An efficient strategy for the purification of cloth-like single walled carbon nanotube soot produced by arc discharge

High purity single walled carbon nanotubes (SWCNTs) were prepared from arc discharge produced cloth-like soot by a new purification strategy, in which liquid oxidation and steam oxidation were combined with a freeze-drying process to remove the metallic and carbonaceous impurities. The process gives a product of >98% purity, which is acquired from a gram-scale dirty raw soot with an overall yield of ∼75% of the SWCNTs. The purity of the samples was characterized by thermogravimetric analysis, scanning and transmission electron microscopy, Raman and Vis-NIR spectroscopy, and magnetometry. A highly pure SWCNT sample with relative purity of 170.4% and I_G/I_D value of 78.92 is achieved. Experiments showed that HNO₃/HCl refluxing combined with freeze-drying is the key process that renders the crude SWCNTs hydrophilic with a large surface area, and thus remarkably increases the efficiency of the steam treatment to remove most of the carbonaceous impurities.     

Impact of nanometal catalysts on the laser vaporization synthesis of single wall carbon nanotubes

The effects of catalyst particle size on the purity, yield, and purification efficiency of single wall carbon nanotubes (SWCNTs) synthesized via pulsed laser vaporization were investigated. The purity of as-produced SWCNT material synthesized using Ni and Co nanometal (∼13 nm diameter) catalyst particles was compared to material synthesized using conventional micronmetal (2-3 μm diameter) particles. The SWCNT material from nanometal catalysts demonstrated a 50% increase in SWCNT purity as assessed by optical absorption spectroscopy and thermogravimetric analysis (TGA). A change in the thermal oxidation properties was also observed with the nanometal-SWCNTs exhibiting a suppression of the exothermic oxidation of post-synthesis catalyst. Statistical analysis of the TGA residue yielded mean post-synthesis catalyst particle diameters of 18 ± 6 nm and 3 ± 1 nm for the micronmetal and nanometal produced material, respectively. When a thermal oxidation profile was performed, the micronmetal-produced material showed the typical decrease in SWCNT purity with increasing oxidation temperature while the nanometal-produced material showed increasing SWCNT purity with increasing temperature. Overall, the use of nanometal catalysts significantly increases synthesis yield and offers novel thermal oxidation procedures to thermally remove carbonaceous impurities without the aid of acid treatments for the development of potential large-scale purification processing.     

Selective oxidation of single-walled carbon nanotubes using carbon dioxide

A simple process for selective removal of carbon from single-walled carbon nanotube samples was developed based on a mild oxidation by carbon dioxide. The reactivity profiles of as prepared and purified nanotube samples were determined using both TG and a related analytical technique, controlled atmosphere programmed temperature oxidation (CAPTO). The complex differential rate curves for weight loss (DTG) or carbon dioxide evolution (CAPTO) could be resolved by a series of Gaussian peaks each associated with carbonaceous species of different reactivity. Comparisons were made between samples as received after preparation by the laser ablation method, after purification by nitric acid oxidation, and both of these after reaction with CO₂. The DTG of as prepared tubes had a broad major peak centered about 410 °C. Mild oxidation of as prepared nanotubes under flowing carbon dioxide at 600 °C preferentially removed more reactive carbon species leaving behind a narrower distribution about the major peak in DTG. In contrast to the as prepared material, the sample that had been purified using nitric acid had a more distinct separation of the major DTG peaks between more and less readily oxidized material. Oxidation of this sample with CO₂ selectively removed the peak associated with the most readily oxidized material. The original CO₂ oxidation experiments performed on the analytical scale were successfully scaled up to a small preparative scale.     

Separation of single-walled carbon nanotubes from graphite by centrifugation in a surfactant or in polymer solutions

Electric arc single-walled carbon nanotubes (SWCNTs) can be separated from their graphitic impurities by a single centrifugation process in a surfactant or in polymer solutions. The purity of SWCNT dispersions, evaluated from near infrared (NIR) spectroscopy measurements, substantially increased after centrifugation at a moderate speed. The supernatant NIR purity was affected by the surfactant choice, following the sequence: sodium cholate ∼ Pluronic F68 > sodium dodecylbenzene sulfonate > Pluronic F127 > sodium dodecyl sulfate. NIR purity was also influenced by the centrifugation speed and the pristine SWCNT concentration in the starting dispersion, but not by the surfactant concentration. SWCNT enrichment was not observed in a pure organic solvent (N,N′-dimethylformamide) under identical centrifugation conditions. X-ray diffraction analysis demonstrated that graphitic impurities were mostly eliminated from SWCNTs during the centrifugation process in a surfactant or in polymer solutions. Thermogravimetric analysis under CO₂ showed that metallic impurities were substantially reduced during the centrifugation process.     

