PhD Thesis Presentation - Controlled Synthesis of Two-dimensional Layered Metal Chalcogenides for Energy Application

Two-dimensional (2D) layered metal chalcogenides (LMCs) have attracted significant research attention for their potential application in energy storage and conversion. However, the understanding of reaction mechanism remains challenging and hurdles the rational design of the structure. This thesis presents strategies to realize the controlled synthesis of 2D LMCs for enhanced electrochemical performance. With the assist from theoretical studies and advanced characterization, we unveil the origin of their electrochemical performance and explore the underlying mechanism referring to the reaction sequence, reversibility and kinetics.

Briefly, the following three topics are covered: 1) Carbon-sandwiched SnS2 nanosheets are fabricated through a hydrogel-embedding method, and demonstrate excellent sodium storage properties. The operando characterizations reveal the high reversibility of the redox reactions as well as the irreversible nanostructure evolution, indicating the significance of engineered carbon support in ensuring the electrode structure stability. 2) We explore the hydrogen evolution reaction (HER) on the basal plane of MoTe2 in both 1T′ and 2H phase, enhanced by creating anion vacancies by combining computational predictions followed with experimental validation. The experimental observations on the intrinsic activity of the vacancy engineered MoTe2 thin films are in good agreement with the prediction from density functional theory (DFT) using grand canonical potential kinetics (GCP-K). Increasing or decreasing the vacancy concentrations eventually reduces HER performance. 3) Through a temperature-dependent chalcogenide substitution reaction, we are able to fabricate a sub-centimeter scale symmetric MoS2/MoTe2(1-x)S2x/MoS2 heterostructure with tunable chemical composition. After revealing the evolution of crystal structure, coordination environment and depth profile as the temperature changing, a Te vacancy-initiated and S diffusion-mediated mechanism is proposed, and verified by the cross section observation from scanning transmission electron microscope (STEM) and DFT. The strategies we have developed and the mechanism we have elucidated in this thesis will provide the new paradigm for material synthesis and guideline for the future design of the advanced catalysts and electrodes.