Fluidized-bed synthesis of sub-millimeter-long single walled carbon nanotube arrays

The rapid growth method for vertically aligned, single walled carbon nanotube (SWCNT) arrays on flat substrates was applied to a fluidized-bed, using ceramic beads as catalyst supports as a means to mass produce sub-millimeter-long SWCNT arrays. Fe/Al₂O_x catalysts were deposited on the surface of Al₂O₃ beads by sputtering and SWCNTs were grown on the beads by chemical vapor deposition (CVD) using C₂H₂ as a feedstock. Scanning electron microscopy and transmission electron microscopy showed that SWCNTs of 2–4 nm in diameter grew and formed vertically aligned arrays of 0.5 mm in height. Thermogravimetric analysis showed that the SWCNTs had a catalyst impurity level below 1 wt.%. Furthermore, they were synthesized at a carbon yield as high as 65 at.% with a gas residence time as short as <0.2 s. Our fluidized-bed CVD, which efficiently utilizes the three-dimensional space of the reactor volume while retaining the characteristics of SWCNTs on substrates, is a promising option for mass-production of high-purity, sub-millimeter-long SWCNT arrays.     

Structure and purity of single walled carbon nanotube samples

Pulsed neutron diffraction has been used to characterize the microscopic structure and purity of single walled carbon nanotubes samples produced by arc discharge. We employed a time of flight diffractometer whose performance in measuring the microscopic structural properties of light-mass materials is well known and recognized. The extended Q-range of the instrument allows for a direct inversion of the data to determine the radial distribution function of the carbon atoms. This is compared with the corresponding function produced by computer simulation. In addition, the absolute calibration of the neutron diffraction data evidences anomalies in the diffraction spectra of the carbon nanotubes, especially at the level of the total scattering section, that could not be observed in previous neutron scattering experiments. These are attributed to the presence of a substantial amount of spurious carbonaceous material that was not quantitatively detected with more conventional diagnostic techniques.     

Low resolution Raman spectroelectrochemistry of single walled carbon nanotube electrodes

A new combination of a low-resolution Raman spectrometer with a minipotentiostat is presented in this work to perform in situ Raman measurements during electrochemical experiments with low-cost instrumentation. The instrumental setup has been used to study the electrochemical oxidation of transparent single walled carbon nanotube (SWCNT) films supported on non-conductive substrates. The spectroelectrochemical response provides the dependence of the characteristic signatures of the SWCNT bundles with the applied potential, which is similar to the response observed for SWCNT films deposited on conducting substrates. The evolution of both the electrical current and the Raman features differs considerably for pristine and oxidized films. The spectroscopic data reveal the occurrence of two kinds of irreversible breakdowns, “oxidative burning” and functionalization of the SWCNTs, in addition to reversible p-doping. 2D-correlation has been applied to analyze the evolution of the spectra with potential and has provided more detailed information than expected from a low spectral resolution spectrometer.     

Synthesis of thin bundled single walled carbon nanotubes and nanohorn hybrids by arc discharge technique in open air atmosphere

Cost-effective synthesis of single walled carbon nanotube (SWCNT) and single walled carbon nanohorn (SWCNH) hybrids, in a single step, by electric arc discharge technique in open air, at lower current densities is reported. The rate of production of the hybrids is 3–5 g/h. The presence of SWCNTs and SWCNHs is confirmed by a transmission electron microscope (TEM). In addition to conventional larger Dahlia-like aggregates of nanohorns, unique nearly-spherical shaped and relatively smaller sized aggregates (mean size ~ 25 nm) of nanohorns are formed along with thin bundles (mean diameter ~ 5.7 nm) of SWCNTs.     

Diameter-control of single-walled carbon nanotubes produced by magnetic field-assisted arc discharge

We have demonstrated a scalable approach to synthesize single-walled carbon nanotubes (SWCNTs) with selected diameter distributions by applying a magnetic field perpendicular to the electric field in the arc plasma. It is found that the purity and orientation of SWCNTs can be controlled by the magnetic field. SWCNTs with different diameter distributions can be separated into two different regions by the applied magnetic field, and the diameter-selection efficiency is improved by modifying the direction of the magnetic field. Our findings suggest that the motion of the catalyst particles with different sizes, positive carbon ions and electrons are significantly influenced by Lorentz forces, resulting in the difference in the growth processes of the SWCNTs due to the collective interactions between the arc plasma and the magnetic field.     

