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

(Ga_1–<i>x</i>Zn_<i>x</i>)(N_1–<i>x</i>O_<i>x</i>) Nanocrystals: Visible Absorbers with Tunable Composition and Absorption Spectra

Bulk oxy­(nitride) (Ga_1–<i>x</i>Zn_<i>x</i>)­(N_1–<i>x</i>O_<i>x</i>) is a promising photocatalyst for water splitting under visible illumination. To realize its solar harvesting potential, it is desirable to minimize its band gap through synthetic control of the value of <i>x</i>. Furthermore, improved photochemical quantum yields may be achievable with nanocrystalline forms of this material. We report the synthesis, structural, and optical characterization of nanocrystals of (Ga_1–<i>x</i>Zn_<i>x</i>)­(N_1–<i>x</i>O_<i>x</i>) with the values of <i>x</i> tunable from 0.30 to 0.87. Band gaps decreased from 2.7 to 2.2 eV over this composition range, which corresponded to a 260% increase in the fraction of solar photons that could be absorbed by the material. We achieved nanoscale morphology and compositional control by employing mixtures of ZnGa₂O₄ and ZnO nanocrystals as synthetic precursors that could be converted to (Ga_1–<i>x</i>Zn_<i>x</i>)­(N_1–<i>x</i>O_<i>x</i>) under NH₃. The high quality of the resulting nanocrystals is encouraging for achieving photochemical water-splitting rates that are competitive with internal carrier recombination pathways

Relationships between Exciton Dissociation and Slow Recombination within ZnSe/CdS and CdSe/CdS Dot-in-Rod Heterostructures

Type-II and quasi type-II heterostructure nanocrystals are known to exhibit extended excited-state lifetimes compared to their single material counterparts because of reduced wave function overlap between the electron and hole. However, due to fast and efficient hole trapping and nonuniform morphologies, the photophysics of dot-in-rod heterostructures are more rich and complex than this simple picture. Using transient absorption spectroscopy, we observe that the behavior of electrons in the CdS “rod” or “bulb” regions of nonuniform ZnSe/CdS and CdSe/CdS dot-in-rods is similar regardless of the “dot” material, which supports previous work demonstrating that hole trapping and particle morphology drive electron dynamics. Furthermore, we show that the longest lived state in these dot-in-rods is not generated by the type-II or quasi type-II band alignment between the dot and the rod, but rather by electron–hole dissociation that occurs due to fast hole trapping in the CdS rod and electron localization to the bulb. We propose that specific variations in particle morphology and surface chemistry determine the mechanism and efficiency of charge separation and recombination in these nanostructures, and therefore impact their excited-state dynamics to a greater extent than the heterostructure energy level alignment alone

Role of Surface-Capping Ligands in Photoexcited Electron Transfer between CdS Nanorods and [FeFe] Hydrogenase and the Subsequent H₂ Generation

Complexes of CdS nanorods and [FeFe] hydrogenase I from Clostridium acetobutylicum have been shown to photochemically produce H₂. This study examines the role of the ligands that passivate the nanocrystal surfaces in the electron transfer from photoexcited CdS to hydrogenase and the H₂ generation that follows. We functionalized CdS nanorods with a series of mercaptocarboxylate surface-capping ligands of varying lengths and measured their photoexcited electron relaxation by transient absorption (TA) spectroscopy before and after hydrogenase adsorption. Rate constants for electron transfer from the nanocrystals to the enzyme, extracted by modeling of TA kinetics, decrease exponentially with ligand length, suggesting that the ligand layer acts as a barrier to charge transfer and controls the degree of electronic coupling. Relative light-driven H₂ production efficiencies follow the relative quantum efficiencies of electron transfer, revealing the critical role of surface-capping ligands in determining the photochemical activity of these nanocrystal–enzyme complexes. Our results suggest that the H₂ production in this system could be maximized with a choice of a surface-capping ligand that decreases the distance between the nanocrystal surface and the electron injection site of the enzyme

Activation Thermodynamics and H/D Kinetic Isotope Effect of the H_ox to H_redH⁺ Transition in [FeFe] Hydrogenase

Molecular complexes between CdSe nanocrystals and Clostridium acetobutylicum [FeFe] hydrogenase I (CaI) enabled light-driven control of electron transfer for spectroscopic detection of redox intermediates during catalytic proton reduction. Here we address the route of electron transfer from CdSe→CaI and activation thermodynamics of the initial step of proton reduction in CaI. The electron paramagnetic spectroscopy of illuminated CdSe:CaI showed how the CaI accessory FeS cluster chain (F-clusters) functions in electron transfer with CdSe. The H_ox→H_redH⁺ reduction step measured by Fourier-transform infrared spectroscopy showed an enthalpy of activation of 19 kJ mol^–1 and a ∼2.5-fold kinetic isotope effect. Overall, these results support electron injection from CdSe into CaI involving F-clusters, and that the H_ox→H_redH⁺ step of catalytic proton reduction in CaI proceeds by a proton-dependent process

Solvents Effects on Charge Transfer from Quantum Dots

To predict and understand the performance of nanodevices in different environments, the influence of the solvent must be explicitly understood. In this Communication, this important but largely unexplored question is addressed through a comparison of quantum dot charge transfer processes occurring in both liquid phase and in vacuum. By comparing solution phase transient absorption spectroscopy and gas-phase photoelectron spectroscopy, we show that hexane, a common nonpolar solvent for quantum dots, has negligible influence on charge transfer dynamics. Our experimental results, supported by insights from theory, indicate that the reorganization energy of nonpolar solvents plays a minimal role in the energy landscape of charge transfer in quantum dot devices. Thus, this study demonstrates that measurements conducted in nonpolar solvents can indeed provide insight into nanodevice performance in a wide variety of environments