Modulating WO3 Crystal Orientation to Suppress Hydroxyl Radicals for Sustainable Solar Water Oxidation

Tungsten trioxide (WO3) is one of the promising semiconductors suitable for photoelectrochemical water oxidation, but its hydroxyl radical (•OH)-induced intrinsic performance degradation remains unclarified. Here, we demonstrate that quenching-treated WO3 with preferred {021} facets shows a highly improved Faradaic efficiency (from 57 to 95%) and its performance stability is more than 36 h relative to that of nontreated WO3 with less than 1-h stability. Using electron paramagnetic resonance (EPR), we find that the •OH could be highly suppressed on the treated WO3 photoanode, while abundant •OH is generated on the nontreated WO3. In situ ultraviolet–visible (UV–Vis) spectroscopy is used to track the presence of surface W–O–O–W intermediates on the treated WO3, suggesting the favorable formation of O–O and thus better oxygen evolution Faradaic efficiency, while the nontreated WO3 favors the formation of •OH, which accumulates on the WO3 surface, thus changing the photoanode/electrolyte interfacial properties and poisoning the oxygen evolution process. This work provides an intrinsic understanding of the degradation of the WO3 photoanode under acidic and neutral conditions.