Photo-Oxidation of Water on Defective Hematite(0001)

Defects are unavoidable and usually originate exotic properties in realistic materials. One of the most fundamental defect-induced properties of a solid surface is its reactivity to adsorbed species. Defects in anodes of electrochemical cells for water splitting could therefore play a critical role in the interatomic interactions at the solvent/solid interfaces and hence in determining the catalytic properties of these materials. Here, by means of density-functional calculations at the PBE+U level, we investigate photo-oxidation of water on defective hematite(0001) substrates which accommodate intrinsic and extrinsic point defects, namely, Fe and O vacancies, as well as N substitutional impurities. In this work, the water oxidation process is assumed to be driven by the redox potential for photogenerated holes with respect to the normal hydrogen electrode. Although iron vacancies do not reduce the overpotential, oxygen vacancies and N impurities lower the overpotential by 0.2−0.3 V compared to the ideal case. These changes are attributed to the coordination loss or the substitution-induced charge states of surface atoms that modify the electronic structure of the surface, thus affecting the relative stability of adsorbed intermediates.