Synthesis Of Single-walled Carbon Nanotubes Research Articles

Nitrogen‐doped SWCNT synthesis using ammonia and carbon monoxide

AbstractGreat effort has been directed into modifying the electrical properties of produced carbon nanotubes by chemical doping and – in parallel – into controlling their chirality. We present a new approach to synthesizing nitrogen‐doped single‐walled carbon nanotubes (SWCNTs), based on gas phase floating catalyst CVD. The nitrogen content in the tubes was verified by transmission electron microscopy/electron‐energy loss spectroscopy (TEM/EELS), and their chiral angle distribution found by electron diffraction (ED) to be skewed toward armchair tubes. Additionally, the optical properties of the material were probed by multi‐frequency Raman spectroscopy and optical absorption.

Synthesis of SWNTs over nanoporous Co-Mo/MgO and using as a catalyst support for selective hydrogenation of syngas to hydrocarbon

Single-wall carbon nanotubes (SWNTs) with high surface area were synthesized over nanoporous Co-Mo/MgO by a chemical vapor deposition (CVD) method. The SWNTs were used as catalyst support for selective hydrogenation of syngas to hydrocarbons. Here an extensive study of Fischer-Tropsch synthesis (FTS) on CNT-supported cobalt catalysts with different amounts of cobalt loading up to 40 wt% is reported. The catalysts were characterized by different methods including N 2 adsorption-desorption, X-ray diffraction, hydrogen chemisorption, inductively coupled plasma (ICP) and temperature-programmed reduction. Enhancement of the reducibility of Co 3O 4 to CoO, CoO to Co o and small cobalt oxide particles, dispersion of the cobalt, and activity and selectivity of FTS were investigated and compared with a conventional support. The CNT supported catalysts achieve a high dispersion and high loading of the active metal, cobalt in particular, so that the bulk formation of cobalt metal, which tends to occur in conventional support, can be avoided. The results showed that the specific activity of CNT supported catalysts increase significantly (there is a two fold increase in CO Conversion per gram of the active metal) with respect to the conventional supported catalyst.

Synthesis and Characterization of Polyurethane-Grafted Single-Walled Carbon Nanotubes via Click Chemistry

Polyurethane (PU)-grafted carbon nanotubes were synthesized by the coupling of alkyne moiety decorated single walled carbon nanotube (SWCNT) with azide moiety containing PU using Cu(I) catalyzed Huisgen [3 + 2] cycloaddition click chemistry. The azide moiety containing poly(s-caprolactone)diol was synthesized by ring-opening polymerization and further used for PU synthesis. Alkyne-functionalizion of SWCNT was completed by the reaction of p-aminophenyl propargyl ether with SWCNT using a solvent free diazotization procedure. Nuclear magnetic resonance, Fourier transform infrared, and Raman spectroscopic measurements confirmed the functionalization of SWCNT. Scanning electron microscopy and transmission electron microscopy images showed an excellent dispersion of SWCNTs, and specially debundling of SWCNTs could be observed due to polymer assisted dispersion. A quantitative grafting was successfully achieved even at high content of functional groups.

Melting of Ni and Fe Nanoparticles: A Molecular Dynamics Study with Application to Carbon Nanotube Synthesis

Molecular dynamics simulations with many-body interatomic potentials are used to study melting of Ni and Fe nanoparticles with diameters that range between 2 and 12 nm. Two different embedded-atom method interatomic potentials are used for each element. The capability of each interatomic potential to model (i) size-dependent melting in nanoparticles and (ii) the bulk melting temperature of Ni or Fe is explored. In agreement with existing theory, molecular dynamics simulations show that the melting temperature of non-supported nanoparticles decreases with decreasing nanoparticle size, displaying a linear relationship with the inverse of nanoparticle diameter. However, molecular dynamics simulations using the interatomic potentials considered in this work provide a lower estimate than existing theory for the sensitivity of the melting temperature to nanoparticle size (slope of linear relationship). Molecular dynamics simulations demonstrate that melting is surface initiated and that a finite temperature range exists in which partial melting of the nanoparticle occurs. This observation is very important in the development of advanced vapor-liquid-solid models for catalyst-assisted single-walled carbon nanotube synthesis.

