WO3/BiVO4 heterojunction photoanodes: Optimized photoelectrochemical performance in relation to both oxides layer thickness

An effective strategy to boost the photoelectrochemical (PEC) performance of photoactive materials consists in combining different semiconductors with complementary characteristics, to build type-II heterojunctions. In particular, WO 3 /BiVO 4 photoanodes exhibit synergistic effects in the photo-oxidation of water into molecular oxygen, usually overwhelming possible recombination paths at work within the heterojunction. We present here a systematic PEC investigation on composite WO 3 /BiVO 4 photoanodes with various WO 3 and BiVO 4 layer thickness (200–800 nm and 40–140 nm, respectively), in comparison with equally thick single WO 3 and BiVO 4 photoanodes, performed under either solar or monochromatic irradiation. We demonstrate that detrimental charge recombination is mainly active under back-side irradiation and is mitigated by minimizing the absorption of both tungsten trioxide and bismuth vanadate layers. Higher photocurrent values are in general attained when the photoanodes are irradiated through the electrolyte/electrode interface, with the best performing photoanodes being composed of ca. 500 nm- and 140 nm-thick WO 3 and BiVO 4 layers, respectively. • Synergistic effects notoriously occur in the WO 3 /BiVO 4 heterojuction photoanode. • Charge recombination may be active in heterojunctions under back-side irradiation. • Both WO 3 and BiVO 4 layers thickness affects the overall heterojunction performance. • Maximal performance is obtained with light directly impinging on the BiVO 4 layer. • Best performing photoanodes operating under back/front- irradiation were identified.