ligand is particularly advantageous due to its versatility in rapidly diffusing toward the interface. It also leads to the formation of robust protonated POSS-NH3+ layers at the interface, facilitating the diffusion of NPSs to the interface and enabling the formation of structured liquids that serve as templates for aerogel production [217]. 6.2.4 Fabrication of hybrid Ti3C2Tx/GO worm-like structured liquids and aerogels The hybrid worm-like system of Ti3C2Tx/GO was generated by preparing hybrid aqueous suspensions of these nanomaterials. Two different composite suspensions were utilized in this phase: a 50wt%:50wt% ratio of GO to Ti3C2Tx and a 20wt%:80wt% ratio of GO to Ti3C2Tx. The suspension was extruded into the hexane-POSS domain using a 200 µm (ID) dispensing needle at the extrusion pressure of 10-15 psi. The formed hybrid liquid templates were frozen at -80 °C for 24 h, followed by lyophilization for 48 h to form worm-like Ti3C2Tx/GO aerogels. It is worth 121 mentioning that the hexane-POSS solution was reusable. In fact, all of the samples used in this study were prepared from a single batch of 200 ml hexane-POSS. 6.2.5 Fabrication of Ti3C2Tx/GO/epoxy nanocomposites For the EMI shielding application, we also prepared Ti3C2Tx/GO/epoxy nanocomposites. To facilitate the infiltration process, epoxy precursors (east coast resin, USA) were infused on the worm-like aerogels, and then the whole system was kept under a vacuum for around 30 min. Finally, the samples were cured in a vacuum oven at 80 °C for 12 h. The prepared Ti3C2Tx/GO/epoxy nanocomposites were then machined into specific shapes with a thickness of 3 mm for EMI shielding measurements. 6.2.6 Characterization The aerogels were characterized with different techniques to confirm their successful formation and desired quality. In this regard, the initial characterizations were performed by using Fourier-transform infrared spectroscopy (FTIR) (NicoletTM iS20) in the spectrum range of 600 to 4000 cm-1, micro-Raman spectroscopy (Bruker Senterra II) equipped with an Olympus U-TV1X-2 microscope, and X-ray diffraction (XRD) (Bruker model D8 Advance). The morphology of the aerogels was analyzed using a Field Emission Scanning Electron Microscope (FESEM) (Tescan Mira 3 XMU) coupled with an Oxford Instruments X-Max energy dispersive spectrometer (EDS) detector. The electrical conductivity of developed aerogels was analyzed using a Loresta GP resistivity meter (Mitsubishi Chemical Co. model MCP-T700). Aqueous suspensions' dynamic interfacial tension (IFT) analysis was acquired via pendant drop by holding a 5 µL droplet into the organic hexane phase with and without POSS molecules. The X-band frequency range (8.2-12.4 GHz) was 122 used to measure the EMI shielding properties. The EMI shielding properties are derived from the guidelines and methods described in Chapter 2 of this thesis. 6.3 Results and discussion 6.3.1 From liquid templates to ultra-lightweight aerogels Conductive and nonresponsive constructs with different morphologies, such as discrete droplets and liquid threads, were fabricated using a 3D printer for microfluidic injection. In the fabrication process, an aqueous suspension of Ti3C2Tx and/or GO was injected into hexane containing 1 mg/ml POSS. For instance, worm-like GO-liquid threads were achieved by streaming a 5-10 mg/ml GO aqueous suspension into hexane containing POSS, where the rapid co-assembly of GO and POSS at the interface generated an elastic interfacial skin around the extruded thread (Figure 6-1). This skin prevented the breakup of the aqueous jet into droplets and suppressed Plateau-Rayleigh instabilities [33,228]. Figure 6-1 Digital image demonstrating tubule formation upon streaming GO aqueous suspension (10 mg/ml) into hexane-POSS. All scale bars correspond to 2 cm. Upon extrusion of the GO suspension into the hexane-POSS medium, the POSS molecules diffuse into the interface, altering the previously negative charge of the hexane/water interface and introducing positive charges by creating positively charged POSS-NH3+ layers at the interface [217]. This transformation reduces the interfacial energy, facilitating the diffusion of the GO flakes 123 toward the interface. As a result, GO flakes swiftly migrate and assemble at the interface due to electrostatic interaction between its deprotonated carboxylic acid functional groups and protonated amine groups of POSS-NH3+ [219]. This process leads to hybrid GO-POSS NPSs assemblies, which encapsulate and envelop the extruded thread, forming an elastic interfacial skin [33,228]. The existence of this interfacial skin is visually demonstrated through expansion/contraction tests conducted in a pendant drop experiment. In this experiment, a droplet of an aqueous dispersion containing 1 mg/ml of GO is suspended in the hexane-POSS domain. As the droplet contracts, its volume diminishes, causing a corresponding decrease in the interfacial area, which applies compressive forces to the interface. In the presence of highly elastic interfacial assemblies, this compressive force induces distinctive wrinkles on the interface (Figure 6-2). These wrinkles indicate that the GO assemblies are securely held in place by POSS, affirming the establishment of a resilient and flexible film at the interface. Consequently, this remarkable phenomenon stabilizes GO liquid threads in the hexane-POSS medium. Figure 6-2 Expansion/contraction test of a 1 mg/ml aqueous dispersion of GO in the hexane-POSS domain. As the droplet contracts, its volume diminishes, resulting in decreased interfacial area and applying compressive forces. Elastic interfacial assemblies respond by forming distinctive wrinkles on the interface. 124 In contrast to the continuous jet observed for the aqueous GO suspension, the aqueous jet of Ti3C2Tx formed droplets, which are referred to as liquid beads (Figures 6-3 (a-b)). The formation of the latter is attributed to the weak co-assembly of Ti3C2Tx-POSS, which is not strong enough to prevent Plateau-Rayleigh instabilities. Consequently, the liquid jet fragments into smaller packets with lower surface energy [33,172]. To further investigate these instabilities, interfacial tension (IFT) measurements were conducted (Figures 6-3(c) and 6-4). Figure 6-3 (a-b) Digital images demonstrating bead formation upon streaming Ti3C2Tx aqueous suspension (10 mg/ml) into hexane-POSS. The co-assembly of Ti3C2Tx and POSS is not efficient enough to prevent Plateau-Rayleigh instabilities, and the liquid jet breaks into smaller packets to reduce surface energy. (c) Interfacial characterizing of NPs aqueous suspension in hexane domain with and without POSS. In part (c), the terms "80wt%" and "50wt%" were assigned to the hybrid systems, referring to suspensions containing 80wt% and 50wt% of Ti3C2Tx, respectively. All scale bars in (a and d) correspond to 2 cm. 125 Figure 6-4 Dynamic interfacial characterizing of NPs aqueous suspension (10 mg/ml) in hexane domain (a) with and (b) without POSS. Particularly in the case of a hexane-POSS medium, measuring the dynamic interfacial tension becomes highly complex due to the rapid assembly of NPs, forming a jammed layer at the interface that resembles skin. This skin exhibits solid-like behavior and quickly distorts the pendant droplet. When the two sides of the droplet do not precisely mirror each other, the tensiometer cannot accurately measure the interfacial tension, resulting in the termination of the experiment. Consequently, we refer to the recorded values for samples in the hexane-POSS medium as "approximate equilibrium interfacial tension." The results of the measurements revealed that the addition of POSS in hexane leads to a significant reduction in the equilibrium IFT of 10 mg/ml Ti3C2Tx against the nonpolar domain, which is approximately 9 mN/m. This is considerably lower than the IFT of the same system in the absence of POSS (approximately 29 mN/m), indicating that the Ti3C2Tx and POSS molecules assemble at the interface to minimize the free energy penalty of exposing the aqueous phase to hexane (as shown schematically in Figure 6-5) [33,217,229]. In this particular scenario, the negatively charged Ti3C2Tx flakes containing an abundance of negative moieties such as -OH, -F, and -O, engage in interactions with the protonated POSS-NH3+ layer present at the interface. This interaction leads to the formation of hybrid overlapping complexes of Ti3C2Tx-POSS NPSs [230]. The jamming of 126 these formed NPSs creates a solid elastic skin that envelops and stabilizes the liquid beads of Ti3C2Tx. Figure 6-5 Schematic illustration of Ti3C2Tx beads formation. It is noteworthy that the Ti3C2Tx-POSS assembly exhibits a remarkable strength that enables it to maintain the shape of liquid beads even after the removal of hexane (Figures 6-6 (a-b)). The created interface acts as a barrier that effectively isolates and compartmentalizes the aqueous Ti3C2Tx suspension, forming distinct bead-like domains. Notably, this occurs even in the absence of a nonpolar domain, which typically keeps the beads separate from one another. FESEM images of freeze-dried Ti3C2Tx aerogel beads also provide evidence of Ti3C2Tx-POSS assembly, wherein a discernible, compact, layered skin forms alongside the porous interior cells of the aerogel (Figures 6-6 (d-f)). This observation provides evidence for the formation of NPS assemblies, even when pure Ti3C2Tx is used. However, the Ti3C2Tx-POSS interfacial skin demonstrates insufficient stability in maintaining the falling stream, leading to the fragmentation of the Ti3C2Tx jet into droplets (as depicted in Figure 6-3 (a-b)). This behavior could be attributed to the slow diffusion of Ti3C2Tx flakes into the interface, leading to loosely packed Ti3C2Tx-POSS assemblies at the interface, as previously described in the literature [231]. 127 Figure 6-6 Microscope images of Ti3C2Tx beads (a) before and (b) after removal of hexane. (c) Digital image of Ti3C2Tx beads 120 minutes after removal of hexane. (d-f) FESEM images of freeze-dried Ti3C2Tx aerogel beads. Scale bars in (a-b), (c), (d-e), and (f) correspond to 1 mm, 1 cm, 200 µm, and 100 µm, respectively. The limited mechanical robustness of Ti3C2Tx-POSS assemblies becomes evident in expansion/contraction tests conducted using pendant drop equipment. After the Ti3C2Tx droplet undergoes contraction, particularly a few seconds after the expansion phase, the droplet often detaches from the injection head. Furthermore, even when droplets manage to remain suspended, a distinct lack of substantial skin formation was observed (Figure 6-7). This occurrence can be explained by the presence of loosely packed Ti3C2Tx-POSS assemblies that cannot withstand the compressive forces experienced during the contraction phase. In contrast, a relatively strong interfacial skin was observed for a 10 mg/ml GO aqueous suspension, with an equilibrium IFT of 128 approximately 0.9 mN/m against the hexane-POSS. In this case, the GO-POSS interface is sturdy enough to stabilize the falling stream of the aqueous GO suspension. The differences in the mechanical strength of the GO and Ti3C2Tx assemblies are the primary reason for the various morphologies observed when jetting their aqueous suspensions into the hexane-POSS phase. Figure 6-7 The conducted expansion/contraction experiments using a 1 mg/ml Ti3C2Tx aqueous suspension in a hexane-POSS medium. (a) We observed that when using approximately a 5 ml droplet, it frequently detached from the injection head with ease a few seconds following the expansion phase. (b) Even after reducing the droplet size to 3 ml, we did not observe a substantial interfacial skin formation for the pure Ti3C2Tx aqueous suspension. Some might argue that the MXene suspension's dark hue hinders skin detection. However, it's important to note that we observed such skin formation in the Ti3C2Tx/GO aqueous suspension, which has the same dark color, as highlighted later in this article. Additionally, during the pendant drop test, we noticed that either pure GO or Ti3C2Tx/GO droplets remained firmly attached to the setup needles, even when the hexane-POSS bath underwent intense agitation. Yet, for the pure 129 Ti3C2Tx aqueous suspension, the droplet readily detached from the needle with the slightest vibration. This finding underscores that MXene alone fails to form robust interfacial skin. The distinctive morphological disparities between the extruded GO and Ti3C2Tx suspensions in the hexane-POSS domain can also be explained by their inherent properties. GO is characterized by a unique dual nature - being both hydrophobic and hydrophilic. This is attributed to its structure, which combines a hydrophobic basal plane and hydrophilic functional groups, such as hydroxyl, carboxylic, and epoxide groups. As a result, GO nanosheets tend to migrate and assemble rapidly at the oil/water interface [33,172,232]. When GO nanosheets migrate into the interface, they act as surfactants, reducing the equilibrium IFT of hexane/water from ≈49 to 7 m /m (Figure 6-4) [33,232]. Additionally, introducing POSS into hexane enables the swift formation of GO-POSS NPSs. This is a result of the rapid migration of GO to the interfaces. The robust structure of POSS molecules, which comprise a cubic inorganic silica-cage core with side chains, considerably bolsters the mechanical strength of the GO-POSS assemblies. Consequently, they prove to be ideal for liquid streaming [33,217]. In contrast, Ti3C2Tx nanosheets possess abundant hydrophilic groups but lack pronounced interfacial activity [233]. As a result, Ti3C2Tx nanosheets tend to remain in the aqueous phase with little inclination to migrate toward the hexane/water interface [231]. For instance, at a concentration of 10 mg/ml and in the absence of POSS in the surrounding hexane phase, the equilibrium IFT of Ti3C2Tx is approximately 29 mN/m, nearly equivalent to that of pure water in hexane. Previous studies have also noted this low interfacial activity, accompanied by the slow migration of Ti3C2Tx to the interface. These studies often employed extended aging periods, e.g., ∼1 h, to encourage the relocation of Ti3C2Tx flakes to the interface [231,234]. This is also evident in streaming aqueous 130 suspensions of Ti3C2Tx into the hexane-POSS phase, where the formation rate of NPSs is not rapid enough to stabilize the falling aqueous stream. We also engineered hybrid aqueous systems by combining GO and Ti3C2Tx NPs for liquid streaming. In these scenarios, we created aqueous tubules resembling those of pure GO by using Ti3C2Tx/GO suspensions. The electrical conductivity of the resulting constructs is primarily attributed to Ti3C2Tx. Concurrently, the elasticity of the NPS assemblies primarily hinges on the interfacial activity of GO. (Figure 6-8). Figure 6-8 Digital image demonstrating tubule formation upon streaming 10 mg/ml Ti3C2Tx/GO aqueous containing 80 wt% Ti3C2Tx. The scale bar corresponds to 2 cm. Figure 6-9 schematically illustrates the electrostatic interaction between the deprotonated carboxylic acid functional groups of GO and the protonated amine groups of POSS-NH3+, which facilitates the swift migration and assembly of GO-POSS NPSs at the interface. This assembly is robust enough to compensate for the loosely packed Ti3C2Tx-POSS assemblies described in the previous section, stabilizing the entire Ti3C2Tx/GO liquid jets. 131 Figure 6-9 (a) Schematic illustration of Ti3C2Tx/GO liquid threads formation. (b) Schematic illustration of NPs-ligand interactions during Ti3C2Tx/GO liquid thread formation. 