A significant improvement of both yield and purity during SWCNT synthesis via the electric arc process

This paper deals with the optimisation of the single walled carbon nanotube (SWCNT) synthesis by the electric arc technique using so-called heterogeneous anodes filled with Ni and Y catalysts along with either graphite (large-grain or small-grain) or diamond powders. The various carbon nanophases produced were analyzed using high-resolution transmission electron microscopy. Plasma physical properties were determined by emission spectroscopy and were correlated to the variation in the carbon products formed. Using large-grain (100 μm) graphite powder corresponded to standard conditions since able to generate impurity-rich SWCNT material resembling that usually described in literature. However, replacing the large-grain graphite powder by small-grain graphite powder (∼1 μm) resulted in a dramatic increase in both the yield and purity of the SWCNTs obtained. On the other hand, a similar result was obtained by using diamond powder (grain size ∼1 μm) instead of the small-grain graphite powder. The results are explained via the erosion modes of the anodes with respect to the apparent density of the powder mixtures filling their cavities. Maintaining a steady plasma composition and a CI/NiI concentration ratio higher than 10⁸ are identified as two conditions required for optimising SWCNT synthesis.     

Structural changes and Raman analysis of single-walled carbon nanotube buckypaper after high current density induced burning

Single-walled carbon nanotube (SWCNT) buckypaper (BP) was exposed to high temperatures with electrical current-driven thermal heating either in the air or a vacuum. High electrical currents generate Joule heating and then cause breakdown of the BP in the air at over 400 °C due to rapid oxidation. In the vacuum, electrical resistive heating can generate temperatures of more than 2000 °C for the samples. Structural changes of SWCNTs after electrical current heating were observed using electron microscopy and Raman spectra. After breakdown of BP, the disorder-induced D-band increased and a smaller diameter related radial-breathing mode was reduced in the high temperature region. Structural transformations of SWCNT to other carbon nanostructures were observed after current-driven high-temperature treatment in the vacuum. In addition, surface-enhanced Raman scattering with intensity enhancement more than ten times was observed in the BP with agglomerated Fe or Ti particles.     

Fabrication of single-walled carbon nanotube Schottky diode with gold contacts modified by asymmetric thiolate molecules

We have fabricated single-walled carbon nanotube (SWCNT) Schottky diodes by asymmetrically modifying the two Au/SWCNT contacts using different thiolate molecules, methanethiol (CH₃SH) and trifluoroethanethiol (CF₃CH₂SH). Characterization has revealed that highly asymmetrical contacts with Schottky barrier heights of ∼190 and ∼40 meV (increased by over 70% and decreased by over 60%, respectively with respect to that of pristine Au/SWCNT contact of ∼110 meV) were achieved for the Au/SWCNT contacts modified by CH₃SH and CF₃CH₂SH, respectively. The performance of our SWCNT Schottky diodes is as follows: the forward and reverse current ratio (I_forward/I_reverse) higher than 10⁴, a forward current as high as ∼5 μA, a reverse leakage current as low as ∼100 pA, and a current ideality factor as low as ∼1.42. This is at least comparable to, if not better than SWCNT Schottky diodes fabricated with asymmetrical metals, where one contact is a metal with a work function lower than that of SWCNTs to yield a Schottky contact, while the other has a work function higher than that of SWCNTs to achieve an ohmic (more near ohmic) contact.     

A comprehensive scenario for commonly used purification procedures of arc-discharge as-produced single-walled carbon nanotubes

The purification of single-walled carbon nanotube (SWCNT) samples was analysed using a multi-technique approach, with structural as well as spectroscopic probes, in order to characterize the samples and to identify important factors for improvement of SWCNT sample quality. The first dry oxidation step (air at 365 °C) is shown to have only a weak selectivity for the removal of the amorphous carbon or weakly organized graphitic species as well as resulting in a partial consumption of the SWCNTs. The functionalization of the SWCNTs is highly specific with formation of carboxyl, hydroxyl and carbonyl groups. On the other hand this oxidation step is highly efficient for the oxidation of the catalytic impurities (Ni, Y) which can be easily removed by subsequent acid treatment. A final high temperature treatment indicates some incomplete restoration of the quality of the SWCNT surface.     

