Abstract
Electrocatalytic water dissociation that produces molecular hydrogen via the hydrogen evolution reaction (HER) may provide a sustainable energy supply for the future, thus the development of highly efficient electrocatalysts for HER has attracted increasing attention over recent years. The exotic and robust metallic topological surface states (TSSs) of topological insulators (TIs) and topological crystalline insulators (TCIs) show promising properties as a platform for novel surface chemistry and catalysis. A heterostructure that consists of such a TI or a TCI and a certain conventional material endows the heterostructure with novel quantum phenomena. However, experimental research aiming to expedite the HER via TIs and TCIs as functional electrocatalysts with a desirable catalytic activity is still an unprecedented field.
This dissertation presents three research works carried out on TI(Bi2Te3), TCI(SnTe), and a transition metal/TCI (Pd/SnTe) heterostructure as highly efficient HER electrocatalysts were experimentally demonstrated along with corresponding theoretical analysis.
In the first work, the Bi2Te3 TI thin films with different thicknesses were fabricated by the molecular beam epitaxy technique and it was found that these thin films exhibit high electrocatalytic activity in HER. The 48 nm Bi2Te3 thin film has the best performance, which is attributed to its largest active area arising from the spiral growth mode of triangular domains as revealed by atomic force microscopy (AFM) imaging. Our theoretical calculations reveal that while pure Bi2Te3 is not a good electrocatalyst, the Bi2Te3 thin films with partially oxidized surfaces or Te vacancies have high HER activity. The existence of the corresponding surface oxides on the Bi2Te3 thin films is supported by X-ray photoelectron spectroscopy (XPS) data. A direct comparison was made between a Bi2Te3 and a Bi2Te3:Fe thin film on their magneto-transport properties and HER performances, which demonstrates that the TSSs play a key role in enhancing the HER performance.
In the second work, the surface orientation dependence on the HER performance of SnTe TCI thin films with orientations along (001), (111) and (211) was studied. Their intrinsic electrochemical activities were determined by the linear sweep voltammetry and cyclic voltammetry measurements, in which the obtained electrochemical surface areas (ECSA) agree well with their surface morphology as revealed by AFM analysis. It was found that the SnTe (001) and (111) surfaces exhibit superior intrinsic activities over those of various topological quantum and nanostructured materials, while the (211) surface shows uncompetitive activity. Our theoretical calculations reveal that while pure (001) and (111) are not good electrocatalysts, the SnTe thin film with Sn vacancies or partially oxidized surfaces, with the latter as evidenced by the XPS analysis, have high HER activity. The overall performance of the (001) and (111) surfaces with TSSs is better than that of the (211) surface without TSSs, which is further confirmed by the weak antilocalization effect in magneto-transport measurements. The high electrocatalytic activity of SnTe (001) and (111) can thus be attributed to the enhanced charge transfer between hydrogen atoms and the TSSs. The absence or fragility of TSSs in the lowly symmetric SnTe (211) explains its relatively low HER performance.
In the third work, high-quality SnTe (001) thin films with different thicknesses were fabricated by the MBE technique, and the 70nm SnTe(001) thin film exhibits the best HER performances among them, which is attributed to its largest active areas as revealed by AFM imaging and the resulted ECSA. A Pd(20nm)/SnTe(70nm) heterostructure and a pure Pd(20nm) sample were fabricated and their HER performance and intrinsic activity were investigated. It was found that the Pd(20nm)/SnTe heterostructure show higher HER activity via its lower overpotential (η), higher exchange current density (j0) and lower charge transfer resistance than those of a Pd(20nm) sample, and the lower η and higher j0 than a commercial Pt foil. The Pd (20nm)/SnTe heterostructure shows a much higher intrinsic activity of per-Pd-site than that of the Pd (20nm) sample, which is also higher than most of the noble-metal-based electrocatalysts for HER reported recently. Our theoretical studies reveal that electrons can transfer from both the Pd surface and the adsorbed hydrogen atom to the TSSs held by the SnTe (001) underlayer, resulting in weaker Pd-H binding strength and more favored 
values, and thus higher HER performance. This work offers a promising substitute to replace commercial Pt foil, which enjoys excellent electrocatalytic performance and intrinsic activity, and the overall usage of Pd in the overlayer of the heterostructure can be as thin as 20nm. The achievement obtained in this thesis study is also expected to stimulate further experimental and theoretical research along the same direction, such as the fabrication and characterisation of an electrocatalyst/photocatalyst system based on a Pd/TI core-shell nanowire structure, where the TI nanowire materials could act as the highly efficient catalysts as well as the linker that provides a high-speed electron migration channel simultaneously.
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