Dan Lupu

Comparative study on different carbon nanotube materials in terms of transparent conductive coatings

We compared conductive transparent carbon nanotube coatings on glass substrates made of differently produced single-wall (SWNT), double-wall, and multiwall carbon nanotubes. The airbrushing approach and the vacuum filtration method were utilized for the fabrication of carbon nanotube films. The optoelectronic performance of the carbon nanotube film was found to strongly depend on many effects including the ratio of metallic-to-semiconducting tubes, dispersion, length, diameter, chirality, wall number, structural defects, and the properties of substrates. The electronic transportability and optical properties of the SWNT network can be significantly altered by chemical doping with thionyl chloride. Hall effect measurements revealed that all of these thin carbon nanotube films are of p-type probably due to the acid reflux-based purification and atmospheric impurities. The competition between variable-range hoping and fluctuation-assisted tunneling in the functionized carbon nanotube system could lead to a crossover behavior in the temperature dependence of the network resistance.

Carbon Nanotubes: Synthesis, Properties, and Applications

This brief review presents a comprehensive outline of the present research status on the fast moving carbon nanotube (CNT) field. It covers a short introduction to the relation between carbon nanotubes, graphite, and other forms of carbon and explains in detail the structure of CNTs. The electronic, electrical, and mechanical properties of CNTs, as well as the most widely used methods for CNT production such as electric arc discharge, laser ablation, and chemical vapor deposition (CVD), are discussed. Some of the CNT applications covered in this review article are: field emission, hydrogen storage, carbon nanotube-based solar cells, and CNT composite materials.

Hydrogen uptake by carbon nanofibers catalyzed by palladium

Abstract Carbon nanofibers of herringbone conformation were obtained by chemical vapor deposition on Pd/La2O3 catalyst, from ethylene—hydrogen mixture. After the removal of La2O3, samples with various Pd/C ratios were obtained by oxidation in air. The hydrogen sorption capacities measured gravimetrically at 10 MPa in pure hydrogen, for six different batches of samples, show a good correlation with the Pd/C ratio reveling a catalytic effect of Pd, which supplies atomic H. A possible charge transfer might lead to the increasing of the H uptake with the increasing Pd/C ratio. A saturation value of 1.5% H (mass) per carbon is reached at rather high Pd/C mole ratio (∼1) for nanofibers with 425– 455 m 2 g −1 BET surface area.

Does the wall number of carbon nanotubes matter as conductive transparent material

As electrically conductive and optically transparent thin coating material, double-wall carbon nanotube network was found to have better transparency-conductance performance as compared with single-wall carbon nanotube (SWNT) and multiwall carbon nanotube (MWNT). The electronic transportability and optical properties of the SWNT films can be altered by chemical modification of thionyl chloride. Additionally, the conductance-temperature dependence analysis revealed that variable-range hopping mechanism dominates the conductance of few wall nanotube mats while fluctuation-assisted tunneling plays a more important role in that of MWNT films.

Surface area and thermal stability effect of the MgO supported catalysts for the synthesis of carbon nanotubes

Surface area (SA) of the oxide support was found to play a major role on the morphology, quality of the single wall carbon nanotubes (SWNTs) growth. Fe–Co bimetal was supported onto MgO powders with different surface areas, and used for the SWNTs synthesis from the pyrolysis of CH4 through radio frequency catalytic chemical vapor deposition (RF-CCVD). The high surface area MgO showed specificity towards the growth of SWNTs even at the very high metal loadings of 17.4%. As the MgO surface areas decreased, the agglomeration of the active metal species on the support, nanotubes with wider diameter distributions and more graphitic walls were obtained. This surface area effect suggested that the high surface area networks were responsible for a more effective confinement of the active metal nanoparticles that impeded their migration during the SWNTs thermal growth process. Therefore, by adjusting the surface area or chemical morphology of the catalyst systems, SWNTs with narrower diameter distribution and specific chiralities can be grown.

CO2 enhanced carbon nanotube synthesis from pyrolysis of hydrocarbons

We report on the role of CO(2) in improving carbon nanotube yield and crystallinity from catalytic chemical vapor deposition of hydrocarbons.

