Structural analysis of carbon nanofibres grown by the floating catalyst method

Submicron vapour grown carbon fibres (carbon nanofibres) grown by the floating catalyst method (Sample A), using hydrogen sulphide and ammonia to suppress soot formation, have been subjected to a comprehensive structural analysis, by high resolution transmission electron microscopy, PEEL, X-ray diffraction and gas adsorption. Results were compared to those obtained from an industrial sample (Sample B). Both fibres were found to be hollow but with different internal and external diameters. Sample A comprised approximately 90% fibres and 10% soot, which could not easily be separated; the fibres consisting of disordered graphene planes (with the c axis approximately perpendicular to the length of the fibre axis) which became more ordered after heat treatment (2730°C). Sample B consisted of fibres with a complex duplex structure also exhibiting disordered graphene planes; upon heat treatment there was a loss of the duplex structure and an increase in graphitic ordering. The graphitisability was similar for both fibres. Although Sample B had the greater diameter it also displayed the highest BET area.     

Online BET analysis of single-wall carbon nanotube growth and its effect on catalyst reactivation

A system combining pulse chemical vapor deposition (CVD) reaction and cryogenic gas sorption (BET) measurement capabilities was designed to allow the sequential synthesis and online analysis of single-wall carbon nanotubes (SWNTs). Cooling treatment in liquid nitrogen (77 K) during BET measurement was found to be efficient for restoring catalysts when deactivation occurs after carbon deposition. By this treatment, the methane conversion could be enhanced by up to seven times, such as from 5.8% to 42.6% mol. When the temperature changes from 850 °C to 77 K, the metal particles on the tip of nanotubes might contract and be separated from the graphite layer of the nanotubes, leading to more active sites on metal particles being exposed. The single point BET analysis of SWNT has been tested as an efficient method for the rapid online analysis of SWNTs produced by CVD.     

Control of the single-wall carbon nanotube mean diameter in sulphur promoted aerosol-assisted chemical vapour deposition

The influence of gas flow on nanotube diameter during the synthesis of high-purity, very long single-wall carbon nanotubes (SWCNT) via aerosol-assisted chemical vapour deposition is reported. The sample morphology, nanotube yield, defect concentration and amount of carbonaceous impurities, as well as the mean diameter and the diameter distribution of the SWCNTs were analysed by combined scanning- and transmission electron microscopy, Fourier Transform Raman spectroscopy and optical absorption spectroscopy. The results show that by using a solution of ferrocene and sulphur in m-xylene the addition of sulphur as a promoter was found to enhance the SWCNT growth and to increase the yield. A reduction of the mean diameter and a change in the diameter distribution are observed when the total gas flow is increased.     

Evolution of directly-spinnable carbon nanotube catalyst structure by recycling analysis

Carbon nanotubes (CNTs), particularly of the substrate-grown directly spinnable variety, have a vast number of potential applications. We have devised a recycling methodology to enable the evolution of catalyst morphology and activity and its effect on CNT growth quality and failure to be studied incrementally. Direct spinnability is particularly sensitive to many variables and so provides verification that each cycle is essentially identical.Two processes, utilising acetylene with and without hydrogen, are studied. Acetylene alone gives four cycles of spinnable CNTs before failing abruptly at the fifth at which point catalyst particle (and CNT) areal density increase while CNT diameter and length fall sharply. In contrast, addition of hydrogen doubles the initial growth rate but spinnability declines on the third cycle and fails on the fourth as catalyst (and CNT) areal density decreases sharply. CNT length also falls although diameter increases.Our observations support a proposed ‘sinking plateau’ model of catalyst behaviour where growth rate is driven by the essentially flat accumulation area or ‘plateau plain’ surrounding a local high spot (active catalyst particle). The growth rate remains stable until the plateau plain drops below the substrate surface through diffusion at the base, at which point it falls sharply.     

Synthesis of single-wall carbon nanotubes and long nanotube ribbons with Ho/Ni as catalyst by arc discharge

The effect of a new bimetallic catalyst Ho/Ni for synthesis of single-walled carbon nanotubes (SWNTs) by arc discharge has been studied. Long ribbons consisting of roughly-aligned SWNT bundles were obtained by a modified arc discharge apparatus. Ribbon lengths can reach as much as 20 cm. Both elements Ho and Ni play important roles in the synthesis of SWNTs with high yield and purity. Changes in the Ho and Ni concentration in the catalyst hardly affect the diameter distribution of SWNTs, but the yield and purity of SWNTs are very sensitive to the concentration. An optimal range of Ho/Ni compositions for synthesis of SWNTs with relatively high purity and yield is given.     

Dry spinning yarns from vertically aligned carbon nanotube arrays produced by an improved floating catalyst chemical vapor deposition method

Carbon nanotube (CNT) yarn was drawn directly from super aligned CNT arrays synthesized by an improved floating catalyst chemical vapor deposition method. The synthesis of aligned CNT was performed as a multi-step interim reactant supply reaction to produce a double-layered CNT array in a horizontal quartz tube reactor. During the growth period, most impurities were blocked on the top surface of the first layer and therefore the top aligned CNT layer was unspinnable. However, the bottom CNT layer was super aligned CNTs, which were with clean surface and a tortuosity factor of 1.07. During the dry spinning process, the tangles, friction, and van der Waals interaction between CNTs served to hold them into CNT yarns. The tensile strength of the as-obtained CNT yarn can be further improved from 0.24 to 0.30 GPa by twisting.     

