Controlled colloidal metal nanoparticles and nanoclusters: Recent applications as cocatalysts for improving photocatalytic water-splitting activity

In recent years, research on the use of metal nanoparticles (NPs) and nanoclusters (NCs) synthesized by liquid-phase reduction in water-splitting photocatalysts has been actively conducted. Water-splitting photocatalysts have been attracting attention because they make it possible to produce hydrogen (H2), which is attractive as a next-generation energy source, from solar energy and water. However, further improvement of water-splitting photocatalysts is required for their practical use in society. Recent studies have demonstrated that the active sites (cocatalysts) of water-splitting photocatalysts can be controlled using the advanced NP/NC syntheses and structural modulation techniques established in the fields of colloid, NP, and NC chemistry and thereby highly active water-splitting photocatalysts can be developed. If such research progresses further, it is expected that a transition to a new society using H2 as the main energy source will become possible. However, such applied research has only just started and examples of such research are currently limited. The purpose of this review is to introduce the importance of controlled colloidal NPs/NCs in research on water-splitting photocatalysis to readers by summarizing the existing research. We hope that this review will raise interest in the application of metal NPs/NCs in water-splitting photocatalysis and that a society actively addresses energy and environmental problems will become a reality as soon as possible.