Science, Irving K. Barber Faculty of (Okanagan)
Chemistry, Department of (Okanagan)
DSpace
UBCO
Mitchell, Emma Louise
Master of Science - MSc
University of British Columbia
The development of reliable renewable energy sources is imperative for a sustainable future. To address this issue, the development of suitable photocatalysts that can produce clean renewable energy at a sustainable cost has become a rapidly expanding research area. Carbon nitride (CNx), an organic semiconductor, is a promising candidate for this role due to its easy synthesis, high stability and tunability. The potential of CNx has been highlighted with reports of highly efficient photocatalytic hydrogen production (quantum yields >50%) under sacrificial conditions. However, the efficiency remains limited for sustainable overall water splitting. To improve the efficiencies, modification of the material’s morphology has been pursued. However, research on the relationship between CNx morphology and photocatalytic efficiency has been inconclusive. Positive correlations as well as variances between CNx photocatalytic activity and surface area have been previously identified. To build a clear relationship between structure-photophysics-activity, different morphologies were prepared and studied. These were mesoporous (meso), macroporous (macro), SBA, and hollow sphere CNx. Progress towards synthesizing hollow sphere CNx was achieved, however, a reproducible synthesis has yet to be accomplished. The trend in photocatalytic hydrogen production of the successfully synthesized materials was as follows meso CNx, < Bulk CNx < SBA CNx < macro CNx. There were differences in the structural and electronic characterization, however, the trends were not consistent with the observed photocatalytic activity. Using Transient Absorption Spectroscopy (TAS), the normalized decay kinetics were found to be similar between all morphologies, indicating that changes in trap state distribution or electronic structure likely do not greatly impact the photocatalytic hydrogen evolution. A direct positive correlation between surface area and photocatalytic activity was observed. The largest increase in surface area of a modified morphology compared to bulk CNx was in macro CNx which had a surface area 1.7-fold larger and an apparent quantum yield (AQY) of 3.6%. These smaller surface area changes observed herein could indicate the morphological modifications do not extend to the nanostructure of the material. Extending CNx modification to the nanostructure of the material could be the key to induce more significant electronic or structural changes which affect the photocatalytic activity.
Carbon Nitride Photocatalysis: An Investigation on the Influence of Morphology by Emma Louise Mitchell B.Sc., Queen’s University, 2019 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE COLLEGE OF GRADUATE STUDIES (Chemistry) THE UNIVERSITY OF BRITISH COLUMBIA (Okanagan) December 2021 © Emma Louise Mitchell, 2021 ii The following individuals certify that they have read, and recommend to the College of Graduate Studies for acceptance, a thesis/dissertation entitled: Carbon Nitride Photocatalysis: An Investigation on the Influence of Morphology submitted by Emma Louise Mitchell in partial fulfillment of the requirements of the degree of Master of Science. Dr. Robert Godin, Faculty of Science Supervisor Dr. Isaac Li, Faculty of Science Supervisory Committee Member Dr. David Jack, Faculty of Science Supervisory Committee Member Dr. Jonathan Holzman, School of Engineering University Examiner iii Abstract The development of reliable renewable energy sources is imperative for a sustainable future. To address this issue, the development of suitable photocatalysts that can produce clean renewable energy at a sustainable cost has become a rapidly expanding research area. Carbon nitride (CNx), an organic semiconductor, is a promising candidate for this role due to its easy synthesis, high stability and tunability. The potential of CNx has been highlighted with reports of highly efficient photocatalytic hydrogen production (quantum yields >50%) under sacrificial conditions. However, the efficiency remains limited for sustainable overall water splitting. To improve the efficiencies, modification of the material’s morphology has been pursued. However, research on the relationship between CNx morphology and photocatalytic efficiency has been inconclusive. Positive correlations as well as variances between CNx photocatalytic activity and surface area have been previously identified. To build a clear relationship between structure-photophysics-activity, different morphologies were prepared and studied. These were mesoporous (meso), macroporous (macro), SBA, and hollow sphere CNx. Progress towards synthesizing hollow sphere CNx was achieved, however, a reproducible synthesis has yet to be accomplished. The trend in photocatalytic hydrogen production of the successfully synthesized materials was as follows meso CNx, < Bulk CNx < SBA CNx < macro CNx. There were differences in the structural and electronic characterization, however, the trends were not consistent with the observed photocatalytic activity. Using Transient Absorption Spectroscopy (TAS), the normalized decay kinetics were found to be similar between all morphologies, indicating that changes in trap state distribution or electronic structure likely do not greatly impact the photocatalytic hydrogen evolution. A direct positive correlation between surface area and photocatalytic activity was observed. The largest increase in surface area of a modified morphology compared to bulk CNx was in macro CNx which had a surface area 1.7-fold larger and an apparent quantum yield (AQY) of 3.6%. These smaller surface area changes observed iv herein could indicate the morphological modifications do not extend to the nanostructure of the material. Extending CNx modification to the nanostructure of the material could be the key to induce more significant electronic or structural changes which affect the photocatalytic activity. v Lay Summary The development of renewable energy sources is essential for a sustainable future. One option for a renewable energy source is utilizing solar energy to produce high energy fuels such as hydrogen. Carbon nitride (CNx) shows promise as a material to generate this type of renewable energy using water and sunlight. The shape of CNx can be controlled (morphological modifications) and can influence the hydrogen production efficiency of CNx. Four different CNx morphologies were studied to investigate the relationship between the material’s morphology and its hydrogen production. Characterization of the samples revealed the connectivity of the atoms and the movement of the charges in the material did not relate to the hydrogen production efficiency. However, there was a direct relationship between the surface area of the material and the amount of hydrogen it produced. This work will help direct future CNx morphology modifications towards achieving higher hydrogen production efficiencies. vi Preface Contributions from Dr. Robert Godin include consultation on experiment planning, data analysis and writing as well as invaluable support and expertise throughout the project. Grady Hewlett contributed to the design of this project. There was collaboration with Destiny Elleanor on the procedure of the hollow sphere carbon nitride synthesis and an image of her hollow sphere synthesis is included in this thesis. Jasper Pankratz was supportive in discussing ideas and data analysis. Dr. Shrestha assisted with the collection of the SEM images. Samples for TEM, XPS and pXRD were sent out and analysed at 4D LABS at Simon Fraser University. Samples for surface area measurements were sent out to MSE Analytical Services. Research curtailments from March 2020 to October 2020 due to COVID-19 directed work towards writing review articles. This resulted in two publications with Abigail Law and Dr. Robert Godin. The first was published on January 7th, 2021, titled Experimental determination of charge carrier dynamics in carbon nitride heterojunctions in Chemical Communications. The second was published on September 19th, 2021, titled Interfacial charge transfer in carbon nitride heterojunctions monitored by optical methods published in Journal of Photochemistry & Photobiology, C: Photochemistry Reviews. A manuscript on the work presented in this thesis is also in progress. vii Table of Contents Abstract .................................................................................................................................... iii Lay Summary ............................................................................................................................ v Preface ...................................................................................................................................... vi Table of Contents .................................................................................................................... vii List of Tables ............................................................................................................................. x List of Figures .......................................................................................................................... xi List of Symbols and Abbreviations ....................................................................................... xv Acknowledgements ............................................................................................................... xix Dedication ............................................................................................................................... xx Chapter 1: Introduction ............................................................................................................ 1 1.1 Semiconductors ........................................................................................................ 2 1.1.1 Organic Semiconductors ..................................................................................... 3 1.1.2 Semiconductor Photocatalysis ............................................................................ 5 1.5.1 Heterojunction Formation ...................................................................................12 1.5.2 CNx Modifications ...............................................................................................14 Chapter 2: Carbon Nitride Characterization .......................................................................... 18 2.1 General Synthesis ...................................................................................................18 2.2 Characterization ......................................................................................................20 2.2.1 Structural Characterization .................................................................................21 2.2.1.1 Fourier-transform Infrared Spectroscopy ....................................................21 2.2.1.2 X-ray Photoelectron Spectroscopy .............................................................23 2.2.1.3 Powder X-ray Diffraction ............................................................................27 2.2.2 Electronic Characterization ................................................................................28 2.2.2.1 UV-Vis Diffuse reflectance Spectroscopy ...................................................28 2.2.2.2 Photoluminescence ....................................................................................32 viii 2.2.3 Morphological Characterization ..........................................................................33 2.2.3.1 Electron Microscopy ...................................................................................33 2.2.3.2 Surface Area ..............................................................................................37 2.3 Charge Carrier Dynamics ........................................................................................38 2.3.1 Transient Absorption Spectroscopy....................................................................38 2.4 Photocatalytic Testing ..............................................................................................41 2.4.1 Hydrogen Evolution ............................................................................................42 2.5 Conclusion ...............................................................................................................43 Chapter 3: Hollow Sphere Carbon Nitride ............................................................................. 45 3.1 Silica Synthesis .......................................................................................................46 3.1.1 Materials and Methods .......................................................................................46 3.2 Hollow Sphere CNx Synthesis ..................................................................................48 3.2.1 Synthesis Variation ............................................................................................48 3.3 Conclusion and Future Directions ............................................................................56 Chapter 4: Conclusion ............................................................................................................ 57 4.1 Surface Area-Photophysics-Activity Relationship .....................................................57 4.2 Future Directions .....................................................................................................60 Chapter 5: Experimental ......................................................................................................... 63 5.1 Synthesis .................................................................................................................63 5.1.1 Carbon Nitride (CNx) ..........................................................................................63 5.1.1.1 Bulk CNx .....................................................................................................63 5.1.1.2 Mesoporous CNx ........................................................................................63 5.1.1.3 Macroporous CNx .......................................................................................64 5.1.1.4 SBA CNx ....................................................................................................64 5.1.1.5 Hollow Spheres ..........................................................................................64 5.1.2 Silica ..................................................................................................................65 ix 5.1.2.1 Silica Spheres ............................................................................................65 5.1.2.2 Silica Spheres Core-Shell ..........................................................................65 5.2 Structural Characterization ......................................................................................66 5.1.2 Fourier-transform Infrared Spectroscopy ............................................................66 5.1.3 X-ray Photoelectron Spectroscopy .....................................................................66 5.1.4 Powder X-ray Diffraction ....................................................................................67 5.3 Electronic Characterization ......................................................................................67 5.3.1 UV-Vis Diffuse Reflectance Spectroscopy (DRS) ...............................................67 5.3.2 Photoluminescence Spectroscopy .....................................................................67 5.4 Morphological Characterization................................................................................68 5.4.1 Scanning Electron Microscopy ...........................................................................68 5.4.2 Transmission Electron Microscopy .....................................................................68 5.4.3 Surface Area ......................................................................................................68 5.5 Charge Carrier Dynamics ........................................................................................68 5.5.1 Transient Absorption Spectroscopy....................................................................68 5.6 Photocatalytic Activity ..............................................................................................69 5.6.1 Hydrogen Evolution ............................................................................................69 Bibliography ............................................................................................................................ 70 x List of Tables Table 1: FTIR peak position of characteristic CNx peaks of each morphology. Positive peak shifts are towards higher wavenumbers and negative peak shifts are towards lower wavenumbers............................................................................................................................23 Table 2: Brunauer–Emmett–Teller (BET) surface measurements for CNx morphologies bulk, meso, macro and SBA CNx .......................................................................................................37 Table 3: Hydrogen evolution rate and apparent quantum efficiency of all four CNx morphologies .................................................................................................................................................42 Table 4: Activity by surface area of the CNx samples with 23mg catalyst used in each trial.......59 xi List of Figures Figure 1: The difference in the bandgap of metal, semiconductor, and insulating solids. VB stands for valence band and CB stands for conduction band and the band gap is the space spanned by the double headed arrow. ....................................................................................... 3 Figure 2: A) The scheme is showing the effect of conjugation on the band gap of molecular crystal semiconductors B) The effect structure has on the band positions of polymer semiconductors17–20. ................................................................................................................... 4 Figure 3: The charge transport mechanisms in an organic semiconductor. This is demonstrating how the charge carriers move. Where VB stands for valence band. ........................................... 5 Figure 4: Left: Diagram showing the different processes in organic semiconductor photocatalysis. Right: The equations which represent the process shown on the left. ................ 6 Figure 5: Steps in photocatalytic water splitting in organic semiconductor. The productive steps 1-4 are in competition with charge recombination. ................................................................... 8 Figure 6: A) Schematic representation of DR TAS setups for µs-TAS. The 1064 nm output of the Nd:YAG is frequency tripled to 355 nm and can pump the OPO to generate pulses with tunable wavelengths. The abbreviations in the diagram include optical parametric oscillator (OPO); mirror (M1); detector (Dt). B) A representative absorption spectrum for the ground and excited states. C) The difference in absorbance of the traces. Photoinduced absorption (PIA) is in blue and the ground state bleaching (GSB) is in brown. .................................................................... 