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Ti-doped α-Fe 2 O 3 (Ti-Fe 2 O 3 ) is regarded as one of the most promising hematite-based photoanodes for photoelectrochemical oxidation of water. However, the sluggish interfacial transfer and rapid recombination of photogenerated holes still limit its efficiency. In this article, we report that an approach of surface modification for this material by electrodeposition in In(NO 3 ) 3 containing aqueous solutions was attempted to overcome these disadvantages. The results show that a thin layer of InOOH was formed on the photoelectrodes, confirmed by X-ray photoelectron spectroscopy. This modification could enhance the photoelectrochemical oxidation rate of water. The reason for the improvement was investigated quantitatively by a combination of multiple (photo-)electrochemical techniques and transient absorption spectroscopy. It is demonstrated that the kinetics of photogenerated holes can be well described by a model of involving photogenerated surface-state mediated charge transfer and recombination. The improvement resulted predominantly from the enhanced interfacial transfer rate constant (k ct ) of the photoholes, despite that the surface recombination rate constant (k sr ) slightly increased. Energetically, the increase in k ct was attributed to more potential drop in the Helmholtz layer. The higher potential drop resulted from more accumulation of photoholes on the surface due to a larger band bending as the flatband potential negatively shifted. Physically, the increase in k ct can be explained by that InOOH acts as a so-called cocatalyst to promote the hole transfer. In addition, the implication of these findings is addressed for further designing efficient Ti-Fe 2 O 3 photoanodes.

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