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 (H2) and oxygen (O2) using light involves two half-reactions: the oxidation of water to O2 and the reduction of protons to H2. 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 H2 evolution and the other for O2 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 H2 evolution half-reaction consist of a photosensitizer, a catalyst, and a sacrificial electron donor. To promote photoinduced electron transfer and photocatalytic H2 production, these three components should be assembled together in a controlled manner. Researchers have struggled to design a photocatalytic system for H2 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]-H2ase mimics and inorganic transition metal salts as catalysts to construct photocatalytic systems with sacrificial electron donors. Covalently linked Re(I) complex-[FeFe]-H2ase mimic dyads and ferrocene-Re(I) complex-[FeFe]-H2ase mimic triads could photocatalyze H2 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]-H2ase mimics as catalysts. These assemblies produced H2 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 Co(2+) ions formed Coh-CdTe hollow nanospheres, and MPA capped-CdSe QDs with Ni(+) ions produced Nih-CdSe/CdS core/shell hybrids in situ in aqueous solutions upon irradiation. The resulting photocatalytic systems proved robust for H2 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.