132 The existence of this strong elastic interfacial skin in the Ti3C2Tx/GO aqueous suspension is evident from the expansion/contraction results observed during the pendant drop experiment. To maintain the visibility of the interfacial layer throughout the experiment, we used a highly diluted suspension of Ti3C2Tx/GO with a solid concentration of 1 mg/ml. This allowed light to pass through the droplet and reveal the interfacial layer. When the droplet contracted within the hexane-POSS domain, noticeable wrinkles appeared on the interface (Figure 6-10). These wrinkles resembled those observed in the pure GO suspension, providing further evidence of the formation of a durable and flexible film at the interface (refer to Figure 6-2). Consequently, this elastic skin plays a vital role in stabilizing the Ti3C2Tx/GO liquid threads in the hexane-POSS medium. Figure 6-10 Contraction test of a 1 mg/ml aqueous dispersion of Ti3C2Tx/GO in the hexane-POSS domain. As the droplet undergoes contraction, its volume decreases, leading to a reduction in interfacial area and applying compressive forces. In response, the elastic interfacial assemblies exhibit a unique pattern of wrinkles along the interface. The addition of GO to Ti3C2Tx suspension also significantly reduced the equilibrium IFT of Ti3C2Tx/GO touching pure hexane, indicating GO's interfacial activity, even in the presence of another NP (Figure 6-4). In the case of hexane-POSS, hybrid systems containing 80 and 50 wt% Ti3C2Tx exhibited equilibrium IFTs of approximately 1 mN/m (Figures 6-4 and 6-3 (c)), confirming that GO and POSS molecules strongly assemble at the interface, leading to the 133 stabilization of Ti3C2Tx/GO streams. The assembly of NPSs at the interface of Ti3C2Tx/GO liquid threads is remarkably strong, as evidenced by the fact that the threads maintain their shape even after the hexane phase is removed from the container (Figure 6-11). The interface acts as a barrier that isolates the Ti3C2Tx/GO suspension inside the tubules, resulting in the formation of distinct liquid threads. Remarkably, this occurs after the removal of nonpolar domain, which typically keeps the liquid threads separate from each other. Figure 6-11 Microscopic images of Ti3C2Tx/GO liquid threads 30 minutes after removal of hexane. The scale bar corresponds to 1 mm. The formation of Ti3C2Tx/GO liquid threads only relies on the rapid co-assembly of NPSs, rather than the bulk rheology of the suspensions, which is the primary origin of the typical filament formation in direct ink writing/printing of hydrogels [11,235]. The flowability of these hybrid suspensions at a solid concentration of 10 mg/ml is evident in Figures 6-12 (a-b). This flowability and absence of viscoelastic properties make liquid streaming more susceptible to pressure alterations compared to conventional writing or printing technologies, where the stream of liquids can break into droplets with a slight increase in pressure [33]. For instance, in the case of 10 mg/ml Ti3C2Tx/GO containing 80 wt% Ti3C2Tx, the aqueous phase stream becomes completely unstable when the pressure increases from 15 to 30 psi at a nozzle diameter of 330 µm (Figure 6-12 (c)). 134 However, due to the liquid-like nature of these suspensions, we observed a relatively fast extrusion of liquid threads without any nozzle blocking, capable of fabricating 3D worm-like and continuous constructs. The unique entangled morphology of continuous tubules creates voids between different parts of Ti3C2Tx/GO liquid threads that are filled with the nonpolar domain. Therefore, upon extrusion of Ti3C2Tx/GO in hexane-POSS, the occupied volume of generated liquid threads is larger than that of those original suspensions (Figures 6-12 (d-e)). The presence of voids can lead to developing secondary porosity within resulting aerogels, proving advantageous for specific applications that demand high surface area and porosity, e.g., gas sorption and EMI shielding. 135 Figure 6-12 The liquid-like behavior (flowability) of Ti3C2Tx/GO (a) 80wt%- 10 mg/ml and (b) 50wt%- 10 mg/ml suspensions. (c) Ti3C2Tx/GO 80wt%- 10 mg/ml suspension is unable to produce tubular filaments upon extrusion at a high pressure (330 μm nozzle at 30 psi extrusion pressure). 2 mL suspension of Ti3C2Tx /GO (d) 80wt%- 10 mg/ml and (e) 50wt%- 10 mg/ml suspensions and their sculpted emulsion counterpart. Upon extrusion, the volume occupied by the liquid threads exceeds that of the original suspensions, which is attributable to the presence of voids between the entangled tubes. Please note that all scale bars in figures (a-e) represent 2 cm. 136 The 3D worm-like liquids of Ti3C2Tx/GO were then transformed into conductive and lightweight aerogels via a one-step freeze-drying process. The resulting aerogels have a density range of approximately 2.8-7 g/cm3 and an electrical conductivity of up to 120 S/m (Figure 6-13). Remarkably, the density of the produced aerogels is so low - even with the 10 mg/ml suspensions - that an electrostatically charged plastic rod can lift and move these structures without making any physical contact (as seen in Figure 6-14). This low density can be attributed to the aerogels' unique morphology, which features micro-scale porosities within the tubular structures and macro-scale porosities between the entangled tubes (as detailed in Figures 6-15). Similar to other freeze-dried aerogels, the micro-scale porosities inside the tubes are a result of ice-crystal formation during the freezing process, which are then locked in place upon freeze-drying (Figure 6-15 (e)). Conversely, the macro-scale porosity is a product of the liquid streaming process, which leaves many voids between the entangled tubes that persist in the aerogels after drying (illustrated in Figures 6-15 (a-d) and 6-15 (f-j)). The FESEM images of Ti3C2Tx/GO worm-like aerogels also display laminated skin that covers the bulk of these tubes. This laminated layer, which originated from the co-assembly of NPs and POSS at the liquid state, can boost the mechanical integrity of the fabricated aerogels [217]. Figure 6-16 shows that the fabricated structures are mechanically robust, can withstand a load much higher than their own weight, and are strong enough to be filled with epoxy without losing structural integrity. 137 Figure 6-13 (a) Electrical conductivity and (b) density of worm-like Ti3C2Tx/GO aerogels. Figure 6-14 The density of the fabricated aerogels, even in the case of 10 mg/ml suspensions, is low enough that an electrostatically charged plastic rod can lift and move the fabricated structures without any physical contact. I to VI delineate the chronological sequence of image capture, with I representing the period when the rod is distant from the sample, and VI signifying the moment when the rod elevates the sample. The scale bar corresponds to 2 cm. 138 Figure 6-15 (a-e) The FESEM images of Ti3C2Tx/GO worm-like aerogels. (f) Digital images of Ti3C2Tx/GO liquid threads and (g) the corresponding microscope image showcasing the entangled morphology of these structures. (h-i) Digital images of Ti3C2Tx/GO worm-like aerogels and (j) the corresponding microscope image showcasing the entangled morphology of these structures. Scale bars in (h-i), (f), (a, g), (b, j), (c-d), and (e) correspond to 2 cm, 1 cm, 2 mm, 1 mm, 500 µm, and 200 µm, respectively. 139 Figure 6-16 (a-e) The FESEM images of Ti3C2Tx/GO worm-like aerogels. (f) Digital images of Ti3C2Tx/GO liquid threads and (g) the corresponding microscope image showcasing the entangled morphology of these structures. (h-i) Digital images of Ti3C2Tx/GO worm-like aerogels and (j) the corresponding microscope image showcasing the entangled morphology of these structures. (k) X-ray diffractogram and (l) micro-Raman spectroscopy analyses of Ti3C2Tx, GO, and their resulting aerogels prepared by Ti3C2Tx/GO 80wt%- 10 mg/ml and 50wt%- 10 mg/ml suspensions. Scale bars in (h-i), (f), (a, g), (b, j), (c-d), and (e) correspond to 2 cm, 1 cm, 2 mm, 1 mm, 500 µm, and 200 µm, respectively. 140 Figures 6-17 (a-b) demonstrate the X-ray diffractogram and micro-Raman spectroscopy analyses of Ti3C2Tx and GO flakes compared with their resulting aerogels prepared by Ti3C2Tx/GO 80wt%- 10 mg/ml and 50wt%- 10 mg/ml suspensions. The X-ray diffractogram of Ti3C2Tx dried at 60 °C demonstrated 2θ peaks of 7.91, 16.94, 26.84, 35.09, and 43.76, attributed to the (002), (004), (006), (008), and (0010) crystalline planes of Ti3C2Tx, respectively [53]. Additionally, the sharp fingerprint peak of GO flakes appeared at the 2θ of 11.15, corresponding to the (001) crystalline plane [32,33]. The X-ray diffractogram of the aerogels obtained from interfacial co-assembly of Ti3C2Tx and GO flakes mainly demonstrated the fingerprint peak of Ti3C2Tx flakes with slight shifts between the 2θ range of 7.7-7.8, attributed to the (002) crystalline plane of Ti3C2Tx. This peak's intensity in the aerogel containing 50 wt% Ti3C2Tx was found to be very low and elevated at a solid concentration of Ti3C2Tx of 80 wt% in the initial suspension. However, the conspicuous fingerprint peak of GO vanished following interfacial co-assembly and aerogel formation. This could be attributed to the interaction of GO with Ti3C2Tx and the ligand during interfacial complexation, as well as the arrangement of GO flakes in the aerogel, which hinders their restacking. This observation aligns well with previous studies investigating the co-assembly of Ti3C2Tx and GO flakes within aerogel frameworks [236,237]. The aerogel containing 50 wt% Ti3C2Tx exhibited characteristic Ti3C2Tx pea s at 2θ values of 27.1, 36.1, and 43.6, corresponding to the (006), (008), and (0010) crystalline planes of Ti3C2Tx. Additional pea s observed at 2θ values of 54.0, 56.0, 63.0, and 68.5 could be ascribed to the ligand due to the assembly of POSS with Ti3C2Tx and GO flakes at interfaces, leading to the formation of a joint complex known as NPSs [238]. However, with a further increase in the Ti3C2Tx concentration to 80 wt% compared to GO, the aerogel system primarily displayed an X-ray diffractogram resembling that of pure Ti3C2Tx. These fingerprint pea s slightly shifted to 2θ values 141 of 16.4, 26.5, and 34.6 for (004), (006), (008) crystalline planes, respectively, indicating the interaction of Ti3C2Tx with GO and the ligand during interfacial complexation. Figure 6-17 (b) presents the micro-Raman spectroscopy results of the developed Ti3C2Tx and GO flakes and their resultant aerogels. As illustrated, the Raman spectrum of GO showed two well-defined peaks at 1351.5 cm-1 and 1602.5 cm-1, attributed to the D- and G-bands of GO flakes, respectively [33]. The ID/IG ratio of GO was measured to be 0.962, indicating the controlled rate of defects in the exfoliated GO flakes. In comparison, the Raman spectrum of Ti3C2Tx did not show any peak in the region related to the D- and G-bands. The hybrid aerogels composed of 50 wt% and 80 wt% Ti3C2Tx displayed two well-defined peaks at approximately 1339 and 1577 cm-1, corresponding to the D- and G- bands of the GO flakes. The ID/IG ratio of the resulting aerogels remained nearly constant at about 1.02, highlighting an increase in structural defects in the GO lattice due to the interaction of GO with Ti3C2Tx flakes during interfacial complexation [236]. Furthermore, with an increased solid concentration of Ti3C2Tx, the corresponding Ti3C2Tx peaks began to emerge with slight shifts in peak position, attributable to the interaction of these flakes with GO and the ligand during interfacial assembly. The results from both the X-ray diffractogram and micro-Raman spectroscopy conclusively confirm the interaction of GO, Ti3C2Tx, and the ligand and the formation of a joint structure during interfacial assembly at the liquid-liquid interfaces. 142 Figure 6-17 (a) X-ray diffractogram and (b) micro-Raman spectroscopy analyses of Ti3C2Tx, GO, and their resulting aerogels prepared by Ti3C2Tx/GO 80wt%- 10 mg/ml and 50wt%- 10 mg/ml suspensions. Figure 6-18 showcases the FTIR spectrum of GO, Ti3C2Tx, GO aerogel, and Ti3C2Tx/GO aerogel. As shown in part (a), the appeared peaks in the FTIR spectrum of GO correspond to the hydroxyl functional groups (-OH) (3422 cm-1), carbonyl (C=O) (1722 cm-1), unoxidized double bond carbon atoms (C=C) (1654 cm-1), C-OH (1416 cm-1), C-O-C (1246 cm-1), and C-O (1110 cm-1) [46]. The existence of these peaks in the FTIR spectrum of GO certifies its hydrophilic nature and facilitates its electrostatic interaction with the ligand, i.e., POSS-NH2, through its carboxylic acid groups [219]. Additionally, as depicted in the FTIR spectrum of Ti3C2Tx, these flakes showcased the two main characteristic peaks at 3349 cm-1 and 1654 cm-1, corresponding to the stretching vibration of hydroxyl functional groups (-OH) (Figure 6-18 (b)). Other peaks in the Ti3C2Tx's FTIR spectrum are attributed to the -CH2 (2912.15 cm-1), C-H (1484.20 cm-1), and -OH (1314.21 cm-1), confirming the formation of Ti3C2Tx with required hydrophilic functional groups [239]. The FTIR spectrum of GO aerogel revealed the corresponding peaks of GO and ligand (POSS-NH2) due to a tight electrostatic bond between them during interfacial assembly (Figure 6-18 (c)). As demonstrated, the appeared peaks at about 3370 cm-1, 1725 cm-1, 1628 cm-1, and 1456 cm-1 are 143 attributed to the hydroxyl (-OH), carbonyl (C=O), C=C unoxidized double bond carbon atoms, and -OH functional groups of GO, respectively, confirming the placement of the GO in the aerogel framework. The other well-defined peaks arise from the ligand corresponding to the stretching vibration of C-H (2873-2953 cm-1) resulting from the six pendant organic arms of POSS-NH2, bending vibration of -CH2CH2CH2- functional groups (1331-1371 cm-1), stretching vibration of Si-C (1222 cm-1), the Si-O-Si stretching vibration of siloxane functional groups (1102 cm-1), i.e., the characteristic peak of silsesquioxane cages, and stretching vibrations of Si-C (834 cm-1) and Si-C-H (736 cm-1) functional groups [240]. The co-existence of GO and POSS-NH2 corresponding peaks indicates the formation of a hybrid aerogel system due to interfacial assembly. The FTIR spectrum of hybrid Ti3C2Tx/GO aerogel (80wt%- 10 mg/ml) showcased the main fingerprint peaks of ligand (POSS-NH2), including -CH2CH2CH2- (1131-1405 cm-1), Si-C (1228 cm-1), Si-O-Si (1096 cm-1), Si-C (839 cm-1), and Si-C-H (736 cm-1), combined with some changes as a result of interaction between Ti3C2Tx, GO, and POSS-NH2 (Figure 6-18 (d)). Accordingly, the appearance of the main fingerprint peaks of GO, including C-OH (1456 cm-1), C=C (1628 cm-1), and C=O (1731 cm-1), along with a meaningful decline in the intensity of hydroxyl (-OH) functional groups (3399 cm-1) and disappearance of -OH group of Ti3C2Tx, reveal the interaction between the constituent components and probable assembly of Ti3C2Tx on GO throughout the aerogel framework. Additionally, peaks in the region 2873-2953 cm-1 arise from the C-H groups of POSS-NH2 and -CH2 groups of Ti3C2Tx. The outcome of the FTIR assessment vividly illuminated the formation of a hybrid complex between the ligand and 2D nanoflakes. 