Applied Science, Faculty of
Engineering, School of (Okanagan)
DSpace
UBCO
Tao, Li
Master of Applied Science - MASc
University of British Columbia
The outbreak of COVID-19 has led to significant demand for Personal Protective Equipment (PPE), such as masks and face shields. While these PPEs can protect people from many infectious agents, they usually are one-time use and non-degradable after disposal, causing a shortage of supply and environmental issues. While many reusable substitutes, such as reusable cloth masks, were developed, their prevention capability might be compromised after prolonged usage. As such, self-disinfecting coatings have been applied to PPEs to improve antibacterial and antiviral properties. However, commercial coating methods, such as spraying and physical vapor deposition, have several limitations for PPE applications, including poor coverage, loose adhesion, and high coating temperature. This thesis work adopted an advanced atomic layer deposition (ALD) technique to fabricate nanoscale self-disinfecting coatings that overcome the above mentioned coating approaches limitations. This work investigated the ALD fabrication, material characterizations, and antimicrobial performance of titanium oxide (TiO₂) and zinc oxide (ZnO), two typical UV-activated self-disinfecting materials. The ALD-deposited TiO₂ and ZnO thin films on the glass substrate had thickness of 81.6 ± 6.6 nm and 312.8 ± 41.3 nm, respectively. Structural analysis by X-ray diffraction, Raman spectroscopy, and X-ray photoelectron microscopy confirmed the successful deposition of TiO₂ and ZnO, most of which were amorphous. The antimicrobial properties of the ALD-TiO₂ and ZnO were evaluated against E. coli, under modified AATCC-100 standard, with commercial TiO₂ and ZnO powders as references. Data indicated that ALD-ZnO could eliminate E. coli during a 12-hour test. All the data were verified through P value test with alpha set to 0.05, and statistically proved the antimicrobial performance of the ALD-ZnO. Moreover, the nanoscale ALD-ZnO exhibited antimicrobial properties similar to commercial ZnO. On the other hand, P-value tests indicated ALD-TiO₂ coating and commercial TiO₂ powders did not generate statistically significant data that suggests antimicrobial activity toward E. coli. The different antimicrobial properties of ALD-ZnO and TiO₂ could be caused by more substantial UV absorption capability of the former than the latter, as revealed by the UV absorption test. It is expected that this work could stimulate the application of the ALD technique for applying self-disinfecting ZnO coatings for PPE.
SYNTHESIS AND CHARACTERIZATIONS OF ANTIMICROBIAL COATING BY ATOMIC LAYER DEPOSITION TECHNIQUE FOR PERSONAL PROTECTIVE EQUIPMENT APPLICATIONS by Li Tao BASc., University of British Columbia, 2020 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE COLLEGE OF GRADUATE STUDIES (Mechanical Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Okanagan) December 2022 © Li Tao, 2022 ii The following individuals certify that they have read, and recommend to the College of Graduate Studies for acceptance, a thesis/dissertation entitled: Synthesis and Characterizations of Antimicrobial Coating by Atomic Layer Deposition Technique for Personal Protective Equipment Applications. submitted by Li Tao. in partial fulfillment of the requirements of the degree of Master of Applied Science. Dr. Jian Liu, School of Engineering Supervisor Dr. Sepideh Pakpour, School of Engineering Co-supervisor Dr. Dimitry Sediako, School of Engineering Supervisory Committee Member Dr. Hadi Mohammadi, School of Engineering Supervisory Committee Member Dr. Liwei Wang, Electrical Engineering, School of Engineering University Examiner iii Abstract The outbreak of COVID-19 has led to significant demand for Personal Protective Equipment (PPE), such as masks and face shields. While these PPEs can protect people from many infectious agents, they usually are one-time use and non-degradable after disposal, causing a shortage of supply and environmental issues. While many reusable substitutes, such as reusable cloth masks, were developed, their prevention capability might be compromised after prolonged usage. As such, self-disinfecting coatings have been applied to PPEs to improve antibacterial and antiviral properties. However, commercial coating methods, such as spraying and physical vapor deposition, have several limitations for PPE applications, including poor coverage, loose adhesion, and high coating temperature. This thesis work adopted an advanced atomic layer deposition (ALD) technique to fabricate nanoscale self-disinfecting coatings that overcome the above mentioned coating approaches limitations. This work investigated the ALD fabrication, material characterizations, and antimicrobial performance of titanium oxide (TiO2) and zinc oxide (ZnO), two typical UV-activated self-disinfecting materials. The ALD-deposited TiO2 and ZnO thin films on the glass substrate had thickness of 81.6 ± 6.6 nm and 312.8 ± 41.3 nm, respectively. Structural analysis by X-ray diffraction, Raman spectroscopy, and X-ray photoelectron microscopy confirmed the successful deposition of TiO2 and ZnO, most of which were amorphous. The antimicrobial properties of the ALD-TiO2 and ZnO were evaluated against E. coli, under modified AATCC-100 standard, with commercial TiO2 and ZnO powders as references. Data indicated that ALD-ZnO could eliminate E. coli during a 12-hour test. All the data were verified through P value test with alpha set to 0.05, and statistically proved the antimicrobial performance of the ALD-ZnO. Moreover, the nanoscale ALD-ZnO exhibited antimicrobial properties similar to commercial ZnO. iv On the other hand, P-value tests indicated ALD-TiO2 coating and commercial TiO2 powders did not generate statistically significant data that suggests antimicrobial activity toward E. coli. The different antimicrobial properties of ALD-ZnO and TiO2 could be caused by more substantial UV absorption capability of the former than the latter, as revealed by the UV absorption test. It is expected that this work could stimulate the application of the ALD technique for applying self-disinfecting ZnO coatings for PPE. v Lay Summary The self-disinfecting coating on personal protective equipment (PPE) could render antimicrobial and antiviral properties, making them effective in protecting people against airborne infectious agents and reusable. This thesis introduced the application of an existing advanced coating technique, atomic layer deposition (ALD), to develop a nanoscale self-disinfecting zinc oxide coating and proved its antimicrobial performance toward a common gram-negative bacteria, E. coli. The ALD zinc oxide coating was thin (about 300 nm), uniform, and conformal on high-surface-area substrates (such as masks) and could be deposited at temperatures from 100 °C down to room temperature. Therefore, the developed ALD zinc oxide self-disinfecting coating has excellent potential for rendering PPE reusability, antimicrobial property, and better protection for people. vi Preface All the antimicrobial-related tests, such as bacteria culturing and test environment setup, were performed at the Biomedical Microbiome Research (BMR) lab at the University of British Columbia Okanagan (UBCO) with the help from Rita Lam and Dr. Kinga Vojnits. Antimicrobial performance-related experiment design and data analysis were performed under the guidance and help form Dr. Kinga Vojnits and Dr. Sepideh Pakpour. Dr. Michael C.P. Wang performed XPS characterization in this work in the 4D Labs at Simon Fraser University. Raman and XRD spectra were performed by Dr. Matthew L. Brown and Evan Hansen at UBCO. UV-Vis tests were performed by Amir Ahmadian from the Nanomaterials and Polymer Nanocomposites Laboratory (NPNL) at UBCO. SEM and EDX characterization tests were performed by Li Tao at the Fipke Laboratory for Trace Element Research (FiLTER) with assistance from Dr. Sudip Shrestha. Film thickness measurements were performed with the help of Chang Liu. All material fabrication and design-related work, including but not limited to ALD-coating synthesis, material preparation, and coating process design, were performed by Li Tao at Advanced Materials for Energy Storage Lab at UBCO, under the supervision of Dr. Jian Liu. All other works presented in this thesis were prepared by me, Li Tao, under the supervision of Dr. Jian Liu and Dr. Sepideh Pakpour. The journey of accomplishing this entire work was guided by my supervisors, Dr. Jian Liu and Dr. Sepideh Pakpour, and the committee members, Dr. Dimitry Sediako and Dr. Hadi Mohammadi. vii Table of Contents Abstract .................................................................................................................................... iii Lay Summary ............................................................................................................................ v Preface ...................................................................................................................................... vi Table of Contents .................................................................................................................... vii List of Tables ............................................................................................................................. x List of Figures .......................................................................................................................... xi Abbreviations ........................................................................................................................ xiv Acknowledgements ............................................................................................................... xvi Dedication ............................................................................................................................ xviii Introduction ........................................................................................................ 1 1.1 General background ............................................................................................... 1 1.2 Research gaps and expectations .......................................................................... 3 1.3 Thesis structure ...................................................................................................... 4 Literature Review ............................................................................................... 5 2.1 Characteristics of self-disinfecting materials ....................................................... 5 2.2 Self-disinfecting materials and working mechanisms ......................................... 5 2.2.1 Intrinsically active antimicrobial materials .................................................. 6 2.2.2 Photo-activated antimicrobial materials ...................................................... 8 2.2.2.1 Titanium dioxide (TiO2) ................................................................................. 8 2.2.2.2 Zinc oxide (ZnO) ..............................................................................................15 2.3 Applying methods of TiO2 and ZnO antimicrobial materials on substrates ......20 2.3.1 Spraying method ..........................................................................................21 2.3.2 Slurry coating methods................................................................................21 2.3.3 Chemical/Physical Vapour Deposition (CVD/PVD) method .......................23 viii 2.3.4 Limitations of current methods ...................................................................25 2.3.5 Atomic Layer Deposition (ALD) technique .................................................26 2.4 Summary ................................................................................................................28 Experimental Procedures .................................................................................30 3.1 Experimental flow design ......................................................................................30 3.2 TiO2 and ZnO coating by atomic layer deposition ...............................................31 3.2.1 TiO2 coating ..................................................................................................32 3.2.2 ZnO coating ..................................................................................................32 3.3 Structure characterizations ..................................................................................32 3.3.1 SEM and EDS ................................................................................................33 3.3.2 XPS ................................................................................................................34 3.3.3 XRD ...............................................................................................................35 3.3.4 Raman spectroscopy ...................................................................................35 3.3.5 UV-VIS ...........................................................................................................36 3.4 Antimicrobial performance evaluation .................................................................36 3.4.1 Test environment and set-up .......................................................................36 3.4.2 Test environment optimization ....................................................................38 3.4.3 AATCC-100 standard assay .........................................................................39 Preparation and Structural Characterization of ALD-TiO2 and ZnO ..............45 4.1 Preparation of TiO2 and ZnO by ALD....................................................................45 4.2 Structural characterizations of ALD-TiO2 and ZnO ..............................................48 4.2.1 Structure and chemical composition of ALD-TiO2 .....................................51 4.2.2 Structure and chemical composition of ALD-ZnO .....................................55 Antimicrobial performance evaluation ............................................................61 5.1 TiO2 antimicrobial performance against E. coli ...................................................61 ix 5.1.1 Comparison of TiO2 antimicrobial performance .........................................63 5.2 ZnO antimicrobial performance against E. coli ...................................................66 5.2.1 ZnO performance discussion ......................................................................72 Conclusions ......................................................................................................74 6.1 Results Summary ..................................................................................................74 6.2 Limitations and future perspective of this work ..................................................75 Appendices ............................................................................................................................. 