9 Figure 7: Band position of carbon nitride and their relation to the potentials of the half reactions of water splitting. Proton reduction (H+/H2) and water oxidation (O2/H2O) vs. NHE. .................11 Figure 8: A) on the left is the chemical structure of heptazine, the repeat unit carbon nitride and on the right is the melon representation of the chemical structure of carbon nitride (CNx). In B) the absorbance (from the Kubelka-Munk transform) and photoluminescence (PL) spectra of CNx. CNx powder is shown in the image inset on the graph. (Reproduced with permission from ref. 42) ........................................................................................................................................12 xii Figure 9: A) An example of a hard templating method to synthesize a modified CNx morphology. B) An example of a soft templating method to synthesize a modified CNx morphology. ............16 Figure 10: The synthesis of different CNx morphologies, bulk, mesoporous, macro, SBA and hollow sphere CNx. A) The addition of silica and cyanamide. B) Calcination of sample at 550°C for 4 hours at 2 °C /min ramp rate. C) Removal of silica with NH4HF2. The C* refers to the NH4HF2 treatment on bulk CNx because this was not to remove silica; this was done to stay consistent with preparation of each sample. ..............................................................................20 Figure 11: Comparison of FTIR Spectra A) is the full spectra of the different CNx morphologies bulk, meso, macro and SBA B) Magnified region of the FTIR spectra. Peak shifts are emphasized with the black dotted line and labelled (a-f) corresponding to peaks shown in Table 1................................................................................................................................................21 Figure 12: A) Full FTIR spectra of macro CNx, macro silica + CNx composites, the silica template B) FTIR spectra region between 1800 cm-1 -600 cm-1 to highlight the silica absorption band at 1090 cm-1. ....................................................................................................................22 Figure 13: The top structure shows the melon model of CNx with highlighted elements which correspond to C 1s and N 1s XPS peak assignments. Panels A-D show the C 1s XPS spectra of the CNx morphologies bulk, meso, macro and SBA CNx. Panels E-H show the N 1s spectra of CNx morphologies bulk, meso, macro and SBA CNx. .................................................................26 Figure 14: Left: pXRD graph of bulk, meso, macro and SBA CNX. right: Table with the angle and d spacing of the 002 pXRD peak. ..............................................................................................28 Figure 15: A) UV-Vis Diffuse Reflectance (DRS) B) Kubelka-Munk transform of the DRS of Bulk, meso, macro and SBA CNx. Where K is the absorption coefficient and S is scattering. .............29 Figure 16: Tauc plots and line of fit (grey dash) used to determine the band gap energy and the extracted band gap on the bottom right of each graph. .............................................................30 xiii Figure 17: Graphs for bulk, meso, macro and SBA CNx plotted to determine the Urbach tail energy by adding a linear fitting (blue) to the linear section (black) of the ln(K/S) vs E. Points excluded in the fitting are in red. ...............................................................................................31 Figure 18: The photoluminescence spectra of bulk, meso, macro and SBA CNx at 405 nm excitation wavelength. ...............................................................................................................32 Figure 19: SEM images of the silica sphere template used to template macro CNx (A 10 kx and B 20 kx) and SBA-15 silica template used to template SBA CNx (C 10 kx and D 20 kx) ............33 Figure 20: SEM images of all CNx morphologies. Panels show Bulk CNx A-D, Meso CNx E-H, Macro CNx I-L and SBA CNx M-P. Each row has an SEM image at 1 kx, 5 kx, 10 kx and 20 kx across the row from left to right. ................................................................................................34 Figure 21: TEM images of Meso CNx. A and B are different magnifications of the same area and C and D are different magnifications of a second area of the same sample. .............................35 Figure 22: Comparison of the CNx morphologies at a magnification of 20kx A) bulk, B) meso (with a TEM image inset outlined in yellow) C) macro and D) SBA CNx. ...................................36 Figure 23: TAS spectra of A) bulk B) meso C) macro and D) SBA CNx .....................................39 Figure 24: The log-log plots of the TAS decay kinetics at 750nm normalized at 4 μs of A) bulk B) meso C) macro and D) SBA CNx ...............................................................................................41 Figure 25: TAS decay kinetics normalized at 4 μs at 750 nm, 850 nm, and 1000 nm. ...............41 Figure 26: Hydrogen evolution obtained for bulk, meso, macro, and SBA CNx samples under photocatalytic conditions. ..........................................................................................................42 Figure 27: Schematic showing the general procedure followed to synthesize CNx HS ..............45 Figure 28: SEM images of Silica core (A) and silica core-shell (B) both synthesized with reagent grade alcohol and crucible lid on during porogen removal both at 40kx magnification. ..............47 Figure 29: SEM images of silica core (A) and silica core-shell (B) both done with 95 % ethanol and crucible lid off during porogen removal at 50kx and 40 kx magnification respectively. ........47 xiv Figure 30: FTIR comparison of silica calcined with no lid (NL) (grey) silica calcined with a lid (WL) (brown), silica not calcined (purple) A) the full spectra, B) zoomed in region of the full spectra from 4000 cm-1 to 2300 cm-1 .........................................................................................48 Figure 31: SEM images of the same HS CNx synthesis done without grinding silica (WL) and without vacuum shown at different magnification (A) at 10 kx and (B) 20 kx magnification respectively. ..............................................................................................................................49 Figure 32: SEM images of A) un-ground core-shell silica, B) ground core-shell silica, C) composite of the silica and cyanamide before the silica was dissolved, D,E) CNx after silica remove at 20 kx magnification and 10 kx magnification respectively. ........................................50 Figure 33: A and B are two different areas of the same HS CNx sample. The synthesis was done under vacuum with the excess cyanamide removed by centrifugation with the GeneVac at 55 C . The images are both at 20 kx magnification. .................................................................51 Figure 34: SEM image of HS CNx, taken and synthesized by Destiny Ellenor’s CNx. Image at 30 kx magnification and reproduced with permission from Destiny Ellenor. ...................................52 Figure 35: SEM images of HS CNx synthesized with 30 C H2O A) 20kx and B) 40kx and HS CNx synthesized with room temperature water, C) 20 kx, D 40kx. .............................................53 Figure 36: SEM images of HS CNx under different synthesis conditions all at 20 kx magnification. Image A and B are comparing different centrifugation speeds of 1000 and 2000 RPM respectively. Images C and D are comparing different centrifugation temperatures achieved by varying wait times before centrifugation at 5 and 4 mins respectively. ...................54 Figure 37: SEM image of HS CNx synthesized with 10 g cyanamide and 2 g of core-shell silica, a calcination ramp rate (4 ˚C/min), with a 5-minute 4000 RPM centrifugation. Different sample spots were imaged in A, B at 20 kx magnification, C at 40kx and D at 60kx magnification. .......55 Figure 38: The Surface Area vs. Activity relationship between the successfully synthesized CNx morphologies investigated in this study. ....................................................................................59 xv List of Symbols and Abbreviations % Percent %Abs(t) Changes in Absorption ˚ Degrees ∝ Proportional ∼ Approximately ≈ Approximately Equal °C Degree Celsius A Acceptor Å Angstrom AQY Apparent Quantum Yield B Constant BET Brunauer-Emmett-Teller BP Black Phosphorus C18TMOS n-octadecyltrimethoxysilane CB Conduction Band CD Carbon Dot cm-1 Wavenumber CNx Carbon Nitride CS Charge Separation CT Charge Transfer CTAB Cetrimonium Bromide D Donor d Distance DCDA Dicyandiamide DOS Density of States DR Diffuse Reflectance DRS Diffuse Reflectance Spectroscopy Dt Detector E Energy Ec Characteristic Energy for Tail of Trap States ef Free Electron Eg Band Gap xvi EIS Electrochemical Impedance Spectroscopy EM Electron Microscopy Eu Urbach Energy e-tr Trapped Electron eV Electron Volt F Faraday’s Constant FL Fluoresce FTIR Fourier-transform Infrared FWHM Full Width Half Max g Grams GHG Greenhouse gas GSB Ground State Bleaching h Planck’s Constant h+tr Trapped Hole hf Free Hole HOMO Highest Occupied Molecular Orbital HS Hollow Sphere hv Light I Incident Photons in vacuo Under Vacuum IR Infrared J Joules k Rate Constant K Absorption Coefficient Kb Boltzmann constant kg Kilograms kJ Kilojoules kx 1000 Times LUMO Lowest Occupied Molecular Orbital M Molar m Meters M1 Mirror M2P Metal Phosphides Macro Macroporous xvii mCD Modified Microwave Method Carbon Dot Meso Mesoporous min Minutes ms Millisecond n Number of Electrons in a Reaction Nd:YAG Neodymium-doped Yttrium Aluminum Garnet NHE Normal Hydrogen Electrode NIR Near Infrared nm Nanometer ns Nanoseconds ntrap(E) Trap State OPO Optical Parametric Oscillator P Power PC Photocatalyst PIA Photoinduced absorption PL Photoluminescence PV Photovoltaic PV Photovoltaic pXRD Powder X-ray Diffraction R Rate R0 Initial Reflectance rec Recombination Rf Reflectance Rf(t) Relative Transient Reflectance RL Relax RPM Revolutions Per Minute rxn Reaction S Scattering s Seconds SBA Santa Barbara Amorphous SC Semiconductor sCD Sonication Assisted Oxidation Carbon Dot SEM Scanning Electron Microscopy T Temperature xviii TAS Transient Absorption Spectroscopy TEM Transmission Electron Microscopy TEOA Triethanolamine TEOS Tetraethoxysilane TRIR Time-resolved Infrared TRMC Time-resolved Microwave Conductance TRPL Time-resolved Photoluminescence UV Ultraviolet v Photon frequency VB Valence band Vis Visible XPS X-ray Photoelectron Spectroscopy α Linear decay αabs Absorbance Coefficient α0 Absorbance Constant γ Electronic Nature of band-to-band Transitions Δ Heat ΔG˚ Gibbs Free Energy θ Angle λ Wavelength μ Micro μM Micromolar μs Microsecond Σ Sum ϕ Quantum Yield xix Acknowledgements I would like to acknowledge the support of Dr. Robert Godin, who has provided invaluable advice. I am extremely grateful for his guidance. I would also like to acknowledge my committee members Dr. Isaac Li and Dr. David Jack for their support throughout this process. Grady Hewlett and Destiny Elleanor for their collaboration in this project. Jasper Pankratz and Matthew McConnachie for their support. Finally, I would also like to thank all the members of the SolarSpec lab throughout my time working there. xx Dedication To my parents, Sarah and Scott 1 Chapter 1: Introduction The need for renewable fuel sources is a critical challenge facing society and is driven by the depletion of non-renewable fuel sources and their environmental impact. Human production and consumption of energy accounts for 78% of Greenhouse gas (GHG) emissions.1 In 2020, the global CO2 emissions dropped by 6.4% due to the COVID-19 crisis.2,3 Though a reduction in emissions was seen, it was only temporary. In 2021 CO2 emissions are set to rise 1.2 billion tonnes, offsetting over 65% of the reduction seen in 2020.4 Current actions to reach the 2°C warming scenario outlined in the United Nations' 2015 Paris Accord are not enough. Global temperature rise is set to be at least 3°C by the end of the century which significantly surpasses the intended 2°C temperature target and the much more desirable 1.5°C senario.2,5 The rapid growth of renewable energy sources to decrease GHG emission and reduce our environmental impact is imperative for a sustainable future.6 To reduce GHG emissions and transition to a low-carbon society we need to develop efficient and reliable renewable energy sources. Solar energy holds impressive potential, as the energy from the sun which reaches the Earth's surface in one hour is larger than the yearly global energy consumption.7 Currently, the leading method of harvesting solar energy is photovoltaic (PV) energy production. This is the direct conversion of solar to electrical energy and is one of the fastest-growing sources of electrical energy in Canada.8 PV energy generation is dependent on environmental factors such as solar availability, and electrical energy storage, both of which present technical challenges. An alternative solar harvesting method is the production of solar fuels. Solar fuels are energy dense chemical fuels made using solar energy. These products, such as hydrogen, can be stored on large scales, and their energy can be released on-demand in fuel cells irrespective of the time of day and solar availability. This approach was inspired by natural photosynthesis, where sunlight can drive thermodynamically unfavourable reactions that produce energy. The production of solar fuels utilizing solar radiation is typically done using a semiconductor material which acts as the photocatalyst, absorbing solar radiation to drive the targeted reaction which produces energy-dense products. Hydrogen is an advantageous product not only due to its high energy density but its application in fuel cells to generate electrical energy which produces only water as a by-product. However, efficient renewable hydrogen production presents a challenge. Current hydrogen production is done using non-renewable fossil fuels, which is undesirable due to the environmental impact. Photocatalytic hydrogen production offers the potential for a renewable hydrogen source, however photocatalytic hydrogen production’s low efficiencies remain a challenge for a commercially 2 viable application. Research needs to be directed towards improving efficiencies while using environmentally friendly materials and methods to develop solar fuels into a commercially viable energy alternative. The use of organic semiconductor materials has gained attention in recent years due to the desire for easily synthesized materials from earth abundant elements. Specifically, carbon nitride (CNx) has been a leading candidate in photocatalysis research with over 20,000 publications since 2016. Important considerations when developing a photocatalyst to produce hydrogen fuel include the semiconductor material, the reaction mechanism, and the charge carrier dynamics. These important factors will be expanded on, exploring organic semiconductors and photocatalysis. This includes the charge carrier dynamics in photocatalysts and the methods used to monitor them, which are highly important for targeting efficiency improvements. Advances in CNx photocatalytic efficiencies have been achieved by forming CNx heterojunctions and CNx modifications. CNx modifications included surface functional group modifications, defect engineering and morphological changes to the material. Overall, many key factors must be considered for the development of a commercially viable photocatalyst for producing hydrogen. CNx morphological modifications done using a hard templating method is the focus of this thesis. An optical method, transient absorption spectroscopy (TAS) was employed to monitor the charge carrier dynamics in different CNx morphologies. This accompanied with electronic and structural characterization techniques have helped develop an understanding of the relationship between structure-photophysics-activity in CNx. This can help direct future research development towards designing an optimal hydrogen evolution photocatalyst. 1.1 Semiconductors The use of PVs and the production of solar fuels to convert solar energy to usable forms would not be possible without semiconductors. Semiconductors were first described by Michael Faraday in 1833 and were further developed after Karl Ferdinand Braun recorded a metal-semiconductor junction device.9 In solid materials like semiconductors there are many interacting atoms. Instead of a distinct highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital (LUMO), the energy levels of all interacting atoms mix and exist as bands. These bands are the valence band (VB) which is lower in energy and filled with electrons, and the conduction band (CB) which is higher in energy and unoccupied in the material's ground state configuration. The energy difference between the VB maxima and CB minima is the material's band gap. A semiconductor can be defined as a material with a band gap which is between 0 eV and approximately less than 4 eV (Figure 1).10 Metals have no band 3 gaps, and materials with band gaps larger than 4 eV are typically considered insulators. Semiconductors fall in between the two (Figure 1). Band to band transitions can occur upon excitation of the material. The band gap of semiconductors falls within the UV-Vis-NIR spectral range of the electromagnetic spectrum and therefore can absorb light. This means photoexcitation of semiconductors can occur, which is when the material absorbs sufficient energy to excite the electrons from the VB to the CB band, a key feature for photocatalysis. Figure 1: The difference in the bandgap of metal, semiconductor, and insulating solids. VB stands for valence band and CB stands for conduction band and the band gap is the space spanned by the double headed arrow. 