Norbert Pfänder

Formation Mechanism of Silver Nanoparticles Stabilized in Glassy Matrices

In any given matrix control over the final particle size distribution requires a constitutive understanding of the mechanisms and kinetics of the particle evolution. In this contribution we report on the formation mechanism of silver nanoparticles embedded in a soda-lime silicate glass matrix. For the silver ion-exchanged glass it is shown that at temperatures below 410 °C only molecular clusters (diameter <1 nm) are forming which are most likely silver dimers. These clusters grow to nanoparticles (diameter >1 nm) by annealing above this threshold temperature of 410 °C. It is evidenced that the growth and thus the final silver nanoparticle size are determined by matrix-assisted reduction mechanisms. As a consequence, particle growth proceeds after the initial formation of stable clusters by addition of silver monomers which diffuse from the glass matrix. This is in contrast to the widely accepted concept of particle growth in metal-glass systems, in which it is assumed that the nanoparticle formation is predominantly governed by Ostwald ripening processes.

Infrared active phonons in single-walled carbon nanotubes

Abstract We present IR reflectance spectra of samples containing single-walled carbon nanotubes and compare them with measurements made on graphite. Weak structures near the graphite modes but shifted towards higher frequencies were found. We assign these structures to IR-active phonons of single-walled nanotubes. Our experimental results suggest that while roughly in agreement with the experiment, further and more accurate calculations of the phonon frequencies of single-walled carbon nanotubes are necessary.

Structural and catalytic properties of sodium and cesium exchanged X and Y zeolites, and germanium-substituted X zeolite

Abstract The conversion of isopropanol in a fixed bed flow reactor was used as a test reaction for a number of faujasite-type zeolites which were modified in order to increase their basicity. Samples included a CsY zeolite with an intact faujasite structure and an exchange degree of nearly 100% prepared by solid-state ion exchange, a CsNaY obtained from CsY through exchange with aqueous NaCl solution, a CsNaX obtained from NaX and aqueous CsCl solution, and a Na(Ge)X, with Si replaced by Ge. At 623 K, an isopropanol partial pressure of 5 kPa in He, and a total feed flow of 90 ml/min, a catalyst mass of 50 mg, initial yields were as follows: NaY: 62% propene, CsNaY: 78% propene, NaCsX: 10% propene, 0.37% acetone, Na(Ge)X: 8% propene, 11% acetone. Conversion in the presence of CsY was 2 adsorption and infrared (IR) spectroscopy were used to probe basicity, and Na(Ge)X was the only sample to form monodentate carbonates upon CO 2 adsorption (bands at 1477 and 1428 cm −1 ). Further characterization with X-ray diffraction (XRD), transmission electron microscopy (TEM), thermal analysis, nitrogen sorption, and isopropanol sorption was necessary to properly interpret catalytic results by identifying samples which contained impurities, had blocked pore systems, or decomposed partially during activation or reaction.

Comprehensive evaluation of in vitro toxicity of three large-scale produced carbon nanotubes on human Jurkat T cells and a comparison to crocidolite asbestos

Abstract This study has evaluated the effects of three industrially relevant multi-walled carbon nanotubes (MWNTs) on human Jurkat T cells and compared them to those of crocidolite asbestos. No overt acute toxicity was observed for all MWNTs tested although signs of oxidative stress were evident. MWNTs did not activate resting Jurkat cells and only slightly stimulated the release of the cytokine interleukin-2 (IL-2) in activated cells. Similar to MWNTs, crocidolite had little toxic effects on Jurkat cells but neither induced the formation of reactive oxygen species nor changes in IL-2 signaling. These findings suggest that, in contrast to many other cell types, T cells are relatively resistant to stress induced by high-aspect ratio particles.

Structural and chemical characterization of N-doped nanocarbons

Abstract Fullerene blacks with 0.7–4.5 at% of nitrogen incorporated into their carbon framework were produced by feeding N2 gas into the center of a carbon arc through a hollow graphite electrode. The formation of toluene-soluble fullerenes is effectively suppressed under these conditions. Optical emission spectroscopy of the arc plasma revealed the presence of N atoms and CN radicals. Chemical composition and stability of the N-doped fullerene blacks were examined and compared to their N-free counterparts produced in pure helium atmosphere by X-ray photoelectron spectroscopy (XPS), electron energy loss spectroscopy and temperature-programmed oxidation with mass-spectrometric detection of the product gases. The nitrogen content was not diminished when N-doped fullerene black was heated to 500 °C in an in situ XPS experiment. Graphitic onions could be formed from n-doped fullerene black by intense electron irradiation in the electron microscope without loss of the nitrogen content. Structural and morphological properties were studied by powder X-ray diffraction, high-resolution transmission electron microscopy and the determination of BET (Brunauen Emme Teller) specific surface areas. The performance of N-doped and undoped fullerene blacks in the removal of ionic contaminants from water was examined in a dynamic adsorption scheme using diammonium chromate(VI) as a test compound.

