Hydrogen production from water using solar energy is one of the most attractive candidates for sustainable clean energy production. Prof. Kazunari Domen has developed various kinds of heterogeneous photocatalysts for splitting of water into hydrogen and oxygen. His major contributions to this field are briefly summarized below.

1) UV light responsive oxide photocatalysts
In 1980, he reported his first photocatalyst for water splitting when he was a Ph.D. student under the supervision of Prof. Kenzi Tamaru at the University of Tokyo. In this photocatalyst, SrTiO₃ powder was modified by NiO/Ni core/shell nanoparticles, which acted as hydrogen evolution sites. It was one of the first examples of a photocatalyst for water splitting that evolved stoichiometric amounts of hydrogen and oxygen. In 1986 and 1997, he reported Ni/K₄Nb₆O₁₇ and NiO/Ni/K₂La₂Ti₃O₁₀ photocatalysts, respectively. Both of these were layered materials with ion-exchange properties, and intercalated water molecules were thought to be involved in the water splitting reaction. These systems were prototypes for various oxide photocatalysts for water splitting and showed a high potential as particulate-type photocatalysts for energy conversion reactions.

2) Visible light responsive non-oxide photocatalysts
For efficient solar energy conversion, it is necessary to develop photocatalysts that operate under visible light. Until about 2000, however, no suitable material was known for inducing a water splitting reaction under visible light. Prof. Domen then found that some oxynitrides, nitrides and oxysulfides were potential candidates for water splitting photocatalysis. These included TaON, Ta₃N₅, LaTiO₂N, Sm₂Ti₂S₂O₅, BaTaO₂N, GaN:ZnO and ZnGeN₂:ZnO solid solutions.
In 2005, he reported that GaN:ZnO modified with RuO₂ could split water into hydrogen and oxygen under visible light at wavelengths up to about 500 nm. Subsequently, Rh_2-xCr_xO₃ was found to be a highly active cocatalyst for hydrogen evolution on GaN:ZnO solid solutions, yielding a quantum efficiency of about 3% at 420 nm. To elucidate the mechanism of hydrogen evolution on the Rh_2-xCr_xO₃ cocatalyst, the Cr₂O₃/Rh core/shell structure was examined. GaN:ZnO modified with Cr₂O₃/Rh showed comparable or even higher activity for water splitting than that achieved with Rh_2-xCr_xO₃. It was found that CrO(OH)· xH₂. O, which is the hydrated form of Cr₂O₃, could transmit H⁺ and H₂ but not O₂. Thus, H⁺ reduction and H₂ formation proceeded on the Rh core, but the reverse reaction could not proceed on the Rh surface, which was one of the reasons for the high activity of the hydrogen evolution site.
Transition metals containing (oxy)nitrides were found to be suitable for the water splitting reaction via a two-step excitation process, the so-called Z-scheme. A combination of Pt/ZrO₂/TaON and Pt/WO₃ coupled with I^-/IO₃^- showed a quantum efficiency of 6.3% at 420 nm for water splitting. The light absorption edges of BaTaO₂N and Ta₃N₅ are located at about 670 and 600 nm, respectively, and these materials were shown to act as effective hydrogen or oxygen evolution photocatalysts in the Z-scheme water splitting reaction after suitable modification. Thus, it is now possible to use a wide range of visible wavelengths for water splitting and hydrogen production.
In summary, Prof. Domen has carried out pioneering work in the field of water splitting on heterogeneous photocatalysts. He developed a wide range of original and unique photocatalysts and demonstrated that such photocatalysis is a promising technique for conversion of solar energy to chemical energy, i.e., solar fuel production.