1.1.1 Organic Semiconductors Organic semiconductors provide a versatile material that can be easily and inexpensively synthesized. They are also composed of earth abundant elements which are typically less toxic than inorganic semiconductors that contain metals or rare earth elements. When developing an environmentally sustainable, commercially viable photocatalyst it is important to consider the toxicity and availability of the material which is why organic materials present some clear advantages. The first organic semiconductor studies were on molecular crystals, such as anthracene, starting in the 1950s.11 The establishment of another class of organic semiconductors, conjugated polymers, occurred in the 1970s.12 Typically organic semiconductor band gaps range from 1.5-3.5 eV, which corresponds to the energy of visible light.12,13 Organic semiconductors have conjugated π-electron systems, creating tunable optoelectronic properties which is beneficial in their utilization for the capture of solar energy.12,14 4 The degree of conjugation (Figure 2A) or structure of the semiconductor can affect the band gap (Figure 2B). Increasing π conjugation and increasing polymer chain length (to an extent) leads to smaller band gaps in organic semiconductors.15 Some of the other factors identified to influence band gap are substituents, bond-length and intermolecular interaction.16 Figure 2: A) The scheme is showing the effect of conjugation on the band gap of molecular crystal semiconductors B) The effect structure has on the band positions of polymer semiconductors17–20. Distinct differences are seen in the optical properties and charge carrier transport in organic semiconductors compared to inorganic semiconductors. A conjugated organic semiconductor, has an inherently low dielectric constant and therefore photoexcitation generates an exciton, which is a coulomb-bound electron hole pair.21,22 The energy required for this electron and hole pair to become a separated electron and hole is known as the exciton binding energy. It is necessary for the exciton to dissociate within the exciton lifetime to generate separated charge carriers. Excitons are more localized in organic semiconductors which results in higher binding energies than in inorganic semiconductors.12,21 Understanding the key steps in how the charges move within the material is important for determining the efficiency of the material. Other important charge transport mechanisms in organic semiconductors can be classified into three 5 categories: band transport, multiple trap and release, and hopping transport (Figure 3).23–25 Band transport occurs in crystalline semiconductors and hopping transport occurs in disordered materials. This is because crystalline semiconductors have crystalline density of states (DOS) with well-defined band edges whereas in amorphous organic semiconductors they are more localized and the DOS can be described with a gaussian or exponential distribution.23,24 Multiple trap and release transport occurs in crystalline semiconductors with some disorder. Hopping transport occurs in amorphous organic semiconductors, such as CNx, which have a tail of states that extend into the band gap. They can be shallow or deep trap states depending on their position relative to the band edge. Charges can become trapped within these states and thermal excitation is necessary for the charges to de-trap. Deep traps can limit photocatalytic efficiencies by preventing the charges’ ability to participate in photocatalytic reactions.26 The pathway the charges take, and their behaviour are both important in understanding the photocatalytic efficiencies of the material. Figure 3: The charge transport mechanisms in an organic semiconductor. This is demonstrating how the charge carriers move. Where VB stands for valence band. 1.1.2 Semiconductor Photocatalysis Observations of photocorrosion and chemical reactions when semiconductors are exposed to light was the starting point for the development of semiconductor photocatalysis.27 The following equations (1.1-1.9) detail a semiconductor photocatalysis scheme.27 6 Figure 4: Left: Diagram showing the different processes in organic semiconductor photocatalysis. Right: The equations which represent the process shown on the left. The first process in organic semiconductor photocatalysis is light absorption (Figure 4 (1.1)). The band gap of the material governs the range of wavelengths of light which the material absorbs. To absorb visible light a band gap lower than 3.2 eV (∼380 nm) is necessary because this is the onset of the visible light. Light absorption results in the photoexcitation of the material and generates an exciton. The exciton can then undergo recombination (Figure 4 (1.2)). Recombination is when the electron and hole come together which can be radiative or non-radiative. The exciton could also dissociate into a separated electron and hole by overcoming the binding energy (Figure 4 (1.3)). The dissociated electron and hole can then be transported to the surface (Figure 4 (1.5)), recombine (Figure 4 (1.6), or become trapped in trap states (Figure 4 (1.4)). The electron/hole stuck in the trap can also undergo recombination (Figure 4 (1.7)). An electron which reaches the material’s surface can participate in surface redox reactions (Figure 4 (1.7-1.8)) or undergo surface recombination. The charge carrier dynamics of a material are critical for understanding photocatalytic efficiency improvements. The productive steps the charges go through in a photocatalytic process can be broken down into 4 general steps: light absorption (Figure 4 (1.1)), charge separation (Figure 4 (1.3)), charge transport (Figure 4 (1.5)), surface redox reaction (Figure 4 (1.7-1.8)). These processes are also in competition with non-productive charge recombination (Figure 4 (1.2, 1.6, 1.7)). The efficiency of each step is determined by the kinetic competition of the productive processes vs. the non-productive processes (Figure 5). Collectively, the pathways taken by charges and their kinetics are referred to as the charge carrier dynamics. For example, a photocatalyst (PC) when photoexcited can either relax or fluoresce, both follow a first order reaction that can be described by the following equations: 7 𝑃𝐶∗𝑟𝑒𝑙𝑎𝑥 (𝑅𝐿)→ 𝑃𝐶 + ∆ 𝑃𝐶∗ 𝐹𝑙𝑢𝑜𝑟𝑒𝑠𝑐𝑒(𝐹𝐿)→ 𝑃𝐶 + ℎ𝑣 The rate of these reaction can be written as: 𝑅𝑅𝐿 = 𝑘𝑅𝐿[𝑃𝐶∗] 𝑅𝐹𝐿 = 𝑘𝐹𝐿[𝑃𝐶∗] The ratio of the products is equal to the ratio of the rate of the two reactions: ∆ℎ𝑣=𝑅𝑅𝐿 𝑅𝐹𝐿 We can now write: ∆ℎ𝑣=𝑘𝑅𝐿[𝑃𝐶∗]𝑘𝐹𝐿[𝑃𝐶∗]=𝑘𝑅𝐿𝑘𝐹𝐿 This simplification shows that the rate constants are the determining factor in the ratio of products. In this example we would like to know how often the charge carriers will fluoresce or relax non-radiatively. The first order kinetics determine the rate constants are proportional to the amount the material will fluoresce and non-radiatively relax. Therefore, the quantum yield will be the rate constant of the process of interest over the rate constant of all processes. For this example, the quantum yield is shown. 𝜙 =𝑘𝑟𝑥𝑛∑𝑘 𝜙 =𝑘𝐹𝐿𝑘𝑅𝐿 + 𝑘𝐹𝐿 Determining the timescales of different photophysical processes and kinetic competition provides insight into limitations and allows for the identification of causes of improvements. Charge recombination typically can occur faster than the productive processes, charge separation, charge transport and surface redox reactions. The overall efficiency is dependent on the yield of each productive step (Figure 5). If one of the productive processes is limiting, the overall reaction yield will be low. Identifying the bottleneck in the productive processes allows for the investigation of targeted improvements to overcome this limitation. Typically, efficiencies of different photocatalysts are compared by reporting the apparent quantum yield (AQY). AQY(%) =nRI This is where the n is the number of electrons in a reaction multiplied by the rate (R) over the incident photons (I). The key information for determining the AQY is the rate of the reaction, which is measured by looking at the amount of the desired product produced over time. 8 Figure 5: Steps in photocatalytic water splitting in organic semiconductor. The productive steps 1-4 are in competition with charge recombination. 1.3 Transient Absorption Spectroscopy (TAS) To determine how fast different processes are taking place and what will dominate the kinetic competition, we need to measure the kinetics. Typically, kinetic studies are performed with electrical or optical techniques.28 Some of these optical methods include transient absorption spectroscopy (TAS),29–31 time-resolved infrared (TRIR) spectroscopy, 32,33 time-resolved microwave conductance (TRMC) spectroscopy,34,35 time-resolved terahertz (tr-THz) spectroscopy,28,36,37 and time-resolved photoluminescence (TRPL) spectroscopy.38,39 The electrical methods include electrochemical impedance spectroscopy (EIS),40 and transient photovoltage or photocurrents.41 For systems such as CNx photocatalysis, the photocatalytic system is commonly a heterogeneous dispersion. Therefore, the use of optical methods for studying the kinetics of CNx photocatalysis are more suitable because the conditions are more like the operating conditions of the system.42,43 TAS is an optical technique that has been used to elucidate the charge carrier dynamics in different types of systems such as solar fuels,42 organic photovoltaics,44 and solar cells.45,46 When the material being investigated is photoexcited, changes in electronic structure and electronic transitions create changes in absorption, which can be measured using TAS. TAS can also measure over a wide range of timescales from femtosecond to second. In the spectrum, electronic transitions that result in absorbance in the ground state and excited state will be monitored. The changes in absorbance in the excited state can be small (>10−2), which is why TAS spectra show the change in absorbance. This allows spectral features such as ground state bleaching (GSB) and photoinduced absorption (PIA) components to be identified (Figure 6B,C). TAS decay kinetics show the excited states over time at a specific wavelength. They 9 typically display an initial high absorbance that decreases over time. Power-law decays of the TAS kinetics indicates the presence of trapped charges within the material and can quantitatively classify the distribution of trap states.26 The TAS spectrum can be built from the decay kinetics done at a range of wavelengths. This is made by plotting the changes in absorption at different times and wavelengths which will generate a TAS spectra. Diffuse reflectance TAS (DR TAS) is necessary for opaque samples where light does not pass through the sample, such as powders. In a typical DR TAS set up, shown in Figure 6, a laser (Nd:YAG) is used to excite the sample and a light source is used to probe the transient changes in absorption produced after photoexcitation (Figure 6). The light is focused on the sample which is reflected and directed towards the monochromator with mirrors.28 The monochromator is used to select the wavelength observed and block out stray light from reaching the detector. Figure 6: A) Schematic representation of DR TAS setups for µs-TAS. The 1064 nm output of the Nd:YAG is frequency tripled to 355 nm and can pump the OPO to generate pulses with tunable wavelengths. The abbreviations in the diagram include optical parametric oscillator (OPO); mirror (M1); detector (Dt). B) A representative absorption spectrum for the ground and excited states. C) The difference in absorbance of the traces. Photoinduced absorption (PIA) is in blue and the ground state bleaching (GSB) is in brown. 10 The changes in absorption (%Abs(t)) can be calculated using the equation as follows:47,48 %Abs(𝑡) = 1 − Rf(𝑡) =𝑅𝑓0 − 𝑅𝑓(𝑡)𝑅𝑓0 Rf(t) is the relative transient reflectance, and Rf0 is the relative reflectance before the sample’s photoexcitation. The changes observed in absorbance are related to changes in the population of the excited states in the sample. TAS can provide important information on the kinetics of the charge carriers and excited state electronics of the material which is important for understanding photocatalytic systems and their efficiencies. 1.4 Photocatalytic Water Splitting Developing the understanding of photocatalytic water splitting systems is an important step towards hydrogen solar fuel production. The concept of producing solar fuels for energy was inspired by natural photosynthesis, where light is used to produce oxygen and organic compounds for energy. The idea is that artificial photocatalytic processes can utilize available solar radiation to produce chemical fuels, which contain a high density of energy. This includes products such as hydrogen fuel or the reduction of carbon dioxide to products such as methanol. In 1972 Honda and Fujishima showed that by irradiating a TiO2 semiconductor electrode, you could reduce the applied potential required to split water.49 This showed that sunlight can be used to facilitate electrochemical reactions, which started research into the area of solar fuel production. Other photocatalytic driven reactions explored include the reduction of CO2 to carbon species such as CO and methanol, as well as the degradation of organic dyes and pollutants. The thermodynamic requirements of the target redox reactions are important to consider when selecting an appropriate semiconductor photocatalyst. Water splitting, the oxidation of H2O to O2 coupled with the reduction H+ to H2, is an unfavorable reaction and requires energy to drive the reaction.50 The half reactions and overall water splitting reaction are shown below: Oxidation 2H2O → O2 + 4H+ + 4 e- 1.4.1 Reduction 2H+ + 2e- → H2 1.4.2 Overall Redox H2O → ½O2 + H2 1.4.3 This reaction has a Gibbs free energy (ΔG˚) of 238 kJ mol−1 which can be related to potential by the number of mols per electron (n) and Faraday’s constant (F) which corresponds to 1.23 eV per electron for water splitting. The equation is shown below. ∆𝐺° = −nF𝐸° 11 This can be done by utilizing photocatalysts with appropriate valence band (VB) and conduction band (CB) positions (Figure 7). An ideal photocatalyst has band positions that straddle the energy level of the desired redox reaction. This is so the VB position has a low enough potential that there is sufficient energy to drive the proton reduction reaction and the holes have high enough potential to oxidize water. Figure 7: Band position of carbon nitride and their relation to the potentials of the half reactions of water splitting. Proton reduction (H+/H2) and water oxidation (O2/H2O) vs. NHE. 1.5 Carbon Nitride Carbon nitride (CNx) is an organic semiconductor material that has shown promise as a photocatalyst for water splitting. Wang et al. first identified CNx as a candidate for photocatalytic hydrogen evolution in 2009.51 Since this discovery, CNx has been at the forefront of photocatalysis research.52 This material can be synthesized by the thermal condensation of nitrogen rich precursors, such as cyanamide, urea or dicyanamide. CNx is made up of carbon and nitrogen which form a π conjugated system of tri-s-triazine rings (Figure 8). This material is chemically and thermally stable, absorbs in the visible region and has a tunable morphology and chemical structure. However, CNx is also limited by fast charge recombination, low surface area, and limited light absorption in the visible range. The tunability of the material allows for optimizations that could help to overcome these disadvantages. Currently, photocatalytic hydrogen production using CNx has exceeded efficiencies of 50% with the use of sacrificial reagents.53 However, the efficiency of overall water splitting with a CNx photocatalyst remains on 12 the order of a few percent.53 Practical sustainable water splitting by CNx (efficiencies > 10% are targeted54) has not been achieved. The high efficiencies attained with sacrificial reagents highlight the potential of utilizing CNx.55,56 Recent advances to improve efficiencies of CNx will be discussed next which will include heterojunctions as well as surface and morphological modifications7 Figure 8: A) on the left is the chemical structure of heptazine, the repeat unit carbon nitride and on the right is the melon representation of the chemical structure of carbon nitride (CNx). In B) the absorbance (from the Kubelka-Munk transform) and photoluminescence (PL) spectra of CNx. CNx powder is shown in the image inset on the graph. (Reproduced with permission from ref. 42) 1.5.1 Heterojunction Formation CNx is restricted by its fast charge recombination, limited light absorption and low charge carrier mobility. For this thesis we pursued modifications of CNx morphology, discussed in section 1.5.2 below. For completeness, here we describe a common alternative method to improve the photocatalytic activity. The utilization of heterojunctions and cocatalysts with CNx is a common method to improve photocatalytic efficiencies. The material used to create a CNx heterojunction is selected based on its potential to improve the limitations of the CNx alone. An example of this would be the use of a material to increase light absorption, broadening the range of visible light which can be utilized. CNx heterojunctions can be made with a wide range of materials such as carbon dots,57–59 metal oxides,60,61 chalcogenides62–64 and many more.43 13 Charge carrier dynamics studies on transition metal phosphides,65,66 black phosphorus,67 and carbon dots (CD)68 have shown notable findings towards developing improved carbon nitride photocatalysts. The first-row transition metals Fe, Co, and Ni phosphides have been studied as materials for CNx heterojunctions.65,66 Metal phosphides (M2P) were combined with sulfur doped CNx and the TRPL decay showed that the formation of the M2P/S-CNx heterojunctions resulted in shorter decay lifetimes compared to the S-CNx alone.65 There was a clear trend of shortened lifetimes associated with increased activity in all the samples studied. Another study also investigated the use of M2Ps, (Fe2P, CoP and Ni2P) to create ternary junctions with sulfide (MoS2) and CNNS.66 The TRPL decay lifetimes show the average lifetime decreased with increasing activity, which was also seen by Sun et al. The trend in decreased lifetimes is suggested to result from charge transfer between CNx and the metal phosphide catalyst, which is presumably related to reduced recombination and improved charge separation. The utilization of black phosphorus (BP) for the creation of a BP/CNx heterojunction was explored using TAS. The results highlighted an example where TAS can provide important information on the mechanism of improvement in the material. It was found that the heterojunction resulted in longer electron decay lifetimes. This was suggested to be due to the formation of trap states created when forming the heterojunction. This was further confirmed by near infrared TAS which concluded there were longer charger carrier lifetimes in BP/CNx compared to BP alone. It was suggested that at the interface of the heterojunction the P-N bonds act as a trap state and combined with the low electronegativity of phosphorus create an active site for proton reduction, improving the photocatalytic hydrogen production. A TAS study was also performed under visible light which mainly monitored electrons in the CNx. This investigation indicated charge transfer occurs from the CNx to the BP. This study highlighted how heterojunctions can result in improved photocatalytic hydrogen evolution using BP/CNx by creating an active site for proton reduction and increasing electron decay lifetimes. A metal-free CD/CNx junction investigation studied two different types of CDs and discovered that the type of CD used resulted in different flows of charges. The different charge flows resulted in significantly different photocatalytic activities.68 The first type of CDs were prepared using a sonication assisted oxidation (sCD) and were found to accept electrons. The second type of CDs were synthesized from urea and citric acid using a modified microwave method (mCD) and were found to accept holes. TAS was used to identify the differences in charge flow resulting in a clear identification of the photocatalytic improvements. TAS showed an increase in the electron:hole ratio in mCD/CNx which can be interpreted as a decrease in the 14 number of holes in CNx, suggesting holes are transferred to the mCD. A decreased TAS signal in the sCD/CNx TAS suggested electron extraction from the CNx to sCD. The mCD/CNx material had a 12-times larger photocatalytic CO2 reduction than the sCD/CNx. The mCD/CNx was found to improve charge separation at the interface. The accumulation of holes on the mCD potentially helped mitigate kinetic issues with the timescale offset of the competing pathways of water oxidation (˜1s) and charge recombination (<μs). Though carbon nitride heterojunctions have been proven to provide valuable efficiency improvements in photocatalytic efficiencies, the complex nature of the systems make understanding the underlying improvement mechanisms difficult. However, some key examples have shown kinetic investigations can provide essential information on their impact on photocatalytic activity. 