FUNDAMENTAL STUDIES FOR THE DESIGN OF TANTALUM NITRIDE PHOTOANODES FOR SOLAR WATER SPLITTING A DISSERTATION SUBMITTED TO THE DEPARTMENT OF CHEMICAL ENGINEERING AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Blaise A. Pinaud October 2013 © 2013 by Blaise Anne Pinaud. All Rights Reserved. Re-distributed by Stanford University under license with the author. This work is licensed under a Creative Commons Attribution- Noncommercial 3.0 United States License. http://creativecommons.org/licenses/by-nc/3.0/us/ This dissertation is online at: http://purl.stanford.edu/gp551gc7846 ii I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Thomas Jaramillo, Primary Adviser I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Stacey Bent I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Mark Brongersma Approved for the Stanford University Committee on Graduate Studies. Patricia J. Gumport, Vice Provost for Graduate Education This signature page was generated electronically upon submission of this dissertation in electronic format. An original signed hard copy of the signature page is on file in University Archives. iii iv Abstract One of today’s greatest challenges is meeting the increasing global energy demand, projected to rise from our current consumption of 18 TW to nearly 26 TW by 2035. A growing environmental awareness of the impact of fossil fuels on many aspects of life, from climate change to personal health, is driving the development of clean, renewable energy sources which must be both technologically and economically viable. The synthesis of chemical fuels such as hydrogen from sustainable energy sources such as solar or wind is an attractive option. Photoelectrochemical (PEC) water splitting is one promising route for hydrogen production. In PEC devices, semiconductor absorbers harvest solar energy to generate excited electrons and holes to drive the hydrogen and oxygen evolution reactions in aqueous electrolytes. The first part of this dissertation focuses on a technoeconomic evaluation of conceptual water splitting plants based on four different reactor designs with the aim of identifying key research needs in the field. A significant finding is that more efficient semiconductor photoelectrodes must be developed to make this technology cost-competitive with existing fossil fuel energy sources. Tantalum nitride (Ta3N5) is a promising photoanode candidate due to its nearly ideal band structure for solar water splitting. The remainder of the dissertation focuses on understanding which properties may limit its performance in order to ultimately design a higher efficiency oxygen-evolving photoanode of this material. An emphasis is placed on developing well-defined sample types and accurate measurement tools to systematically study the fundamental structural, optical, electronic, and photoelectrochemical properties of Ta3N5 photoanodes. Photoelectrochemical measurements on Ta3N5 thin films grown via thermal oxidation and nitridation of Ta foils reveal that their photoactivity is strongly correlated with increased surface area, suggesting poor hole transport. While the thermal conversion of Ta foils is facile, it is difficult to control the surface morphology which hinders the systematic study of material properties. Work then shifts to the development of an improved sample architecture for the synthesis of flat, crack-free films of tightly controlled thickness. It is v achieved by converting evaporated Ta layers with or without a Pt contact supported on fused silica to form Ta3N5/Pt/fused silica or Ta3N5/fused silica. These samples enable quantitative assessment of the electronic conductivity and optical absorption of Ta3N5. Results from the literature suggest the presence of impurity phases in some Ta3N5 photoelectrodes. We therefore seek to control the synthesis of phase-pure materials through an understanding of the effect of nitridation temperature and the underlying substrate on film quality. We discover that temperature has little consequence on the crystallinity and absorption properties of Ta3N5 synthesized on fused silica but that the presence of mobile Ta atoms in Ta foil substrates can result in the formation of reduced nitride phases (e.g. Ta2N, Ta5N6) at temperatures at or above 1000°C. The localization of the Ta2N phase at the Ta3N5/Ta foil interface is demonstrated with grazing incidence x- ray scattering measurements. We next turn our attention to a well-known issue with Ta3N5, its degradation under illumination to Ta2O5 which blocks transport of the photogenerated holes to the surface. Several catalysts for the oxygen evolution reaction (OER) are deposited on the Ta3N5 surface with the aim of stabilizing the material and improving the water oxidation kinetics. While the precious metal-based catalysts Pt, RuO2, and IrO2 have higher inherent catalytic activity than the novel CoTiOx catalyst developed in