Charge Carrier Dynamics in Hematite Photoanodes for Solar Water Oxidation

Abstract

Although the field of solar water splitting is now forty years old, in recent years there has been an upsurge of research in this area, with the aim of using sunlight to produce hydrogen cheaply and efficiently. Hematite (α-Fe2O3) is of particular interest as a photoanode material for solar water splitting, due to its optimum band gap (2.0-2.2 eV) and visible light absorption and stability. Various modifications – including nanostructuring and doping – have been investigated as routes to improved efficiencies, thought to be limited by long visible light absorption depths, low charge carrier mobilities and slow hole-transfer kinetics. Additionally, an anodic applied bias is required for water oxidation to occur on hematite. Improved understanding of the role of applied bias and the processes limiting the performance of hematite photoanodes will lead to more directed routes to photoanode architectures with increased efficiencies. This Thesis describes the results of transient absorption spectroscopy studies, in conjunction with photoelectrochemical measurements, of hematite photoanodes. Transient absorption spectroscopy on microsecond-second timescales allows direct monitoring of the recombination, trapping and reaction of photogenerated holes, both in isolated hematite films, and in photoanodes in a fully functional photoelectrochemical cell. Transient photocurrent measurements probe electron extraction from the photoanode on microsecond-millisecond timescales. The charge carrier dynamics are found to be strongly dependent on the electron density, which is controlled by applied electrical bias. The photocurrent generated is found to correlate with the population of long-lived holes, determined by the kinetics of electron-hole recombination. Generally, effects which lower electron density result in retarded electron-hole recombination kinetics, increasing the population of long-lived holes and hence increasing the photocurrent. Following an introduction and review of the literature, the first results chapter reports that the effect of a positive applied bias is to retard the otherwise dominant electron-hole recombination, increasing the lifetime of photogenerated holes such that water oxidation can occur. The relative timescales of recombination, electron extraction and water oxidation as a function of applied bias are discussed in the following chapter, in conjunction with the results of excitation density studies. The third results chapter compares the charge carrier dynamics in photoanodes with different nanomorphologies. The fourth results chapter discusses the effect of an energetic trap state on charge carrier dynamics, while the effects of surface treatment with cobalt, which is shown to retard recombination at low applied bias, is reported in the final results chapter. Overall conclusions are drawn and the implications of these for photoelectrode design are discussed.