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

Tungsten oxide (WO 3 ) 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 WO 3 film was altered, and their photoelectrochemical performances were compared. Anodization of the tungsten film in fluoride-containing H 3 PO 4 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 (V RHE ) for annealed terrace-like WO 3, which was almost 3 times larger than that of annealed porous WO 3 . As a photoanode, terrace-like WO 3 films could generate hydrogen at 58 μmol cm –2 h –1 under simulated solar light, while porous WO 3 films produced hydrogen with a slower rate of 28 μmol cm –2 h –1 . The terrace-like WO 3 films experience higher photoelectrochemical performance and incident photon-to-electron conversion efficiency (IPCE) than porous WO 3 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 WO 3 films for PEC water splitting.