The synthesis of covalent bonded single‐walled carbon nanotube/polyvinylimidazole composites by in situ polymerization and their physical characterization

Single‐walled carbon nanotube (SWCNT) polyvinylimidazole (PVI) composites have been prepared by in situ emulsion polymerization. Dispersion of raw SWCNTs in the PVI matrix was improved by surface modification of the SWCNTs using nitric acid treatment and air oxidation. The carbonyl‐terminated SWCNTs were covalently bonded to PVI by in situ polymerization and the SWCNT/PVI composite was thus obtained. The morphological and structural characterizations of the surface‐functionalized SWCNTs and SWCNT/PVI composites were carried out by Fourier transform infrared spectroscopy, X‐ray diffraction, conductivity measurements, scanning, and transmission electron microscopy. Thermograms of the materials were determined by the differential scanning calorimetry technique. The characterization results indicate that PVI was covalently bonded to SWCNTs and a new material was then obtained. The functionalized SWCNTs showed homogenous dispersion in the composites, whereas purified SWCNT resulted in poor dispersion and nanotube agglomeration. SWCNT/PVI composites exhibited chemical stability enhancement in many common solvents. I–V curves of the samples exhibit an ohmic character. Conductivity values for pure SWCNTs, pure PVI and SWCNT/PVI composite were measured to be 3.47, 2.11 × 10⁻⁹, and 2.3 × 10⁻³ S/m, respectively. Because of resonance, a large dielectric constant is obtained for SWCNT/PVI composite, which is not observed for ordinary materials. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Optical absorption response of chemically modified single-walled carbon nanotubes upon ultracentrifugation in various dispersants

Arc discharge single-walled carbon nanotubes (SWCNTs) were modified through different oxidative treatments and functionalization reactions. The modified SWCNT powders were dispersed in four different aqueous media and purified by ultracentrifugation. Extinction coefficients of the modified SWCNTs depended on the SWCNT type but did not depend on the dispersion medium. According to visible/near infrared spectroscopy, the purity of all the modified SWCNT dispersions substantially improved after ultracentrifugation; however, the spectrum profile, the degree of purity and the centrifugation yield were influenced by the SWCNT type, the surface functional groups and the dispersion medium. Semi-quantitative purity indexes calculated from optical absorption spectra were supported by transmission electron microscopy observations. Contents in metal impurities were analyzed by energy dispersive X-ray spectroscopy. SWCNT samples processed by oxidative acid treatments and ultracentrifugation showed metal contents of lower than 0.5 wt%.     

Influence of the soluble organic fraction on the thermal behaviour, texture and surface chemistry of diesel exhaust soot

Diesel soot samples collected on a SiC filter while using or not an oxidation catalyst, were solvent extracted or heat treated under inert atmosphere. All studied soot were characterised by XPS, DRIFTS, TGA-MS, Py-GC-MS and gas sorption. In the first part, the obtained results are discussed in order to show the influence of an oxidation catalyst on some properties of the raw soot materials, such as their soluble organic fraction (SOF) content and composition, their surface chemistry and textural characteristics. Then, it is seen that the thermal decomposition of SOF adsorbed on the particulate matter leads under inert atmosphere to the formation of a microporous carbonaceous layer at 600 °C. On the other hand, textural analyses have revealed that the structure of the raw soot material is non-porous in nature, as subsequent extractions in DCM and toluene do not seem to create any porosity within the samples. Finally, we have described and discussed the composition of the particulate matter as a function of its respective components, namely SOF, volatile organic fraction (VOF), carbonaceous matrix and ashes.     

Moderate anisotropy in the electrical conductivity of bulk MWCNT/epoxy composites

Bulk aligned multi-walled carbon nanotube films and their epoxy composites were prepared and their DC and AC conductivity studied. Nanotube films of up to 2 mm thickness were grown by catalytic chemical vapor deposition. Composites of nanotubes were made by infiltrating the films with a commercial epoxy. DC electrical resistivities in the axial direction of as-grown and purified films were found to be ∼1.2 Ωmm and ∼3.4 Ωmm, respectively. For the transverse direction the resistivity values were higher only with a factor of ∼2. In the case of composites, anisotropy is more pronounced showing more than an order of magnitude higher resistivity in the transverse direction (∼71.4 Ωmm) as compared to the axial value (∼4.2 Ωmm). AC behavior of the films investigated between 1 MHz and 3 GHz shows the presence of inductive and capacitive components at frequencies above ∼100 MHz. The moderate anisotropy for both DC and AC electrical properties are explained on the basis of the films’ structure combined with percolation theory and equivalent circuit models.     

