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

Abstract 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.