A. Burian

Diamond nanoparticles to carbon onions transformation: X-ray diffraction studies

Abstract Carbon onions prepared by high temperature annealing of ultradispersed diamond nanoparticles of about 5 nm in average diameter have been studied by X-ray diffraction using synchrotron radiation. The X-ray diffraction patterns show transformation of the diamond nanoparticles with sp 3 bonds into spherical carbon onions containing remaining diamond-like core and then into polyhedral onions with facets on their outer part and pure sp 2 graphitic bonds. The prepared onions form concentric-shell particles which comprise of about ten shells with an intershell distance of 0.35–0.36 nm. The large intershell distance suggests a considerable reduction in intershell interaction when compared to perfect graphite. The X-ray data are related to the previously performed studies by electron energy-loss spectroscopy and electron spin resonance.

Paracrystalline structure of activated carbons

Structural studies by means of neutron diffraction of activated carbons, prepared from a polymer of phenol formaldehyde resin by carbonization and activation processes, with variable porosity, are presented. The neutron scattering data were recorded over the range of the scattering vector Q from 2.5 to 500 nm-1. The structure of activated carbons has been described in terms of disordered graphite-like layers with very weak interlayer correlations. The model has been generated by computer simulations and its validity has been tested by comparison of the experimental and calculated intensity functions. Modelling studies have shown that the model containing 3-4 layers each about 2 nm in diameter accounts for the experimental data and that graphite layers are randomly translated and rotated, according to the turbostratic structure. Near-neighbour carbon-carbon distances of about 0.139 nm and 0.154 nm have been determined. The Debye-Waller factor exp (-Q2σ2/2) with σ = σ0(r)1/2 suggests a paracrystalline structure within a single layer. The value of the interlayer spacing of 0.36 nm has been found from paracrystalline simulations of the layer arrangement in the c-axis direction. The high quality of the experimental data has enabled determination of the coordination numbers, the interatomic distances and their standard deviations using a curve-fitting procedure over the Q-range from 250 nm to 500 nm, providing structural information about short- and intermediate-range ordering.

Structural studies of disordered carbons by high-energy X-ray diffraction

X-ray diffraction measurements were carried out on three samples of disordered, commercially produced carbons, AX21, CXV and BP71, on the ID15B beam-line at the European Synchrotron Radiation Facility (ESRF), Grenoble. Intensity data were converted to pair correlation functions via the Fourier transform. The results obtained show that the structure of the studied samples consists of one–four graphite-like layers, stacked without spatial correlations. The size of the ordered regions is in the range of 9–16 Å. The atomic arrangement within an individual layer can be described in terms of the paracrystalline ordering, in which lattice distortions propagate proportionally to the square root of interatomic distances. The paracrystalline structure was simulated by introducing the Stone–Wales defects (pair of two pentagons and two heptagons), randomly distributed in the network. The resulting structures were relaxed using the reactive empirical bond order potential for carbon–carbon interaction and the Lennard-Jones potential with parameters for interlayer interactions. Such defects lead to curvature of individual layers.

Structural modeling of dahlia-type single-walled carbon nanohorn aggregates by molecular dynamics.

The structure of dahlia-type single-walled carbon nanohorn aggregates has been modeled by classical molecular dynamics simulations, and the validity of the model has been verified by neutron diffraction. Computer-generated models consisted of an outer part formed from single-walled carbon nanohorns with diameters of 20-50 Å and a length of 400 Å and an inner turbostratic graphite-like core with a diameter of 130 Å. The diffracted intensity and the pair correlation function computed for such a constructed model are in good agreement with the neutron diffraction experimental data. The proposed turbostratic inner core explains the occurrence of the additional (002) and (004) graphitic peaks in the diffraction pattern of the studied sample and provides information about the interior structure of the dahlia-type aggregates.

Structural studies of activated carbons by pulsed neutron diffraction

Pulsed neutron diffraction results are presented for a series of activated carbons with a variable microporosity. The results cover a wide Q-range extending up to 50 A-1 and enable the pair correlation function to be evaluated with good spatial resolution. The diffraction pattern shows a broad oscillatory behaviour representative of a highly defective crystal structure and differs considerably from graphite. Detailed investigation shows that the local atomic structure is similar to that of graphite and exhibits the expected hexagonal network but that the first peak arising from the carbon-carbon bond has two components, indicating a quinoidal distortion of the network. The correlation length within the graphene sheet is short ranged and the inter-layer spacing also displays some disorder with an increased spacing compared with graphite. The data provide a good basis for the detailed modelling of the structure and exhibit characteristics that could not have been easily observed by other techniques.

