Gas‐Selective Remote Plasma Engineering of WO ₃ Photoanodes for Solar Water Splitting

ABSTRACT The photoelectrochemical (PEC) performance of WO ₃ photoanodes is highly sensitive to near‐surface defect chemistry, yet the role of plasma‐gas chemistry in governing oxygen‐vacancy states and interfacial energetics remains insufficiently understood. Here, we show that radio frequency (RF) remote plasma enables gas‐selective control over the near‐surface defect structure of WO ₃ , leading to markedly different water‐oxidation behavior without altering the bulk crystal structure. Under AM 1.5G illumination, Ar‐treated WO ₃ delivers the best overall performance, exhibiting a photocurrent of 0.78 mA cm ⁻² at 1.23 V _RHE , more than twice that of O ₂ ‐treated WO ₃ . Ar treatment generates the most oxygen‐vacancy‐rich surface, the highest W ⁵⁺ content, and a marked increase in donor density. In contrast, O ₂ plasma passivates vacancy‐related states, shifts the onset and flat‐band potentials to more positive values, and gives the poorest PEC response, while N ₂ plasma yields intermediate behavior. Cathodoluminescence provides further insight into the nature of the plasma‐modified defect states: Ar treatment strongly enhances the red luminescence band at ∼1.85 eV, indicating selective enrichment of reactive near‐surface vacancy states. We propose a defect‐assisted PEC model in which plasma‐generated near‐surface vacancy states increase donor density, band bending, and interfacial hole transfer, leading to more efficient utilization of photogenerated charge carriers.