Molecular Catalysts Immobilized on Semiconductor Photosensitizers for Proton Reduction toward Visible-Light-Driven Overall Water Splitting.

Photocatalytic or photoelectrochemical hydrogen production by water splitting is one of the key reactions for the development of an energy supply that enables a clean energy system for a future sustainable society. Utilization of solar photon energy for the uphill water splitting reaction is a promising technology, and therefore many systems using semiconductor photocatalysts and semiconductor photoelectrodes for the reaction producing hydrogen and dioxygen in a 2:1 stoichiometric ratio have been reported. In these systems, molecular catalysts are also considered to be feasible; recently, systems based on molecular catalysts conjugated with semiconductor photosensitizers have been used for photoinduced hydrogen generation by proton reduction. Additionally, there are reports that the so-called Z-scheme (two-step photoexcitation) mechanism realizes the solar-driven uphill reaction by overall water splitting. Although the number of these reports is still small compared to those of all-inorganic systems, the advantages of molecular cocatalysts and its immobilization on a semiconductor are attractive. This Minireview provides a brief overview of approaches and recent research progress toward molecular catalysts immobilized on semiconductor photocatalysts and photoelectrodes for solar-driven hydrogen production with the stoichiometric uphill reaction of hydrogen and oxygen generation.