144 Figure 6-18 The FTIR spectrum of (a) GO, (b) Ti3C2Tx, (c) GO aerogel, and (d) Ti3C2Tx/GO aerogel 80wt%- 10 mg/ml. 145 The chemical functionalities of the 2D nanoflakes and aerogels were also examined using X-ray photoelectron spectroscopy (XPS) (Figure 6-19 to 6-23). As shown in Figure 6-19, the XPS graphs demonstrated distinct peaks at approximately 282-285 eV, 398-402 eV, and 530-531 eV, which correspond to C1s, N1s, and O1s, respectively. Notably, the Ti3C2Tx nanoflakes and hybrid Ti3C2Tx/GO aerogel exhibited an additional peak at 456 eV, indicating the presence of Ti 2P. Upon deconvolution, the C1s peak of GO revealed two distinct peaks (Figure 6-20 (a)). The first peak, observed at 285.05 eV is attributed to sp2 carbon atoms, which serve as a distinctive fingerprint for carbon atoms within a heterocyclic arrangement. The second peak, appearing at 286.02 eV corresponds to the presence of C-O bonds [241]. Analyzing the O1s peak of GO uncovered two well-defined peaks at 530.37 eV and 530.86 eV, representing the O-C=O and C=O functional groups, respectively (Figure 6-20 (b)) [242]. This finding suggests that GO has the capability to undergo deprotonation through its pre-existing carboxylic acid functional groups. Additionally, examination of the N1s peak of GO (Figure 6-20 (c)) confirmed the presence of pyrrolic nitrogen within the GO structure, which was detected at 399.8 eV [243]. Figures 6-19 (b) and 6-21 provide the XPS survey of Ti3C2Tx [244]. Specifically, Figure 6-21 (a) presents the deconvoluted C1s peak of Ti3C2Tx, which exhibits three prominent peaks corresponding to Ti-C (281.83 eV), C-C (284.86 eV), and C-O (287.19 eV) [245]. The deconvoluted O1s peak of Ti3C2Tx, depicted in Figure 6-21 (b), confirms the presence of Ti-O (530.13 eV) and Ti-OH (531.91 eV) within the Ti3C2Tx's structure. Moreover, the Ti 2P fitted peak of Ti3C2Tx, as shown in Figure 6-21 (c), reveals three well-defined peaks at 455.23, 456.55, and 459.15 eV, corresponding to Ti-C, Ti-O, and Ti ions with a +4 formal valence (TiO2-xFx) resulting from surface oxidation of Ti3C2Tx. Further analysis of the N1s deconvoluted peak of Ti3C2Tx, presented in Figure 6-21 (d), demonstrates the existence of three main nitrogen-based functionalities, namely 146 N-Ti (396.91 eV), N-H (399.74 eV), and quaternary nitrogen (401.09 eV) [246]. These findings further confirm the formation of Ti3C2Tx nanoflakes possessing the essential functionalities required for interfacial assembly alongside GO at the interface. Figures 6-19 (c) and 6-22 demonstrate the XPS survey of GO aerogels, revealing the successful assembly of the GO in the aerogel framework. As evidenced in Figure 6-22 (a), the deconvoluted C1s peak of GO aerogel showed the fingerprint peaks of GO at 285.09 and 286.08 eV, attributed to the sp2 carbon atoms and C-O, respectively. Likewise, the O1s (Figure 6-22 (b)) and N1s (Figure 6-22 (c)) peaks also showed the existence of C=O (530.87 eV) and pyrrolic nitrogen (400.03 eV) in the GO aerogel. Similarly, hybrid Ti3C2Tx/GO aerogel showed the footmark of corresponding functional groups of GO and Ti3C2Tx in the XPS survey of the aerogel (Figure 6-19 (d) and 6-23). The deconvoluted C1s of Ti3C2Tx/GO aerogel (Figure 6-23 (a)) revealed the existence of the C-C group of Ti3C2Tx at 284.91 eV, whereas the deconvoluted O1s peak (Figure 6-23 (b)) showed C=O (530.62 eV), and Ti-O (529.69 eV) functionalities, originated from the co-existence of GO and Ti3C2Tx in the aerogel framework. The deconvoluted Ti 2P peak of hybrid aerogels showed the existence of Ti-C (455.15 eV) and Ti-O (456.32 eV) in this structure (Figure 6-23 (c)). Furthermore, the deconvoluted broad N1s peak of Ti3C2Tx/GO aerogel also revealed the possibility of the co-existence of pyrrolic nitrogen of GO and N-H group of Ti3C2Tx at 399.30 eV (Figure 6-23 (d)). These outcomes justified the interaction of GO and Ti3C2Tx during interfacial assembly and the formation of hybrid aerogels. 147 Figure 6-19 XPS analysis of (a) GO, (b) Ti3C2Tx, (c) GO aerogel, and (d) Ti3C2Tx/ GO aerogel. 148 Figure 6-20 High-resolution deconvoluted spectra of (a) C1s, (b) O1s, and (c) N1s of GO. 149 Figure 6-21 High-resolution deconvoluted spectra of (a) C1s, (b) O1s, (c) Ti 2P, and (d) N1s of Ti3C2Tx. 150 Figure 6-22 High-resolution deconvoluted spectra of (a) C1s, (b) O1s, (c) Ti 2P, and (d) N1s of GO aerogel. 151 Figure 6-23 High-resolution deconvoluted spectra of (a) C1s, (b) O1s, (c) Ti 2P, and (d) N1s of hybrid Ti3C2Tx/GO aerogel. 152 6.3.2 EMI shielding of Ti3C2Tx/GO worm-like aerogels The EMI shielding properties of the specially formulated aerogels, distinguished by their unique morphology, are demonstrated in Figures 6-24 to 6-26. These aerogels have the capability to reflect a certain segment of the power of electromagnetic waves. This reflective capacity is due to the impedance mismatch between the aerogels' interface and the free space [11,36,206]. The wave impedance of an electromagnetic wave, defined as the ratio between the transverse components of the electric and magnetic fields, approximates 377 Ω in free space. However, when it comes to conductive materials, the impedance is significantly lower. This indicates a diminished ability for electromagnetic noise to propagate through these conductive structures. In such instances, the energy of the electromagnetic waves is transferred to the electrons and other charged particles within the shield, inducing them to oscillate. These forced oscillations cause the charges to interact with the incident waves, reflecting them as a scattered field, a process known as reflection [11,36]. In addition to this reflective phenomenon, a segment of the electromagnetic waves is also absorbed within the aerogels, leading to energy dissipation in the form of heat. This secondary mechanism, known as absorption, occurs in GO due to electrical dipoles and in Ti3C2Tx with finite electrical conductivity [11,31,36]. 153 Figure 6-24 EMI shielding characteristics of Ti3C2Tx/GO worm-like aerogels prepared by 80wt%- 10 mg/ml and 80wt%- 5 mg/ml suspensions. 154 Figure 6-25 EMI shielding characteristics of Ti3C2Tx/GO aerogels prepared by 5 mg/ml Ti3C2Tx /GO aqueous containing 20 wt% GO, printed with a 200 μm nozzle. I and II refer to the data obtained from two different sides of the samples. The thickness of these samples was 3 mm. Three different samples were tested: (a, b) sample one, (c, d) sample two, and (e, f) sample three. 155 Figure 6-26 EMI shielding characteristics of Ti3C2Tx/GO aerogels prepared by 10 mg/ml Ti3C2Tx /GO aqueous containing 20 wt% GO, printed with a 200 μm nozzle. I and II refer to the data obtained from two different sides of the samples. The thickness of these samples was 3 mm. Three different samples were tested: (a, b) sample one, (c, d) sample two, and (e, f) sample three. 156 The worm-like aerogels with multiscale porosities possess a multitude of interfaces. These interfaces facilitate multiple internal reflections within the constructs, effectively extending the propagation path length of the waves before transmission. This extended propagation increases the probability of interactions between the shielding materials and the electromagnetic waves [11,36]. These multiple internal reflections, referred to as internal scattering, augment the dissipation of electromagnetic waves and enhance the aerogels' absorption capacity, as schematically depicted in Figure 6-27. As a result of these characteristics, the worm-like aerogels, specifically those formed with Ti3C2Tx/GO containing 80 wt% Ti3C2Tx, demonstrate remarkable EMI shielding effectiveness (SET). This effectiveness is evident with measurements of 44.7 and 69.2 dB in structures prepared with 5 and 10 mg/ml suspensions, respectively (Figures 6-24 to 6-26). Figure 6-27 Schematic illustration of internal scattering in Ti3C2Tx/GO aerogels from macro-to micro-scale. Table 6-1 and Figure 6-28 compare these worm-like structures' electrical conductivity, density, SET, and specific EMI shielding effectiveness (SSE/t = SET/(density×thickness)) with previous best-of-its-class EMI shields. The exceptional EMI shielding characteristics, e.g., /t ≈ 33,000-50,000 dB cm2/g, at relatively low density, e.g., 𝜌 =3-7 mg/cm3, render our aerogels ideal candidates for both defense and commercial applications, which require SET = 30 dB. 157 Figure 6-28 A comparison of the SSE/t vs. density of the Ti3C2Tx/GO worm-like aerogels with the previous best practices in the literature. 158 Table 6-1 A comparison of the SSE/t vs. density of the Ti3C2Tx/GO worm-like aerogels with the previous best practices in the literature. Material Thickness (mm) Electrical conductivity (S/m) EMI SE (dB) Density (mg/cm3) SSE/t (dB cm2/g) Ref. rGO 2.5 4.85 40.2 4.5 35730 [247] rGO aerogel film 0.12 2.5×105 ~70 410 25600 [248] rGO aerogel 2.5 179 27.6 5.5 20070 [249] rGO/epoxy 4 95 35 4 21800 [250] rGO/PDMSa 5 0.5 65 4.2 29500 [251] rGO/PDMS ~1.5 - 53 122 2890 [252] rGO/melamine 40 0.026 37.2 11 850 [253] CNTs/cellulose 2.5 1.8 20.8 95 875 [254] CNTs/carbonized cellulose 2 263.4 96.4 30.5 15800 [255] CNTs/ANFsb 0.168 230 22.7 43 33528 [256] Ti3C2Tx 2 2,200 ~62 6.3 49500 [187] Ti3C2Tx 1 - 70.6 11 64182 [38] Ti3C2Tx 1 - 44.8 6 81455 [38] Ti2CTx 1 - 69.2 11 62909 [38] Ti3CNTx 1 - 54.1 11 49182 [38] Ti3C2Tx/carbonized wood 3 3520 71.3 197 1200 [257] Ti3C2Tx /CAc 0.026 1.3 54.3 1190 17500 [215] Ti3C2Tx foam 0.006 5.8×105 32 390 136752 [186] rGO/ Ti3C2Tx 3 1,000 50.7 46 36737 [188] Ti3C2Tx/AgNWsd 2 2,450 52.6 490 5313 [189] Ti3C2Tx/SAe/PDMS 2 2,211 72 20 18000 [190] Ti3C2Tx/CNTs 3 9,430 104 420 8253 [191] Ti3C2Tx/rGO 2 1,085 52.7 440 5288 [19] C/SiC nanowires 4 223 ~47 590 198.8 [258] C/cenosphere 10 940 ~47 32 142 [259] Stainless-steel fiber/PPf 3.1 ~0.5 48 640 242 [260] Phthalonitrile-based carbon foam 2.4 240 51.2 154 1705 [261] GO/ Ti3C2Tx worm-like structure 3 20 44.7 ~3 49670 This study GO/ Ti3C2Tx worm-like structure 3 120 69.2 ~7 32953 This Study apolydimethylsiloxane, bAramid nanofiber, cCalcium alginate, dSilver nanowires, eSodium alginate, fpolypropylene A significant aspect of the worm-like aerogels is their capacity to achieve exceptional EMI shielding performance without chemical or thermal reduction. Traditionally, these techniques have been employed in past research to enhance the electrical conductivity and EMI shielding properties 159 of constructs [19,38]. However, with our worm-like Ti3C2Tx/GO aerogels, the absorption contribution to shielding effectiveness is sufficiently robust that no further treatment is required. A comparison of results detailed in Table 6-1 emphasizes the crucial role absorption plays in our structures. We have achieved a higher SET for Ti3C2Tx/GO aerogels compared to structures with greater electrical conductivity. This can be attributed to the numerous interfaces within the aerogels, ranging from micro to macro scales, which significantly augment the absorption capacity. Figures 6-24 to 6-26 illustrate that the worm-like structures are capable of absorbing between 30-45% of the incident waves, thereby significantly enhancing the SET of these constructs. These findings underscore the role of morphology in EMI shielding, where enhancing the absorption characteristics of aerogels leads to improved performance at a lower cost. We further expanded our study by creating Ti3C2Tx/GO/epoxy nanocomposites. This was achieved by infusing epoxy precursors into the worm-like aerogels. These nanocomposites exhibited an impressive SET of 31.7-51.4 dB, achieved at an extremely low filler loading of approximately 0.3-0.7 wt% (Figures 6-29 to 6-32). Moreover, Figure 6-29 (b) illustrates that the absorption coefficient (A) surpassed the reflection coefficient (R) – with A approximately 52-54% – signifying a dominant absorption behavior. This extraordinary performance is likely attributable to the interfaces within the nanocomposites, which considerably enhance both the absorption and SET of these shields. Leveraging this EMI shielding mechanism, our structures attained remarkable EMI shielding at significantly lower filler loadings compared to previously reported conductive nanocomposites. This advancement paves the way for new opportunities in the realm of cost-effective EMI shields, as depicted in Figure 6-33 and Table 6-2. 160 Figure 6-29 EMI shielding characteristics of Ti3C2Tx/GO/epoxy nanocomposites prepared by infusing epoxy precursor into worm-like aerogels. The parent aerogels were made by 80wt%- 10 mg/ml and 80wt%- 5 mg/ml suspensions. 161 Figure 6-30 EMI shielding characteristics of epoxy/ Ti3C2Tx /GO composites prepared by 5 mg/ml Ti3C2Tx /GO aqueous containing 20 wt% GO, printed with a 200 μm nozzle. I and II refer to the data obtained from two different sides of the samples. The thickness of these samples was 3 mm. Three different samples were tested: (a, b) sample one, (c, d) sample two, and (e, f) sample three. 162 Figure 6-31 EMI shielding characteristics of epoxy/Ti3C2Tx/GO composites prepared by 10 mg/ml Ti3C2Tx /GO aqueous containing 20 wt% GO, printed with a 200 μm nozzle. I and II refer to the data obtained from two different sides of the samples. The thickness of these samples was 3 mm. Three different samples were tested: (a, b) sample one, (c, d) sample two, and (e, f) sample three. 163 Figure 6-32 EMI shielding characteristics of pure epoxy. I and II refer to the data obtained from two different sides of the samples. The thickness of these samples was 3 mm. Three different samples were tested: (a, b) sample one, (c, d) sample two, and (e, f) sample three. 164 Figure 6-33 A comparison of the SET vs. filler loading of the Ti3C2Tx/GO/epoxy nanocomposites with the previous best practices in the literature. 165 Table 6-2 A comparison of the SET vs. filler loading of the Ti3C2Tx/GO/epoxy nanocomposites with the previous best practices in the literature. Material Filler Loading (wt%) Thickness (mm) EMI SE (dB) EMI SE/(Thickness× Filler loading) (dB/cm) Ref. rGO/Fe3O4/PEIa 10 2.5 41.5 1660 [262] Graphene/ PEI 10 2.3 19.7 856.5 [263] Graphene/PDMSb 0.8 1 20 25000 [264] Graphene/PSc 30 2.5 29.3 390.6 [127] Annealed few-layered Ti3C2Tx/epoxy 15 2 41 1366.6 [265] SWCNTsd/PURe 15 2 25 833.3 [266] rGO/epoxy 15 2 21 700 [267] MWCNT Fe3O4 @Ag/epoxy 15 2 35 1166.6 [268] Multilayered Ti3C2Tx/wax 60 2 39 325 [269] Annealed multilayered Ti3C2Tx/wax 90 1 32 355.5 [270] Fe3O4@rGO/ MWCNTs/ WPU 11.2 0.8 35.9 4006.7 [271] rGO/PDMS 0.42 5 42 20000 [251] Epoxy/wood derived carbon 7 2 27.8 1985.7 [272] rGO/SWCNTs/PDMS 0.28 2 34 60714.2 [273] CBf/TPUg 15 2 20 666.6 [274] CNTs/TPU 10 2 22 1100 [274] rGO/ PS 4 2.5 31.5 3150 [275] rGO/epoxy 1 3 30 10000 [276] CNTs/PVDFh 3 1.1 16.7 5060.6 [277] CNTs/epoxy 0.66 2 33 25000 [278] rGO/epoxy 0.33 2 27 40909.09 [250] MWCNTs/PSi 20 2 30 750 [175] rGO/PMMAj 19 2.4 25 548.2 [135] MWCNT/Fe3O4/PLAk 15 - 22 - [279] CB/epoxy 30 1 44 1466.6 [280] Branched carbon nanotubes/epoxy 1 1.8 34.5 19166.6 [281] Liquid metal/PDMS 15 3 ~38 844.4 [282] Ti3C2Tx/PS 6.2 2 62 5000 [192] Ti3C2Tx/PS 4 2 40 5000 [192] Ti3C2Tx/PS 2.4 2 33 6875 [192] Ti3C2Tx/GO/epoxy ~0.3 3 31.7 35222.2 This work Ti3C2Tx/GO/epoxy ~0.7 3 51.4 24476.1 This work Notably, achieving higher SET in a composite with minimal filler loading and lower thickness offers distinct advantages in terms of weight and cost considerations [283]. When comparing 166 composites, if one composite achieves the required shielding effectiveness with a low filler loading while another necessitates a filler loading exceeding 50 wt%, it becomes evident that the latter composite would likely be prohibitively expensive and less suitable for practical applications. In order to highlight the effectiveness of our approach in developing cost-efficient EMI shields, we carried out a comparative analysis of the normalized SET values, i.e., Normalized 𝑆𝐸𝑇 =𝑆𝐸𝑇(Thickness× Filler loading) in (dB/cm), pitting our samples against previously established cutting-edge methods (Figure 6-34 and Table 6-2). This in-depth comparison serves to underline the superiority of our methodology. Despite their extremely low filler loading and reasonable thickness, our nanocomposites achieved remarkably high SET values. Notably, achieving a higher SET in a nanocomposite with minimal filler loading and reduced thickness presents considerable advantages concerning weight and cost considerations. Figure 6-34 A comparison of the normalized SET vs. SET of the Ti3C2Tx/GO/epoxy nanocomposites with the previous best practices in the literature. 