The role of hydroxyl and atomic oxygen in multiwall carbon nanotube growth

Multiwall carbon nanotubes (CNTs) were grown by the plasma-enhanced chemical vapor deposition (PECVD) method in downstream on the p-Si (100) substrate. Besides precursors, methane as the carbon source and hydrogen as the ablation, oxygen or H₂O was alternatively inlet into the reactive chamber at the pressure of 0.05 MPa. Given characterizations of the tube structure and tube mass weight, the role of radical atomic O, hydroxyl and perhydroxyl in multiwall CNT growth was explored. In addition to a small amount of O₂ (∼0.67%) or H₂O (∼0.1%), it was found that a high quantity of pure nanotubes can be grown in the downstream. However, no nanotube could be formed or even the carbon matrix generated when the concentration of O₂ or H₂O exceeded a proper value in the mixture. The mechanism of multiwall CNT growth controlled by active radicals was explored in this paper.     

The role of hydroxyl and atomic oxygen in multiwall carbon nanotube growth

Multiwall carbon nanotubes (CNTs) were grown by the plasma-enhanced chemical vapor deposition (PECVD) method in downstream on the p-Si (100) substrate. Besides precursors, methane as the carbon source and hydrogen as the ablation, oxygen or H₂O was alternatively inlet into the reactive chamber at the pressure of 0.05 MPa. Given characterizations of the tube structure and tube mass weight, the role of radical atomic O, hydroxyl and perhydroxyl in multiwall CNT growth was explored. In addition to a small amount of O₂ (∼0.67%) or H₂O (∼0.1%), it was found that a high quantity of pure nanotubes can be grown in the downstream. However, no nanotube could be formed or even the carbon matrix generated when the concentration of O₂ or H₂O exceeded a proper value in the mixture. The mechanism of multiwall CNT growth controlled by active radicals was explored in this paper.     

Synthesis of high aspect-ratio carbon nanotube "flying carpets" from nanostructured flake substrates

We present a robust method for synthesis of aligned, single-walled carbon nanotube (CNT) "flying carpets" from nanostructured alumina flakes. Roll-to-roll e-beam deposition is utilized to produce the flakes, and hot filament chemical vapor deposition is utilized to grow dense, aligned carbon nanotubes from the flakes with remarkably high CNT yields. The flakes are captured inside a mesh cage and freely suspended in the gas flow during growth. Optical characterization indicates the presence of high quality, small diameter single-walled carbon nanotubes.     

Investigation of influence of synthesis parameters on length and purity of the carbon nanotubes

In this article, a systematic study was conducted to understand the influences of various synthesis parameters, such as catalyst pretreatment time, growth time, growth temperature, reaction gas flow rate on length and quality of the carbon nanotubes grown by thermal chemical vapor deposition (TCVD). Carbon nanotube (CNT) grown on Fe deposited on silicon substrates were characterized by scanning electron microscope and Raman spectroscopy. It was found that all of the synthesis parameters investigated had effects on both length and quality of the carbon nanotube. After optimizing the various thermal chemical vapor deposition synthesis parameters, long carbon nanotube arrays of up to 150 microm in length were successfully synthesized and possess the potential application in multi-level interconnects.     

Carbon Nanotube Synthesis via the Catalytic CVD Method: A Review on the Effect of Reaction Parameters

This review covers the results obtained in carbon nanotube synthesis by chemical vapor deposition. Parameters such as catalysts, supports, carbon precursors, reaction time, temperature and gas flow rates that are used in the production of carbon nanotubes are discussed throughout the text. Purification of the synthesized carbon nanotubes and methods utilized for cost reduction were also explored.     

