Photoswitchable Activity of Mixed Bismuth Oxide (BiO _x ) for Water Splitting in Neutral Media

Unary bismuth oxide (BiO _x ), containing both Bi³⁺ and Bi⁵⁺ species, exhibits a photoswitchable response that, to the best of our knowledge, has not yet been reported. The material has attracted increasing attention as an anode for the oxygen evolution reaction (OER), often in combination with additional elements to enhance electrochemical performance. This study investigates the electrodeposition of BiO _x and its unexplored photoswitchable properties as an anode for solar-driven enhanced water splitting. Illumination with a xenon lamp equipped with solar simulation filters reduced the deposition time from 90 to 10 min without altering the amperometric profile, offering significant energy savings and potential for scale-up. Structural and morphological characterization were performed using scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) and X-ray diffraction (XRD). Chopped light voltammetry revealed that the photocurrent in neutral media has a mixed photo- and electrocatalytic contribution. Under illumination, the BiO _x electrode achieved a total current density of 2.5 mA cm^-2, 140 mV lower than in dark conditions. Electrochemical impedance spectroscopy (EIS) revealed a decrease in depletion layer resistance with illumination, and an electrolyte-free impedance measurement indicated that changes in intrinsic conductivity also contribute to the PEC response. Overall, illumination enhances BiO _x electrocatalytic water splitting, with a particularly pronounced effect on thin-layer films.