our lab, the latter outperforms the three others both in terms of increasing the photocurrent of and stabilizing Ta3N5. This result highlights the important role of the catalyst/semiconductor interface. Lastly, the knowledge of the optimal synthesis conditions, hole and electron transport lengths, and absorption depth is combined to design a core-shell Ta-Ta3N5 photoanode. Several approaches are explored for the nanostructuring of the Ta scaffold on which the Ta3N5 shell can be grown thermally or through anodization. Nanosphere lithography to form a Cr mask followed by reactive ion etching transfers a porous pattern to the Ta foil. A short oxidation time is crucial for limiting the thickness of the shell and preserving the Ta core. We demonstrate a proof-of-principle Ta-Ta3N5 core-shell and discuss means by which to increase the overall aspect ratio. vi In summary, this dissertation covers fundamental studies of the properties of Ta3N5 to design and develop a high performance photoanode which will hopefully enable more efficient solar water splitting devices for the production of hydrogen fuel. vii viii Acknowledgements Graduate school is not always an easy undertaking but the wonderful support of a wide network of people – colleagues, professors, friends, and family – have made my time at Stanford infinitely more productive and enjoyable. I would like to start by acknowledging my terrific adviser, Professor Tom Jaramillo. I vividly remember debating which research group at Stanford would be the right fit for me and I was ultimately drawn to Tom’s enthusiasm and drive to not simply run experiments, but to rigorously design the right tests to draw meaningful conclusions. I truly appreciated his willingness to allow students to get hands on experience with every aspect of research in the group, from synthesis to characterization to testing. I am a significantly more well-rounded researcher for my time spent under Tom’s guidance. While your choice of adviser and research direction play a major role in your graduate experience, it is with your fellow group members that you spend most of your waking hours. I could not have asked for a better group of people than my lab mates in the Jaramillo group! Our once small group grew significantly over the years, with each new member bringing something to the mix, but I would like to acknowledge those who particularly impacted my time at Stanford. First and foremost, I must acknowledge the incredible mentorship I received from Dr. Zhebo Chen. I joined the group with only a basic knowledge of electrochemistry and absolutely no understanding of semiconductor physics. He patiently taught me the important concepts and showed me how to run the experiments correctly. Despite little overlap in our research projects, Dr. Kendra Kuhl has also been an excellent mentor and an incredible friend. Dr. Yelena Gorlin’s keen insights into electrochemical phenomena and attention to detail pushed me to think deeply about my data. Dr. Jakob Kibsgaard’s superior graphic design skills helped elevate the quality of many of my presentations and figures. Perhaps even more importantly, his positive attitude made it a pleasure to come into the office each morning. Dr. Peter Vesborg was an invaluable mentor for work on tantalum nitride and oxynitrides, always providing excellent guidance when I did not know what to try next. I was fortunate enough to be a member of a very talented ix photoelectrochemistry sub-group. Jesse Benck, Linsey Seitz, and Pong Chakthranont are not only incredibly intelligent people, they are fantastic friends. If I was having a tough day in lab where nothing seemed to work, Jesse, Linsey, and Pong were always willing to help troubleshoot and remind me of the greater goal of our work. I would like to also thank David Abram, my fellow classmate who joined the Jaramillo group. Together we learned the ropes in lab and shared many great adventures! Whether lounging with a beer at a group social out on the lawn or competing for the almond during Danish Christmas lunch, group members Dr. Benjamin Reinecke, Etosha Cave, Ariel Jackson, Desmond Ng, Toru Hatsukade, Ieva Narkeviciute, Tommy Hellstern, Jeremy Feaster, Dr. Arnold Forman, Dr. Chris Hahn, Dr. Maureen Tang, and Dr. Samuel Fleischman have made my daily life all that much more fun. I would also like to acknowledge all of the other visiting professors, post-doctoral scholars, master’s students, and undergraduates who worked in the lab. Lastly, I was fortunate enough to have the opportunity to mentor several undergraduate and graduate students. Desmond Ng and Ieva Narkeviciute were both eager rotation students who, happily, chose to join our group. Jared O’Leary was an undergraduate student who worked diligently on the very difficult problem of synthesizing phase pure tantalum oxynitride; his unfailing enthusiasm and work ethic were much appreciated.