Modeling of the carbon nanotube chemical vapor deposition process using methane and acetylene precursor gases

Chemical vapor deposition of carbon nanotubes (CNTs) in a horizontal tube-flow reactor has been investigated with a fully coupled reactor-scale computational model. The model combined conservation of mass, momentum, and energy equations with gas-phase and surface chemical reactions to describe the evolution of a hydrogen and hydrocarbon feed-stream as it underwent heating and reactions throughout the reactor. Investigation was directed toward steady state deposition onto iron nanoparticles via methane and hydrogen as well as feed-streams consisting of acetylene and hydrogen. The model determines gas-phase velocity, temperature, and concentration profiles as well as surface concentrations of adsorbed species and CNT growth rate along the entire length of the reactor. The results of this work determine deposition limiting regimes for growth via methane and acetylene, demonstrate the need to tune reactor wall temperature to specific inlet molar ratios to achieve optimal CNT growth, and demonstrate the large effect that active site specification can have on calculated growth rate.     

Evaluation of the optimal carrier gas flow rate for the carbon nanotubes growth

Arrays of aligned carbon nanotubes (CNTs) were synthesized in a gas-phase flow reactor by thermal decomposition of reaction mixture (2% solution of ferrocene in toluene) on the surface of silicon substrates heated to 800°C. Variation of the height of the CNT array as a function of position of the substrate in the reactor and carrier gas flow rate was registered. The difference in the obtained dependences and temperature distribution in the reactor points to the necessity of taking into account the change in the concentration of the active carbon component in the gas mixture. An expression associating the parameters of synthesis and thickness of the CNT array being formed on the substrate is offered.     

Nanodimensional carbon materials formed in a gas discharge plasma

We have experimentally studied the formation of nanodimensional carbon materials from a methane-hydrogen gas mixture activated by a dc discharge. The range of discharge voltages and currents ensuring stable deposition of carbon films was determined. Data on the carbon-containing components of the activated gas phase were obtained by in situ optical emission spectroscopy of the gas discharge plasma. It is shown that the formation of nanodiamond and nanographite particles, as well as carbon nanotubes, in the deposited films is correlated with the presence of C₂ carbon dimers in the gas phase. A mechanism of the noncatalytic formation of carbon nanotubes from platelike graphite nanoparticles is proposed.     

Functionalization of gold-coated carbon nanotubes with self-assembled monolayers of thiolates

Multi-walled carbon nanotubes (CNT) were synthesized by chemical vapor deposition using Co–Fe as a catalyst and ethylene as a carbon source. Afterward, a simple method combining wet-chemistry and chemical reduction was used to prepare carbon nanotube/gold material (CNT/Au). Pristine nanotubes and CNT/Au were characterized by transmission electron microscopy micrographs. It appeared that gold formed nanoparticles on CNTs endings and their sidewalls. Further functionalization was carried out by using thiols of different chemical properties and molecule sizes. Thiols formed self-assembled monolayer on gold surface that led to formation of CNT/gold/thiol-functionalized material. The amounts of chemisorbed thiols were measured by elemental analysis and thermogravimetry.     

Synthesis and properties of poly(hexamethylene terephthalate)/multiwall carbon nanotubes nanocomposites

Poly(hexamethylene terephthalate) (PHT)/carbon nanotubes (CNT) nanocomposites containing 1% and 3% (w/w) of filler were prepared by two procedures: in situ ring-opening polymerization of hexamethylene terephthalate cyclic oligomers in the presence of CNT and melt blending of PHT/CNT mixtures. Arc discharge multiwalled carbon nanotubes, both pristine (MWCNT) and hydroxyl functionalized (MWCNT-OH), were used. The objective was to evaluate the effect of preparation procedure, nanotube side-wall functionalization and amount of nanotube loaded on properties of PHT. All nanocomposites showed an efficient distribution of the carbon nanotubes within the PHT matrix but interfacial adhesion and reinforcement effect was dependent on both functionalization and nanotubes loading. Significant differences in thermal stability and mechanical properties ascribable to functionalization and processing were observed among the prepared nanocomposites. All the prepared nanocomposites showed enhanced crystallizability due to CNT nucleating effects although changes in melting and glass transition temperatures were not significant.     

