Quantum Dot-Sensitized Photoreduction of CO in Water with Turnover Number > 80,000.

Climate change and global energy demands motivate the search for sustainable transformations of carbon dioxide (CO) to storable liquid fuels. Photocatalysis is a pathway for direct conversion of CO to CO, one step within light-powered reaction networks that could, if efficient enough, transform the solar energy conversion landscape. To date, the best performing photocatalytic CO reduction systems operate in nonaqueous solvents, but technologically viable solar fuels networks will likely operate in water. Here we demonstrate catalytic photoreduction of CO to CO in pure water at pH 6-7 with an unprecedented combination of performance parameters: turnover number (TON(CO)) = 72,484-84,101, quantum yield (QY) = 0.96-3.39%, and selectivity () > 99%, using CuInS colloidal quantum dots (QDs) as photosensitizers and a Co-porphyrin catalyst. At higher catalyst concentration, the system reaches QY = 3.53-5.23%. The performance of the QD-driven system greatly exceeds that of the benchmark aqueous system (926 turnovers with a quantum yield of 0.81% and selectivity of 82%), due primarily to (i) electrostatic attraction of the QD to the catalyst, which promotes fast multielectron delivery and colocalization of protons, CO, and catalyst at the source of photoelectrons, and (ii) termination of the QD's ligand shell with free amines, which capture CO as carbamic acid that serves as a reservoir for CO, effectively increasing its solubility in water, and lowers the onset potential for catalytic CO reduction by the Co-porphyrin. The breakthrough efficiency achieved in this work represents a nonincremental step in the realization of reaction networks for direct solar-to-fuel conversion.