1.5.2 CNx Modifications The modification of CNx surface, functional groups and morphology are important methods used to improve photocatalytic efficiency. Modifications can affect active sites on the material, charge carrier dynamics, and surface area, which can all affect photocatalytic production. These methods can include approaches such as surface functional group modification, defect engineering and nanostructure engineering.7,69 Surface functional group modification is done by altering the functional groups on the surface of the CNx through changes in the synthesis procedures or post-synthetic modifications. By investigating different degrees of polymerization and how the structural features affect CNx photocatalytic hydrogen evolution, Lau et al. ascribed the superior hydrogen evolution of melem oligomers to the exposure of active amine groups.70 This highlights the importance of understanding the active functional groups in a material and the importance of CNx polymers chain termination for improving the photocatalytic activity. This research group also investigated modifications of functional groups in CNx by another method which showed the addition of KSCN resulted in the addition of cyanamide groups on the surface of CNx.71 Their results showed the cyanamide modified CNx had a 12 fold increase in photocatalytic hydrogen production compared to unmodified CNx with a Pt cocatalyst. This increased activity is ascribed to the increased number of cyanamide active sites (-N-C≡N). The cyanamide is suggested to have better coordination with the Pt cocatalyst giving the increased activity. Surface group modification can provide information into key structural features which improve photocatalytic activity in CNx. 15 Defect engineering incorporates the addition of impurities (doping), vacancies in the materials, as well as any other defects which can be introduced into the material. Defect engineering can improve photocatalytic activity by influencing the electronic and chemical structure of CNx. Point defects are the typical category of defects employed with CNx. Point defects can include vacancies, interstitial defects and replacement defects.72 The addition of nitrogen vacancies has been shown to tune the band of CNx by narrowing band gap and improving light absorption.73,74 Carbon vacancies in CNx have also been suggested to extend light absorption into the NIR and can create active site for Pt deposition.75,76 Both types of vacancies have been shown to improve photocatalytic efficiency of CNx. Nanostructure engineering involves different morphological modifications to CNx. This is a promising strategy for improving photocatalytic efficiencies. This includes the synthesis of CNx into different shapes such as nanotubes,77,78 spheres,79,80 nanosheets sheets,81,82 and other morphological modifications.7,69 Methods used to achieve these types of modifications typically use either a hard or soft templating method. The soft templating methods used to make CNx utilize surfactants, ionic liquids, and gas bubbles to direct the assembly of the material into the desired shape.69 The amphophilic properties of surfactants allow for the self-assembly of the material into the desired shape. Some examples of soft templating in CNx synthesis are the use of a non-ionic surfactant, triton X-100 and melamine sulfate precursor.83 This produced nanoporous CNx with a pore size range of 45 -60 nm depending on the synthesis conditions. The nanoporous CNx resulted in large surface area and improved photocatalytic dye degradation compared to bulk CNx. Another method that has been investigated is the use of ionic liquids, which are salts that are in a liquid state under 100°C, to modify the morphology.69 Ionic liquids provide methods of creating mesoporous and textured CNx while allowing for the potential to incorporate doping with phosphorus, boron or fluorine.84,85 The third soft templating method involves the use of gas bubbles for surface regulation.69 This concept is based on the formation of by-products during the CNx synthesis. When CNx is polymerized through a heating procedure, ammonia is evolved. When using urea precursors, which contain oxygen, water is evolved in the CNx synthesis procedure, and these bubbles can create morphological modification by creating pores in the CNx. The other method of morphological modification is hard templating. This method involves casting the CNx precursors into a pre-synthesized solid mold. The template-precursor composite is then subjected to the synthesis procedure to obtain the desired material. Hard templating methods have some advantages as it is easier to control the shape of the mold to obtain the desired morphology. Hard templating can also present drawbacks with the removal of the 16 template. Some templates which have been used include anodic alumina oxides and silica. Silica is the more commonly used method, as its shape is easily tunable and can be synthesized into a variety of arrangements for hard templating. Silica has been previously used as a template for the creation of spheres,79,80 hollow spheres,86,87 and mesoporous CNx.88–93 Sun et al. synthesized hollow spheres (HS) CNx by creating a silica template with a solid silica core and a mesoporous outer silica shell.86 The CNx HS achieved hydrogen evolution over 24 times improved compared to bulk CNx. It is suggested that this is due to the HS ability to have multiple reflections within the structure, resulting in increased path length for increased light absorption and also resulting in differences in structure condensation compared to bulk CNx. Spherical and mesostructured CNx have also been successful with the hard templating methods exhibiting over 45-fold and 5-fold improvements compared to their bulk CNx comparison respectively.80,92 Figure 9: A) An example of a hard templating method to synthesize a modified CNx morphology. B) An example of a soft templating method to synthesize a modified CNx morphology. 1.6 Research Objectives CNx nanostructure surface modification investigations into the importance of morphology, focusing on surface area to achieve higher photocatalytic efficiencies, have been inconclusive.94–96 Positive correlations, as well as variances between CNx photoactivity and surface area, have been previously identified. Improved photocatalytic efficiencies are commonly attributed to increased surface area due to an increase in active sites, improved charge separation and transfer ability.88,97,98 The use of a hard templating silica method to create 17 different CNx morphology has shown positive correlations between increased surface area and increased photocatalytic activity.88,99 However, they have also shown inconsistent correlations between surface area and activity.86,100 Currently it is known that surface area is an important component for high photocatalytic activity but it has been shown that other factors are also important. Other factors which are currently under consideration are structure condensation, charge mobility and light absorption. The presence of the silica template during CNx polymerization could affect the electronic structure and the charge carrier dynamics of the material, which could create the inconsistencies seen in surface area activity correlations. Understanding photocatalytic enhancements caused by improved morphology is critical to developing design guidelines to devise a CNx photocatalyst with high activity. I set out to rationalize the complex influence of morphology, identify the sources of activity loss or gain, and build clear structure-photophysics-activity relationships. To accomplish this, different CNx morphologies including bulk CNx, mesoporous CNx, large pore CNx, SBA CNx and hollow sphere CNx samples were prepared. The modified morphologies were prepared using a silica hard templating method. Brunauer-Emmett-Teller (BET) surface area measurements and electron microscopy (EM) were used to confirm the morphological modifications of CNx. The charge carrier dynamics in the different CNx morphologies were investigated using Transient Absorption Spectroscopy (TAS). TAS can monitor the trapped states as well as identify changes in electronic structure in the different CNx morphologies. The use of BET, EM, TAS, and monitoring the H2 production of the different CNx morphologies will help to build a CNx structure-photophysics-activity relationship. This information is needed to understand the limitations and mechanisms of improvement in the different CNx morphologies and produce an optimal hydrogen evolution photocatalyst. 18 Chapter 2: Carbon Nitride Characterization 2.1 General Synthesis To establish a strong surface area-photophysics-activity relationship, different CNx morphologies were synthesized. Previously Li et al. identified that different CNx morphologies synthesized using a hard templating method showed a non-linear surface area-activity relationship.100 In their study the SBA CNx and mesoporous CNx had surface area increases of 1.3-fold and 15-fold, and activity increases of 34-fold and 33-fold, respectively, compared to bulk CNx. The complex relationship between the surface area and activity of CNx prompted this thesis project on the charge carrier dynamics of different CNx morphologies to gain more insight into this unclear area of research. A hard templating synthesis model was employed to create four different CNx morphologies along with a bulk CNx reference. The hard templating material used was silica (SiO2), due to its robust nature, facile formation and the availability of a variety of mold shapes. This method of modifying the morphologies involved the addition of CNx precursors within or surrounding silica molds. The samples were then calcined at the desired temperature resulting in the thermal condensation of the nitrogen rich precursor into CNx. The silica is dissolved using ammonium bifluoride leaving behind the CNx product. The silica molds used were silica spheres (12 nm diameter), larger silica spheres (300 nm diameter), mesostructured SBA-15 silica and dense core-porous shell silica spheres to make mesoporous (meso CNx), macroporous (macro CNx), and SBA CNx and hollow sphere (HS CNx) CNx morphologies, respectively. The consistency of the preparation method between CNx samples was critical as the precursor, calcination rate, and temperature can all affect the polymerization of CNx.101–103 This would result in differences in the observed photocatalytic activities. Figure 10 shows the scheme for the synthesis of all CNx morphologies studied. All CNx samples were prepared by the calcination of 2 grams of cyanamide in an alumina crucible with a lid at 550 °C for four hours in a muffle furnace using a heating ramp rate of 2°C/min. This was consistent throughout all samples to remove inconsistencies with sample preparation. Briefly the CNx morphological modifications were performed as follows. Mesoporous (meso) CNx was synthesized using a cyanamide/silica mixture. This procedure used a colloidal silica solution (40% wt. LUDOX) with 12 nm spheres to create approximately 12 nm pores in the CNx material. The cyanamide was heated to 70 °C and the aqueous LUDOX solution was added dropwise. The mixture was then calcined at 550 °C to form the meso CNx product after etching of the silica mold. The next morphology, macro CNx, was synthesized by 19 mixing the 300 nm silica spheres with molten cyanamide which was then dried, calcined, and etched. The SBA CNx was synthesized using a solution of water, SBA-15 silica, and cyanamide that was then dried, calcined, and etched. SBA-15 silica has hexagonally arranged porous channels,104 which were 10 nm in diameter in this case, to create cylindrical tube shaped CNx. The final morphology on the right side of Figure 10 is hollow spheres CNx. The first step was to synthesize the silica mold which was a dense silica core with a mesoporous outer shell. This silica was mixed with molten cyanamide under sonication and vacuum then dried and calcined. These five morphologies, bulk, meso, macro, SBA and hollow sphere, were the focus of the morphological investigations into the surface area-photophysics-activity relationship. In this chapter the characterization of bulk, meso, macro and SBA CNx will be discussed. The synthesis of the hollow sphere morphology was not successful. Further optimizations to establish a reproducible protocol are required and will be discussed in Chapter 3. 20 Figure 10: The synthesis of different CNx morphologies, bulk, mesoporous, macro, SBA and hollow sphere CNx. A) The addition of silica and cyanamide. B) Calcination of sample at 550°C for 4 hours at 2 °C /min ramp rate. C) Removal of silica with NH4HF2. The C* refers to the NH4HF2 treatment on bulk CNx because this was not to remove silica; this was done to stay consistent with preparation of each sample. 2.2 Characterization In this section the characterization of bulk, meso, macro and SBA CNx morphologies are discussed. The bulk, meso and SBA were made in triplicate and the macro sample in duplicate, to confirm reproducibility. Structural and electronic characterizations were performed on each 21 morphology to confirm the successful synthesis of the desired morphology, as well as the optical and electronic properties of the materials. 2.2.1 Structural Characterization This section includes the structural characterization methods Fourier-transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and powder X-ray diffraction (pXRD). 2.2.1.1 Fourier-transform Infrared Spectroscopy Fourier-transform infrared (FTIR) spectroscopy was used to confirm the chemical structure and production of CNx for the different morphologies synthesized. The IR wavelengths absorbed correlate to different bond vibrations in the molecule. The FTIR spectra of all CNx morphologies present similar peaks (Figure 11). The broad band observed around 3100 cm-1 is due to bridging NH groups, terminal NH2 group and OH groups in CNx from partial condensation and adsorbed water.105–107 The peaks at 1200 cm-1 to 1650 cm-1 are stretching vibration modes seen for aromatic CN heterocycles typically observed with CNx.86,92,93,108,109 The sharp peak observed at 804 cm-1 for all samples is characteristic of the breathing mode for triazine and heptazine based structures.105,109–112 This is a critical diagnostic peak for identifying the formation of CNx, as the heptazine units are the main chemical structural component of the material. Figure 11: Comparison of FTIR Spectra A) is the full spectra of the different CNx morphologies bulk, meso, macro and SBA B) Magnified region of the FTIR spectra. Peak shifts are emphasized with the black dotted line and labelled (a-f) corresponding to peaks shown in Table 1. The removal of the silica template was also confirmed by FTIR. The SiO2 FTIR spectrum shows strong absorption bands at 1090 cm-1 and 795 cm-1 for asymmetric and symmetric vibration of SiO2, respectively.113 These are clearly seen in the silica FTIR shown in Figure 12 22 for the silica spheres used for the synthesis of macro CNx. The FTIR spectra of the silica, the CNx/silica composite pre-NH4HF2 etching and the CNx after the etching are shown in Figure 12. The FTIR spectrum of the CNx composite (CNx and silica) displays the characteristic bands for silica at ∼1100 cm -1. After NH4HF2 etching, the characteristic silica band was not present in the CNx product. This is a clear indication the silica was dissolved in the etching process leaving only CNx with the modified morphology. The confirmation of silica removal using FTIR was performed for all CNx morphologies modified using a silica template and confirmed effective template removal. Figure 12: A) Full FTIR spectra of macro CNx, macro silica + CNx composites, the silica template B) FTIR spectra region between 1800 cm-1 -600 cm-1 to highlight the silica absorption band at 1090 cm-1. The FTIR spectra of all four morphologies were compared closely, shown in Figure 11 and Table 1. It is evident that all the peaks are similar across all samples. For example, the 804 cm-1 peak typically found in CNx is the same for all samples. However, some differences were observed. The macro and SBA morphologies showed shifted peaks compared to bulk CNx in the region from 1200 cm-1 to 1650 cm-1. The peak at approximately 1200 cm-1 (peak b) was shifted 4 cm-1 and 8 cm-1 towards higher wavenumbers for macro and SBA CNx, respectively, compared to bulk CNx. Peak shifts towards higher wavenumbers were also seen at 1400 cm-1 (peak d) where macro and SBA CNx are shifted 2 cm-1 and 8 cm-1. SBA CNx also showed peak shifts at 1550 cm-1 (peak e) which was shifted 10 cm-1 (to higher wavenumber) and at 1620 cm-1 (peak f) of 19 cm-1 (to lower wavenumber). Full details on the peak shifts can be seen in Table 1. As described above, these peaks are due to aromatic CN heterocycle vibration modes. The shifts may be due to differences in electronic structure which could result from changes in the connectivity due to hydrogen bonding and interlayer stacking in each sample. The silica mold could influence the CNx polymerization which will affect the connectivity of the material and result in these small FTIR shifts observed. In the FTIR of the CNx-silica composites the most 23 prominent silica peaks appear in the SBA CNx which is consistent with the most significant peak shifts seen in the SBA CNx FTIR. This supports that the silica maybe have affected polymerization and therefore connectivity of the material resulting in the peak shifts. Table 1: FTIR peak position of characteristic CNx peaks of each morphology. Positive peak shifts are towards higher wavenumbers and negative peak shifts are towards lower wavenumbers. Peak approximation (cm-1) Peak label in Figure 11 Bulk CNx peak position (cm-1) Mesoporous CNx Peak Shift* (cm-1) Macro CNx Peak Shift *(cm-1) SBA CNx Peak Shift* (cm-1) 800 a 804 0 0 0 1200 b 1228 0 4 8 1300 c 1315 -2 0 -2 1400 d 1402 0 2 8 1550 e 1556 0 0 10 1620 f 1622 4 0 -19 *Compared to Bulk CNx 2.2.1.2 X-ray Photoelectron Spectroscopy X-ray photoelectron spectroscopy (XPS) gives information on the chemical bonding in samples, identifying the types of elements and information on the element’s substituents. The surface of the material is irradiated with X-rays and an electron is ejected (photoelectron). The kinetic energy of the photoelectron, energy of the incident photon and the work function of the spectrometer can be used to determine the binding energy of the electron.114,115 The survey scan of all four morphologies showed the samples all consisted of mainly carbon and nitrogen, as expected, with some oxygen components from adsorbed water, CO2 or possibly chemical defects. The ratio of C/N determined from the survey scan ranged from 0.98-1.13. The high carbon content is likely due to adsorbed impurities on the surface such as CO2 and carbon from the carbon tape used for analysis. These impurities can create adventitious carbon in the samples. The macro and SBA CNx have the highest C/N content of 1.09 and 1.13 respectively presumably due to their higher surface areas (Table 2) allowing more carbon impurities to be adsorbed on the surface. The C 1s XPS showed three main peaks in all four samples (Figure 13A-D). The peak seen at 285 eV is attributed to C-C bonds from carbon impurities and is used as an internal reference. The peak seen around 286 eV is attributed to C-O bonds. These adventitious carbon 24 peaks can arise from the carbon tape used in the analysis process and contamination of the sample surface.105 The most prominent peak seen in all morphologies at approximately 288 eV is due to the N-C=N motif in the heptazine rings of CNx. Generally, the relative elemental composition of the sample can be determined by the ratios of the relative peak areas. In the CNx C 1s XPS quantitative analysis of the peak is more ambiguous due to absorbed surface impurities. However, the higher ratio of N-C=N motif peak areas to the adventitious carbon is consistent with the expected structure. The ratio of C-C and C-O contributions in the SBA and macro samples are higher than those in the meso and bulk CNx. This suggests there are a higher number of impurities adsorbed on the surface or possibly chemical defects in the macro and SBA CNx, which is consistent with the higher C/N ratio observed in the survey scan for these morphologies. The higher surface area of these two materials is likely a contributing factor which would increase surface impurities, however macro CNx has a lower ratio of C-C and C-O contributions than SBA but a higher surface area. This could indicate more chemical defects or higher transferred impurities from the carbon tape to SBA sample. From the N 1s spectra of CNx nitrogen assignments can be made which correspond to CNx structure. The first most prominent peak observed around 399 eV is attributed to the pyridinic (C=N-C) nitrogen in CNx. This is the most prevalent nitrogen form in the CNx structure. The next peak situated around 400 eV is due to NH and/or NH2 groups. The final peak is from the centre amine (Nx). As previously mentioned, the relative peak areas can be used to quantitatively analyze the relative peak areas of the different forms of nitrogen. The N 1 XPS was more ambiguous than expected with many variations of nitrogen assignments seen throughout literature with 2 to 4 peaks fitted to nitrogen XPS plots of CNx.116–122 In the N 1s XPS of these samples the fitting of three peaks in the main XPS peak was determined to be the most appropriate fitting with a 4th peak identified as π-π interaction in the material which was observed in the N 1s XPS spectra of all four samples.119 The relative peak areas in N 1s XPS of CNx can be used to identify branching and polymerization.122 Previously it was shown that in more condensed CNx structures the relative peak areas of Nx where higher and the NHx would be lower.122 The linkage of the heptazine subunits when there is incomplete polymerization is through NH, whereas the Nx amine is the linkage of the heptazine subunits when fully condensed. When analyzing the XPS N 1s of these morphologies it is evident that the SBA CNx contains the smallest Nx amine peak and a larger NHx peak. This suggests a lower degree of polymerization in SBA CNx. The polymerization may be limited by its confinement within the silica mold creating a polymer with more NHx. The ratio of the Nx amine and amino relative peak areas of bulk, meso and macro CNx range from 0.65-1. 