Transient optical studies of metal oxides for water oxidation

Abstract

As the next energy crisis looms threateningly before us and we teeter on the edge of the irreparable and undeniably catastrophic loss of global ecosystems as a result of man-made climate change, the quest for alternative fuels is of paramount importance. Solar water splitting is an active area of research which seeks to produce renewable hydrogen fuel from two abundant resources: water and sunlight. However, it is the formation molecular oxygen, the necessary by-product of water splitting, that presents the major chemical challenge, both in terms of kinetics and thermodynamics. As such, high-performing, stable and Earth-abundant materials for water oxidation are highly sought after. The focus of this thesis is the understanding of two such materials. While Chapter 1 gives a more detailed overview of the current environmental situation, the challenges in global energy and fuel supply, and the field of artificial photosynthesis, Chapter 2 introduces transition metal oxides, the workhorses of the water oxidation reaction. The frequently studied oxides are discussed and particular focus is given to current kinetic understanding and performance limitations. The methods applied in this thesis are then detailed in Chapter 3. This body of research concerns the spectroscopic analysis of two metal oxide systems for water oxidation: tungsten trioxide (WO3) and mixed nickel-iron oxides, which become oxyhydroxides during catalysis (Nix Fey OOH). WO3, a visible and ultraviolet light absorber, has been studied as a photoanode for photoelectrochemical water oxidation from the first conceptualisation of this reaction in the 1970s. Despite many years of intensive research, much regarding the mechanism and factors limiting the performance of this robust and relatively abundant oxide remain unclear and are frequently debated. Chapter 4 sets the stage for WO3 within the growing field of water oxidation, with direct comparison to other metal oxides. The kinetics of water oxidation and electron extraction are examined, revealing some unexpected trends. The timescale of water oxidation is found to be remarkably fast, t50% < 1 ms, while electron extraction is limited by trap-mediated transport. In Chapter 5, I delve deeper into this complex material to understand the role of the most common intrinsic defects to transition metal oxides: oxygen vacancies. This chapter begins by probing the initial charge separation of photogenerated carriers on ultrafast timescales, through which I uncover that electrons trap into defect states on pre-picosecond timescales. I then go on to examine the effects of altering band-bending before investigating samples with different oxygen content to deduce the importance of the resultant defect states generated. The space-charge layer was found to boost the attainable concentration of surface holes from ultrafast timescales, while an intermediate concentration of oxygen vacancies was deemed vital to adequately separate photogenerated charges. I conclude by highlighting the wider significance that defect control has across all timescales monitored, from picoseconds to seconds, and emphasise this as a means to the betterment of existing photoanodes for water oxidation. In the final results chapter, Chapter 6, I examine a different approach to water oxidation. This chapter explores Ni/Fe oxyhydroxides; dark catalytic materials that can be used as co-catalysts in conjunction with a photoanode (such as WO3) or employed independently for dark electrolysis using renewably-generated electricity. This chapter presents spectroelectrochemical analyses and examines the rate law for water oxidation on these materials. In particular, the relationship between nickel and iron (the latter an often unintended dopant of the former) is examined, with the aim of unearthing the origin of the synergistic benefit observed when both metals are present. I find that the reactive intermediates accumulated under catalytic conditions are nickel centred at low iron concentrations, but become iron-centred at greater Fe:Ni ratios. However, the rate order with respect to these species is four in each case, suggesting a similar catalytic mechanism between all samples examined. In Chapter 7, I conclude by summarising this body of work and discuss the impact it may have on the next steps in water oxidation research. Finally, I give my insights into the role that transitional metal oxides may have in the future of solar energy conversion.