Aqueous Photogeneration of H2 with CdSe Nanocrystals and Nickel Catalysts: Electron Transfer Dynamics.

CdSe quantum dots (QDs) and simple aqueous Ni(2+) salts in the presence of a sacrificial electron donor form a highly efficient, active, and robust system for photochemical reduction of protons to molecular hydrogen in water. Using ultrafast transient absorption (TA) spectroscopy, the electron transfer (ET) processes from the QDs to the Ni catalysts have been characterized. CdSe QDs transfer photoexcited electrons to a Ni-dihydrolipoic acid (Ni-DHLA) catalyst complex extremely fast and with high efficiency: the amplitude-weighted average ET lifetime is 69 ± 2 ps, and ∼90% of the ultrafast TA signal is assigned to ET processes. The impacts of Auger recombination, QD size and shelling on ET are also reported. These results help clarify the reasons for the exceptional photocatalytic H2 activity of the CdSe QD/Ni-DHLA system and suggest direction for further improvements of the system.