Solar-Driven Water Splitting over a BaTaO2N Photoanode Enhanced by Annealing in Argon

BaTaO2N prepared by the nitridation of Ba5Ta4O15 drives photoelectrochemical water oxidation in response to photoexcitation up to 660 nm. However, a high concentration of defects and an amorphous surface promote the recombination of photogenerated holes and electrons in this material, thus reducing its performance. In this work, annealing in an Ar flow is used to activate the BaTaO2N surface and thus improve its water oxidation activity. The results show that annealing at 1073 K both crystallizes the amorphous BaTaO2N surface and increases the bulk crystallinity. Following surface modification to enhance charge separation during the photoreaction, a photoanode made of the annealed BaTaO2N generates an unprecedented photocurrent of 6.5 mA cm–2 at 1.23 VRHE during sunlight-driven water oxidation and retains 79% of the initial photocurrent over 24 h. The half-cell solar-to-hydrogen energy conversion efficiency reaches 1.4% at 0.88 VRHE, representing the highest value yet reported for any perovskite-type oxynitride with intense visible light absorption. This remarkable improvement demonstrates that a surface treatment based on annealing in Ar effectively enhances the photoreaction over oxynitrides that otherwise tend to have amorphous surfaces.