Raman scattering studies of the graphitization process in anthracene- and saccharose-based carbons

Abstract We report Raman scattering studies of the graphitization process in a series of carbons produced by the pyrolysis of saccharose and anthracene and then annealed at 1000, 1900 and 2300°C. Using 488 nm light, the first- and second-order Raman spectra of six samples have been examined in the range 50-4000cm−1. The low-frequency Raman range 50-lOOOcn−1 has been carefully examined using the laser excitation wavelengths 457.2, 476.5, 488.0 and 514.3 nm. A dependence of the Raman spectra in the first- and second-order ranges on annealing temperature and degree and size of ordered domains has been observed and correlated with neutron diffraction observations, indicating almost complete graphitization of the carbon prepared from anthracene and a disordered structure of the saccharose carbon at 2300°C. The occurrence of a sharp peak at about 464cm−1 and broadened peaks in the range 75-210cm−1, which shift with excitation energy as result of the one-dimensional auantum confinement effect of electrons in such structures, provide evidence for the presence of fullerene- and nanotube-like elements in the investigated materials. The presence of curved elements with odd-membered rings at an early stage of preparation and their sizeable increase at higher temperatures distinguish non-graphitizing from graphitizing carbons.

Modelling of glass-like carbon structure and its experimental verification by neutron and X-ray diffraction

Glass-like carbon is a well known carbon form that still poses many challenges for structural characterization owing to a very complex internal atomic organization. Recent research suggests that glassy carbon has a fullerene-related structure that evolves with the synthesis temperature. This article reports on direct evidence of curved planes in glassy carbons using neutron and X-ray diffraction measurements and their analysis in real space using the atomic pair distribution function formalism. Changes in the structure including the degree of curvature of the non-graphitizing glassy carbons as a function of the pyrolysis temperature in the range 800–2500°C (1073–2773 K) are studied using optimized models of the atomic structure. Averaged models of single coherent scattering domains as well as larger structural fragments consisting of thousands of atoms were relaxed using classical molecular dynamics. For such models the diffraction intensities and the pair distribution functions were computed. The compatibility of the computer-generated models was verified by comparison of the simulations with the experimental diffraction data in both reciprocal and real spaces. On the basis of features of the developed structural models for glass-like carbons, the origin of the properties such as high strength and hardness and low gas permeability can be better understood.

Conversion of Natural Tannin to Hydrothermal and Graphene-Like Carbons Studied by Wide-Angle X-ray Scattering

The atomic structure of carbon materials prepared from natural tannin by two different techniques, high-temperature pyrolysis and low-temperature hydrothermal carbonization, was studied by wide-angle X-ray scattering. The obtained diffraction data were converted to the real space representation in the form of pair distribution functions. The X-ray photoelectron spectroscopy measurements provided information about the chemical state of carbon in tannin-based materials that was used to construct final structural models of the investigated samples. The results of the experimental data in both reciprocal and real spaces were compared with computer simulations based on the PM7 semiempirical quantum chemical method. Using the collected detailed information, structural models of the tannin-based carbons were proposed. The characteristics of the investigated materials at the atomic level were discussed in relation to their preparation method. The rearrangement of the tannin molecular structure and its transformation to graphene-like structure was described. The structure of tannin-based carbons pyrolyzed at 900 °C exhibited coherently scattering domains about 20 Å in size, consisting of two defected atomic layers and resembling a graphene-like arrangement.

The atomic scale structure of CXV carbon: wide-angle x-ray scattering and modeling studies.

The disordered structure of commercially available CXV activated carbon produced from finely powdered wood-based carbon has been studied using the wide-angle x-ray scattering technique, molecular dynamics and density functional theory simulations. The x-ray scattering data has been converted to the real space representation in the form of the pair correlation function via the Fourier transform. Geometry optimizations using classical molecular dynamics based on the reactive empirical bond order potential and density functional theory at the B3LYP/6-31g* level have been performed to generate nanoscale models of CXV carbon consistent with the experimental data. The final model of the structure comprises four chain-like and buckled graphitic layers containing a small percentage of four-fold coordinated atoms (sp(3) defects) in each layer. The presence of non-hexagonal rings in the atomic arrangement has been also considered.

Structural studies of amorphous In–Se by EXAFS

Amorphous In–Se films containing 50, 60 and 66 at.% Se, prepared by vacuum evaporation, have been studied by extended X-ray absorption fine structure (EXAFS) at the In K-edge. A least-squares fitting procedure has been used to derive structural information about the In environment. The results indicate that In is tetrahedrally coordinated and for In50Se50 each In atom has one In nearest neighbor at 2.75 A and three Se nearest neighbors at 2.63 A. The number of In–In bonds decreases with increasing Se content.