Revathi Bacsa

Specific surface area of carbon nanotubes and bundles of carbon nanotubes

The theoretical external specific surface area of single- and multi-walled carbon nanotubes and of carbon nanotube bundles is calculated as a function of their characteristics (diameter, number of walls, number of nanotubes in a bundle). The results are reported in diagrams and tables useful to correlate the microscopic characteristics and the specific surface area of samples. The calculated values are in good agreement with the microscopic characteristics and the specific surface area measurements which have been previously reported in the literature. The specific surface area is a macroscopic parameter which can be helpful to adjust the synthesis conditions of carbon nanotubes.

High specific surface area carbon nanotubes from catalytic chemical vapor deposition process

A carbon nanotube specimen with a carbon content of 83 wt.% (95 vol.%) and a specific surface area equal to 790 m2/g (corresponding to 948 m2/g of carbon) is prepared by a catalytic chemical vapor deposition method. The nanotubes, 90% of which are single- and double-walled, are individual rather than in bundles. High-resolution electron microscopy shows a diameter distribution in the range 0.8–5 nm and Raman spectroscopy shows a high proportion of tubular carbon. Both techniques reveal a maximum in the inner wall diameter distribution close to 1.2 nm.

CCVD synthesis of carbon nanotubes from (Mg, Co, Mo)O catalysts: influence of the proportions of cobalt and molybdenum

Carbon nanotubes have been synthesised by catalytic chemical vapour deposition of a H2–CH4 mixture (18 mol% CH4) over (Mg,Co,Mo)O catalysts. The total amount of cobalt and molybdenum has been kept constant at 1 cat% and the proportion of molybdenum with respect to cobalt has been varied from x(Mo) = 0.25–1.0. This variation has important effects on both the yield and the nature (number of walls, straight walls or bamboo-like structures) of the carbon nanotubes. It also has an influence on the purity of the samples (amount of encapsulated metal particles, presence or not of amorphous carbon deposits). For x = 0.25, the nanotubes were mainly double- and triple-walled (inner diameter less than 3 nm); samples prepared from catalysts with higher molybdenum ratios contained larger multi-walled carbon nanotubes (inner diameter up to 9 nm), having up to 13 concentric walls. It is proposed that different growth mechanisms may occur depending on the initial composition of the catalyst.

Theoretical and Experimental Studies on the Carbon‐Nanotube Surface Oxidation by Nitric Acid: Interplay between Functionalization and Vacancy Enlargement

The nitric acid oxidation of multiwalled carbon nanotubes leading to surface carboxylic groups has been investigated both experimentally and theoretically. The experimental results show that such a reaction involves the initial rapid formation of carbonyl groups, which are then transformed into phenol or carboxylic groups. At room temperature, this reaction takes place on the most reactive carbon atoms. At higher temperatures a different mechanism would operate, as evidenced by the difference in activation energies. Experimental data can be partially related to first-principles calculations, showing a multistep functionalization mechanism. The theoretical aspects of the present article have led us to propose the most efficient pathway leading to carboxylic acid functional groups on the surface. Starting from mono-vacancies, it ends up with the synergistic formation of dangling -COOH groups and the enlargement of the vacancies.

Chirality of internal metallic and semiconducting carbon nanotubes

We have assigned the chirality of the internal tubes of double walled carbon nanotubes grown by catalytic chemical vapor deposition using the high sensitivity of the radial breathing ~RB! mode in inelastic lightscattering experiments. The deduced chirality corresponds to several semiconducting and only two metallic internal tubes. The RB modes are systematically shifted to higher energies when compared to theoretical values. The difference between experimental and theoretical energies of the RB modes of metallic tubes and semiconducting tubes are discussed in terms of the reduced interlayer distance between the internal and the external tube and electronic resonance effects. We find several pairs of RB modes corresponding to different diameters of internal and external tubes.

Synergistic effect between few layer graphene and carbon nanotube supports for palladium catalyzing electrochemical oxidation of alcohols