Influence of the gas pressure on single-wall carbon nanotube formation

Experiments and modeling are performed to predict the effect of gas pressure on species distributions and nanotube growth rate under specific conditions of synthesis of single-wall carbon nanotubes (SWCNTs) by arc discharge. Numerical results are compared with experiments in order to find a consistent correlation between the nanotube growth and the pressure. We use argon and helium as buffer gases with a total pressure varied between 0.1 and 1 bar. We experimentally observe that both the anode erosion rate and the Brunauer-Emmett-Teller (BET) surface area of the as-produced nanotube soot material are very sensitive to the total gas pressure in the reactor.     

Continuous production of single-walled carbon nanotubes using a supported floating catalyst

Using catalytic decomposition, a technique for the production of singe-walled carbon nanotubes (SWCNTs) is reported with a production rate up to 6 g h⁻¹ after purification, and scaling capability up to 220 g h⁻¹. This is achieved by injection of pre-prepared alumina supported catalyst powder into a modified vertical floating reactor. The product is collected in several cyclones connected in series. Wide range Raman studies (laser excitations from λ = 1064 to 488 nm) and temperature programmed oxidation measurements of the samples collected from the different cyclones show that SWCNTs were separated in situ by tube diameter. This is attributed to the different residual times of the catalyst in the reaction zone depending on particle diameter. A series of computational fluid dynamics calculations of the flow and heat transfer in the reactor, as well as modeling of catalyst particle transport reveals the parametrical dependence of the process.     

Microscopic mechanism of reinforcement in single-wall carbon nanotube/polypropylene nanocomposite

We analyzed mechanical properties and structure of polypropylene fibers with different concentrations of single-wall carbon nanotubes (SWNTs) and draw down ratios (DDR). Tensile tests show a three times increase in the Young's modulus with addition of only 1 wt% SWNT, and much diminished increase of modulus with further increase in SWNT concentration. Microscopic study of the mechanism of reinforcement by SWNT included Raman spectroscopy and wide-angle X-ray diffraction (WAXD). The results show linear transfer of the applied stress from the polymer matrix to SWNT. Analysis of WAXD data demonstrates formation of a β-crystal phase in polypropylene matrix under the strain.     

Enhancement of electrochemiluminesence of lucigenin by ascorbic acid at single-wall carbon nanotube film-modified glassy carbon electrode

The electrochemiluminescent behavior of lucigenin on a single-wall carbon nanotube/DMF film-modified glassy carbon electrode was studied in this paper. Comparing with the bare glassy carbon electrode, the electrochemiluminescent of lucigenin at modified electrode is more stable and without tedious procedure for clean-up the surface of modified electrode. It has been found that ascorbic acid could enhance the electrochemiluminescent intensity of lucigenin greatly at this modified electrode. Based on which, a new sensitive and simple electrochemiluminescent method for determination of ascorbic acid could be developed. The condition for the determination of ascorbic acid was optimized. Under the optimized condition, the enhanced electrochemiluminescent intensity versus ascorbic acid concentration was linear in the range of 1.0 × 10⁻⁸ to 4.0 × 10⁻⁶ mol/L with a detection limit of 2.0 × 10⁻¹⁰ mol/L, and the relative standard derivation for 1.0 × 10⁻⁷ mol/L ascorbic acid was 3.8% (n = 8). The possible mechanism was also discussed.     

Tuning the photophysical properties of soluble single-wall carbon nanotube derivatives by co-functionalization with organic molecules

The photophysical characterization of three soluble derivatives of single-wall carbon nanotubes (SWCNTs), functionalized with poly(ethylene glycol) (PEG), co-functionalized with PEG and aminofluorene and co-functionalized with PEG and aminoanthracene is reported. The peculiar excellent solubility of these derivatives allows, for the first time in covalently functionalized SWCNTs, the study of their excitation dynamics by monitoring the near-infra-red emission. Moreover, the aminoanthracene derivative shows higher photoluminescence efficiency in the visible range than the aminofluorene derivative, demonstrating the possibility to tune extensively the photophysical properties of these functionalized SWCNTs.     

State of the catalyst during carbon nanotube growth

We review our in-situ X-ray photoemission (XPS) and in-situ transmission electron microscopy studies which determined that the catalyst is in the metallic state for Fe, Co and Ni catalysts. We show that the existence of surface carbide phases in related catalytic reactions could account for the observation of carbide peaks in XPS. The observed catalytic activity of gold is discussed in terms of carbon solubility, reaction rates, and surface coordination numbers.     

