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

Abstract

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 TON_SHI of 1.1 × 10⁶ under mild (RT, pH = 7.35) conditions.