76 Appendix A ......................................................................................................................76 Reference ................................................................................................................................ 79 x List of Tables Table 1 Gram-negative bacteria inhibited by TiO2 .....................................................................11 Table 2 Gram-positive bacteria inhibited by TiO2.......................................................................13 Table 3 Virus inhibited by TiO2 .................................................................................................14 Table 4 Gram-negative bacteria inhibited by ZnO .....................................................................17 Table 5 Gram-positive bacteria inhibited by ZnO .......................................................................19 Table 6 Viruses inhibited by ZnO ..............................................................................................20 Table 7 Film thicknesses of ALD-TiO2 and ZnO on glass measured by a surface profilometer..50 xi List of Figures Figure 1.1 Possible ways of infection in daily life [2]. Copyright © 2019 Elsevier B.V ................. 1 Figure 2.1 (a) Photo-activated antimicrobial materials, and (b) intrinsically antimicrobial active materials [2]. Copyright © 2019 Elsevier B.V. ............................................................................. 6 Figure 2.2 The antibacterial mechanism of silver nanoparticles [31]. Copyright © 2020 Yin et al. .................................................................................................................................................. 7 Figure 2.3 Photocatalytic reaction of TiO2 and the ROS disinfection mechanism towards microorganisms [51]. Copyright © 2014 Elsevier B.V ................................................................. 9 Figure 2.4 ZnO disinfection mechanisms [104] © Dimapilis EAS, et al used under Attribution-NonCommercial-NoDerivatives 4.0 International .......................................................................16 Figure 2.5 a) Static brush coating b) Sponge roller coating [138]. Reproduced with permission Copyright © 2007 Taylor & Francis Group .................................................................................22 Figure 2.6 A schematic diagram of the CVD coating [140]. Copyright © 2003 Elsevier B.V .......23 Figure 2.7 Schematic illustration of the physical vapor deposition process [143]. Copyright © 2018 Elsevier B.V .....................................................................................................................24 Figure 2.8 Schematic representation of ALD using self-limiting surface chemistry and an AB binary reaction sequence [144]. Copyright © 2010, American Chemical Society . .....................26 Figure 2.9 Comparison among sol-gel, chemical vapor deposition (CVD), atomic layer deposition (ALD), and physical vapor deposition (PVD) methods for film depositions [149]. Copyright © 2002 Elsevier B.V ..................................................................................................27 Figure 2.10 Example of ALD- Al2O3 films deposited on 3D Si wafers with trench structures [150]. Copyright © 1999 Elsevier B.V ........................................................................................28 Figure 3.1 Overall flow of the experiment ..................................................................................30 Figure 3.2 Atomic Layer Deposition (ALD) system. ...................................................................31 xii Figure 3.3 Biosafety Cabinet used for antimicrobial property testing in this work. ......................37 Figure 3.4 The light chamber test environment with humidity, temperature, light intensity control, monitor, and recording system. .................................................................................................38 Figure 3.5 Light chamber optimization process .........................................................................39 Figure 3.6 Logarithmic growth phase of E. coli O157 under the scanning electron microscope [153]. Copyright © 2018 Wei and Zhao .....................................................................................40 Figure 3.7 Plated Agar plates in a dark incubator ......................................................................42 Figure 3.8 Typical agar plates with E. coli colonies growing ......................................................44 Figure 4.1 One ALD cycle of TiO2 on the glass substrate by alternatively introducing tetrakis(dimethylamino)titanium (TDMAT) and water (H2O). ......................................................45 Figure 4.2 One ALD cycle of ZnO on the glass substrate by alternatively introducing diethylzinc (DEZ) and water (H2O). .............................................................................................................47 Figure 4.3 Visual comparison of a) bare glass substrate, b) ALD-TiO2 on glass, and c) ALD-ZnO on glass. ...................................................................................................................................49 Figure 4.4 (a-c) SEM images and EDX elemental mapping (d) Ti and (e) O elements of ALD-TiO2 on glass. ...........................................................................................................................51 Figure 4.5 (a) XPS survey spectrum and high-resolution XPS spectra of (b) Ti 2p, (c) O1s, and (d) C1s for ALD-TiO2 on glass. ..................................................................................................53 Figure 4.6 (a) XRD and (b) Raman spectra of commercial TiO2 nano-powders (anatase ≥ 99.9%, NanoArc), ALD-TiO2 on glass, and the bare glass substrate. ........................................54 Figure 4.7 (a-c) SEM images and EDX elemental mapping (d) Zn and (e) O elements of ALD-ZnO on glass. ...........................................................................................................................56 Figure 4.8 (a) XPS survey spectrum and high-resolution XPS spectra of (b) Zn 2p, (c) O1s, and (d) C1s for ALD-ZnO on the glass substrate. ............................................................................57 xiii Figure 4.9 (a) XRD and (b) Raman spectra of commercial ZnO powders (GRADE CR-4, Ever Zinc), ALD-ZnO on glass, and bare glass substrates. ...............................................................58 Figure 4.10 UV-visible absorbance spectra of ALD-ZnO on glass, ALD-TiO2 on glass, and bare glass. ........................................................................................................................................59 Figure 5.1 E. coli colonies counting test result of commercial TiO2 nanoparticles ......................62 Figure 5.2 E. coli colonies counting test result of ALD-TiO2 coated glass ..................................63 Figure 5.3 Photo summary of (a) the control sample and (b) the commercial ZnO powder sample. .....................................................................................................................................67 Figure 5.4 E. coli colonies counting test result of commercial ZnO powders .............................68 Figure 5.5 Photo summary of the control sample (a), untreated glass (b), and ALD-ZnO coated glass (c). ...................................................................................................................................69 Figure 5.6 Count comparison between control sample, uncoated glass sample, ALD-ZnO coated glass sample and combined box plot. ............................................................................71 Figure S1 (a-c) SEM images and EDX elemental mapping (d) Ti and (e) O elements of commercial TiO2 powders (anatase ≥99.9%, NanoArc). ..........................................................76 Figure S2 (a-c) SEM images and EDX elemental mapping (d) Zn and (e) O elements of commercial ZnO powders (GRADE CR-4, Ever Zinc). ..............................................................77 Figure S3 (a-c) SEM images and EDX elemental mapping (d) Zn and (e) O elements of ALD-ZnO on cloth. ............................................................................................................................78 xiv Abbreviations AATCC American Association of Textile Chemists and Colorists ALD Atomic Layer Deposition ATCC American Type Culture Collection BR Bacteriostatic reduction CFU Colony-forming unit COVID-19 Corona Virus Disease 19 CVD Chemical Vapour Deposition DNA Deoxyribonucleic acid DEZ Diethylzinc EC Electrochromic E.coli Escherichia coli EDX/EDS Energy-dispersive X-ray spectroscopy GPC Growth per cycle PPE Personal Protective Equipment TiO2 Titanium dioxide PCD Photocatalytic disinfection PVD Physical Vapour Deposition ROS Reactive oxygen species SCCM Standard cubic centimeters per minute SEM Scanning Electron Microscope TDMAT Tetrakis(dimethylamino)titanium UFP Ultra Fine Particles UV-VIS Ultraviolet–visible spectroscopy XPS X-ray photoelectron spectroscopy xv XRD X-ray powder diffraction ZnO Zinc Oxide xvi Acknowledgements I am most grateful to all the faculty, staff, and fellow students at UBC who offered help and encouraged me to continue this work. I want to thank my supervisors, Dr. Jian Liu and Dr. Sepideh Pakpour, for helping me work through this project, offering critical and professional guidance, providing me with deep insight into science, and guiding me through this cross-discipline project. Also, I especially want to thank Dr. Kinga Vojnits for helping me dive into the field of microbiology and offering plenty of help with experimental operations and principle understandings. It is my great honor to do research with them. Thank my committee members, Dr. Dimitry Sediako and Dr. Hadi Mohammadi, for providing valuable and critical feedback and questions throughout the research which taught me how to view the bigger image and have a more profound understanding of research. Thank you for your time and energy in my research. Dr. Sudip Shrestha and all other technicians for providing help with the SEM and EDX data collection. Thank Amir Ahmadian for helping obtain the UV-Vis data. Thank Dr. Michael C.P. Wang for assisting with the XPS characterization and data collection. Thank Dr. Matthew L. Brown and Evan Hansen for helping with the XRD and Raman characterization and data collection. Special thanks to Chang Liu for helping with the coating thickness measurement and data collection and Rita Lam for providing help with antimicrobial tests setup, operation, and data collection. Thank my fellow students and colleague, Dr. Xuejun Lu, Keqi Qu, Zhenrui Wu, Evan Hansen, Chang Liu, Amir Ahmadian, and Yue Zhang, for your friendship, support, and advice. It was a pleasure to work with you. In addition, I want to thank all the staff that has been supportive of the School of Engineering. I acknowledge the funding received from the Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Foundation for Innovation (CFI), B.C. xvii Knowledge Development Fund (BCKDF), the University of British Columbia (UBC), Peter Wall Institute for Advanced Studies, and Myant Inc. At last, special thanks to all my family members that have been supportive throughout this project, especially during the difficult time of COVID-19. Without their financial, moral, and emotional support, it would be impossible for me to continue my education journey; it is my most extraordinary luck to be part of this family. xviii Dedication This thesis is dedicated to all my family, friends, and mentors who helped me reach where I am. To the love of science and technology and the dream of helping create a different world. I appreciate all for keeping me going for all these years. 1 Introduction This chapter will introduce the importance of antimicrobial materials and their role in today’s society. It will also review current antimicrobial material preparation methods and present their major drawbacks, which lead to the need to use new antimicrobial material deposition methods, i.e., atomic layer deposition (ALD). Two typical antimicrobial materials, i.e., TiO2 and ZnO, are also introduced, along with their background and advantages. The need for new methods to produce these two metal oxide materials and the potential benefits of ALD production methods are explained. 1.1 General background Contamination caused by viruses and bacteria is always considered a significant threat to the general public's health. This is a particularly crucial problem for areas like medical service, food processing, and public transportation as they pose a high risk of spreading harmful viruses and bacteria [1] (Figure 1.1). Figure 1.1 Possible ways of infection in daily life [2]. Copyright © 2019 Elsevier B.V 2 As human health and disease control concepts started to gain more awareness among the public [3], especially with the current impact of Corona Virus Disease 19 (COVID-19), the demand for new antibacterial and disinfection techniques also dramatically increased. Traditional cleaning methods are usually conducted by wiping contaminated surfaces with liquid cleaners, which are generally time-consuming and only effective for a short period. Moreover, not only are some cleaners hard to be obtained by the general public, but they can also be toxic to people and the environment [4]. Improper usage of aggressive compounds can even increase the potential of creating a resistant bacteria and virus-favored environment that can decrease future disinfection efficiency [5]. Recent investigations focus on developing inexpensive and safe antimicrobial materials [6]. Among many newly developed antimicrobial materials, photocatalytic self-disinfecting metal oxide materials have caught great attention due to their low toxicity, long life, and good performance. Among various metal oxide materials, titanium dioxide (TiO2) and zinc oxide (ZnO) were particularly caught our interest. Both TiO2 and ZnO are chemically and physically stable, meaning they are unlikely to be affected while applied. They also have a relatively low toxic level to human where leaching of such material during use or into the environment causes much less negative impact than many other materials [7, 8]. In addition, both these materials are compatiable with different producing processes such as chemical vapor deposition (CVD) and the atomic layer deposition (ALD) processes, which provides them with flexibility during synthesis. 