High-Voltage Etching-Induced Terrace-like WO₃ Photoanode for Efficient Photoelectrochemical Water Splitting

Tungsten oxide (WO3) is a strong candidate as the photoanode in photoelectrochemical (PEC) water splitting owing to its moderate band gap (2.6-3.2 eV) for wide light absorption and stable physicochemical properties. This work reports a morphology engineering strategy to fabricate voltage-induced terrace-like tungsten oxide (TW) film and its high-voltage performance as a photoanode for PEC water splitting. By controlling the anodization voltage, the morphology of the anodic WO3 film was altered, and their photoelectrochemical performances were compared. Anodization of the tungsten film in fluoride-containing H3PO4 leads to the growth of either irregular porous tungsten oxide layers or terrace-like tungsten oxide layers, depending on the anodic potentials. The possible growth mechanism of the porous oxide layer during anodization is proposed with a combination of the field-assisted dissolution model and the oxygen-bubble mold model. Under light irradiation, the photocurrent density was 3.0 mA cm(-2) at 1.23 V vs RHE (VRHE) for annealed terrace-like WO3, which was almost 3 times larger than that of annealed porous WO3. As a photoanode, terrace-like WO3 films could generate hydrogen at 58 mu mol cm(-2) h(-1) under simulated solar light, while porous WO3 films produced hydrogen with a slower rate of 28 mu mol cm(-2) h(-1). The terrace-like WO3 films experience higher photoelectrochemical performance and incident photon-to-electron conversion efficiency (IPCE) than porous WO3 due to more regular nanostructure, smaller band gap, fast charge-transfer rate, and alleviated recombination rate. This study shows that morphology control can effectively enhance the photocurrent density and IPCE of photoanodes and suggests a practical direction for finding the optimal WO3 films for PEC water splitting.