Boosted charge carrier dynamics in WO3 photoanodes engineered through a one-step electrochemical post-treatment for efficient photoelectrochemical water splitting

• Oxygen-vacancy-rich WO 3 film is fabricated via the electrochemical post-treatment. • The enriched oxygen defects increase the overall carrier density and band bending. • Electrochemical post-treatment boosts photoexciton efficiency and WOR kinetics. • WO 3-x -Re60 photoanode shows improved photoresponse of 1.1 mA/cm 2 at 1.23 V RHE . • PEC enhancement mechanism and charge transfer energetics are discussed in detail. Strengthening intrinsic charge transport via defect engineering is an effective strategy to optimize the photoelectrochemical (PEC) water splitting properties of tungsten oxide (WO 3 ) photoanode. In this work, a modified and effective electrochemical post-treatment approach has been developed to introduce abundant oxygen vacancies into highly-ordered WO 3 nanoplate arrays, yielding oxygen-deficient WO 3-x photoanode. Electrochemical tests and energy band structural analyses reveal that the generated oxygen vacancies, acting as shallow donors, significantly increase the overall carrier density, electrical conductivity, and built-in electric field (band bending), thereby contributing to the promotion of both bulk charge separation and interfacial charge injection efficiency (up to ∼ 90 % at 1 V RHE ). As a result, the optimized WO 3-x -Re60 photoanode demonstrates a dramatically improved photoresponse, delivering a maximum photocurrent density of 1.1 mA/cm 2 at 1.23 V RHE , representing a 440 % increment over the pristine WO 3 , and is simultaneously accompanied by a negatively shifted onset potential of 0.24 V and boosted photon-to-current conversion efficiency. Overall, this work elucidates the critical role of oxygen vacancies in governing charge-carrier dynamics in WO 3 and demonstrates a practical defect-engineering strategy for designing efficient photoanodes toward solar-driven water oxidation.