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.     

Effect of laser intensity on yield and physical characteristics of single wall carbon nanotubes produced by the Nd:YAG laser vaporization method

We report on the effect of the Nd:YAG laser intensity on diameter distribution, yield and physical characteristics of single-wall carbon nanotubes (SWNTs) while comparing three different laser configurations (namely: (i) single 532 nm pulse; (ii) single 1064 nm pulse; and (iii) 532 nm followed by the 1064 nm double pulse). The carbon SWNTs were synthesized at a furnace temperature of 1150 °C and characterized by means of laser micro-Raman spectroscopy and high resolution transmission electron microscopy (HRTEM). Regardless of the laser configuration used, it is found that both the yield and the structural characteristics of the SWNTs are highly sensitive to the laser intensity. Indeed, by combining Raman analyses together with HRTEM observations we were able to point out the existence of an optimal laser intensity which leads not only to the highest yield of SWNTs and the largest bundles but also to the lowest level of amorphous and, or disordered sp² carbon in the deposits. While the optimal laser intensity was found to increase from 1.7 to 2.9×10⁹ W/cm² when the laser wavelength is changed from 1064 to 532 nm, the double pulse configuration offered a larger process latitude since high yield of SWNTs was obtained over the (0.8–3.5)×10⁹ W/cm² laser intensity range centered around the optimal value of 2.3×10⁹ W/cm². Moreover, it is shown that the increase of the laser intensity (from 0.5 to 5.6×10⁹ W/cm²) favors the growth of large nanotubes (1.4 nm-diam.) to the detriment of smaller ones (1.1 nm-diam.). A tendency to form larger nanotubes was also observed when increasing the furnace temperature from 1000 to 1150 °C. Finally, the laser intensity effect is interpreted in terms of near-surface or deep laser energy absorption in the graphite target.     

Structure changes of single-wall carbon nanotubes and single-wall carbon nanohorns caused by heat treatment

Raman spectra and transmission electron microscope images showed that diameter enlargement of HiPco, a kind of single-wall carbon nanotube, accompanied by tube-wall corrugation was caused by heat treatment (HT) at 1000 to 1700 °C. Further enlargement accompanied by straightening of the tube walls and incorporation of carbon fragments within the tubes became obvious after HT at 1800 to 1900 °C. The transformation of some single-wall carbon nanotubes into multi-wall nanotubes was observed after HT at 2000 °C, and most single-wall tubes were transformed into multi-wall ones by HT at 2400 °C. What influence the Fe contained in the HiPco tubes had on these structure changes was unclear; similar changes were observed in single-wall carbon nanohorns that did not contain any metal. This indicates that thermally induced changes in the structure of single-wall carbon nanotubes can occur without a metal catalyst. Heat treatment increased the integrity of the nanotube-papers, and this increase may have been due to tube-tube interconnections created by HT.     

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.     

Production of controlled architectures of aligned carbon nanotubes by an injection chemical vapour deposition method

High purity, aligned multi-wall carbon nanotube films were grown on quartz substrates by injecting a solution of ferrocene in toluene into a suitable reaction furnace. The injection CVD method allows excellent control of the catalyst to carbon ratio. The detailed study presented here demonstrates how such a system can be used to control the nanotube diameter, length, alignment and yield by manipulating the experimental parameters. Primary growth was found to occur via a base growth mechanism, although overgrowths of single wall carbon nanotubes were obtained under certain conditions. Such a method also allows nanotubes of various packing densities to be produced which may be useful for specific applications such as electrodes.     

Mechanical and morphological characterization of polymer-carbon nanocomposites from functionalized carbon nanotubes

Carbon nanotubes were functionalized with poly(vinyl alcohol) (PVA). The water-soluble PVA-functionalized carbon nanotubes were then embedded into PVA matrix via a wet-casting method, resulting in polymer-carbon nanocomposite films with homogeneous nanotube dispersion. Composites with pristine and functionalized nanotubes were tested in tension. It was found that the mechanical properties of these nanocomposite films were significantly improved compared to the neat polymer film. Functionalization allowed good distribution of the nanotubes in the matrix, leading to higher film strength. Scanning electron microscopy shows an apparent good wetting of the nanotubes by the PVA matrix. These results are supportive of good interfacial bonding between the functionalized carbon nanotubes and the hosting polymer matrix.     

