Effect of Acid Treatment on Boosting the Photoelectrochemical Performance of Doped and Codoped α-FeO Photoanodes

Acid treatment of Ti-doped α-FeO 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 α-FeO 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 α-FeO 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 ( the reversible hydrogen electrode) and a reduction in the potential onset by ∼60 mV after HCl hydrothermal treatment. Anatase TiO was detected by Raman spectroscopy on the Ti-doped α-FeO 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 -doped α-FeO and an excessive treatment time for the -doped α-FeO 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 α-FeO was determined by revealing its working mechanism.