Identification of surface states on TiO2 modified hematite photoelectrodes for enhanced solar water splitting

Hematite is a promising material for solar water splitting, but poor charge carrier mobility and recombination limit the photocatalytic efficiency. Surface states take part in charge carrier transfer, trapping and recombination processes. Two different surface states are proposed in literature, one involved with the water splitting reaction and the other acting as a recombination center. In this research the modification of the surface states by atomic layer deposition of sub-monolayer amount of TiO2 and by post-annealing treatment was studied. The effects of these treatments on the chemical composition, charge transfer dynamics and photoelectrochemical (PEC) properties were determined with XPS, transient absorption spectroscopy, and PEC measurements. Results show that the post-annealing causes diffusion of Ti into the hematite surface layers which results in an increase in the saturation photocurrent and an anodic shift in the photocurrent onset potential. Without post-annealing, the separate TiO2 phase on the hematite surface results in delayed charge carrier dynamics and shows a second surface state capacitance peak in the impedance spectroscopy analysis. This result support the model in which surface states observed with impedance spectroscopy from TiO2/Fe2O3 system are directly involved with the water oxidation reaction mechanism and not to the charge carrier recombination.