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Structural Insights into Poly(Heptazine Imides): A Light-Storing Carbon Nitride Material for Dark Photocatalysis
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Microwave Synthesis of Spinel MgFe 2 O 4 Nanoparticles and the Effect of Annealing on Photocatalysis
Through microwave heating in ethanol and with subsequent annealing, crystalline MgFe 2 O 4 nanoparticles are produced rapidly and in high yields >99%. Under varied annealing temperatures, the degree of Mg and Fe site inversion changes the optical, electronic, and composition of the nanoparticles. Here, a small particle size of ~10 nm is achievable with the aid of an ammonium salt mineralizer that caps the particles during nucleation and growth. Particles with the lowest inversion parameter and limited sintering upon annealing (at 600 ºC) exhibit the greatest production of hydroxyl radicals under visible light illumination. As such, these particles also facilitate the degradation of methylene blue dye faster than those particles annealed at higher temperature and show a rate constant of 0.061 h –1 for degrading 10 ppm methylene blue with 10 mg catalyst.
Toward Metal-free Photocatalysis: Photochemical Regeneration of Organic Hydride Donors Using Phenazine-Based Photosensitizers
Earth-abundant chromophores and catalysts are important molecular building blocks for artificial photosynthesis applications. Our team previously reported that metal-free hydride donors, such as biomimetic benzoimidazole-based motifs, can reduce CO 2 selectively to the formate ion and that they can be electrochemically regenerated using the proton-coupled mechanism. To enable direct utilization of solar energy, we report here the photochemical regeneration of a benzoimidazole-based hydride donor using a phenazine-based metal-free chromophore. The photochemical regeneration was investigated under different experimental conditions involving varying sacrificial donors, proton donors, solvents, and component concentrations. The best hydride regeneration yield of 50% was obtained with phenol as a proton source and thiophenolate as a sacrificial electron donor. The mechanism of photochemical regeneration was studied using steady-state and time-resolved UV/Vis spectroscopies. Based on the results of these studies, we hypothesize that the initial photoinduced electron transfer from photoexcited phenazine chromophores involves the benzoimidazole cation and that this process is likely coupled with proton transfer to generate protonated benzoimidazole-based radical cation. The second photoinduced electron transfer is hypothesized to generate the hydride form. Furthermore, our findings provide the requisite information for the future development of reductive photocatalysts for solar energy and light-harvesting applications utilizing earth-abundant metal-free materials.
Strong Potential Gradients and Electron Confinement in ZnO Nanoparticle Films: Implications for Charge-Carrier Transport and Photocatalysis
We report that zinc oxide (ZnO) nanomaterials are promising components for chemical and biological sensors and photocatalytic conversion and operate as electron collectors in photovoltaic technologies. Many of these applications involve nanostructures in contact with liquids or exposed to ambient atmosphere. Under these conditions, single-crystal ZnO surfaces are known to form narrow electron accumulation layers with few nanometer spatial penetration into the bulk. A key question is to what extent such pronounced surface potential gradients can develop in the nanophases of ZnO, where they would dominate the catalytic activity by modulating charge-carrier mobility and lifetimes. Here, we follow the temperature-dependent surface electronic structure of nanoporous ZnO with photoemission spectroscopy to reveal a sizable, spatially averaged downward band bending for the hydroxylated state and a conservative upper bound of < 6 nm for the spatial extent of the associated potential gradient. This nanoscale confinement of conduction-band electrons to the nanoparticle film surface is crucial for a microscopic understanding and further optimization of charge transport and photocatalytic function in complex ZnO nanomaterials.
Generalizable Organic-to-Aqueous Phase Transfer of a Au 18 Nanocluster with Luminescence Enhancement and Robust Photocatalysis in Water
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Cooperative Stereoinduction in Asymmetric Photocatalysis
Stereoinduction in complex organic reactions often involves the influence of multiple stereocontrol elements. The interaction among these can often result in the observation of significant cooperative effects that afford different rates and selectivities between the matched and mismatched sets of stereodifferentiating chiral elements. The elucidation of matched/mismatched effects in ground-state chemical reactions was a critically important theme in the maturation of modern stereocontrolled synthesis. The development of robust methods for the control of photochemical reactions, however, is a relatively recent development, and similar cooperative stereocontrolling effects in excited-state enantioselective photoreactions have not previously been documented. Herein, we describe a tandem chiral photocatalyst/Brønsted acid strategy for highly enantioselective [2 + 2] photocycloadditions of vinylpyridines. Importantly, the matched and mismatched chiral catalyst pairs exhibit different reaction rates and enantioselectivities across a range of coupling partners. We observe no evidence of ground-state interactions between the catalysts and conclude that these effects arise from their cooperative behavior in a transient excited-state assembly. Furthermore, these results suggest that similar matched/mismatched effects might be important in other classes of enantioselective dual-catalytic photochemical reactions.
Engineered disorder in CO 2 photocatalysis
Light harvesting, separation of charge carriers, and surface reactions are three fundamental steps that are essential for an efficient photocatalyst. Here we show that these steps in the TiO 2 can be boosted simultaneously by disorder engineering. A solid-state reduction reaction between sodium and TiO 2 forms a core-shell c-TiO 2 @a-TiO 2-x (OH) y heterostructure, comprised of HO-Ti-[O]-Ti surface frustrated Lewis pairs (SFLPs) embedded in an amorphous shell surrounding a crystalline core, which enables a new genre of chemical reactivity. Specifically, these SFLPs heterolytically dissociate dihydrogen at room temperature to form charge-balancing protonated hydroxyl groups and hydrides at unsaturated titanium surface sites, which display high reactivity towards CO 2 reduction. This crystalline-amorphous heterostructure also boosts light absorption, charge carrier separation and transfer to SFLPs, while prolonged carrier lifetimes and photothermal heat generation further enhance reactivity. The collective results of this study motivate a general approach for catalytically generating sustainable chemicals and fuels through engineered disorder in heterogeneous CO 2 photocatalysts.