25 The largest Nx amine peak is seen in the meso CNx with a smaller peak area of the amino group indicating this could be the most polymerized morphology with the bulk and macro following with 0.65 and 0.85 relative peak areas Nx/NHx respectively. However, this analysis should be taken with caution. There is a variety of peak numbers and assignments seen in literature for CNx N 1s XPS. Further studies to conclusively confirm these suggestions about the XPS are necessary in future work. 26 Figure 13: The top structure shows the melon model of CNx with highlighted elements which correspond to C 1s and N 1s XPS peak assignments. Panels A-D show the C 1s XPS spectra of the CNx morphologies bulk, meso, macro and SBA CNx. Panels E-H show the N 1s spectra of CNx morphologies bulk, meso, macro and SBA CNx. 27 2.2.1.3 Powder X-ray Diffraction Powder x-ray diffraction (pXRD) allows for further insight on structural comparison of the different CNx morphologies and can be seen in Figure 14. In general, the position of pXRD peaks is determined by the periodicity of the material’s structure and the intensity of the peak depends on the scattering within the repeating units.123 The main peak (002) observed around 27.4˚ is due to interplanar stacking of the CNx sheets.51,102,124,125 The interlayer distance (d) can be calculated giving the distance between the stacking of CNx sheets in the material. This can be calculated using Braggs Law as follows: 𝑑 =𝜆2𝑠𝑖𝑛(𝜃) Where the lambda is the wavelength of the incident X-ray radiation (1.5406 Å) and θ is the x-ray diffraction angle. The (002) peak for SBA CNx is slightly shifted to higher angles giving a smaller interlayer spacing value which could indicate there is denser packing of the CNx sheets in this morphology. Another less prominent peak (100) can also be seen around 13˚ giving d = 0.68 nm which is due to in plane spacing between the heptazine units.124,126 The peak is more prominent in the bulk, meso and macro CNx than the SBA 15 CNx. The SBA CNx has approximately 10 nm wide tubes and the loss of the peak may be due to size dependent peak broadening.125,127 Overall the pXRD showed similar spacing in each morphology, indicating the addition of the silica templating did not significantly affect interplanar stacking between CNx sheets. Interestingly, the full width half max (FWHM) of the pXRD can be used to provide insight into the crystallinity of the material. The FWHM of the bulk, meso and macro CNx was 1.78˚, 1.97 and 1.90˚ respectively, However, the FWHM of the SBA CNx was 2.72˚ which could support the XPS conclusion that the SBA is less polymerized with more NHx groups due to the confinement of the synthesis within the silica molds. 28 Figure 14: Left: pXRD graph of bulk, meso, macro and SBA CNX. right: Table with the angle and d spacing of the 002 pXRD peak. 2.2.2 Electronic Characterization Electronic characterization techniques were performed on bulk, meso, macro and SBA CNx as well. This included UV-Vis Diffuse reflectance Spectroscopy (DRS) and Photoluminescence Spectroscopy. 2.2.2.1 UV-Vis Diffuse reflectance Spectroscopy The use of UV-Vis Diffuse Reflectance Spectroscopy (DRS) allows for insight into the different electronic properties of an opaque material. The reflectance is obtained by measuring the light which is not absorbed by the material. In the range of wavelengths that the material absorbs the reflectance measurements are low and when the material does not absorb reflectance values are close to 100% (Figure 15). The relationship between the diffuse reflection and the absorbance of the material can be described using Kubelka-Munk theory.128,129 The equation developed can be seen below. 𝐹(𝑅𝑓∞) =(1 − 𝑅𝑓∞)22𝑅𝑓∞=𝐾𝑆 Where the 𝑅𝑓∞ is the reflectance relative to a non-absorbing standard, K is the absorption coefficient and S is the scattering coefficient. The scattering coefficient is constant and based on the assumption that the scattering is isotropic, evenly scattered in all directions and not dependent on wavelength.130 This equation can then be taken one step further using the Tauc method to determine the band gap of the material. 131 The band gap is critical for identifying the 29 materials range of absorbance and relative positions of the VB and CB. The Tauc method uses the equation as follows. (𝛼𝑎𝑏𝑠 ∙ ℎ𝑣)1𝛾 = 𝐵(ℎ𝑣 − 𝐸𝑔) Where αabs is the absorbance coefficient, h is Planck’s constant, v is photon frequency, B is a constant, Eg is the band gap and γ represents the electronic nature of the band-to-band transitions. The bands can be direct and have no difference in momentum between the initial and final state or the transition can be indirect and have differences in momentum .132–134 CNx has a direct band gap and the appropriate value for γ is ½. The Kubelka-Munk transform K/S can replace αabs in the equation because of the scattering coefficient assumption and this relation can be plotted as (𝐾𝑆∙ ℎ𝑣)2 vs ℎ𝑣 where the band gap is determined from a linear trend given at the area with the maximum slope by extrapolating the x intercept of the tangent.131 Figure 15: A) UV-Vis Diffuse Reflectance (DRS) B) Kubelka-Munk transform of the DRS of Bulk, meso, macro and SBA CNx. Where K is the absorption coefficient and S is scattering. 30 Figure 16: Tauc plots and line of fit (grey dash) used to determine the band gap energy and the extracted band gap on the bottom right of each graph. The reflectance measurements and the K/S transform in Figure 15 show that all morphologies absorbed light with wavelengths under approximately 450 nm. Looking at the K/S transform, it appears the absorbance of the SBA CNx is blue shifted compared to bulk and the meso which appear similar. The meso CNx displays a tail in absorbance seen in the 450-550 nm region of the spectra. This could suggest the existence of a larger trap state distribution in the meso CNx which results in sub-band electronic transitions.135,136 The Tauc plots for the morphologies were then constructed (Figure 16) to obtain quantitative information on the materials’ band gaps. In literature CNx typically has a band gap of 2.7 eV.51,137 The band gap for the bulk CNx in this work was calculated to be 2.88 eV. Meso CNx had a band gap of 2.80 eV. The macro and SBA have slightly larger bandgaps of 2.95 and 3.03 eV, respectively. The larger band gap of SBA CNx is consistent with literature reports of an increased band gap in this modified morphology which could be due to confinement during the CNx polymerization.92,138 31 Figure 17: Graphs for bulk, meso, macro and SBA CNx plotted to determine the Urbach tail energy by adding a linear fitting (blue) to the linear section (black) of the ln(K/S) vs E. Points excluded in the fitting are in red. The Tauc plots showed a tail of absorbance at energy levels lower than the determined band gaps for all the morphologies. This tail is known as the Urbach tail and can provided information on the energy of defect states extending into the band gap.139 The Urbach equation is as follows.140,141 𝛼𝑎𝑏𝑠 = 𝛼0exp (𝐸𝐸𝑢) Where αabs is the absorbance coefficient, α0 is an absorbance constant, E is photon energy and Eu is the Urbach energy. The Urbach tail energy can be calculated by adding a linear fitting to the linear section of the 𝑙𝑛(𝐾𝑆) vs E below the band gap (Figure 17). The slope gives 1/Eu and therefore can give information on the energy of possible defect states. There was a limited linear region of the Urbach tail indicating that something more complex in the tail states and the optical transition involving the trap states may be occurring (Figure 17). The relative Urbach tail energies were reported due to the limitations of using DRS, where the absorbance coefficient can not be directly calculated. The Urbach tail energies relative to bulk CNx for meso, macro and SBA CNx were 1.9, 1.5 and 1.1 respectively. This suggests that the trap states extend 1.9-fold deeper in the band gap for meso CNx compared to bulk. 32 2.2.2.2 Photoluminescence The Photoluminescence (PL) spectra of the CNx morphologies gives information on the emissive charge carriers in the different morphologies. The PL of the material can be affected by the other charge carrier pathways in kinetic competition such as non-radiative recombination and charge trapping. The charge carrier dynamics of the material are critical in understanding the overall efficiency. Common factors affecting PL include the distribution of trap states in the material. Typically trap states are non-emissive and when more charges become trapped in non-emissive states the PL decreases. Other factors to consider are that decreased PL could suggest there are less charges available to participate in the non-reactive emissive states and are instead available to participate in photocatalytic reaction. Under 405 nm excitation, bulk CNx exhibited the most intense PL peak which was centered around 476 nm. The macro CNx had a similar position around 477 nm, however, the peak intensity was lower than that of bulk CNx. This is indicative of a lower amount of emissive recombination in the macro CNx. The SBA CNx had a slightly blue shifted peak at 460 nm which is consistent with the higher band gap obtained using DRS. The peak intensity of SBA CNx is similar to that of macro CNx. The PL of the meso CNx is significantly quenched compared to the other CNx samples and the maximum emission is around 507 nm. This could be due to a wider distribution of trap states which is also suggested to cause the tail of absorbance seen in the DRS. Less than 10% of charges in CNx radiatively recombine so this analysis is only looking at a fraction of charges in the material.142 This can make definite conclusions from the PL limited as PL is not monitoring charges which are reaching active catalytic sites.43 Figure 18: The photoluminescence spectra of bulk, meso, macro and SBA CNx at 405 nm excitation wavelength. 33 2.2.3 Morphological Characterization In the following section the bulk, mesoporous, macro and SBA CNx morphological modifications will be discussed. Techniques used to identify the morphological modifications include scanning electron microscopy (SEM) and transmission electron microscopy (TEM). 2.2.3.1 Electron Microscopy Electron microscopy allows images of the different CNx morphologies to be taken and confirmed. Images for bulk, meso, macro and SBA CNx were taken and can be seen in varied magnification in Figure 20. To quantitatively analyze the size of the morphological modifications and silica templates, measurements of the electron microscopy images were done by averaging at least 10 measurements. Figure 19: SEM images of the silica sphere template used to template macro CNx (A 10 kx and B 20 kx) and SBA-15 silica template used to template SBA CNx (C 10 kx and D 20 kx) 34 Figure 20: SEM images of all CNx morphologies. Panels show Bulk CNx A-D, Meso CNx E-H, Macro CNx I-L and SBA CNx M-P. Each row has an SEM image at 1 kx, 5 kx, 10 kx and 20 kx across the row from left to right. In Figure 20, panel A shows the amorphous nature of bulk CNx. These images are useful as reference to compare with the modified morphologies. The SEM of the mesoporous CNx is seen in panel B. The expected pore size for the mesoporous morphology is 12 nm, however SEM utilizes secondary electrons to create the observed SEM image which does not provide high enough magnification to observe this pore size.143 Transmission electron microscopy (TEM) images use a thin sample where the electrons are transmitted through the sample which can 35 provide high magnification and here was used to confirm the presence of the pores. These TEM images (Figure 21) show the expected mesoporous morphologies where the pores are the lighter circular patches throughout the samples. The average pore size was measured to be 10 ± 2 nm which is consistent with the diameter of the silica used. Figure 21: TEM images of Meso CNx. A and B are different magnifications of the same area and C and D are different magnifications of a second area of the same sample. The macro CNx was synthesized using silica particles synthesized using the Stöber silica method. The silica spheres were measured to be 303 ± 23 nm in diameter (Figure 19). The CNx resulting from this silica template can be seen in Figure 22C. The average pore size of this material was measured to be 245 ± 35 nm. This is consistent with the size of the silica spheres used. The fourth CNx morphology SBA was templated using SBA-15 mesostructured silica which is hexagonally mesostructured with a pore size of approximately 10 nm. The SBA-15 silica template is constructed of 10 nm pores which are hexagonally packed in a tubular fiber which form larger bundles (277 nm ± 76nm) which are visible in SEM (Figure 19 C, D). The SBA 36 CNx gave an average bundle size of 180 ± 40 nm (Figure 22D). This is smaller than the width of the mold and consistent with the CNX polymerizing within the SBA-15 silica because the polymerization of CNx is constrained to inside the silica. Figure 22: Comparison of the CNx morphologies at a magnification of 20kx A) bulk, B) meso (with a TEM image inset outlined in yellow) C) macro and D) SBA CNx. Overall, the synthesises of the bulk, meso, macro and SBA CNx morphologies were confirmed using SEM images and the nanostructure modification were measured using TEM. A comparison of all four morphologies can be seen in Figure 22. 37 2.2.3.2 Surface Area Brunauer–Emmett–Teller (BET) analysis allows for measurements of the surface area of a sample. At low temperatures the interaction between surfaces and gas molecules are weak, leading to the physisorption of gas molecules on the surface of the material. By adjusting pressure, the amount of gas adsorbed will be affected.144–146 BET uses multilayer adsorption by looking at the relative pressure compared to the volume of adsorbed gas to determine surface area. The surface area measurements for the CNx morphologies can be seen in Table 2. The surface area of bulk CNx is consistent with previous literature reports of bulk CNx surface area which is commonly around 10 m2/g.101,147 The surface area of the modified morphologies compared to the bulk are not as significantly different as expected. According to literature reports the surface area of 12 nm mesoporous CNx can be over 26-fold larger than the bulk CNx.93,100 Li et al. conducted a study comparing DCDA, Bulk CNx and cyanamide -based modified morphologies which showed a surface area trend of Bulk < SBA (5 nm pore size) < Meso (12 nm) CNx. The trend observed in this study is different and shows bulk ≈ meso < SBA CNx < Macro. The influence of sample preparation on CNx is a potential factor as different methods such as the silica precursors used, changes in porosity, the size the silica could change the surface area of the material. Other factors to consider as well are calcination temperature and precursors which could all influence the surface area of the material. It has been shown that urea derived CNx typically has higher surface are in comparison to CNx derived from dicyanamide, thiourea and melamine precursors due to the formation of larger pores when using urea.101,102 These factors contribute to the differences seen in our modified morphological surface areas compared to other literature reports. Table 2: Brunauer–Emmett–Teller (BET) surface measurements for CNx morphologies bulk, meso, macro and SBA CNx Sample BET Surface Area (m2/g) Bulk CNx 12.1 Meso CNx 11.7 Macro CNx 20.9 SBA CNx 15.6 38 2.3 Charge Carrier Dynamics The charge carrier dynamics of a material can be insightful in understanding the pathways of the charge carriers in the photocatalyst. The pathway the charge carriers take, and their kinetic competition is vital in understanding the efficiencies of a material. The optical method used to understand the charge carrier dynamics in the different CNx morphologies was TAS. 2.3.1 Transient Absorption Spectroscopy TAS measurements were obtained for the four successful CNx morphologies. TAS can provide information on the excited state population in a material and be beneficial in understanding the photophysics of a photocatalyst. The complex kinetics in CNx make identifying sources of gains or losses in the photocatalysis difficult and ultimately hinder the achievement of an optimal design. Using spectroscopic and kinetic TAS studies the elucidation of complex mechanisms can be achieved which can help to develop an understanding of the charge carrier dynamics of CNx. The absorption features typically seen in TAS spectra are from ground state beaching (GSB), photoinduced absorption (PIA) and PL emission. GSB presents as a negative TAS spectra signal due to the absorption in ground states resulting in the loss of available electrons in the VB. PIA presents itself as a positive absorption feature due to a electrons/holes moving to higher energy states within the band (VB or CB). Emission absorption features can present themselves as a negative feature due to the emission of a photon increasing the light observed after excitation. The TAS spectra (Figure 23) for each sample show strong absorption in the 700-800 nm range which is typical of photogenerated electrons in CNx.26,148,149 39 Figure 23: TAS spectra of A) bulk B) meso C) macro and