Charge carrier dynamics and water oxidation photoelectrochemical performance of bismuth vanadate

Abstract

In this thesis, the charge carrier dynamics that govern the photoelectrochemical water oxidation performance of bismuth vanadate are investigated using an array of time-resolved transient and steady-state absorption spectroscopic techniques. Chapter 1 outlines the motivation for this work and gives a brief introduction to the field, highlighting key research on bismuth vanadate photoanodes relevant to this work. The experimental methods used herein are detailed in chapter 2. Chapter 3, the first results chapter of this thesis, explores the role of structural defects such as oxygen vacancies, on the photoelectrochemical performance. The electronic occupancy of the defect states are modulated in situ electrochemically, thermally and optically, which allows the determination of their energetics and monitor the resulting impact on charge carrier kinetics. The results from this chapter provide insight into how the occupancy of the sub-bandgap electronic states affects charge carrier recombination, trapping, and transport. Occupied sub-bandgap states are observed to trap photogenerated holes in the bulk of the material, whereas the unoccupied counterparts (predominantly within the space-charge layer) function as electron traps which facilitate a thermally activated electron transport through a trapping / de-trapping mechanism, with an activation energy of ~0.2 eV. Chapter 4 investigates the charge transfer and charge extraction processes in the tungsten oxide/bismuth vanadate heterojunction photoanodes, at timescales relevant to water oxidation (μs – s). The enhanced performance of the heterojunction, in relation to the individual counterparts, is attributed to sub – microsecond electron transfer across the materials, leading to spatial separation of charge which minimises recombination losses. The role of applied bias on the charge carrier kinetics of heterojunction is also investigated. Chapter 5 focuses on the electrochemical water oxidation performance of FeOOH, Ni(Fe)OOH and FeOOHNiOOH electrocatalysts, and investigates the kinetics of water oxidation catalysis under neutral conditions. These electrocatalysts, when functionalised on bismuth vanadate photoanodes exhibit significant improvements in the photoelectrochemical water oxidation performance of mesoporous bismuth vanadate. The origin of this enhancement is explored, and is observed to be related to fast hole transfer from bismuth vanadate to the catalyst layer, minimising recombination losses.