Role of Heterojunctions in Controlling Carrier Separation in Two-Layer Fe₂O₃/BiVO₄ Anodes for Photoelectrochemical Water Oxidation

The Fe2O3/BiVO4 heterojunction was previously reported to exhibit a higher efficiency in water splitting applications than Fe2O3 and BiVO4 anodes alone. In the present study, we elucidate the carrier activation process in the Fe2O3/BiVO4 heterojunction based on photoelectrochemical and carrier dynamics analyses. The intraband states of Fe2O3 and BiVO4 were found to be closely related to the carrier transitions in the Fe2O3/BiVO4 heterojunction. Impedance spectroscopy analysis showed that the minimum resistance was about 0.6 V versus the reversible hydrogen electrode; this was attributed to the injection of photogenerated electrons into the Fe3+/Fe2+ intraband states of Fe2O3 in the Fe2O3/BiVO4 heterojunction, which enhanced the electrical conductivity. Spectroelectrochemical evaluation showed that in the bias-induced spectra of the Fe2O3/BiVO4 anode, only changes in the BiVO4 overlayer in contact with the electrolyte were observed. The sub-conduction band of Fe2O3 was found to be around 450–525 nm from the thermal change spectrum. The photogenerated charge carrier dynamics was studied through transient absorption spectroscopy and shown to contain the features of both Fe2O3 and BiVO4. The photoholes were observed to be trapped in the V4+/V5+ intraband states on the surface of BiVO4 at the picosecond time scale, whereas the photogenerated electrons were injected into the Fe3+/Fe2+ intraband states at the microsecond time scale, leading to enhanced carrier separation. Photoinduced absorption spectroscopy results showed significantly increased accumulation of photogenerated holes on the Fe2O3/BiVO4 anode surface, which supported the improved carrier separation. Thus, the inherent energy potential difference in the valence band prevents backflow of holes to further enhance water oxidation on the anode surface.