Heterojunction electrodes were fabricated by layer-by-layer deposition of WO3 and BiVO4 on a conducting glass, and investigated for photoelectrochemical water oxidation under simulated solar light. The electrode with the optimal composition of four layers of WO3 covered by a single layer of BiVO4 showed enhanced photoactivity by 74% relative to bare WO3 and 730% relative to bare BiVO4. According to the flat band potential and optical band gap measurements, both semiconductors can absorb visible light and have band edge positions that allow the transfer of photoelectrons from BiVO4 to WO3. The electrochemical impedance spectroscopy revealed poor charge transfer characteristics of BiVO4, which accounts for the low photoactivity of bare BiVO4. The measurements of the incident photon-to-current conversion efficiency spectra showed that the heterojunction electrode utilized effectively light up to 540 nm covering absorption by both WO3 and BiVO4 layers. Thus, in heterojunction electrodes, the photogenerated electrons in BiVO4 are transferred to WO3 layers with good charge transport characteristics and contribute to the high photoactivity. They combine merits of the two semiconductors, i.e. excellent charge transport characteristics of WO3 and good light absorption capability of BiVO4 for enhanced photoactivity.
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