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Dynamics of Photogenerated Charge Carriers in WO3/BiVO4 Heterojunction Photoanodes

Grigioni, Ivan, Stamplecoskie, Kevin G., Selli, Elena, Kamat, Prashant V.
The Journal of Physical Chemistry C 2015 v.119 no.36 pp. 20792-20800
bismuth, electrodes, electron transfer, photocatalysis, photocatalysts, physical chemistry, spectroscopy, synergism, tungsten oxide
Bismuth vanadate (BiVO₄) 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 BiVO₄ band gap excitation are effective for water splitting, especially when BiVO₄ is combined with other metal oxides such as WO₃. We report a solution processed method for designing transparent WO₃/BiVO₄ heterojunction electrodes and observe a synergistic effect on the photoelectrochemical activity of WO₃/BiVO₄, with the combined system performing dramatically better than either individual component. Using ultrafast transient absorption spectroscopy, we elucidated the electronic interaction between WO₃ and excited BiVO₄. Moreover, the photocatalytic reduction of thionine by WO₃/BiVO₄ 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 WO₃/BiVO₄ film a shifted quasi-Fermi level results, due to electronic equilibration between the two materials. The better performance of WO₃/BiVO₄ heterojunction electrodes is thus a consequence of the electron injection from BiVO₄ into WO₃, followed by back electron transfer from WO₃ to the holes in BiVO₄.