Balancing electron transfer rate and driving force for efficient photocatalytic hydrogen production in CdSe/CdS nanorod-[NiFe] hydrogenase assemblies

We describe a hybrid photocatalytic system for hydrogen production consisting of nanocrystalline CdSe/CdS dot-in-rod (DIR) structures coupled to [NiFe] soluble hydrogenase I (SHI) from Pyrococcus furiosus. Electrons are shuttled to the catalyst by a redox mediator, either methyl viologen (MV2+, E-0 = -446 mV vs. NHE) or propyl-bridged 2-2'-bipyridinium (PDQ(2+), E-0 = -550 mV vs. NHE). We demonstrate nearly equal photoreduction efficiencies for the two mediators, despite extracting similar to 100 mV of additional driving force for proton reduction by PDQ(2+). Femtosecond to microsecond transient absorption reveals that while electron transfer (ET) from the DIR to PDQ(2+) is slower than for MV2+, in both cases the ET process is complete by 1 ns and thus it efficiently outcompetes radiative decay. Long-lived charge separation is observed for both mediators, resulting in similar net efficiencies of photoreduction. Whereas both mediators are readily photoreduced, only PDQ(2+) yields measurable H-2 production, demonstrating the importance of optimizing the electron shuttling pathway to take advantage of the available reducing power of the DIR excited state. H-2 production in the PDQ(2+) system is highly efficient, with an internal quantum efficiency (IQE) as high as 77% and a TONSHI of 1.1 x 10(6) under mild (RT, pH = 7.35) conditions.