The synergistic effect of cation mixing in mesoporous Bi_xFe_1-xVO₄ heterojunction photoanodes for solar water splitting

Mixed metal vanadates are an interesting class of materials due to their favorable bandgap for visible light absorption and their catalytic activity. Here, we report a novel Bi_xFe_1-xVO₄ mixture system fabricated by electrospinning, which upon annealing is composed of triclinic FeVO₄ and monoclinic BiVO₄ phases. The mixture demonstrates extended optical absorption and a clear bandgap shift as compared with a pure BiVO₄. This is also accompanied by an extended wavelength range for its photoactivity as evident from the incident photon-to-current efficiency. In addition, the mixture with a Bi/(Bi + Fe) ratio of 0.5 (i.e., x = 0.5) shows superior charge transfer and charge separation efficiency. The improved charge transfer efficiency is attributed to the higher catalytic activity of the mixed cation, while the presence of a BiVO₄/FeVO₄ heterojunction enhances the charge separation. The formation of the heterojunction is verified through detailed microscopic investigations revealing BiVO₄ particles intimately surrounded by FeVO₄. Our results demonstrate the advantage of establishing a mixture of complex metal oxides in extending optical absorption and boosting the photoelectrochemical performance.