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 (MV 2+ , 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 ∼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 MV 2+ , 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 TON SHI of 1.1 × 10 6 under mild (RT, pH = 7.35) conditions.