Chemical and Electronic Properties of the O₂ and H₂O/Electrode Interface of an Electrochemical Cell for Water Splitting

The properties of a material are governed by its microscopic structure, and the investigation of structure-property relationships has been the focus of material science. Group III-V compound semiconductors have been demonstrated to be promising candidates for incorporating into devices for solar cell, photoelectrochemical water splitting and CO₂ reduction applications. The surface structural properties of III-V semiconductors are often affected by their interactions with their environment, such as the surrounding electrolyte, which is usually an aqueous solution exposed to an ambient environment. For controllable design and fabrication of high-quality III-V semiconductor-based devices, knowledge of environmental molecule/III-V semiconductor interfacial chemical and electronic properties is highly desirable, especially that gained from studies performed under close to practical operational conditions. However, previous studies have been limited by the complexity of the system or the availability of suitable instrumentation, and all the experimental data in literature are performed under ex-situ and ultra-high vacuum conditions.

In the frame of my PhD thesis, I aim to investigate the chemical and electronic properties at the environmental molecule (H₂O and O₂)/III-V semiconductor interface under in-situ or operando conditions using ambient pressure X-ray photoelectron spectroscopy (AP-XPS). Chapter 1 introduces background information for solar water splitting, the motivations of the thesis, the details of the major instrumentation (AP-XPS) and the overall goal of the thesis; Chapter 2 and 3 present the studies of the interfacial chemistry of O₂ and H₂O/III-V semiconductors, respectively; Chapter 4 focuses on the investigation of the interfacial chemical and electronic property relationship in a III-V based photoelectrode; Chapter 5 is an extension of the previous three chapters, where a real electrochemical device for water splitting is designed, fabricated and incorporated into the AP-XPS system, and properties including the H₂O/device interfacial chemical, electrical and catalytic features are investigated under operational conditions.

The present study has important implications for the fundamental understanding of environmental molecule (H₂O or O₂)/III-V semiconductor interactions and for the practical design and fabrication of a future high-quality solar water splitting device.