D) SBA CNx The absorptance at times shorter than 100 μs below 650 nm were removed from all spectra. Absorptance rise times can be affected by emission which typically occurs near the band gap. Therefore, the shorter wavelengths with stronger emission influences are not an accurate representation of the absorption in the sample and were omitted from the spectra. The spectral shape in all morphologies appeared similar with peak absorptance seen in the modified morphology samples seen around 700-750 nm in all spectra. There was a slightly broader and higher absorbance seen in the bulk sample from 650-950 nm at 5 and 10 μs compared to the modified morphologies. The TAS decay kinetics of the samples can provide information on the excited state population for the material. Typically, hydrogen evolution occurs on similar time scales as were measured and the decay kinetics can give information on the trap state distribution and potentially reactive charges.150,151 The slope of the log-log plots of the TAS kinetic has been suggested to be proportional to the trap state energetic distribution of the sample.26 The slope of the linear decay (α) can be determined when the log-log plots display a power lay decay. The power law decay can be attributed to recombination affected by trapping and de-trapping processes.152,153 The relationship between α and the density of available traps state 𝑛𝑡𝑟𝑎𝑝(𝐸) is written as follows: 𝑛𝑡𝑟𝑎𝑝(𝐸) ∝ 𝑒𝑥𝑝(−𝛼𝐸𝐾𝑏𝑇) 40 The available traps states are at an energy (E) and the temperature is represented by (T) and the Boltzmann constant by (𝐾𝑏). This relationship indicates that the smaller α the larger the trap state distribution. The decay kinetics were investigated at 750 nm for each sample (Figure 24). The log-log plots displayed power law decay. After fitting the linear section with a straight line of best fit to determine the α for each sample, the trap state distributions were found to be similar across all samples (Figure 24). The largest trap state distribution is suggested to be in meso CNx with an α of 0.32 and the smallest in SBA CNx with an α of 0.39. The α value can also be used to determine the characteristic energy of the of the tail of trap states (Ec), a measure of the energetic distribution of trap states, using the equation as follows.154,155 𝐸𝑐 =𝑘𝑏𝑇𝛼 The characteristic energies of the tail of trap states were calculated to be 66 meV, 79 meV, 70 meV and 65 meV for bulk, meso, macro and SBA CNx respectively. Values proportional to the distribution energy of the tail states were also calculated using the Urbach equation from the DRS characterization (Chapter 2.2.2.1). The trends seen in the tail state distributions are similar between both the TAS and Urbach tail analysis methods. The meso CNx had the deepest trap state energy with both the TAS and with the Urbach tail analysis methods, suggesting lower energy defect states in this morphology. This is followed by the macro CNx which had the second highest characteristic energy in both analysis methods as well. The tail energy determined by TAS indicates bulk and SBA CNx have similar tail energies of 66 meV and 65 meV respectively. The relative Urbach tail energies of these two morphologies were similar as well, with SBA being 1.1-fold larger than bulk. Overall, both methods suggested meso CNx has the deepest trap energy, followed by macro, then SBA and bulk CNx which had similar trap state energy. 41 Figure 24: The log-log plots of the TAS decay kinetics at 750nm normalized at 4 μs of A) bulk B) meso C) macro and D) SBA CNx The decay kinetics were normalized at 4 μs and compared to identify any differences in the decays that may affect the photocatalytic efficiencies (Figure 25). The TAS decay kinetics were normalized and compared at 750 nm 850 nm and 1000 nm for each morphology and showed that the decay kinetics were similar. Figure 25: TAS decay kinetics normalized at 4 μs at 750 nm, 850 nm, and 1000 nm. 2.4 Photocatalytic Testing The photocatalytic testing of all CNx materials was done by measuring the rate of hydrogen evolution from each sample. A hole scavenger, triethanolamine (TEOA) was used due to the slow kinetics of water oxidation and to promote photocatalytic hydrogen production. Water oxidation is typically a slower reaction and would limit the production of our target product, H2. A 3% wt. Pt cocatalyst was deposited on the surface of the CNx as Pt is an active and common 42 catalyst for hydrogen evolution.156 These photocatalytic studies allow for the determination of the effects that the morphology modifications have on the photocatalytic hydrogen evolution. 2.4.1 Hydrogen Evolution The photocatalytic hydrogen production of all four morphologies can be seen in Figure 26. All samples were done in at least duplicate trials. The results show the macro CNx had the highest hydrogen evolution rate of 9×10-4 μmol/s and an AQY of 3.6%. The hydrogen evolution rate for bulk, meso and SBA CNx were 6.1×10-4 μmol/s, 7×10-4 μmol/s, 9×10-4 μmol/s respectively. The AQY obtained of CNx are consistent with literature reports which show the AQY of CNx can range from 0.1-3% confirming the obtained AQY values are reasonable.19,157 Figure 26: Hydrogen evolution obtained for bulk, meso, macro, and SBA CNx samples under photocatalytic conditions. Table 3: Hydrogen evolution rate and apparent quantum efficiency of all four CNx morphologies Sample Average H2 evolution rate (μmol s-1) AQY (%) Bulk CNx 6.1×10-4 ± 5×10-5 1.7 ± 0.1 Meso CNx 7×10-4 ± 2×10-4 1.9 ± 0.5 Macro CNx 1.3×10-3 ± 2×10-4 3.6 ± 0.5 SBA CNx* 9×10-4 ± 1×10-4 2.6 ± 0.3 *Performed in triplicate 43 The apparent quantum yield (AQY) was determined for each sample. The samples were irradiated with 405 nm light with a power density of approximately 10.75 mW/cm2 to replicate solar illumination conditions. The calculations were performed as follows. First the energy of a photon from 405 nm light was calculated 𝐸𝑡 =ℎ𝑐𝜆= (6.63 × 10−34𝑚2𝑘𝑔𝑠 )× (2.998 × 108𝑚𝑠 )4.05 × 10−7𝑚= 4.90 × 10−19 𝐽𝑝ℎ𝑜𝑡𝑜𝑛 Next the total energy per second was calculated from the power density of the light 𝐸𝑡 = 𝑃 ∗ 𝐴𝑟𝑒𝑎 = (10.75 𝑚𝑊𝑐𝑚2 × 2 𝑐𝑚2)/1000 = 0.0215 𝐽𝑠 The total number of photons were then calculated 𝐼 =0.0215 𝐽𝑠4.90 × 10−19 𝐽𝑝ℎ𝑜𝑡𝑜𝑛= 4.38 × 1016 𝑝ℎ𝑜𝑡𝑜𝑛𝑠𝑠 The above value was then used to calculate the AQY 𝐴𝑄𝑌 =𝑛𝑅𝐼 Where R is the rate of hydrogen evolution in mol/s and n is the number of mols needed for the reaction, which in this case is 2 for proton reduction and I is the total number of photons hitting the samples per second. 2.5 Conclusion In conclusion, a thorough analysis was performed on the bulk, meso, macro and SBA CNx samples which included extensive characterization as well as charge carrier dynamics and activity investigations. The characterization methods used included structural, electronic, and morphological characterization. The CNx structure was investigated with the techniques of FTIR, XPS and pXRD. The FTIR confirmed the presence of key CNx peaks as well as highlighted some small peak shifts which may have resulted due to influences of the silica. The XPS also confirmed the structure of CNx with expected characteristic carbon and nitrogen peaks seen throughout the samples. Varied N 1s contributions were observed which may highlight different 44 degrees of polymerization within the samples, however, future investigations into XPS assignments are suggested. The final structural characterization technique, pXRD, indicated similar interlayer packing between the materials in all the samples. The electronic characterization included the UV-Vis DRS and PL measurements. The DRS measurement showed that the materials had similar absorbance ranges and band gaps which were consistent with literature values for CNx materials. Differences observed in PL spectra show that there are changes in the radiative recombination between the materials, however, the origin of this difference is unknown and will be discussed further in Chapter 4. The morphological modifications were confirmed using SEM and TEM to obtain high magnification images of the samples. The macro, SBA and bulk morphologies were observed and confirmed using SEM and the mesoporous morphology confirmed using TEM. The surface area measurements showed that the morphological modification did not make significant surface area changes, however, they did increase the surface area up to 1.7-fold compared of bulk CNx. The charge carrier dynamics indicated similar excited state compositions in all morphological modifications. The photocatalytic activity showed that the macro CNx is the most active with an AQY of 3.6 ± 0.5 %. Overall, the discussion of the activity, charge carrier dynamics and characterization results will be discussed further in Chapter 4. 45 Chapter 3: Hollow Sphere Carbon Nitride A fifth carbon nitride morphology, CNx hollow spheres (HS) was a target morphology for this surface area-photophysics activity investigation. The synthesis of this morphology involves the polymerization of the CNx precursor within a silica mold similar to the SBA CNx morphology, with the target shape being a CNx hollow sphere in this case. Significant progress was made towards finalizing the synthesis procedure of this morphology; however, a successful reproducible synthesis procedure of the HS CNx was not achieved. The procedure for the HS CNx synthesis was based on a procedure done by Sun et al.86 HS CNx has also been synthesized in other literature reports as well.87,158–163 Common to these procedures is that the silica mold is synthesized by first creating a solid silica sphere core using the Stöber silica method, then a mesoporous silica shell is grown onto the core. The pores in the outer silica shell allow the CNx precursor, cyanamide, to infiltrate the mold. After the silica mold is synthesized the cyanamide and the silica are heated and mixed under vacuum with sonication. The cyanamide/silica mixture is isolated using centrifugation and then dried. The sample is then calcined, resulting in a CNx silica composite. The silica mold is removed from the calcined composite using a 4M NH4HF2 treatment, same as the other silica modified CNx morphologies. The HS CNx procedure can be broken down into two steps. First the synthesis of the silica mold and second the formation of the HS CNx. Challenges occurred in both of these steps and will be discussed further in this chapter as well as future directions for finalizing the HS CNx synthesis procedure. Figure 27: Schematic showing the general procedure followed to synthesize CNx HS 46 3.1 Silica Synthesis The synthesis of the silica mold initially follows a Stöber silica synthesis which is followed by the addition of a mesoporous silica shell. The procedural details were as follows; the catalyst, aqueous ammonia (Sigma-Aldrich, 28 -30 wt.%) was added to a solution of 58.5 g ethanol and 10 g of water. The solution was then mixed at 30˚C for 30 mins to create a homogeneous mixture. Then tetraethoxysilane (TEOS; Alfa Aesar, 99.9%), the silica precursor, was added to the solution, after vigorous stirring the solution was left standing to allow the formation of the spherical silica core. After the formation of the silica core, the mesoporous outer shell was added. To create the mesoporous outer shell a mixture of TEOS (5.83 g) and a porogen n-octadecyltrimethoxysilane (C18TMOS, 2.62 g) was added to the silica core solution dropwise while stirring. The C18TMOS is added to form the pores - it has a long alkyl chain that creates the pores. This carbonaceous material is removed by calcining which leaves the pores in the mold. After the TEOS + C18TMOS addition, the solution was left standing for 3 hours to allow for the formation of the outer shell. The silica core-shells were then isolated by centrifugation and calcined to remove the C18TMOS porogen. The final step was the neutralization of the calcined product with 1M HCl which helps improve the silica’s interaction with the cyanamide precursor. 3.1.1 Materials and Methods The synthesis of the silica core shell presented two challenges: the materials used and the formation of the pores on the silica shell. One of the required reagents was ethanol. Initially, reagent grade alcohol was used. This is a mixture of approximately 90% ethyl alcohol and 10% isopropyl and methyl alcohol. The use of this product created silica cores of 320 nm ± 20 nm and core shell-silica of 540 nm ± 20 nm seen in Figure 28. 47 Figure 28: SEM images of Silica core (A) and silica core-shell (B) both synthesized with reagent grade alcohol and crucible lid on during porogen removal both at 40kx magnification. The solvent choice, an alcoholic medium in the Stöber synthesis of silica is important for the final particle size.164 The silica created with reagent alcohol was then compared to silica synthesized with ethanol. The 95 % ethanol synthesized silica is shown in Figure 29 and was measured to be 290 nm ± 20 nm with a core-shell size of 520 ± 20 nm. Figure 29: SEM images of silica core (A) and silica core-shell (B) both done with 95 % ethanol and crucible lid off during porogen removal at 50kx and 40 kx magnification respectively. Overall, it was concluded that the ethanol synthesized silica was to be used for further synthesis due to the more ordered nature of the silica cores. The other important part of the silica synthesis was the formation of the porous outer shell, these pores allow the cyanamide into the silica mold. These pores are created by adding C18 TMOS to the outer core and then removing it through calcination. To confirm the successful removal of the porogen the FTIR spectra was taken of the silica (Figure 30). The FTIR of silica pre-calcination presents two sharp peaks around 2850 cm-1 and 2920 cm-1. These peaks are characteristics of C-H stretches which are a result of the C18TMOS porogen. In the first silica synthesises the lid was always kept on (with lid (WL)) the crucible. This was a common practice in other synthesises and was carried over to the silica synthesis. The FTIR obtained of the WL calcined silica shows clear C-H stretches similar to those in the uncalcined product. The FTIR obtained of the silica synthesized with the lid off shows there are no C-H stretches. It was concluded that the calcination with lid on was preventing the removal of the porogen. This is likely due to the combustion reaction required for 48 the removal of the porogen requiring oxygen to occur. The addition of the lid may prevent the free flow of gas which would limit the removal of the porogen. Figure 30: FTIR comparison of silica calcined with no lid (NL) (grey) silica calcined with a lid (WL) (brown), silica not calcined (purple) A) the full spectra, B) zoomed in region of the full spectra from 4000 cm-1 to 2300 cm-1 Overall, after trials with the different alcohols and the successful removal of the porogen, it was concluded the synthesis of porous silica sphere template was successful. It was assumed the shell was sufficiently porous however this was never experimentally verified by TEM. The next step in the HS CNx procedure was to successfully infiltrate the silica pores with the CNx precursor to form the desired CNx morphology. 3.2 Hollow Sphere CNx Synthesis The hollow spheres synthesis procedure involves mixing of the cyanamide precursor and the previously synthesized silica followed by calcination and silica removal to produce hollow sphere CNx. The details of this procedure were as follows, the cyanamide precursor was melted at 60 ˚C and mixed with the silica for 3 hours then sonicated for 2 hours at 60 ˚C under vacuum and left to stir overnight at 60˚C. The mixture was centrifuged the next day to isolate the silica particles infiltrated with cyanamide and remove excess cyanamide. The sample was then dried and calcined to obtain the silica CNx composite. The silica was dissolved using NH4HF2. This procedure can be broken up into two key sections, the infiltration of cyanamide into the silica pores, during the mixing and sonication of the cyanamide/silica mixture, and the removal of the excess cyanamide by centrifugation prior to calcination. 3.2.1 Synthesis Variation Challenges arose in both the pore infiltration and excess cyanamide removal sections of this procedure. The initial CNx synthesis which was done with WL reagent alcohol silica yielded 49 no HS CNx (Figure 31). The SEM images show pore indentations where the silica template was, however, the majority of the material appeared to be bulk CNx. These results indicated the silica template was not well dispersed throughout the cyanamide/silica mixture and this may have been impeding the cyanamide’s ability to infiltrate the pores. This was also likely influenced by incomplete porogen removal as this was done with WL silica. Other potential factors preventing the synthesis may have been the synthesis conditions. In literature this procedure was performed in vacuo,86 in this case it was not. Both factors seem to be important, performing the reaction under vacuum could allow the pores to be vacated which would improve the cyanamide’s ability to enter the pores. To improve the dispersal of the silica throughout the solution the silica template was ground prior to its mixture with cyanamide. In the next synthesis the ground silica was used, and the sonication and mixing process was performed in vacuo (Figure 32). Figure 31: SEM images of the same HS CNx synthesis done without grinding silica (WL) and without vacuum shown at different magnification (A) at 10 kx and (B) 20 kx magnification respectively. It was confirmed the silica was not crushed in the grinding process which would have destroyed the mold by comparing two samples synthesized in the same batch one of which was ground and one which was not ground (Figure 32A, B). This showed the grinding with a mortar and pestle did not affect the nanostructure of the silica and grinding was therefore adopted into the procedure. 50 Figure 32: SEM images of A) un-ground core-shell silica, B) ground core-shell silica, C) composite of the silica and cyanamide before the silica was dissolved, D,E) CNx after silica remove at 20 kx magnification and 10 kx magnification respectively. These results showed a more uniform distribution of silica throughout the sample, however the successful formation of the CNx hollow spheres remained unachieved. These synthesis results showed the CNx product contained some partial sphere shapes and intended divots where the silica was. The composite of the silica and cyanamide before the silica was dissolved was also imaged (Figure 32C). This revealed there still appeared to be a significant excess cyanamide present in the composite surrounding the silica molds. The removal of the excess cyanamide then became an area of interest as it was clear from the CNx composite (Figure 32C) there was excess cyanamide present. This excess cyanamide may have been creating the CNx pores seen in the material. When excess cyanamide surrounds the silica core-shell it may polymerize around the silica core-shell creating the observed pores. The removal of excess cyanamide posed some challenges due the 44 C melting point of cyanamide. In our original procedure, the cyanamide and silica mixture was centrifuged for 10 minutes at 4000 RPM. However, due to its melting point the cyanamide solidified during centrifugation preventing the decanting of the excess cyanamide. For the next synthesis excess 51 cyanamide removal was done using a GeneVac (Genevac EZ-2 Series) which can centrifuge sample at warmer temperature 55 C. However, this still resulted in porous CNx instead of HS CNx (Figure 33). Figure 33: A and B are two different areas of the same HS CNx sample. The synthesis was done under vacuum with the excess cyanamide removed by centrifugation with the GeneVac at 55 C . The images are both at 20 kx magnification. The CNx SEM images showed half spheres, suggesting the warm centrifugation may have allowed the cyanamide to wick out of the pores. This led to the testing of different cyanamide removal factors such as centrifugation times, temperature, and solvent additions. At this time, a former group member Destiny Ellenor began working on a parallel project and investigated the addition of solvent (H2O) to remove excess cyanamide. The silica used for this was a core-shell silica synthesized using a different porogen, cetrimonium bromide (CTAB) according to a procedure by Chen.163 Destiny Ellenor was able to successfully synthesize one batch of CNx hollow spheres (Figure 34) using H2O to remove excess cyanamide. 