Metal Complex/Semiconductor Hybrid Photoelectrocatalysts and Photocatalysts for CO₂ Reduction

Reductive conversion of CO₂ into energy-added molecules has been an important subject in various fields including materials chemistry, catalysis, electrochemistry, and photochemistry, from viewpoints of both decreasing CO₂ concentration and gaining energy and carbon resources. Among the various methods and schemes proposed, visible-light-driven CO₂ reduction in combination with water oxidation, one of the representative models of artificial photosynthesis, is an attractive solution because it enables abundant water and inexhaustible solar energy to be used to produce value-added chemicals. Molecular metal complexes and semiconductors are promising candidates for photocatalysts that can reduce CO₂ to CO, formate, formaldehyde, or other hydrocarbons. Although both molecular metal complexes and semiconductors have strengths and weaknesses, their weaknesses (low oxidation ability and low selectivity for reduction reactions) can be overcome via the construction of a suitable molecule/semiconductor hybrid material. However, facilitating electron transfer at the molecule/semiconductor junction while suppressing unfavorable back electron transfer events is challenging.<br/>Here, our design principle for developing the hybrid photoelectrodes and photocatalysts will be reported, starting from introduction of selective electrocatalysis and photocatalysis of CO₂ reduction by a metal complex catalyst in aqueous solution. Subsequently, application of these metal-complex photocatalysts into a hybrid photoelectrode with semiconductor materials for photoelectrochemical CO₂ reduction will be presented. Simple hybrid photocatalysts directly connecting metal complexes and semiconductor particles, which facilitate photocatalytic CO₂ reduction via interfacial electron transfer without the aid of electrochemical techniques, will also be reported. Finally, we will report our recent approach for constructing hybrid photocatalysts with design-flexible conjugated polymers as a light absorber, a photoelectron transporter, and a suitable reaction center by site-selective modification of metal-complex catalysts.