3 1.2 Research gaps and expectations However, commercial TiO2 and ZnO, are typically found as solid powder materials, making applications of such materials as a self-disinfecting agent more difficult than liquid chemicals such as ethanol. To ensure the functionality of these materials, they are often made into slurries or mixed with binding agents, then pasted, painted, or mixed over or within desired application surface [9-13]. The disadvantages of such applying method are apparent. Mixing metal oxide powders with other materials will reduce the contact between metal oxides and microorganisms, lowering general antimicrobial efficiency. Depending on the strength of the binding material, the coating may also be worn out over time, making the disinfecting performance much less effective. More importantly, the binding material will also likely block the microstructure of metal oxide powders, decreasing overall performance [14-16]. To overcome the abovementioned limitations, a coating method that provides enough coating binding strength, large surface area, and free microstructures is needed. Herein, nano-layer metal oxide coatings by the Atomic Layer Deposition (ALD) technique are introduced, and their characteristics and performance as photocatalytic self-disinfecting coatings will be investigated. In this MASc thesis, we aimed to create a photoactivated self-disinfecting nano-coating with low toxicity through ALD technology. We have choosen and investigated the photocatalytic antimicrobial performance of TiO2 and ZnO ALD nano-coatings. The effect of different coating materials was evaluated through various characterizations and AATCC-100, an industrial standard antimicrobial tests. This research explores the path of applying ALD technology in the antimicrobial area, especially the possibility of creating photoactivated self-disinfecting ultra-thin coatings for everyday applications. 4 1.3 Thesis structure Chapter 1 of this thesis provides some general background information regarding antimicrobial methods and materials, especially developments under the influence of the recent global pandemic caused by COVID-19. It outlines the research gap and the approach this research takes to fill the gap. Chapter 2 provides a much more comprehensive introduction to antimicrobial material and self-disinfecting mechanisms. It also introduces the synthesis of some of the common self-disinfecting materials and the fundamental of the ALD technique. Chapter 3 provides a systematic insight into this study's main experimental procedures and methods, such as the material fabrication process, characterization methods, and antimicrobial performance evaluation procedure. Chapter 4 covers the ALD deposition processes and structural characterizations of TiO2 and ZnO. Chapter 5 covers the antimicrobial test results of ALD-TiO2 and ZnO. Chapter 6 provides a comprehensive summary of current achievements, research limitations, and future suggestions for this work. 5 Literature Review This chapter first reviews the recent development of photocatalytic antimicrobial materials, their corresponding synthesis methods, and possible disinfecting mechanisms. Then this chapter compares and discusses the advantages and limitations of these materials for antimicrobial applications. After that, this chapter introduces the fundamentals and principles of the atomic layer deposition (ALD) technique and its potential in fabricating antimicrobial coating materials for the state-of-the-art progress of ALD materials’ photocatalytic process. 2.1 Characteristics of self-disinfecting materials A wide range of materials, such as heavy metals (silver and copper), metal oxides (TiO2 and ZnO), and chemical solutions (such as ethanol and hydrogen peroxide), have antimicrobial properties [17-19]. These materials have many applications, such as water disinfection, air purification, and sterilizing medical equipment. Self-disinfecting material is a special category of antimicrobial materials that kills microorganisms when activated by certain conditions such as heat and light irradiation. Their working mechanisms are usually passive and without assistance from other chemicals, making their antimicrobial process low-toxicity, non-aggressive, and continuous. These unique characteristics helped self-disinfecting materials gain more and more attention during the past decades [2, 20]. They started to be applied to daily objects such as public seats, door handles, and walls [21-25]. 2.2 Self-disinfecting materials and working mechanisms There are two main categories of self-disinfecting materials, i.e., intrinsically active antimicrobial materials, and photo-activated antimicrobial materials, depending on whether 6 activation is required or not, as schematically shown in Figure 2.1 The following sections introduce their main characteristics and working mechanisms for killing microorganisms. Figure 2.1 (a) Photo-activated antimicrobial materials, and (b) intrinsically antimicrobial active materials [2]. Copyright © 2019 Elsevier B.V 2.2.1 Intrinsically active antimicrobial materials Some materials possess intrinsic antimicrobial properties and do no require activation (such as light) because the materials by themselves have the natural ability to eliminate microorganisms. The most known intrinsically active antimicrobial materials include heavy metals (silver, copper) or polymers (triclosan). Effective mechanisms of self-disinfecting materials toward different microorganisms are usually different, and most of them are still under debate. For instance, heavy metals, such as silver and copper, are known for their antimicrobial capabilities. Silver products with various morphologies, such as nanoparticles and nano films [26], have been researched for a long time. However, when explaining its antimicrobial mechanisms, some groups proposed that the 7 antimicrobial behavior is a result of generated free radicals [27, 28], while many others suggested the inactivation of various vital enzymes by released silver ions may be the cause [29, 30]. (Figure.2.2) Figure 2.2 The antibacterial mechanism of silver nanoparticles [31]. Copyright © 2020 Yin et al Another example of a commonly used antimicrobial material is called Triclosan (C12H7Cl3O2). It can be found in toothpaste, soap, and some self-disinfecting coatings. It is widely accepted that Trislosan’s germicide activities are achieved by targeting bacteria's inner membrane and affecting multiple enoyls [32-35]. However, the performance of Triclosan was found to suffer from the paradoxical effect, meaning that once it reached a certain concentration threshold, further increase of concentration would decrease its antimicrobial efficiency. Even though the paradoxical effect was found more than 70 years ago, explanations and 8 understandings of such an effect remain relatively poor [36-38]. In addition, while being effective against bacteria such as E Coli and Salmonella species, P aeruginosa was found to be resistant to Triclosan [39, 40]. 2.2.2 Photo-activated antimicrobial materials Photo-activated, also sometimes referred to as photocatalytic, antimicrobial materials such as TiO2 and ZnO have also been extensively studied in recent decades. It is believed that when certain compounds are exposed to light that contains sufficient energy and at the correct wavelengths, reactions that generate cytotoxic species will occur and kill microorganisms [41, 42]. The unique behaviors of photocatalytic materials enable them for utilization in touchless and permanent applications. A few antimicrobial mechanisms, such as the generation of ROS during the photocatalytic process and released metal ions attacking cells, were proposed. However, the exact mechanisms of how these photocatalytic materials kill microorganisms are still unclear and under research. Among different photocatalytic antimicrobial materials, TiO2 and ZnO are the most popular photo-activated materials due to their low production cost, excellent antimicrobial ability towards pathogens, and low toxicity to humans[43]. 2.2.2.1 Titanium dioxide (TiO2) Due to its low toxicity, low cost, and good stability, titanium dioxide, sometimes referred to as Titanium (IV) or TiO2, has been widely applied in industries such as air treatment, health products, and agricultural processes [44, 45]. Its unique, organic contaminants removal property is mainly believed to be enabled by a series of photocatalytic reactions, which were first found around 100 years ago [46]. However, the actual microorganisms killing capability was not formally studied until 1985 by Matsunaga’s group [47, 48]. Recently, the number of TiO2-related 9 photocatalytic disinfection (PCD) publications has increased as its been used more and more as a photocatalytic antimicrobial material [24]. TiO2 generally can be found in three crystalline phases, i.e., anatase, brookite, and rutile [49]. For antimicrobial applications, anatase TiO2 plays a vital role since it is considered to have the best photocatalytic property among all three TiO2 phases [50, 51]. Figure 2.3 Photocatalytic reaction of TiO2 and the ROS disinfection mechanism towards microorganisms [52]. Copyright © 2014 Elsevier B.V The photocatalytic process happens when TiO2 is irradiated with light that has photon energy equal to or greater than the TiO2 bandgap (hν ≥ 3.20 eV) [53, 54]. As shown in Figure.2.3, absorption of sufficient electromagnetic radiation will create a positively charged hole in the valence band (VB) by exciting a valence band electron to the conduction band (CB) (equation 2.1). The holes can be filled by electrons from nearby molecules and become mobile when the filling process repeats [3]. When electrons and holes contact species that are absorbed on the TiO2 surface, a reduction-oxidation reaction will happen [55]. Reactive oxygen 10 species (ROS), such as •OH (equation 2.2) and O2-• (equation 2.3), are generated during this process which could create H2O2 (equation 2.4), hydroperoxyl (equation 2.5), and further hydroxyl (equation 2.6) radicals [56]. ROS like •OH and •OOH are believed to be responsible for the inactivation of microorganisms by degrading them into H2O and CO2 through reactions (equation 2.7) [57]. TiO2 + hv → TiO2 + e- +h+ (2.1) h+ + H2O → •OH + H+ (2.2) e- + O2 → O2-• (2.3) •OH + •OH → H2O2 (2.4) O2-• + H+ → •OOH (2.5) O2-• + H2O2 → •OH + OH- + O2 (2.6) Microorganisms + ROS → Inactivation + H2O + CO2 (2.7) It is widely believed that the TiO2-generated ROS act as a strong oxidants for many microorganisms [58]. A few research groups found damage to cell DNA and considered it the cause of cell death [59]. However, the damages are more likely to be an event after the damage to cell structure, cell death, and leakage of cell content [60]. Until this thesis is written, TiO2 has proven effective in killing various virus and bacteria groups through the PCD process. Some bacterias and virus that are reported to be eliminated by TiO2, are listed in Table.2.1, Table.2.2, and Table.2.3. The bacterias are categorized into two typical bacterial groups based on their peptidoglycan thickness : Gram-positive and Gram-negative. The corresponding TiO2 synthesis methods or material used, such as electrospun, sol-gel, and bio- 11 synthesis, are also included. However, it is worth mentioning that the volume of profound studies regarding TiO2 disinfection performance under complicated virus/bacteria environments remains low, and some of the proposed antimicrobial behavior and mechanisms are contradictory. Table 1 Gram-negative bacteria inhibited by TiO2 Bacteria TiO2 antimicrobial materials Ref. Year Acinetobacter TiO2 suspension [61] 2006 Escherichia coli Biosynthesized TiO2 [62] 2020 Escherichia coli Sigma-Aldrich P25 TiO2 [63] 2020 Salmonella enterica S. enterica/ M. luteus mixture Escherichia coli Lactobacillus acidophilus TiO2 [64] 2020 Shigella spp. Klebsiella spp. Pseudomonas spp. Proteus spp. Pseudomonas aeruginosa Electrospun TiO2 nanofibers [65] 2020 Escherichia coli Microwave-assisted hydrothermal TiO2 (phases, anatase, brookite) [66] 2020 Escherichia coli (ATTCC® 11775™) TiO2 (E171) [67] 2020 Pseudomonas aeruginosa TiO2 nanoparticles from peel extract of Citrus aurantium fruit [68] 2020 Klebsiella pneumonia TiO2 nanoparticles from peel extract of Citrus aurantium fruit [68] 2020 Pseudomonas aeruginosa Sea water treated TiO2 nanoparticle [69] 2020 Escherichia coli (ATCC 11775) TiO2 E171 [70] 2020 Anabaena TiO2-coated hollow Pyrex glass beads [71] 2005 12 Bacteria TiO2 antimicrobial materials Ref. Year Bacteroides fragilis, strain 638R TiO2 on orthopedic implants [72] 2007 Escherichia coli, strain ATCC 25922 Staphylococcus aureus, strain ATCC 25923 coliform bacteria TiO2 suspension [73] 1995 Edwardsiella tarda TiO2 coated glass slides [74] 2008 Salmonella enterica var. Enteridis (ATCC 13076) NM101 and NM105 TiO2 suspensions [75] 2019 Escherichia coli (ATCC 25922) NM101 and NM105 TiO2 suspensions [75] 2019 Staphylococcus aureus (ATCC 29247) Bacillus cereus (ATCC 29247) Escherichia coli (ATCC 8739) TiO2-coated glass UV photocatalytic [76] 2013 Serratia marcescens (ATCC 13880) multidrug-resistant Acinetobacter baumannii (MDRA) multidrug-resistant Pseudomonas aeruginosa (MDRP) NDM-1 producing K. pneumonia (BAA-2146) Proteus vulgaris (MTCC No: 1771) Sol-gel prepared TiO2 [77] 2015 Pseudomonas aeruginosa (MTCC No: 424) Escherchia coli Cannabis sativa leaves extract TiO2 nanoparticles [78] 2021 Escherichia coli (ATCC 25922) Commercial Aeroxide P25 TiO2 nanoparticles [79] 2021 Escherichia coli (ATCC 25922) Polyacrylonitrile and TiO2 composite electrospun nanofibers [80] 2021 Citrobacter freundii Azadirachta indica twigs, Ficus benghalensis and Syzygium aromaticum synthesized TiO2 nanoparticle [81] 2020 13 Table 2 Gram-positive bacteria inhibited by TiO2 Bacteria TiO2 antimicrobial materials Ref. Year Micrococcus Sp. Biosynthesized TiO2 [62] 2020 Staphylococcus aureus Biosynthesized TiO2 nanoparticle [62] 2020 Micrococcus luteus Sigma-Aldrich P25 TiO2 [63] 2020 E. coli/ M. luteus mixture Sigma-Aldrich P25 TiO2 [63] 2020 methicillin-resistant Staphylococcus aureus Electrospun TiO2 nanofibers [65] 2020 Staphylococcus aureus Microwave-assisted hydrothermal TiO2 (phases, anatase, brookite) [66] 2020 Lactobacillus rhamnosus GG (ATTCC® 53103™) TiO2 (E171) [67] 2020 Staphylococcus epidermidis TiO2 nanoparticles from peel extract of Citrus aurantium fruit [68] 2020 Staphylococcus aureus Staphylococcus aureus Sea water treated TiO2 nanoparticle [69] 2020 Lactobacillus rhamnosus GG (ATCC 53103) TiO2 E171 [70] 2020 Bacillus subtilis PS533 TiO2-based cast film [82] 2020 Lactobacillus casei (ATCC 393) NM101 and NM105 TiO2 suspensions [75] 2019 Lactobacillus delbrueckii subsp. bulgaricus (ATCC 11842), Lactobacillus lactis subsp. lactis (ATCC 19435) Lactobacillus acidophilus (ATCC 4356) methicillin-resistant Staphylococcus aureus (MRSA) TiO2-coated glass UV photocatalytic [76] 2013 vancomycin-resistant Enterococcus spp. (VRE) penicillin-resistant Streptococcus pneumoniae (PRSP) Bacillus subtilis (MTCC No: 10619) Sol-gel prepared TiO2 [77] 2015 Streptococcus pneumonia (MTCC No: 2672) Staphylococcus aureus (MTCC No: 96) 14 Bacteria TiO2 antimicrobial materials Ref. Year Enterococcus faecalis (MTCC 439) Reactive plasma