Plasma-Induced Oxygen Vacancies in Ultrathin Hematite Nanoflakes Promoting Photoelectrochemical Water Oxidation

The incorporation of oxygen vacancies in hematite has been investigated as a promising route to improve oxygen evolution reaction activity of hematite photoanodes used in photoelectrochemical water oxidation. However, introducing oxygen vacancies intentionally in a-Fe2O3 for active solar water splitting through facile and effective methods remains a challenge. Herein, air plasma treatment is shown to produce oxygen vacancies in a-Fe2O3, and ultrathin a-Fe2O3 nanoflakes are used to investigate the effect of oxygen vacancies on the performance of photoelectrochemical oxygen oxidation. Increasing the plasma treatment duration and power is found to increase the density of oxygen vacancies and leads to a significant enhancement of the photocurrent response. The nanoflake photoanode with the optimized plasma treatment yields an incident photo-to-current conversion efficiency of 35.4% at 350 nm under 1.6 V vs RHE without resorting to any other cocatalysts, an efficiency far exceeding that of the pristine a-Fe2O3 nanoflakes (similar to 2.2%). Evidence for the presence of high density of oxygen vacancies confined in nanoflakes is clarified by X-ray photoelectron spectroscopy. The increased number of oxygen vacancies after plasma treatment resulting in an increased carrier density is interpreted as the main cause for the enhanced oxygen evolution reaction activity.