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Investigation Into High Efficiency Visible Light Photocatalysts for Water Reduction and Oxidation

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Investigation Into High Efficiency Visible Light Photocatalysts for Water Reduction and Oxidation Investigation into high efficiency visible light photocatalysts for water reduction and oxidation David James Martin 2014 A thesis submitted for the partial fulfilment of the requirements for the degree of Doctor of Philosophy at University College London Department of Chemical Engineering, University College London, Torrington Place, London, WC1E 7JE 1 Declaration I, David James Martin, confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. ……………………………………………… Signature ……………………………………………… Date 2 I. Acknowledgements Firstly, I would like to thank my supervisor, Dr Junwang Tang for his unparalleled guidance, expertise and insight throughout the entire project. Throughout the ups and downs, he remained focused and determined, two characteristics which have definitely rubbed off on me. He will remain a lifelong mentor and friend. I would also like to thank my second supervisor, Prof. Jawwad Darr, for many beneficial discussions, and for being very supportive in difficult times. I’m eternally grateful to Mark Turmaine and Jim Davy for helping me with SEM, Steve Firth for help with TEM/FTIR/Raman spectroscopy, Martin Vickers with XRD, and Rob Gruar for helping me with the most annoying instrument ever (ZetaSizer Nano). A special mention goes to Xiaowei Chen and the humble Juan Jose Delgado for consistently collaborating via their excellent TEM expertise. A huge mention must go to Dr Naoto Umezawa and Prof. Jinhua Ye (National Institute of Materials Science, ‘NIMS’, Japan). Prof. Ye, who very kindly let me undertake a short research internship in her group, taught me in a very short space of time, to not only think outside the box, but also to make sure the box has the correct space group reflection conditions – so you know where the edges are! In my first international collaboration, Naoto was not only a good friend, but a patient and thoughtful man who had real faith that our work would be complementary and hence publishable. We had many discussions both personal and professional, and he will remain another lifelong friend. I thank them both for making my study in Japan thoroughly enjoyable. I would also like to thank the forever enthusiastic and forward-thinking Dr Stephen Shevlin, who was heavily involved in the second collaboration. His insight into DFT and TDDFT studies complimented and explained some of the experimental work I completed on carbon nitride. On a personal level Stephen also taught me about the ups and downs of reviewers, which I’ll never forget. Kaipei Qiu was also extremely helpful in contributing to a publication. Prof. Z. Xiao Guo, who co-supervised the collaboration with Dr Junwang Tang, was also precise, thoughtful and insightful throughout. Dr Albertus Handoko and Dr Savio Moniz were two of the best post-docs a student could have. Before Albertus arrived, working without a PDRA was frankly a little difficult. Sometimes it’s really beneficial to have 3 somebody who can give you a quick answer, rather than search for it for hours. Definitely a friend for life, and despite him moving away from the group, I’m sure our paths will cross again soon. The people who made my PhD actually fun in moments, kept me sane and helped with work during the later hours; Ben, Phil, Rhod, Seamus, Lawerence, Mayo, Amal, Chara, Noor, Mithila, Miggy, Eria, Moz, Toby, Jay, Vidal, Erik and all the rest of you guys. To my fiancé Catariya, who was there throughout the good times and bad - I couldn’t have done it without you. Finally, to my family; Mum, Dad, brothers, Nan and Granddad – you literally kept me going and I will never forget the sacrifices made. 4 II. Abstract Solar water splitting using an inorganic semiconductor photocatalyst is viewed as one of the most exciting and environmentally friendly ways of producing clean renewable fuels such as hydrogen from abundant resources. Currently, there are many diverse semiconductors that have been developed, the majority for half reactions in the presence of sacrificial reagents. However, for industrial facilitation, there exists an essential, non- debatable trifecta of being robust, cheap and efficient for overall water splitting. To date, no system has combined all three, with most examples missing at least one of the necessary trio. Therefore one of the current challenges of the field is to develop low cost, highly efficient and stable photocatalysts for industrial scale-up use. In order to achieve that aim, researchers must focus on novel semiconductors to improve efficiencies and also understand the fundamental mechanisms. The primary focus of this thesis then, is to investigate some of