synthesized nano-sized TiO2 powder with 365nm UV [83] 2013 Staphylococcus aureus Cannabis sativa leaves extract TiO2 nanoparticles [78] 2021 Bacillus subtilis Staphylococcus Aureus (ATCC 25923) Commercial Aeroxide P25 TiO2 nanoparticles [79] 2021 Streptococcus Pyogenes (ATCC 19615) Bacillus sp. Polyacrylonitrile and TiO2 composites electrospun nanofibers [80] 2021 Streptococcus mutans Azadirachta indica twigs, Ficus benghalensis and Syzygium aromaticum synthesized TiO2 nanoparticle [81] 2020 Table 3 Virus inhibited by TiO2 Virus TiO2 antimicrobial materials Ref. Year Influenza virus (IFV) [76] 2013 Norovirus (NoV) HSV-1 (Herpes simplex virus) TiO2 thin films on glass by plasma enhanced chemical vapour deposition (PECVD) [84] 2007 Bovine leukemia virus protein gp51 Semiconducting TiO2 layer [85] 2018 Influenza A TiO2 ~DNA nanocomposites [86] 2015 hepatitis C virus infection nanosized TiO2 [87] 2021 SARS-CoV-2 Newcastle virus (NDV) Titanium dioxide nano-colloids [88] 2019 Murine norovirus (MNV-1) CW1 UV-assisted TiO2 photocatalytic reactor [89] 2017 T4 TiO2 thin films [90] 2008 SARS coronavirus Photocatalytic titanium apatite filter (PTAF) [91] 2004 Vaccinia TiO2 film [92] 2010 Influenza A/H3N2 MS2 bacteriophage Phase-mixed TiO2 nanoparticles [93] 2021 Murine norovirus Influenza virus Zika virus vector Aedes aegypti Azadirachta indica twigs, Ficus benghalensis and Syzygium aromaticum synthesized TiO2 nanoparticle [81] 2020 PRD1 bench-scale and pilot-scale TiO2 [94] 2008 15 2.2.2.2 Zinc oxide (ZnO) Zinc oxide (ZnO) is another very widely used and commonly seen photocatalyst metal oxide material. Due to its relatively low price, stable chemical properties, high adsorption, and good catalytic efficiency, ZnO has been extensively used as a pollution removal agent, skin care product additive, and germicide additive [95-97]. When comparing ZnO with TiO2 regarding their photocatalytic properties, It can be found that they have similar bandgaps (3.37 eV for ZnO, 3.20 eV for TiO2) [98]. While TiO2 can mainly only be activated by UV-A lights with a wavelength of 315~400 nm, ZnO can be activated by irradiation of both UV-A and UV-B light, which has a wavelength of 280~400 nm [96]. As a result, ZnO has sometimes been considered a better photocatalytic organic pollutants degradation material than TiO2 [99-101]. The actual photocatalytic process of ZnO is similar to the TiO2 photoactivation process. When ZnO is irradiated by light with proper wavelength and contains higher energy than ZnO’s band gap, holes (h+) and electrons (e-) will be generated in the (VB) and conduction band (CB) (equation 2.8) [102]. These holes may recombine or migrates to the surface and start to produce hydroxyl radicals by reacting with adsorbed water and/or hydroxide ions. (equations 2.9 and 2.10). Electrons in the conduction band will react with oxygen and produce superoxide radical anion (equation 2.11), which then produces hydrogen peroxide (equation 2.12) [103]. Superoxide radicals will then react with hydrogen peroxide and form hydroxyl radicals (equation 2.13). ROS generated during or at the end of these reaction processes, such as hydroxyl radicals (• OH.), will then act over microorganisms adsorb on the surface of ZnO and convert them into intermediates which eventually will be converted into harmless matters such as CO2, H2O (equation 2.14). 16 ZnO + hv → ZnO + e- +h+ (2.8) h+ + H2O → •OH + H+ (2.9) h+ + OH- → •OH (2.10) e- + O2 → O2-• (2.11) O2-• + H+ + HO2• → H2O2 + O2 (2.12) O2-• + H2O2 → •OH + OH- + O2 (2.13) Microorganisms + ROS → Inactivation + H2O + CO2 (2.14) Figure 2.4 ZnO disinfection mechanisms [104] © Dimapilis EAS, et al used under Attribution-NonCommercial-NoDerivatives 4.0 International Similar to the antimicrobial process of TiO2, many different mechanisms have been proposed to explain the antimicrobial behavior of ZnO. Some of the antimicrobial mechanisms, 17 including the ROS reaction with organic elements, damage to the membrane cell wall due to Zinc ion adhesion, and membrane cell wall penetration caused by ROS and zinc ions [105] [106], have been reported and widely accepted (Figure 2.4). Synthesis methods and targeting microorganisms of ZnO material share some similarities with TiO2 materials. Some of these investigated microorganisms are listed in the following tables and categorized into Gram-positive bacteria, Gram-negative bacteria, and viruses (Table.2.4, Table.2.5, and Table.2.6). Material or specific synthesis methods used for killing each microorganism, such as hydrothermal ZnO nanotubes, ZnO nanorods, and green synthesized ZnO nanopowder are also included. Up to the date this thesis was written, there are still no definitive explanations for ZnO’s antimicrobial behavior because of the difficulty of evaluating the effect of each mechanism, as they may co-occur [107]. Table 4 Gram-negative bacteria inhibited by ZnO Bacteria Zn antimicrobial materials Ref. Year Escherichia coli DH5α Aqueous solution produced ZnO nanoparticles [108] 2006 E. coli O111 ZnO nanoparticles [109] 2011 E. coli 1313 ZnO nanoparticles [110] 2009 E. coli O157:H7 ZnO nanoparticles [111] 2009 Salmonella typhimurium Uncapped ZnO Small Nanorods Long Nanorods [112] 2013 Proteus vulgaris Klebsiella pneumoniae Enterobacter aerogenes ZnO nanorods [113] 2013 Aerobacter aerogenes Escherichia coli Nanorods Nanospheres Nanoflower [114] 2013 18 Bacteria Zn antimicrobial materials Ref. Year Pseudomonas aeruginosa Cauliflower shape nano ZnO [115] 2015 Proteus mirabilis Escherichia coli ZnO micro-flowers [116] 2010 Salmonella typhimurium Klebsiella pneumoniae Escherichia coli ZnO nano-flowers [117] 2013 Escherichia coli Commercial ZnO nanopowder [118] 2012 Pseudomonas aeruginosa Klebsiella pneumoniae Proteus vulgaris Acinetobacter sp. Hydrothermal-produced ZnO nanotubes [119] 2015 Escherichia coli Klebsiella pneumonia Proteus mirabilis Pseudomonas aeruginosa Salmonella sp. Escherichia coli ZnO nanoparticles and quantum dots [120] 2014 Escherichia coli ZnO thin film [121] 2014 Pseudomonas aeruginosa Epigallocatechin gallate (EGCG) assisted green synthesis ZnO nanopowder [122] 2015 Klebsiella aerogenes Escherichia coli Klebsiella aerogenes Cassiafistula plant extract synthesized ZnO nanoparticles [123] 2015 Escherichia coli Pseudomonas desmolyticum Salmonella typhimurium Hydrothermal ZnO nanoparticles [124] 2014 19 Table 5 Gram-positive bacteria inhibited by ZnO Bacteria Zn antimicrobial materials Ref. Year Bacillus subtilis ZnO nanorods [113] 2013 Staphylococcus aureus Staphylococcus aureus ZnO nanorods [112] 2013 Staphylococcus epidermidis Nanorods Nanospheres Nanoflower [114] 2013 Staphylococcus aureus Cauliflower shape nano ZnO [115] 2015 Bacillus subtilis Staphylococcus aureus ZnO micro-flowers [116] 2010 Staphylococcus aureus ZnO nano-flowers [117] 2013 Bacillus subtilus Commercial ZnO nanopowder [118] 2012 Bacillus megaterium Staphylococcus aureus Sarcina lutea Bacillus sp. ZnO nanotubes [119] 2015 Micrococcus spp. Staphylococcus aureus Staphylococcus epidermidis Streptococcus pneumonia MRSA Staphylococcus aureus Epigallocatechin gallate (EGCG) assisted green synthesis ZnO nanopowder [122] 2015 Listeria monocytogenes Commercial ZnO nanoparticles [125] 2013 Staphylococcus aureus Bio-synthesized ZnO nanoparticles with curry leaf extracts [126] 2013 Bacillus subtilis Staphyloccus aureus 20 Table 6 Viruses inhibited by ZnO Virus Note Ref. Year Herpes simplex virus type 1 (HSV-1) Polyethylene glycol (PEG)-coated ZnO nanoparticle [127] 2018 Herpes simplex virus type 2 (HSV-2) Influenza virus (H1N1) Coronavirus (COVID-19) Berberine (BER)/ZnO complex [128] 2021 Bacteriophage MS2 virus (ATCC 15597-B1) ZnO nano-spines sprouted activated-carbon nanofibers [129] 2022 Herpes simplex virus type-2 (HSV-2) Zinc oxide tetrapods (ZnOTs) [130] 2012 Herpes simplex virus type-1 (HSV-1) ZnO micro–nano structures [131] 2011 Human cytomegalovirus (HCMV) Polysaccharide encapsulated ZnO nanoparticle [132] 2022 Tobacco mosaic virus (TMV) ZnO nanoparticles [133] 2020 2.3 Applying methods of TiO2 and ZnO antimicrobial materials on substrates TiO2 and ZnO can be synthesized through different methods and produce very different resulting materials. Although their chemical composition may be the same, different synthesis processes will likely create different morphologies and generates different physical or chemical properties. Because of the difference in material properties, these materials need to be applied through different methods to make proper usage and benefit from their antimicrobial characteristics. Because ZnO and TiO2 have similar properties, such as chemical stability, low toxicity, and photocatalytic capability, they can often be applied using similar methods. A few typical application methods, such as liquid spray, slurry coating, and vapor deposition methods, are reviewed. 21 2.3.1 Spraying method A common spray method for applying both TiO2 and ZnO to surfaces is the suspension spraying method. A suspension is a form of a mixture in which particles are dispersed in fluid instead of dissolved. It can be obtained by mixing TiO2 or ZnO powders, usually Ultra Fine Particles (UFP), into liquid such as water and creating suspension mixtures. These suspensions can then be applied to surfaces such as bed sheets, masks, and gloves through conventional spray-coating methods such as air spray [134]. Another spray method, called the cold spray method, can also create desired coatings on solid surfaces such as steel, aluminum, and stainless steel. The cold spray works by injecting fine particles, in this case, TiO2 or ZnO particles, into a stream of hight-speed gas and accelerating through a nozzle, then hitting a suitable substrate such as steel. The particles are expected to deform and create a bond with the substrate when they impact the surface at high speed [135]. However, the drawbacks of these spray methods are also apparent. For instance, it is tough for the suspension spray method to deposit the coating evenly, and it can be challenging to use such methods to apply on smooth surfaces such as glass. As for the cold spray method, it is an aggressive process that requires the substrate to have strong mechanical properties. The bonding quality of the cold sprayed coatings also highly varied depending on substrate hardness, temperature, and surface chemicals. Moreover, creating these spray coatings generates lots of free-floating UFP, which can be dangerous when humans are exposed to it [136]. 2.3.2 Slurry coating methods Another way to apply powder materials like TiO2 and ZnO is to prepare them into slurries and then paste the slurry to desired substrates. This method is preferable for creating thick 22 coatings [137] and has wide application in different products such a paint, glue, and makeup. Slurries can be made by mixing TiO2 or ZnO powders with binding solutions such as polyesters and polyurethanes to desired viscous and applied to surfaces through different methods. Two of the commonly used methods are the static brushing and sponge roller methods (Figure 2.5). The static brushing method injects slurry into a fixed brush at a designed rate while the substrate moves under the brush, and the sponge roller method essentially replaces the brush with a roller to enable better coverage [138]. Figure 2.5 a) Static brushing method b) Sponge roller method [138]. Copyright © 2007 Taylor & Francis Group While slurry coating methods such as brushing can produce thick and durable coatings and have lower requirements for substrates, the disadvantage of applying slurries are also apparent. For instance, the thickness of the coating can often be hard to control, and the final coatings are often thicker than coatings created by other methods, such as spray coating. The thick coating also has low flexibility after solidification, making it challenging to apply on soft 23 surfaces such as cloth. In addition, slurries can be hard to be used to complex geometries, which lowers their coverage and narrows their applications [138]. 2.3.3 Chemical/Physical Vapour Deposition (CVD/PVD) method Chemical Vapour Deposition (CVD) is one of the most flexible and simple modern techniques that enables the deposition of materials as a thin solid film on substrates [139]. As illustrated in Figure 2.6, the process initiates by passing gaseous materials into a reacting chamber, which is heated to the necessary temperature. Inside the chamber, triggered by conditions such as temperature gradient, a reaction will happen on or near the substrate surface and form a coating layer. Unreacted reactive gas will exit the reacting chamber with the help of carrying gas, positive pressure, or, sometimes, the use of a vacuum pump. The CVD process can produce high purity coatings with sufficient structural control and coverage [140, 141]. Figure 2.6 A schematic diagram of the CVD coating [140]. Copyright © 2003 Elsevier B.V Physical Vapour Deposition (PVD) is another widely used thin film and surface coating technique [142]. As the example of sputtering PVD indicated in Figure 2.7, the process starts by dislodging desired deposition material’s molecules through physical methods such as gaseous ions impact (plasma). The sputtered molecules will then contact and condense on the 24 substrate's surface, which eventually forms a layer of coating [143]. Due to promising flexibility, low cost, and good corrosion resistance, PVD coatings have various industrial applications, such as car parts coating, hardware coating, and food packing. Figure 2.7 Schematic illustration of the physical vapor deposition process [143]. Copyright © 2018 Elsevier B.V While both CVD and PVD are among some of the most promising coating techniques, they also have several disadvantages. For instance, the CVD and PVD method has relatively low control over coating thickness because there is no direct way of precisely controlling the deposition process or monitoring coating thickness. As a result, the deposition of ultra-thin, nano-scale coatings remains difficult for these techniques. In addition, both of these methods have limited surface coverage capability and uniformity control. For example, the PVD method has a limited ability to cover surfaces with complex geometries, and the CVD method often creates uneven coatings over different orientated surfaces. 25 2.3.4 Limitations of current methods Although most of the methods mentioned above are widely accepted and studied, they all have their significant drawbacks. For instance, spray and slurry methods are mainly applied through traditional methods such as brushing and spraying, which have low control over the coating quality and limited coating efficiency. In comparison, CVD and PVD methods have a much-improved coating quality and better-controlled coating process, but coating coverage and evenness are limited when facing complicated surfaces. A list of significant disadvantages of the above-mentioned methods are listed below: • The coating has low-quality control • Poor coating coverage on the whole surface of substrates • Unable to fully cover complicated geometries and surfaces • pollutive process of producing coatings • Coating material has limited efficiency (i.e., photocatalytic effect) • Unable to create nano-level coating with good evenness These disadvantages limited not only the efficiency of coating material but also limited applications of coatings. To overcome these difficulties, new methods of deposition coatings are needed. 