Enhanced solar-driven water splitting performance using oxygen vacancy rich ZnO photoanodes

This work reports a facile two-step method of producing highly efficient ZnO photoanodes for photoelectrochemical (PEC) water splitting under solar light conditions and describes the role of surface oxygen vacancies (VO) in their enhanced PEC performance. The photoanode fabrication involves post-growth oxidation of a metallic Zn layer, which produces a nanostructured ZnO film consisting of ∼50 nm diameter nanorods containing a high concentration of VO defects. The PEC activity of the ZnO films is investigated by studying water oxidation in an aqueous electrolyte under simulated solar illumination. The relationship of PEC and charge transfer characteristics of the ZnO photoanodes with ionized surface VO defects is established using cathodoluminescence, X-ray photoemission, voltammetry, electrochemical impedance spectroscopy and chronoamperometry. The combined results show that the photoanode fabricated in this work possesses a high surface density of ionized VO states that facilitate the effective transportation of holes for water oxidation. It is found that the photoanode exhibits an exceptional photocurrent density of 1.14 mA/cm2 at 1.23 VRHE, being one of the best performances reported in the literature for ZnO-based photoelectrodes so far. Our results demonstrate a simple, low-cost method for fabricating highly efficient VO rich ZnO-based PEC photoanodes that is suitable for large scale production.