Enhancement of the Efficiency of Photocatalytic Reduction of Protons to Hydrogen via Molecular Assembly

CONSPECTUS: One of the best solutions for meeting future energy demands is the conversion of water into hydrogen fuel using solar energy. The splitting of water into molecular hydrogen (H-2) and oxygen (O-2) using light involves two half-reactions: the oxidation of water to O-2 and the reduction of protons to H-2. To take advantage of the full range of the solar spectrum, researchers have extensively investigated artificial photosynthesis systems consisting of two photosensitizers and two catalysts with a Z-configuration: one photosensitizer-catalyst pair for H-2 evolution and the other for O-2 evolution. This type of complete artificial photosynthesis system is difficult to build and optimize; therefore, researchers typically study the reductive half-reaction and the oxidative half-reaction separately. To study the two half-reactions, researchers use a sacrificial electron donor to provide electrons for the reductive half-reaction, and a sacrificial electron acceptor to capture electrons for the oxidative half-reaction. After optimization, they can eliminate the added donors and acceptors as the two half reactions are coupled to a complete photocatalytic water spitting system. Most photocatalytic systems for the H-2 evolution half-reaction consist of a photosensitizer, a catalyst, and a sacrificial electron donor. To promote photoinduced electron transfer and photocatalytic H, production, these three components should be assembled together in a controlled manner. Researchers have struggled to design a photocatalytic system for H-2 evolution that uses earth-abundant materials and is both efficient and durable. This Account reviews advances our laboratory has made in the development of new systems for photocatalytic H evolution that uses earth-abundant materials and is both efficient and durable. We used organometallic complexes and quantum-confined semiconductor nanocrystals (QDs) as photosensitizers, and [FeFe]-H(2)ase mimics and inorganic transition metal salts as catalysts to construct photocatalytic systems with sacrificial electron donors. Covalently linked Re(I) complex-[FeFe]-H(2)ase mimic dyads and ferrocene-Re(I) complex-[FeFe]-H(2)ase mimic triads could photocatalyze H-2 production in organic solutions, but these photocatalytic systems tended to decompose. We also constructed several assemblies of CdTe and CdSe QDs as photosensitizers with [FeFe]-H(2)ase mimics as catalysts. These assemblies produced H-2 in aqueous solutions photocatalytically and efficiently, with turnover numbers (TONs) up to tens of thousands. Assemblies of 3-mercaptopropionic acid (MPA)-capped CdTe QDs with Co2+ ions formed Co-h-CdTe hollow nanospheres, and MPA capped-CdSe QDs with Ni+ ions produced Ni-h-CdSe/CdS core/shell hybrids in situ in aqueous solutions upon irradiation. The resulting photocatalytic systems proved robust for H-2 evolution. These systems showed excellent activity and impressive durability in the photocatalytic reaction, suggesting that they can serve as a valuable part of an overall water splitting system.