26 2.3.5 Atomic Layer Deposition (ALD) technique Figure 2.8 Schematic representation of ALD using self-limiting surface chemistry and an AB binary reaction sequence [144]. Copyright © 2010, American Chemical Society Introduced by Suntola in 1977, Atomic Layer Deposition (ALD) is a thin film deposition method based on the sequential use of self-limiting surface reactions, which could produce ultra-thin, highly uniform, and conformal films [145] [146]. Different from other vapor deposition methods, the ALD process introduces precursor vapors into the reaction chamber alternatively, one at a time, separated by purging periods [147] [148]. Each precursor exposure saturates the substrate surface, resulting in the self-limiting film growth by ALD. The film growth by ALD takes place in a cyclic manner. In a simple binary case (as shown in Figure 2.8), one ALD cycle consists of four steps: 1) exposure of the first precursor (A) to the substrate surface until the surface reaction is complete; 2) purge of the reaction chamber to remove byproducts and unreacted precursor A; 3) exposure of the second precursor (B) to the sample surface, and 4) purge of the reaction chamber. The thickness of thin films deposited by one ALD cycle is consistant. Therefore, the targeted thin film thickness can be precisely controlled by repeating different ALD cycles. 27 Figure 2.9 Comparison among sol-gel, chemical vapor deposition (CVD), atomic layer deposition (ALD), and physical vapor deposition (PVD) methods for film depositions [149]. Copyright © 2002 Elsevier B.V ALD offers many advantages over other film deposition methods in preparing uniform and conformal coatings on complex or temperature-sensitive substrates. Firstly, among available deposition techniques, ALD can achieve the highest film coverage on 3D substrates, even high-aspect-ratio structures (Figure 2.9). Figure 2.10. shows examples of uniform Al2O3 films deposited by ALD on 3D Si wafers with trench structure. In contrast, the physical vapor deposition (PVD) method has poor step coverage on 3D substrates due to its line-of-sight deposition nature. Secondly, ALD can achieve a uniform and conformal coating of the thin films on 3D substrates. The precursor vapor could diffuse into the whole surface, and the self-limiting surface reactions ensure the film thickness is the same in each ALD cycle. In comparison, sol-gel or chemical vapor deposition (CVD) methods have less control over the film thickness at the nanoscale level. The deposited thin films end up non-uniform across the whole substrate surface. Thirdly, ALD is featured with low-temperature deposition, usually from room temperature to 350 °C, making it suitable for coating thin films on temperature-sensitive substrates, such as polymers and fabrics used in PPE. Last but not least, because the ALD 28 technique is essentially a special chemical deposition process, it is very easy to scale up its production scale to fit industrial needs. Also, due to its fine control over film thickness and capability of producing ultra-thin films, the consumption of reaction chemicals is low, making it a more material efficient film deposition method. Therefore, ALD has great potential for applying nanoscale antimicrobial material coatings on PPE. Figure 2.10 Example of ALD- Al2O3 films deposited on 3D Si wafers with trench structures [150]. Copyright © 1999 Elsevier B.V 2.4 Summary To satisfy the need of self-disinfecting coatings, the newly developed self-disinfecting coatings should have the ability to enable high coverage over complicated structures, good antimicrobial capability, and enough durability. In this section, recent developments of TiO2 and ZnO antimicrobial properties, synthesis techniques and their disadvantages, and advantages of the ALD technique are provided. Based on provided summaries, selected metal oxides: TiO2 and ZnO, are considered superior candidates to be fabricated into self-disinfecting coatings. Not only have they demonstrated promising antimicrobial properties towards various types of bacteria and viruses, 29 but they also can be synthesized through different methods with different morphologies. However, the antimicrobial performance of most commercially available TIO2 and ZnO are greatly limited by various disadvantages, such as unevenness of material, small surface area, and difficulty in application. These disadvantages can be mostly avoided with the introduction of the ALD technique. Therefore, the ALD technique seems promising for producing high efficiency, high coverage, and ultra-thin ZnO or TiO2 antimicrobial self-disinfecting coatings. 30 Experimental Procedures This chapter first details the synthesis process for the ALD-TiO2 and ZnO coatings and related characterization methods. Then it introduces antimicrobial performance evaluation methods, including their operation procedure, material used, and standards. 3.1 Experimental flow design The general experimental and optimization processes can be divided into two sections, i.e., ALD fabrication and antimicrobial performance evaluation. The synthesis and characterization of ALD TiO2 and ZnO coatings will be introduced in the ALD fabrication section. The antimicrobial evaluation section will explain how the antimicrobial properties of metal oxides are evaluation and their corresponding testing environment setups. Figure 3.1 Overall flow of the experiment As shown in Figure 3.1, the material fabrication and antimicrobial test processes work alternately. The antimicrobial materials will be fabricated first, then proceed for materials structural characterization and antimicrobial testing. Data from antimicrobial tests and material characterizations will be analyzed and compared to make further decisions on optimizing 31 antimicrobial materials by ALD processes. The optimization loop will complete once the antimicrobial performance of the metal oxides is satisfactory. When the experimental process has been established and optimized, such a procedure will be repeated about three times to obtain enough data for a summary. 3.2 TiO2 and ZnO coating by atomic layer deposition The deposition of TiO2 and ZnO was performed on a plasma-enhanced atomic layer deposition system (GEMStar XT-R, Arridance, USA), as shown in Figure 3.2. The ALD system provides a precisely controlled reacting environment with many adjustable parameters such as operating temperature, reacting time, and chemical inject sequence. By installing different chemical materials and adjusting parameter settings, different kinds of coatings with different thicknesses and phases can be created through this ALD system. Figure 3.2 Atomic Layer Deposition (ALD) system 32 3.2.1 TiO2 coating Glass pieces (Thermo Scientific cover, 22 mm x 22 mm, by width and length, and 0.18~0.25 mm thickness) from VWR category CA48367-139 were purchased and used as coating substrates for ALD-TiO2 to be deposited on,. Tetrakis(dimethylamino)titanium (TDMAT), and water (H2O) were loaded into the ALD system as the metal precursor and oxidizer, respectively. The coating process was performed at 150 °C by alternatively introducing TDMAT and H2O into the ALD reaction chamber, where the glass substrates were placed. The thickness of the coating will later be measured, and coating cycles will be adjusted based on the thickness level. 3.2.2 ZnO coating The fabrication of ZnO coating also started by placing glass substrates (Thermo Scientific, CA48367-139) into the ALD reacting chamber. Diethylzinc (DEZ, ≥96%, VWR) and H2O were loaded into the ALD system as the metal precursor and oxidizer, respectively. The coating process was performed by alternatively introducing DEZ and H2O into the ALD reaction chamber at 150°C. Similarly, the thickness of ZnO coating was controlled by measuring coating thickness and adjusting ALD cycles. 3.3 Structure characterizations The physical and chemical properties of ALD-TiO2 and ZnO coatings were investigated using various material characterization methods below. Commercial TiO2 (anatase ≥99.9%, NanoArc) and ZnO (GRADE CR-4, Ever Zinc) powders were included for comparison. The morphologies of all the samples were observed by scanning electron microscopy (SEM), and the elemental distribution was analyzed by Energy dispersive X-ray (EDS) spectroscopy. X-ray 33 photoelectron spectroscopy (XPS) was applied to examine the elemental composition and chemical bonding on the surface of ALD-TiO2 and ZnO. X-ray diffraction (XRD) and Raman spectroscopy were used to analyze the phases and crystallinity of ALD-TiO2 and ZnO compared to commercial TiO2 and ZnO powders. Ultraviolet-visible spectroscopy (UV-VIS) was adopted to evaluate light absorption at different wavelengths and estimate the potential light wavelength for the photocatalytic reaction of ALD-TiO2 and ZnO. The fundamental of these characterizations and their functions in this thesis is briefly introduced below. 3.3.1 SEM and EDS SEM is one of the most common methods that are widely used in today’s research and industry areas to observe the morphology of the samples. Like many other electron microscopies, SEM shines the sample with a beam of electrons. When these electrons hit the sample surface, they interact with the sample atom and produce various signals. In most cases, the secondary electron will be collected by an Everhart-Thornley detector, also known as a secondary electron detector. A computer will process the detected secondary electron position and intensity, which then be used to produce an image of material topography. EDS is a test that usually comes along with the SEM equipment. Although it is always done on the same machine as SEM, it uses a different detector from the secondary electron detector. The EDS detector detects the sample emitted X-rays after the sample has been excited by a beam of electrons. The EDS is often used for sample elemental analysis and chemical characterization. It is a relatively straightforward method and works well on most materials. Although EDS is often used for detecting elements in the bulk sample, its accuracy is limited. 34 3.3.2 XPS XPS is an effective material characterization method mainly used to identify surface chemical composition. The detection depth of XPS is limited to the surface or near-surface, as a result, it can avoid most interference from materials’ deeper layers. Therefore, XPS is particularly useful for indentifying composition of thin coatings or surfaces and capabile of providing results with higher accuracy when compared with EDX. The principle of XPS uses the photoelectric effect, which was discovered in 1887. In 1907, the first XPS spectrum was recorded with a Röntgen tube, Helmholtz coils, a magnetic field hemisphere, and photographic plates [151]. The key to this technology is the detection of ejected electrons from a limited depth of the sample. The sample is often irradiated by a bean of X-rays, such as Al Kα or Mg Kα sources, and results in an escape of electrons from sample surface atoms. The number of escaped electrons and their kinetic energy is different between chemicals. A detector will measure this information simultaneously, and after calculating the electron intensity and binding energy, an XPS spectrum is generated. There are a few application differences between the EDS and the XPS methods. One of the main differences is their accuracy. In most cases, the EDS method is good enough to determine the existence of an element; however, its accuracy is not high sufficient for detailed studies or elements with low concentrations. In comparison, the XPS method has much higher accuracy, ranging from the parts per thousand range to the parts per million range (ppm). Higher accuracy enables the detection of lower concentrated materials, which can be very beneficial when studying nano-scaled material modifications. Another advantage is the XPS method only detects material composition at a surface level (about 10-200 Å). This property is desired when studying the properties of ultra-thin coatings because it eliminates undesired data 35 from a deeper level of target materials. Last but not least, the XPS method can also detect the material chemical bonding conditions. While two elements can exist on one piece of material, they can be in many bonding conditions, resulting in many different chemical and physical properties. 3.3.3 XRD XRD is a common rapid method often used to determine the phase and crystallinity of the targeted materials. X-rays will need to be generated by a cathode ray tube and pass through a filter to produce monochromatic X-rays. The monochromatic X-rays will then be directed toward the sample and interact with the target crystalline material. During this process, diffracted rays will be generated, and a detector will collect these diffracted rays. Information will be collected when conditions satisfy Bragg's Law [152]: nλ = 2d sin θ (3.1) Where n is an integer, d indicates the distance between the atomic layers, λ represents the wavelength of the incident X-ray beam, and θ stands for the angles of incidence. Collected information will then be processed, generating a spectrum with diffraction peaks. The diffraction peaks can be converted into the material d-spacing, which, when compared to a standard reference database, determines the crystalline phase of a material. 3.3.4 Raman spectroscopy Raman spectroscopy is commonly used to provide material chemical structure information and molecule identifications. This non-destructive test is performed by shining monochromatic light, generally from a beam of laser, to the sample. Interactions between the 36 laser light and sample molecular vibrations will cause the energy of laser photons to shift. A lens will collect emitted electromagnetic radiation from where the laser shines at the sample and process it through multiple filters. Results were plotted on an X-Y axis where Raman shift will be on the X axis and intensity will be on the Y axis. 3.3.5 UV-VIS UV-VIS is a relatively simple and inexpensive quantitative method to measure a substance’s absorption of light. This is achieved by shining a beam of light on the sample and measuring the intensity of light that passes through. A reference sample or simply the blank sample will also be tested at the same time to provide comparisons. The detector will measure the intensity of passed-through light under different wavelengths and plot a diagram where the relationship between intensity and wavelength are linked. In most cases, the UV-VIS test can show how much light and within what wavelength will the sample material absorb. 