Photocatalytic overall water splitting into H<sub>2</sub> and O<sub>2</sub> is expected to be a promising method for the efficient utilization of solar energy. The design of optimal photocatalyst structures is a key to efficient overall water splitting, and the development of photocatalysts which can efficiently convert large portion of visible light spectrum has been required. Recently, a series of complex perovskite type transition metal oxynitrides, LaMg<sub>x</sub>T <sub>1-x</sub>O<sub>1+3x</sub>N<sub>2-3x</sub>, was developed as photocatalysts for direct water splitting operable at wide wavelength of visible light. In addition two-step excitation water splitting via a novel photocatalytic device termed as photocatalyst sheet was developed. This consists of two types of semiconductors (hydrogen evolution photocatalyst and oxygen evolution photocatalyst) particles embedded in a conductive layer, and showed high efficiency for overall water splitting. These recent advances in photocatalytic water splitting were introduced.
Photocatalytic overall water splitting promises to enable a sustainable large-scale hydrogen-based energy system using
solar light, and great attention has been paid to the development of photocatalysts. It is necessary to develop
photocatalysts that function under visible light to utilize sunlight efficiently. We have proposed non-oxide materials as
candidates for visible-light-driven photocatalysts for overall water splitting, and this manuscript presents our recent
research in photocatalyst development. Some oxynitride photocatalysts, modified with appropriate cocatalysts, showed
performance for overall water splitting under visible light irradiation. The modification with cocatalysts drastically
improved the efficiency of photocatalytic reactions, indicating the importance of controlling the surface active sites.
Two-step excitation systems, known as Z-schemes, can significantly expand the range of light available for water
splitting to longer wavelengths.