We present a surface corrosion method to shift the photocurrent onset potential cathodically for water oxidation on a Ti⁴⁺ doped Fe₂O₃ by about 100 mV. After the surface treatment, the doped hematite photoanodes showed a similar photocurrent onset potential to the lowest values obtained by loading with electrocatalysts or depositing functional over-layers. Moreover, the cathodic shift of the onset potential was preserved well even after a long operating time. The results indicated the effectiveness of this simple surface treatment. In order to make clear the reason for the onset potential shift, the doped hematite samples before and after surface corrosion were investigated by SEM, X-ray photoelectron spectroscopy (XPS), inductively coupled plasma mass spectrometry (ICP-MS), photoluminescence spectroscopy (PL), electrochemical impedance spectroscopy (EIS), Mott–Schottky and so on. Based on the experimental evidence, we proposed a new mechanism for the onset potential shift. The cathodic shift of the onset potential was due to decreasing the back reaction, but not accelerating water oxidation kinetics, passivating surface states or ions adsorption as reported in the previous studies. This strategy of suppressing the back reaction can offer a reference to reduce the overpotential for other photoelectrodes.

Photoelectrochemical water splitting is an attractive method to produce H₂ fuel from solar energy and water. Ion doping with higher valence states was used widely to enhance the photocurrent of an n-type oxide semiconductor. In this study, the different doping sites and the photoelectrochemical properties of Mo⁶⁺, W⁶⁺ and Sn⁴⁺-doped BiVO₄ were studied systematically. The results suggested that Mo⁶⁺ or W⁶⁺-doped BiVO₄ had a much higher photocurrent while the photocurrent of Sn⁴⁺-doped BiVO₄ did not change obviously. Raman and XPS were used to identify the doping sites in the BiVO₄ crystal lattice. It was found that Mo or W substituted V sites but Sn did not substitute Bi sites. Results of theoretical calculation indicated that a higher formation energy and lower solubility of impurity ions led to serious SnO₂ segregation on the surface of the Sn⁴⁺-doped BiVO₄ thin film, which was the main reason for the poor performance of Sn-doped BiVO₄. The higher formation energy of Sn⁴⁺ came from the large mismatch of ion radius and different outer shell electron distribution. These results can offer guidance in choosing suitable doping ions for other semiconductor photoelectrodes.