Conductivity and mechanical properties of well-dispersed single-wall carbon nanotube/polystyrene composite

The morphologies, electrical and mechanical properties and structure of polystyrene (PS) composites with varying concentrations of single-wall carbon nanotubes (SWNT) are analyzed. Using Raman spectroscopy and electron microscopy, we demonstrate that initial thermal annealing of SWNT significantly improves their dispersion in PS. In dielectric measurements, the annealed SWNT/PS composites show higher electrical conductivity and a lower percolation threshold (less than 0.3 wt%) than the raw SWNT/PS composites, which provides further evidence of good dispersion of the annealed SWNT in PS. Raman spectra of composites under tension show good transfer of an applied stress from the polymer matrix to SWNT. However, mechanical moduli of the annealed SWNT/PS composites are only increased slightly. The reason for this discrepancy remains unclear.     

Microscopic mechanisms for the catalyst assisted growth of single-wall carbon nanotubes

Whatever the synthesis technique used, the growth of ropes of single-wall carbon nanotubes requires the assistance of a metallic catalyst. In this paper, the role played by the catalyst is studied both experimentally and theoretically. Experimentally, the similarities between the samples synthesized from different techniques suggest a common growth mechanism proceeding via the precipitation of excess carbon on metallic nanoparticles. In this paper, the correlation between ropes and catalytic particles is investigated in detail in the case of the Ni-Y catalyst used in the arc discharge technique by combining high resolution transmission electron microscopy, X-ray and electron energy loss spectroscopy. It is shown that the ropes are always found attached to metallic particles about ten times larger than the tube diameter. A further remarkable proof of this relationship is provided by the chemical analyses of the metallic particles. These are found to be free of carbon and to always display the same Ni:Y composition range, whatever the initial Ni:Y composition of the catalyst mixture used in the synthesis, whereas the composition of other particles is highly dispersed. These experimental results support a mechanism of formation based on a vapor-liquid-solid model, in which the tubes of a given bundle nucleate in a cooperative manner and grow at the surface of a same metallic particle. This phenomenological scheme is supported by quantum molecular dynamics simulations which show that carbon atoms are incorporated at the root of a growing tube by a diffusion-segregation process occurring at the surface of the catalytic particle.     

Electrochemical durability of single-wall carbon nanotube electrode against anodic oxidation in water

The electrochemical capability of single-wall carbon nanotube (SWCNT) electrodes is investigated to establish their reliability in practical applications. Direct current (DC) voltage of +10 V is applied across the SWCNT anode and Pt cathode in water, and the electrochemical fracturing behavior of SWCNTs is analyzed using transmission electron microscopy and atomic force microscopy. A considerable number of short SWCNTs, with lengths of less than 200 nm, are observed to be electrochemically generated. This result suggests that the anodic corrosion of SWCNTs occurs even in water, a non-electrolyte liquid. Raman spectroscopy and a comparison study of the anodization behavior of SWCNTs with narrow (0.9 nm) and wide (1.8 nm) diameters indicate that the durability of narrow SWCNTs is lower than that of the wide SWCNTs.     

3D microfabrication of single-wall carbon nanotube/polymer composites by two-photon polymerization lithography

We present a method to develop single-wall carbon nanotube (SWCNT)/polymer composites into arbitrary three-dimensional micro/nano structures. Our approach, based on two-photon polymerization lithography, allows one to fabricate three-dimensional SWCNT/polymer composites with a minimum spatial resolution of a few hundreds nm. A near-infrared femtosecond pulsed laser beam was focused onto a SWCNT-dispersed photo resin, and the laser light solidified a nanometric volume of the resin. The focus spot was three-dimensionally scanned, resulting in the fabrication of arbitrary shapes of SWCNT/polymer composites. SWCNTs were uniformly distributed throughout the whole structures, even in a few hundreds nm thick nanowires. Furthermore, we also found an intriguing phenomenon that SWCNTs were self-aligned in polymer nanostructures, promising improvements in mechanical and electrical properties. Our method has great potential to open up a wide range of applications such as micro- and nanoelectromechanical systems, micro/nano actuators, sensors, and photonics devices based on CNTs.     

Increasing the length of a single-wall carbon nanotube forest by adding titanium to a catalytic substrate

The effect of titanium (Ti) added to the top layer of an aluminum (Al)/iron (Fe)/Al (bottom) sandwich catalytic substrate was studied. The Ti caused a significant lengthening in the single-wall carbon nanotube forest produced by chemical vapor deposition (CVD). In general, particles of iron oxide on the Al/Fe/Al catalytic substrate are formed by exposure to the atmosphere during deposition process of the substrate. The particles of iron oxide are metalized during pretreatment under a reductive gas before the growth of the nanotubes with nano-sized dispersion stabilized. On this process, the metallization and the stabilization of nano-sized iron oxide particles occur on the basis of the oxygen affinity in the top aluminum oxide layer, with the ionization tendency Al > Fe. It is thought that the addition of Ti increases the oxygen affinity of the catalytic substrate, since Ti has a stronger ionization tendency than Al. After optimizing the quantity of Ti added to the top layer, we successfully fabricated a millimeter-long, small-diameter, single-wall carbon nanotube forest.