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Abstract: Here we describe synthesis of very unusual multi-walled carbon nanotubes through a catalytic chemical vapor deposition method using a floating reactant method and subsequent thermal treatment up to 2600°C in a large quantity. Main characteristics of these nanotubes are (1) relatively wide distribution of diameters ranging from 20 to 70nm and linear, long macro-morphology (aspect ratio >100), (2) highly straight and crystalline layers, (3) high purity through removal of metallic impurity, (4) very low interlayer spacing (0.3385nm) and low R value (I D/I G =0.0717), (5) high G′ intensity over intensity of G band (G′/G=0.85) and strongly negative magnetoresistance value of −1.08% at 77K and 1T. The unusual microstructure of thin multi-walled carbon nanotubes with a partially facetted cross-sectional shape caused by thermal treatment is mainly ascribed to abrupt density changes (from 1.89 to 2.1g/cm3) within a confined nanosized space, accompanying with the phase separation. [Copyright &y& Elsevier]

Systematic studies of structural changes in stacked cup carbon nanofibers by heat treatment from 1800 to 3000 °C are carried out. The most prominent feature upon heat treatment of these nanofibers is the formation of energetically stable loops between adjacent active end planes both on the inner and outer surfaces. The appearance of the jagged outer and inner surfaces at 3000 °C is due to a combinational effect, perhaps caused by improved stacking within domains connected by loops having limited crystallite size, accompanied by structural reorganization between domains. Consequently, a low degree of graphitizability is ascribed to this unusual stacked cup morphology of pristine carbon nanofibers. [Copyright &y& Elsevier]

Electron paramagnetic resonance (EPR) study of air-physisorbed defective carbon nano-onions evidences in favor of microwave assisted formation of weakly-bound paramagnetic complexes comprising negatively-charged O 2 − ions and edge carbon atoms carrying π-electronic spins. These complexes being located on the graphene edges are stable at low temperatures but irreversibly dissociate at temperatures above 50–60 K. These EPR findings are justified by density functional theory (DFT) calculations demonstrating transfer of an electron from the zigzag edge of graphene-like material to oxygen molecule physisorbed on the graphene sheet edge. This charge transfer causes changing the spin state of the adsorbed oxygen molecule from S = 1 to S = 1/2 one. DFT calculations show significant changes of adsorption energy of oxygen molecule and robustness of the charge transfer to variations of the graphene-like substrate morphology (flat and corrugated mono- and bi-layered graphene) as well as edges’ passivation. The presence of H- and COOH- terminated edge carbon sites with such a corrugated substrate morphology allows formation of ZE–O 2 − paramagnetic complexes characterized by small (<50 meV) binding energies and also explains their irreversible dissociation as revealed by EPR. [ABSTRACT FROM AUTHOR]

Abstract: π-Electronic edge-localized spin magnetism in defective carbon nano-onions (DCNOs) derived from detonation nanodiamonds was investigated in terms of molecular oxygen adsorption and microwave (MW) irradiation effect. At low MW power (P MW ⩽200μW) electron paramagnetic resonance (EPR) spectra of air-contained DCNO samples show progressive broadening of the g =2.0014(2) edge-spin signal on temperature decrease. Below 140K, the edge-spin signal becomes unobservable and the only weak narrow g =2.0022(1) signal ascribed to sp 3-like defects is observed. Upon higher power (P MW >20mW) MW-irradiation at T <60K, that weak signal demonstrates unusual enhancement in peak/integral intensities and line broadening accompanied with upward deviations from the conventional saturation behavior. On abrupt reduction of P MW to low (P MW <1mW) levels, a new strong narrow signal with the same g =2.0022(1) emerges. Below 60K the intensity of the strong signal shows non-Curie–Weiss behavior. This EPR signal is attributed to new magnetic entities originating from antiferromagnetically coupled dimers of edge-spin and negatively charged O2 − molecule. The O2 − molecules, formed by charge transfer from π-electronic reservoir of underlaid nano-graphene sheet, appear in the vicinity of nano-graphene edges in the process of desorption/adsorption of oxygen molecules caused by strong MW-irradiation and subsequent abrupt reduction of P MW. [Copyright &y& Elsevier]