167 6.4 Conclusion This chapter presents a new method for creating cost-effective aerogels based on Ti3C2Tx/GO worm-like with micro- and macro-scale porosities. Our approach uses interfacial jamming of NPSs to stabilize aqueous suspensions of Ti3C2Tx/GO in a nonpolar medium. This produces filamentous liquids that are used as a template for ultra-flyweight aerogel fabrication. The resulting structures are comparable to, or better than, some of the best available porous EMI shields, with the important added benefit that no chemical or thermal reduction or pre-processing is required. This includes exceptional EMI shielding effectiveness, specific EMI shielding effectiveness, and ultra-low density of 44.7-69.2 dB, 33,000-50,000 dB cm2/g, and 3-7 mg/cm3, respectively. Furthermore, these aerogels produced highly effective EMI shields at a low filler loading of 0.3-0.7 wt%, when used as a backbone for conductive epoxy nanocomposites. Overall, our results suggest that liquid templating offers a unique alternative to traditional aerogel pre-treatment methods, such as freeze casting, chemically induced assemblies, and interfacial electrochemical self-assemblies, allowing the creation of aerogels with multiscale porosities and lower density at a lower cost. 168 Chapter 7: Functional Janus structured liquids and aerogels Janus structures have unique properties due to their distinct functionalities on opposing faces, but have yet to be realized with flowing liquids. We demonstrate such Janus liquids with a customizable distribution of nanoparticles (NPs) throughout their structures by joining two aqueous streams of NP dispersions in an apolar liquid. Using this anisotropic integration platform, different magnetic, conductive, or non-responsive NPs can be spatially confined to opposite sides of the original interface using magnetic graphene oxide (mGO)/GO, Ti3C2Tx/GO, or GO suspensions. The resultant Janus liquids can be used as templates for versatile, responsive, and mechanically robust aerogels suitable for piezoresistive sensing, human motion monitoring, and electromagnetic interference (EMI) shielding with a tuned absorption mechanism. The EMI shields exhibit superior performance in wave absorption compared to existing counterparts, boasting an average SET value of approximately 51 dB, SER of around 0.4 dB, and an A value of 0.91. This enhanced performance is attributed to their high porosity spanning micro- to macro-scales, coupled with non-interfering magnetic and conductive networks facilitated by the Janus architecture. 7.1 Introduction The self-assembly and jamming of nanoparticles (NPs) at the liquid-liquid interface is an emerging platform for the fabrication of soft functional materials, where the inherent properties of NPs can be integrated into the final structures [219,220]. In practice, interfacial jamming can be realized when the functionalized NPs dispersed in one liquid interact with polymeric/oligomeric ligands with complementary functionality dissolved in a second immiscible liquid, forming what has been termed NP surfactants (NPSs) [33,219,222,223]. This process arrests the reduction in the interfacial area, stabilizing highly non-equilibrium shapes of liquids. Due to the nature of jamming, external stimuli 169 can be used to control the collective properties via manipulating the binding of the NPSs to the interface, imparting responsiveness to the final constructs [224]. Reconfigurable on-demand, interfacial jamming allows the fabrication of novel all-liquid systems, e.g., reconfigurable discrete droplets [284], molded liquid objects [285], and bicontinuous interfacially jammed emulsion gels (bijels) [286], with unique characteristics and applications. Liquid streaming is a new process where a jetted stream of one liquid in an external immiscible liquid can be stabilized by interfacial jamming, forming tubular liquid threads [33,172,219]. For the efficient generation of these tubular all-liquid structures, the rate of NPSs formation, assembly, and jamming must be faster than the relaxation of fluids, while their binding energy must be high enough to impart mechanical stability [228,287,288]. This process generates a robust skin around the tubular jetted liquid threads that maintains the integrity of this class of structured liquids [33,219,288]. Taking advantage of liquid jetting, several new concepts of structured liquids, e.g., all-liquid printing [228,231,288] and 3D water in oil tubular emulsions, with tunable morphology and domain size [33], were put forward. But, so far, little advantage has been taken from the combination of functional NPs and liquid streaming to generate constructs with a specific function in soft robotics [289], microfluidics [288,290], or sensing [231,291]. Here, we report a one-step fabrication of Janus structured liquids with an anisotropic distribution of NPs. Our approach relies on stabilizing and then merging two aqueous streams of NPs in a nonpolar liquid containing complementary functionalized ligands. The resulting liquid threads consist of two distinct sections, each composed of specific groups of NPs. This unique duality inherent in Janus liquids affords an opportunity to assign distinct functionalities to opposing sides of the structures and fine-tune them independently. As a proof of concept and to highlight the future potential of these unique constructs, magnetic, conductive, or non-responsive NPs were 170 sequestered in opposite sides of these constructs using magnetic graphene oxide (mGO)/GO, Ti3C2Tx/GO, or GO suspensions, allowing the formation of multi-responsive soft materials that can be heterogeneously tailored from the micro- to macro-scale. Such precise control over the functionality of Janus constructs becomes even more pronounced when these structured liquids are used as templates, enabling the development of customized compositions and arrangements for task-oriented aerogels. The potential applications of Janus aerogels are vast, especially when a deliberate connection is established between the functionality of the Janus building blocks and the ultimate application. For instance, the development of Janus aerogels with non-interfering magnetic/conductive opposing sections represents a breakthrough in electromagnetic interference (EMI) shielding. By integrating alternating magnetic/conductive domains within the aerogel structure, Janus constructs surpass existing shielding materials, offering superior wave absorption properties. This addresses a long-standing challenge in EMI shielding and positions Janus aerogels as highly promising candidates for next-generation shielding materials. This integration platform is also used to make functional aerogels flexible, where functionality, e.g., electrical conductivity, is customized through one side of the structures, and mechanical flexibility is mainly derived from the opposite side. This adjustable strategy opens up numerous opportunities in pressure sensing and human motion monitoring. 7.2 Experimental section 7.2.1 Materials Synthesis of recipes and characterization of GO and Ti3C2Tx flakes were discussed in Chapter 3. A mixture of hexane (97%) and PSS-[3-(2-Aminoethyl)amino] propyl-heptaisobutyl substituted (POSS) was used in liquid streaming, both supplied by Sigma-Aldrich. 171 7.2.2 Fabrication of structured liquids and aerogels 7.2.2.1 Ti3C2Tx beads Ti3C2Tx aqueous suspensions with a concentration of 5-10 mg/ml were loaded into a plastic syringe and extruded into the hexane containing 1 mg/ml POSS using a 3D Bioprinter (Allevi, USA). In the case of the 200 µm (ID) dispensing needle, the extrusion pressure was set at 10-15 psi. Upon injection, the aqueous jet of Ti3C2Tx breaks into discrete droplets, forming what we have called liquid beads. Such a structure can also be achieved with the aid of a simple compressor or extrusion of the aqueous phase by hand. Ti3C2Tx liquid beads were used for aerogel fabrication. To achieve this, samples were kept in a freezer overnight, and then the hexane phase was removed. Finally, the frozen aqueous phase was freeze-dried for two days to achieve MXene aerogel beads. 7.2.2.2 GO worm-like structures GO-liquid threads with a worm-like morphology were achieved by streaming a 5-10 mg/ml GO aqueous suspension into hexane containing POSS. Here, the rapid co-assembly of GO and POSS at the interface generates an elastic interfacial skin around the extruded thread that prevents the breakup of an aqueous jet into droplets, i.e., suppressing Plateau-Rayleigh instabilities. In the case of the 200 µm (ID) dispensing needle, the extrusion pressure was set at 15-30 psi. Such a structure can be achieved with the aid of a 3D Bioprinter, a simple compressor, or extrusion of the aqueous phase by hand. GO worm-like aerogels were fabricated through a similar procedure explained in section 7.2.2.1. 7.2.2.3 Ti3C2Tx/GO worm-like structures Deciphering the inherent characteristics of GO and Ti3C2Tx, we also designed hybrid aqueous systems containing both NPs for liquid streaming. Here, aqueous tubules like that of pure GO were achieved for Ti3C2Tx/GO inks. Unless otherwise specified, we employed a gauge 27 dispensing 172 needle with an inner diameter of 200 µm, along with an extrusion pressure of 10-15 psi for the fabrication of these structures. Such a structure can be achieved with the aid of a 3D Bioprinter, a simple compressor, or extrusion of the aqueous phase by hand. Ti3C2Tx/GO worm-like aerogels were fabricated through a similar procedure explained in section 7.2.2.1. Four types of Ti3C2Tx/GO inks based on the different ratios of NPs and different solid contents are prepared: • Ti3C2Tx/GO inks with a total solid concentration of 10 mg/ml containing 20 and 80 wt% of GO (weight of GO compared to the total weight of NPs). • Ti3C2Tx/GO inks with a total solid concentration of 5 mg/ml containing 20 and 80 wt% of GO (weight of GO compared to the total weight of NPs). 7.2.2.4 Janus liquids and aerogels Janus liquid threads were fabricated by merging two streams of structured liquids in the hexane/POSS domain. In the case of magnetic/conductive structures, two different streams of 10 mg/ml Ti3C2Tx/GO (20 wt% GO) and 10 mg/ml mGO/GO (50 wt% GO) were merged. However, for the case of conductive/non-responsive constructs, two streams of 10 mg/ml Ti3C2Tx/GO (20 wt% GO) and 10 mg/ml GO were merged to form the Janus constructs. The injection heads were placed in a way that the two streams collide and merge into Janus liquids threads. In the case of the 200 µm (ID) dispensing needle, the extrusion pressure was set at 10-15 psi for Ti3C2Tx/GO and 15-20 psi for mGO/GO or pure GO inks. Interestingly, the ratio of opposing parts in the Janus liquid threads can be controlled by tuning the streaming parameters. For instance, using different needles for each ink, i.e., 200 µm for Ti3C2Tx/GO and 400 µm for mGO/GO, we fabricated Janus structures in which the magnetic parts' volume is larger than that containing Ti3C2Tx. The extrusion process involved adjusting the 173 pressure of Ti3C2Tx/GO from 20 to 15 psi. Janus structures can be achieved with the aid of a 3D Bioprinter that has two injection heads, a simple compressor, or extrusion of aqueous phase by hand. Janus aerogels were fabricated through a similar procedure explained in section 7.2.2.1. For magnetic/conductive Janus EMI shields, 6 ml of conductive ink [10 mg/ml Ti3C2Tx/GO (20 wt% GO)] and 8 ml of magnetic ink [10 mg/ml mGO/GO (50 wt% GO)] were used. Both inks were extruded through 200 µm (ID) dispensing needles. At the beginning of the process, only mGO/GO ink was extruded for around 3s (almost 1 ml of mGO/GO ink). After that, both conductive and magnetic inks were simultaneously extruded and merged to form Janus liquid treads. The volume of magnetic and conductive inks was selected in a way that the conductive ink finished slightly sooner than the magnetic ink, and therefore, the top surface of the sculpted structure mainly covers with magnetic filaments. After freeze-drying, the prepared magnetic/conductive aerogels with a thickness of ~5 mm were used as free-standing EMI absorbers. We also developed a unique EMI trap using the magnetic/conductive Janus aerogels. This structure consists of a 3mm Janus aerogel covered with 0.05 mm conductive copper tape from one side. The Janus aerogel of this shield is designed in a way that the magnetic parts' volume is larger than the conductive domain, i.e., the mGO/GO (50 wt% GO) and Ti3C2Tx/GO (20 wt% GO) inks were extruded from 400 and 200 µm nozzle, respectively. In this case, the impedance mismatch between this Janus structure and free space is even lower than the Janus aerogels prepared in the previous section, allowing the incident waves to penetrate the structure with minimum reflection. Janus aerogels of Ti3C2Tx/GO (20 wt% GO): GO were adopted for pressure sensing. The solid concentration of both inks was 10 mg/ml, and 200 µm (ID) dispensing needles were used for both aqueous suspensions. After freeze-drying, two copper wires were attached to the two sides of the 174 constructs, and the whole structure was covered by double-sided elastomeric tapes. Afterward, a multichannel potentiostat (Metrohm Autolab, the Netherlands) was employed to investigate the current vs. time behavior of the printed sensors at a constant voltage of 1 V. The behavior of the sensor at various compression conditions was evaluated with a tensile apparatus (Instron universal testing machine, USA). Finally, the Janus sensors were employed for real-time human motion and health monitoring. Written consent from all participants was obtained prior to the research. 7.2.3 Characterization The as-developed aerogels were assessed via diverse analyses to confirm their successful fabrication with the desired quality. Accordingly, the general characterizations were conducted via Fourier-transform infrared spectroscopy (FTIR) (Nicolet-S20) within the range of 600-4000 cm-1, X-ray diffraction (XRD) (Brucker D8 Advance), and micro-Raman spectroscopy (Brucker Senterra II) equipped with an Olympus U-TV1X-2 microscope. Additionally, the morphological assessments of the developed aerogels were conducted via a Field Emission Scanning Electron Microscope (FESEM) (Tescan Mira 3 XMU) equipped with an Oxford Instruments X-Max energy dispersive spectrometer (EDS) detector. The electrical conductivity of the developed aerogels was measured via a Loresta GP resistivity meter (Mitsubishi Chemical Co. model MCP-T700). The EMI shielding properties are derived from the guidelines and methods described in Chapter 2 of this thesis. The 3D morphology of Janus nanofibers and the interface of filaments with each other were assessed via a 3D laser scanning microscope (Olympus model LEXT OLS5000). X-Ray micro-computed tomography (micro-CT) images of the prepared aerogels were obtained using 3D X-ray microscopes (ZEISS Xradia 520 Versa). The interfacial rheology and dynamic interfacial tension (IFT) tests of aquatic GO or Ti3C2Tx-based inks were assessed by holding the 5 µL of aquatic 175 droplets of in s’ suspensions in the organic hexane phase. Accordingly, the dynamic interfacial tension assessments in the pure hexane were performed in 1 h. This time did not prolong because of the rapid interfacial solidification and loss of sphericity of the drop in the host aquatic media containing POSS and hexane. 