Influence of the Potential on the Charge-Transfer Rate Constant of Photooxidation of Water over α-Fe2O3 and Ti-Doped α-Fe2O3

It has been reported that applying a certain external anodic potential over un-doped (alpha-Fe2O3) and Ti-doped alpha-Fe2O3 (Ti-Fe2O3) electrodes can improve the photocurrent or the photoelectrochemical oxidation rate of water. However, it is assumed in the literature that the potential drops completely across the side of the solid semiconductor (band edge pinning), and the influence of the potential on the interfacial charge-transfer rate constant is rarely reported. In this article, the impact of the applied potential on the interfacial charge-transfer rate constant during photoelectrochemical oxidation of water over the two electrodes was investigated by electrochemical impedance spectroscopy. The results showed that by increasing the applied anodic potential, the interfacial charge-transfer rate constants for both electrodes were increased. The smaller increase in the magnitude of the rate constant than determined by theory indicates that not all of the applied potential drops across the Helmholtz layer, but takes place in both the space charge and Helmholtz layers simultaneously (Fermi level pinning). The results of the surface-state capacitance measurements suggested that the photo-generated charge can be accumulated in the surface states, resulting in the re-distribution of the potential at the interface and an improvement in the rate constant. Under the same applied potential, the higher the light intensity is, the more the photogenerated holes accumulated in the surface states. This causes an increase in the potential drop across the Helmholtz layer and consequently increases the charge-transfer rate constant. Compared with the alpha-Fe2O3, the improvement of the charge-transfer rate constant by the anodic potential is more obvious.