On the mechanism of carbon nanotube formation: I. The thermodynamics of formation of carbon molten drops in a metallic catalyst

Carbon molten drops in a metallic catalyst are known to be nucleation centers for carbon nanotubes. The problem of the kinetics of condensation of such drops in wide concentration ranges of carbon and metal vapors is considered. The equilibrium distribution of the drops over the size and mole fraction of the components is obtained. The main result is the calculation of the quasi-steady-state rate of condensation of the molten drops in a supersaturated carbon vapor. This result forms the basis for the calculation of the characteristics of explosive and rapid condensation of the vapor upon its cooling. This calculation is performed in the next part of this work.     

On the mechanism of carbon nanotube formation in electrochemical processes

Quantum-chemical methods are used to analyze the mechanism of carbon nanotube formation in the electrochemical bath, where tiny fragments of graphene planes are in the environment of atoms and ions of alkali metals and halogens. In the optimal configuration, alkali metal atoms move toward the edge of a graphene fragment, whereas halogen atoms remain at the sites of their initial attachment. When the graphene fragments “burdened” by alkali metal and halogen atoms interact with each other, the overall graphene configuration twists in a natural way into a nanotube-like open-end structure.     

On the mechanism of carbon nanotube formation: II. The kinetics of explosive condensation of carbon molten drops in a metallic catalyst

The preliminary stage of the formation of carbon nanotubes by the vapor-liquid-drop mechanism is considered as applied to the condensation of drops from carbon and metal vapors. The problem of the condensation of molten drops is solved for a wide concentration range for both vapors at a condensation temperature. It is shown that, at very high concentrations of the metal vapor (10¹⁸–10¹⁹ cm⁻³) and high temperatures (about 0.3 eV), peculiar heterogeneous condensation of the drops can occur at huge supersaturation of the carbon vapor and the saturated metal vapor. This problem of the condensation of the binary vapor is of methodical interest. This condensation is shown to be unrealizable in real experiment at the parameters of the carbon and metal vapors; it virtually merges with the homogeneous condensation of the metal vapor. The maximum concentration of the carbon vapor below which carbon condenses into drops and above which carbon condenses into amorphous soot particles is calculated. The calculation makes it possible to propose a new approach to the controlled growth of carbon nanotubes.     

Ultra-long carbon nanotube growth on catalyst

Ultra-long carbon nanotube growth with Fe particles sitting at tip end as a function of reaction time, reaction temperature, diameter of the carbon nanotube, damping factor of the system, and the type of catalyst in chemical vapor deposition is investigated by using a theoretical analysis on the phonon vibration of the system. Simulations demonstrate that metal cluster makes and keeps the carbon atoms at tip end reactive. So carbon nanotube grows more than 4 cm. In addition, results show carbon nanotubes with larger diameter grow lesser owing to higher damping factors. In addition, effect of temperature on growth is discussed and it is shown that there is an optimum temperature for growth process. Lastly, dependence of type of catalyst on growth process is investigated.     

The role of precursor gases on the surface restructuring of catalyst films during carbon nanotube growth

Catalyst films undergo considerable surface morphology restructuring prior to carbon nanotube nucleation, deeply influencing the nanostructures obtained. Here we study the influence of different gaseous atmospheres on the structure of thin Fe films. The morphology is influenced by process temperature and substrate interactions and varying the gas type and pressure can control the average catalyst island height.     

Microscopic formation mechanism of nanotube peapods

Using molecular dynamics calculations, we investigate the absorption of a C(60) molecule in a (10,10) nanotube either through the open end or a large defect in the tube wall as possible scenarios for the hierarchical self-assembly of (C(60))(n)@(10,10) "nano-peapods." We find the absorption through a defect to be significantly more efficient than the end-on absorption. This process occurs most likely within a narrow launch velocity range for the fullerene that agrees well with the observed optimum temperature window for peapod formation.     

Pattern formation on carbon nanotube surfaces

Calculations of fluorine binding and migration on carbon nanotube surfaces show that fluorine forms varying surface superlattices at increasing temperatures. The ordering transition is controlled by the surface migration barrier for fluorine atoms to pass through next neighbor sites on the nanotube, explaining the transition from semi-ionic low coverage to covalent high coverage fluorination observed experimentally for gas phase fluorination between 200 and 250 degrees C. The effect of solvents on fluorine binding and surface diffusion is explored.     

