Dynamics of Photogenerated Charge Carriers in WO3/BiVO4 Heterojunction Photoanodes

Bismuth vanadate (BiVO 4 ) with a band gap of ∼2.4 eV has emerged as one of the visible photocatalysts that can absorb light below 520 nm. The electron/hole pairs that are generated following BiVO 4 band gap excitation are effective for water splitting, especially when BiVO 4 is combined with other metal oxides such as WO 3 . We report a solution processed method for designing transparent WO 3 /BiVO 4 heterojunction electrodes and observe a synergistic effect on the photoelectrochemical activity of WO 3 /BiVO 4, with the combined system performing dramatically better than either individual component. Using ultrafast transient absorption spectroscopy, we elucidated the electronic interaction between WO 3 and excited BiVO 4 . Moreover, the photocatalytic reduction of thionine by WO 3 /BiVO 4 as well as by each individual oxide component is used to track electron injection processes and determine the energetics of the studied systems. In the composite WO 3 /BiVO 4 film a shifted quasi-Fermi level results, due to electronic equilibration between the two materials. The better performance of WO 3 /BiVO 4 heterojunction electrodes is thus a consequence of the electron injection from BiVO 4 into WO 3, followed by back electron transfer from WO 3 to the holes in BiVO 4 .