Single-wall carbon nanotubes prepared with different kinds of Ni-Co catalysts: Raman and optical spectrum analysis

The addition of acetates and nitrates for the synthesis of single-wall carbon nanotubes via laser ablation was explored. Targets containing nominal amounts of acetates or nitrates in addition to Ni and Co catalysts were compared to a standard target containing only Ni and Co at temperatures ranging from 1000 to 1200 °C. The as produced web-like soot was characterized by transmission electron microscopy, Raman spectroscopy and optical absorption spectroscopy. All samples showed a linear increase in SWCNT mean diameter with temperature; however, the rate of mean diameter change with temperature differed for the various targets, more so at lower temperatures. The addition of nitrates improved the SWCNT relative yield over all the temperatures used, whereas the inclusion of acetates improved the relative yield only at lower temperatures. The above results were discussed and analyzed according to the porous structure of the targets resulting from the decomposition of the acetates and nitrates and consequential thermal diffusion changes.     

Temperature and catalyst effects on the production of amorphous carbon nanotubes by a modified arc discharge

Large amounts of amorphous carbon nanotubes (ACNTs) were prepared with Co-Ni alloy powders as catalyst in hydrogen gas atmosphere by a modified arc discharging furnace which can control temperature during the electric arcing process. The experimental results indicate that the cooperative function of temperature and catalyst plays an important role in the soot production rate and the relative ACNT purity. When temperature increases from 25 °C to 700 °C, the soot production rate increases from around 1 g/h to 8 g/h, the best relative ACNT purity at 600 °C can reach up to 99% compared to the room temperature sample. Without catalyst, only plate graphite is formed at 25 °C and very few carbon nanotubes are found when temperature increases to 600 °C. TEM, SEM, HRTEM and XRD analysis showed that the as-prepared carbon nanotubes are almost amorphous. The soot production rate is 8 g/h and diameter range of amorphous carbon nanotubes is about 7-20 nm, respectively.     

Raman scattering study of the thermal conversion of bundled carbon nanotubes into graphitic nanoribbons

Raman scattering is used to study the temperature-driven structural transformations of bundled single-walled carbon nanotubes (SWCNTs) observed in HiPCO and ARC synthesis by electron microscopy, i.e., tube-tube coalescence ∼1300-1400 °C, coalesced tubes to multi-walled tubes (MWCNT) at ∼1600-1800 °C and finally (only ARC tubes) MWCNT to graphitic nanoribbons (GNRs) at ∼1800 °C. All these transformations occurred in vacuum. Here, we present the details of these transformations as seen through the “eyes” of Raman scattering via changes in the radial (R) SWCNT band, the G-band (and its substructure) and the relative intensity of the disorder-induced D- and D′-band scattering. The Raman spectrum of GNRs is also discussed in detail. For 514.5 nm laser excitation, five relatively broad GNR Raman bands are observed: 1350, 1580, 1620, 2702 and 3250 cm⁻¹. A Knight plot is used to estimate the GNR width and we find w ∼ 9 nm, which is in reasonable agreement with the estimate of 7.6 nm based on TEM and the model that a GNR is a collapsed MWCNT.     

Effects of a Pt/Ce0.68Zr0.32O2 catalyst and NO2 on the kinetics of diesel soot oxidation from thermogravimetric analyses

In this study, the reactivity of well-characterized diesel soot samples is investigated by thermogravimetry under different kinds of oxidizing atmospheres (20% O₂ or 10% O₂ + 700 ppm NO₂) either under catalyzed or non-catalyzed conditions. Whatever the atmosphere used, the catalyst Pt/ceria-zirconia was able to lower significantly the ignition temperature of soot, but the catalytic effect was found to be more pronounced when the oxidation process was assisted by NOx. This is due mainly to the efficiency of both catalyst components (the noble metal and the OSC material) in recycling the NO released after attack of the soot by NO₂. By contrast, the NO₂ is of very limited use in the absence of catalyst under our experimental conditions. The global kinetic parameters representative of the carbonaceous matrix oxidation are determined using a methodological approach combining thermogravimetric experiments and non-linear multivariate regression. The kinetic parameters obtained are consistent both with the literature results and the postulated mechanistic pathways for soot oxidation assisted or not by NOx.     

