Designing a strategy for fabrication of single-walled carbon nanotube via CH4/N2 gas by the chemical vapor deposition method

Fe/Mo catalysts supported on alumina are suggested for the growth of single-walled carbon nanotubes (SWCNTs) by chemical vapor deposition (CVD) in methane (CH₄). One obvious synergistic advantage identified by molybdenum (Mo) is that by controlling the reaction temperature, it eliminates the need to activate the catalyst in hydrogen gas (H₂). So as activation of the growth catalyst in H₂ is a costly and extra step in the CVD method, this is a significant result of the present study. Herein, the sample quality and the dependence of carbon mass yield on CVD growth conditions are disserted. The activity of the catalyst is perused in the form of oxide under CH₄ flow (160 sccm) and in the temperature range of 680 to 1000 °C. We came to the conclusion that the Fe/Mo/Al₂O₃ oxide catalyst is activated at temperatures as low as 800 °C. Under these conditions (800 °C, 160 sccm of CH₄), the temperature is not enough for the tube to grow, but by increasing it, the desired conditions can be achieved (900 –1000 °C).     

O2, CH4 and CO2 gas retentions by acid smectites before and after thermal treatment

Acid smectites in natural condition and after thermal treatment up to 900 °C were studied for their O₂, CH₄ and CO₂ gas retentions at 25 °C and 1 kg/cm². Two smectites, one dioctahedral and one trioctahedral, were treated with 5.0 N sulphuric acid solution for 15 and 60 min. The gas adsorptions by the smectites increased after acid treatments. The O₂ (0.014–0.030 mmol/g) and CH₄ (0.045–0.063 mmol/g) gas retention values were small whereas the values corresponding to CO₂ (0.206–0.357 mmol/g) retentions were high for acid smectites. The acid trioctahedral smectite showed higher gas adsorption values than acid dioctahedral smectite. The CO₂ gas adsorption values by acid smectites heated up to around 600 °C, decreased around 10% and 20% for acid trioctahedral and dioctahedral smectites, respectively. After that drastically decreased and at 900 °C both acid smectites showed very small adsorption values. The chemical composition, the structure and the texture of the smectites influenced the gas retention.     

Chemically deposited zinc oxide thin film gas sensor

Zinc oxide (ZnO) thin films were prepared by a low cost chemical deposition technique using sodium zincate bath. Structural characterizations by X-ray diffraction technique (XRD) and scanning electron microscopy (SEM) indicate the formation of ZnO films, containing 0.05–0.50 m size crystallites, with preferred c-axis orientation. The electrical conductance of the ZnO films became stable and reproducible in the 300–450 K temperature range after repeated thermal cyclings in air. Palladium sensitised ZnO films were exposed to toxic and combustible gases e.g., hydrogen (H₂), liquid petroleum gas (LPG), methane (CH₄) and hydrogen sulphide (H₂S) at a minimum operating temperature of 150 °C; which was well below the normal operating temperature range of 200–400 °C, typically reported in literature for ceramic gas sensors. The response of the ZnO thin film sensors at 150 °C, was found to be significant, even for parts per million level concentrations of CH₄ (50 ppm) and H₂S (15 ppm).     

Bulk preparation and characterization of mesoporous carbon nanotubes by catalytic decomposition of cyclohexane on sol–gel prepared Ni–Mo–Mg oxide catalyst

Multiwalled carbon nanotubes were synthesized using Ni–Mo–Mg oxide catalyst prepared by sol–gel technique. Carbon nanotubes were formed in situ by the reduction of nickel oxide (NiO) and molybdenum oxide (MoO₃) to Ni and Mo by a gas mixture of nitrogen, hydrogen and cyclohexane at 750 °C. Scanning Electron Microscopy (SEM) was used to confirm the formation of carbon nanotubes (CNTs). The pore size distribution of carbon nanotubes (CNTs) was investigated by N₂ adsorption and desorption. It was found that the pore size fell into the mesopore range: 2 < d < 50 nm. Interpretation was also made using Raman spectroscopy, Diffuse reflectance spectroscopy, X-ray diffraction and ESR spectra. This method is found to produce a very high yield weighing over 20 times of the catalyst. Based on the experimental conditions and results obtained a possible growth mechanism of the carbon nanotubes is proposed.     

Low-pressure synthesis and characterization of multiphase SiC by HWCVD using CH4/SiH4

Silicon carbide (SiC) thin films were deposited by low-pressure hot wire chemical vapor deposition (HWCVD) technique using SiH₄ and CH₄ gas precursors with no hydrogen dilution. Spectroscopic and structural properties of the films deposited at various methane flow rate (10-100 sccm) and low silane flow rate of 0.5 sccm were investigated. The use of low methane flow rate resulted in a sharp and intense Si-C peak in the Fourier transform infrared (FTIR) absorption spectra. The XRD spectra of the films showed the formation of SiC crystallites at low methane flow rate. The Raman spectroscopy measurements showed the coexistence of a-Si and SiC phases in the films. Increase in methane flow rate increased the carbon incorporation and deposition rate of the SiC films but also promoted the formation of amorphous Si and SiC phases in the films.     