Effect of catalyst preparation on the yield of carbon nanotube growth

Multi-wall carbon nanotubes (MWCNTs) were synthesized by catalytic chemical vapor deposition (CVD) on catalytic iron nanoparticles dispersed in a silica matrix, prepared by sol gel method. In this contribution, variation of gelation condition on catalyst structure and its influence on the yield of carbon nanotubes growth was studied. The precursor utilized were tetraethyl-orthosilicate and iron nitrate. The sols were dried at two different temperatures in air (25 or 80 °C) and then treated at 450 °C for 10 h. The xerogels were introduced into the chamber and reduced in a hydrogen/nitrogen (10%v/v) atmosphere at 600 °C. MWCNTs were formed by deposition of carbon atoms from decomposition of acetylene at 700 °C. The system gelled at RT shows a yield of 100% respect to initial catalyst mass whereas the yield of that gelled at 80 °C was lower than 10%. Different crystalline phases are observed for both catalysts in each step of the process. Moreover, TPR analysis shows that iron oxide can be efficiently reduced to metallic iron only in the system gelled at room temperature. Carbon nanotubes display a diameter of about 25–40 nm and several micron lengths. The growth mechanism of MWCNTs is base growth mode for both catalysts.     

Magnetic properties of core-shell catalyst nanoparticles for carbon nanotube growth

Two types of core-shell nanoparticles have been prepared by laser pyrolysis using Fe(CO)₅ and C₂H₂ or [(CH₃)₃Si]₂O as precursors and C₂H₄ as sensitizer. The first type (about 4 nm diameter) - produced by the decomposition of Fe(CO)₅ in the presence of C₂H₄ and C₂H₂ - consists of Fe cores protected by graphenic layers. The second type (mean particle size of about 14 nm) consists also of Fe cores, yet covered by few nm thick γ-Fe₂O₃/porous polycarbosiloxane shells resulted from the [(CH₃)₃Si]₂O decomposition and superficial oxidation after air exposure. The hysteresis loops suggest a room temperature superparamagnetic behavior of the Fe-C nanopowder and a weak ferromagnetic one for larger particles in the Fe-Fe₂O₃-polymer sample. Both types of nanoparticles were finally used as a catalyst for the carbon nanotube growth by seeding Si(100) substrates via drop-casting method. CNTs were grown by Hot-Filament Direct.Current PE CVD technique from C₂H₂ and H₂ at 980 K. It is suggested that the increased density and orientation degree observed for the multiwall nanotubes grown from Fe-Fe₂O₃-polymer nanoparticles could be due to their magnetic behavior and surface composition.     

On the compressive response of carbon nanotube tangles

The nonlinear bulk compressibility of entangled multiwalled carbon nanotubes is studied. The analogy with textile fibre assemblies is explored by means of the well established van Wyk model. In view of the small diameter of the nanotubes, the possible effect of adhesive van der Waals interactions at tube-tube contacts is analysed. It is found, however, that the contribution of adhesive contacts to the bulk stress should be negligible. Compression experiments are performed on multi-walled carbon nanotubes and show that van Wyk's model is able to describe the response, although the values of the dimensionless parameter k of van Wyk's model were lower than expected. There is indeed no indication that van der Waals interactions play any significant role.     

Structural and morphological dependence of carbon nanotube arrays on catalyst aggregation

Catalyst aggregation affects the growth of carbon nanotube (CNT) arrays in terms of tubular structures, waviness, entanglement, lengths, and growth density etc., which are important issues for application developments. We present a systematic correlation between the aggregation of catalyst on the SiO₂/Si substrate and the structure and morphology of CNT arrays. The thickness of the catalyst film has a direct effect on the areal density of the catalytic particles and then the alignment of the CNT array. Introducing alumina as buffer layer and annealing the catalyst film at low pressure are two effective approaches to downsize the catalyst particles and then the diameter, wall number of the CNTs. Both the size and areal density of the catalyst also change with the CNT growth in accordance with Ostwald ripening process, with the bottom of the CNT array varying from well-aligned to disordered and adhesion between catalyst particles and the substrate getting enhanced. Strategies including tuning the thickness of the catalyst film, changing buffer layer, controlling on the growth time and the system pressure were used to regulate the aggregation of the catalyst. CNT arrays from disordered to well-aligned, from multi-walled to few-walled and further to single-walled were reproducibly synthesized by chemical vapor deposition of acetylene.     

Single-walled carbon nanotube formation with double laser vaporization technique

Single-walled carbon nanotubes (SWNTs) were prepared with double laser vaporization of a graphite target and a metal/alloy target inside an electric furnace at 1200 ^°C ambient temperature with 500 torr Ar gas atmosphere. Each target was vaporized simultaneously with a different Nd:YAG laser. Several kinds of metal/alloy target (Ni, Co, Fe, and permalloy) were tested in order to see the difference in the resulting SWNT yield and the diameter distribution of them. The Raman spectra of SWNT-containing soot prepared by use of this technique with permalloy/carbon system indicated that permalloy gives almost the same yield as compared with Ni/Co carbon composite rod with single laser vaporization technique, though the diameter distribution of them is slightly different. Also, time-resolved images of the plume by carbon and permalloy nanoparticles after laser vaporization were collected using a high-speed video camera. These images suggest that the hot plumes due to carbon and permalloy nanoparticles do not mix together so extensively, at least in a few hundred microseconds after laser vaporization. The effect of time delay between two laser pulses on the yield and the diameter distribution of SWNTs was also presented and discussed.     

