Synthesis, characterization, and applications of 1T'-phase group VIB transition metal dichalcogenides

Preparation of layered transition metal dichalcogenides (TMDs) with unconventional metastable phases has increased in importance thanks to their fascinating physicochemical properties that are promising in various practical applications. Many considerable and rapid advances in the development of synthesis and preparation methods for this new family of layered TMDs have been made in recent decades, yet a universal, reliable, and comprehensive strategy to obtain metastable layered TMDs with high yield, high phase purity, and absence of thermodynamically stable phase impurity is still lacking. Therefore, it is desirable to design a preparation method that can successfully address these issues using the existing understanding of the metastable layered TMDs, which will subsequently open up many opportunities for in-depth studies on their intrinsic properties and the exploration of their potential practical applications. In the effort to achieve this goal, this thesis focuses specifically on the development of a bottom-up synthesis approach for group VIB layered TMDs with metastable 1T′ phase, the evaluation of the phase purity of the products, the investigation of their crystal structures and intrinsic properties, and their applications in other fields of materials science research. Firstly, a general and facile synthetic method was developed to prepare metastable group VIB layered TMD crystals including 1T′-phase WS2, WSe2, MoS2, MoSe2, WS2xSe2(1-x), and MoS2xSe2(1-x) with the use of alkali metal-based precursors. The distorted structure, the high phase purity, and the metastable nature of the as-prepared 1T′-TMD crystals were confirmed with various characterization techniques such as X-ray diffraction, transmission electron microscopy, Raman spectroscopy, etc. Importantly, the complete structure solutions of the metastable 1T′-phase WS2, WSe2, MoS2, and MoSe2 were obtained with single-crystal X-ray diffraction technique, which laid a foundation for future in-depth studies on structure-related properties of these materials. A proposed formation mechanism of the metastable 1T′ phase among group VIB TMDs was also discussed, which could be beneficial for the development of synthetic methodologies to prepare layered TMDs with unconventional phases. Overall, this work broadens the understanding of the formation of metastable 1T′ phase among group VIB layered TMDs and their alloys, provides an effective strategy to explore their intrinsic properties and applications, and paves a way for the development of preparation methods for various layered TMDs with novel unconventional crystal structures. Secondly, a facile synthetic process was developed to study the anisotropic growth of Au nanostructures on metastable 1T′-MoS2 and 1T′-WS2 nanosheets, which have an anisotropic quasi-1D lattice arrangement on their basal planes. The 2D ultrathin 1T′-MoS2 and 1T′-WS2 nanosheets were prepared by applying the lithium intercalation and exfoliation method to their corresponding 1T′-TMD bulk crystals. The formation of the anisotropic pattern of the metal nanostructures was successfully realized on the 1T′-MoS2 nanosheets and further investigated with varying reaction time, reaction temperature, and growth substrate, which provides an important insight into the nucleation and formation of the anisotropic Au nanostructures. Additionally, the obtained 1T′-MoS2/Au heterostructure exhibited significantly enhanced electrocatalytic performance toward hydrogen evolution reaction in aqueous 0.5 M H2SO4 electrolyte. The incorporation of Au showed a substantial improvement in the intrinsic activity and the charge-transport properties of the heterostructure compared with the as-prepared 1T′-MoS2 nanosheets. This work demonstrates the ability of metastable 1T′-phase layered TMDs in controlling the growth behavior of other materials and the promising potential of the resulting heterostructure in electrocatalysis application.