7.3 Results and discussion 7.3.1 Shaping GO non-responsive liquid threads The fabrication of Janus liquids requires: 1) jetting separate liquid threads, 2) bringing the liquid threads together during jetting, 3) and merging the threads. Three different 2D NPs, including graphene oxide (GO), GO decorated with Fe3O4 or magnetic GO (mGO), and highly conductive Ti3C2Tx, were prepared, dispersed in water, and jetted. We initially produced non-responsive liquid threads by streaming 5-10 mg/ml aqueous GO suspensions into hexane containing 1 mg/ml PSS-[3-(2-aminoethyl) amino]propyl-heptaisobutyl substituted POSS (POSS-NH2) (Figures 7-1 and 7-2). Here, the rapid co-assembly of GO and POSS at the interface forms an interfacial skin that wraps the jetted liquid thread [33,228]. This interfacial skin prevents the jet from breaking up into droplets, suppressing Plateau-Rayleigh instabilities [219,222]. The interfacial layer imparts mechanical stability to these highly non-equilibrium-shaped aqueous threads, allowing them to remain intact even after being agitated or aged for months (Figure 7-2). The formation of the non-responsive liquid threads is based on the dual hydrophobic/hydrophilic nature of GO, i.e., the hydrophobic basal plane and hydrophilic functionality such as hydroxyl, carboxylic, and epoxy groups, which promotes the very rapid migration and assembly of these NPs at the oil/water interface [33,172,232]. When GO is in the aqueous phase, both sides of the hydrophobic basal planes are in contact with water; however, upon migration to and assembly at the interface, 176 not only are the bare water oil contacts removed by the insertion of the GO, but one-half of the non-favorable GO basal plane/water contacts are eliminated [33]. This represents a significant gain in the free energy and a reduction in the equilibrium interfacial tension (IFT) from ~49 to 30 mN/m (refer to Figure 7-3) [33,232]. The POSS in the oil phase rapidly interacts with the hydrophilic functional groups on GO, making this exposed plane of the GO more hydrophobic, anchoring the GO/POSS, i.e., essentially sheet-like NPSs, to the interface, dramatically reducing the IFT to 3.25 mN/m. The rigid structure of POSS, a cubic inorganic silica-cage core with side chains, significantly increases the mechanical integrity of the GO/POSS assemblies, making them ideal for stabilizing a jetted aqueous thread [33,217]. In order to further evaluate the GO/POSS interfacial layers, the worm-like liquid threads of GO were freeze-dried and assessed by a field emission scanning electron microscope (FESEM). A compact and laminated skin covering the bulk of these highly porous tubes was observed in these constructs (Figures 7-1 (b-e)). Figure 7-1 (a) Digital images demonstrating tubule formation upon streaming GO aqueous suspension (10 mg/ml) into hexane-POSS. (b-e) FESEM images of worm-like aerogels of GO. Scale bars in (a), (b), (c), (d), and (e) correspond to 2 cm, 2000, 1000, 500, and 100 µm, respectively. 177 Figure 7-2 (a) Digital image demonstrating tubule formation upon streaming GO aqueous suspension (5 mg/ml) into hexane-POSS. (b) GO-liquid threads with a worm-like morphology after three months of aging. All scale bars in (a-b) correspond to 2 cm. 7.3.2 Shaping mGO/GO magnetic liquid threads Using a mixed suspension of GO and mGO, magnetic liquid threads could be produced in a similar manner. The jetting of mGO/GO suspension in hexane containing POSS and the response of the magnetic liquid tubules to an external magnetic field are shown in Figures 7-4 (a-b). The mixed aqueous suspension of mGO and GO contained 50 wt% of GO and mGO, with a total solid concentration of 10 mg/ml. We note that liquid threads could be produced with only mGO. However, upon freeze-drying, relatively poor structural stability and high volume shrinkage were observed due to the weak interfacial skin (Figures 7-4 (c-d)) [33]. Both were significantly improved in the hybrid system. 178 Figure 7-3 Interfacial characterization of NPs aqueous suspension in hexane domain (a) without and (b, c) with POSS. The aqueous suspension of Ti3C2Tx (10 mg/ml), GO (10 mg/ml), and hybrid systems containing 20 and 50 wt% GO, i.e., 2 and 5 mg/ml of GO in 10 mg/ml Ti3C2Tx/GO suspensions, were tested in this experiment. (d) The average equilibrium IFT of the aqueous suspensions/hexane with and without POSS. (e) The schematic illustration of NPSs assemblies in the presence of POSS results in lower equilibrium IFT of the aqueous suspensions/hexane. Contraction of a pendent drop of (f) GO (10 mg/ml); (g) Ti3C2Tx (10 mg/ml); (h) Ti3C2Tx/GO 80 wt%; and (i) Ti3C2Tx/GO 50 wt% in hexane with and without 1 mg/mL POSS. 179 Figure 7-4 (a) The process of streaming mGO/GO 50 wt% and their response to an external magnetic field. (b) The prepared mGO/GO showed enhanced structural integrity and minimum shrinkage upon freeze-drying. (c) Digital image demonstrating tubule formation upon streaming 10 mg/ml mGO aqueous suspension. The mGO aqueous suspension is capable of fabricating liquid threads. (d) However, upon freeze-drying, relatively low structural stability and high-volume shrinkage for mGO aerogels were observed. All scale bars in (a-d) correspond to 2 cm. 180 7.3.3 Shaping Ti3C2Tx/GO conductive liquid threads Ti3C2Tx was chosen as a benchmark for generating conductive liquid threads due to its high electrical conductivity and ability to be processed in water, unlike other types of conductive nanomaterials [194]. However, when pure Ti3C2Tx is jetted into the hexane/POSS domain, it breaks into small droplets, forming liquid beads (Figures 7-5 and 7-6). Unlike GO, which acts as a surfactant and rapidly assembles at the interface and provides structural stability, Ti3C2Tx is negatively charged and does not assemble at the oil/water interface initially [231]. At a Ti3C2Tx concentration of 10 mg/ml and without POSS in the surrounding hexane phase, the equilibrium IFT is ~48 mN/m, almost equal to that of pure water against hexane, which is ~49 mN/m (refer to Figure 7-3). In this case, the POSS must first assemble at the interface, followed by diffusion of the Ti3C2Tx that interacts with the assembled POSS [231,234,289]. While POSS significantly reduces the interfacial energy, the binding energy per POSS is not sufficient to withstand the compressive forces exerted when droplets begin to form. Consequently, the stream breaks up into droplets [33,172]. A solution of POSS in hexane against pure water has an IFT of ~18 mN/m, demonstrating the surfactant-like characteristics of POSS [172,219]. However, with POSS being dissolved in hexane, the equilibrium IFT of 10 mg/ml Ti3C2Tx in water against hexane containing POSS dramatically decreased to ~10 mN/m (Figures7-3 (c-d)), confirming the formation and assembly of the Ti3C2Tx/POSS NPSs at the interface [33,217,229]. Evidence of the interfacial Ti3C2Tx/POSS assembly is also found in the FESEM images of freeze-dried Ti3C2Tx aerogel beads, where a compact, layered skin, distinct from the porous interior of the cells forming the aerogel, is evident. This indicates the formation of NPS assemblies, even when using pure Ti3C2Tx (Figures 7-5 (c-e) and 7-6 (d-h)). We note that pure Ti3C2Tx also assembles at the interface, though the assemblies are 181 much weaker in comparison to those formed with GO, which has an equilibrium IFT of ~3.25 mN/m against a solution of POSS in hexane (Figure 7-3 (b)). Figure 7-5 (a) Digital and (b) confocal images of Ti3C2Tx beads. (c-d) FESEM images of Ti3C2Tx aerogel beads. The red arrows in (d-e) highlight the laminated skin that covers the bulk of aerogels. Scale bars in (a), (b), (c, d), and (e) correspond to 2 cm, 1000, 500, and 100 µm, respectively. Figure 7-6 (a) Digital image demonstrating bead formation upon streaming Ti3C2Tx aqueous suspension (10 mg/ml) into hexane-POSS. Ti3C2Tx beads (b) before and (c) after three months of aging. (d-h) FESEM images of Ti3C2Tx aerogel beads. These structures showcase laminated skin covering the bulk of aerogels. Scale bars in (a-c), (e), (d, f), (g), and (h) corresponded to 2cm, 400, 200, 50, and 10 µm, respectively. 182 7.3.4 Shaping Ti3C2Tx/GO conductive liquid threads Ti3C2Tx was chosen as a benchmark for generating conductive liquid threads due to its high electrical conductivity and ability to be processed in water, unlike other types of conductive nanomaterials [194]. However, when pure Ti3C2Tx is jetted into the hexane/POSS domain, it breaks into small droplets, forming liquid beads (Figures 7-5 and 7-6). Unlike GO, which acts as a surfactant and rapidly assembles at the interface and provides structural stability, Ti3C2Tx is negatively charged and does not assemble at the oil/water interface initially [231]. At a Ti3C2Tx concentration of 10 mg/ml and without POSS in the surrounding hexane phase, the equilibrium IFT is ~48 mN/m, almost equal to that of pure water against hexane, which is ~49 mN/m (Figure 7-3 (a)). In this case, the POSS must first assemble at the interface, followed by diffusion of the Ti3C2Tx that interacts with the assembled POSS [231,234,289]. While POSS significantly reduces the interfacial energy, the binding energy per POSS is not sufficient to withstand the compressive forces exerted when droplets begin to form. Consequently, the stream breaks up into droplets [33,172]. A solution of POSS in hexane against pure water has an IFT of ~18 mN/m, demonstrating the surfactant-like characteristics of POSS [172,219]. However, with POSS being dissolved in hexane, the equilibrium IFT of 10 mg/ml Ti3C2Tx in water against hexane containing POSS dramatically decreased to ~10 mN/m (Figures 7-3 (c-d)), confirming the formation and assembly of the Ti3C2Tx/POSS NPSs at the interface [33,217,229]. Evidence of the interfacial Ti3C2Tx/POSS assembly is also found in the FESEM images of freeze-dried Ti3C2Tx aerogel beads, where a compact, layered skin, distinct from the porous interior of the cells forming the aerogel, is evident. This indicates the formation of NPS assemblies, even when using pure Ti3C2Tx (Figures 7-5 and 7-6). We note that pure Ti3C2Tx also assembles at the interface, though the assemblies are much weaker 183 in comparison to those formed with GO, which has an equilibrium IFT of ~3.25 mN/m against a solution of POSS in hexane (Figure 7-3 (b)). To address the stability of MXene jetted streams, a mixture of GO and Ti3C2Tx was used that yielded aqueous tubules similar to that found with GO, where the functionality of the final constructs, i.e., electrical conductivity, was attained from Ti3C2Tx and the interfacial NPSs assemblies mainly relied on the GO’s interfacial activity (Figure 7-7 (a)). The equilibrium IFT of aqueous Ti3C2Tx/GO suspensions against pure hexane was considerably reduced, when compared to pure Ti3C2Tx suspension, demonstrating the superior interfacial activity of GO (Figure 7-3 (a)). This reduction was even more evident when POSS was added to the hexane, where, for suspensions of 20 and 50 wt% GO, i.e., 2 and 5 mg/ml of GO in 10 mg/ml Ti3C2Tx/GO suspensions, the IFTs were ~ 2.75 and 2.5 mN/m, respectively (Figure 7-3 (b-d)). These results confirm that the GO and POSS strongly assemble at the interface and stabilize the Ti3C2Tx/GO streams. The formation of interfacial Ti3C2Tx/GO/POSS assemblies is also evident in FESEM images of jetted Ti3C2Tx/GO tubules, where a laminated skin covers the jetted stream of these highly porous tubes (Figures 7-7 (b-e)). Figures 7-8 to 7-11 also demonstrate the potential for precise adjustment of the dimensions of aqueous tubes and the macro-scale voids between Ti3C2Tx/GO tubules through a liquid streaming approach. As illustrated in Figures 7-8 and 7-9, the size of the extruded tubules can be varied from 450 to 850 µm by only using needles with different gauge numbers ranging from 21 to 15. Additionally, the presence of macro-scale voids among the Ti3C2Tx/GO tubules is clearly evident in the cross-sectional X-Ray micro-computed tomography (micro-CT) images of these structures (Figures 7-10 and 7-11). The size and distribution of these macro-scale pores can be regulated by altering the dimensions of the extruded filaments. This remarkable flexibility in controlling the 184 macro-scale porosity of Ti3C2Tx/GO worm-like structures opens up exciting possibilities for diverse applications, including EMI shielding, oil absorption, and gas absorption/detection, where precise control over macro-to-micro scale porosities is of paramount importance. Figure 7-7 (a) Digital images demonstrating tubule formation upon streaming 10 mg/ml Ti3C2Tx /GO aqueous containing 20 wt% GO. The rapid assembly of NPSs suppresses the Plateau-Rayleigh instabilities and stabilizes the aqueous jet of Ti3C2Tx/GO. (b-e) FESEM images of Ti3C2Tx/GO worm-like aerogels. The red arrow in (d) highlights the laminated skin that covers the bulk of aerogels. Scale bars in (a), (b), (c), (d), and (e) correspond to 2 cm, 2000, 500, 200, and 100 µm, respectively. 185 Figure 7-8 FESEM images of bulk and filaments of Ti3C2Tx/GO (20 wt% GO) aerogels generated via liquid streaming approach using needles with different gauge numbers of (a) 21, (b) 18, and (c) 15. In the case of these samples, the extrusion pressure was fixed at 15 psi. Scale bars equal (I) 2 mm and (II-III) 1 mm. 186 Figure 7-9 Filament diameter of Ti3C2Tx/GO (20 wt% GO) aerogels generated via liquid streaming approach using needles with different gauge numbers of (a) 21, (b) 18, and (c) 15. Average filament diameter of Ti3C2Tx/GO (20 wt% GO) aerogels prepared at different needle gauges. In the case of these samples, the extrusion pressure was fixed at 15 psi. Figure 7-10 Cross-sectional micro-CT images of Ti3C2Tx/GO (20 wt% GO) aerogels prepared via gauge 15 needles. In the case of these samples, the extrusion pressure was fixed at 15 psi. 187 Figure 7-11 Cross-sectional micro-CT images of Ti3C2Tx/GO (20 wt% GO) aerogels prepared via gauge 21 needles. In the case of these samples, the extrusion pressure was fixed at 15 psi. It's worth emphasizing that, when employing smaller needles for extruding filaments, the section of the sample situated nearer to the bottom of the holding vials demonstrates a denser composition with diminished macro-scale voids, as evident in images I to VI. 7.3.5 Fabrication of Janus liquids and aerogels Magnetic/conductive and conductive/non-responsive Janus liquid tubules were fabricated by merging aqueous streams of 10 mg/ml Ti3C2Tx/GO (20 wt% GO) and mGO/GO (50 wt% GO) or pure GO (Figure 7-12). The heterogeneous content of the opposing sections of the Janus threads is shown in Figure 7-13. Interestingly, the ratio of magnetic and conductive or non-responsive parts of the Janus liquid tubules can be readily controlled by tuning the jetting parameters. For example, using needles with different inner diameters for jetting each suspension, e.g., 200 µm for Ti3C2Tx/GO and 400 µm for mGO/GO, enabled the fabrication of Janus liquid tubules where the volume of the magnetic portion was larger than that of the Ti3C2Tx suspension. 188 The Janus structured liquids were transformed to lightweight magnetic/conductive aerogels, e.g., 𝜌 ~ 7 mg/cm3 in the case of Ti3C2Tx/GO:mGO/GO structures printed with 200 µm needles (Figures 7-12 (c-d)). The electrical conductivity of the Janus aerogels arises from the Ti3C2Tx-containing sections, while the magnetic response is attributed to mGO present on the opposite side of the filaments. Compared to conventional magnetic/conductive composite aerogels, the functionalities of Janus aerogels arise from different macroscopic sections of the constructs and can be easily tuned independently, i.e., alternating layers of magnetic and conductive NPs. In this way, we addressed the current challenge of electrically conductive network rupture upon the introduction of less-conductive magnetic particles [64,292]. 