On the Interfacial Electronic Structure Origin of Efficiency Enhancement in Hematite Photoanodes

The electronic structure origins of interfacial losses in hematite photoanodes and the cause of the literature-reported order-of-magnitude photocurrent increase upon short high-temperature annealing are investigated. Synchrotron-based soft X-ray absorption spectroscopy is used to probe the unoccupied states at and near the interface between hematite (alpha-Fe2O3) and fluorine-doped tin oxide (FTO). Oxygen K-edge and iron L-edge absorption spectra indicate that the interfacial interaction reduces the degree of p-d hybridization and alters the crystal field in alpha-Fe2O3. The interface is found to be associated with a distribution of unoccupied oxygen p-hybridized states located below the lowest unoccupied iron 3d states in alpha-Fe2O3 (just below the conduction band minimum), which are eliminated with high-temperature processing. These data facilitate future efforts to engineer favorable interfacial compositions and associated electrochemical potential gradients within photoanodes, which are required to efficiently separate charge carriers in operating photoelectrochemical systems such as solar cells and photocatalytic devices.