the newest photocatalysts for water photooxidation, reduction, and overall water splitting. In doing so, the thesis aids to shed light on the mechanisms behind what makes certain photocatalysts either efficient or inefficient. Firstly, test station was set up to analyse gaseous products such as hydrogen and oxygen produced from photocatalytic water splitting, by using a custom made high purity borosilicate reactor in conjunction with a gas chromatography unit. Gaseous products could be measured with very small sampling error (<1%), which improved the throughput of experiments. The photooxidation of water using a novel faceted form of Ag3PO4 was investigated. A novel synthetic method was created that made it possible to control the exposing facets of silver phosphate in the absence of surfactants to yield tetrahedral crystals composed entirely of {111} facets. It was found that due to high surface energy of {111}, and low hole (h+) mass in the 111 direction, Ag3PO4 tetrahedral crystals could outperform all other low index facets for the oxidation of water under visible light. The quantum yield was found to be nearly unity at 400 nm, and over 80% at 500 nm. With the exception of Ag3PO4 tetrahedral crystals, no photocatalyst has exhibited quantum efficiencies reaching 100% under visible irradiation. Therefore, the strategy of morphology control of a photocatalyst, led by DFT calculations of surface energy and charge carrier mobility, in order to boost photooxidation yield has been demonstrated to 5 be very successful, and could be applied to improve other semiconductors in future research. Hydrogen production from water was further studied using the only known robust organic photocatalyst, graphitic carbon nitride (g-C3N4). It was discovered that using a novel preparation method, urea derived g-C3N4 can achieve a quantum yield of 26% at 400 nm for hydrogen production from water; an order of magnitude greater than previously reported in the literature (3.75%). The stark difference in activity is due to the polymerisation status, and consequently the surface protonation status as evidenced by XPS. As the surface protonation decreases, and polymerisation increases, the performance of graphitic carbon nitride for hydrogen production increases. The rate of hydrogen production with respect to BET specific surface area was also found to be non-correlating; a juxtaposition of conventional photocatalysts whose activity is enhanced with larger surface areas - believed to be because of an increase in surface active sites. Finally, overall water splitting was probed using Z-scheme systems comprising of a redox mediator, hydrogen evolution photocatalyst, and oxygen evolution photocatalyst. Ag3PO4 was found not be not suitable for current Z-scheme systems, as it is unstable in the pH ranges required, and also reacts with both of the best known electron mediators used in Z- schemes, as evidenced by XRD, TEM, and EDX studies. However, it has been demonstrated that urea derived g-C3N4 can participate in a Z-scheme system, when combined with either WO3 or BiVO4 – the first example of its kind, resulting in a stable system for an overall water splitting operated under both visible light irradiation and full arc irradiation. Further studies shows water splitting rates are influenced by a combination of pH, concentration of redox mediator, and mass ratio between photocatalysts. The solar-to-hydrogen conversion of the most efficient system was experimentally verified to be ca. 0.1%. It is postulated that the surface properties of urea derived graphitic carbon nitride are related to the adsorption of redox ions, however, further work is required to confirm these assumptions. 6 III. Publications & conferences Publications 1. Efficient visible driven photocatalyst, silver phosphate: performance, understanding and perspective. David James Martin, Liu Guigao, Jinhua Ye and Junwang Tang. Chemical Society Reviews, in preparation, 2014 2. Visible Light-Driven Pure Water Splitting by a Nature-Inspired Organic Semiconductor-Based System. David James Martin, Philip James Thomas Reardon, Savio J.A. Moniz, and Junwang Tang. Journal of the American Chemical Society, 2014, 136 (36), 12568 -12571 3. Highly Efficient H2 Evolution from Water under visible light by Structure-Controlled Graphitic Carbon Nitride. David James Martin, Kaipei Qiu, Stephen Andrew Shevlin, Albertus Denny Handoko, Xiaowei Chen, Zheng Xiao Guo & Junwang Tang. Angewandte Chemie International Edition, 2014, 53 (35), 9240-9245 4. Facet engineered Ag3PO4 for efficient water photooxidation. David James Martin, Naoto Umezawa, Xiaowei Chen, Jinhua Ye and Junwang Tang. Energy & Environmental Science, 2013, 6, 3380-3386 5. H2 and O2 Evolution from Water Half-Splitting Reactions by Graphitic Carbon Nitride Materials. A. Belen Jorge, David James Martin, Mandeep
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