Experimental evidence for the early nucleation of single-walled carbon nanotubes

In situ optical emission spectroscopy (OES) was used to investigate the spatial and temporal evolution of C 2 , the electron density ( N e ) and the electron temperature ( T e ) of laser induced plasmas in the laser-furnace method of synthesizing single-walled carbon nanotubes (SWCNTs). The intensities of the spectral maps of C 2 , N e and T e showed a strong temperature dependence. The frequency of sharp fluctuations which appear as hot spots in the spectral maps of N e and T e increased as the furnace temperature was increased from 1073 to 1273 K. We suggest that the appearance of hot spots is a result of the heat of formation of SWCNTs.

Cap Formation Engineering: From Opened C60 to Single-Walled Carbon Nanotubes

Structural control, and especially chirality control, remains a significant challenge in the synthesis of single-walled carbon nanotubes (SWNTs). We report herein a rational approach to engineering fullerene caps for growing SWNTs with controlled structures via chemical vapor deposition (CVD). Opening of fullerendione via thermal oxidation yields hemispherical caps which can initiate SWNT growth at their open ends. The size and structure of these caps can be engineered by tuning the temperature of thermal oxidation. Results show that pregrowth treatment of the cap is indispensable to successful growth of SWNTs. The temperature used for thermal oxidation strongly affects the size and structure of the cap and further determines the diameter distribution of the as-grown SWNTs. Stronger oxidation treatments (450 degrees C oxidation in air) promote formation of thinner SWNTs, while weaker oxidation treatments (350 degrees C oxidation in air) lead to wider SWNTs. Interestingly, SWNTs made using fullerene caps show steplike diameter distributions relative to SWNTs catalyzed by Fe nanoparticles. This cap engineering using opened C(60) provides a potential approach to grow SWNTs with controlled structures.

Position-controlled synthesis of single-walled carbon nanotubes on a transparent substrate by laser-induced chemical vapor deposition

The synthesis of single-walled carbon nanotubes (SWCNTs) on a transparent substrate with multiple-catalyst layer (Fe/Al/Cr: 0.5/15/500nm) using laser-induced chemical vapor deposition is reported. Ethylene (C2H4) mixed with hydrogen (H2) and a continuous wave Nd:YVO4 laser (532nm) were used as the precursor gas and the irradiation source, respectively. It was found that the density and quality of the SWCNT dots varied sensitively to laser irradiance and chamber pressure. From subsequent micro-Raman analyses at different excitation sources (488, 514, 633, and 785nm), the diameters of the SWCNTs were estimated to be within the range of 0.8–2nm and that the SWCNT dots were composed of both semiconducting and metallic SWCNTs. It is demonstrated that an array of SWCNT dots can be fabricated at precisely controlled positions of a transparent substrate at room temperature with no need of catalysis patterning.

Characterization of diffusion flames for synthesis of single-walled carbon nanotubes

Recent studies have shown that Fe/Si/O catalysts on the fuel side of an oxygen-enriched inverse diffusion flame produce micron-length single-walled carbon nanotubes at rapid rates (>100 μm/s). Despite the favorable catalyst/flame interaction for nanotube nucleation and growth, the catalyst lifetimes are only a few milliseconds. To increase catalyst lifetime and hence, carbon nanotube length, it is necessary to know how the local environment changes as the catalyst moves through the flame. A 2-D computational fluid dynamics model with detailed chemistry is employed to investigate the nature of the flame environment along various catalyst trajectories. The results indicate that temperature and species concentrations do not change significantly along individual catalyst trajectories, although not all trajectories experience the same environment due to the steep gradients in the radial direction. On the other hand, analysis of catalyst particle composition before and after nanotube growth shows that catalyst oxygen content decreases significantly during nanotube growth. This change in catalyst composition could affect the relative rates of carbon supply versus removal from the catalyst surface, such that carbon encapsulation and thus poisoning of the catalyst is favored after sufficient time. The results of this work indicate that catalyst deactivation, not a changing catalyst environment, is responsible for rapid encapsulation of the catalyst by amorphous carbon and thus, the short catalyst lifetimes observed in oxygen-enriched diffusion flames.