Reaction of NO with carbonaceous materials:: III. Influence of the structure of the materials

Abstract Four carbonaceous materials: carbon black, activated carbon, fullerene black and graphite, were employed in the NO–carbon reaction in order to study its structure sensitivity. The carbon structures were characterized by TEM and XRD analysis. The results obtained indicate that the internal structure of the samples employed greatly affects the reactivity of the samples towards NO. The reaction of NO with graphite shows a higher reaction rate per unit surface area than the reaction with the three amorphous carbons. The differences in reactivity between graphite and the amorphous carbons point to the presence of two kinds of active sites attributed to defective carbon sites and the carbon sites at the basal planes, respectively. The relative concentration of the different active sites depends on the structure of carbonaceous materials. The results obtained strongly suggest that the reaction between NO and carbon is a structure sensitive gas–solid reaction.

Synthesis and characterization of the titanium doped nanostructural V2O5

Abstract Using scanning and analytical transmission electron microscopies (TEM), the morphology and structure of nanostructurally assembled V 2 O 5 doped with Ti has been studied. It was found that the bulk structure of the oxide particles crystallized in rod-like shape is of the V 2 O 5 type whereas Ti atoms are located mainly on the thin surface layer of the rods. Such surface coating is nonuniform and contains up to 3 at.% of titanium. Modification of the oxide sample with titanium atoms seems to stabilize the V 2 O 5 structure against electron beam irradiation.

Bismuth Hexagons: Facile Mass Synthesis, Stability and Applications

A unique direct electrodeposition technique involving very high current densities, high voltages and high electrolyte concentrations is applied for highly selective mass synthesis of stable, isolable, surfactant-free, single-crystalline Bi hexagons on a Cu wire at room temperature. A formation mechanism of the hexagons is proposed. The morphology, phase purity, and crystallinity of the material are well characterized by FESEM, AFM, TEM, SAED, EDX, XRD, and Raman spectroscopy. The thermal stability of the material under intense electron beam and intense laser light irradiation is studied. The chemical stability of elemental Bi in nitric acid shows different dissolution rates for different morphologies. This effect enables a second way for the selective fabrication of Bi hexagons. Bi hexagons can be oxidized exclusively to α-Bi(2)O(3) hexagons. The Bi hexagons are found to be promising for thermoelectric applications. They are also catalytically active, inducing the reduction of 4-nitrophenol to 4-aminophenol. This electrodeposition methodology has also been demonstrated to be applicable for synthesis of bismuth-based bimetallic hybrid composites for advanced applications.

Intermediate product regulation in tandem solid catalysts with multimodal porosity for high-yield synthetic fuel production

Tandem catalysis is an attractive strategy to intensify chemical technologies. However, simultaneous control over the individual and concerted catalyst performances poses a challenge. We demonstrate that enhanced pore transport within a Co/Al2 O3 Fischer-Tropsch (FT) catalyst with hierarchical porosity enables its tandem integration with a Pt/ZSM-5 zeolitic hydrotreating catalyst in a spatially distant fashion that allows for catalyst-specific temperature adjustment. Nevertheless, this system resembles the case of close active-site proximity by mitigating secondary reactions of primary FT α-olefin products. This approach enables the combination of in situ dewaxing with a minimum production of gaseous hydrocarbons (18 wt %) and an up to twofold higher (50 wt %) selectivity to middle distillates compared to tandem pairs based on benchmark mesoporous FT catalysts. An overall 80 % selectivity to liquid hydrocarbons from syngas is attained in one step, attesting to the potential of this strategy for increasing the carbon efficiency in intensified gas-to-liquid technologies.

Microstructural properties of CVD-grown CuGaSe2 based thin film solar cells

Abstract Microstructural aspects of interfaces involved in CuGaSe 2 (CGSe) based thin film solar cells have been investigated. High resolution transmission electron microscopy and scanning energy dispersive X-ray techniques have been employed for the analysis of complete solar cell cross-sections, revealing details at nanometer scale of the soda lime glass/Mo/MoSe 2 /CGSe/CdS/i:ZnO/Ga:ZnO heterostructure making up the complete devices.