We use a great deal of energy in our daily lives, and more than 80% of our energy resources depend on fossil fuel resources. The continuing development of a society reliant on fossil resources may result in energy and environmental problems on a global scale due to resource depletion, destabilization of supply systems, and increased emissions of gases, such as nitrogen oxides, sulfur oxides, and carbon dioxide. These emissions have a greenhouse effect and cause acid rain.

Hydrogen is attracting attention as a new energy medium alternative to fossil fuels. Hydrogen emits only water as a byproduct when burned under ideal conditions and can be stored for a long time as a fuel with high energy density. It is also an important raw material in the chemical industry and can be used to recycle carbon dioxide and nitrogen to produce useful compounds. At present, however, hydrogen is produced from fossil resources. To solve the energy and environmental issues, it is necessary to develop technology that can produce hydrogen without using fossil resources.

The decomposition of water into hydrogen and oxygen using solar energy and photocatalysts is studied as a means of sustainable hydrogen production. The photocatalyst used here is a semiconductor. By absorbing light, negatively charged electrons and positively charged holes are generated, triggering redox reactions associated with decomposition of water. As the reaction proceeds with renewable solar energy, it is expected to enable the production of hydrogen without consuming fossil resources. There has been a great deal of research regarding such chemical processes that make use of solar energy to convert low-energy substances into high-energy substances, which are referred to as artificial photosynthesis.

To make effective use of solar energy, it is necessary to develop a photocatalyst that absorbs visible light and exhibits activity. At the Domen-Hisatomi lab at Shinshu University, we mainly research particulate oxynitrides, nitrides, and oxysulfide semiconductors as photocatalytic materials for water splitting (Fig. 1). Many of these materials are known to exhibit intense long-wavelength visible light absorption. To activate these photocatalytic materials, our laboratory is investigating the detailed correlation between material synthesis methods, physical properties, activation methods, and photocatalytic activity. To make the knowledge obtained in the laboratory useful in the real world, the production of photocatalytic reactors that can be deployed on a large scale is also under investigation.