Photofunctional molecular assembly for artificial photosynthesis: Beyond a simple dye sensitization strategy

Abstract

Photocatalytic water splitting is a desirable reaction that produces a clean and sustainable energy resource—hydrogen (H2)—from solar light and water, a process called artificial photosynthesis. Since the discovery of the Honda–Fujishima effect, significant developments have been made in the field of metal-oxide-based semiconductor photocatalysts, to achieve high solar-to-hydrogen energy conversion efficiencies suitable for practical applications (>10%). One promising approach for utilizing the entire spectral region of solar light in the water-splitting reaction is to construct a Z-scheme photocatalytic system using a combination of O2 and H2 evolving photocatalysts with a redox-reversible electron mediator. However, these Z-scheme photocatalysts are still limited by backward electron transfer and back reactions at the photocatalyst–mediator interface. In contrast, indispensable molecules and clusters for natural photosynthesis are precisely arranged at the optimal position in protein scaffolds supported by flexible lipid bilayer membranes to effectively promote/suppress forward/backward reaction. The large gap between artificial and natural photosynthesis suggests the requirement of judicious reaction field design at the molecular level. Therefore, in this review, we focus on the photofunctional molecular assemblies present on the surfaces of rigid semiconductors or soft lipid membranes, to overcome the backward reactions in Z-scheme water-splitting photocatalysis. Recent studies clearly indicate that these approaches, based on the dye-sensitization mechanism, may provide powerful techniques not only to improve the photoinduced charge-separation efficiency, but also to regulate the reaction field and pathway in the photocatalytic O2 and H2 evolution reactions. Further, the integration of external-stimuli-responsive structures and artificial photosynthesis may provide a new methodology for photocatalytic activity control by mesoscale structure changes, similar to the changes observed in the grana structure in natural photosynthesis.