PNNL

Abstract

Transition metal oxide materials have attracted much attention for photoelectrochemical water splitting, but problems remain, e.g. the sluggish transport of excess charge carriers in these materials, which is not well understood. Here we use periodic, spin-constrained and gap-optimised hybrid density functional theory to uncover the nature and transport mechanism of holes and excess electrons in a widely used water splitting material, bulk-hematite (a-Fe2O3). We find that upon ionisation the hole relaxes from a delocalized band state to a polaron localised on a single iron atom with localisation induced by tetragonal distortion of the 6 surrounding iron-oxygen bonds. This distortion is responsible for sluggish hopping transport in the Fe-bilayer, characterised by an activation energy of 70 meV and a hole mobility of 0.031 cm2/Vs. By contrast, the excess electron induces a smaller distortion of the iron-oxygen bonds resulting in delocalisation over two neighbouring Fe units. We find that 2-site delocalisation is advantageous for charge transport due to the larger spatial displacements per transfer step. As a result, the electron mobility is predicted to be a factor of 3 higher than the hole mobility, 0.098 cm2/Vs, in qualitative agreement with experimental observations. Our study demonstrates that constrained DFT is a very powerful tool for the prediction of charge transfer rates and mobilities in application-relevant oxide materials.