Scalable fabrication of carbon nanotube/polymer nanocomposite membranes for high flux gas transport

We present a simple, fast, and practical route to vertically align carbon nanotubes on a porous support using a combination of self-assembly and filtration methods. The advantage of this approach is that it can be easily scaled up to large surface areas, allowing the fabrication of membranes for practical gas separation applications. The gas transport properties of thus constructed nanotube/polymer nanocomposite membranes are analogous to those of carbon nanotube membranes grown by chemical vapor deposition. This paper shows the first data for transport of gas mixtures through carbon nanotube membranes. The permeation of gas mixtures through the membranes exhibits different properties than those observed using single-gas experiments, confirming that non-Knudsen transport occurs.     

Interaction of methane with carbon nanotube thin films: role of defects and oxygen adsorption

This paper deals with the dependence of the electrical conductance on the presence of structural defects and of molecular oxygen adsorbates in carbon nanotube (CNT) thin films for gas molecule detection. Our results show that oxygen contamination may be responsible for the reported sensitivity of the electronic and transport properties to methane at room temperature. In particular, the sample exhibits a crossover from decreasing to increasing electrical resistance vs. methane concentration depending on the surrounding atmosphere. The obtained results show that when the nanotube walls contain topological defects, oxygen molecules become chemisorbed. We suggest that the conductivity type of the CNT can be changed from p-type to n-type by adsorption of O₂ acting as an electron and donor doping the CNTs, which has p-type semiconductor character in the outgassed state. The obtained results demonstrate that nanotubes could be used as sensitive chemical gas sensor likewise indicate that intrinsic properties measured on as-grown nanotubes may be severely changed by extrinsic oxidative treatments.     

Effect of filling on the compressibility of carbon nanotubes: predictions from molecular dynamics simulations

The compressibility of filled and empty (10, 10) carbon nanotubes (CNTs) is examined using classical molecular dynamics simulations. The filled nanotubes contain C60, CH4, Ne, n-C4H10, and n-C4H7 molecules that are covalently cross-linked to the inner CNT walls. In addition, nanotubes filled with either a hydrogen-terminated carbon nanowire or a carbon nanotube of comparable diameter is also considered. The forces on the atoms are calculated using a many-body reactive empirical bond-order potential and the adaptive intermolecular reactive empirical bond-order potential for hydrocarbons. The butane-filled system shows a unique yielding behavior prior to buckling that has not been observed previously. Cross-linking the molecules to the inner CNT walls is not predicted to affect the stiffness of the filled nanotube systems and removes the yielding response. The mechanical response of the nanowire filled CNT is remarkably similar to the response of the similarly sized multiwalled CNT.     

Effect of Ni loading and reaction temperature on the formation of carbon nanotubes from methane catalytic decomposition over Ni/SiO<Subscript>2</Subscript>

Since their discovery carbon nanotubes (CNT) have attracted much attention due to their singular physical, mechanical and chemical properties. Catalytic chemical vapor deposition (CCVD) of hydrocarbons over metal catalysts is the most promising method for the synthesis of CNT, because of the advantages of low cost and large-scale production and the relatively low temperature used in the process, compared to the other methods (laser ablation and discharge between graphite electrodes). In this study, CNT were synthesized by CCVD using Ni supported on SiO₂ as a catalyst. The carbon deposited in the reaction was analyzed by Raman spectroscopy, thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The effects of reaction temperature and Ni loading on the carbon nanotube formation were evaluated. The catalyst with 5% Ni favored high yield of CNT at lower temperature, with abundant “multi-walled carbon nanotubes” (MWNTs) at 625 °C, while single-walled carbon nanotubes (SWNTs) and MWNTs were obtained at 650 °C. With an increase in the reaction temperature a marked decrease in the yield of CNT was observed, probably due to the sintering of the catalyst. The catalyst with 1% Ni gave SWNTs with a high degree of order at all reaction temperatures, but in low quantity.     