Nanocrystallized steel surface by micro-shot peening for catalyst-free carbon nanotube growth

Nanocrystallized steel surface by micro-shot peening (MSP) were applied to carbon nanotube growth in this study. Micro-shot peening treatment severely deformed steel surface and nanocrystallized surface layer was formed by the plastic deformation. The grain sizes of the nanocrystallized layer were 10-30 nm after 300 s of MSP treatment. On the nanocrystallized surface, carbon nanotubes were formed with thermal chemical vapour deposition without catalysts. Before carbon nanotube growth, the nanocrystallized steel surface was reduced with H₂/N₂ gas at 600 °C. The carbon nanotube growth was performed at 600 °C with C₂H₂ gas carried by H₂/N₂ gas. The carbon nanotubes formed on the nano-structured surface was multi-walled carbon nanotube and the diameter was 10-20 nm. The reduction process before carbon nanotube growth was essential to form carbon nanotubes on the nanocrystallized surface with MSP.     

Chemical vapour growth of HfC whiskers and their morphology

HfC whiskers were prepared from a gas mixture of HfCl₄ + CH₄ + H₂ + Ar in the presence of metal impurities, and the growth conditions and morphology were examined. The HfC whiskers preferentially grew at an H/Cl ratio of above 8, an HfCl₄ gas flow rate of 10–20 standard cm³ min^–1, a CH₄ flow rate of 10–20 standard cm³ min^–1, and at temperatures above 1050 °C. HfC whiskers, 60–170 m long, with a ball-like tip and periodically varying diameters, were obtained at 1250 °C using a cobalt impurity.     

Linear antenna microwave plasma CVD deposition of diamond films over large areas

Diamond thin films were grown by linear antenna microwave plasma CVD process over large areas (up to 20 × 10 cm²) from a hydrogen based gas mixture. The influence of the gas composition (H₂, CH₄, CO₂) and total gas pressure (0.1 and 2 mbar) on the film growth is presented. For CH₄/H₂ gas mixtures, the surface crystal size does not show dependence on the methane concentration and total pressure and remains below 50 nm as observed by SEM. Adding CO₂ (up to 10%) significantly improves the growth rate. However, still no significant change of morphology is observed on films grown at 2 mbar. The crucial improvement of the diamond film purity (as detected by Raman spectroscopy) and crystal size is found for deposition at 0.1 mbar. In this case, crystals are as large as 500 nm and the growth rate increases up to 38 nm/h.     

Vapor-phase formation of single-helix carbon microcoils by using WS2 catalyst and the morphologies

Very regular single-helix carbon microcoils of outer coil diameter 1–;3 m, inner coil diameter 0.5–;2 m, and coil pitch 1–;2 m were prepared by the WS₂-catalyzed pyrolysis of acetylene in the presence of thiophene impurity. The effects of reaction temperature and thiophene gas flow rate on the growth of single-helix carbon microcoils and the morphology were examined in detail. The optimum reaction temperature was 780°C, and optimum gas flow rate of thiophene, acetylene, hydrogen and argon for obtaining regular single-helix carbon coils with a constant coil diameter were 0.2, 40, 90, and 30 sccm, respectively. The formation mechanism of the single-helix carbon microcoils is discussed.     

Theoretical investigation of methane adsorption onto boron nitride and carbon nanotubes

Methane adsorption onto single-wall boron nitride nanotubes (BNNTs) and carbon nanotubes (CNTs) was studied using the density functional theory within the generalized gradient approximation. The structural optimization of several bonding configurations for a CH₄ molecule approaching the outer surface of the (8,0) BNNT and (8,0) CNT shows that the CH₄ molecule is preferentially adsorbed onto the CNT with a binding energy of −2.84 kcal mol⁻¹. A comparative study of nanotubes with different diameters (curvatures) reveals that the methane adsorptive capability for the exterior surface increases for wider CNTs and decreases for wider BNNTs. The introduction of defects in the BNNT significantly enhances methane adsorption. We also examined the possibility of binding a bilayer or a single layer of methane molecules and found that methane molecules preferentially adsorb as a single layer onto either BNNTs or CNTs. However, bilayer adsorption is feasible for CNTs and defective BNNTs and requires binding energies of −3.00 and −1.44 kcal mol⁻¹ per adsorbed CH₄ molecule, respectively. Our first-principles findings indicate that BNNTs might be an unsuitable material for natural gas storage.     

