Growth of carbon nanotubes by open-air laser-induced chemical vapor deposition

Carbon nanotubes have remarkable mechanical, electronic and electrochemical properties, but the full potential for application will be realized only if the growth of high quantity and quality carbon nanotubes can be optimized and well controlled. In this study, carbon nanotubes have been successfully grown on fused quartz rods by a novel open-air laser-induced chemical vapor deposition (LCVD) technique with gold palladium nanoparticles as catalyst material. In this LCVD technique, a curtain of inert nitrogen gas was used to shield the deposition zone from the surrounding environment and allows the growth of the nanotubes to occur under open-air conditions. A 35-W continuous CO₂ laser was used as a heat source to induce a local temperature rise on the substrate surface covered with metal nanoparticles, subsequently resulting in deposition of multi-wall carbon nanotubes. The carbon nanotubes deposited in this study are derived from a precursor mixture that consists of propane and hydrogen, and are in tangled form with different diameters (10-250 nm) and structures. Raman spectroscopy, transmission and scanning electron microscopy are used to investigate the microstructure and composition of the carbon nanotubes.     

Possible tactics to improve the growth of single-walled carbon nanotubes by chemical vapor deposition

The growth time, growth mode and the method of preparing the supported catalysts play an important role in the growth of single-walled nanotubes (SWNTs). Their effects on the chemical vapor deposition (CVD) growth of SWNTs with MgO-supported catalysts were investigated in this study. It is shown that the growth rate of SWNTs was large during the initial few minutes of growth, however the quality of the tubes was low owing to the formation of many defects. Long term growth may favor the formation of tubes with high quality and high yield, but the introduction of other forms of carbon (impurities) is also unavoidable. There was a balance between the increase in yield and quality and sacrifice of the purity during growth of SWNTs. MgO-supported catalysts prepared by the co-precipitation method were found to be more effective for the synthesis of SWNTs than those prepared by the widely used impregnation method. The size and dispersion state of the catalyst were found to be crucial in enhancing the growth of SWNTs. In addition, growth on the surface of SWNTs over nanosized catalyst films was shown to be more favorable for the synthesis of tube products with higher quality, yield and purity.     

Chemical vapour deposition of pyrolytic carbon on carbon nanotubes: Part 3: Growth mechanisms

As a first step to identify the growth mechanism of various pyrolytic carbon deposit morphologies onto multiwall carbon nanotubes (MWNTs) presented in earlier papers, we determined their growth chronology by carrying-out synthesis experiments involving a large time range. We propose that the formation of any of the deposit morphologies is the consequence of the primary formation of hydrocarbon liquid droplets in the gas phase and their subsequent deposition onto the MWNTs. This makes the formation mechanisms of the various deposit morphologies depend on physical phenomena related to the wetting of nanotube surfaces by the droplets, where the [droplet diameter]/[nanotube diameter] ratio plays an important role. The droplets are the result of the recombination of species issued from the cracking of the gaseous precursor (methane), and their characteristics (number, size, and aromaticity) depend on experimental parameters such as temperature, time of flight, and gas phase composition. The results bring a new light to the currently admitted hypotheses for the mechanisms of pyrolytic carbon deposition, and revitalise the liquid droplet theory formerly proposed by Grisdale in the 1950s, at least in the range of conditions investigated.     

Coiled carbon nanotubes growth via reduced-pressure catalytic chemical vapor deposition

Coiled carbon nanotubes were prepared by catalytic chemical vapor deposition (CCVD) on finely divided Co nano-particles supported on silica gel under reduced pressure and relatively low gas flow rates. The morphology and the graphitization of the coil tube, coil bend, and coil node of the coiled carbon nanotubes were examined by transmission electron microscope (TEM). The influence of pH value, reaction pressure, and flow rate of C₂H₂ on the growth of the coiled carbon nanotubes were also discussed. With the drastic reduction in the consumption of C₂H₂ and lower required pressure with the modified CCVD approach, the amount of amorphous carbon coated on the carbon nanotubes was shown to be greatly reduced. Most importantly, this method offers a preferable alternative for the efficient, environment-friendly and safer growth of coiled carbon nanotubes.     

Control of diameter distribution of single-walled carbon nanotubes using the zeolite-CCVD method at atmospheric pressure

Single-walled carbon nanotubes (SWCNTs) have been synthesized on zeolite powder with Fe/Co catalysts by a catalytic chemical alcohol-vapor deposition (CCAVD). We have first used a cold wall reactor at the atmospheric pressure, the system having been modified for the zeolite-CCAVD specifications by the use of radio-frequency heating. The G/D ratio (∼25), estimated by analysis of Raman spectroscopy, obtained here is equivalent to that by the conventional CCAVD method under reduced pressure, indicating the high purity of the present specimen. The estimated diameter distributions of the SWCNTs obtained at synthesis temperatures of 900, 1000 °C and constant ethanol temperature of 0 °C are 0.9-1.8 and 1.2-2.2 nm, respectively, whereas that of synthesized at synthesis temperature of 900 °C and ethanol temperatures of 40 °C ranges form 0.8 to 1.4 nm. The diameter distribution shifts towards larger diameters as the synthesis temperature is increased and the carbon supply rate (ethanol temperature) decreases, from which we suggest a selective growth model due to a competition between deposition and etching of carbon atoms.     

