Identifying the Spectroelectrochemical Characteristics of Hematite Photoanodes for Water Oxidation

Achieving efficient solar water splitting using hematite (α-Fe2O3), one of the most promising candidates for photoanodes, requires photogenerated holes to be efficiently used for water oxidation. However, this goal is obstructed by multiple undesirable recombination processes, as well as insufficient fundamental mechanistic understandings of water oxidation kinetics, particularly as to the nature of reaction pathways and possible reaction intermediates. Here we spectroelectro-chemically identify some of the most critical interfacial processes which determine the photoelectrocatalytic efficiencies of water oxidation, for hematite films with varied surface properties by tailoring the doping level of titanium. The spectroscopic signals of the processes inactive for water oxidation, including oxidation of intra-gap Fe2+ states and Fermi level pinning, are successfully distinguished from that of the active reaction intermediate, Fe(IV)=O. In addition, our kinetic analyses reveal two water oxidation pathways, of which the direct hole transfer mechanism becomes dominant over the surface states-mediated mechanism when the hematite surface is reconstructed by high levels of titanium dopants.