Synthesis and Electrocatalytic Performance of Two-dimensional Semiconductors

Abstract

Two-dimensional (2D) Semiconductors show considerable potential in electrocatalysis, due to their high density of surface active sites, excellent mechanical properties, and tunable electrical conductivity. However, further development for exploring their potential faces various challenges, including the lack of robust chemical synthetic methods, the need for more functionalizing strategies of 2D semiconductors, and the limitation of experimental evaluation techniques of electrochemical properties on the nanoscale. Hence, we have been committing to expand the research scope of the electrocatalytic performance of 2D semiconductors.

Contents of this thesis are organized as follows. Chapter 1 offers an introduction on the topics of 2D semiconductors, 2D structural engineering, and on-chip electrochemical microcell (OCEM). In Chapter 2, we report a surfactant-free liquid-phase synthesis of high-quality 2D tellurene based on ultrasonication-assisted exfoliation of metastable 1T'-MoTe₂. The as-prepared tellurene nanosheets exhibit excellent single crystallinity and clean surface with typical thicknesses of 10-50 nm and lateral lengths of up to 100 μm. Furthermore, a unique growth mechanism based on the atomic escape of Te atoms from metastable transition metal dichalcogenides and guided 2D growth in the liquid phase was proposed and verified. 2D tellurene-based field-effect transistors show ultrahigh hole mobility exceeding 1000 cm2·V^-1·s^-1 at room temperature attributing to the high crystallinity and surfactant-free surface, and exceptional chemical and operational stability using both solid-state dielectric and liquid-state electrical double layer. The facile ultrasonication-assisted synthesis of high-quality tellurene paves the way for further exploration of elemental 2D materials (E2DMs) and expands the scope of liquid-phase exfoliation (LPE) methodology towards the controlled wet-chemical synthesis of functional nanomaterials.

In Chapter 3, I report a reversible phase transition in the semi-metallic 1T′-WS₂ driven by proton intercalation and deintercalation, resulting in a newly discovered semiconducting WS₂ with a novel unconventional phase, denoted as 1T′d phase. Impressively, an on/off ratio of > 106 has been achieved during the phase transition of WS₂ from the semi-metallic 1T′ phase to the semiconducting 1T′d phase. Our work not only provides a unique insight into the phase transition of TMDs via the proton intercalation, but also opens possibilities to tune their physicochemical properties for various applications.

In Chapter 4, I report an OCEM platform specially tailored to investigate electrocatalytic oxygen reduction reaction (ORR) at a microscopic level by introducing electrolyte convection through a microfluidic flow cell. The setup is firstly established on gold microelectrodes and later successfully applied to investigate how Ar-plasma treatment affects the ORR activities of 2H MoS₂. We found that Ar-plasma treatment significantly enhances the ORR performance of MoS₂ nanosheet owing to the introduction of surface defects. Our study paves the way for highly efficient microscopic investigation of diffusion-controlled electrocatalytic reactions.

Finally, Chapter 5 offers conclusions for my work. The list of publications published be the author of this thesis is provided in the Appendix.

Date of Award	19 Aug 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Qiyuan HE (Supervisor)