Visible-light photoconversion of carbon dioxide into organic acids in an aqueous solution of carbon dots.

Carbon "quantum" dots (or carbon dots) have emerged as a new class of optical nanomaterials. Beyond the widely reported bright fluorescence emissions in carbon dots, their excellent photoinduced redox properties that resemble those found in conventional semiconductor nanostructures are equally valuable, with photon-electron conversion applications from photovoltaics to CO2 photocatalytic reduction. In this work we used gold-doped carbon dots from controlled synthesis as water-soluble catalysts for a closer examination of the visible-light photoconversion of CO2 into small organic acids, including acetic acid (for which the reduction requires many more electrons than that for formic acid) and, more interestingly, for the significantly enhanced photoconversion with higher CO2 pressures over an aqueous solution of the photocatalysts. The results demonstrate the nanoscale semiconductor-equivalent nature of carbon dots, with excellent potential in energy conversion applications.

Efficient fluorescence quenching in carbon dots by surface-doped metals--disruption of excited state redox processes and mechanistic implications.

The carbon dots in this study were small carbon nanoparticles with the particle surface functionalized by oligomeric poly(ethylene glycol) diamine molecules. Upon photoexcitation, the brightly fluorescent carbon dots in aqueous solution served the function of excellent electron donors to reduce platinum(IV) and gold(III) compounds into their corresponding metals to be deposited on the dot surface. The deposited metals even in very small amounts were found to have dramatic quenching effects on the fluorescence emission intensities, but essentially no effects on the observed fluorescence decays. The obviously exclusive near-neighbor static quenching could be attributed to the disruption of electron-hole radiative recombinations (otherwise responsible for the fluorescence emissions in carbon dots). The results provide important evidence for the availability of photogenerated electrons that could be harvested for productive purposes, which in turn supports the current mechanistic framework on fluorescence emission and photoinduced redox properties of carbon dots.

Carbon nanoparticles as visible-light photocatalysts for efficient CO2 conversion and beyond.

Increasing atmospheric CO(2) levels have generated much concern, driving the ongoing carbon sequestration effort. A compelling CO(2) sequestration option is its photocatalytic conversion to hydrocarbons, for which the use of solar irradiation represents an ultimate solution. Here we report a new strategy of using surface-functionalized small carbon nanoparticles to harvest visible photons for subsequent charge separation on the particle surface in order to drive the efficient photocatalytic process. The aqueous solubility of the catalysts enables photoreduction under more desirable homogeneous reaction conditions. Beyond CO(2) conversion, the nanoscale carbon-based photocatalysts are also useful for the photogeneration of H(2) from water under similar conditions.

Carbon nanoparticles as chromophores for photon harvesting and photoconversion.

Carbon nanomaterials have generated a tremendous amount of attention in the scientific community. While most of the research and development efforts have been on fullerenes, carbon nanotubes, and graphene sheets, carbon nanoparticles (which are often considered as impurities or unwanted complications in the other carbon nanomaterials) have recently emerged as a unique class of highly fluorescent nano-dots. However, little or no attention has been paid to potential uses of carbon nanoparticles as chromophores in photochemical reactions or for photon harvesting and photoconversion in general. In the study reported herein we demonstrate the chromophore-equivalent functions of aqueous-suspended small carbon nanoparticles in harvesting visible photons for the reductive coating of the nanoparticles with silver and gold and, as a result, the preparation of unique carbon-noble-metal core-shell nanostructures.

Aluminum nanoparticles capped by polymerization of alkyl-substituted epoxides: ratio-dependent stability and particle size.

We report here on the polymerization of epoxide monomers on incipient aluminum nanoparticle cores and the effects of changing the epoxide-capping precursor and the metallic monomer ratio on the resultant stability and particle size of passivated and capped aluminum nanoparticles. When altering the ratio of aluminum to cap monomer precursor, nanoparticles capped with epoxydodecane, epoxyhexane, and epoxyisobutane show a clear decreasing trend in stability with decreasing alkane substituent length. The nanoparticle core size was unaffected by cap ratio or composition. PXRD (powder X-ray diffraction) and DSC/TGA (differential scanning calorimetry/thermal gravimetric analysis) confirm the presence of successfully passivated face-centered cubic (fcc) aluminum nanoparticles. We also report preliminary results from ATR-FTIR (attenuated total reflectance-Fourier transform infrared), (13)C CPMAS (cross-polarization/magic-angle spinning), and (27)Al MAS solid-state NMR (nuclear magnetic resonance) measurements. The most stable aluminum nanoparticle-polyether core-shell nanoparticles are found at an Al:monomer mole ratio of 10:1 with an active Al(0) content of 94%.

Synthesis and characterization of srilankite nanowires.

We describe the synthesis and characterization of srilankite (Ti2ZrO6) nanowires. The nanowires are produced via hydrothermal synthesis with a TiO2/ZrO2 mixture under alkaline conditions. The zirconium titanate nanowires have median diameters of 60 nm and median lengths of 800 nm with the (022) axis along the length of the nanowire. Electron microscopy, energy dispersive X-ray spectroscopy, powder X-ray diffraction, and electron diffraction are used to characterize the phases and compare nanowires produced with varying molar ratios of Ti and Zr. Electron diffraction patterns produced from single nanowires show highly crystalline nanowires displaying a compositional-ordering superlattice structure with Zr concentrated in bands within the crystal structure. This is in contrast to naturally occurring bulk srilankite where Zr and Ti are randomly substituted within the crystal lattice. Streaking is observed in the electron diffraction patterns suggesting short-range ordering within the superlattice structure.