Influence of carbon structure of the anode on the production of graphite in single-walled carbon nanotube soot synthesized by arc discharge using a Fe–Ni–S catalyst

We investigated the production of the graphite contained in the soot of single-walled carbon nanotubes (SWCNTs) synthesized using the arc discharge method with a poorly graphitized carbon (PGC) rod in comparison to a graphite rod. A PGC rod was produced using a mixture of coal tar and carbon black and was heat treated to 1000 °C. The rod was packed with a mixture of iron (Fe), nickel (Ni), sulfur (S), and PGC and used for the production of SWCNT soot using arc discharge. From the results of X-ray diffraction and electron microscopy, the amount of graphite in the SWCNT soot synthesized by PGC rod was lower than that by graphite rod. The production of graphite in the soot was found to be dependent on the carbon structure of the anode and the current density of arc discharge.     

Electrical conductivity of single walled and multiwalled carbon nanotube containing wool fibers

The main purpose of this study was producing conductive wool fabric applying carbon nanotubes. Raw and oxidized wool samples were treated with carbon nanotubes in the impregnating bath in the presence of citric acid as a crosslinking agent and sodium hypophosphite as a catalyst while sonicating them in the ultrasonic bath. Electrical resistance, washing durability, and color variation of treated samples were assessed. Through SEM images, the surface morphology of treated samples was studied confirming the surface coating through carbon nanotubes. According to the results, the electrical resistance of treated wool with carbon nanotubes reduced substantially. However, the single‐walled carbon nanotubes are more useful to increase the conductivity. In addition, the wool color changed into gray after the treatment. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

A molecular dynamics simulation of methane adsorption in single walled carbon nanotube bundles

The physisorption of methane in idealized bundles of single walled carbon nanotubes (SWCNT) is investigated in detail in this work employing computational. Several aspects related to the possible application of nanotubes as fuel gas containers are analyzed employing molecular dynamics simulations. The influence of the nanotube diameter on the adsorption capacity of the material and the distribution of the adsorbate are examined by considering bundles of carbon nanotubes with different morphologies. An increase of the load capacity with the nanotube diameter is observed, together with a qualitative change in the distribution of the adsorbed molecules. The effect of porosity is also studied from the point of view of the nanotube separation, finding that this leads to a significant increase in storage capacity in the case of bundles made of small diameter nanotubes. The role of temperature as a possible uptake/release triggering variable is also examined.     

Computer simulation of hydrogen physisorption in a Li-doped single walled carbon nanotube array

Hydrogen physisorption in a Li-doped single walled carbon nanotube (SWCNT) array is investigated by grand canonical Monte Carlo simulation. The optimization of hydrogen storage capacity at normal temperature and moderate pressure as a function of Li doping arrangement, doping-site position, doping ratio, and SWCNT array configuration is discussed and explained.     

Two possible emission mechanisms involved in the arc discharge method of carbon nanotube preparation

By investigating the morphologies and microstructures of the cathode deposits prepared by self-sustained arc discharge between graphite rods, we consider that there are two electron emission mechanisms occurring on the cathode: field emission and thermionic emission. The former occurs mainly on the edge of the growing surface, by which we can explain the formation of the outer hard shell of the cathode deposit; while the latter occurs mainly on the growing surface except for the edge area and it is the main cause for the growth of carbon nanotubes.     

Aluminum nitride‐single walled carbon nanotube nanocomposite with superior electrical and thermal conductivities

Development of aluminum nitride (AlN)‐single walled carbon nanotube (SWCNT) ceramic‐matrix composite containing 1‐6 vol% SWCNT by hot pressing has been reported in this article. The composites containing 6 vol% SWCNT are dense (~99% relative density) and show high dc electrical conductivity (200 Sm^?1) and thermal conductivity (62 Wm^?1K^?1) at room temperature. SWCNTs contain mostly metallic variety tubes obtained by controlled processing of the pristine tubes before incorporation into the ceramic matrix. Raman spectroscopy and field emission scanning electron microscopy (FESEM) of the fracture surface of the samples show the excellent survivability of the SWCNTs even after high‐temperature hot pressing. The results indicate the possibility of preparation of AlN nanocomposite for use in plasma devices and electromagnetic shielding.     

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.     