Effect of fibre coating and geometry on the tensile properties of hybrid carbon nanotube coated carbon fibre reinforced composite

Hierarchically structured hybrid composites are ideal engineered materials to carry loads and stresses due to their high in-plane specific mechanical properties. Growing carbon nanotubes (CNTs) on the surface of high performance carbon fibres (CFs) provides a means to tailor the mechanical properties of the fibre–resin interface of a composite. The growth of CNT on CF was conducted via floating catalyst chemical vapor deposition (CVD). The mechanical properties of the resultant fibres, carbon nanotube (CNT) density and alignment morphology were shown to depend on the CNT growth temperature, growth time, carrier gas flow rate, catalyst amount, and atmospheric conditions within the CVD chamber. Carbon nanotube coated carbon fibre reinforced polypropylene (CNT-CF/PP) composites were fabricated and characterized. A combination of Halpin–Tsai equations, Voigt–Reuss model, rule of mixture and Krenchel approach were used in hierarchy to predict the mechanical properties of randomly oriented short fibre reinforced composite. A fractographic analysis was carried out in which the fibre orientation distribution has been analyzed on the composite fracture surfaces with Scanning Electron Microscope (SEM) and image processing software. Finally, the discrepancies between the predicted and experimental values are explained.     

Nanotube-Based Chemical and Biomolecular Sensors

We present a brief review about recent results regarding carbon nanotube （CNT）-based chemical and biomolecular sensors. For the fabrication of CNT-based sensors, devices containing CNT channels between two metal electrodes are first fabricated usually via chemical vapor deposition （CVD） process or ＂surface programmed assembly＂ method. Then, the CNT surfaces are often functionalized to enhance the selectivity of the sensors. Using this process, highly-sensitive CNT-based sensors can be fabricated for the selective detection of various chemical and biological molecules such as hydrogen, ammonia, carbon monoxide, chlorine gas, DNA, glucose, alcohol, and proteins.     

CVD growth of carbon nanotubes on thin-film Ni20Ti35N45 alloy catalyst

The possibility of forming carbon nanotube (CNT) arrays on a Ni–Ti–N catalytic alloy with low nickel content by chemical vapor deposition (CVD) is demonstrated. Adding nitrogen to the Ni–Ti alloy composition favors the formation of TiN compound and segregation of Ni on the surface, where it produces a catalytic effect on the CNT growth. It is found that, using CVD from acetylene gas phase at a substrate temperature of 650°C, a CNT array of 9-µm height can be grown for 2 min.     

Reactor length-scale modeling of chemical vapor deposition of carbon nanotubes

Chemical Vapor Deposition (CVD) of carbon nanotubes from a gas mixture consisting of methane (carbon precursor) and hydrogen (a carrier gas) in the presence of cobalt, nickel or iron catalytic particles in a cylindrical reactor is modeled at the reactor length-scale by solving a continuum-based coupled boundary-layer laminar-flow hydrodynamics, heat-transfer, gas-phase chemistry and surface chemistry problem. The model allows determination of the gas-phase fields for temperature, velocity, and various species as well as the surface-species coverages and the carbon deposition rate. Various available experimental and theoretical assessments are used to construct the necessary database for gas-phase and surface chemistry and gas-phase transport parameters. A reasonably good agreement is found between the model predicted and the experimentally measured carbon nanotubes deposition rates over a relatively large range of processing conditions.     

Carbon arc plasma as a source of nanotubes: emission spectroscopy and formation mechanism

Diagnostics of carbon arc plasma by optical emission spectroscopy during the synthesis of carbon nanotubes is reviewed. Spatial distributions of temperature and C2 radicals in different plasmas are presented. The influence of gas pressure, anode composition, and reaction environment is discussed. Mechanisms of carbon nanotube formation are reviewed, with an emphasis on surface diffusion processes and catalytic effects.     

Coral-like amorphous carbon nanotubes synthesized by a modified arc discharge

A coral-like amorphous carbon nanotube was prepared by a modified arc discharging furnace in hydrogen atmosphere with a mixture of Mo-Co₂O₃-Mg powders as catalyst at 600°C. This carbon nanotube presented a microscopic coral-like by SEM observation and amorphous structure of nanotubes by HRTEM observation. The XRD diffraction and Raman pattern presented noncrystal characteristics compared to the normal graphite structure. We believed that these results may be affected by the “synergistic” effect of catalyst, atmosphere, and temperature in the synthesis process. The possible explanations to the formation mechanism of this novel carbon nanotube have also been proposed.