Carbon quantum dots and applications in photocatalytic energy conversion.

Quantum dots (QDs) generally refer to nanoscale particles of conventional semiconductors that are subject to the quantum-confinement effect, though other nanomaterials of similar optical and redox properties are also named as QDs even in the absence of strictly defined quantum confinement. Among such nanomaterials that have attracted tremendous recent interest are carbon dots, which are small carbon nanoparticles with some form of surface passivation, and graphene quantum dots in various configurations. In this article, we highlight these carbon-based QDs by focusing on their syntheses, on their photoexcited state properties and redox processes, and on their applications as photocatalysts in visible-light carbon dioxide reduction and in water-splitting, as well as on their mechanistic similarities and differences.

A threshold-based approach to calorimetry in helium droplets: measurement of binding energies of water clusters.

Helium droplet beam methods have emerged as a versatile technique that can be used to assemble a wide variety of atomic and molecular clusters. We have developed a method to measure the binding energies of clusters assembled in helium droplets by determining the minimum droplet sizes required to assemble and detect selected clusters in the spectrum of the doped droplet beam. The differences in the droplet sizes required between the various multimers are then used to estimate the incremental binding energies. We have applied this method to measure the binding energies of cyclic water clusters from the dimer to the tetramer. We obtain measured values of D(0) that are in agreement with theoretical estimates to within ∼20%. Our results suggest that this threshold-based approach should be generally applicable using either mass spectrometry or optical spectroscopy techniques for detection, provided that the clusters selected for study are at least as strongly bound as those of water, and that a peak in the overall spectrum of the beam corresponding only to the cluster chosen (at least in the vicinity of the threshold) can be located.

Spontaneous hydrogen generation from organic-capped Al nanoparticles and water.

The development of technologies that would lead toward the adoption of a hydrogen economy requires readily available, safe, and environmentally friendly access to hydrogen. This can be achieved using the aluminum-water reaction; however, the protective nature and stability of aluminum oxide is a clear detriment to its application. Here, we demonstrate the spontaneous generation of hydrogen gas from ordinary room-temperature tap water when combined with aluminum-oleic acid core-shell nanoparticles obtained via sonochemistry. The reaction is found to be near-complete (>95% yield hydrogen) with a tunable rate from 6.4x10(-4) to 0.01 g of H2/s/g of Al and to yield an environmentally benign byproduct. The potential of these nanoparticles as a source of hydrogen gas for power generation is demonstrated using a simple fuel cell with an applied load.

Capping and passivation of aluminum nanoparticles using alkyl-substituted epoxides.

We report here on the synthesis and passivation of small (20-30 nm) aluminum nanoparticles using alkyl-substituted epoxides as capping agents. FTIR and 13C NMR spectroscopy indicate that the epoxides polymerize to form a polyether cap on the surfaces of the aluminum nanoparticles. Nanoparticles capped with epoxyhexane and epoxydodecane are stable in air, but particles capped with epoxyisobutane are pyrophoric. TEM images show spherical Al particles. Powder X-ray diffraction shows the presence of crystalline Al. Titrimetric analysis of the core-shell nanostructures in air reveals that 96% of the total aluminum present is active (unoxidized) aluminum.

Formation and properties of stabilized aluminum nanoparticles.

The wet-chemical synthesis of aluminum nanoparticles was investigated systematically by using dimethylethylamine alane and 1-methylpyrrolidine alane as precursors and molecules with one or a pair of carboxylic acid groups as surface passivation agents. Dimethylethylamine alane was more reactive, capable of yielding well-defined and dispersed aluminum nanoparticles. 1-Methylpyrrolidine alane was less reactive and more complex in the catalytic decomposition reaction, for which various experimental parameters and conditions were used and evaluated. The results suggested that the passivation agent played dual roles of trapping aluminum particles to keep them nanoscale during the alane decomposition and protecting the aluminum nanoparticles postproduction from surface oxidation and that an appropriate balance between the rate of alane decomposition (depending more sensitively on the reaction temperature) and the timing in the introduction of the passivation agent into the reaction mixture was critical to the desired product mixes and/or morphologies. Some fundamental and technical issues on the alane decomposition and the protection of the resulting aluminum nanoparticles are discussed.

Formation of protein-metal oxide nanostructures by the sonochemical method: observation of nanofibers and nanoneedles.

The sonochemical reaction of iron pentacarbonyl is explored in water and in water with the protein BSA (bovine serum albumen). In water, the reaction is found to produce spherical nanoparticles of magnetite (Fe3O4) with a particle size distribution of <10 to approximately 60 nm. In water with BSA, the reaction produces either nanofibers or nanoneedles, depending on the concentration of BSA. The nanofiber and nanoneedle samples are found to be mixtures of goethite, lepidocrocite, and hematite (alpha-FeOOH, gamma-FeOOH, and alpha-Fe2O3, respectively). The sonochemical reaction of iron pentacarbonyl with BSA in water is thought to proceed through the thermal decomposition mechanism for iron pentacarbonyl with BSA acting as a templating agent.