Activated carbon based selective purification of medical grade NO starting from arc discharge method

Four activated carbons were tested under normal temperature and pressure to selectively adsorb nitrogen dioxide (NO₂) from medical nitric oxide (NO) formed by arc discharge. The samples’ pore structures were characterized by an automatic specific surface area and porosity analyzer based on the Brunauer–Emmett–Teller method, t-plot, and the Barrett–Joyner–Halenda method. Surface chemical properties both before and after adsorption, as well as the resultant nitro-compound (C-NO₂ or nitrate) after adsorption, were analyzed using the classic Boehm titration method and Fourier transform infrared spectroscopy. The selective adsorption amounts of NO₂ and NO were evaluated using specially designed equations. It was found that, with their large surface areas and highly acidic groups, spherical activated carbon from wood and activated carbon fibers had the best selective adsorption of NO₂. It was also shown that the large difference in acid–basic surface groups and the molecular polarity between NO and NO₂ accounted for their adsorption on different functional groups, and that those acidic groups enhanced the selective adsorption of NO₂. The research on regeneration and re-adsorption showed that regeneration had an obvious effect on the adsorption of NO₂, but little effect on NO.     

Fluffy carbon nanotubes produced by shearing vertically aligned carbon nanotube arrays

Fluffy carbon nanotubes (CNTs), which are cotton-like macroscopic structures, are obtained by simple high-speed shearing of vertically aligned CNT (VACNT) arrays. The fluffy CNTs are composed of CNT bundles with a diameter of several micrometers, and have an extremely low apparent density of 3-10 g/L. A requisite for their formation is the alignment of CNTs in the initial array. The shear between the rotor and the arrays tears the arrays along the axial direction and this results in their dispersion into low density fluffy CNTs.     

Simultaneous voltammetric determination of dopamine and adenosine using a single walled carbon nanotube – Modified glassy carbon electrode

The simultaneous voltammetric determination of adenosine (ADS) and dopamine (DA) using a single wall carbon nanotube (SWCNT) modified glassy carbon electrode (GCE) is reported. This has physiological importance in controlling Parkinson’s disease. In phosphate buffer medium of pH 7.2, the concentration vs. peak current plots were linear in the range 1–100 μM for ADS and DA. A comparison of the voltammetric response of DA at acid-treated (purified and super-purified) and untreated SWCNT modified GCE indicates that the oxidation peak current of DA decreases considerably at the treated SWCNT modified GCE. This indicates that metallic impurities in nanotubes play a large role in enhancing the electrochemical current. The detection limit (3σ) and sensitivity observed for ADS and DA were 34.7 μM, 7 μM and 9.5 nA μM⁻¹, 77.9 nA μM⁻¹, respectively.     

Selective laser treatment and laser patterning of metallic and semiconducting nanotubes in single walled carbon nanotube films

Single wall carbon nanotubes (SWCNT) synthesized using mass production methods such as pulsed arc deposition consist of a mixture of metallic and semiconducting nanotubes. In this work, we report on an approach for the selective removal of either metallic or semiconducting SWCNT by a heat-treatment process with cw-lasers and pulsed lasers with specific wavelengths. The results show that using ultraviolet–visible radiation (with wavelengths between 473 nm and 632 nm) it is possible to remove predominantly metallic nanotubes. In contrast, near infrared lasers with 785 nm and 1064 nm wavelengths can be used to remove predominantly the semiconducting nanotubes. Finally, the fabrication of SWCNT films with an anisotropic distribution of metallic and semiconducting nanotubes is demonstrated using a direct laser interference pattering method.     

Selective destruction of individual single walled carbon nanotubes by laser irradiation

We demonstrate selective burnout of individual carbon nanotubes that are electronically resonant with the incident laser energy. Raman spectroscopy and atomic force microscopy are used to quantify the burnout of nanotubes. The threshold laser power for rapid burnout is found to occur between 0.4 and 0.9 W/μm². At lower laser powers of 80 mW/μm², the burnout depends linearly on time, over tens of minutes. Non-resonant nanotubes could not be burned out even with high laser power or long exposure times. This preferential burnout of resonant nanotubes demonstrates the possibility of selective removal of metallic nanotubes from an inhomogeneous sample.     

Optical anisotropy of single walled carbon nanotubes investigated by spectroscopic ellipsometry

The optical properties of single walled carbon nanotube (SWCNT) films, produced by vacuum filtration, are correlated with their diameter and their in plane preferred orientation by coupling transmission and scanning electron microscopy to conventional ellipsometry. We focused on the optical anisotropy of this material and we demonstrated that it was originated from a breakdown in the selection rules.