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Sol-gel-derived aerogels are three-dimensional, nanoscale materials that combine large surface areas and high porosities. These traits make them useful for any rate-critical chemical process, particularly sensing or electrochemical applications, once physical or chemical moieties are incorporated into the gels to add their functionality into the ultraporous scaffold. Incorporating biomolecules into aerogels has been challenging due to the inability of most biomolecules to remain structurally intact within the gels during the necessary supercritical fluid processing. However, the heme protein cytochrome c (cyt. c) forms self-organized superstructures around gold (or silver) nanoparticles in buffer that can be encapsulated within silica and processed to form aerogels in which cyt. c retains its characteristic visible absorption. The gold (or silver) nanoparticle-nucleated superstructures protect the majority of the protein from the harsh physicochemical conditions necessary to form an aerogel. The Au∼cyt. c superstructures exhibit rapid gas-phase recognition of nitric oxide (NO) within the aerogel matrix, as facilitated by the high-quality pore structure of the aerogel, and remain viable for weeks at room temperature.

The self-limiting redox reaction of carbon nanofoam substrates with permanganate at room temperature in neutral-pH solutions produces conformal nanoscale MnO2 deposits throughout the macroscopic thickness (∼0.17 mm) of the nanofoam structure. The nanoscale MnO2 morphology ranges from ∼10 nm layered ribbons and rods for a 4 h deposition to ∼20 nm polycrystalline nanoparticles that form at long deposition times (20 h). The through-connected pore network of the carbon nanofoam is maintained at all deposition times (5 mm to 20 h), although the average pore size shifts to smaller values and the cumulative pore volume decreases as the MnO2 coatings grow and thicken within the nanofoam structure. The electrochemical capacitance of the resulting hybrid electrode Structure 15 dominated by the pseudocapacitance of the MnO2 and increases with MnO2 loading (a function of the exposure time in permanganate), particularly at low charge-discharge rates and at ac frequencies <0.1 Hz. The significant enhancement in mass-, volume-, and footprint-nonnalized capacitance at high MnO2 mass loadings is accompanied by a modest increase in the Warburg resistance that develops as the pore size and void volume of the nanofoam substrate are reduced by internal MnO2 deposition [ABSTRACT FROM AUTHOR]

• TiO 2 aerogels consistently stabilize low-valent Cu due to Ti3+ content. • Low-temp CO oxidation activity improves when low-valent Cu is stabilized by aerogels. • Cu/aerogel catalysts do not require pre-treatment prior to CO oxidation reaction. • Low-valent Cu persists in Cu/aerogel catalyst in practical operating conditions. We exploit interfacial charge transfer from titania (TiO 2) to copper (Cu) to design catalytic Cu/TiO 2 composite aerogels that shift the chemical state of Cu nanoparticles away from Cu2+, making them highly active for low-temperature CO oxidation. The high degree of interfacial contact between ∼2–3 nm–diameter Cu particles and the networked ∼10 nm–diameter TiO 2 particles in ultraporous aerogel stabilizes a high ratio of Cu0/1+:Cu2+. The reduced nature of Cu in Cu/TiO 2 aerogels is evidenced by a strong surface plasmon resonance in its diffuse reflectance UV–vis spectrum, by its X-ray photoelectron spectral features, and by infrared spectroscopic evidence of CO binding at the catalyst surface. In contrast, when larger diameter (∼50–60 nm), non–networked TiO 2 particles are used to support Cu nanoparticles, the single planar nanoscale interface between Cu and the support particle stabilizes a much lower fraction of low-valent Cu. The Cu0/1+ speciation stabilized within the aerogel catalyzes low-temperature CO oxidation (<100 °C) at high conversion rates and does not necessitate high-temperature activation in a reducing gas stream—performance that the non-networked catalyst cannot meet. Our work demonstrates how nanoscale interfacial materials design can be exploited to create active Cu nanoparticle–based catalysts that are stable under practical conditions. [ABSTRACT FROM AUTHOR]

We use plasmonic Au-TiO2 aerogels as a platform in which to marry synthetically thickened particle-particle junctions in TiO2 aerogel networks to Au∥TiO2 interfaces and then investigate their cooperative influence on photocatalytic hydrogen (H2) generation under both broadband (i.e., UV + visible light) and visible-only excitation. In doing so, we elucidate the dual functions that incorporated Au can play as a water reduction cocatalyst and as a plasmonic sensitizer. We also photodeposit non-plasmonic Pt cocatalyst nanoparticles into our composite aerogels in order to leverage the catalytic water-reducing abilities of Pt. This Au-TiO2/Pt arrangement in three dimensions effectively utilizes conduction-band electrons injected into the TiO2 aerogel network upon exciting the Au SPR at the Au∥TiO2 interface. The extensive nanostructured high surface-area oxide network in the aerogel provides a matrix that spatially separates yet electrochemically connects plasmonic nanoparticle sensitizers and metal nanoparticle catalysts, further enhancing solar-fuels photochemistry. We compare the photocatalytic rates of H2 generation with and without Pt cocatalysts added to Au-TiO2 aerogels and demonstrate electrochemical linkage of the SPR-generated carriers at the Au∥TiO2 interfaces to downfield Pt nanoparticle cocatalysts. Finally, we investigate visible light-stimulated generation of conduction band electrons in Au-TiO2 and TiO2 aerogels using ultrafast visible pump/IR probe spectroscopy. Substantially more electrons are produced at Au-TiO2 aerogels due to the incorporated SPR-active Au nanoparticle, whereas the smaller population of electrons generated at Au-free TiO2 aerogels likely originate at shallow traps in the high surface-area mesoporous aerogel. [ABSTRACT FROM AUTHOR]

Abstract: Wrapping a monoparticulate layer of 2–3-nm RuO2 around the fibers comprising porous SiO2 filter paper produces a conductive nanoshell in which ∼90% of the RuO2 units are surface-sited, creating the equivalent of a supported single-unit layer of the oxide. The room temperature electronic conductivity, normalized for the dimensions of the total RuO2(SiO2) object, increases with calcination temperature reaching a maximum of 830mScm−1 for a 200°C-calcined paper, and then sharply decreases at higher temperatures as the nanoparticles in the ultrathin RuO2 shell ripen and disrupt the connectivity. The calcination temperature also influences the electrochemical capacitance, which is optimized at 150°C with a RuO2-normalized specific capacitance of 850Fg−1. Calcining the RuO2(SiO2) papers to temperatures ⩾250°C reduces the electrochemical capacitance due to structural ordering and ripening of the RuO2 nanoparticles composing the coating. The electrochemical capacitance and the magnitude of the electronic conductivity of the RuO2(SiO2) paper are unaffected by exposure to air, humidified air, Ar, humidified Ar, or methanol-saturated Ar at 25°C. Exposing the papers at room temperature to either pure H2 or humidified H2 significantly reduces the pseudocapacitance and electronic conductivity. X-ray photoelectron spectroscopy confirms reduction of the RuO2 to Ru and scanning electron microscopy demonstrates shrinkage-induced stress cracking and disruption of the H2-reacted nanoscale coating. These results indicate that the RuO2(SiO2) architecture can serve as a rugged, inexpensive, and conductive gas-diffusion scaffold for the design of a carbon- and ionomer-free anode for direct methanol fuel cells. [Copyright &y& Elsevier]