Cellular structures of carbon nanotubes in a polymer matrix improve properties relative to composites with dispersed nanotubes

A new processing method has been developed to combine a polymer and single wall carbon nanotubes (SWCNTs) to form electrically conductive composites with desirable rheological and mechanical properties. The process involves coating polystyrene (PS) pellets with SWCNTs and then hot pressing to make a contiguous, cellular SWCNT structure. By this method, the electrical percolation threshold decreases and the electrical conductivity increases significantly as compared to composites with well-dispersed SWCNTs. For example, a SWCNT/PS composite with 0.5 wt% nanotubes made by this coated particle process (CPP) has an electrical conductivity of ∼3 × 10⁻⁴ S/cm, while a well-dispersed composite made by a coagulation method with the same SWCNT amount has an electrical conductivity of only ∼10⁻⁸ S/cm. The rheological properties of the composite with a macroscopic cellular SWCNT structure are comparable to PS, while the well-dispersed composite exhibits a solid-like behavior, indicating that the composites made by this new CPP are more processable. In addition, the mechanical properties of the CPP-made composite decrease only slightly, as compared with PS. Relative to the common approach of seeking better dispersion, this new fabrication method provides an important alternative means to higher electrical conductivity in SWCNT/polymer composites. Our straightforward particle coating and pressing method avoids organic solvents and is suitable for large-scale, inexpensive processing using a wide variety of polymers and nanoparticles.     

Comprehensive kinetic characterization of the oxidation and gasification of model and real diesel soot by nitrogen oxides and oxygen under engine exhaust conditions: Measurement, Langmuir-Hinshelwood, and Arrhenius parameters

The reaction kinetics of the oxidation and gasification of four types of model and real diesel soot (light and heavy duty vehicle engine soot, graphite spark discharge soot, hexabenzocoronene) by nitrogen oxides and oxygen have been characterized for a wide range of conditions relevant for modern diesel engine exhaust and continuously regenerating particle trapping or filter systems (0-20% O₂, 0-800 ppm NO₂, 0-250 ppm NO, 0-8% H₂O, 303-773 K, space velocities 1.3 × 10⁴-5 × 10⁵ h⁻¹). Soot oxidation and NO₂ adsorption experiments have been performed in a model catalytic system with temperature controlled flat bed reactors, novel aerosol particle deposition structures, and sensitive multicomponent gas analysis by FTIR spectroscopy. The experimental results have been analyzed and parameterized by means of a simple carbon mass-based pseudo-first-order rate equation, a shrinking core model, oxidant-specific rate coefficients, Langmuir-Hinshelwood formalisms (maximum rate coefficients and effective adsorption equilibrium constants), and Arrhenius equations (effective activation energies and pre-exponential factors), which allow to describe the rate of reaction as a function of carbon mass conversion, oxidant concentrations, and temperature. At temperatures up to 723 K the reaction was driven primarily by NO₂ and enhanced by O₂ and H₂O. Within the technically relevant concentration range the reaction rates were nearly independent of O₂ and H₂O variations, while the NO₂ concentration dependence followed a Langmuir-Hinshelwood mechanism (saturation above ∼200 ppm). Reaction stoichiometry (NO₂ consumption, CO and CO₂ formation) and rate coefficients indicate that the reactions of NO₂ and O₂ with soot proceed in parallel and are additive without significant non-linear interferences. The reactivity of the investigated diesel soot and model substances was positively correlated with their oxygen mass fraction and negatively correlated with their carbon mass fraction.     

An STM study of phosphoric acid inhibition of the oxidation of HOPG and carbon catalyzed by alkali salts

The role of phosphoric acid as an inhibitor in the oxidation of HOPG and as a neutralizer of alkali salt catalysts is examined using scanning tunneling microscopy, supported by thermogravimetric analysis of carbon powder samples. HOPG samples were oxidized in air primarily at 700 °C, with a few samples oxidized at 800 °C. Reaction time was 20 min. Powder samples were oxidized for 5 min at temperatures ranging from 500 °C to 900 °C and rates of oxidation were determined. STM images of impurity deposits and oxidized samples are presented and analyzed. Two alkali salts are examined, sodium hydroxide and potassium acetate, and both catalyze oxidation at 700 °C. Phosphoric acid proves to be an inhibitor at 700 °C but begins to lose its inhibiting effect at 800 °C. It also demonstrates neutralization of potassium acetate at 700 °C but results for NaOH/phosphoric acid mixtures are less conclusive.     

Size-controlled synthesis of graphene oxide sheets on a large scale using chemical exfoliation

The synthesis of graphene oxide (GO) sheets with controlled size on a large scale was developed using chemical exfoliation by simply controlling the oxidation and exfoliation procedure. The GO samples prepared under different conditions, which all have excellent water dispersion, are characterized by thermal gravimetric analysis, Ultraviolet-visible spectroscopy, X-ray diffraction and atomic force microscopy. It is found that as longer oxidation times and more oxidants are used, the mean size of the GO sheets, which has a Gaussian distribution, decreases from ∼59,000 to ∼550 nm².