Observations of novel carbon nanotubes with multiple hollow cores

Usually carbon nanotubes (CNTs) containing only one hollow core are obtained from the catalytic decomposition of hydrocarbons when hydrocarbon gases flow straight into the reaction tube. However, unusual carbon nanotubes with multiple hollow cores were observed when the gas-feed method was changed in an attempt to increase CNT production yield using a floating catalyst method. The fraction of multicored carbon nanotubes can be as high as 60%. The formation of such an unusual structure is ascribed to the introduction of pentagon and heptagon defects to the CNTs in the growth process, owing to the change of gas-feed method. This finding enriches the family of CNTs and could be helpful in understanding the CNT formation mechanism.     

Synthesis of vertically aligned carbon nanotubes on metal deposited quartz plates

Without plasma aid, we have successfully synthesized vertically aligned carbon nanotubes (CNTs) on iron-, cobalt- or nickel-deposited quartz plates by chemical vapor deposition with ethylenediamine as a precursor. The amine serves as both etching reagent for the formation of metal nanoparticles and carbon source for the growth of aligned carbon nanotubes. The carbon nanotubes were vertically aligned in high density on a large area of the plain silica substrates. The density and diameter of CNTs is determined by the thickness of the deposited metal film and the length of the tubes can be controlled by varying the reaction time. High-resolution transmission electron microscopy analysis reveals that the synthesized CNTs are multiwalled with a bamboo-like structure. Energy dispersive X-ray spectra demonstrate that the CNTs are formed as tip growths. Raman spectrum provides definite evidence that the prepared CNTs are multiwalled graphitic structure.     

Intermetallic catalyst for carbon nanotubes (CNTs) growth by thermal chemical vapor deposition method

The methane conversion and carbon yield of the chemical vapor deposition (CVD) reaction suggests that the optimum reaction conditions of the formation of multi-wall carbon nanotubes (MWCNTs) can be obtained by using a 50 mg of nano-MgNi alloy under pyrolysis of the pure CH₄ gas with the flow rate about 100-120 cm³/min at 650 °C for 30 min. Raman results indicate the CNTs are in multi-wall structure, since no single-wall characteristic features appearing in the 200-400 cm⁻¹ region. This is consistent with those of the XRD and TGA findings. Under selected condition, the carbon yield and the CNTs purity can reach up to 1231% and 92% in the presence of hydrogen. It is presumable that the presence of hydrogen in the pyrolysis of CH₄ prevents the deactivation of catalysts and enhances the graphitization degree of CNTs. In addition, the presence of Mg metal in the alloy can prevent the aggregation of the Ni metal and forms the active Mg₂Ni phase to enhance the CH₄ pyrolysis to form CNTs. After the purification procedures with both air oxidation at 550 °C and HCl treatments, the final purified yield and purity of CNT reach to 73.2% and (98.04 ± 0.2)% respectively.     

Effect of hydrocarbons precursors on the formation of carbon nanotubes in chemical vapor deposition

High-temperature decomposition of hydrocarbons may lead to the formation of carbon deposits. However in our present studies, we found that the morphology of carbon deposits over MgO supported Fe catalyst during chemical vapor deposition (CVD) process was closely related to the thermodynamic properties and chemical structures of hydrocarbon precursors. Six kinds of hydrocarbons (methane, hexane, cyclohexane, benzene, naphthalene and anthracene) were used as carbon precursors in this study. Methane which has a pretty simple composition and is more chemically stable was favorable for the formation of high-purity single walled carbon nanotubes (SWNTs). For high-molecular weight hydrocarbons, it was found that the chemical structures rather than thermodynamic properties of carbon precursors would play an important role in nanotube formation. Specifically, the CVD processes of aromatic molecules such as benzene, naphthalene and anthracene inclined to the growth of SWNTs. While the cases of aliphatic and cyclic hydrocarbon molecules seemed a little more complicated. Based on different pyrolytic behaviors of carbon precursors and formation mechanism of SWNTs and multi-walled carbon nanotubes (MWNTs), a possible explanation of the difference in CVD products was also proposed.     

Selective formation of carbon nanotubes over Co-modified beta zeolite by CCVD

Multiwall carbon nanotubes containing carbon-zeolite composite materials have been obtained by ethylene catalytic chemical vapor deposition (CCVD) on Co-modified beta zeolite powder. High carbon selectivity to nanotubes was achieved. The combined physical characterisation through several techniques, such as TEM, HRTEM, SEM, XRD, N₂ adsorption-desorption, allows to determine the geometric characteristics of nanotubes and their dependence of catalyst and synthesis conditions.     

