Hydrogen-treated hematite nanostructures with low onset potential for highly efficient solar water oxidation

Hydrogen-treated hematite nanostructures were prepared by a simple pyrolysis of NaBH4 in a crucible. The H2-treated hematite photoelectrode showed high efficiency for solar water oxidation with a photocurrent of 2.28 mA cm−2 at 1.23 V vs. RHE, which was over 2.5 times higher than that for pristine hematite (0.88 mA cm−2). The significant improvement of the photocurrent can be attributed to increased oxygen vacancies after the H2 treatment. Moreover, the onset potential for H2-treated hematite was low and when compared to the hematite photoelectrode treated in an oxygen-deficient atmosphere to produce oxygen vacancies, a cathodic shift of the onset potential was observed by about 120 mV (from 0.99 to 0.87 V vs. RHE). The cathodic shift of the onset potential was attributed to the surface effect of H2 treatment while the oxygen-deficiency treatment mainly affected the bulk, which was confirmed by X-ray absorption spectroscopy. The results also suggest that the presence of surface defect states of Fe2+ in hematite is not the reason for high onset potential described in the literature. The H2-treated hematite with high efficiency could be used as a good starting material to achieve better performance for practical applications with further modifications such as surface catalysts or elemental doping.