Abstract: Compositional changes of the residual iron compounds in isothermally treated catalytic chemical vapor deposition derived multi-walled carbon nanotubes have been monitored using 57Fe transmission Mössbauer spectroscopy and X-ray fluorescence. The iron phases entrapped in the as-synthesized carbon nanotubes consist of γ-iron, α-iron, Fe3C and Fe1−x S. The Fe1−x S phase decomposes completely around 1500°C while the iron carbide phase decomposes in the temperature range of 1500–2400°C. The obtained apparent activation energy of ca. 76kcal/mol suggests that the entrapped iron was removed via a diffusion process during thermal treatment. [Copyright &y& Elsevier]

Abstract: Systematic isothermal annealing has been carried out on catalytic chemical vapor deposition-derived highly disordered multi-walled carbon nanotubes in order to understand the structural evolution of carbon nanotubes kinetically. After specified isothermal time the crystallographic parameters strongly depend on annealing temperature, and can be divided into three stages. Rapid structural enhancement in the range from 1800 to 2200°C originates from the abrupt evolution of the residual catalytic compounds and is seen as partially aligned small fringes along the tube length. The lack of further structural ordering at temperatures above 2400°C and the low activation energy (ca. 90kJ/mol) are thought to be the result of the limited space available for further structural development considering the nano-sized multi-walled carbon nanotubes. [Copyright &y& Elsevier]

Abstract: The time evolution of graphitization was analyzed based on the structural and magnetic properties of nanodiamond samples annealed at 1600°C for various time intervals. High resolution TEM and XRD show that the nanodiamond particles are converted to spherical onions for short annealing time intervals, and then they are completely transformed to polyhedral nanographite through the annealing for 120min. The in-plane orbital susceptibility χ orb (300K) analyzed on the basis of Kotosonov’s equation remains fairly constant in the range −5.5×10−6 to −5.9×10−6 emu/g, suggesting that the coherent scattering region (CSR) and the Fermi energy are independent of the annealing times. It also indicates the presence of serious defects affecting the electronic structure of nanographite. As the annealing time increases from 2 to 120min, the Pauli paramagnetic susceptibility decreases. The broad ESR signal is associated with the spins localized on the edge states of the π-electron graphite network. The ESR linewidth becomes nine times less as the heat-treatment time rises from 2 to 120min. The relatively large linewidth is associated with the scattering of π-electrons by the edge phonon modes and magnetic interaction between the edge-localized spins. During continuous annealing the nanoparticle structure changes from more defective to less defective and the number of edge-localized spins in a particle drops from ca. 2 to ca. 1 despite the CSR size and the Fermi energy remain practically the same. [Copyright &y& Elsevier]

A morphological characterization of activated AR-resin was carried out. The time and temperature effect of the EDLC (electric double layer capacitor) properties of AR-resin were investigated. In order to clarify the relation between the electric double layer capacitance and the ion mobility in the organic solvent, a computer simulation was used to calculate the possible solvation size of Et4NBF4 in propylene carbonate. The sample (HTT 700) activated at a heat treatment temperature (HTT) of 700 °C for 2 h has a specific capacitance as high as 35.27 F g−1 (equivalent to 141.08 F g−1 for a single electrode), when it is charged to 2.5 V and discharged at 1 mA cm−2. The capacitance increase by using a longer activation time at each HTT was as follows: 2.3 times at HTT 600, 1.6 times at HTT 700, 1.3 times at HTT 800 and 1.2 times at HTT 900. This variation could be forecast from image analysis using a TEM photograph. Furthermore, a possible ion size of electrolyte (Et4NBF4) in a solvent was calculated and it is related to the mobility of charge carriers in a micropore. [Copyright &y& Elsevier]