Optical emission spectroscopy study of the influence of nitrogen on carbon nanotube growth

In-situ optical emission spectroscopy was used to systematically study the influence of nitrogen on the growth of carbon nanotubes (CNTs) by microwave-plasma enhanced chemical vapor deposition at different CH₄ concentrations in a CH₄/N₂ gas mixture. The results show that CN and C₂ co-exist in the plasma and emission intensities of the two species are correlated. The morphology and microstructure of the samples vary with the CH₄ concentration. Well aligned nanotubes are obtained at 20% CH₄. With the participation of N₂, the CNTs present a polymerized nanobell structure. More importantly, the length and thickness of each nanobell can be modulated by varying the CH₄ concentration. Without N₂, conventional cylindrical CNTs are obtained.     

Electrolytic, TEM and Raman studies on the production of carbon nanotubes in molten NaCl

The production of carbon nanotubes (CNTs) by the electrolysis of molten NaCl was investigated by examining the effect of electrolysis duration, current density and voltage. It was found that as the electrolysis was run for longer periods the cathode eroded, changing the current density and consequently preventing nanotube production. The electrolysis was also inhibited by the anode effect and the formation of a sodium layer on the top of the electrolyte. The cell was modified to avoid these difficulties and then optimised under voltage control. Minimum and optimum voltages and current densities were found for CNT production. However, it was discovered that the percentage of nanotube produce still fell as the electrolysis progressed despite minimising the variation in the current density. The nanomaterial produced was studied by TEM. In particular, it was observed that half of the nanotubes were coated with amorphous carbon, suggesting a two-stage growth process. No link, though, was established between the growth conditions and the morphology of the nanotubes. Raman spectroscopy showed that the quality of the nanotubes was comparable to those produced by the CVD route. Titration was used to establish the uptake of sodium into the cathodes, providing evidence for the intercalation growth mechanism.     

High quality double wall carbon nanotubes with a defined diameter distribution by chemical vapor deposition from alcohol

We have synthesized double wall carbon nanotubes (DWNTs) with few defects and little amorphous carbon by hot wall chemical vapor deposition (CVD) of alcohol. Catalysts for the DWNT growth were made from cobalt and molybdenum acetates. Scanning electron microscopy, transmission electron microscopy, multi frequency resonance Raman spectroscopy and optical absorption spectroscopy were used for characterization of the product with regard to DWNT yield, the nanotube diameter distribution, defect concentration and amorphous carbon content. Base pressures lower than 1 × 10⁻⁵ mbar in the CVD reactor considerably suppress defects in the DWNTs. Optimized growth conditions for DWNT formation are presented.     

SERS studies on single-walled carbon nanotubes submitted to chemical transformation with sulfuric acid

Surface-enhanced Raman scattering (SERS) at 676.44 nm and 1064 nm excitation wavelengths was used to investigate chemical transformation of single-walled carbon nanotubes (SWNTs) deposited on a gold support. Sulfuric acid was used as the chemical reagent. Special attention was paid to the changes in the Raman bands associated to radial and tangential vibration modes. Partial restoration of the Raman spectra by a subsequent alkaline treatment indicates a transformation with a certain degree of reversibility. The recovery reaction achieved with a 0.5 M KOH solution showed that the variations of tangential and radial band groups are not correlated. The intensity changes of the radial bands is a principal indicator for the chemical transformation of the SWNTs. Particular attention was paid to radial bands at 164 and 176 cm⁻¹, observed with 1064 nm and 676.44 nm excitation wavelength, respectively, and their 14 cm⁻¹ up-shifted replicas i.e. the bands at 178 and 190 cm⁻¹. A different behavior of these bands in the anti-Stokes side was observed.     

Continuous deposition of carbon nanotubes on a moving substrate by open-air laser-induced chemical vapor deposition

Continuous deposition of carbon nanotubes under open-air conditions on a moving fused quartz substrate is achieved by pyrolytic laser-induced chemical vapor deposition. A CO₂ laser is used to heat a traversing fused quartz rod covered with bimetallic nanoparticles. Pyrolysis of hydrocarbon precursor gas occurs and subsequently gives rise to rapid growth of a multi-wall carbon nanotube forest on the substrate surface. A “mushroom-like” nanotube pillar is observed, where a random orientation of carbon nanotubes is located at the top of the pillars while the growth is more aligned near the base. The typical carbon nanotube deposition rate achieved in this study is approximately 50 μm/s. At high power laser irradiation, various carbon microstructures are formed as a result of excessive formation of amorphous carbon on the substrate. High-resolution transmission and scanning electron microscopy, and X-ray energy-dispersive spectrometry are used to investigate the deposition rate, microstructure, and chemical composition of the deposited carbon nanotubes.