Growth of nanoscale carbon structures and their corresponding hydrogen uptake properties

Carbon nanostructures have been synthesized using the chemical vapor deposition technique (CVD) on different catalysts, using ethylene, acetylene, or methane as the hydrocarbons. Morphological characterizations obtained using a scanning electron microscope (SEM) showed that the reaction products are carbon nanofibers (CNF) with an outer diameter that depends on the reaction conditions and nature of the reactants. Hydrogen uptake measurements, performed volumetrically in a Sievert-type installation, showed the quantity of desorbed hydrogen (for pressure intervals ranging from 1 to 100 bars) depends on the synthesis conditions and the treatment preceding the hydrogen absorption process. For carbon nanotubes that were preparedaccording to literature guidelines and obtained from ethylene on a Ni:Cu catalyst, the amounts of absorbed hydrogen were less than 1% by weight. carbon nanostructures chemical vapor deposition hydrogen absorption SEM

Effects of the Fe–Co interaction on the growth of multiwall carbon nanotubes

The influences of active species Fe-Co composition on the growth of carbon nanotubes (CNTs) were systemically investigated. CNTs were grown from the pyrolytic decomposition of C(2)H(2) over Fe-Co/CaCO(3) catalysts by radio frequency chemical vapor deposition (CVD). The catalyst stoichiometry was found to strongly influence the carbon deposition rate as well as the nanotube crystallinity characteristics. Compared to the CNTs synthesized over the Co/CaCO(3) catalyst, those produced by Fe-containing catalysts have less amorphous carbon. The maximum yield of high-quality CNTs was achieved at the Fe/Co atomic ratio of 2:1 due to a suitable concentration of benzene generated from acetylene CVD on such catalytic system. Fe and Co can form alloy and therefore the d-electron interaction between Fe and Co was believed to play an important role in the CNT growth.

Hydrogen Storage in Carbon-Based Nanostructured Materials

Carbon nanostructures represent a revolution in science and hold the potential for a large range of applications because of their interesting electrical, mechanical, and optical properties. Multiwall carbon nanotubes and carbon nanofibers of herringbone formation were grown by chemical vapor deposition on different catalysts from a number of hydrocarbon sources. After the total or particle removal of the catalyst system, the carbon nanostructures were analyzed for hydrogen uptake. Six samples of nanofibers grown on a Pd-based catalyst system (with a surface area of 425–455 m2/g) were controlled oxidized in air, such that they had different ratios of Pd/C varying from 0.05 to 0.9 mole ratio. The hydrogen uptake experiments were performed volumetrically in a Sievert-type installation and showed that the quantity of desorbed hydrogen (for pressure intervals ranging from 1 to 100 bars) by the carbon nanostructures free of any metal catalyst particles was between 0.04 and 0.33% by weight. For the samples of nanofibers that contained Pd in various Pd/C ratios, palladium revealed catalytic properties and supplied atomic hydrogen at the Pd/C interface by dissociating the H2 molecules. The results show a direct correlation between the Pd/C ratio and the quantity of hydrogen absorbed by these samples. A saturation value of about 1.5 wt.% was reached for a high ratio of about 1:1 of Pd/C. The multiwall carbon nanotubes grown on a Fe:Co:CaCO3 catalytic system and purified by acid cleaning and air oxidation showed a hydrogen uptake value of 0.1 to 0.2 wt.%.

Hydrogen absorption and hydride electrode behaviour of the Laves phase ZrV1.5-xCrxNi1.5

Abstract The substitution of chromium for vanadium in ZrV1.5−xCrxNi1.5 leads to a cubic C15 phase as the main phase with a low content of the secondary phase Zr7Ni10. The lattice parameters suggest a complex redistribution of V, Ni or Cr between the B and A sites of the Laves phase AB2, which does not depend only on the Cr content. The pressure–composition isotherms show sloping plateaux, alloy hydrogen contents between 3.35 and 4.25 H/mol and desorption enthalpies of ΔH=41.8–50.2 kJ/mol H2 at constant H concentration (1.8 H/mol). Discharge capacities of between 113 and 222 mAh/g have been obtained for the hydride electrodes at a 160 mA/g discharge current; for the Cr-substituted alloys the discharge capacities increase with x.