189 Figure 7-12 (a) Schematic illustration of Janus liquid threads formation. In this graphical representation, GO and Ti3C2Tx nanosheets are represented by gold and black colors, respectively. (b) Magnetic/conductive Janus structures were fabricated by merging streams of Ti3C2Tx/GO (20 wt% GO) and mGO/GO (50 wt% GO). (c-d) Digital images of Ti3C2Tx/GO:mGO/GO magnetic/conductive aerogels. All scale bares correspond to 2cm. 190 Figure 7-13 (a-d) Confocal images of the Janus liquid threads prepared by merging streams of Ti3C2Tx/GO-10 mg/ml (20 wt% GO) and mGO/GO-10 mg/ml (50 wt% GO) inks. In Figures a-b, 200 µm nozzles were used for both inks. However, in Figures c-d, we used different needles for each ink, i.e., 200 µm for Ti3C2Tx/GO and 400 µm for mGO/GO, and fabricated Janus structures in which the magnetic parts' volume is larger than that containing Ti3C2Tx. All scale bars correspond to 500 µm. The morphology of the Janus structures was investigated by FESEM and energy-dispersive X-ray spectroscopy (EDX) analysis and is shown in Figures 7-14 to 7-18. The Janus aerogels show dual porosities, which is the characteristic of worm-like aerogels, i.e., macro-scale porosities between the Janus tubes and micro-scale porosities within the tubules. Figures 7-15 and 7-16 illustrate the controllable manipulation of tube size and the subsequent connections and entanglements between tubules in Janus structures achieved by employing needles of different sizes during the streaming process. Additionally, to emphasize the micro-scale porosities within these aerogels, Figures 7-17 and 7-18 display FESEM images of hierarchical aerogels fabricated from pure GO ink, mGO/GO (20 wt% GO), and Ti3C2Tx/GO (20 wt% GO) suspensions. It's noteworthy that the formation of these micro-scale pores results from the crystallization of ice during the freezing process, which is subsequently preserved upon freeze-drying. Furthermore, the heterogeneous distribution of NPs throughout the Ti3C2Tx/GO:mGO/GO and Ti3C2Tx/GO:GO tubules is evident in EDX analysis, where a distinct boundary on the surface of the Janus aerogels filament is observed (Figure 7-14). 191 The interfacial skin of these Janus worm-like constructs plays a crucial role in determining their mechanical stability [33]. In comparison to worm-like aerogels of Ti3C2Tx/GO, both Janus aerogels of Ti3C2Tx/GO:GO and Ti3C2Tx:mGO/GO showed enhanced compressibility and mechanical stability (Figure 7-19). These characteristics can be explained by the formation of a GO network throughout the tubes and the altered stress distribution on Ti3C2Tx/GO parts that have a semi-cylindrical shape in the case of Janus tubes [33,293]. Figure 7-14 FESEM and EDX analysis of (a-e) Ti3C2Tx/GO:mGO/GO and (f-j) Ti3C2Tx/GO:GO Janus aerogels. All scale bars correspond to 1000 µm. 192 Figure 7-15 FESEM images of Ti3C2Tx/GO (20 wt% GO):GO Janus aerogels generated using needles with different gauge numbers of (a) 21 and (b) 18. Scale bars correspond to (I) 2 mm and (II-III) 1 mm. Figure 7-16 Filament diameter distribution of Ti3C2Tx/GO (20 wt% GO):GO Janus aerogels generated using needles with different gauge numbers of (a) 21 and (b) 18. (c) Average filament diameter of Ti3C2Tx/GO (20 wt% GO):GO Janus aerogels prepared by needles with different gauge numbers. 193 Figure 7-17 Micro-scale porosity of the hierarchical aerogels made of (a) 10 mg/ml GO, (b) 10 mg/ml mGO/GO (50 wt% GO), and (c)10 mg/ml Ti3C2Tx/GO (20 wt% GO) suspensions at (I-II) 500 µm, and (III) 200 µm scale bars. 194 Figure 7-18 Micro-scale pore size distribution of aerogels made of (a) 10 mg/ml GO, (b) 10 mg/ml mGO/GO (50 wt% GO), and (c)10 mg/ml Ti3C2Tx/GO (20 wt% GO) suspensions. Average micro-scale porosities of generated aerogels out of 10 mg/ml GO, mGO/GO, and Ti3C2Tx/GO suspensions. Figure 7-19 A comparison between the compressibility of various fabricated aerogels. (a) Worm-like aerogels of pure GO showcased elastic behavior, (b) while Ti3C2Tx/GO worm-like aerogels cannot elastically recover after compression. The Janus aerogels of (c, e) Ti3C2Tx/GO:mGO/GO and (d) Ti3C2Tx/GO:GO also showcased exceptional compressibility. (e) The Janus aerogel of Ti3C2Tx/GO:mGO/GO was capable of pushing a ~20 g sample upward after compression. All scale bars in (a-e) correspond to 2 cm. 195 7.3.6 Janus EMI shields with absorption-dominant characteristics Before delving into the discussion of the EMI shielding characteristics of the prepared samples, it is crucial to emphasize that, particularly for worm-like aerogels, a comprehensive evaluation of EMI shielding characteristics is required for each sample, considering assessments from both sides. This necessity arises due to the potential macro-scale inhomogeneity in worm-like aerogels prepared through liquid streaming processes. Consequently, for all the aerogels presented in this chapter, we conducted EMI shielding assessments on three different samples, each evaluated from both sides. This extensive dataset, collected within the X-band frequencies, enables the calculation of a realistic average value and standard deviation for each aerogel. This approach offers an effective representation of the inherent EMI shielding capability within these worm-like structures. In the first step, an examination of the EMI shielding characteristics of the worm-like structures composed of Ti3C2Tx/GO (20 wt% GO), mGO/GO (50 wt% GO), and pure GO is conducted. It is noteworthy that both mGO/GO (50 wt% GO) and pure GO fail to exhibit the impedance mismatch characteristics crucial for effective EMI shielding, primarily due to their lack of electrical conductivity. Consequently, these structures are considered unsuitable for mitigating unwanted electromagnetic interference, as exemplified in Figures 7-20 to 7-22. In contrast, the conductive worm-like structures of Ti3C2Tx/GO (20 wt% GO) demonstrate an exceptional shielding effectiveness of 69.2 dB, a noteworthy achievement for lightweight electromagnetic shields (Figure 7-20). Importantly, this EMI shielding behavior is achieved without the need for thermal or chemical reduction processes. However, it is crucial to emphasize that the conductive properties of these structures, characterized by an electrical conductivity of approximately 120±10 S/m, result in a high reflectance index of approximately 70%, 196 corresponding to an absorptance of around 30%. This high reflectance introduces secondary reflection issues, a challenge that we endeavor to address through the development of Janus aerogels. To underscore the future potential of the Janus liquid constructs, magnetic/conductive Janus aerogels were used to address a key challenge in EMI shielding: the high reflection index. The Janus aerogels of Ti3C2Tx/GO (20 wt% GO) and mGO/GO (50 wt% GO), jetted with two 200 µm nozzles, showed exceptional absorption-dominant shielding behavior, e.g., SET=23.4 dB, SER=0.8 dB, and A=83% (Figures 7-23 (a) and 7-24). This unique behavior can be explained by the high porosity ranging from the micro- to macro-scales and alternating magnetic/conductive domains of the aerogels, creating numerous interfaces within a small volume. Here, by incorporating a magnetic component (mGO/GO), Janus aerogels ensure magnetic loss, while the conductive element (Ti3C2Tx/GO) contributes to dielectric loss, enabling efficient absorption and dissipation of electromagnetic waves. Due to the very low electrical conductivity of magnetic domains, their impedance almost matches free space and allows the penetration of electromagnetic waves into the bulk of aerogels [294]. Even with conductive domains, a large portion of electromagnetic waves can pass through due to the shallow thickness of conductive sections that always have magnetic domains next to them [11,36]. Upon penetrating these aerogels, the energy of electromagnetic waves gets dissipated due to numerous interfaces arranged inside these structures from micro- to macro-scales and prolonged the interaction of the waves with electrical/magnetic dipoles and domains with finite electrical conductivity. In addition, the Janus structures have numerous interfaces between conductive and magnetic domains with impedance match/mismatch characteristics that can further boost the scattering and absorption of electromagnetic waves. This unique absorption 197 mechanism, schematically represented in Figure 7-23 (b), not only safeguards human health and sensitive equipment from electromagnetic waves but also minimizes the secondary EMI pollution, which is as detrimental as the original waves. Using the magnetic/conductive Janus aerogels, we also developed a unique EMI trap with SET=51 dB, SER=0.4 dB, and A=91%. This structure is comprised of a 3 mm Janus aerogel covered with 0.05 mm conductive copper tape from one side. From the aerogel sides, we observed a unique absorption behavior, while the copper side mainly reflected the electromagnetic waves (Figures 7-23 (c-d) and 7-24). The Janus aerogel was prepared so that the magnetic section's volume fraction was larger than that of the conductive section; specifically, mGO/GO (50 wt% GO) and Ti3C2Tx/GO (20 wt% GO) suspensions were jetted from 400 and 200 µm nozzles, respectively. Therefore, the impedance mismatch between this Janus structure and free space was even lower than the Janus aerogels described previously, allowing the incident waves to penetrate the structure with minimum reflection. Once the electromagnetic waves penetrate the structure, their energy gets dissipated due to the presence of multi-scale porosities and alternating magnetic/conductive layers that significantly boost the internal scattering. This construct is essentially a trap for electromagnetic waves. Figure 7-23 € and Table 7-1 compare the SET and A of this shield with others. The exceptional EMI shielding performance and unique absorption-dominant behavior make these structures ideal for medical and military applications, where SET >30 dB and a low reflection index are of utmost importance [11,31]. 198 Figure 7-20 (a, b) EMI shielding characteristics of the worm-like structures composed of Ti3C2Tx/GO (20 wt% GO), mGO/GO (50 wt% GO), and pure GO. (c) Electrical conductivity of Ti3C2Tx/GO (20 wt% GO) worm-like aerogels. The thickness of these aerogels is 3mm. 199 Figure 7-21 EMI shielding characteristics of pure GO aerogels, printed with a 200 μm nozzles. I and II refer to the data obtained from two different sides of the samples. The thickness of these samples was 3 mm. Three different samples were tested: (a, b) sample one, (c, d) sample two, and (e, f) sample three. I II I II I III II I III II200 Figure 7-22 EMI shielding characteristics of mGO/GO (50 wt% GO) aerogels, printed with a 200 μm nozzles. I and II refer to the data obtained from two different sides of the samples. The thickness of these samples was 3 mm. Three different samples were tested: (a, b) sample one, (c, d) sample two, and (e, f) sample three. I III II I III II I III II201 Figure 7-23 (a) EMI shielding characteristics of Ti3C2Tx/GO (20 wt% GO) and mGO/GO (50 wt% GO) Janus aerogels prepared by two 200 µm nozzles. (b) Schematic illustration of internal scattering in Ti3C2Tx/GO (20 wt% GO) and mGO/GO (50 wt% GO) Janus aerogels from macro-to micro-scale. (c, d) EMI shielding characteristics of copper coated magnetic/conductive Janus aerogels. This shield consists of a 3 mm Janus aerogel covered with 0.05 mm conductive copper tape from one side. The Janus aerogel of this shield is designed in a way that the magnetic parts' volume is larger than the conductive domain. (c) summarizes the EMI shielding characteristics of the aerogel sides, while (d) showcases those characteristics that belong to the copper side of the shield. (e) A comparison of the SET vs. A of the magnetic/conductive Janus structures with the previous best practices in the literature. For more information, see Table 7-1. 202 Figure 7-24 EMI shielding characteristics of Ti3C2Tx/GO (20 wt% GO) and mGO/GO (50 wt% GO) Janus aerogels, printed with two 200 μm nozzles. I and II refer to the data obtained from two different sides of the samples. The thickness of these samples was 5 mm. Three different samples were tested: (a, b) sample one, (c, d) sample two, and (e, f) sample three. I II I II I II I III III II 203 Figure 7-25 EMI shielding characteristics of copper coated magnetic/conductive Janus aerogels. This shield consists of a 3 mm Janus aerogel covered with 0.05 mm conductive copper tape from one side. The Janus aerogel of this shield is designed in a way that the magnetic parts' volume is larger than the conductive domain. I and II refer to the data obtained from two different sides of the samples: (I) Janus aerogel side and (II) copper side. Three different samples were tested: (a, b) sample one, (c, d) sample two, and (e, f) sample three. I II I III II I II I III II 204 Table 7-1 A comparison of the SET vs. A of the magnetic/conductive Janus structures with the previous best practices in the literature. In the case that R and A coefficients were not directly reported in the papers, these coefficients are calculated based on the formulations explained in section three of this file. ample Thic ness (mm) MI (dB) R (dB) R A Ref. C Ts–multilayered graphene edge plane core–shell hybrid foams 1.6 38.4 3.9 0.59 0.41 [295] Ti3C2Tx 0.001 46.1 13.5 ~0.96 ~0.04 [13] 3 graphene fibers 0.0263 56 ~9 ~0.89 0.11 [296] rGO film 0.0084 20 ~8 ~0.84 ~0.16 [297] Glucose derived anisotropically oriented carbon films (GCFs) 0.00048 21.72 7.96 0.84 0.16 [298] Ti3C2Tx/P a 2 61.2 6.5 ~0.78 ~0.22 [192] M C Ts/ PUb foam 2.3 50.5 ~9 ~0.89 ~0.11 [299] PCCc/MXene/PVAd phase change composite film 0.048 43.1 13.17 ~0.96 ~0.04 [300] C Ts sponge/epoxy 2 44 5 ~0.68 ~0.32 [278] C Ts/ Ti3C2Tx /latex 1.1 52 ~19 ~0.98 ~0.02 [301] A Fse reinforced PVA hydrogels and a layer of Ag sf/PVA ~0.3 52 16 ~0.97 ~0.03 [302] Pristine Ti3C2Tx 0.014 82 ~20 ~0.99 ~0.01 [303] rin led Ti3C2Tx 0.02 68 27 0.99> <0.01 [304] Ag s decorated leather 0.5 55 ~8 ~0.84 ~0.16 [305] Ti3C2Tx aerogel/epoxy 2 34.5 ~6.5 ~0.78 ~0.22 [217] Fe3O4/PVA composite electrospun nanofibers in the top and bottom layers and Ti3C2Tx/PVA composite electrospun nanofibers in the middle layer 0.075 40 ~13 ~0.96 0.04 [306] Ag s wrapped carbon hybrid sponge 3 70.1 ~11 ~0.91 ~0.09 [307] M C Ts/Fe3O4@Ag/ epoxy 2 35 4.5 0.65 0.35 [268] Graphene /Fe3O4 papers 0.25 24 8 0.84 0.16 [308] M C Ts/PLLAg 1.5 30 4 0.6 0.4 [309] C Ts/GTRh 1 30 6 0.75 0.25 [310] Fe3O4@rGO/PMMAi 2.9 29.3 4 0.6 0.4 [64] rGO foam 0.9 42.3 10 0.9 0.1 [311] M C Ts/PV F 1 22.5 6.3 0.77 0.23 [312] Graphene /M C Ts/PMMA 2 36 6.67 0.78 0.22 [313] C Ts/rGO foam 2 31.2 5.9 0.74 0.26 [314] Multilayer Fe3O4@rGO/M C Ts / PU 0.8 36 1.46 0.27 0.73 [271] 205 Multilayer Ti3C2Tx/C Fj 0.035 39.6 17.7 0.98 0.02 [123] rGO/P M 5 26 2.99 0.49 0.51 [33] Ti2CTx/PVA 0.3 21 11 0.91 0.09 [122] Ti2CTx/PVA foam 5 28 2 0.37 0.63 [122] Ti3C2Tx/P OT:P aerogels 5 28 3 0.5 0.5 [315] Ti3C2Tx/ polyaniline/ carbon fiber 0.55 26.5 5.7 0.74 0.26 [316] Multilayer Ti3C2Tx/PVA 0.027 44.4 8.3 0.85 0.15 [126] Ti3C2Tx/A F 0.018 55.9 10.3 0.9 0.1 [317] Polyaniline Grafted Ti3C2Tx/PV F 1.7 33.3 5.2 0.7 0.3 [318] ilver/electrospun nylon nanofiber 0.005 77.6 13 ~0.95 ~0.5 [319] FGQFl 0.2 ~48 ~9.6 ~0.89 ~0.11 [320] FGQF 1.12 106 ~10 ~0.9 ~0.1 [320] acre li e graphene/PIm 0.01 43.8 ~13.3 ~0.95 ~0.05 [145] MXene functionalized P OT:P m hydrogel 0.295 51.7 3.7 0.58 0.42 [21] Ti3C2Tx /M C Ts/ rFe12O19 0.04 62.9 ~12 ~0.94 ~0.06 [321] Ionic liquid doped P OT:P hydrogel 0.046 54 ~18.4 ~0.98 <0.02 [322] creen printed Ti3C2Tx /Xanthan Gum hybrids 0.012 40.1 ~15 ~0.97 ~0.03 [323] Cu s /graphene core–shell aerogels 5 43.7 2.86 ~0.48 ~0.52 [324] Compressed Cu s /graphene core–shell aerogels 5 31.28 4.11 ~0.61 ~0.39 [324] M C Ts/ PU 2.3 46.7 ~7.2 ~0.8 ~0.2 [325] Ti3C2Tx/gelation 0.004 53.6 ~19.2 ~0.98 <0.02 [326] Ti3C2Tx 0.005 57.4 17.5 ~0.98 ~0.02 [5] Ti3C Tx 0.005 49.7 17 ~0.98 ~0.02 [5] Honeycomb porous graphene/Ag s 0.0483 61.8 ~14.5 ~0.96 ~0.04 [327] 2 Cu nanosheets 0.063 105 ~14 ~0.96 ~0.04 [328] Ag s/ anocellulose aerogels 2 70.5 ~17 ~0.98 ~0.02 [329] C Ts/A Fs hybrid aerogel 0.568 54.4 ~8 ~0.84 ~0.16 [256] Polyolefin Composites with a Ti3C2Tx/graphene framewor 1.85 61 7.7 0.83 0.17 [330] Ti3C2Tx/C F 0.074 25.8 5.3 ~0.71 ~0.29 [138] Ti3C2Tx/polydopamine hybrids 0.007 58.4 ~16 ~0.97 ~0.03 [153] Graphite film ~0.0004 27.8 ~5 ~0.69 ~0.31 [331] rGO/(C iC)12 4 70.2 ~12 ~0.94 ~0.06 [332] rGO wrapped Ag s networ s ~0.0001 35 8.1 ~0.84 ~0.16 [15] Ti3C2Tx/rGO aerogel 2 56.4 5.7 0.73 0.27 [19] Laminated A F/C Ts 0.024 36 ~14 ~0.96 ~0.04 [333] Ti3C2Tx /A F 0.037 48 ~18 ~0.98 ~0.02 [334] ensified C Ts film ~0.015 101 ~20 ~0.99 ~0.01 [335] CFn iCo2O4–4F 0.034 53 ~8.2 0.85 0.15 [336] CF iCo 0.034 45 ~10 0.90 0.1 [336] CF 0.032 45 ~16 0.97 0.03 [336] 206 Polydopamine modified expanded graphite/PI 0.5 101 ~19 ~0.98 ~0.02 [337] Recycled carbon fibers/ Ti3C2Tx 0.343 45.7 ~15 ~0.97 ~0.03 [338] Carbonized wood/ i/C Ts 48.2 2.8 ~0.47 ~0.53 [339] Ti3C2Tx/ magnetic GO Janus aerogel 5 23.4 0.83 0.17 0.83 This study Ti3C2Tx/ magnetic GO Janus aerogel coated with cu tape 3 51 0.4 0.089 0.91 This study apolystyrene, bwater‐borne polyurethane, cphase change capsules, dpolyvinyl alcohol, eAramid nanofibers, fsilver nanowires, gpoly(L lactide), hground tire rubber, ipoly (methyl methacrylate), jcellulose nanofiber, poly(3,4 ethylenedioxythiophene) polystyrene sulfonate, lferromagnetic graphene quartz fiber, m polyimide, nCarbon fabric 7.3.7 Janus piezoresistive sensors for human motion monitoring. The Janus aerogels of Ti3C2Tx/GO (20 wt% GO) and GO show excellent reversible compressive behavior, where the structures that were compressed up to 70% rapidly recovered to their original shape after release (Figure 7-26 (a-b)). The stress-strain curves of these Janus aerogels at different sets of strains show mechanical robustness, which is crucial for high-performance piezoresistive sensors (Figure 7-26 (a)). Fatigue hysteresis tests were performed on the prepared aerogels, subjecting them to 100 compressive cycles at large strains of 40% and 50%. The results showed minor relaxation, where the aerogels maintained ~80% of their initial Young's moduli and maximum stresses (Figure 7-27). Besides, the interfacial skin that covers the bulk of the aerogels maintains the mechanical integrity of these structures that remained intact after 100 compressive cycles, indicating their mechanical robustness (Figure 7-28). The cyclic piezoresistive sensing performance of the Janus aerogel under different compressive strains is shown in Figure 7-26 (c). Upon compression, the density of the interconnected conductive sections of the Janus aerogels on micro- and macro-scales increases, causing an exponential increase in conductance. By increasing the compressive strain to 60%, face-to-face contact between adjacent conductive domains becomes more effective, leading to a higher 207 sensitivity. Additionally, it is important to mention that the inclusion of GO on the opposite side of the Janus aerogels was aimed at improving the overall mechanical properties and compressibility, which cannot be achieved solely with the worm-like Ti3C2Tx/GO (20 wt% GO), as depicted in Figure 7-19, along with their corresponding discussion. Figure 7-26 (a) The stress-strain curves of the Janus aerogels of Ti3C2Tx/GO (20 wt% GO):GO at different sets of strains. (b) Digital images showcasing the compressibility of Ti3C2Tx/GO (20 wt% GO):GO aerogels. (c) The cyclic piezoresistive sensing performance of the Janus aerogel under different compression strains. The scale bar in (b) corresponds to 2 cm. 208 Figure 7-27 Fatigue hysteresis tests of Ti3C2Tx/GO (20 wt% GO) and GO Janus aerogels. The samples were compressed up to (a) 40% and (b) 50% strain for 100 cycles. The results showed minor relaxation at those strain levels, where the aerogels maintained ~80% of their initial Young's moduli and maximum stresses. Figure 7-28 SEM images of Janus aerogels of Ti3C2Tx/GO (20 wt% GO) and GO after 100 compression cycles. The sample shown in (c) was compressed during SEM imaging. All scale bars in (a-c) correspond to 1 mm. 209 The performance of pressure sensors is evaluated based on their sensitivity, i.e., 𝑆 =∆𝐼𝐼0∆𝑃, and response/recovery times, which are calculated to be 1-8.18 kPa-1 and 300/250 ms for the Janus aerogels, respectively (Figure 7-29 (a-c)) [141,340]. These characteristics are comparable to some of the best sensors available with the significant advantage that no chemical/thermal reduction or pre-processing, e.g., freeze-casting, is needed for the Janus aerogels (see Table 7-2 and the corresponding discussion). Consequently, these interfacially driven morphologies open up numerous opportunities in pressure sensing and human motion monitoring. The Janus aerogels were also used as touch sensors, where the touch strength and frequency can be obtained by analyzing the signal curves (Figures 7-29 (d-e)). To assess the durability of these sensors, five participants were asked to touch the sensor, and the response was recorded over the course of the experiment (Figure 7-30). Even after 2,000 compression cycles, the baseline of the sensor and response/recovery signals were stable, underscoring their potential for applications ranging from wearable electronics to human-machine interfaces. The response of the sensors remained stable for up to three months (Figure 7-31). Only the oxidation of Ti3C2Tx lowered the baseline of the sensor, which is translated into a lower electrical conductivity [11,341]. However, the Janus aerogels’ response/recovery signals and piezoresistive sensing performances remained stable. 210 Figure 7-29 The (a) sensitivity and (b-c) response/recovery times of the fabricated sensor. The recorded signal for (d) soft and (e) hard press of the sensor. 211 Table 7-2 A comparison between the main characteristics of our sensor and those structures reported in the previous best practices in the literature. Material Sensitivity/GF Detection range Stability Preparation method Ref rGO aerogel 0.46 kPa−1 0.5–8 kPa 4200 Solvothermal (Chemical reduction) and thermal annealing [342] N-doped rGO sponge 1.33 kPa−1 30%-40% compression 3000 Hydrothermal (Chemical reduction and doping) and thermal annealing [343] rGO sponge 0.79–1.46 50% compression 500 Emulsion assembly alongside chemical reduction [344] rGO aerogel - up to 90% compression - 3D printing and thermal annealing [345] Biomimetic rGO aerogel - 0–50% 6000 Bidirectional freezing technique and thermal annealing [346] MoS2/graphene/Ecoflex 6.06 kPa−1 - 4000 Growing graphene on Ni foam, immersing in Ecoflex, and etching the Ni template [347] RGO-PUa 0.26 kPa−1 0–10 kPa 10,000 Dip coating PU foam in GO suspension and hydrothermal reduction [348] 3D graphene–PDMSb hollow structure 15.9 kPa−1 - - Growing graphene on Ni foam, immersing in PDMS, and etching the Ni template [349] rGO paper 17.2 kPa−1 0–20 kPa 300 Immersing a paper in GO suspension and thermal reduction [350] Wood-derived carbon aerogel 5.16 kPa−1 0–16.89 kPa 1000 Carbonization of alkali lignin/CNFc aerogel [351] Hollow Ti3C2Tx spheres/rGO aerogel 609 kPa−1 0-10 kPa 6000 Fabrication of PSd microspheres, electrostatic self-assembly of Ti3C2Tx on PS, mixing with go suspension, and thermal reduction and removing the PS spheres via annealing [352] Ti3C2Tx/CNCe aerogel 114.6 kPa−1 50 Pa−10 Pa 10000 Freeze casting, annealing, and carbonization [353] Ti3C2Tx/ carboxyated carbon nanotubes/ carboymethyl chitosan 0.18-3.84 kPa−1 0–12.4 kPa- 32.8–80.9 kPa 1000 Freeze casting and dipping in Ti3C2Tx [354] Ti3C2Tx/carbon nanofiber 65 kPa−1 up to 95% compression 5000 Freeze casting [355] Ti3C2Tx/rGO/PIf aerogel 0.08 kPa−1 0-180 kPa 10000 Freeze-drying and thermal reduction [356] PI nanofiber / Ti3C2Tx composite aerogel 0.002-0.014 kPa−1 up to 90% compression 1000 PI nanofiber was first treated with O2 plasma, mixed with Ti3C2Tx, and then freeze dried to form [141] 212 aerogel. The prepared aerogel was then annealed at high temperature. Ti3C2Tx/rGO 4.05-22.56 kPa−1 1 - 3.5 kPa 10000 Freeze-drying and thermal reduction [23] RGo/PVAg 1.32-7.48 kPa−1 0-3 kPa 1000 Bidirectional freezing and thermal reduction. [357] Wave-layered carbon aerogels 10.08 kPa−1 0-10 kPa 3000 A mixture of glucose and dicyandiamide was first carbonized into multi-layered nanosheets and then dispersed in CNF suspension. A multi-step processing, including directional freeze-casting, freeze-drying, and carbonization was used to prepare the carbon aerogels. [340] Ti3C2Tx/GO Janus aerogel 1-8.18 kPa−1 0~11 kPa 2000 at real conditions Liquid-liquid sculpting without any freeze casting or thermal/chemical reduction This study 213 Figure 7-30 The durability of the sensor after 2,000 compression cycles at real conditions. To assess the durability of these sensors, five different participants were asked to touch the sensor, and its response was recorded over the course of the experiment. 214 Figure 7-31 The recorded response and digital images of the sensing performance of the fabricated structures (a-b) before and (c-d) after three months. All scale bars in (b, d) correspond to 2 cm. The Janus aerogels of Ti3C2Tx/GO were also used for real-time human motion and health monitoring. Stable, reproducible signals were produced from the repeated bending of fingers and contraction-relaxation of biceps muscles, both large-scale body movements (Figures 7-32 (a-b)). Subtle motions, including the movement of facial muscles, talking, and coughing, were also monitored. The Janus aerogel devices could distinguish the vocalization of different letters and even polysyllabic words when attached to the skin outside of the larynx of a human subject (Figures 7-32 (c) and 7-33). The response of this system to coughing is shown in Figure 7-32 (d), demonstrating the feasibility of our sensor for health monitoring devices. The sensitivity to facial movement is shown in Figure 7-32 (e) for multiple cycles of contraction and relaxation of forehead 215 muscles. This response can possibly be used for human emotion recognition as one of the many potential applications of Janus aerogels. Figure 7-32 The capability of the sensor for monitoring (a) finger movements and (b) contraction-relaxation biceps muscles, both as large-scale body movements. The recorded response for various subtle human motions, including (c) talking, (d) coughing, and (e) facial muscle movements. Figure 7-33 The capability of the fabricated device to distinguish different alphabets and even polysyllabic words when attached to the skin outside of the larynx of a human subject. 216 7.4 Conclusion In summary, Janus all-liquid systems with anisotropic and customizable distributions of NPs were developed through interfacial assembly and jamming of NPs at the liquid-liquid interface. These Janus liquids offer greater versatility and tailorability when compared to conventional composites, as they allow for the precise, independent tuning of different functionalities on the different sides of the Janus architecture, resulting in multi-responsive soft materials with well-defined characteristics. The potential of these anisotropic platforms has been demonstrated through proof-of-concept applications such as absorption-dominant EMI shielding, piezoresistive sensing, and human motion monitoring. We anticipate that the Janus liquid approach will have many more potential uses in areas such as dual-encapsulation of cells and active matter, vessels for biphasic chemical synthesis and chemical separations, multi-modal sensors, and all-liquid robots, where the simultaneous encoding of multiple functionalities in a single construct is highly beneficial. Overall, our results suggest that Janus liquids are unique alternatives to traditional composites, blends, and grafted systems, where different functionalities can be achieved simultaneously in a single structure without interference. 217 Chapter 8: Conclusion and future work In this thesis, we have delved into a wide array of methods and structural design perspectives in the pursuit of crafting solutions for electromagnetic interference (EMI) shielding and strain/pressure sensors. The structures we developed encompassed laminated thin films, 3D printed structures, worm-like aerogels, and polymer nanocomposites. Each of these structures was strategically chosen for its intrinsic properties, tailored to serve either EMI shielding or strain/pressure sensing across various scenarios. Section 8.1 furnishes a comprehensive summary of the research conducted and underscores the distinctiveness of each research endeavor presented in the preceding chapters. Section 8.2 underscores the gaps and challenges associated with each developed structure. Finally, in Section 8.3, we outline the avenues recommended for future research, which have been gleaned from the insights discussed here. 8.1 Summary of main findings • In Chapter 3, we embarked on the synthesis processes, recipes, and characterization of three distinct types of nanomaterials: Ti3C2Tx, graphene oxide (GO), and magnetic GO (mGO). These nanomaterials constitute the cornerstone of our research and serve as our primary tools for designing various engineered structures. The characteristics highlighted in Chapter 3 unequivocally demonstrate the exceptional quality of these nanomaterials. Furthermore, they furnish a clear roadmap for synthesizing these pivotal nanomaterials, which find applications in a wide array of fields, including EMI shielding, sensing, environmental remediation and monitoring, thermal management, heating, and beyond. • In Chapter 4, we devised a straightforward yet versatile drop-casting method, complemented by co-treatment with sulfuric acid and methanol, to produce highly conductive nanocomposite films of Ti3C2Tx/PEDOT:PSS. Although the uniformity of our 218 drop-cast nanocomposite films is on par with those produced using conventional techniques like vacuum-assisted filtration (VAF), our method boasts significantly reduced manufacturing costs and offers a more straightforward preparation process. Notably, the resulting nanocomposite film demonstrated excellent mechanical properties, maintaining its EMI shielding effectiveness even after 500 bending cycles. Additionally, we introduced a wet-transfer technique to adapt these drop-casted films onto complex geometries with curves, angles, and corners. This flexible, transferable, and highly conductive EMI shielding material holds promise for applications in flexible electronics and EMI shields designed for intricate geometries. Despite the simplicity of the processing method employed in Chapter 4 to create laminated EMI shields, the resulting structures exhibited reflection-dominant EMI shielding characteristics, which may constrain their application in advanced EMI shielding contexts. However, this initial phase of research and development served as a valuable learning experience, imparting fundamental knowledge of EMI shielding measurements and calibration, the handling of nanomaterials and polymers, and fostering a deeper understanding of coating chemistry. These foundational insights have proven invaluable for the subsequent chapters of this thesis. • Chapter 5 was dedicated to the 3D printing of conductive structures for applications in EMI shielding and strain sensing. We tackled two prominent challenges associated with extrusion printing: its limited printing resolution and its difficulty in creating structures with simultaneous high electrical conductivity and mechanical flexibility. Our first achievement was the development of aqueous, additive-free inks containing Ti3C2Tx and