3.4 Antimicrobial performance evaluation To assess the degree of antimicrobial activity of each material, the AATCC (American Association of Textile Chemists and Colorists)-100 standard, which has become the industry standard for fabric antimicrobial tests, was followed with some modifications. 3.4.1 Test environment and set-up The microorganism-related operations, such as bacteria culturing, planting, and diluting, were performed in a Biosafety Cabinet (Thermo Scientific 1300 series A2) (Figure 3.3). 37 Figure 3.3 Biosafety Cabinet used for antimicrobial property testing in this work The light-irradiation and antimicrobial processes were performed by placing the prepared samples inside a custom-made light chamber (Figure 3.4). Glass panels of this chamber are Electrochromic (EC) smart windows glass containing electrochromic coatings that can adjust the light passing rate by the tint control switch. A xenon arc high-power lamp (Sciencetech) can generate a light spectrum similar to natural sunlight, simulating solar irradiations. Actual light passing into the chamber is controlled by changing the tint levels of the glass. A program-controlled humidifier was used to control the relative humidity level inside the chamber, and data were monitored by an environment monitoring sensor (HOBO ® MX Analog/Temp/RH/Light (MX1104)). 38 Figure 3.4 The light chamber test environment with humidity, temperature, light intensity control, monitor, and recording system To eliminate potential contaminations, all equipment was cleaned with 70% ethanol or autoclaved at 120 °C for 30 minutes before being used. All samples, such as glass pieces, TiO2, and ZnO powders, were also autoclaved at 120 °C for 30 minutes before being used for any tests. 3.4.2 Test environment optimization Because the growth of microorganisms can be primarily affected by the surrounding environment, an optimization scheme is designed to tune up the testing environment, as shown in Figure 3.5. Light intensity, temperature, and relative humidity are three parameters that can be monitored and controlled in the light chamber setup. By correlating bacteria growth behaviors 39 with the above three parameters, the best growing condition for E. coli was determined at 25 °C, 40% relative humidity, and tint level 2. These parameters were used in all following tests. Figure 3.5 Light chamber optimization process 3.4.3 AATCC-100 standard assay This test method provides a quantitative procedure for comparing and evaluating the degree of antibacterial activity after exposure to the test bacteria on the test material compared directly against an untreated control. Escherichia coli (ATCC 11229), a common human bacteria, was chosen as the test organism. Escherichia coli (E. coli) (Figure 3.6) is a rod-shaped, fast-growing, commonly seen, and widely used gram-negative bacteria in microbiology studies. 40 Figure 3.6 Logarithmic growth phase of E. coli O157 under the scanning electron microscope [153]. Copyright © 2018 Wei and Zhao E. coli was first described in 1885 by a german pediatrician named Theodor Escherich [154] and is now considered one of the best-understood cellular forms. [155]. E. coli normally lives in the intestines of healthy people and animals and helps in food digestion processes. Although E. coli are harmless in most cases, certain strains can cause symptoms, including diarrhea and stomach pain. As a versatile bacteria with a very short growing period on various nutrients, E. coli is commonly used in academic research, pharmacy, and biotechnology industries [156]. It is possible to claim that E. coli is one of the most important model organisms in the field of biology [157]. E. coli stock available in our lab was purchased from American Type Culture Collection (ATCC). Fresh bacteria inocula were prepared by mixing E. coli stock (20 µm) into the autoclaved nutrient broth (5 ml, BD 234000) inside a standard sterilized centrifugal tube (10 ml). For contamination control, the centrifugal tube with E. coli and another tube with only nutrient 41 media were then placed into a shaker at 37 °C with a shaking frequency of 90~150 rpm. After 12 hours, both tubes were removed from the shaking machine, and their Optical Density (OD) values were measured with the reference of standard autoclaved nutrient broth. The commercial TiO2 and ZnO powders were added to the same nutrient broth mentioned above with a concentration of 4g/L. Such powder-mixed nutrient broth solution was sent to an autoclave at 120 °C for 30 minutes to remove biological contaminations. Once cooled down to room temperature, cultured E. coli was added until reaching a concentration of 1x105 cells/ml. The solution was then divided into 50-mm diameter Petri dishes with 7 ml in each dish. For ALD-TiO2 or ZnO coated glass samples, glass pieces were autoclaved at 120 °C for 30 minutes and placed inside a 50 mm diameter petri dish, submerged in 7 ml of E. coli nutrient broth solution with 1x105 cellsml concentration. These Petri dishes were placed into a desired artificial growing environment with the optimized parameters (25 °C, 40% relative humidity, and tint level 2) for 12 hours. Nutrient agar plates were prepared by pouring 20-ml autoclaved nutrient agar solution inside 100 mm diameter sterilized petri dishes. Once the liquid nutrient agar solution was solidified, petri dishes were sealed and kept at 4 °C in a freezer for future usage. 42 Figure 3.7 Plated Agar plates in a dark incubator After 12 hours of exposure, samples were removed from the growing environment, and serial dilutions were made with autoclaved nutrient broth solution. Dilutions of (10-2, 10-4, and 10-6) were subjected to colony counting tests. For this, 100 ul were spread onto the previously prepared and stored agar plates and then incubated in a dark incubator (Figure 3.7) with temperature controlled at 37º C for 24 h. During this period, if the injected solution contains a sufficient amount of bacteria, bacteria colonies form on the surface of the agar as visible dots (Figure 3.8). After incubation, bacterial colonies were counted and recorded as colony forming units (CFU/mL). The percent reduction in the number of colonies in treated samples compared to the untreated samples gives the antibacterial activity of the test samples. 43 Antimicrobial activity or % bacteriostatic reduction was calculated according to Eq. 1: 100(B-A)/B=BR (1) Where: BR = % bacteriostatic reduction, A = the number of bacteria recovered from the inoculated treated test specimen incubated over the 24 h contact period, and B = the number of bacteria recovered from the inoculated untreated test specimen incubated over the 24 h contact period. Collected data was organized and processed with necessary statistical analysis techniques, such as the P-value test, a hypothesis testing method, to help validate data and make conclusions. In case of this work, the P-value test is mainly used to compare and validate data sets between control samples and experimental samples, such as commercial TiO2 and ALD-ZnO coated glass. The null hypothesis is set that there are no difference between two selected data sets where the alternative hypothesis is there is a significant difference. The alpha value is set at 0.05 or 5%, which is a standard setting for the P-value test. 44 Figure 3.8 Typical agar plates with E. coli colonies growing For each colony counting test, a control sample group, referred to as the control group, was included. A control group refers to a group of samples going through the same operation as all testing samples. However, it only contained selected test microorganisms, in this case, E. coli. The control was the referring standard of each test set because the bacteria in the control group grow under the same test environment without any interference. Including the control group in each test group, allows one to easily compare other samples in the same test set with the control group and judge the growth of bacteria, which simultaneously indicates the testing material’s antimicrobial capability. 45 Preparation and Structural Characterization of ALD-TiO2 and ZnO This section will explain and summarize characterization data of both commercial materials and ALD-produced coatings. Various methods, such as SEM, EDX, and XPS, will be included, and the importance of their data will be analyzed. 4.1 Preparation of TiO2 and ZnO by ALD Figure 4.1 One ALD cycle of TiO2 on the glass substrate by alternatively introducing tetrakis(dimethylamino)titanium (TDMAT) and water (H2O). TiO2 thin film was deposited on the glass substrate at 150 °C in the ALD chamber by alternatively introducing TDMAT and H2O precursors. The surface reactions for TiO2 deposition with TDMAT and H2O can be separated into two half-reactions, which are described as follows [158, 159]: |-OH* + Ti(NCH3)2)4(g) → |-OH-Ti(NCH3)2)3* + NH(CH3)2(g) (4.1) |-O-Ti(NCH3)2)3* + 2H2O(g) → |-TiO2-OH* + 3NH(CH3)2(g) (4.2) 46 Where the symbol “|” represents the substrate surface, “*” denotes the surface species and “(g)” standards for a gaseous phase. The overall reaction is: Ti(NCH3)2)4(g) + 2H2O(g) → TiO2 + 4C2H6(g) (4.3) One ALD cycle consists of the following four steps (Figure 4.1): Step 1: pulse TDMAT vapor for 0.5 seconds and hold it for 5 seconds in the chamber to complete the surface reaction (Equation 4.1): Step 2: purge the ALD chamber with an Ar gas (50 sccm) to remove reaction byproducts [HN(CH3)2] and residual TDMAT; Step 3: pulse H2O vapor for 0.2 seconds and hold it for 5 seconds in the ALD chamber to complete the surface reaction (Equation 4.2); Step 4: pulse the ALD chamber with an Ar gas (50 sccm) for 15 seconds to remove reaction byproducts [HN(CH3)2] and unreacted H2O. The TiO2 thin film was deposited on the glass substrate by repeating the above ALD cycle 1,500 times (or 1,500 ALD cycles). The obtained sample is donated as ALD-TiO2 and used for further structural and antimicrobial characterizations. 47 Figure 4.2 One ALD cycle of ZnO on the glass substrate by alternatively introducing diethylzinc (DEZ) and water (H2O). The deposition of ZnO by ALD follows a similar process as ALD-TiO2 but with a different metal precursor. Specifically, ZnO thin film was coated on the glass substrate at 150 °C by introducing diethylzinc (DEZ) and water (H2O) sequentially. The surface reactions for ZnO with DEZ and H2O can be described in two half-reactions below [148, 158, 160]: |-OH* + Zn(C2H5)2(g) → |-O-Zn(C2H5)*+C2H6(g) (4.4) |-O-Zn(C2H5)* + H2O(g) → |-O-Zn-OH* +C2H6(g) (4.5) Where the symbol “|” represents the substrate surface, “*” denotes the surface species, and “(g)” standards for a gaseous phase. The overall reaction is: 48 Zn(C2H5)2(g) + H2O(g) → ZnO + 2C2H6(g) (4.6) One ALD cycle of ZnO consists of four steps (Figure 4.2): Step 1: pulse DEZ vapor for 0.5 seconds and hold it for 5 seconds in the chamber for a complete surface reaction (Equation 4.4): Step 2: purge the ALD chamber with an Ar gas (50 sccm) to remove reaction byproduct C2H6 and excessive DEZ; Step 3: pulse H2O vapor for 0.2 seconds and hold it for 5 seconds in the ALD chamber (Equation 4.5); Step 4: pulse the ALD chamber with an Ar gas (50 sccm) for 15 seconds to remove the reaction byproduct C2H6 and extra H2O. The ALD-ZnO on the glass substrate was produced by repeating the above ALD cycle 1,000 times. The obtained sample is designated ALD-ZnO and used for structural characterization (Section 4.2) and antimicrobial evaluation (Chapter 5). 4.2 Structural characterizations of ALD-TiO2 and ZnO The as-prepared ALD-TiO2 and ZnO samples are characterized in terms of their thickness, morphology, phase, and chemical composition, and the results are presented as follows. 49 Figure 4.3 Visual comparison of a) bare glass substrate, b) ALD-TiO2 on glass, and c) ALD-ZnO on glass. Figure 4.3 visually compares ALD-TiO2 and ALD-ZnO on glass, with bare glass as a reference. The sample color turns yellowish for ALD-TiO2 and blue for ALD-ZnO on glass. Compared to bare glass, both ALD-TiO2 and ALD-ZnO on glass are still semitransparent, suggesting the thin nature of the metal oxide coating layers. The thicknesses of ALD-TiO2 and ALD-ZnO on the glass substrate were measured using a surface profilometer. Five measurements were conducted at five random locations of each sample, and the film thickness results are listed in Table 7. The ALD-TiO2 film thickness is determined to be 69.9, 82.2, 88.1, 87.5, and 80.2 nm at sites A, B, C, D, and E, respectively. The average film thickness is calculated as 81.6 nm in average with 6.6 nm standard deviation, yielding a growth rate per cycle (GPC) of about 0.054 nm/cycle for the ALD-TiO2 deposited with TDMAT and H2O at 150 °C. The GPC of 0.054 nm/cycle agrees with the value reported for TiO2 thin films deposited by ALD using the same precursors previously [159]. The ALD-ZnO film thickness is measured as 331.8, 382.5, 261.8, 292.2, and 312.9 nm for sites A, B, C, D, and E, respectively. The average film thickness is 312.8 nm in average with a standard deviation of 41.3 nm, giving a GPC of about 0.156 nm/cycle for the ALD-ZnO deposited with DEZ and H2O at 150 °C. For comparison, the GPC of ALD-ZnO from the DEZ 50 and H2O process was about 0.18-0.20 nm/cycle within a temperature window of 110-170 °C. The high standard deviation of ALD-ZnO film might be due to the incomplete purging of DEZ and/or H2O during the deposition process. The unremoved precursors could physically absorb the film surface and react with the other precursor in the following half-reaction, causing higher film thickness than expected. The film thickness uniformity can be improved by prolonging the purge time to remove the unreacted precursors entirely or reducing the DEZ (or H2O) pulse time. The GPC of ALD-ZnO (0.156 nm) is substantially higher than that of ALD-TiO2 (0.054 nm), mainly due to the different reactivity of the surface reactions, DEZ/H2O for ALD-ZnO (Equations 4.3) and TDMAT/H2O for ALD-TiO2 (Equations 4.6). The reaction of DEZ with H2O is exothermic. It should be noted that the deposition temperature of ALD-ZnO and ALD-TiO2 could be further lowered to 100 °C [148] and 50 °C [159], respectively, to accommodate temperature-sensitive substrates (such as textiles and polymers for PPE) [161, 162] Moreover, the film thickness can also be adjusted by controlling the ALD cycle numbers. Table 7 Film thicknesses of ALD-TiO2 and ZnO on glass measured by a surface profilometer. Film thickness on the glass substrates (nm) ALD-TiO2 ALD-ZnO Site A 69.9 331.8 Site B 82.2 382.5 Site C 88.1 261.8 Site D 87.5 292.2 Site E 80.2 312.9 Average 81.6 312.8 Stanadrd deviation 6.6 41.3 51 4.2.1 Structure and chemical composition of ALD-TiO2 The as-prepared ALD-TiO2 film was characterized by SEM, XRD, and XPS to analyze its morphology, phase, and chemical composition, respectively. Commercial TiO2 nano-powders (anatase ≥99.9%, NanoArc) are included as the control sample for structural characterization and antimicrobial evaluation (discussed in Chapter 5). The structural characterization results are discussed as follows. Figure 4.4 (a-c) SEM images and EDX elemental mapping (d) Ti and (e) O elements of ALD-TiO2 on glass. 