Synthesis, characterization, and electrical properties of nitrogen-doped single-walled carbon nanotubes with different nitrogen content

Nitrogen-doped single walled carbon nanotubes (SWCNTs) have been synthesized via the thermal decomposition of ferrocene using different ratios of acetonitrile/ethanol feedstock mixtures during the chemical vapor deposition process. The experiments were performed at 950 °C and 2 bar. The concentration of acetonitrile in the mixtures was varied from 0% to 100%. High resolution transmission electron microscopy and Raman spectroscopical measurements revealed the formation of SWCNTs for all mixtures. X-ray photoelectron spectroscopical analysis show nitrogen doping levels of up to 2 at.%. The doping levels increase as the acetonitrile concentration increases. The nitrogen incorporation is predominantly in the pyridine form. Electrical conductivity measurements show the dependence of conductivity as a function of nitrogen incorporation in the SWCNTs.

Impact of Thermodiffusion on Carbon Nanotube Growth by Chemical Vapor Deposition

Thermal diffusion, the process by which a multicomponent mixture develops a concentration gradient when exposed to a temperature gradient, has been studied in order to understand if its inclusion is warranted in the modeling of single-wall carbon nanotubes (SWNTs) synthesis by thermal chemical vapor deposition (CVD). A fully coupled reactor-scale model employing conservation of mass, momentum, species, and energy equations with detailed gas phase and surface reaction mechanisms has been utilized to describe the evolution of hydrogen and hydrocarbon feed streams as they undergo transport, as well as homogeneous and heterogeneous chemical reaction within a CVD reactor. Steady state velocity, temperature, and concentration fields within the reactor volume are determined, as well as concentrations of adsorbed species and SWNT growth rates. The effect of thermodiffusion in differing reactor conditions has been investigated to understand the impact on SWNT growth. Thermal diffusion can have a significant impact on SWNT growth, and the first approximation of the thermal diffusion factor, based on the Chapman–Enskog molecular theory, is sufficient for modeling thermophoretic behavior within a CVD reactor. This effect can be facilitatory or inhibitory, based on the thermal and mass flux conditions. The results of this investigation are useful in order to optimize model and reactor designs to promote optimal SWNT deposition rates.

Diameter-Dependent Solubility of Single-Walled Carbon Nanotubes

We study the solubility and dispersibility of as-produced and purified HiPco single-walled carbon nanotubes (SWNTs). Variation in specific operating conditions of the HiPco process are found to lead to significant differences in the respective SWNT solubilities in oleum and surfactant suspensions. The diameter distributions of SWNTs dispersed in surfactant solutions are batch-dependent, as evidenced by luminescence and Raman spectroscopies, but are identical for metallic and semiconducting SWNTs within a batch. We thus find that small diameter SWNTs disperse at higher concentration in aqueous surfactants and dissolve at higher concentration in oleum than do large-diameter SWNTs. These results highlight the importance of controlling SWNT synthesis methods in order to optimize processes dependent on solubility, including macroscopic processing such as fiber spinning, material reinforcement, and films production, as well as for fundamental research in type selective chemistry, optoelectronics, and nanophotonics.

