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, WO3/BiVO4 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 WO3/BiVO4 photoanodes with various WO3 and BiVO4 layer thickness (200-800 nm and 40-140 nm, respectively), in comparison with equally thick single WO3 and BiVO4 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 WO3 and BiVO4 layers, respectively.