Dynamics of photogenerated charge carriers in WO3/BiVO4 heterojunction photoanodes

Photoelectrochemical water splitting is a promising way to convert and store solar energy into hydrogen as fuel. Among semiconductor materials, metal oxides are commonly used as photoanodes due to their bandgap positions and high stability under oxidative conditions. BiVO4 and WO3 are widely studied n-type metal oxides with small optical bandgaps (ca. 2.5 and 2.7 eV respectively). While BiVO4 alone exhibits poor electron transport, by combining it with WO3 highly enhanced photoelectrochemical performances have been obtained due to increased charge carrier separation in the photoanode. Individual WO3 and BiVO4 have been synthesized by solution processed techniques and the photoanodes, including the WO3/BiVO4 heterojunction system, have been prepared by spin coating. The so obtained electrodes were tested in the reaction of photoelectrochemical water splitting. The photocurrent of the WO3/BiVO4 electrode was highly increased as compared to those of the two pure WO3 and BiVO4 electrodes. Femtosecond transient absorption spectro-scopy was used to investigate the charge carrier dynamics of the studied photoanodes. The spectra of BiVO4 could be assigned to trapped electrons (460-600 nm) and trapped holes (600-850 nm). Electron injection from BiVO4 to WO3 was observed in the heterojunction system, as shown by the faster decay of the 470 nm signal assigned to trapped electrons. Thionine photoreduction experiments carried out under continuous irradiation at 400 nm confirmed the electron injection process between excited BiVO4 and WO3.