Ti3C2Tx/PEDOT:PSS, with rheological properties ideal for high-resolution extrusion 219 printing. Notably, the printed tracks of pure Ti3C2Tx and the composite ink containing 80 wt% MXene exhibited minimal spreading, almost matching the inner diameter of the extrusion nozzle, resulting in only a 25% filament spread. Furthermore, the thickness and width variation of the printed tracks were within 20% of the average values, highlighting the exceptional printing resolution we achieved. Our efforts also led to the creation of composite structures using MXenes, which displayed both high electrical conductivity and mechanical flexibility. For instance, a structure comprising 80 wt% Ti3C2Tx exhibited a conductivity of 1,600 ± 400 S/cm even after enduring 3,000 bending cycles. This knowledge was then applied to the development of flexible, lightweight EMI shields, just micrometers thick, on double-sided tapes. These printed shields could adapt to complex geometries and demonstrated remarkable EMI shielding properties. Additionally, we utilized printed grids of pure Ti3C2Tx to craft sensitive strain sensors with an impressive gauge factor of 11,300 at a 4% strain, capable of monitoring a wide range of subtle human movements, including breathing, facial muscle movements, speech, and the flexing and relaxing of bicep muscles. It's worth noting that our research marked one of the pioneering studies to employ the 3D printing of MXenes as a versatile platform for creating cost-effective multifunctional constructs. However, the EMI shields we developed exhibited reflection-dominant behavior, and the monitoring range of the 3D-printed structures was limited to subtle movements. These limitations stem from the inherent characteristics of 3D-printed laminated structures. For instance, their high conductivity leads to significant impedance mismatch, resulting in reflection-dominant properties. Moreover, the absence of internal interfaces within these structures hampers their ability to absorb electromagnetic waves 220 effectively. On the other hand, the strain-sensing capability of these sensors relies on induced fractures intentionally designed within the structures, which enhances sensitivity but narrows the monitoring range. Consequently, to achieve improved EMI shielding absorption and sensors with a broader operating range, we shifted our focus to aerogel-based conductive structures in the subsequent sections. • In Chapter 6, we introduce a novel method for creating cost-effective aerogels based on Ti3C2Tx/GO with worm-like structures featuring micro- and macro-scale porosities. Our approach leverages the interfacial jamming of nanoparticle surfactants (NPSs) to stabilize aqueous suspensions of Ti3C2Tx/GO within a nonpolar medium. This process generates filamentous liquids that serve as templates for the fabrication of ultra-lightweight aerogels. The resultant structures exhibit performance comparable to or exceeding that of some of the best available porous EMI shields. Importantly, this method eliminates the need for chemical or thermal reduction or extensive pre-processing. Interestingly, the developed aerogels offer exceptional EMI shielding effectiveness, specific EMI shielding effectiveness, and ultra-low density. Moreover, these aerogels produced highly effective EMI shields at a low filler loading of 0.3-0.7 wt%, when used as a backbone for conductive epoxy nanocomposites. While the developed aerogels initially exhibit reflection-dominant behavior (reflectance greater than absorption coefficient), we observed a significant increase in the absorbance factor due to the presence of multiscale porosities and numerous interfaces within these structures, enhancing their ability to absorb electromagnetic waves. Interestingly, we achieved an absorption-dominant regime in the case of epoxy nanocomposite EMI shields developed in this chapter. This clearly underscores the significance of these worm-like 221 structures in ushering in a new generation of EMI shields. Therefore, in the subsequent chapter, we will delve further into this class of soft materials to explore their full potential. • In Chapter 7, we embarked on the development of Janus all-liquid systems with customizable distributions of NPs using interfacial assembly and NP jamming at the liquid-liquid interface. These Janus liquids offer enhanced versatility and tailorability when compared to conventional composites. They enable precise and independent tuning of distinct functionalities on each side of the Janus architecture, resulting in the creation of multi-responsive soft materials with well-defined characteristics. The potential of these anisotropic platforms has been vividly demonstrated through proof-of-concept applications, including absorption-dominant EMI shielding, piezoresistive sensing, and human motion monitoring. Notably, the Janus EMI shields have exhibited superior performance in terms of wave absorption, boasting an impressive SET of approximately 51 dB, SER of around 0.4 dB, and an absorption coefficient (A) of 0.91. These exceptional properties arise from the high porosity spanning micro- to macro-scales and the non-interfering magnetic and conductive networks facilitated by the Janus architecture. Furthermore, we addressed longstanding issues associated with the mechanical characteristics of MXene-based aerogels through the development of Janus MXene/GO materials. This breakthrough allowed us to create Janus pressure sensors for piezoresistive sensing and human motion monitoring, featuring remarkable sensitivity and an exceptional operating range. In summary, our findings underscore that the Janus liquid concept represents a unique alternative to traditional composite, blend, and grafted system 222 fabrication approaches. It empowers the simultaneous achievement of diverse functionalities within a single structure without interference. 8.2 Analyzing the pros and cons of the developed synthetic structures in this thesis Prior to presenting recommendations for the future development of advanced synthetic structures for EMI shielding and strain/pressure sensing, it is crucial to thoroughly assess both the positive and negative attributes of the structures devised in this study. In Chapter 4, we introduced a straightforward yet versatile drop-casting method for producing highly conductive nanocomposite films of Ti3C2Tx/PEDOT:PSS. These films serve as cost-effective and lightweight EMI shields adaptable to complex geometries. However, these thin films exhibit a reflection-dominant EMI shielding mechanism, limiting their application in advanced military and medical electronics. Moving to Chapter 5, 3D printed structures of Ti3C2Tx/PEDOT:PSS were designed for EMI shielding and strain sensing applications. Noteworthy advancements include minimal material consumption, excellent mechanical characteristics, and the ability to fabricate intricate geometries. Nonetheless, the 3D printed structures also demonstrated a reflection-dominant EMI shielding mechanism and a restricted detection range for strain sensors, making them unsuitable for effectively monitoring large human motions. Chapters 6 and 7 present a novel strategy for aerogel fabrication to address the inherent challenge of synthetic EMI shields being reflection-dominant. In Chapter 6, we introduced the concept of liquid streaming, creating wormlike aerogels of Ti3C2Tx/GO with exceptional EMI shielding characteristics. These aerogels, when utilized as a backbone for conductive epoxy polymer nanocomposites, exhibited absorption-dominant EMI shielding at an extremely low filler content. 223 However, the Ti3C2Tx/GO worm-like aerogels alone lacked mechanical compressibility, limiting their application in flexible electronics and strain/pressure sensing. Chapter 7 delves into the design of Janus aerogels based on the liquid streaming concept, significantly enhancing the mechanical characteristics of the previously developed worm-like aerogels. This improvement allowed for the creation of highly sensitive and mechanically robust aerogels with an acceptable detection range. Furthermore, these aerogels displayed record-high absorption-dominant characteristics in EMI shielding. Nevertheless, further investigation is required to fully unlock the potential of Janus structures. This includes exploring how the arrangement of nanoparticles in the nanoscale improves the mechanical characteristics of these aerogels and developing a methodology to isolate single Janus filaments for studying their individual mechanical and electrical properties. 8.3 Future work recommendations In Chapter 4, we introduced a straightforward and versatile drop-casting method for the creation of highly conductive Ti3C2Tx/PEDOT:PSS laminated structures. These structures have significant potential for EMI shielding in compact and wearable electronics, and we have outlined several avenues for their future development and study. • Examining the long-term stability of Ti3C2Tx/PEDOT:PSS laminated structures under varying temperature and humidity conditions is a noteworthy aspect for future research. This is particularly crucial when comparing the stability of these composite structures with that of pure Ti3C2Tx. Moreover, assessing the chemical degradation of these structures using different chemical characterization techniques (initially designed for studying pure MXene flakes), would provide us with valuable insights. 224 • One key aspect to explore in future research is the impact of thickness variation on the EMI shielding characteristics of these laminated structures. These structures operate on the principle of impedance mismatch and the reflection of electromagnetic waves. Reflection occurs at the interfaces within these structures, and the most effective wave attenuation typically happens around a characteristic depth known as the skin depth. If a drop-casted structure maintains a consistent thickness throughout that exceeds the skin depth, it can effectively reflect electromagnetic waves. This means that uniformity of thickness throughout these structures is not necessarily a concern, as long as the minimum thickness is greater than the skin depth. Researchers can thus focus on the development of cost-effective methods for EMI shielding without worrying too much about thickness uniformity. • Another intriguing project to consider, building on the results presented in Chapter 4, is the development of conductive water-based paints for fabricating thin EMI shields. Imagine the simplicity of using a brush to create EMI shields in complex geometries, even on large objects. • Additionally, we recommend exploring the processing of Ti3C2Tx/PEDOT:PSS inks using blade coating techniques. The presence of shear forces in this processing method can help align MXene flakes, enhancing the electrical conductivity of these structures. Furthermore, PEDOT:PSS can serve as an adhesive to bond MXene flakes together, thereby improving the mechanical characteristics of the laminated structures. This innovative approach holds promise for future research in the field. 225 In Chapter 5, we harnessed additive-free inks containing Ti3C2Tx and Ti3C2Tx/PEDOT:PSS to craft EMI shields and strain sensors. In the following discussions, we outline several ideas closely aligned with this project. • One promising avenue to explore is the utilization of 3D printing for the development of multilayer structures. We propose the creation of such structures with distinct layers, where the conductivity of the top layer is significantly lower than that of the layers beneath. In this configuration, electromagnetic waves can pass through the less conductive layers with minimal reflection. When these waves encounter the highly conductive layer beneath, they reflect back into the structure and dissipate through ohmic losses. This structural design, known as gradient impedance design, has been previously reported but has not yet been fully explored through 3D printing methods, which offer advantages in material efficiency. • Another innovative approach involves the development of free-standing aerogels composed of Ti3C2Tx and Ti3C2Tx/PEDOT:PSS for EMI shielding. In this scenario, the printed structures would undergo a freezing process and then undergo freeze-drying over several days. Directional freezing could be particularly intriguing in this context. Imagine a 3D-printed structure slowly freezing from the bottom using liquid nitrogen. The formation of ice crystals could exert pressure, aligning the MXene flakes and ultimately enhancing the electrical conductivity and mechanical properties of these aerogels. This method holds promise for creating highly effective EMI shielding materials. In Chapter 6, we introduced the development of worm-like aerogels made of Ti3C2Tx/GO through the interfacial assembly of nanoparticles at the liquid-liquid interface. We believe that the following suggestions can significantly enhance this topic: 226 • It would be valuable to create worm-like structures with different filament diameters and compare their EMI shielding characteristics. Reducing the filament size can increase entanglement and the presence of macro-scale porosities. Investigating the impact of these parameters on EMI shielding is of paramount importance to gain a deeper understanding of their effects. • Consider exploring the assembly of fibrous nanostructures, such as cellulose nanofibers (CNFs), with various sizes and geometries within the realm of worm-like structures. Initial findings indicate that the combination of CNFs and GO in liquid streaming can significantly enhance the compressibility of resulting aerogels. Delving into the mechanisms and interactions behind this process is a fascinating avenue, especially when considering how fibrous nanostructures can wrap around 2D nanomaterials like GO to improve their connection and stabilize the 3D network. This research could yield valuable insights into enhancing the properties of worm-like aerogels. In Chapter 7, we presented Janus all-liquid systems, showcasing customizable distributions of NPs through interfacial assembly and NP jamming at the liquid-liquid interface. This method allows for precise and independent tuning of distinct functionalities on each side of the Janus architecture, leading to the creation of multi-responsive soft materials with well-defined characteristics. We have identified several future ideas for this project: • Additional research is necessary to fully realize the potential of Janus structures. This entails delving into the impact of nanoparticle arrangement at the nanoscale on the mechanical properties of these aerogels. Additionally, a methodology needs to be developed for isolating single Janus filaments, allowing for a detailed examination of their 227 individual mechanical and electrical attributes. Furthermore, it is crucial to comprehend how temperature and humidity influence the electrical conductivity and EMI shielding characteristics of these structures. Exploring the development of advanced temperature and humidity sensors based on Janus aerogels emerges as an intriguing avenue for future investigations. • Exploring the 3D printing of these Janus structures within a liquid environment holds significant promise. To achieve this, we suggest utilizing a viscoelastic bath that can securely hold the printed filaments in place while facilitating interfacial assembly. 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Thesis/Dissertation
eng
Mechanical Engineering
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Arjmand, Mohammad
Structural design and fabrication of electrically conductive materials for electromagnetic interference shielding and strain/pressure sensing
Text