Effect of acid treatment on boosting the photoelectrochemical performance of doped and codoped -Fe₂O₃ photoanodes

Acid treatment of Ti-doped alpha-Fe2O3 photoanode can reduce the onset potential and promote the photocurrent density for photoelectrochemical (PEC) water splitting reaction. However, the inner mechanism of how this occurs has not yet been clarified. This report compares the effect of HCl hydrothermal treatment on alpha-Fe2O3 photoanodes doped with Ge, Pt, Ti, and Sn or codoped with TiGe, TiPt, and TiSn. The findings show that the promotion effect of HCl hydrothermal treatment was far less significant on the Ge-, Pt-, and Sn-doped alpha-Fe2O3 than on the Ti-doped one. In contrast, the codoped photoanodes could realize a lift in the photocurrent of up to 39% at 1.23 V-RHE (versus the reversible hydrogen electrode) and a reduction in the potential onset by similar to 60 mV after HCl hydrothermal treatment. Anatase TiO2 was detected by Raman spectroscopy on the Ti-doped alpha-Fe2O3 with adequate treatment in HCl solution. Thus, the performance promotion by acid treatment was ascribed to the surface-concentrated Ti-O bonds acting as a passivation layer that could increase the charge-capture capacity and reduce the charge-transfer resistance, as demonstrated by the potential-modulated electrochemical impedance spectroscopy results. HCl treatment of the in situ-doped alpha-Fe2O3 and an excessive treatment time for the ex situ-doped alpha-Fe2O3 caused an inhibition in the PEC performance, which could be attributed to the adverse effect of lattice defects induced by acid corrosion. The application scope of HCl treatment on the doped alpha-Fe2O3 was determined by revealing its working mechanism.