Simulation-Aided Design and Synthesis of Hierarchically Porous Membranes

Free-standing silica membranes with hierarchical porosity (ca. 300 nm macropores surrounded by 6–8 nm mesopores) and controllable mesopore architecture were prepared by a dual-templating method, with the structural design aided by mesoscale simulation. To create a two-dimensional, hexagonal macropore array, polymeric colloidal hemisphere arrays were synthesized by a two-step annealing process starting with non-close-packed polystyrene sphere arrays on silicon coated with a sacrificial alumina layer. A silica precursor containing a poly­(ethylene) oxide–poly­(propylene oxide)–poly­(ethylene) oxide (PEO–PPO–PEO) triblock-copolymer surfactant as template for mesopore creation was spin-coated onto the support and aged and then converted into the free-standing membranes by dissolving both templates and the alumina layer. To test the hypothesis that the mesopore architecture may be influenced by confinement of the surfactant-containing precursor solution in the colloidal array and by its interactions with the polymeric colloids, the system was studied theoretically by dissipative particle dynamics (DPD) simulations and experimentally by examining the pore structures of silica membranes via electron microscopy. The DPD simulations demonstrated that, while only tilted columnar structure can be formed through tuning the interaction with the substrate, perfect alignment of 2D hexagonal micelles perpendicular to the plane of the membrane is achievable by confinement between parallel walls that interact preferentially with the hydrophilic components (PEO blocks, silicate, and solvent). The simulations predicted that this alignment could be maintained across a span of up to 10 columns of micelles, the same length scale defined by the colloidal array. In the actual membranes, we manipulated the mesopore alignment by tuning the solvent polarity relative to the polar surface characteristics of the colloidal hemispheres. With methanol as a solvent, columnar mesopores parallel to the substrate were observed; with a methanol–water mixed solvent, individual spherical mesopores were present; and with water as the only solvent, twisted columnar structures were seen

Charge Transfer Dynamics between Photoexcited CdS Nanorods and Mononuclear Ru Water-Oxidation Catalysts

We describe the charge transfer interactions between photoexcited CdS nanorods and mononuclear water oxidation catalysts derived from the [Ru­(bpy)­(tpy)­Cl]⁺ parent structure. Upon excitation, hole transfer from CdS oxidizes the catalyst (Ru²⁺ → Ru³⁺) on a 100 ps to 1 ns timescale. This is followed by 10–100 ns electron transfer (ET) that reduces the Ru³⁺ center. The relatively slow ET dynamics may provide opportunities for the accumulation of multiple holes at the catalyst, which is necessary for water oxidation

(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

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

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

Photocatalytic Regeneration of Nicotinamide Cofactors by Quantum Dot–Enzyme Biohybrid Complexes

We report the characterization of biohybrid complexes of CdSe quantum dots and ferredoxin NADP⁺-reductase for photocatalytic regeneration of NADPH. Illumination with visible light led to reduction of NADP⁺ to NADPH, with an apparent <i>k</i>_cat of 1400 h^–1. Regeneration of NADPH was coupled to reduction of aldehydes to alcohols catalyzed by a NADPH-dependent alcohol dehydrogenase, with each NADPH molecule recycled an average of 7.5 times. The quantum yield both of NADPH and alcohol production were 5–6% for both products. Light-driven NADPH regeneration was also demonstrated in a multienzyme system, showing the capacity of QD-FNR complexes to drive continuous NADPH-dependent transformations

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

Electron Transfer Kinetics in CdS Nanorod–[FeFe]-Hydrogenase Complexes and Implications for Photochemical H₂ Generation

This Article describes the electron transfer (ET) kinetics in complexes of CdS nanorods (CdS NRs) and [FeFe]-hydrogenase I from Clostridium acetobutylicum (CaI). In the presence of an electron donor, these complexes produce H₂ photochemically with quantum yields of up to 20%. Kinetics of ET from CdS NRs to CaI play a critical role in the overall photochemical reactivity, as the quantum efficiency of ET defines the upper limit on the quantum yield of H₂ generation. We investigated the competitiveness of ET with the electron relaxation pathways in CdS NRs by directly measuring the rate and quantum efficiency of ET from photoexcited CdS NRs to CaI using transient absorption spectroscopy. This technique is uniquely suited to decouple CdS→CaI ET from the processes occurring in the enzyme during H₂ production. We found that the ET rate constant (<i>k</i>_ET) and the electron relaxation rate constant in CdS NRs (<i>k</i>_CdS) were comparable, with values of 10⁷ s^–1, resulting in a quantum efficiency of ET of 42% for complexes with the average CaI:CdS NR molar ratio of 1:1. Given the direct competition between the two processes that occur with similar rates, we propose that gains in efficiencies of H₂ production could be achieved by increasing <i>k</i>_ET and/or decreasing <i>k</i>_CdS through structural modifications of the nanocrystals. When catalytically inactive forms of CaI were used in CdS–CaI complexes, ET behavior was akin to that observed with active CaI, demonstrating that electron injection occurs at a distal iron–sulfur cluster and is followed by transport through a series of accessory iron–sulfur clusters to the active site of CaI. Using insights from this time-resolved spectroscopic study, we discuss the intricate kinetic pathways involved in photochemical H₂ generation in CdS–CaI complexes, and we examine how the relationship between the electron injection rate and the other kinetic processes relates to the overall H₂ production efficiency

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