Gas-Selective Remote Plasma Engineering of WO3 Photoanodes for Solar Water Splitting

Abstract

The photoelectrochemical (PEC) performance of WO₃ 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 WO₃, leading to markedly different water-oxidation behavior without altering the bulk crystal structure. Under AM 1.5G illumination, Ar-treated WO₃ delivers the best overall performance, exhibiting a photocurrent of 0.78 mA cm⁻² at 1.23 V_RHE, more than twice that of O₂-treated WO₃. Ar treatment generates the most oxygen-vacancy-rich surface, the highest W⁵⁺ content, and a marked increase in donor density. In contrast, O₂ plasma passivates vacancy-related states, shifts the onset and flat-band potentials to more positive values, and gives the poorest PEC response, while N₂ 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.