Carbon nanotubes synthesis in microwave plasma torch at atmospheric pressure

Well aligned multi-walled carbon nanotubes were synthesized at atmospheric pressure using a microwave plasma torch on silicon substrates with silicon oxide buffer layer and catalyst overlayer in the mixture of argon, hydrogen and methane. Iron or nickel was used as catalysts. The optimum substrate temperature for the deposition on Si/SiO₂/Fe substrates was about 970 K. In this case SEM micrographs of the deposits revealed a presence of vertically aligned nanotubes with the diameters around 15 nm. TEM micrographs showed a presence of amorphous carbon particles in the samples and some defects in the wall structure of the produced nanotubes. In Raman spectra two peaks at 1332 and 1584 cm⁻¹ were observed. The CNTs were also synthesized on the substrates without SiO₂ buffer layer but their quality was lower. The synthesis with Ni instead of Fe catalyst required lower temperature and the alignment of the nanotubes was worse. The deposition process was monitored by optical emission spectroscopy. Atomic lines of hydrogen and argon, an emission of CN due to a presence of nitrogen impurities from atmosphere, a weak molecular band of CH and strong C₂ emission were detected in the spectra.     

Effect of growth temperature on the CVD grown Fe filled multi-walled carbon nanotubes using a modified photoresist

Fe filled carbon nanotubes were synthesized by atmospheric pressure chemical vapor deposition using a simple mixture of iron(III) acetylacetonate (Fe(acac)₃) with a conventional photoresist and the effect of growth temperature (550-950 °C) on Fe filled nanotubes has been studied. Scanning electron microscopy results show that, as the growth temperature increases from 550 to 950 °C, the average diameter of the nanotubes increases while their number density decreases. High resolution transmission electron microscopy along with energy dispersive X-ray investigation shows that the nanotubes have a multi-walled structure with partial Fe filling for all growth temperatures. The graphitic nature of the nanotubes was observed via X-ray diffraction pattern. Raman analysis demonstrates that the degree of graphitization of the carbon nanotubes depends upon the growth temperature.     

Effect of dilution gases in methane on the deposition of diamond-like carbon in a microwave discharge II: Effect of hydrogen

The effect of hydrogen as a dilution gas on the deposition of diamond-like carbon by the decomposition of methane in a microwave discharge was studied from surface analysis of the substrate and from plasma diagnostics. When carbon deposited from a CH₄-Ar plasma and consisting of large amounts of graphite and small amounts of diamond, was placed in the hydrogen plasma chemical sputtering of carbon to form hydrocarbons and adsorption of hydrogen on the carbon substrate were observed. The reaction occured only on graphite and not on diamond. The effects of hydrogen as a dilution gas on the deposition of diamond-like carbon from CH₄-H₂ plasma are to cause the formation of CH₃ radicals in the plasma, the removal of graphite from the deposit and the adsorption of atomic hydrogen on the deposit as an active participant in the diamond crystallization process.     

Physical activation and characterization of multi-walled carbon nanotubes catalytically synthesized from methane

A series of treatment processes were employed to purify and then physically activate the multi-walled carbon nanotubes obtained using catalytic decomposition of methane. In order to characterize and compare the activation effect, the carbon fibers were also treated by the same activation processes. The results showed that the normal physical activation by CO₂ or steam has not too much effect on the surface area of purified multi-walled carbon nanotubes, in particular, the carbon nanotubes were burned when using the poignant activation conditions. However, the surface area of carbon fibers availably etched in the same activation processes is much increased. In addition, the mechanisms of physical activation on multi-walled carbon nanotubes and carbon fibers have been investigated.     

Possibility of methane decomposition in a gas discharge

The possibility of using a gas discharge such as a corona discharge or a barrier discharge for decomposition of methane in different gaseous mixtures is investigated theoretically. The effect of preheating of the gas to a temperature of 1200 K on the degree of methane conversion in the discharge is studied. A kinetic model that describes the processes of methane decomposition and oxidation in CH₄/CO₂, CH₄/H₂O, and CH₄/O₂ mixtures is developed. The effect of the discharge parameters and gas additives on the efficiency of methane decomposition is investigated. The optimum temperature of the mixture, particle lifetime, and initial concentration of oxygen for the production of hydrogen molecules are found. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 71, No. 6, pp. 1016–1023, November–December, 1998.     

