Hermann Sauer

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.

Pt/ceria thin film model catalysts after high-temperature reduction: a (HR)TEM study

Well-facetted Pt particles, epitaxially grown on (001) NaCl single crystal surfaces, were supported by thin films of crystalline ceria. Structural and morphological changes due to calcination and reduction have been studied after treating in oxygen (673K, 1h) and hydrogen exposure at increasing temperatures from 673 to 1073K. These changes were followed by (HR)TEM, electron diffraction and EELS, and possible structures were selected with help of density functional calculations. Reduction at 723K causes the transformation of the initially half octahedral Pt particles into cube or platelet shapes with a double lattice periodicity in both HRTEM image and electron diffraction pattern. This can be attributed to the hydrogen-influenced topotactic formation of a Pt3Ce alloy phase of high thermodynamic stability as confirmed by DFT calculations. Further heating of the ceria-based catalyst in hydrogen up to 1073K results in clustering of metal particles on large flat areas of the sintered support with lattice periodicities between 0.8 and 1.2 nm. EELS and X-ray diffraction both confirm partial reduction of ceria and indicate the formation of chemically stable Ce suboxides.

Formation of chains of graphitic nanoparticles by heating fullerene blacks covered with thin metal films

Strongly bent, fibrous carbon nanostructures with outer diameters usually between 35 and 90 nm were generated by the pyrolysis above 800°C of a fullerene black which had been covered with a thin nickel film. Transmission electron microscopy (TEM) and electron energy loss spectrometry (EELS) revealed that the material consists of chains of hollow elongated multi-wall graphitic nanoparticles. The worm-like nanostructures only arose from the nickel-covered parts of the fullerene black as checked with scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDX). Attempts to generate similar structures from commercial carbon blacks, arc-produced fullerene-free carbon blacks and glassy carbon failed. Substituting nickel by cobalt or molybdenum also did not lead to the generation of fibrous structures. Additionally, the pyrolysis-induced changes of metal films on graphite and glassy carbon and the direct interaction of nickel films with fullerene vapour were investigated. A growth model for the nanochains is proposed, which includes the diffusion of carbon through a metal particle, the segregation of carbon and the repeated surface melting and solidification of the metal.

Chains of carbon nanoparticles from the interaction of fullerenes with thin metal films

Different fullerene blacks were produced by the arc-discharge vaporization of graphite in various experimental conditions. Pressed pellets of these materials were sputtered with thin metal films. After the subsequent pyrolysis up to 1200 °C the samples were studied with SEM. Wormlike nanostructures arose from the nickel covered parts of the fullerene containing pellets. TEM and EELS showed that chains of hollow elongated carbon nanostructures were generated with outer diameters usually between 35 and 90 nm. A model for the formation of the chains is proposed considering diffusion of carbon through a metal particle and surface melting of the metal.