Current and Arc Pushing Force Effects on the Synthesis of Single-Walled Carbon Nanotubes by Arc Discharge

The current and arc pushing force of arcing process effects on the synthesis of single-walled carbon nanotubes were investigated by a temperature-controlled arc discharging furnace with Co-Ni alloy powder as catalyst at 600 degrees C. By experimental investigations, the optimal parameters of the process were obtained. The appropriate conditions included the current and arc pushing force of arcing process was 100 A and 80 A respectively, the buffer gas was helium and the gas pressure was kept at 500 torr. The experimental results indicated that the cooperative function of current and arc pushing force of arcing process played an important role in the production rate and purity of single-walled carbon nanotubes. The uniform diameter of single-walled carbon nanotubes was about 1.27 nm and the production rate was about 7 g/h and the purity was around 70%.

Parametric Study of Alcohol Catalytic Chemical Vapor Deposition for Controlled Synthesis of Vertically Aligned Single-Walled Carbon Nanotubes

In this study we examine catalyst preparation and chemical vapor deposition (CVD) parameters related to synthesis of single-walled carbon nanotubes (SWNTs) by alcohol catalytic CVD. We show that modifying the catalyst recipe considerably changes the average SWNT diameter, and vertically aligned arrays with an average diameter of 1.5 nm were obtained. The height of vertically aligned SWNT arrays can be significantly enhanced by surface modification of the substrate prior to dip-coating, although this produces SWNTs with larger diameters at the root of the array. We demonstrate patterned growth by combining this method with suppression of SWNT synthesis by formation of a hydrophobic surface. We also consider the effects of ethanol flow rate and thermal decomposition on the chemical environment at the substrate. The growth process is considerably altered by the extent of ethanol decomposition, with sudden termination of the growth occurring in the extreme low-flow (complete decomposition) case.

Low-Temperature Synthesis of Single-Walled Carbon Nanotubes by Alcohol Gas Source Growth in High Vacuum

Carbon nanotube (CNT) growth was carried out on SiO2/Si substrates with a Co catalyst using an alcohol gas source method in an ultra-high vacuum chamber. The resulting CNTs were characterized by scanning electron microscopy (SEM), Raman spectroscopy and transmission electron microscopy (TEM). Reducing the ethanol pressure decreased the optimum growth temperature for maximum yield, enabling single-walled carbon nanotube (SWNT) growth at 400 degrees C. By employing an Al2Ox buffer layer, SWNT yield increased several times, even at 400 degrees C. Under TEM observation, the Co particle size on the Al2Ox layers did not show a significant dependence on the growth temperature between 400 and 700 degrees C. Raman and TEM results confirmed activation of Co particles with larger diameter (>1 nm) by the Al2Ox buffer layer.

Synthesis of Single-Walled Carbon Nanotubes Through Micropores of Surface-Treated Zeolites by Catalyst-Supported Chemical Vapor Deposition

Cobalt nanoparticle-loaded Y-type zeolite (HSZ390HUA) without outer-surface Co aggregates is successfully prepared by Co impregnation reaction followed by surface treatment using tetraethylammoniumbromide having larger molecule size than the orifice size of the Y-type zeolite. Single-wall carbon nanotubes (SWCNTs) have been synthesized by ethanol catalyst-supported chemical vapor deposition at 973 K for 10 min using as-produced and surface-treated Co-loaded zeolites. The SWCNTs synthesized are characterized by transmission electron microscopy, Raman spectroscopy and photoluminescence spectroscopy. The products synthesized by using the as-produced Co-loaded zeolites are mostly multi-wall carbon nanotubes (MWCNTs) with larger diameter than the orifice of zeolite and carbon nanocapsules (CNCs). In contrast, SWCNTs with diameters of 0.66-0.85 nm, which are approximately the same as that of the orifice size, are selectively synthesized by using the surface-treated Co-loaded zeolites.