Abstract Few layer graphene (FLG), multi-walled carbon nanotubes (CNTs) and a nanotube-graphene composite (CNT-FLG) were used as supports for palladium nanoparticles. The catalysts, which were characterized by transmission electron microscopy, Raman spectroscopy and X-ray diffraction, were used as anodes in the electrooxidation of ethanol, ethylene glycol and glycerol in half cells and in passive direct ethanol fuel cells. Upon Pd deposition, a stronger interaction was found to occur between the metal and the nanotube-graphene composite and the particle size was significantly smaller in this material (6.3 nm), comparing with nanotubes and graphene alone (8 and 8.4 nm, respectively). Cyclic voltammetry experiments conducted with Pd/CNT, Pd/FLG and Pd/CNT-FLG in 10 wt% ethanol and 2 M KOH solution, showed high specific currents of 1.48,2.29 and 2.51 mA·µg−1Pd, respectively. Moreover, the results obtained for ethylene glycol and glycerol oxidation highlighted the excellent electrocatalytic activity of Pd/CNT-FLG in terms of peak current density (up to 3.70 mA·µg−1Pd for ethylene glycol and 1.84 mA·μg−1Pd for glycerol, respectively). Accordingly, Pd/CNT-FLG can be considered as the best performing one among the electrocatalysts ever reported for ethylene glycol oxidation, especially considering the low metal loading used in this work. Direct ethanol fuel cells at room temperature were studied by obtaining power density curves and undertaking galvanostatic experiments. The power density outputs using Pd/CNT, Pd/FLG and Pd/CNT-FLG were 12.1, 16.3 and 18.4 mW·cm−2, respectively. A remarkable activity for ethanol electrooxidation was shown by Pd/CNT-FLG anode catalyst. In a constant current experiment, the direct ethanol fuel cell containing Pd/CNT-FLG could continuously deliver 20 mA·cm−2 for 9.5 h during the conversion of ethanol into acetate of 30%, and the energy released from the cell was about 574 J.

Spectroscopic detection of carbon nanotube interaction with amphiphilic molecules in epoxy resin composites

Incorporation of carbon nanotubes into epoxy resin composites has the effect of increasing electrical conductivity at low percolation levels. An amphiphilic molecule such as palmitic acid has been used to increase the surface contact area and to improve the dispersion of the carbon nanotube bundles in the prepolymer. The chemical environment of the dispersed nanotubes has been probed using vibrational Raman spectroscopy. Spectroscopic Raman maps on sample surfaces (60×60μm2) with ratios of nanotubes to palmitic acid varying from 1:2 to 2:1 by weight, have been recorded to test the uniformity of the dispersion. Substantial spatial inhomogeneities have been observed in the G-band shift and an additional spectral band at 1450cm−1. The 1450cm−1 band has been attributed to the CH3 group of the amphiphilic molecules adsorbed onto the nanotube surface. The maps are correlated with the measured electrical conductivity values. The highest conductivity has been observed for the best dispersed nanotubes and nanotube...

Oxidized few layer graphene and graphite as metal-free catalysts for aqueous sulfide oxidation

Few layer graphene and natural graphite were chemically modified by oxidation with HNO3 at 80 and 140 °C and used to promote the oxidation of sulfide ions in aqueous solutions. TEM, potentiometric titration, XRD, BET and Raman spectroscopy show that HNO3 treatment even at 140 °C did not modify the graphene and graphite morphologies but produced significant amounts of different oxygen surface groups. The presence of these groups on few layer graphene and graphite strongly increased the Saq2− oxidation, showing activities comparable to a high surface area activated carbon with a similar amount of oxygen surface groups. The obtained results suggest that the sulfide oxidation efficiency requires a balance between two important effects, i.e. the presence of oxygen functionalities to initiate the Saq2− oxidation and the electrical conductivity that is important to further transfer the electrons removed from sulfide.

Effect of the Carbon Support on the Catalytic Activity of Ruthenium‐Magnetite Catalysts for p‐Chloronitrobenzene Hydrogenation

Few‐layer graphene, carbon nanotubes, and N‐doped carbon nanotubes have been used to support Ru and/or Fe3O4 nanoparticles. The materials were prepared by a new single‐step sustainable method that involves: 1) a simple procedure, 2) the use of propanol as a solvent, 3) short reaction times, 4) an efficient and energy‐saving heat source, and 5) the absence of any high‐boiling‐point stabilizing agent or organic surfactant. The supported nanoparticles were characterized by TEM, XRD, inductively coupled plasma optical emission spectroscopy, and SQUID magnetometer analyses and employed in the selective hydrogenation of p‐chloronitrobenzene to p‐chloroaniline with very promising results. The hydrogenation runs evidence the marked influence of the diverse carbon nanostructures on the reaction rate, and highlights the promising activity of nanocatalysts supported on few‐layer graphene.

Laser Induced Modifications of Carbon Nanotube Composite Surfaces

Carbon nanotubes epoxy composites have been processed with high power laser pulses and micro-machined with a single crystal diamond tool. The effect of the dispersion of the carbon nanotubes (CNTs, 0.4 wt %) in the epoxy resin and carbon nanotube interaction with the composite matrix have been probed using spectroscopic Raman mapping. While the micro-machined surface maintains a good electrical conductivity after machining, the surface is poorly conductive after laser ablation. Laser processing (power 150 J/pulse, 1064 nm) transforms the surface of the carbon nanotube nanocomposite up to a distance of 25 µm. AFM images show that the diamond machined surface reduces the composite surface roughness.