Non-hydrolytic Sol-gel (NHSG) Synthesis of Transition Metal Sulfides and Theoretical Investigations
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 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 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.
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