Insight into Charge Separation in WO₃/BiVO₄ Heterojunction for Solar Water Splitting

Recently, the WO₃/BiVO₄ heterojunction has shown promising photoelectrochemical (PEC) water splitting activity based on its charge transfer and light absorption capability, and notable enhancement of the photocurrent has been achieved via morphological modification of WO₃. We developed a graft copolymer-assisted protocol for the synthesis of WO₃ mesoporous thin films on a transparent conducting electrode, wherein the particle size, particle shape, and thickness of the WO₃ layer were controlled by tuning the interactions in the polymer/sol-gel hybrid. The PEC performance of the WO₃ mesoporous photoanodes with various morphologies and the individual heterojunctions with BiVO₄ (WO₃/BiVO₄) were characterized by measuring the photocurrents in the absence/presence of hole scavengers using light absorption spectroscopy and intensity-modulated photocurrent spectroscopy. The morphology of the WO₃ photoanode directly influenced the charge separation efficiency within the WO₃ layer and concomitant charge collection efficiency in the WO₃/BiVO₄ heterojunction, showing the smaller sized nanosphere WO₃ layer showed higher values than did the plate-like or rod-like one. Notably, we observed that photocurrent density of WO₃/BiVO₄ was not dependent on the thickness of WO₃ film or its charge collection time, implying slow charge flow from BiVO₄ to WO₃ can be a crucial issue in determining the photocurrent, rather than the charge separation within the nanosphere WO₃ layer.