Ana M. Benito

Sensitivity of single wall carbon nanotubes to oxidative processing: structural modification, intercalation and functionalisation

Abstract The effect of oxidation on modification of single wall carbon nanotubes (SWCNTs) through successive purification steps has been studied. The efficient elimination of metal impurities has been followed by induced coupled plasma spectroscopy. Upon acid treatment, Raman spectroscopy clearly proofed that HNO3 molecules were intercalated into the bundles of SWCNTs. At the same time, SWCNTs also have suffered a high degree of degradation and defects were introduced. The subsequent thermal processes led to the removal of further defect carbon materials and to the almost complete de-intercalation of the HNO3 molecules. Changes in the structure of the SWCNT bundles have been observed by transmission electron microscopy. While bundles tend to separate upon acid treatment, after the complete purification process, the remaining SWCNTs tend to form thick bundles again. The existence of functional groups in the raw single wall carbon nanotubes material and their modification and almost complete removal after the final annealing step has been studied by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and temperature programmed desorption. Nitrogen adsorption isotherms analysed according to Brunauer–Emmet–Teller showed important changes in the pore volume and surface area through the purification steps.

Pyrolytically grown BxCyNz nanomaterials: nanofibres and nanotubes

Pyrolysis of CH3CN · BCl3 at ca. 900–1000°C over Co powder generates novel graphitic BxCyNz nanofibres and nanotubes possessing a range of morphologies (e.g. curled, branched and bent). In these experiments the metal particles play an important role in the growth since nanotube formation appears to occur at the metal surface. High resolution electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) studies suggest that the stoichiometry of the filaments is ca. [BC2Nz]n (z = 0.3−0.6). Higher relative concentrations of carbon occur at the tips where the metal particle is found. A possible growth mechanism for these new materials is proposed.

Production of high-density single-walled nanotube material by a simple laser-ablation method

Abstract A continuous-wave 250 W CO2-laser operating at 10.6 μm has been employed to evaporate graphite/bi-metal targets in a vertical evaporation chamber. Without the help of an additional furnace, web-like soot material has been easily produced. This contains high densities of bundles of single-walled nanotubes (SWNTs). Electron microscopy, Raman spectroscopy and neutron diffraction show the high quality of the SWNT material. The use of this simple laser-ablation system offers additional possibilities to study experimental parameters important for the formation of SWNTs leading to a better understanding of its growth mechanism.

Synthesis of a new polyaniline/nanotube composite: “in-situ” polymerisation and charge transfer through site-selective interaction

A new polyaniline/multi-wall carbon nanotube (PANI/MWNT) composite has been successfully synthesized by an “in-situ” polymerisation process; Raman studies indicate a site-selective interaction between the quinoid ring of the polymer and the MWNTs opening the way for charge transfer processes; transport measurements clearly reveal drastic changes in the electronic behaviour confirming the formation of a true composite material with enhanced electronic properties.

Single-Walled Carbon Nanotubes as Electrodes in Supercapacitors

7 pages.-- PACS: 82.47.Uv; 82.45.Fk; 85.35.Kt; 68.43.Mn; 82.45.Yz; 82.45.Gj; 81.40.Gh; 78.30.Na

Synthesis and characterization of new polyaniline/nanotube composites

Abstract New polyaniline/nanotube (PANI/NT) composites have been synthesized by “in situ” polymerization processes using both multi-wall carbon nanotubes (MWNTs) and single-wall carbon nanotubes (SWNTs) in concentrations ranging from 2 to 50 wt.%. Although no structural changes are observed using MWNTs above a concentration of 20 wt.%, the in situ synthesis results in electronic interactions between nanotubes and the quinoid ring of PANI leading to enhanced electronic properties and thus to the formation of a genuine PANI/MWNT composite material. On the other hand, using SWNTs favors the formation of inhomogeneous mixtures rather than of a homogeneous composite materials, independent of the SWNT concentration. X-ray diffraction, Raman and transport measurements show the different behavior of both classes of nanotubes in PANI/NT materials. The difficulties in the formation of a true PANI/SWNT composite are related to the far more complex structure of the SWNT material itself, i.e. to the presence of entangled bundles of SWNTs, amorphous carbon and even catalytic metal particles.

Modifications of single-wall carbon nanotubes upon oxidative purification treatments

A systematic characterization of single-wall carbon nanotube (SWCNT) material after successive purification steps, including reflux treatment with nitric acid, air oxidation, and annealing, has been performed. Inductively coupled plasma–optical emission spectroscopy shows that a considerable reduction of the metal impurities by up to 95% can be obtained by the nitric acid reflux treatment. During this process, Raman spectroscopy clearly proves that HNO3 molecules are intercalated into the bundles of SWCNTs. At the same time, SWCNTs have suffered a high degree of degradation and defects are being introduced. The subsequent thermal processes lead to the removal of further defect carbon materials and to the almost complete de-intercalation of the HNO3 molecules. Transmission electron microscopy reveals that the remaining SWCNT bundles tend to form thick bundles. Thus the applied purification process results in a high-purity SWCNT material with a drastically reduced content of metal nanoparticles and composed of large bundles of SWCNTs.

Carbon nanotubes production by catalytic pyrolysis of benzene

Abstract Pyrolysis of benzene at ca 600–900 °C over Ni powder generated different types of carbon nanostructures possessing a wide range of morphologies. The effects of temperature and time on carbon nanotubes growth were evaluated. The deposited carbon yield was measured, and the quality of the nanotubes was analyzed by transmission electron microscopy (TEM).

A soluble and highly functional polyaniline–carbon nanotube composite

A completely soluble polyaniline-multi-wall carbon nanotube (CNT–PANI) composite with drastically enhanced conductivity, improved thermal stability, and luminescent behaviour, has been synthesized. The presence of straight multi-wall carbon nanotubes during the polymerization of aniline induces the formation of a more planar conformation of polyaniline which acts as coating layer for the carbon nanotubes and leads to favourable interaction between the constituents. The polyaniline-coated multi-wall carbon nanotubes align into bundles and form a three-dimensional network in the overall composite. A highly functional carbon nanotube composite completely soluble in n-methylpyrrolidinone (NMP) exhibiting all the favourable processing and transformation possibilities of PANI has been obtained. These findings have important consequences for practical technological applications, especially for the development of opto-electronic devices.

Thermal cracking of coal residues: Kinetics of asphaltene decomposition

Abstract An asphaltenic residue from a synthetic crude obtained by coal liquefaction was processed by thermal cracking. The kinetics of formation of oil + gas and coke (toluene-insoluble) from conversion of the asphaltenic fraction were determined. A three-lump model is proposed which considers parallel reactions for oil + gas and coke formation. Conversion data fitted second-order kinetics throughout for asphaltene conversion and oil + gas and coke formation. Delplot analysis indicated that under the most severe experimental conditions, asphaltenes and oils participated in secondary coke-forming reactions which are not included in the model. The activation energies ranged from 63.94 to 86.13 kJ mol −1 , the highest being that for asphaltene conversion. The highest asphaltene conversion, 55.9 wt%, was obtained at 475°C and 40 min reaction time. At short reaction times, the asphaltenic fraction converted to coke (∼30 wt%) was much less than that converted to oil (∼70 wt%) at the four temperatures used, whereas at long reaction times this trend was reversed, especially at 475°C. Structural analysis showed lower aromaticity and higher H/C and N/C ratios in the oil from the products than from the feed.