52 SEM was used to observe the morphology of ALD-TiO2 on the glass substrate, and EDX analysis was performed to confirm the elemental distribution. The results are displayed in Figure 4.4 (ALD-TiO2). Due to the non-conductive nature of the glass substrate, Pt coating was applied on ALD-TiO2 to improve the sample’s electronic conductivity and obtain high-quality SEM pictures. As seen in Figure 4.4 (a-c), the ALD-TiO2 maintains a flat surface of the glass substrate, suggesting the excellent uniformity of the TiO2 film, as also indicated by the low thickness standard deviation (6.6 nm) Table 1. The white particles, circled in Figure 4.4 (a), are Pt particles applied by sputtering before SEM characterization. EDX mapping on the rectangular area of ALD-TiO2 (Figure 4.4 (c)) confirms the existence of Ti and O elements across the area, as seen in Figures 4.4 (d) and (e). Commercial TiO2 powders (anatase ≥99.9%, NanoArc) are included for comparison. The SEM and EDX results are included in Figure S1 in Appendix. 53 Figure 4.5 (a) XPS survey spectrum and high-resolution XPS spectra of (b) Ti 2p, (c) O1s, and (d) C1s for ALD-TiO2 on glass. XPS characterization was adopted to provide a better surface chemical composition of the ALD-TiO2 on glass, and the results are displayed in Figure 4.5. As shown in Figure 4.5 (a), the XPS survey spectrum reveals the presence of Ti, O, and C elements with an atomic percentage of 22.1, 58.8, and 19.1 at%, respectively, for the ALD TiO2. The high concentration of C element in ALD-deposited films has been reported before and is due to residual and unreacted ligands from TDMAT, and adventitious C absorbed on the surface [ref]. The Gaussian fitting curve of the Ti2p spectrum consists of two peaks at 464.4 eV and 458.6 eV, which can be attributed to Ti2p3/2 and Ti2p1/2, respectively. The peak positions and the 5.8 eV peak separation (ΔBE) of the Ti 2p doublet agree well with the energy reported for TiO2 [163]. Deconvolution of the O1s spectrum results in two peaks, 532.0 eV and 529.9 eV, which correspond to Ti-OH and Ti-O bonding, respectively [164], as shown in Figure 4.5 (c). The Ti-O bonding is clearly from ALD-TiO2, while Ti-OH results from the hydroxyl group (-OH) remaining 54 on the ALD-TiO2 surface after ALD coating (Figure 4.1 and Equation 4.2). Decomposition of the C1s spectrum leads to two main peaks at 288.4 eV and 284.8 eV, which are assignable to C=O and C-C bonding, respectively [165, 166]. Figure 4.6 (a) XRD and (b) Raman spectra of commercial TiO2 nano-powders (anatase ≥ 99.9%, NanoArc), ALD-TiO2 on glass, and the bare glass substrate. Since XPS is a surface-sensitive characterization technique, XRD and Raman spectroscopy were further used to verify the bulk structure of ALD-TiO2, and the results are given in Figure 4.6. Bare glass and commercial TiO2 powders are included for comparison. As shown in Figure 4.6, commercial TiO2 nano-powders possess strong and sharp peaks, suggesting their high crystallinity. Most of these diffraction peaks can be assigned to anatase TiO2 (JCPSD PDF No. 21-1272), while the remaining peaks belong to rutile TiO2 (JCPSD PDF No. 21-1276). The TiO2 nano-powder XRD pattern provides a good baseline for indexing the ALD-TiO2 sample. The XRD pattern of ALD-TiO2 on glass exhibits one broad peak ranging from 20 to 35 degrees, which agrees well with that of bare glass. In addition, ALD-TiO2 55 on glass shows three weak-intensity peaks at 25.4°, 48.1°, and 55.3°, which could be assigned to the (101), (200), and (105) planes of anatase TiO2, respectively. The weak XRD peaks for ALD-TiO2 could be due to the small thickness of the ALD-TiO2 film (81.6 ± 6.6 nm) and its amorphous nature in large. The structure of ALD-TiO2 film might remain disordered in the long-range, owing to the relatively low deposition temperature (150 °C). The structure of ALD-TiO2 on glass is further verified by Raman characterization. As shown in Figure 4.6, the Raman frequencies of TiO2 nano-powders are 144, 393, 517, and 638 cm-1, which correspond to the Eg, B1g, A1g, and Eg active modes, respectively, for tetragonal anatase TiO2 [167, 168] The 144 cm-1 band is the strongest of all the observed Raman bands for anatase TiO2. Notedly, the ALD-TiO2 on glass exhibits one prominent peak centered at 144 cm-1 belonging to anatase TiO2. The XRD and Raman results reveal that the ALD-TiO2 from the TDMAT/H2O process has a tetragonal anatase structure, and the majority of ALD-TiO2 is amorphous. In summary, TiO2 thin film was successfully deposited by ALD from TDMAT/H2O process at 150 °C. The as-deposited ALD-TiO2 film exhibited excellent uniformity on the glass substrate. XRD and XPS analysis revealed that ALD-TiO2 possessed a tetragonal anatase phase but was largely amorphous. 4.2.2 Structure and chemical composition of ALD-ZnO The morphology of ALD-ZnO on glass is shown in Figure 4.7. As seen, ALD-ZnO possesses a slightly rough surface than ALD-TiO2. Nevertheless, except for the Pt coating particles, the ALD-ZnO film still shows good uniformity in the local areas, Figures 4.7 (a-c). EDX 56 analysis verifies the uniform distribution of Zn and O elements on the glass substrate (Figure 4.7 (d, e)). Commercial ZnO powders (GRADE CR-4, Ever Zinc) are included for comparison. The SEM and EDX results are included in Figure S1 in Appendix. Figure 4.7 (a-c) SEM images and EDX elemental mapping (d) Zn and (e) O elements of ALD-ZnO on glass. 57 Figure 4.8 (a) XPS survey spectrum and high-resolution XPS spectra of (b) Zn 2p, (c) O1s, and (d) C1s for ALD-ZnO on the glass substrate. To confirm its chemical composition and bonding, XPS characterization was performed on the ALD-ZnO glass, and the results are displayed in Figure 4.8. XPS survey indicates the existence of Zn (15.4 at%), O (55.3 at%), and C (29.3%) in the ALD-ZnO film. A high-resolution scan on the Zn 2p spectrum (Figure 4.8 (b)) shows two distinct peaks at 1021.2 and 1044.2 eV, corresponding to Zn 2p1/2 and Zn 2p3/2 of Zn-O with spin-orbit splitting energy of 23.0 eV (ΔBE), respectively. This result is in agreement with the value of ZnO reported previously [169, 170]. Deconvolution of the O1s spectrum results in two peaks at 530.2 eV and 531.6 eV, as shown in Figure 4.8 (c). The former belongs to the Zn-O bonding in ZnO, while the latter is due to the -OH group on the ALD-ZnO surface [171]. A large amount of C impurity (29.3%) is detected in the ALD-ZnO film, and high-resolution XPS analysis reveals the coexistence of C=O (288.7 eV) and C-C (284.8 eV). 58 Figure 4.9 (a) XRD and (b) Raman spectra of commercial ZnO powders (GRADE CR-4, Ever Zinc), ALD-ZnO on glass, and bare glass substrates. The crystal structure of ALD-ZnO is further analyzed using XRD and Raman to reveal its bulk information, and the results are presented in Figure 4.9, together with commercial ZnO powders and glass substrates as references. The XRD pattern of ALD-ZnO clearly shows three peaks at 31.8°, 34.4°, and 36.3°, which can be indexed as the (100), (002), and (101) planes of ZnO (JPCDS PDF No. 36-1451), respectively. It should be noted that the relative peak intensity of (100), (002), and (101) changes for ALD-ZnO compared to commercial ZnO powders. This could be due to the different forms of the ZnO, i.e., thin film for ALD-ZnO and powders for commercial ZnO powders, prepared by different methods. It is speculated that (002) is the preferred growth direction for ZnO thin film deposited by ALD. Moreover, Figure 4.9 (a) shows that the ALD-ZnO diffraction peaks are strong and shield the broad diffraction peak from the glass substrate. The Raman spectra in Figure 4.9 (b) disclose relatively weak peaks at 99 and 438 cm-1 for ALD-ZnO on glass, in sharp contrast with commercial ZnO powders. The broad background in the Raman spectrum results from the glass substrate. 59 In summary, the above characterization results confirm the successful deposition of ZnO thin film on the glass substrate from the DEZ/H2O ALD process. XPS, XRD, and Raman analysis verify the chemical composition and crystal structure of ALD-ZnO. Figure 4.10 UV-visible absorbance spectra of ALD-ZnO on glass, ALD-TiO2 on glass, and bare glass. The TiO2 and ZnO material have to interact with photons to establish photocatalytic processes, which results in light absorption. To study the effect of photocatalytic behavior and light absorption properties, the Ultraviolet-visible spectroscopy (UV-VIS) test was performed to measure the absorption of UV light on ALD-ZnO and TiO2 on the glass substrate at room temperature, and the results are compared in Figure 4.10. As shown, the bare glass sample demonstrates no UV absorption until below 320-nm wavelength, which then has a steady but low absorption level at around 0.75. Such behavior can be explained as the selected glass pieces having some natural UV absorption capability even when uncoated. The ALD-TiO2 on glass has a slightly higher UV absorption level than the bare glass when the wavelength is above 400 nm. The absorption level started to increase below 400nm wavelength and reached 60 a peak value of around 1.2 at a wavelength of about 320 nm. The absorption level then started to drop and stabilized slightly lower than the glass sample. In contrast, the ALD-ZnO glass shows the rapid growth of absorption level at around 38 0nm wavelength and reached an absorption level of about 4.8. It can be found that although having a relatively high absorption level compared with other samples, the actual data line fluctuated, and such characteristics existed until the minimum wavelength could be tested. It can be concluded that the ALD-ZnO has the highest average absorption level and largest absorption wavelength among all three samples tested. In summary, TiO2 and ZnO thin films were successfully deposited on the glass substrates using the ALD processes of TDMAT/H2O and DEZ/H2O, respectively. The thickness of the ALD-TiO2 and ALD-ZnO thin films was determined as 81.6 ± 6.6 nm and 312.8 ± 41.3 nm, respectively. The thickness difference was mainly due to the different ALD growth rates of ALD-TiO2 (0.54 Å/cycle) and ALD-ZnO (1.56 Å/cycle). XPS, XRD, and Raman spectroscopy analysis confirmed the crystal structures and chemical compositions of ALD-TiO2 and ALD-ZnO. UV-VIS characterization revealed a strong UV absorption capability of ALD-ZnO compared to ALD-TiO2 and bare glass substrate. 61 Antimicrobial performance evaluation This section will summarize the antimicrobial performance of ALD-TiO2 and ZnO prepared in Chapter 4 and explain the possible reasons for different bacteria- eliminating behavior observed in these materials. 5.1 TiO2 antimicrobial performance against E. coli The antimicrobial performance of commercial TiO2 nanoparticles against E. coli was obtained through the colonies counting test mentioned in the above sections, where the end data is organized and presented in Figure 5.1. As indicated, the control sample (labeled as Ctrl) showed a larger data distribution (from around 6x1010 to above 9x1010 CFU/ml) than the commercial TiO2 particles (labeled as TiO2) (from around 5.5 x 1010 to above 7x1010 CFU/ml). However, their medians, both around 7x1010 CFU/ml, did not pose as much difference as the data distribution. A P test with the alpha value set at 0.05 was performed to statistically verify the relationship between the control and the commercial TiO2 samples with the null hypothesis that they are dependent. The calculated P value(indicated on the top of Figure 5.1) for the control group and the commercial TiO2 group is 0.3647, which is larger than the set alpha value of 0.05, supporting the null hypothesis that these two samples are not significantly different. Such results can be used as evidence to prove that commercial TiO2 was not able to make a significant difference in the growth of E. coli or that it did not show enough antimicrobial behavior (BR = 13.3 %) against E. coli. 62 Figure 5.1 E. coli colonies counting test result of commercial TiO2 nanoparticles The same test and data analysis technique were used when evaluating the antimicrobial performance of ALD-TiO2-coated glass. The results are shown in Figure 5.1. A total of three different samples were involved in this test, which are the control (Ctrl), the Untreated glass (Untreated glass), and the ALD TiO2 coated glass (TiO2 glass). The plotted figure indicated a large difference in the data distribution between the three samples, with uncoated glass data concentrated around 2.5x1012 CFU/ml and the TiO2 glass data spans around 1x1012 and 1.5x1013 CFU/ml. P value test was conducted between control and uncoated glass, uncoated glass and TiO2 glass, control, and TiO2 glass separately. 63 Figure 5.2 E. coli colonies counting test result of ALD-TiO2 coated glass The P value between the control and the untreated glass is 0.9839, which is larger than the set alpha value of 0.05, indicating that these two samples are related. Similarly, the P value for TiO2 glass and untreated glass (0.3499), TiO2, and control (0.2993) are all larger than the 0.05 alpha value, indicating that there is no significant difference between these samples. With the support of the P value, a conclusion can be drawn that neither the presence of untreated glass or ALT TiO2 glass significantly altered the growth of E. coli. Thus, no significant antimicrobial activity was observed, with a calculated BR value of 21.2 %. 5.1.1 Comparison of TiO2 antimicrobial performance As summarized in Chapter 5, under the setup mentioned above, commercial TiO2 powder and ALD-TiO2 glass pieces did not generate enough statistical evidence to prove their antimicrobial capability against E. coli. Such a result is different from results claimed by some other groups. 