Methanation of CO2 with the oxygen-deficient Ni(II)-ferrite under dynamic conditions

The continuous methanation of CO₂ has been accomplished over hydrogen-reduced Ni(II)-bearing ferrite (Ni_xFe_3–xO_4–; x=0.39, > 0) in a mixed gas flow of CO₂ and H₂ at 250–375 °C. The yield and the selectivity for the methanation were larger than 50% and 95%, respectively, at the initial stage of the process. They decreased to 31% and 89%, respectively, after 6 h methanation. The innovative results can be ascribed to the use of the new material; hydrogen-reduced Ni(II)-bearing ferrite. Its formation was evinced by chemical analyses and the increase in the lattice constant; the lattice constant of the Ni(II)-bearing ferrite (a₀ 0.8375 nm) was enlarged to 0.8379 nm by hydrogen reduction. The enlarged lattice constant was not changed during the methanation. These findings suggest that the methanation occurs at the oxygen-deficient site of the hydrogen-reduced Ni(II)-bearing ferrite, as well as the formation of water by combination of the incorporated oxygens with hydrogen. The methanation consists of three steps of the elementary reaction. First, the oxygen-deficient sites are formed by hydrogen reduction; second, CO₂ is reduced to elementary carbon and two oxygen ions which are incorporated into the oxygen-deficient sites; and third, the carbon deposited on the surface of the reduced ferrite is selectively hydrogenated to CH₄.     

Synthesis and properties of magnetic multi-walled carbon nanotubes loaded with Fe4N nanoparticles

High saturation magnetization (>90 emu/g) multi-walled carbon nanotubes (MWCNTs) and Fe₄N nanoparticles composite were successfully synthesized by gas nitriding at 550 °C. Almost all Fe₄N nanoparticles were evenly distributed inside the carbon nanotubes and formed a special composite microstructure. This composite microstructure shows excellent soft magnetic property, structural stability, and chemical stability at room temperature. The investigations of electromagnetic and microwave absorption performances indicate that microwave absorbing capacity of low frequency band of MWCNTs were greatly improved by addition of Fe₄N nanoparticles.     

Carbon dioxide decomposition into carbon with the rhodium-bearing magnetite activated by H2-reduction

The CO₂ decomposition into carbon with the rhodium-bearing activated magnetite (Rh-AM) was studied in comparison with the activated magnetite (AM). The Rh-AM and the AM were prepared by flowing hydrogen gas through the rhodium-bearing magnetite (Rh-M) and the magnetite (M), respectively. The rate of activation of the Rh-M to the Rh-AM was about three times higher than that of the M to the AM at 300 °C. The reactivity for the CO₂ decomposition into carbon with the Rh-AM (70% CO₂ was decomposed in 40 min) was higher than that with the AM (30% in 40 min) at 300 °C. The Rh-M was activated to the Rh-AM at a lower temperature of 250 °C, and the Rh-AM decomposed CO₂ into carbon at 250 °C. On the other hand, the M was little activated at 250 °C.     

Synthesis of carbon nanotube films by thermal CVD in the presence of supported catalyst particles. Part I: The silicon substrate/nanotube film interface

The interface between the silicon substrate and a carbon nanotube film grown by thermal CVD with acetylene (C₂H₂) and hydrogen at 750 or 900 °C has been characterized by high resolution and analytical transmission electron microscopy, including electron spectroscopic imaging. Silicon (0 0 2) substrates coated with a thin (2.8 nm) iron film were heat treated in the CVD furnace at the deposition temperature in a mixture of flowing argon and hydrogen whereby nanosized particles of (Fe,Si)₃O₄ formed. These particles were reduced to catalytic iron silicides with the –(Fe, Si), ₂–Fe₂Si and ₁–Fe₂Si structures during CVD at 900 °C, and multi-wall carbon nanotubes grew from supported particles via a base-growth mechanism. A limited number of intermediate iron carbides, hexagonal and orthorhombic Fe₇C₃, were also present on the substrate surface after CVD at 900 °C. The reduction of the preformed (Fe, Si)₃O₄ particles during thermal CVD at 750 °C was accompanied by disintegration leading to the formation of a number of smaller (<5 and up to 10 nm) iron and silicon containing particles. It is believed that the formation of these small particles is a prerequisite for the growth of aligned multi-wall carbon nanotube films.     

Effect of additive gases on synthesis of organic compounds from carbon dioxide using non-thermal plasma produced by atmospheric surface discharges

Reduction and recycling of carbon dioxide (CO₂) were performed using a non-thermal plasma produced by a surface discharge at atmospheric pressure. Useful hydrocarbons (CHs) such as dimethyl ether and methane were produced at the discharge voltage of 11 kV, when hydrogen (H₂) gas was mixed with CO₂ and the mixture ratio was 50%. The conversion of CO₂ to the CHs mixing with water vapor of 50% requires a higher discharge voltage of 12 kV. The conversion ratios to the hydrocarbons were several percentage in both H₂ mixture and water vapor mixture cases.