Characterization of Photochemical Processes for H2 Production by CdS Nanorod–[FeFe] Hydrogenase Complexes

We have developed complexes of CdS nanorods capped with 3-mercaptopropionic acid (MPA) and Clostridium acetobutylicum [FeFe]-hydrogenase I (CaI) that photocatalyze reduction of H+ to H2 at a CaI turnover frequency of 380–900 s–1 and photon conversion efficiencies of up to 20% under illumination at 405 nm. In this paper, we focus on the compositional and mechanistic aspects of CdS:CaI complexes that control the photochemical conversion of solar energy into H2. Self-assembly of CdS with CaI was driven by electrostatics, demonstrated as the inhibition of ferredoxin-mediated H2 evolution by CaI. Production of H2 by CdS:CaI was observed only under illumination and only in the presence of a sacrificial donor. We explored the effects of the CdS:CaI molar ratio, sacrificial donor concentration, and light intensity on photocatalytic H2 production, which were interpreted on the basis of contributions to electron transfer, hole transfer, or rate of photon absorption, respectively. Each parameter was found to have pronounced effects on the CdS:CaI photocatalytic activity. Specifically, we found that under 405 nm light at an intensity equivalent to total AM 1.5 solar flux, H2 production was limited by the rate of photon absorption (∼1 ms–1) and not by the turnover of CaI. Complexes were capable of H2 production for up to 4 h with a total turnover number of 106 before photocatalytic activity was lost. This loss correlated with inactivation of CaI, resulting from the photo-oxidation of the CdS capping ligand MPA.