Colloidal Semiconductor Nanocrystal Photocatalysts: Teaching an Old Dot New Tricks
Photochemical syntheses, such as the light-driven reduction of protons to H2 (i.e. artificial photosynthesis) that involve purely molecular systems typically suffer from short lifetimes because of decomposition of the light-absorbing molecule, such as an organic dye, making these systems impractical. A complementary approach to entirely molecular systems would be to incorporate nanoscale based systems, such as semiconductor nanocrystals, into the photochemical process as the light-harvesting element. Nanocrystals offer several potential advantages for photocatalytic applications including durability, electronic states and redox potentials that are size tunable, and the ability to store and deliver multiple electrons. We will present studies of a robust and highly active system for solar hydrogen generation in water that uses CdSe nanocrystals capped with dihydrolipoic acid (DHLA) as the solar energy harvester and a soluble Ni2+-DHLA catalyst for proton reduction. Under appropriate conditions, the nanocrystal-based system has undiminished activity for at least 360 hours with about a million turnovers. Ultrafast optical spectroscopy studies of electron transfer from the nanoparticles to a Ni-catalyst reveals the optimal nanoparticle size and shape for photochemical proton reduction, which was confirmed with photocatalytic experiments. Finally, we will also present recent measurements of carbon-carbon bond formation driven photochemically using CdSe nanocrystals. A single-sized CdSe nanocrystal (3.0 ± 0.2 nm) can replace several different dye catalysts needed for five different photoredox reactions (beta-alkylation,beta-aminoalkylation, dehalogenation, amine arylation, and decarboxylative radical formation). Even without optimization of the nanocrystals or the reaction conditions, efficiencies rivaling the best available metal dyes were obtained. The facile customization afforded by nanocrystals for photoredox catalysis suggests they could have important applications in future syntheses of novel pharmaceuticals.