Metal oxide & oxyhydroxide materials for solar water oxidation
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Solar water splitting -- using electromagnetic energy to convert water to hydrogen and oxygen gas -- combines energy harvest and storage into one step, and brings with it the potential to lower the total cost of the processes. A disadvantage of the combined system is the difficulty of finding materials that satisfy all the necessary constraints. In this dissertation we focus on materials for the water oxidation reaction (photoanodes), both seeking out new materials and modifying promising ones in an attempt to improve their characteristics. We first synthesized nanostructured, Si-doped Fe₂O₃ to clarify the mechanism by which Si acts to improve the photo-oxidation performance. It is found that Si does not increase the bulk conductivity but instead segregates to the surface and passivates it during annealing, improving the efficiency of charge transfer to the solution phase. Continuing work on abundant, Fe-based materials, we built on oxygen evolution reaction (OER) work on FeOOH. We apply an anodic electrodeposition technique using N-methylimidazole as an electrolyte additive. The resulting FeOOH films are high quality and can act as a protective layer even for thicknesses on the order of 10 nm and above. The FeOOH films have a high activity relative to other materials that can be deposited under similar conditions. We demonstrate the utility of these films by depositing on commercial solar cells and achieving one of the highest reported solar to hydrogen efficiencies. We continue investigating the FeOOH system by doping with Ni and find increased activity in alkaline conditions, at the expense of providing less protection from the solution. We then synthesize thin films of Ag₃VO₄ using successive ionic layer adsorption and reaction (SILAR). This represents the first thin film study of Ag₃VO₄ for solar water oxidation and we show the material thermodynamically prefers p-type behavior and that it has an unfavorably located cathodic instability, rendering it difficult to use as a photoanode. Finally, we look at the applicability of using sub-bandgap illumination to improve charge transport in BiVO₄, and more generally in materials in which charge carriers form small polarons -- a common feature of many metal oxides.