52 Figure 34: SEM image of HS CNx, taken and synthesized by Destiny Ellenor’s CNx. Image at 30 kx magnification and reproduced with permission from Destiny Ellenor. Reproducing this result was difficult and investigations began into the factors involved in the successful synthesis, which included the addition of water, temperature, and centrifugation. This successful result was obtained using a procedure as follows, the cyanamide and silica were mixed at 65 C then sonicated under vacuum for 3 hours and left to stir overnight at 65˚C. The final product was placed in a centrifuge tube and rinsed with warm water then centrifuged at 4000 RPM for 5 mins and left to dry. This prompted modifications to the original procedure outlined above where work from here on was now performed by melting the cyanamide and silica together at 65 C followed by a 3-hour sonication period and the silica was synthesized with the CTAB porogen. After sonication the sample was left to stir at 65 C overnight. However, this successful synthesis was unable to be successfully reproduced, by Destiny or myself. The first variable tested was the addition of water at varied temperatures to allow for the removal of excess cyanamide in the mixture. This would allow for room temperature centrifugation due to the solubility of cyanamide in water and was used in the successful attempt. Water temperatures of 40 C, 30 C and room temperature (∼22 C) were tried. The results showed that at 40 C there was no product, as the remaining product after calcination was only silica. This indicated the 40 C water was too warm and was likely washing away all the cyanamide in the pores as well as the excess. The lower temperature waters were then tested (Figure 35). Both these tests yielded similar results showing half formed spheres. There is the potential that the addition of water resulted in too much cyanamide being removed and portions of the cyanamide in the pores were being washed away. 53 Figure 35: SEM images of HS CNx synthesized with 30 C H2O A) 20kx and B) 40kx and HS CNx synthesized with room temperature water, C) 20 kx, D 40kx. The next factors considered were the centrifugation speed (no H2O addition) and the temperature of the samples. First low centrifugation speeds were tested to determine if centrifugation could be a factor in the synthesis. This was tested due to the potential of high forces wicking the cyanamide out of the pores. Low centrifugation speeds of 1000 and 2000 RPMs were tested at 30 second time intervals. The purpose of this experiment was to see if the low centrifugation time and speed could remove enough excess cyanamide as well as prevent the removal of the cyanamide from the pores. The results of these tests are shown in Figure 36 which shows that each centrifugation speed results in similar yields with only a few milligrams produced (< 10 mg). This result indicated that the centrifugation speed was likely not a factor as the majority of the cyanamide was removed at the lower speeds and centrifugation time. 54 Figure 36: SEM images of HS CNx under different synthesis conditions all at 20 kx magnification. Image A and B are comparing different centrifugation speeds of 1000 and 2000 RPM respectively. Images C and D are comparing different centrifugation temperatures achieved by varying wait times before centrifugation at 5 and 4 mins respectively. With this information the last factor to check was temperature. It was hypothesized that the process of cooling the sample was imperative for allowing the cyanamide to remain in the pores while also allowing the excess to be removed. The time which the sample took to solidify was assessed. It was found the samples took approximately 10 minutes to solidify when cooling at room temperature. First, a 5-minute wait time was tested before centrifugation, and the sample became hard and more difficult to work with. This was the longest wait time which was tested. Next a 4-minute trial was also performed before centrifugation of the sample. The results of these tests are shown in Figure 36C, D. They both appeared to have more excess cyanamide in the product with higher yields than the previous sample. 55 After many trials and variables changed the question of where to proceed yielded no easy answer. Thus, it was decided to return to the original paper and attempt an exact replicate of their procedure to see if the variables tested had drifted too far from the original procedure. This involved a large-scale synthesis, a faster calcination ramp rate (4 ˚C/min), with a 5-minute 4000 RPM centrifugation and silica made with the C18TMOS porogen. The results of this trial can be seen in Figure 37. This was the closest procedure developed to reproducibly form HS CNx. Figure 37: SEM image of HS CNx synthesized with 10 g cyanamide and 2 g of core-shell silica, a calcination ramp rate (4 ˚C/min), with a 5-minute 4000 RPM centrifugation. Different sample spots were imaged in A, B at 20 kx magnification, C at 40kx and D at 60kx magnification. These results were unexpected. The main factors changed in this synthesis compared to other were the scale, and the ramp rate of the synthesis. The previous successful synthesis by Destiny was done at the smaller scale of 2 grams of cyanamide whereas 10 grams was used in 56 this synthesis. The ramp rate was then also investigated by completing a similar synthesis on a smaller scale using the same silica: cyanamide ratio as previously reported (2 g cyanamide) with the higher ramp rate. However, this resulted in only porous CNx as previously seen. Synthesis attempts to achieve a reproducible HS CNx morphology were ultimately ended. The cyanamide removal process was thoroughly studied. Time constraints limited the development and execution of a new approach. The macro CNx was developed while the HS CNx synthesis was being investigated. The macro morphology was reproducible and was used as the fourth morphology this this inquiry on morphology’s relationship to photocatalytic activity instead of HS CNx. 3.3 Conclusion and Future Directions Overall, progress towards a successful hollow spheres synthesis was made, however, an optimized procedure has not been finalized. This synthesis investigation revealed key variables in the silica synthesis which included the successful removal of the porogen from the silica template and the imperativeness of grinding the silica template to create an even dispersion of silica in the molten cyanamide mixture. The addition of in vacuo conditions and the excess cyanamide removal were important considerations for a successful HS CNx synthesis. The cyanamide removal process was thoroughly studied and conclusions were drawn on many of the variables in this process. It can be concluded the centrifugation speed was not a factor affecting synthesis, however, the temperature of the mixture during centrifugation is important. When the sample is left to cool the cyanamide removal is less efficient and centrifugation of the sample while warm is more successful at removing the excess cyanamide. The addition of a solvent is too effective at dissolving the cyanamide and removes cyanamide from the pores as well as the outside of the mold. Future cyanamide removal procedures should include short centrifugation times at 4000 RPM. This shorter time frame will allow the sample temperature to remain more stable while still removing all the excess cyanamide. Future work should focus on other factors not thoroughly investigated in this work. The importance of the ramp rate and the scale of the synthesis were potential factors which were considered after the larger scale synthesis with the higher ramp rate. Dehkordi et al. completed a 12-hour silica acidification procedure in HCl to improve cyanamide pore infiltration.87 This was not commonly reported however to improve the silica coordination with the CNx precursor and could be an area of research to explore before finalizing this synthesis. 57 Chapter 4: Conclusion This study focused on building our understanding of the effect of CNx morphology on the charge carrier dynamics and photocatalytic hydrogen production. The morphological modifications pursued were mesoporous, macroporous, SBA and hollow sphere (HS) CNx. These morphologies were studied along with bulk CNx as a reference. The meso CNx, macro CNx, and SBA CNx were successfully synthesized and characterized using structural and electronic characterization techniques. The charge carrier dynamics were studied using TAS and the photocatalytic hydrogen evolution of these materials were measured. Progress was made in the synthesis of the HS CNx morphology however, challenges remain when synthesizing the HS morphology. 4.1 Surface Area-Photophysics-Activity Relationship To build the surface area-photophysics-activity relationship, the activity trend needed to be established. The photocatalytic hydrogen production utilizing a hole scavenger (triethanolamine) and Pt cocatalyst resulted in the activity trend as follows: bulk < meso < SBA < macro CNx. To identify the source of the improved H2 evolution efficiencies seen in the modified morphologies, characterization techniques and charge carrier dynamics studies were pursued. The structural characterization, FTIR, XPS and pXRD, showed some differences between the different CNx morphologies, although no clear trend could be linked to that observed in photocatalytic activity. The FTIR spectra showed the same peaks, demonstrating key features of CNx in all samples, such as the heptazine breathing mode at 804 cm-1. It was identified in the FTIR spectra that there were some shifts in the peak positions, which could suggest changes in the electronic structure. However, the trend in these shifts was not consistent with the observed activity, with the largest shifts observed in the SBA CNx. The C 1s XPS results showed the expected C=N-C motif in all four morphologies. The N 1s XPS also had similar peaks in each CNx morphology, however differences in the relative peak areas were observed. The relative peak areas can provide valuable quantitative information on the components of the CNx structure. A variety of assignments of N 1s were seen in literature, which made it challenging to determine conclusive arguments about the relative peak area ratios. The fitting of three components in the main N 1s peak was commonly seen for CNx and best fit the spectral of results of this study.116,118–120 The relative peak area differed between the different morphologies suggesting there may be different degrees of polymerization 58 between each morphology. This indicated the SBA CNx could have a lower degree of polymerization and meso CNx had the highest, which was different than the trends observed in photocatalytic activity. Electronic characterization techniques were performed using UV-Vis DRS. The band gaps of the materials were similar for each morphology and ranged from 2.88-3.03 eV. The largest band gap was found to be in the SBA CNx (3.03 eV), macro CNx had a band gap of 2.95 eV, bulk CNx had a band gap of 2.88 eV and the smallest band gap was found in the meso CNx (2.80 eV). Overall, these characterizations showed trends that were inconsistent with the trends observed in photocatalytic activity. This suggests that any changes observed in these structural and electronic characterizations are not the main contributors to the photocatalytic activity differences between the materials. The charge carrier dynamics in the CNx morphologies were investigated using TAS. These measurements probed the excited state population in the materials and provided insight into factors affecting the photocatalytic efficiency of the material. The normalized TAS decay at 750 nm, 850 nm and 1000 nm all showed similar decay kinetics and the calculated distribution of trap states were 0.32, 0.36, 0.38 and 0.39 for meso, maco, bulk and SBA CNx respectively. The trap state energies calculated with TAS and from the DRS displayed similar trends with the deepest energy found in meso CNx followed by macro CNx and lower trap energy found in SBA and bulk CNx. The trap state kinetics can be a good indication of activity trends because the time scales measured are similar to the time required for proton reduction, which is on the μs-ms time scale.150,151 The trend in trap state distribution was not consistent with the trend observed in photocatalytic efficiency. Given the TAS kinetics, changes in the excited state dynamics of the CNx morphologies are presumably not the main source of the observed activity changes. Differences between the CNx materials were clearly seen in the morphology, the surface area, and the photoluminescence. The SEM and TEM images showed morphological modifications in the silica templated CNx material. The meso pores were clearly observed in TEM images. SEM was used to see the pores for the macro CNx and the tubular bundles in SBA templated CNx. The BET surface area of bulk, meso, macro and SBA CNx were determined to be 12.1, 11.7, 20.9 and 15.6 m2/g, respectively. The meso and bulk CNx had very similar surface areas. The SBA and macro CNx had surface areas which were 1.3-fold and 1.7-fold larger than bulk CNx respectively. In other morphological studies, meso CNx has been 59 reported to increase the surface area 32-fold,93 and the SBA CNx morphology has showed increases of up to 19-fold compared to the bulk CNx.138 Interestingly, a study by Li et al. showed a 1.3-fold increase in the surface area of SBA templated CNx which is similar to surface area increase in the SBA CNx observed in this work.100 These smaller surface area changes observed herein could indicate that morphological modifications do not extend to nanostructuring the material. Considering the similar structural characterization, material band gaps and smaller relative surface area changes, we propose that the characteristics of these modified CNx’s approach those of bulk CNx. Surface area is known to be an important factor in photocatalysis because an increase in surface area typically leads to an increased number of active sites for the photocatalytic reaction.165,166 When comparing the surface area with hydrogen evolution rate a striking direct positive correlation is seen (Figure 38). Figure 38: The Surface Area vs. Activity relationship between the successfully synthesized CNx morphologies investigated in this study. Table 4: Activity by surface area of the CNx samples with 23mg catalyst used in each trial. Sample Average H2 evolution rate (μmol s-1) BET Surface Area (m2/g) Activity by Surface Area (μmol s-1 m-2) Bulk CNx 6.1×10-4 ± 5×10-5 12.1 2.18 × 103 Meso CNx 7×10-4 ± 2×10-4 11.7 2.60 × 103 Macro CNx 1.3×10-3 ± 2×10-4 20.9 2.69 × 103 SBA CNx 9×10-4 ± 1×10-4 15.6 2.61 × 103 60 This direct correlation suggests that the surface area is influencing photocatalytic activity. This indicates the limitation within the photocatalytic efficiency of the materials studied is the charges reaching the surface. This suggests any changes in electronic properties are not dictating the hydrogen evolution rate. The scale of the morphological modification performed here is larger. This could suggest nanostructuring the material on the same length scales as the charge carrier diffusion length may be important for improving hydrogen evolution efficiencies. The other characterization technique which showed a clear difference in the different CNx morphologies was the photoluminescence. The most intense PL peak was centred at 476 nm and was from bulk CNx. The SBA CNx and macro CNx had similar peak intensities, which were lower than that of bulk CNx, with peak positions of around 460 nm and 477 nm respectively. The meso CNx displayed the lowest PL emission with a larger peak shift. Clearly meso CNx had the lowest PL and bulk CNx had the most radiative recombination. This trend is drastically different than the trends seen in photocatalytic activity and surface area. This characterization technique was not a clear indicator of reactive charge carriers in the material considering the vastly different trend observed in the hydrogen production. PL charges in CNx account for less than 10% of the charge carriers, therefore it is reasonable that there is not an obvious direct connection between PL and photocatalytic activity.43,142,167 However, changes within the structure of the material are affecting the PL kinetics which opens an inquiry as to the reason for these changes. In conclusion, this study showed there was a direct correlation between the surface area and the activity of the photocatalysts. The morphological changes did not produce significantly improved surface areas suggesting the materials are not nanostructured. The TAS kinetics showed no changes to the excited state kinetics of the materials indicating electronic changes did not significantly affect the photocatalytic efficiencies. However, the PL showed significant differences between the materials. 