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+, E0 = −446 mV vs. NHE) or propyl-bridged 2-2′-bipyridinium (PDQ2+, E0 = −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 PDQ2+. Femtosecond to microsecond transient absorption reveals that while electron transfer (ET) from the DIR to PDQ2+ 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 PDQ2+ yields measurable H2 production, demonstrating the importance of optimizing the electron shuttling pathway to take advantage of the available reducing power of the DIR excited state. H2 production in the PDQ2+ system is highly efficient, with an internal quantum efficiency (IQE) as high as 77% and a TONSHI of 1.1 × 106 under mild (RT, pH = 7.35) conditions.