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.     

Size-selected Ni catalyst islands for single-walled carbon nanotube arrays

Many properties of single-walled carbon nanotube (SWCNT) arrays are determined by the size and surface coverage of the metal catalyst islands from which they are nucleated. Methods using thermal fragmentation of continuous metal films frequently fail to produce size-uniform islands. Hybrid numerical simulations are used to propose a new approach to controlled self-assembly of Ni islands of the required size and surface coverage using tailored gas-phase generated nanocluster fluxes and adjusted surface temperatures. It is shown that a maximum surface coverage of 0.359 by 0.96–1.02 nm Ni catalyst islands can be achieved at a low surface temperature of 500 K. Optimized growth of Ni catalyst islands can lead to fabrication of size-uniform SWCNT arrays, suitable for numerous nanoelectronic applications. This approach is deterministic and is applicable to a range of nanoassemblies where high surface coverage and island size uniformity are required.

     

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.     

单壁碳纳米管储氢研究

摘要 利用Gaussian03程序计算出C-H键的键能是1.88eV,键长是0.113nm。已知H-H键能是4.748eV,键长是0.074nm。显然, H-H键能大于C-H键的键能,所以在常温常压下碳纳米管储氢时,以物理吸附H2分子为主,化学形式的C-H键吸附为辅。另外,利用LJ势能函数,计算了H2分子在碳纳米管中C原子所成的六边形中心正上方、C原子正上方以及相邻两C原子中间正上方时H2分子与碳纳米管之间的势能。得到无论管内、管外或者两端,都是H2分子在C原子所成的六边形中心正上方时能量最低。且在管内时H2分子距离管壁的距离是0.320nm,在管外时距离管壁的距离是0.309nm;在两端的管内时距离管壁的距离是0.324nm,在两端的管外时距离管壁的距离是0.313nm。     

单壁碳纳米管储氢研究

利用Gaussian03程序计算出C-H键的键能是1.88 eV,键长是0.113 nm.已知H-H键能是4.748 eV,键长是0.074 nm.显然,H-H键能大于C-H键的键能,所以在常温常压下碳纳米管储氢时,以物理吸附H_2分子为主,化学形式的C-H键吸附为辅.另外,利用LJ势能函数,计算了H_2分子在碳纳米管中C原子所成的六边形中心正上方、C原子正上方以及相邻两C原子中间正上方时H_2分子与碳纳米管之间的势能.得到无论H_2分子是被吸附到管内或管外,还是被吸附到中间区域或两端区域,都是H_2分子在C原子所成的六边形中心正上方时能量最低.当H_2分子被吸附到碳纳米管中间区域时,管内和管外的H_2分子距管壁的距离分别是0.320 nm和0.309 nm;而当H_2分子被吸附到碳纳米管两端区域时,这两个距离分别是0.324 nm和0.313 nm.     

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.     

Surfactant-assisted reflux synthesis, characterization and formation mechanism of carbon nanotube/europium hydroxide core-shell nanowires

Carbon nanotube (CNT)/europium hydroxide core-shell nanowires were prepared easily on a large scale under the boiling reflux of water assisted by the surfactant, sodium polystyrenesulfonate (SPS). The core-shell nanowires are characterized by transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectrum. A possible formation mechanism has been suggested as follows: The phenyl rings of SPS can react with the carbon ring of CNTs to form the π-π noncovalent bond, which makes the SPS cover the surface of CNTs entirely, and thus the surface of modified CNTs is negatively charged, which repel with each other resulting in the good dispersion. In addition, the negatively charged surface of CNTs adsorbs europium ions (positive). The adsorbed europium ions in situ react with OH⁻ ions to create europium hydroxide nanoparticles, and subsequently, the nanoparticles fuse together to form a dense coating layer on CNTs.     

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.