An intensity-modulated photocurrent spectroscopy study of the charge carrier dynamics of WO3/BiVO4 heterojunction systems

Synergistic effects as a result of combining different photoactive materials provide a promising pathway to improve the photoelectrochemical performance in heterojunction devices for solar water splitting. The photoelectrochemical characteristics of tungsten trioxide (WO3) and bismuth vanadate (BiVO4) have been studied, comparing the single-phase materials with two heterojunction systems based on a WO3 underlayer and a thin, spin-coated BiVO4 top layer, using two types of underlayer: (i) a nanorod-based WO3 substrate prepared by hydrothermal methods (nr-WO3/BiVO4); and (ii) a spin-coated thin, compact WO3 substrate (pl-WO3/BiVO4). Intensity-modulated photocurrent spectroscopy (IMPS) was used to determine the charge separation efficiency, internal and external quantum efficiencies, and process time constants for the four systems in a phosphate buffer at pH 7, both without and with an added hole scavenger, Na2SO3. The heterojunction systems show excellent performance, which is ascribed to the superior capability of WO3 to extract photogenerated electrons from the BiVO4 film that acts as the main absorber. IMPS convincingly shows that the heterojunction configuration prevents surface recombination at the BiVO4/electrolyte interface, as well as the detrimental surface modification generally observed for WO3 photoelectrodes. The improved electron transport properties of the nr-WO3 substrate and the large area of the heterojunction interface result in better performance for nr-WO3/BiVO4, where at 455 nm water photo-oxidation is quantitative. For the single-phase BiVO4 photoelectrodes, an interesting photocurrent switching phenomenon is observed in the presence of the hole scavenger, indicating the intricate interplay between electron trapping and hole charge transfer; this phenomenon is prevented by the heterojunction structure because of the rapid electron extraction by WO3. These results show that IMPS gives detailed information on the reasons for the excellent performance of heterojunction systems, providing opportunities to design new, more efficient solar water splitting systems.