Water splitting to form hydrogen and oxygen over a heterogeneous photocatalyst using solar energy is a promising process for clean and renewable hydrogen production. In recent years, numerous attempts have been made for the development of photocatalysts that work under visible light irradiation to efficiently utilize solar energy. This article reviews recent research progress in the development of visible light-driven photocatalysts, focusing on the refinement of oxynitride materials. They harvest visible photons (~450–700 nm) and work as stable photocatalysts for water reduction and oxidation under visible light. Oxynitrides with d0 electronic configuration can be successfully applied to a two-step water-splitting system, which can harvest a wide range of visible photons (~660 nm), in the presence of an iodate/iodide shuttle redox mediator. Also d10-type oxynitrides of GaN–ZnO and ZnGeN2–ZnO solid solutions can achieve functionality as photocatalysts for overall water-splitting under visible light without noticeable degradation.

Photoelectrochemical (PEC) water splitting for hydrogen production is a promising technology that uses sunlight and water to produce renewable hydrogen with oxygen as a by-product. In the expanding field of PEC hydrogen production, the use of standardized screening methods and reporting has emerged as a necessity. This article is intended to provide guidance on key practices in characterization of PEC materials and proper reporting of efficiencies. Presented here are the definitions of various efficiency values that pertain to PEC, with an emphasis on the importance of solar-to-hydrogen efficiency, as well as a flow chart with standard procedures for PEC characterization techniques for planar photoelectrode materials (i.e., not suspensions of particles) with a focus on single band gap absorbers. These guidelines serve as a foundation and prelude to a much more complete and in-depth discussion of PEC techniques and procedures presented elsewhere.

Powders of TiO2 doped with a metal ion and N species were prepared by a polymerized complex method and the visible-light photocatalytic activities of the products are investigated. Of the metal ions studied (K+, Ca2+, Sr2+, Ba2+, Nb5+, Fe3+, Zn2+, and Al3+), the photocatalyst prepared with Sr2+ exhibits the highest activity for acetaldehyde decomposition under visible-light irradiation. Results obtained from x-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) analyses suggest that the doped N species reside at interstitial lattice positions in the catalyst. It was also found by XPS and ESR measurements that the doped N species combine with lattice oxygen to give rise to a paramagnetic property. The visible-light response of the catalyst is driven by the formation of paramagnetic N species at interstitial positions in the TiO2 lattice.

A thin film composed of an ion-exchangeable layered oxide HxTi(2-x/4)⃞x/4O4 (HTiO) (where the open square, ⃞, is vacancy) is studied as an electrode material. The thin film prepared on an indium tin oxide substrate functions as an electrode for the redox reaction of Ru(bpy)32+/Ru(bpy)33+ without noticeable degradation. Lack of reaction in the presence of Fe(CN)64−/Fe(CN)63− indicates that reaction on this electrode occurs in the interlayers between titanate sheets. A Ru(bpy)32+-intercalated form of the electrode generates photocurrent under visible light irradiation.

The interlayer space of a thin film of layered titanate, Cs0.68Ti1.83‪0.17O4, was successfully expanded by SiO2 pillaring. Ion exchange of the Cs ions in the interlayer to alkylammoniuim cations, n-CnH2n+1NH3+ (n = 8, 12, 18), expanded the interlayer space, and enabled intercalation of tetraethylorthosilicate. X-ray diffraction and the cross section of transmission electron microscopy images revealed that tetraethylorthosilicate-treated thin film maintained the expansion of interlayer space by SiO2 pillaring after calcination at 773 K. X-ray photoelectron spectroscopy after etching the thin film about 100 nm from the surface further confirmed the existence of SiO2 in the interlayer space.

Fine powders of semiconducting oxides loaded with deposited metal and/or metaloxide particles have been widely used as heterogeneous photocatalysts for innumerable chemical reactions. Among the many photocatalytic reactions, the splitting of water assisted by light has become one of the most active areas in heterogeneous photocatalysis, since it can be a promising chemical route for energy renewal and energy storage. The photocatalytic splitting of water on TiO2 electrodes, discovered by Fujishima and Honda in 1972, is a prototypic example of this technique, and there is a vast body of literature describing the potential application of TiO2-based photocatalysts for water decomposition. This has brought about a burst of research related to the development of many other photocatalytic systems.

A layered perovskite type oxide, K2La2Ti3O10, was prepared by a gel technique using the polymerized complex (PC) method. A single phase K2La2Ti3O10 was obtained by adding twice the potassium required for stoichiometry. The Ni–K2La2Ti3O10 catalyst prepared by the PC method exhibited a higher photocatalytic activity for decomposition of H2O into H2 and O2 than that prepared by a conventional solid state reaction.

A new porous material was prepared from a layered compound, K4Nb6O17, through the exfoliation of its layers. A composite of the niobate sheets and MgO particles were obtained by precipitating the exfoliated two-dimensional niobate sheets with MgO fine particles. Porous niobium oxide was obtained by removal of the MgO particles from the composite after thermal treatment. It had a large surface area and showed higher photocatalytic activity than the original H+/K4Nb6O17 for H2 evolution from various aqueous alcohol solutions.