Role of Heterojunctionsin Controlling Carrier Separationin 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.