Hydrothermal surface engineering of anodic $WO_3$ photoelectrode by simultaneous iron doping and $Fe_3O_4/FeWO_4$ formation

This study reports a hydrothermal surface modification approach to porous anodized $WO_{3}$ to enhance its photoelectrochemical water oxidation performance. This results in the Fe doping of monoclinic $WO_{3}$ and the simultaneous formation of Fe-containing phases, such as $FeWO_{4}$ and $Fe_{3}O_{4}$. The photocurrent generated at the surface-engineered electrodes was double that of pure $WO_{3}$ with long-term stability. The enhancement is attributable to the creation of oxygen vacancies due to Fe doping and the formation of the heterojunction between $WO_{3}$ and $FeWO_{4}$, a p-type semiconductor, which likely improved the charge carrier lifetime and charge transfer properties. Incident photon to current efficiency (IPCE) measurements revealed enhanced visible light performance, supported by the observed red shift in the light absorption edge. This work is one of the few explorations of $WO_{3}$ photoanodes with an opaque metal substrate that involves fabrication of a light-facing overlayer at the surface. Characterization of the fabricated electrodes was carried out using X-ray diffraction (XRD), scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and diffuse reflectance spectroscopy (UV–Vis DRS). Photoelectrochemical studies were conducted using linear voltammetry, amperometry, and electrochemical impedance spectroscopy (Nyquist, Bode, and Mott–Schottky plots).