Purification process for single-wall carbon nanotubes

Single-wall carbon nanotubes (SWNTs) have exceptional strength and stiffness and high thermal and electrical conductivity, making them excellent candidates for aerospace structural materials. However, one of the most fundamental challenges is purifying the SWNTs. The purpose of this study was to develop a simple purification process for SWNTs, along with an understanding of the purification process. In addition, uncomplicated analytical methods were sought to screen and compare various purification methods. In this study, we demonstrate an easy method of cleaning SWNTs and evaluating their purity. The cleaning method, which employed oxidative heat treatment followed by acid reflux, was straightforward, inexpensive, and fairly effective. The purification mechanism was determined to be, first, that much of the non-nanotube carbon and iron catalyst was oxidized and, second, that the acid washing removed the iron oxide, leaving relatively pure SWNTs. Also, it was shown that a combination of thermal gravimetric analysis and Raman spectroscopy, both of which take only a few minutes and require little sample preparation, are sufficient as qualitative screening tools to determine the relative purity of SWNTs. Other analytical techniques were used to verify the validity of the screening techniques.     

Purification of multi-walled carbon nanotubes through microwave heating of nitric acid in a closed vessel

We demonstrate that microwave-assisted heating in 5 mL of nitric acid eliminates impurities, such as amorphous carbon, carbon nanoparticles, and metals, from multi-walled carbon nanotubes (MWNTs). Heating the closed reaction vessel under microwave irradiation at 160 °C for 30 min is a very effective means of purifying the MWNTs. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images confirm that these reaction conditions are beneficial for removing the impurities and ensuring that the MWNTs remain intact. In contrast, a purification temperature of 180 °C provides too strongly oxidizing conditions that destroy the MWNTs. The ratio of the G and D bands in the Raman spectra also confirms that a temperature of 160 °C is optimal. The defect peak that we observed in the differential thermogravimetry (DTG) analysis of the raw material was not present after microwave purification. The presence of metal impurities in the MWNTs can be reduced significantly when using this method.     

Catalytic growth of carbon nanotubes through CHNO explosive detonation

Multi-walled carbon nanotubes (CNTs) have been efficiently synthesized by a self-heating detonation process, operated at low loading densities of picric acid (PA), which acts as the explosive to provide needed high temperatures and parts of carbon sources. Paraffin or benzene provides additional carbon source for tube assembling and hydrogen source to capture oxygen in PA to form H₂O and thus to survive some carbon species from oxidation. Cobalt nanoparticles, in situ formed from a detonation-assisted decomposition and reduction of cobalt acetate, show good catalytic activity for nanotube nucleation and growth and for disproportionation reaction of CO generated from the PA detonation. The nanotubes and catalyst particles are characterized by SEM, TEM, EDX, SAED, XRD, and Raman spectroscopy techniques. Some tubes are well crystallized but others have lots of structural defects, especially for the tubes with thin walls and bamboo-like shapes. The catalyst particles show conical shapes and exhibit a fcc crystalline structure of parent cobalt. These data also experimentally show that tube growth is at a very high rate and suggest that it is possible for a large-scale synthesis of CNTs under high-density and high-pressure conditions.     

Structure of carbon nanotubes probed by local and global probes

A review is proposed of different techniques available today for the characterization of the atomic structure of carbon nanotubes. This review covers the electron microscopies, various diffraction techniques, scanning probe microscopies, and optical spectroscopies, including Raman scattering. The advantages and limitations of the characterization techniques are discussed.     

Selective oxidation of single-walled carbon nanotubes using carbon dioxide

A simple process for selective removal of carbon from single-walled carbon nanotube samples was developed based on a mild oxidation by carbon dioxide. The reactivity profiles of as prepared and purified nanotube samples were determined using both TG and a related analytical technique, controlled atmosphere programmed temperature oxidation (CAPTO). The complex differential rate curves for weight loss (DTG) or carbon dioxide evolution (CAPTO) could be resolved by a series of Gaussian peaks each associated with carbonaceous species of different reactivity. Comparisons were made between samples as received after preparation by the laser ablation method, after purification by nitric acid oxidation, and both of these after reaction with CO₂. The DTG of as prepared tubes had a broad major peak centered about 410 °C. Mild oxidation of as prepared nanotubes under flowing carbon dioxide at 600 °C preferentially removed more reactive carbon species leaving behind a narrower distribution about the major peak in DTG. In contrast to the as prepared material, the sample that had been purified using nitric acid had a more distinct separation of the major DTG peaks between more and less readily oxidized material. Oxidation of this sample with CO₂ selectively removed the peak associated with the most readily oxidized material. The original CO₂ oxidation experiments performed on the analytical scale were successfully scaled up to a small preparative scale.     