Effect of Vaporization Temperature on the Diameter and Chiral Angle Distributions of Single-Walled Carbon Nanotubes

Pulsed laser vaporization synthesis of single-wall carbon nanotubes on Co/Ni and Rh/Pd catalysts was explored with respect to variations in the production temperature. The nanotube type populations were determined via photoluminescence, UV-Vis-NIR absorption and Raman spectroscopy. It was found that lowered production temperature leads to smaller nanotube diameters and exceptionally narrow (n, m) type distributions, with marked preference towards large chiral angles for both catalysts. Interestingly, larger nanotube diameters tend to be associated with larger chiral angles. These results demonstrate that PLV production technique can provide at least partial control over the nanotube (n, m) populations. In addition, these results have implications for the understanding the nanotube nucleation mechanism in the laser oven. SWCNT synthesized at lower temperatures appear quite attractive as a starting material for nanotube type separation experiments.

Synthesis of single-walled carbon nanotubes, their ropes and books

The carbon nanotube (CNT) is a model system of one-dimensional nanomaterials, which shows numerous unique properties and vast potential applications. The synthesis and assembly of high-quality CNTs are the basis for studies of their properties and applications. In this contribution, we briefly review our works on the controlled synthesis of single-walled CNTs (SWCNTs) and double-walled CNTs via floating catalyst chemical vapor deposition and arc-discharge methods. The in situ assembly of such CNTs into ordered macrostructures with various dimensions including ropes, films, and buckybooks are also presented. Finally, we introduced our recent findings on metal-catalyst-free growth of SWCNTs. (C) 2010 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.

Study on the effect of the metal–support (Fe-MgO and Pt-MgO) interaction in alcohol-CVD synthesis of carbon nanotubes

This study presents the effect of the metal–support interaction in two systems: (1) iron particle, and (2) platinum particles, being supported on magnesium oxide (MgO) nanopowder in alcohol-CVD process for carbon nanotubes (CNTs) growth. The employment of the different metals but the same substrate (with equal molar ratio) resulted in the synthesis of single-walled CNTs (SWCNTs) or double-walled CNTs (DWCNTs), using iron and platinum, respectively. Furthermore, along with the prolongation of the process time, the decrease of the mean nanotubes diameter in case of iron-catalyzed materials was detected. Interestingly, the extention of the growth time in the synthesis using Pt/MgO resulted in the synthesis of the thicker mean nanotubes diameter. However, for both applied catalytic systems the reduction of the diameter distribution of the tubes and the increase of relative purity of the samples upon the growth time increase were detected.

Experimental and Numerical Studies of Heat Convection in the Synthesis of Single-Walled Carbon Nanotubes by Arc Vaporization

Numerical analysis on heat convection in the synthesis of single-walled carbon nanotubes (SWNTs) by arc vaporization is carried out to calculate the cooling rate of carbon clusters and the temperature distribution of the buffer gas (helium) during the synthesis. The effects of three factors, gravity (G = 1, 2, and 3g0, where 1g0 is normal earth gravity), input power (P = 500, 1000, 2000, and 3000 W), and buffer gas pressure ( p = 40, 50, 80, and 100 kPa) are investigated. By comparing the calculated results with those of a previous synthesis experiment under gravity conditions of G = 1, 2, and 3g0, it is clarified that differences in the cooling rate and the shape of the high-temperature region of the buffer gas cause considerable changes in both the yield and quality of SWNTs produced. The similarity of the heat convection under different input powers and buffer gas pressures is also considered.

Synthesis of SWNTs from charcoal by arc-discharging

With charcoal as carbon source and FeS (20wt%) as catalyst, single-wall carbon nanotubes (SWNTs) were successfully produced by dc arc-discharge method. The analysis by scanning electron microscopy, high-resolution transmission electron microscopy and Raman spectroscopy clearly demonstrated that the SWNTs developed from charcoal achieved a high purity with diameter about 1.2 nm, and FTIR measurement showed the inner structural transformation of carbon source, in which functional groups and grapheme sheets were decomposed and then ordered into nanotubes. By this easy-get and relatively low cost material, the experimenal results clearly indicate that charcoal has the opportunity of reducing the cost of SWNTs production.