Dynamics of Photogenerated Charge Carriers in WO₃/BiVO₄ Heterojunction Photoanodes

Abstract

Bismuth\nvanadate (BiVO₄) with a band gap of ∼2.4\neV has emerged as one of the visible photocatalysts that can absorb\nlight below 520 nm. The electron/hole pairs that are generated following\nBiVO₄ band gap excitation are effective for water splitting,\nespecially when BiVO₄ is combined with other metal oxides\nsuch as WO₃. We report a solution processed method for\ndesigning transparent WO₃/BiVO₄ heterojunction\nelectrodes and observe a synergistic effect on the photoelectrochemical\nactivity of WO₃/BiVO₄, with the combined system\nperforming dramatically better than either individual component. Using\nultrafast transient absorption spectroscopy, we elucidated the electronic\ninteraction between WO₃ and excited BiVO₄. Moreover,\nthe photocatalytic reduction of thionine by WO₃/BiVO₄ as well as by each individual oxide component is used to\ntrack electron injection processes and determine the energetics of\nthe studied systems. In the composite WO₃/BiVO₄ film a shifted quasi-Fermi level results, due to electronic equilibration\nbetween the two materials. The better performance of WO₃/BiVO₄ heterojunction electrodes is thus a consequence\nof the electron injection from BiVO₄ into WO₃, followed by back electron transfer from WO₃ to the holes\nin BiVO₄.