59th Annual Report on Research 2014

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Reports: DNI1052423-DNI10: Sol-Gel Assembly of Hollow Metal Particles Into a New Class of Porous Nanostructures and Their Application in Heterogeneous Catalysis

The 2013/2014 fiscal year is the second year of funding for proposed research. The ACS-PRF award is used to support a postdoctoral fellow for 7 months, summer support for one graduate student, one month summer salary for the PI, and to purchase chemicals and supplies. In addition, two graduate, one undergraduate and two high school students and a high school Chemistry teacher (Mr. Rajendra Jaini) were also involved with the proposed research. This award has already made significant impact on PI career and other participants. So far we have published three articles in high impact journals including J. Am. Chem. Soc. and Chem. Mater.  Significant accomplishments achieved over the past two years were presented at 65^th Southeastern Regional Meeting of the American Chemical Society, Atlanta, GA. The objective of proposed research is to investigate the sol-gel assembly of metal colloids into high-surface-area, highly-conducting, aerogel superstructures via oxidation-induced assembly of thiol-stabilized nanoparticles (NPs). To this end, we have recently reported the synthesis of transparent and opaque Ag aerogels via chemical oxidation of glutathione (GSH) stabilized Ag NPs (J. Am. Chem. Soc. 2014, 136, 7993–8002). Precursor Ag colloids with varying size, shell thickness, and faceted structures were produced by fast chemical (NaBH₄) reduction of preformed Ag₂O NPs (Figure 1A). Precursor Ag₂O templates were prepared by the reaction of AgNO₃ with NaOH in the presence of GSH surfactant. For the first time, we have successfully demonstrated the facile tunability of size, shell thickness, and consequently the plasmonic properties (470-570 nm) of hollow Ag NPs by varying the molar ratio of reactants. As-prepared Ag hollows display a progression in color as the plasmonic bands are red-shifted with increasing size and shell thickness. Interestingly, with increasing molar ratio of AgNO₃/glutathione, hexagonal-shaped Ag hollows displaying high-energy facets were produced (Figure 1B inset). In contrast, when the reduction reactions are performed slowly at ambient conditions significantly smaller Ag NPs (3-5 nm) were formed (Figure 1C inset). As-prepared Ag colloids were concentrated by centrifuge filtration and the gelation is induced by introduction of 1% tetranitromethane (50-350 mL). It was found that the direct cross-linking of Ag colloids into hydrogel superstructures relies in part on the kinetics by which the active sites for the assembly become available on the NP surface. The number of active sites generated has been controlled by varying the oxidant/glutathione molar ratio (X), which governs the aggregation kinetics and the ultimate size of the fractal NP clusters produced in the aerogels. Hence, the macroscopic properties (conductivity, transparency and opacity, surface area and porosity) of the aerogel superstructures have been tuned either by varying the size, shape, and concentration of the NP precursor or the X value. When X exceeds the critical value of 7.7, in-situ generated HNO₃ acid byproduct facilitate the oxidative etching of Ag from the fractal NP clusters, reducing their size below the wavelength of visible light yielding transparent Ag aerogels. The degree of transparency in metal monoliths has been successfully tuned by varying the X value and the concentration of the metal sol. Notably, the majority of NPs present in the transparent Ag aerogels are < 1 nm in size (Figure 1C), which ensures the production of significantly smaller clusters (0.1-0.8 mm) at the gel point leading to optical transparency. However, to the best of our knowledge synthesis of such smaller NPs is difficult, which has potentially limited the fabrication of transparent conducting metal monoliths and sol-gel derived thin films. Nonetheless, the innovative sol-gel strategy developed in our lab offers a unique approach to overcome the above issue via transformation of even larger Ag hollows into smaller NPs via progressive dissolution of Ag from corresponding hydrogel superstructures. The resulting Ag hydrogels were dried supercritically to produce monolithic Ag aerogels. The monoliths of Ag exhibit densities as low as 0.04 g/cm³, representing 0.38% of the bulk density of Ag. Energy dispersive spectroscopic studies indicate that the sulfur content is reduced from ~17% to ~3% (NPs vs. aerogels) consistent with the oxidative removal of GSH and direct cross-linking of Ag colloids. As a preliminary result, the resistance found for non-optimized Ag monoliths (~4 mm) is in the range of 5-10 Ohm indicating the highly conducting nature of Ag monoliths. The assembly of Ag colloids has no apparent impact on the structure, crystallinity, and the oxidation state of NPs that make up the gel frameworks. Powder X-ray diffraction patterns and X-ray photoelectron spectra of the Ag monoliths are characteristic of cubic Ag⁽⁰⁾ phase with a decrease in crystallite size owing to oxidative etching of Ag. TEM images of the opaque Ag aerogels consist of an inter-connected network of precursor hollows that are directly cross-linked into fractal clusters with average size in the range of 0.8-2 mm (Figure 1B). The presence of meso-to-macro-pore network with a wide range of pore diameters is clearly visible in the both gel materials. The surface area obtained for opaque Ag monoliths is 160 m²/g while that of transparent Ag monoliths is 182 m²/g. In general, a systematic decrease in surface area with increasing outer diameter and shell thickness of precursor hollows is observed for opaque superstructures. In contrast, the precursor hollow- or nano-particles are significantly less porous and exhibit surface areas in the range of 3.1-7.6 m²/g. The enormously high surface area obtained for opaque Ag aerogels is attributed to low density, highly porous architecture of the aerogel, which provides the superior accessibility of molecules to both inner and outer surfaces of the hollows, offering new perspectives for future catalytic, electrocatalytic, and sensor applications. In the future, we will extend this innovative strategy for direct self-supported assembly of other catalytic metals (Au/Ag, Pt/Ag, and Pd/Ag) into high-surface-area, highly-conducting, transparent and opaque aerogel superstructures. We will utilize GSH-coated Ag colloids as templates for the growth of bi-metallic alloy NPs via galvanic displacement. The de-alloying of Ag by in-situ generated acids will be utilized to produce transparent, conducting superstructures. As-prepared metal aerogels will also be investigated in alcohol oxidation fuel cells and surface enhanced Raman scattering based chemical sensing applications.