Tuning the ion permeability of an Al2O3 coating layer on Fe2O3 photoanodes for improved photoelectrochemical water oxidation

Inevitable surface defects on alpha-Fe2O3 photoanodes, a promising candidate for photoelectrochemical (PEC) water splitting with excellent stability and absorption in the visible light region, greatly limit their PEC performance for water oxidization. Surface modification is an effective strategy to decrease the negative effects of the surface defects. However, a challenge in understanding the nature of surface modification still remains. Here, we found that an ion-permeable Al2O3 coating layer can effectively prevent oxygen diffusion to the photoanode surface, thus suppressing the back reaction from oxygen reduction. The ion-permeable Al2O3 leads to ineffective surface state passivation due to the possible interface corrosion effect of OH- ions that decrease the interactions between Al2O3 and Fe2O3. In contrast, the ion-impermeable Al2O3 layer exhibits strong interactions with Fe2O3 and is mainly responsible for both surface state passivation and back reaction suppression, improving the photocurrent onset potential. Our results indicated that the modification of ion-impermeable passivation layers is a potential effective strategy to enhance the PEC performance.