64 Various groups have studied the antimicrobial performance of different TiO2 materials and found them to have the sufficient antimicrobial capability to be addressed in their publications. However, upon further investigation, we found that many of their testing methods, involved materials and experimental setups are very different from what has been used in this thesis. For example, J.A. Rengifo-Herrera and C. Pulgarin tested the antimicrobial performance of two commercial TiO2 powders, Degussa P25 and TKP 102 (supplied by Tayca Corporation and Degussa AG), under simulated UV-light and reported antimicrobial behavior of both materials [172]. However, upon further reading of their paper, it can be found that they used different bacteria culturing processes and illumination processes than this thesis. Their E. coli was prepared using a centrifugation process with custom-made saline solution (8 mg mL−1 NaCl and 0.8 mg mL−1 KCl in Milli-Q water, pH 7 by addition of HCl or NaOH). The same solution was also used during other test processes, which was exposed under the Hanau Suntest (AM-1) solar simulator for 4 hours. Saowaluk Boonyod et al. investigated the commercial P25 powder antimicrobial performance against E. coli when irradiated by UV and fluorescent light [173]. When mixing 50 mg TiO2 powder in 100 mL E. coli conidial suspension (105 CFU/ml) and exposed to UV light, E. coli has a survival rate below 20 % after around 30 minutes. It took around 4 hours to have the E. coli survival rate dropped under 20 % when the same solution was exposed to fluorescent light. This work claimed TiO2’s capability of killing E. coli under both UV and fluorescent light and showed the effect of different lighting over antimicrobial efficiency. It was concluded that commercial TiO2 powder has a more substantial antimicrobial effect against E. coli when exposed to UV light, making the effect time eight times shorter. However, Boonyod’s paper used a TiO2 powder concentration of 50mg/100ml (0.5g/L), which is eight times lower than the concentration used in this thesis (4g/L). In addition, Boonyod’s group exposed an entire 100 ml 65 of TiO2 mixed solution under light irradiation, while in this thesis, only 7ml portioned sample dishes were exposed under the light. The exposure time of Boonyod’s samples is also much shorter (4h) maximum for fluorescent light) compared with this thesis (12 h). ALD TiO2 produced using TiCl4 and H2O under an N2 environment was tested by V.Scuderi et al., and its antimicrobial capability against E. coli was evaluated [174]. They declared that their ALD TiO2 could increase E. coli 's survival rate to 61% after exposing E. coli to the film for 1 hour. They also compared the performance of ALD TiO2 produced under both 200 °C and 300°C, concluding that although higher processing temperatures can enhance TiO2's photocatalytic properties, it significantly reduces its antimicrobial capabilities. When comparing our work with other groups’ works, we found that the environment and sample requirements to activate TiO2’s antimicrobial capability are very strict. Further investigation indicated that several groups had observed similar issues when trying to trigger TiO2’s antimicrobial reaction. Joanna Podporska-Carroll et al. reported a significantly compromised Degussa P25 antimicrobial performance when testing under their setup [175]. They have determined that the lack of observable antimicrobial behavior was due to numerous factors such as source and type of TiO2, biological tests, environmental conditions, and other experimental conditions. They claimed that differences in these factors would affect ROS generation, thus impacting TiO2’s antimicrobial performance. Yemmireddy et al. also made similar conclusions in their paper that various causes can easily alter the antimicrobial performance of TiO2 [176]. They specifically pointed out that the bacteria culture harvesting methods and the volume of testing suspensions can significantly impact TiO2 antimicrobial behavior, which several studies did not account for in their paper. As in our case, the antimicrobial test procedure was a relatively strict and standard industrial standard (AATCC- 66 100), which is not commonly used in the works where TiO2 showed antimicrobial behavior. In addition, considering the low UV absorption level and narrow effective UV wavelength TiO2 samples demonstrated, it is possible that TiO2’s antimicrobial property could not be effectively activated withour setup. Overall, it was found that the activation of TiO2 antimicrobial behavior requires very specific conditions and is very sensitive to environmental changes. In this thesis, we observed that with the current material and setup, we could not make TiO2 show observable antimicrobial properties against E. coli. Such results are likely caused by the TiO2 nanopowder we sourced E. coli source, the bacterial preparation process, the test protocol been used, and variations in test environments. 5.2 ZnO antimicrobial performance against E. coli Similar to TiO2-related tests, the antimicrobial performance of ZnO against E. coli was tested and summarized under the same method, and the results are presented in the following figures. As indicated in Figure 5.3, all nine control dishes contain plenty of E. coli colonies, while the commercial ZnO powder samples have no colonies at all. Based on the photographs, it can be concluded that the commercial ZnO powder successfully eliminated the growth of E. coli and provided a very consistent performance. a b 67 Figure 5.3 Photo summary of (a) the control sample and (b) the commercial ZnO powder sample. As shown in Figure 5.4, the control (Ctrl) sample’s data varies between 1x109 CFU/ml and 4x109 CFU/ml, while the commercial ZnO sample (ZnO) remains at 0. A P-value test was conducted to ensure the difference between these two samples is significant, which returns a value smaller than 0.0001. Because the calculated P value is significantly smaller than the selected 0.05 alpha value, it is safe to conclude that these two samples are quite different. Consequently, commercial ZnO has a strong effect on the growth of E. coli with a calculated 100 % BR. 68 Figure 5.4 E. coli colonies counting test result of commercial ZnO powders ALD-ZnO coated glass pieces and uncoated glass pieces were also tested to evaluate the antimicrobial performance of the coating and to form a comparison group with the presence of untreated glass. The quantity of both coated and untreated glass pieces were the same; their results are presented in Figure 5.5 and Figure 5.6. As indicated in Figure 5.5, both the control group and untreated glass group showed E. coli colonies growth. However, the uncoated glass group had some uneven growth patterns in some of the pertri dishes. Upon analysis, the uneven colonies distribution was likely caused by water condensation inside the petri dish, washing away plated E. coli. The water condensation issue was likely caused by unstable room humidity conditions and extreme temperature changes when storing petri dishes inside the low-temperature freezer. Based on the number of data points obtained from each sample set, these condensation-affected data can be compromised. 69 Figure 5.5 Photo summary of the control sample (a), untreated glass (b), and ALD-ZnO coated glass (c). As indicated in Figure 5.6, all three samples have different visual data ranges. Similar to previous tests, the P value test was performed for every two samples with the alpha set at 0.05, and results are noted in the figure. Calculated results indicate that P values between untreated glass and control, ZnO glass and control, and untreated glass and control, are all smaller than 0.0001, which can be considered significantly smaller than the 0.05 alpha value. Unlike previous ALD TiO2 samples tests, the P value for untreated glass and control samples is smaller than 0.0001, which suggests these two samples statistically have a significant difference. Combined with the fact that the data range of untreated glass is visually lower than the control sample, the presence of untreated glass statistically demonstrated a limitation over E. coli’s growth. However, because the P value between untreated glass and the ZnO samples is also significantly smaller than 0.05, and knowing that all data points for ZnO a b c 70 glass are at 0, the untreated glass sample still poses a substantially higher count than the ZnO glass sample, resulting in 46.5 % and 100 % BR, respectively. Such results can be used to support that these three samples are statistically significantly different from each other. Considering that the ZnO sample is the only sample that 100 % eliminated E. coli, it is safe to conclude that under the current setup, ALD-produced ZnO coating has the capability to prevent the growth of E. coli. 71 Figure 5.6 Count comparison between control sample, uncoated glass sample, ALD-ZnO coated glass sample and combined box plot. A few important conclusions can be drawn based on all data collected and presented in the ZnO performance analysis section. First, when tested under the environment mentioned earlier in this thesis, commercial-grade ZnO powder material has a very strong antimicrobial behavior against E. coli. It completely eliminated the growth of E. coli. Second, the glass substrate we used during the ALD coating process, which is also referred to as uncoated glass, has some unstable effects on E. coli. However, we can not conclude it has antimicrobial capability against E. coli. Third, the ALD-ZnOcoated glasses pieces also eliminated the growth 72 of E. coli, which indicates similar antimicrobial performance to the commercial grade ZnO material. Combining all these conclusions, it is safe to claim that when against E. coli, the ALD-created ZnO nanocoating has at least the same antimicrobial capability as commercial-graded ZnO powder when glass pieces are used as a coating substrate. 5.2.1 ZnO performance discussion Both commercial ZnO powder and ALD-ZnO coated glass pieces showed complete elimination of E. coli under the test environment mentioned in this thesis. Such results match claims made by many other groups who have reported the antimicrobial behavior of ZnO. The antimicrobial performance of commercial grade ZnO nanoparticles made by Sigma Aldrich, Germany, was evaluated by R.K.Dutta et al. They found that the antimicrobial behavior against E. coli depends on the concentration of ZnO in culture media. Under their setup, a minimum inhibitory concentration of 15 mg/100mL was needed, and when the concentration reached 45 mg/mL, the E. coli was eliminated [177]. Zhang et al. tested ZnO nanoparticles from Nanostructured & Amorphous Materials (USA) against E. coli and obtained observable antimicrobial behavior [178]. They suggested that Zn+2 ions released into the ZnO suspension were not the leading cause of antimicrobial activities where the generation of ROS is the dominant mechanism. They also urged the existence of ZnO nanoparticles may also have caused physical damage to the E. coli lipids, which results in a leakage of pyridine or protons between the interior and exterior of the vesicle, causing cell death. Later work performed by Jiang et al., in which Zhang also participated, further proved the physical and chemical damage ZnO nanoparticles did to E. coli cells [179]. 73 However, it is worth mentioning that both these works were performed in the dark to suppress the effect of photocatalytic reactions, which shows that under certain setups, ZnO nanoparticles can show antimicrobial behavior against E. coli without a photocatalytic process. ALD-ZnO coating produced by DEZ and H2O was tested by Kang-HeePark et al., and its antimicrobial capability was addressed in their paper [180]. Although their targeting bacteria is S. aureus instead of E. coli, the effect of UV light was explicitly considered and tested. They concluded that under their setup, the ALD-produced ZnO coating had similar photocatalytic properties to ZnO nanoparticles which ROS are produced mainly when the ZnO film is UV-A irradiated. Overall, test results indicated that both commercial ZnO nanopowder and ALD ZnO film can eliminate E. coli when subjected to 12 hours testing period. Both samples showed the same observable antimicrobial performance during our test, which aligns with claims made by other research groups. 74 Conclusions 6.1 Results Summary The main goal of this thesis is to develop a photocatalytic self-disinfecting coating for PPEs. When applied to different surfaces such as glass, plastic, and fabric, a such coating can provide antimicrobial properties against various types of microorganisms, with better coverage, longevity, and uniformity than conventional coatings. To achieve these goals, we test the antimicrobial performance of two commercial metal oxide powders: TiO2 and ZnO. We then produced the ALD coating of these materials on glass pieces and evaluated their antimicrobial capability against E. coli. After analyzing experimental data, some conclusions are drawn and listed as follows: ⚫ Proven by various material characterization results, both ALD-TiO2 and ALD-ZnO coatings were successfully created on the glass substrates by ALD. ⚫ Both commercial TiO2 powders and ALD-TiO2 coatings could not show any significant antimicrobial behavior under the above mentioned test environment. ⚫ TiO2 was found to have stringent activation requirements for its photocatalytic antimicrobial properties. ⚫ Under the same test environments in which TiO2 materials were tested, commercial ZnO powders and ALD-ZnO-coated glass pieces showed significant antimicrobial behavior against E. coli. ⚫ Both commercial ZnO powders and ALD-ZnO coated glass pieces showed complete elimination of E. coli, indicating that under our setup, the ALD-ZnO coating had at least a similar antimicrobial performance to the commercial ZnO powder. ⚫ During trial production, ALD-ZnO coating was also successfully created on cloth surfaces. 75 In addition, at the end of this thesis, we attempted to apply ALD-ZnO coatings on a small piece of cloth. Some obtained characterization data are included in Appendix A (Figure S3). Initial data has proven the success of coating the ALD-ZnO material on the cloth. However, due to various limitations, we could not test or further investigate the property of such material in this thesis. 6.2 Limitations and future perspective of this work Although we successfully demonstrated both commercial ZnO powders and ALD-ZnO coated glass to eliminate E. coli, there are still many other details that can be explored in the future: • Test whether the ALD-ZnO-coated cloth material will show antimicrobial behavior when tested under the same environment mentioned in this thesis • Test ZnO-related materials against different types of bacteria and evaluate corresponding antimicrobial performance. • Test ZnO-related materials under different environments, including but not limited to environment temperatures, light intensity, and humidity. • Test whether the ALD-ZnO coating will leach into test solutions • If ALD-ZnO-coated cloth has sufficient antimicrobial capability and durability 76 Appendices Appendix A Figure S1 (a-c) SEM images and EDX elemental mapping (d) Ti and (e) O elements of commercial TiO2 powders (anatase ≥99.9%, NanoArc). 77 Figure S2 (a-c) SEM images and EDX elemental mapping (d) Zn and (e) O elements of commercial ZnO powders (GRADE CR-4, Ever Zinc). 78 Figure S3 (a-c) SEM images and EDX elemental mapping (d) Zn and (e) O elements of ALD-ZnO on cloth. 79 Reference [1] S. 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Thesis/Dissertation
eng
Mechanical Engineering
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Liu, Jian
Pakpour, Sepideh
Synthesis and characterizations of antimicrobial coating by atomic layer deposition technique for personal protective equipment applications
Text