Elimination of metal catalyst and carbon-like impurities from single-wall carbon nanotube raw material

The purification of as-produced single-wall carbon nanotube (SWCNT) material is one important step in order to make the material optimally suited for a number of potential applications. We present a purification procedure based upon oxidation of the raw material in oxygen atmosphere at elevated temperatures and a subsequent treatment in HCl. It is shown that this procedure results in the removal of the majority of the impurities comprising carbonaceous species and metal catalyst particles. The purification and the evolution of SWCNT material using this procedure are monitored using optical absorption spectroscopy, transmission electron microscopy including electron energy-loss spectroscopy as well as electron diffraction. Furthermore, the method has a sufficiently high yield of about 50% to be applicable for a large-scale purification. PACS 81.05.-t; 81.20.-n; 81.07.-b     

Structure and thermal properties of supported catalyst clusters for single-walled carbon nanotube growth

Structure and thermal properties of supported iron clusters were studied using molecular dynamics simulations. When supported clusters are in the liquid state, their surfaces have spherical curvature, whereas solid clusters form a layered crystalline structure. The cluster freezing (melting) point increases dramatically with increasing cluster-substrate interaction strength, and rapid diffusion of cluster surface atoms is observed below the freezing point.     

Possible role of charge transport in enhanced carbon nanotube growth

We consider the role of electric fields during metal-catalysed thermal chemical vapour deposition growth of carbon nanotubes and show that enhanced growth occurs from a negatively biased electrode. An electric field, applied externally to the growing tubes and/or generated as a result of electron emission or self-biasing, may strongly affect the carbon supply through the catalyst nanoparticle, enhancing the growth rate. Different aspects of the growth process are analysed: the nature of the nanoparticle catalysis, carbon dissolution kinetics, electron emission from the nanotube tips, charge transport in the nanotube–catalytic nanoparticle system and carbon drift and diffusion through the catalyst under the action of the electric field. A fundamental tenet for modelling of charge-transport dynamics during the nanotube growth process is proposed. PACS 81.07.De; 81.15.Gh     

Quantum-mechanical investigation of field-emission mechanism of a micrometer-long single-walled carbon nanotube

A quantum-mechanical simulation is carried out to investigate the charge distribution and electrostatic potential along a 1 microm long (5,5) single-walled carbon nanotube under realistic field-emission experimental conditions. A single layer of carbon atoms is found sufficient to shield most of the electric field except at the tip where strong field penetration occurs. The penetration leads to a nonlinear decrease of potential barrier for emission, which is equally responsible for the low threshold voltage besides the well-known geometrical field enhancement factor.     

The effects of catalyst treatment on fast growth of millimeter-long multi-walled carbon nanotube arrays

An optimized strategy was developed for fast growth of millimeter-long CNT arrays using chemical vapor deposition (CVD). Growth temperature of 800 °C was firstly determined, and catalyst heat treatment conditions were then optimized to probe the full potential of growth rate. 1.5 mm long CNT arrays were obtained in 10 min under optimized growth and catalyst heat treatment conditions. The growth rate of CNT arrays strongly depends on the growth temperature and catalyst heat treatment. Insufficient reduction could not reduce iron oxide into metallic state or/and crack down catalyst film into particles, but excessive treatment may result in large particles due to Ostwald ripening process. This method would offer more freedoms in designing the fast growth of high-purity, long CNT arrays.     

Single-walled carbon nanotube formation on iron oxide catalysts in diffusion flames

Single-walled carbon nanotubes (SWCNTs) are shown to grow rapidly on iron oxide catalysts on the fuel side of an inverse ethylene diffusion flame. The pathway of carbon in the flame is controlled by the flame structure, leading to formation of SWCNTs free of polycyclic aromatic hydrocarbons (PAH) or soot. By using a combination of oxygen-enrichment and fuel dilution, fuel oxidation is favored over pyrolysis, PAH growth, and subsequent soot formation. The inverse configuration of the flame prevents burnout of the SWCNTs while providing a long carbon-rich region for nanotube formation. Furthermore, flame structure is used to control oxidation of the catalyst particles. Iron sub-oxide catalysts are highly active toward SWCNT formation while Fe and Fe₂O₃ catalysts are less active. This can be understood by considering the effects of particle oxidation on the dissociative adsorption of gas-phase hydrocarbons. The optimum catalyst particle composition and flame conditions were determined in near real-time using a scanning mobility particle sizer (SMPS) to measure the catalyst and SWCNT size distributions. In addition, SMPS results were combined with flame velocity measurement to measure SWCNT growth rates. SWCNTs were found to grow at rates of over 100 μm/s.