4.2 Future Directions The observed surface area-photophysics-activity relationship displayed a direct positive linear surface area-activity trend and similar charge carrier kinetics between samples. The activity measurements were obtained using 3% wt. Pt and a hole scavenger (TEOA). The oxidation of the hole scavenger occurs in the reaction monitored instead of water oxidation. This is a photocatalytic system which removes the kinetic challenges associated with water oxidation and shifts focus to the hydrogen reduction ability of the material. For photocatalytic systems, which are not limited by the absorption coefficients, carrier lifetimes, or interfacial charge-61 transfer kinetics, photocatalytic efficiencies are only limited by charges reaching the surface, and therefore, their surface area.168 The direct correlation between surface area and photocatalytic activity observed in this work suggests that these assumptions are valid for the photocatalytic system here. In photosynthetic systems (without the use of sacrificial reagents) the limitation of the system changes from charges reaching the surface to the interface and cocatalysts of the system.168 Photosynthetic systems are limited by electrochemical selectivity; it prevents back reactions such as the reduction of oxygen to superoxide. The changes in morphology could have different effects on the electrochemical selectivity. Electrochemical kinetic limitations could also become apparent under water splitting conditions, due to the slow kinetics of water oxidation (>10 ms-1 s).169–171 Future research could be directed at testing the modified CNx morphologies in a photosynthetic system, where the charges reaching the surface is not the system’s limitation. This could be insightful for identifying the impact of modifying the CNx morphologies when the limitations of the system are changed. Investigations into activity in non-sacrificial conditions could provide interesting insights on the morphological changes when the rate of charges reaching the surface is no longer the bottleneck. One consideration is the PL which is obtained under non-sacrificial conditions, without hole scavengers, solvent or a cocatalyst. The changes observed in the PL present some interesting concepts for future research. It is clear there is a reduction in PL in the modified morphologies compared to bulk CNx. A reduction in PL can be an indicator of changes in the distribution of trap state as trap states can be non-radiative.26 The trap state distribution determined from the TAS kinetics does show meso CNx with the largest trap state distribution and PL shows the lowest PL intensity, however the trend is not consistent between all samples. PL is performed with just the powdered material, whereas the photocatalytic measurements are performed with a Pt cocatalyst. Potentially, PL measurements of the CNx with the cocatalyst deposited will show a trend more similar to the observed activity. This could be another future direction of research which could help to understand these trends in PL observed. Branching out from this research, attempts at synthesizing nanostructured CNx materials could be look at in a similar investigation to assess how nanostructuring the material affects the electronics. If the charge carrier diffusion length of carbon nitride was on a length scale of 10 nm, the charge carrier lifetime would have to be almost 4 µs due to the low charge carrier mobility of CNx (10-6 to 10-4 cm2 V-1s-1).172 The lifetime of charge carriers in CNx is more typically around 10 ns, and given the charge carrier mobility, the charge carrier diffusion length would be 0.2-1.6 nm.172 Changes of the morphology on scales larger than this isn’t expected to change 62 the excited state dynamics as, from the charges’ point of view, the diffusion process to reach the surface remains the same. To target changes to excited state dynamics to improve photocatalytic hydrogen evolution we suggest targeting modifications on < 10 nm scales. This could provide insight into which types of nanostructuring influence the kinetics while providing increased surface area. Methods of nanostructuring CNx that could be investigated include mesoporous morphologies with higher silica-precursor ratios and smaller pore sizes.90,173,174 Other nanomorphology options are also often performed with not only silica templating but the addition of other solvents, which would add a layer of complexity to the analysis as the solvent addition may also play a role in changing the photocatalytic efficiency of the material.175,176 Examples include 5 nm mesoporous CNx synthesized using a template method where TEOS and HCl are added to the cyanamide precursor and calcined together creating silica spheres in situ. This creates mesoporous CNx nanosheets with a surface area of 152 m2g−1 (bulk CNx BET comparison not reported) and activity improvements compared to bulk CNx for dye degradation over 9.3-fold improved.175 Another example of nanostructuring was performed using a supramolecular method to synthesize a mesoporous CNx with hierarchical spheres. These modified morphologies have surface areas ranging from 55 to 58 m2g−1 (bulk CNx BET comparison not reported) and the activity of the undoped CNx mesoporous spheres was over 5.3-fold improved compared to bulk.176 Another potential area for future research is how does the morphologies affect the hydrophobicity of CNx. The surface morphologies can change the materials hydrophobicity.177,178 Inducing surface curvature on carbon surfaces on a < 2 nm scale has been shown to increase the hydrophilicity of the material.179 Creating spherical CNx morphologies could improve hydrophilicity of CNx by introducing similar surface curvatures. Surface roughness have been show incease hydrophobicity and material in shapes like lotus leave or geckos feet can be super hydrophobic178 This concept should be investigated for CNx when creating design guideline for CNx morphologies because the interaction of the CNx surface and water is an important for photocatalytic water splitting. In conclusion, this study has clearly shown that the modified morphologies have a direct correlation to photocatalytic activity. This shows that the limitations with this material are the charges reaching the surface, which is consistent with a photocatalytic system using sacrificial reagents. Future directions of research into these morphologies should involve investigations into the effect of using a photosynthetic system without sacrificial reagents which will change the system limitations from charges reaching the surface. 63 Chapter 5: Experimental 5.1 Synthesis The synthesis details of all CNx morphologies and silica materials. 5.1.1 Carbon Nitride (CNx) All CNx materials were synthesized with 2 g cyanamide (Sigma Aldrich, 99%) in a 20 mL alumina crucible and calcined in a muffle furnace with the crucible lid on. All samples were calcined at 550 °C for 4 hours at a ramp rate of 2 °C/min. The washing procedure employed for all CNx samples was as follows. The CNx sample was ground and placed into a 50 mL conical tube. 50 mL of deionized H2O (dH2O) was added to the conical tube then sonicated for 5 mins. The samples were then centrifuged at 4000 RPM at 22 °C for 10 mins and the supernatant was decanted. This process was then repeated with 1M KOH, 1M HCl, acetonitrile, dH2O, 1M KOH, 1M HCl and water. The samples were transferred to a crystallizing dish and dried overnight at 70 °C. 5.1.1.1 Bulk CNx To synthesize bulk carbon nitride, the cyanamide was placed into a crucible. The crucible was placed into a muffle furnace with the crucible lid on. The sample was calcined at 550 °C for 4 hours at a ramp rate of 2 °C/min. Then samples were ground and treated with 4M NH4HF2 for 48-hours. The CNx was isolated by centrifuging at 4000 RPM at 22 °C for 10 mins. The samples were then washed, dried and ground. Typical yields were 20-30%. 5.1.1.2 Mesoporous CNx To synthesize mesoporous carbon nitride, cyanamide was melted at 70 °C.89 Then 2.5 g LUDOX HS40 was added dropwise over 2 mins. The mixture was then stirred for 5 min then transferred to a crucible preheated to 70 °C. The crucible was placed in the muffle furnace at 70 °C, which was then set to hold at 550 °C for 4 hours at a ramp rate of 2 °C/min. The CNx-silica composite was ground and then treated with 4M NH4HF2 for 48-hours. The sample was then washed using the washing procedure described above. Typical yields were 40%. 64 5.1.1.3 Macroporous CNx To synthesize macro CNx cyanamide was melted at 70 °C. Then 1 g of silica spheres were added to the mixture and stirred for 30 mins. The sample was then transferred to a crystallizing dish and dried for ~4 h. The sample was calcined at 550 °C for 4 hours at a ramp rate of 2 °C/min. The CNx-silica composite was ground and then treated with 4M NH4HF2 for 48-hours. The final samples were then washed, dried and ground. Typical yields were 20-25%. 5.1.1.4 SBA CNx To synthesize SBA carbon nitride, the cyanamide was mixed with 3.625 mL of dH2O in a 50 mL round bottom flask for 10 mins until the solution was homogeneous.92,180 Then 0.375g of SBA-15 silica (Sigma Aldrich) was added. The solution was mixed for 1h at ambient temperature. The sample was then transferred to a 50mL conical tube and centrifuged for 10 mins at 4000 RPM. The supernatant was decanted, and the silica loaded cyanamide was placed in a crystallizing dish and dried for 72 h. The sample was then calcined at 550 °C for 4 hours at a ramp rate of 2 °C/min. The CNx-silica composite was ground and the treated with 4M NH4HF2 for 48-hours. The final samples were washed, dried and ground. Typical yields were 20-30%. 5.1.1.5 Hollow Spheres 1. To synthesize HS CNx, cyanamide was melted at 60 °C and then 0.4 g of the previously synthesized silica core-shell was added.86 The sample was mixed at 60 °C for 3 h. The samples were then sonicated at 60 °C for 2 h, then stirred overnight at 60 °C. The sample was then transferred to a 50mL conical tube and centrifuged for 10 mins at 4000 RPM. The supernatant was decanted, and the silica loaded cyanamide was placed in a crystallizing dish and dried overnight. The sample was then calcined at 550 °C for 4 hours at a ramp rate of 2 °C/min. The CNx-silica composite was ground and then treated with 4M NH4HF2 for 48-hours. Then final samples were washed, dried and ground. 2. To synthesize HS CNx, cyanamide is melted at 65 °C and then 0.4 g of the previous synthesized silica core-shell was added.86 The sample was sonicated at 65 °C for 3 h, then stirred overnight at 65 °C. The sample was then transferred to a 50mL conical tube and centrifuged for 10 mins at 4000 RPM. The supernatant was decanted, and the silica loaded cyanamide was placed in a crystallizing dish and dried overnight. The sample was then calcined 65 at 550 °C for 4 hours at a ramp rate of 2 °C/min. The CNx-silica composite was ground and then treated with 4M NH4HF2 for 48-hours. Then final samples were washed, dried and ground. 5.1.2 Silica Silica was synthesized for two of the morphologies, the macroporous CNx and the hollow sphere carbon nitride. 5.1.2.1 Silica Spheres The silica spheres used to synthesise the macro CNx were synthesized as follows.86 First 117 g of 95% ethanol was mixed with 20 g of Millipore H2O at 30 °C for 5 mins, then 6.8 g of 28-30% aq. ammonia was added into the solution and stirred for 30 mins. Then 11.2 mL of tetraethoxysilane (TEOS) was added to the solution with vigorous stirring. Then stirring was stopped and the solution was left stationary for 1 h at 30 °C. The sample was then transferred to a 50mL conical tube and centrifuged for 10 mins at 4000 RPM. The supernatant was decanted, and the silica particles isolated. They were transferred to a crystallizing dish and dried at 70 °C. The sample was then placed in a 50 mL conical tube and 50 mL of 1M HCl solution was added and left to sit for 3 hours. The sample was centrifuged for 10 mins at 4000 RPM and the HCl was decanted. The silica was then dried at 80 °C in a crystallizing dish overnight. 5.1.2.2 Silica Spheres Core-Shell 1. The silica spheres with C18TMOS porogen used to synthesize the hollow sphere CNx were synthesized as follows.86 First 58.5 g of 95% ethanol was mixed with 10 g of Millipore H2O at 30 °C for 5 mins, then 3.10 g of 28-30% aqueous ammonia was added into the solution and stirred for 30 mins. Then 5.6 mL of tetraethoxysilane (TEOS) was added to the solution with vigorous stirring. Then stirring was stopped and the solution was left stationary for 1 hours at 30 °C. Then a mixture of TEOS (5.83 g) and a porogen n-octadecyltrimethoxysilane (C18TMOS, 2.62 g) was added to the solution dropwise over 15 mins with stirring. The solution was left to sit for 3 hours at ambient temperature. The sample was then centrifuged for 10 mins at 4000 RPM and dried at 70 °C in a crystallizing dish overnight. The sample was then placed in an alumina crucible and calcined at 550 °C for 6 hours at a ramp rate of 2 °C/min. The sample was then placed in a 50 mL conical tube and 50 mL of 1M HCl solution was added and left to sit for 3 hours. The sample was centrifuged for 10 mins at 4000 RPM and the HCl was decanted. The silica was dried at 80 °C in a crystallizing dish. 66 2. The silica spheres with CTAB porogen used to synthesis the hollow sphere CNx were synthesized as follows.163 First 50 mL of 95% ethanol was mixed with 2.5 mL of 28-30% aqueous ammonia and stirred for 30 mins. Then 1.5 mL of tetraethoxysilane (TEOS) was added to the solution with vigorous stirring. Then stirring was stopped and the solution was left stationary for 24 h. The silica cores were isolated by centrifugation for 10 mins at 4000 RPM. Then 100mg of the isolated silica core was dispersed in a EtOH (12.6 mL), Millipore H2O (30mL), CTAB (160 mg) solution by sonication the solution until the silica core were dispersed, Then the solution was stirred for 30 mins. Then 0.286 mL of TEOS was added with stirring and left to stir overnight. The sample was centrifuged for 10 mins at 4000 RPM and dried at 70 °C in a crystallizing dish overnight. The sample was then placed in an alumina crucible and calcined at 550 °C for 4 hours at a ramp rate of 2 °C/min. The sample was then placed in a 50 mL conical tube and 50 mL of 1M HCl solution was added and left to sit for 3 hours. The sample was centrifuged for 10 mins at 4000 RPM and the HCl was decanted. The silica was dried at 80 °C in a crystallizing dish. 5.2 Structural Characterization Experimental details on the structural characterization techniques, FTIR, XPS and pXRD. 5.1.2 Fourier-transform Infrared Spectroscopy All FTIR spectra were obtained as follows. Powder samples were placed onto an attenuated total reflectance (ATR) crystal and compressed under a constant flow of nitrogen. All spectra were taken from 525 cm-1 to 4500 cm-1 with 32 scans. The instrument used was a Thermo Electron Nicolet 6700 Spectrometer with a XT-KBr beamsplitter and a SmartOrbit diamond attenuated total reflectance (ATR) attachment. Thermo Electron’s OMNIC software was used with SmartOrbit experimental settings to obtain data. 5.1.3 X-ray Photoelectron Spectroscopy The X-ray photoelectron spectroscopy (XPS) analysis was performed using a Kratos Analytical Axis ULTRA spectrometer containing a DLD spectrometer using a monochromatic aluminum source (AlKα, 1486.6 eV) operating at 150 W (10 mA emission current and 15 kV HT). The sample area analyzed was 700 x 300 µm2. The survey scans were obtained at a 1 eV step size, pass energy of 160 eV, and averaged over 2 scans. Narrow scans were obtained at a 67 0.1 eV step size, a pass energy of 20 eV, and averaged over 3 scans. Energy scale linearity was calibrated using Al and Mg X-ray sources on Argon sputter cleaned gold and copper substrates. The calibration procedure was performed in accordance with the ISO 15472 international procedure. 5.1.4 Powder X-ray Diffraction Powder X-ray diffraction (pXRD) patterns were acquired with a Bruker D8 Advance diffractometer using Cu Kα radiation (λ = 0.15418 nm) and a LynxEye detector operating in one-dimension mode. Copper X-rays were generated from a copper target by electron bombardment at 40 kV and 40 mA. A 1.0-mm divergence slit was placed between the X-ray source and the sample. Samples were mounted on quartz, a low background, sample holder. 5.3 Electronic Characterization Experimental details on the electronic characterization techniques, UV-vis DRS and photoluminescence. 5.3.1 UV-Vis Diffuse Reflectance Spectroscopy (DRS) The UV-Vis diffuse reflectance spectroscopy of the CNx morphology was obtained by first preparing the samples. To prepare the samples a thin layer was pressed between a quartz and glass slide. The detector used was Ocean Optics Maya 2000 PRO spectrometer. The samples were placed under a 50 mm integrating sphere with an Ocean Optics deuterium/halogen UV-Vis light source on. The integrating sphere was fit with a white reflective top. The spectrometer was set to a slit width of 200 µm and experimental settings of 100 scans at 100 ms integration time with a boxcar width of 3 and with electric dark and nonlinearity correction settings on. 5.3.2 Photoluminescence Spectroscopy The PL spectra samples were prepared similarly to the DRS by putting the sample between a quartz and glass slide. The detector used was Ocean Optics Maya 2000 PRO spectrometer with a slit width of 200 µm. The samples were placed under a 50 mm integrating sphere with an excitation wavelength of 405 nm. The current of a 405 nm laser diode was set to 40 mA and the diode temperature set at 20 ˚C. An optical trap was installed on the integrating sphere to capture specular reflectance. 100 scans were averaged with an integration time of 300 ms each, with electric dark and nonlinearity correction settings on. 68 5.4 Morphological Characterization Click or tap here to enter text. 5.4.1 Scanning Electron Microscopy SEM was obtained of powdered samples. First a thin layer of sample was stuck onto the circle sample holder. Then a 10 nm Pt layer was sputter coated onto the sample using a Cressington sputter coater. The instrument used was a Tescan Mira 3 XMU scanning electron microscope and the images were taken with working distance of 14-15 mm, with a beam intensity of 15.00, and an accelerating voltage of 20.0 kV. 5.4.2 Transmission Electron Microscopy The Samples for TEM imaging were prepared by sonicating the samples with IPA (ACS Plus grade), which were then drop casted onto a lacey carbon grid (200 mesh, Ted Pella Inc). The TEM images were acquired on a FEI Tecnai Osiris S/TEM system equipped with Bruker SuperX EDX detectors and operated at 200 keV. 5.4.3 Surface Area The Brunauer-Emmett-Teller (BET) surface area measurements were done using a ASAP 2460 from Micromeritics instrument. The samples were degassed at 150˚C for 4 hours. 5.5 Charge Carrier Dynamics Click or tap here to enter text. 5.5.1 Transient Absorption Spectroscopy The TAS measurements were obtained by first preparing the samples. The ground powders were packed into a 1cm quartz cuvette. A Surelite continuum Nd:YAG laser (1064 nm fundamental output) was frequency tripled to generate laser pules at 355 nm which was used to excite the samples. A 95 W oriel lamp light source was used to probe the transient changes in absorption produced after photoexcitation. The light was focused on the sample, which is reflected and directed towards an Oriel Cornerstone monochromator with mirrors. The signal was averaged over 256 scans for each wavelength from 450-1000 at 50 nm intervals. Thorlabs band pass filters were used to filter out undesired wavelengths. The laser’s 355 nm fluence was 153 uJ/cm2 with area of 0.49 cm2. 69 5.6 Photocatalytic Activity Each sample was measured in at least duplicate. 5.6.1 Hydrogen Evolution First the solution for photocatalysis, the hole scavenger solution was prepared. This was a 11.5 mL solution of Millipore H2O and 1.5 mL of the hole scavenger triethanolamine (TEOA).51,181,182 This solution was sonicated for 5 min with 23 mg of the CNx photocatalyst. Then 3% wt Pt was added from a previously prepared Pt salt (H2PtCl6) stock solution. The sample in a quartz cuvette, was purged with argon for 10 mins then irradiated with a 405 nm LED at 10.75 mW/cm2 for approximately 1-2 hours to photodeposit the Pt onto the surface of CNx. The irradiation was stopped, and the samples were purged again with argon for 10-30 mins until a stable baseline signal from the Unisense hydrogen microprobe sensor was achieved. Then the light was turned on again and the sample was irradiated with 10.75 mW/cm2 and the rate of hydrogen evolution was measured. The H2 evolution rate for one sample was typically measured at least twice. After the first H2 hydrogen evolution measurement, the LED was turned off and the sample purged with argon for 10 mins to remove any H2 that was produced. The sample was then purged and the LED was turned back on and the hydrogen evolution rate was measured again. The H2 sensor used was a Unisense hydrogen microprobe sensor which was placed in the solution for measurements. 70 Bibliography (1) Canada, N. R. Energy-Facts; Natural Resources Canada, 2020. (2) Environment, U. N. Emissions Gap Report 2020; 2020. (3) COVID Curbed Carbon Emissions in 2020 — but Not by Much. (4) International Energy Agency. 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Thesis/Dissertation
eng
Chemistry
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Godin, Robert
Carbon nitride photocatalysis : an investigation on the influence of morphology
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