New continuous gas-phase synthesis of high purity carbon nanotubes by a thermal plasma jet

We have developed a new gas-phase synthesis technique to produce carbon nanotubes (CNTs) with a continuous process and at high temperature, by using a thermal plasma jet. A thermal plasma jet was generated by applying a direct current of 100-300 A, using Ar as the plasma gas with a flow rate of ∼6 ksccm. The temperature of the thermal plasma jet was very high (∼10⁴ K) and the velocity was very fast (∼100 m/s). Fe(CO)₅ and CO were used as a catalyst precursor and carbon source, respectively. The yield of CNTs was dramatically increased by attaching a helical extension reactor at the end of the plasma nozzle. High purity (∼80%) CNTs were produced with a continuous process by using a thermal plasma jet with helical extension reactor equipment. The number of CNT walls produced was critically affected by the hydrogen gas injected as an auxiliary plasma gas. Without hydrogen gas, single-walled carbon nanotubes whose diameter was about 1 nm were mostly produced while with hydrogen gas double-walled carbon nanotubes (about 4 nm in diameter) were predominantly produced, with small amount of 3- and 4-walled carbon nanotubes.     

Synthesis and Raman characterization of boron-doped single-walled carbon nanotubes

A systematic study was carried out to dope single-walled carbon nanotube (SWNT) bundles with varying amounts of boron using the pulsed laser vaporization technique. Targets containing boron concentrations ranging from 0.5 to 10 at.% boron were prepared by mixing elemental boron with carbon paste and the Co/Ni catalysts. The laser-generated products that were obtained from these targets were characterized by high resolution transmission electron microscopy, electron energy loss spectroscopy (EELS), thermoelectric power (TEP) measurements, and Raman scattering experiments. Electron microscopy and Raman studies revealed that the presence of various levels of boron concentration in the target strongly affected the products that were prepared. SWNTs were found in the products prepared from targets containing up through 3 at.% boron, and high resolution EELS estimated that less than 0.05-0.1 at.% boron is present in the SWNT lattice. The absence of SWNT bundles in the products derived from targets containing more than 3 at.% boron implies that the presence of excess boron in the carbon plume severely inhibits the carbon nanotube growth. The overall effect of the boron incorporation primarily leads to: (i) a systematic increase in intensity of the disorder-induced band (D-band) upon boron doping, with increasing D-band intensity observed for higher doping levels, (ii) a systematic downshift in the G′-band frequency due the relatively weaker C-B bond, and (iii) a non-linear variation in the RBM and G′-band intensities which is attributed to shifts in resonance conditions in the doped tubes. Resonant Raman spectroscopy thus provides large changes in the intensity of prominent features even when the dopant concentration is below the detectable limit of EELS (0.05-0.1 at.%). Thermoelectric power data also provide complementary evidence for the presence of a small boron concentration in the SWNT lattice which transforms the SWNTs into a permanently p-type material.     

Transformation of fullerene peapods to double-walled carbon nanotubes induced by UV radiation

Double-walled carbon nanotubes were prepared by XeCl-laser irradiation of fullerene (C₆₀ or C₇₀) peapods. Raman spectroscopy evidences less defect structure of outer tubes, as compared to those in double-walled carbon nanotubes grown by thermal treatment of peapods. The diameter distribution also differs from that of the thermally prepared nanotubes. At the given laser fluence, the conversion of C₇₀@SWCNT into double-walled carbon nanotubes was more efficient than the corresponding conversion of C₆₀@SWCNT.     

Structure modification of single-wall carbon nanotubes

Various purification processes were applied to single-wall carbon nanotubes synthesized by metal catalyzed laser ablation. Structure modifications introduced by these processes were investigated by high-resolution transmission electron microscopy and Raman spectroscopy. An apparent structure modification after purification was the increase of bundle size although breaking of nanotubes and a change of nanotube diameter distribution were also observed. More vigorous attacking of single-wall carbon nanotube structure was identified by a strong mixed-acid treatment.