Synthesis and Mechanism Study of 1T'-WS₂

Abstract

Metastable transition metal dichalcogenides (TMDs) have lately caught the attention of scientists due to their distinct properties compared to their thermodynamically stable counterparts. This opened a whole new research area, since apart from the classic parameters such as composition and thickness, the phase also became a critical and tuneable parameter. Similar to how carbon exhibits different properties in different allotropes, the properties of TMD metastable phases show differences compared to those of the stable 2H-phase (for example, the 2H-MoS₂ is a well-known semiconductor, while the 1T-MoS_² and 1T′-MoS₂ are metallic and semimetallic, respectively). This allows for new applications of these materials, which, so far, have shown improved results compared to the 2H-phase in areas such as catalysis (1T-MoS₂ nanosheets proved to have superior catalytic performance in the hydrogen evolution reaction) and superconductors (1T′-MoS₂ was observed to exhibit superconductivity). To date, the main focus was on group VI B transition metal dichalcogenides, whose pure metastable phases were obtained as single crystals and were thoroughly characterised. However, we must attempt to understand the formation mechanism of these metastable phases in order to widen the library of metastable materials to other compounds.

This thesis is focused on the better understanding of 1T′-WS₂, as a model compound for metastable VI-B transition metal dichalcogenides and is structured into four chapters. The first chapter presents a thorough literature review, commencing with an introduction to the present research progress into TMDs and thio-oxo-metalates, followed by the current progress in Raman studies of both thermodynamically stable and metastable TMDs. Next, the metastable materials are discussed, together with the advantages of using these materials compared to their stable counterparts. Followed by the current strategies for synthesizing metastable TMDs and their current applications. At the end of this chapter, the research purpose, motivation, and aims of this thesis are introduced.

The first chapter of experimental results elucidates the vibration modes shown in the Raman spectrum of 1T′-WS₂. By using both non-polarised Raman and angle-polarised Raman, we investigated, on one hand, the Raman spectrum of 1T′-WS₂ by combining experimental results with a statistical approach and fitting, and, on the other hand, assigning each peak to its corresponding vibration mode, based on theoretical considerations and results of the angle-polarised Raman measurements in both parallel and perpendicular configuration.

The following chapter is focused on the mechanistic insights of the 1T′-WS₂ formation using the salt-assisted synthesis. The main conclusions were that the reaction goes through a stable tungsten (VI) intermediate, which we postulate that is a thio-oxo-tungstate. The gas-solid salt-assisted method is, up to date, the ideal method to synthesise metastable TMDs, since it allows the synthesis of pure, crystalline compounds. On the other hand, this method shows great versatility, allowing the synthesis of 1T′-MoS₂, MoSe₂, WS₂, WSe₂, as well as alloys such as 1T′-WS_2xSe_2(1-x) and 1T′-MoS_2xSe_2(1-x). While certain mechanistic aspects were proposed in the past, there was no thorough attempt to understand the formation of this metastable phase. Thus, this chapter made significant progress in understanding the mechanism of this reaction.

In the last experimental chapter, we analysed the crystallisation of thio-oxo-tungstates from the direct sulphuration synthesis, and we were able to unveil a correlation between the nuclearity of the resulting thio-oxo-tungstate and the speed of diffusion. While we focused on the mononuclear species, the resulting polynuclear thio-oxo-tungstates also showed interesting structures, obtaining crystal structures of an undecanuclear species, an uncapped dodecahedron, as well as a tridecanuclear species, a single (thio)-tungstate moiety caged into a dodeca-thio-oxo-tungstate. Finally, the resulting thio-oxo-tungstates were successfully used as precursors to access the 1T′-phase of WS₂.

The final chapter presents the conclusions and the outlook of the work. The Raman study presented in Chapter 2 shows the suitability of this method to characterise TMDs and to unequivocally assign the phase of the resulted materials. Future work may prove that this non-destructive method is also suitable to accurately determine the composition of complex materials, comprising either multiple phases (2H and 1T′-TMD) or alloys (for example Mo_xW_1-xS₂ or WS_2xSe_2-2x). The mechanistic study of 1T′-WS₂ formation proves that the salt-assisted synthesis can be extended to other starting materials besides the metal (IV) sulphide, thus extending the applicability of this synthesis to materials for which the metal sulphide is not easily available, either due to the cost or the commercial availability. Moreover, in corroboration with the final chapter, which shows that the 1T′ phase can be accessed using thio-oxo-metalates, it opens a whole new approach to synthesising metastable materials by starting from thio-oxo-metalates and, perhaps, other coordination compounds. The synthesis of poly-oxo-thio-metalates has already been extensively researched so these compounds can be rationally designed to control the resulting material. For example, heterometallic poly-oxo-metalates can prove key to the synthesis of 1T′-TMD alloys with finely controlled composition.

Date of Award	29 May 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Hua ZHANG (Supervisor)