A Thesis Entitled Non-hydrolytic Sol-gel (NHSG) Synthesis of Transition Metal Sulfides and Theoretical Investigations Xiuquan Zhou Submitted to the Graduate Faculty as partial fulfillment of the requirements for The Master of Science Degree in Chemistry _________________________________ Dr. Cora Lind-Kovacs, Committee Chair _________________________________ Dr. Terry Bigioni, Committee Member _________________________________ Dr. Eric Findsen, Committee Member _________________________________ Dr. Sanjay Khare, Committee Member _________________________________ Dr. Patricia Komuniecki Dean of College of Graduate Studies The University of Toledo May 2013 An Abstract of Non-hydrolytic Sol-gel (NHSG) Synthesis of Transition Metal Sulfides and Theoretical Investigations by Xiuquan Zhou Submitted to the Graduate Faculty as partial fulfillment of the requirements for The Master of Science Degree in Chemistry The University of Toledo May 2013 Non-hydrolytic sol-gel (NHSG) synthesis provides an elegant approach to many solid-state materials, which was originally developed for the preparation of oxides.1 As they do not require high temperatures like conventional solid-state routes, access to thermodynamically metastable materials, which cannot be prepared through traditional solid-state routes, is possible. In this project, NHSG chemistry is explored for the synthesis of binary metal sulfides. Sulfides, some of which are thermally unstable and highly oxygen sensitive, have applications in many areas, such as solar cells, catalysts, sensors, lubricants, semiconductors etc.2 Despite the widespread use of sulfides, they have been studied much less comprehensively than oxides. One of the reasons is the difficulty in synthesis because metastable sulfides are intolerant to high temperatures and oxygen. In NHSG routes, the reaction of a metal halide with a thioether is used to form a metal sulfide iii network at low temperatures. This can give access to materials that are not accessible by high temperature routes, and may lead to the discovery of new phases.3 Because synthesis of sulfides through NHSG is an unexplored area, the synthesis conditions must be optimized for each metal. In this thesis, the Cu-S and Ta-S systems were thoroughly explored using NHSG techniques. Amorphous tantalum sulfides were obtained in as-recovered samples, and heat treatments of such amorphous precursors resulted in crystallization of 1-TaS2 and 3R-TaS2 phases. For the synthesis of copper sulfides, precise phase selection of five different polymorphs, metastable hexagonal chalcocite, monoclinic chalcocite, djurleite, low digenite and covellite, was achieved by fine tuning synthetic parameters. In addition, a continuous phase evolution from copper rich phases towards copper deficient phases between chalcocites and djurleite was observed. All polymorphs could be obtained as nanoparticles. Theoretical studies can complement experimental approaches, and may aid in understanding polymorph stabilities. In addition, theory can elucidate phase transition pathways between polymorphs, which may help design synthetic approaches to specific phases. As a starting project, pressure-induced phase transitions from the NaCl-type (B1) to the CsCl-type (B2) structure in BaS, BaSe and BaTe were studied using ab initio density functional theory computations in the local density approximation. The Buerger4 and WTM5 mechanisms were explored by mapping the enthalpy contours in two and four dimensional configuration space for the two mechanisms, respectively. Transition pressures for BaS, BaSe and BaTe were determined to be 5.5 GPa, 4.9 GPa and 3.4 GPa, respectively. From these configuration space landscapes, a low enthalpy barrier path was constructed for the transitions to proceed at three different pressures. We obtained iv barriers of 0.18, 0.16 and 0.15 eV/pair (17.4, 15.4 and 14.5 kJ/mol) for the Buerger mechanism, and 0.13, 0.13 and 0.12 eV/pair (12.5, 12.5 and 11.6 kJ/mol) for the WTM mechanism at the transition pressures for BaS, BaSe and BaTe, respectively, indicating that the WTM mechanism is slightly more favorable in these compounds. v Acknowledgements A conclusion of my study at the University of Toledo (UT) will soon be reached, although the results will be quite different from what I expected when I first came here. From both bitter and sweet experiences, I indeed have learned much more than what I expected as well. I would like to thank those who have helped and supported me in the past, as nothing was achieved alone. First and most, I would like to thank my advisor Dr. Cora Lind-Kovacs for her guidance and inspiration. I would not have decided to continue to do research in the field of solid-state chemistry if I had not worked for her. She not only taught me valuable knowledge of sciences, but also taught me how to be a scientist. Admittedly, not all experiences were pleasant, but it was those unpleasant ones that revealed mistakes and helped me to change. I did not fully realize the importance of the role of an advisor until I reached the lowest point of my life, as many things I had taken for granted. A good advisor is a like a beacon that can guide lost ships to a safe port from rough seas in storms. Fortunately, I had a very good advisor, and for that matter, I will always be grateful. I would like to thank Dr. Sanjay Khare, not only for the financial support of my last semester at UT, but also for leading me into the world of theoretical physics. He taught me a valuable lesson that a seemingly fine paper from a reputable group in a vi respectful journal could contain many vital mistakes by iterating a work related to our research. He also provided us laughter with many of his witty jokes, such as "always trying to roar like a tiger, even though end up with a meow". I would like to thank my committee members, Dr. Terry Bigioni and Eric Findsen. I would like to thank Pannee for all the patient help with the instruments, Steve for the help with glassware, and Youming and Tom for fixing many equipments. I would like to thank Jason Roehl, Dr. Khare's graduate student, for teaching me how to use VASP and working together with me. Not for Jason, our publication on Journal of Physics: Condensed Matter would not have been possible. I would also like to thank Jason for showing me how to be a professional graduate student. I would like to thank both current and former members of Lind-Kovacs group for eating, working and laughing together. I enjoyed and appreciated their friendships, especially for Xiaodong and Nate. In addition, I want to thank Nathalie Pedoussaut, Anne Soldat, Christophe Heinrich, Martin Kluenker and Derek Mull as their work will be included in our papers. I would like to thank the University of Toledo and all the faculty and stuff at the department of chemistry. I want to thank National Science Foundation (NSF) for funding our work on sulfides (DMR 1005911), providing computational resources (CNS 0855134 and CMMI 1234777) and providing SEM (CRIF 0840474). I would like to thank Ohio Supercomputer Center (OSC) for providing additional computational resources and the Advanced Photon Source (APS) for 16 mail-in samples. vii viii Table of Contents An Abstract of .................................................................................................................. iii Acknowledgements .......................................................................................................... vi Table of Contents ............................................................................................................. ix List of Tables .................................................................................................................. xiii List of Figures ................................................................................................................. xvi 1. Introduction ............................................................................................................... 1 1.1 Transition Metal Sulfides ..................................................................................... 1 1.2 Literature Routes to Metal Sulfides ..................................................................... 7 1.2.1 Traditional High Temperature Routes .......................................................... 8 1.2.2 Solution Based Routes .................................................................................. 9 1.2.3 Other Routes ............................................................................................... 10 1.3 Non-hydrolytic Sol-gel (NHSG) Methods ......................................................... 11 1.4 Theoretical Modeling in Solid State Chemistry ................................................. 14 1.5 Goals of Thesis Research ................................................................................... 16 2. Experimental Methods ........................................................................................... 18 2.1. Synthetic Method ............................................................................................... 18 2.1.1 Room Temperature Route ........................................................................... 19 2.1.2 Solvo-thermal Route ................................................................................... 20 2.1.3 Heat Treatment............................................................................................ 21 ix 2.2. Characterization Methods .................................................................................