Insight\ninto Charge Separation in WO₃/BiVO₄ Heterojunction\nfor Solar Water Splitting

Abstract

Recently, the WO₃/BiVO₄ heterojunction has shown promising photoelectrochemical (PEC) water splitting\nactivity based on its charge transfer and light absorption capability,\nand notable enhancement of the photocurrent has been achieved via\nmorphological modification of WO₃. We developed a graft\ncopolymer-assisted protocol for the synthesis of WO₃ mesoporous\nthin films on a transparent conducting electrode, wherein the particle\nsize, particle shape, and thickness of the WO₃ layer were\ncontrolled by tuning the interactions in the polymer/sol–gel\nhybrid. The PEC performance of the WO₃ mesoporous photoanodes\nwith various morphologies and the individual heterojunctions with\nBiVO₄ (WO₃/BiVO₄) were characterized\nby measuring the photocurrents in the absence/presence of hole scavengers\nusing light absorption spectroscopy and intensity-modulated photocurrent\nspectroscopy. The morphology of the WO₃ photoanode directly\ninfluenced the charge separation efficiency within the WO₃ layer and concomitant charge collection efficiency in the WO₃/BiVO₄ heterojunction, showing the smaller sized\nnanosphere WO₃ layer showed higher values than did the\nplate-like or rod-like one. Notably, we observed that photocurrent\ndensity of WO₃/BiVO₄ was not dependent on the\nthickness of WO₃ film or its charge collection time, implying\nslow charge flow from BiVO₄ to WO₃ can be a\ncrucial issue in determining the photocurrent, rather than the charge\nseparation within the nanosphere WO₃ layer.