Abstract

One of the simplest methods for splitting water into H2 and O2 with solar energy entails the use of a particulate-type semiconductor photocatalyst. To harness solar energy efficiently, a new water-splitting photocatalyst that is active over a wider range of the visible spectrum has been developed. In particular, a complex perovskite-type oxynitride, LaMgxTa1xO1+3xN23x (x 1/3), can be employed for overall water splitting at wavelengths of up to 600 nm. Two effective strategies for overall water splitting were developed. The first entails the compositional fine-tuning of a photocatalyst to adjust the bandgap energy and position by forming a series of LaMgxTa1xO1+3xN23x solid solutions. The second method is based on the surface coating of the photocatalyst with a layer of amorphous oxyhydroxide to control the surface redox reactions. By combining these two strategies, the degradation of the photocatalyst and the reverse reaction could be prevented, resulting in successful overall water splitting.

Direct water splitting into H2 and O2 on semiconductor photocatalysts is the most fundamental artificial photosynthetic reaction. A high-efficiency photocatalytic reaction using sunlight would offer an attractive means of producing clean and renewable energy. This would also be expected to be the most scalable and cost-effective route for large-scale renewable hydrogen production.[1] Although various kinds of photocatalysts for water splitting have been reported to date, most of these are only active under UV irradiation, and only a few have been demonstrated to operate under visible light,[2] up to a wavelength of approximately 500 nm.[3] Thus, the development of water-splitting photocatalysts with a narrower bandgap and a larger wavelength range is a significant target for efficient solar hydrogen production.