Chirality fingerprinting and geometrical determination of single-walled carbon nanotubes: Analysis of fine structure of X-ray diffraction pattern

Single-walled carbon nanotubes (SWCNTs) are seamless cylindrical tubes consisting of carbon atoms with diameters ranging from less than one nanometer to a few nanometers. The arrangement of carbon atoms in a SWCNT is uniquely specified by using a pair of integers (n, m) referred to as the chiral indices. While the detailed structures, such as a carbon–carbon bond length, should be important, they have not been fully clarified yet. In this work, we examine the possibility of powder X-ray diffraction (XRD) method to characterize structures of SWCNTs. It is found that the XRD is a useful tool to “fingerprint” the chiral indices of bulk SWCNT samples. Besides, we find that information on the detailed structure within a SWCNT can be obtained from the XRD pattern. The application to a highly concentrated SWCNTs clarifies that the (6,5) SWCNT is expanded along the radial direction compared to that of ideal rolling up structure of graphene, with a negligible change along the tube axis.     

New metric for evaluating the purity of single-walled carbon nanotubes using ultraviolet–visible-near infrared absorption spectroscopy

Accurate purity evaluation of single-walled carbon nanotubes (SWCNTs) forms the basis for an improvement of material quality during synthesis and purification. We propose a novel purity evaluation method of as-produced and purified SWCNTs by using ultraviolet–visible-near infrared absorption spectroscopy. Our approach integrates the absorption peaks from the interband electronic transitions of semiconducting and metallic tubes after the linear subtraction of π-plasmon contribution in optical absorption spectra from SWCNT-containing samples. For the constructed sample sets prepared by mixing designed amounts of highly pure SWCNTs and carbonaceous impurities, this method has demonstrated quite a good closeness of the measured purities to the designed ones. Our novel purity-evaluation method has also verified its usability for a set of arc-produced samples having different SWCNT purities, prepared by isothermal oxidation at 275∼475 °C.     

Purification of single-walled carbon nanotubes using a fixed bed reactor packed with zirconia beads

Single-walled carbon nanotube (SWCNT) soot produced by arc discharge was purified through gas and liquid phase oxidations. In the gas-phase oxidation, zirconia beads with different diameters of 1, 5, and 10 mm were packed together with raw SWCNT soot inside a vertical quartz tube to enhance air flow uniformity and an exposed surface area of the raw soot during thermal oxidation in air. A decrease of the bead sizes led to such a stronger oxidation of carbonaceous impurities that ∼10 wt.% higher weight loss was then achieved with the 1 mm beads than without them. A subsequent HNO₃ treatment and the second thermal oxidation were engaged to improve further the purity of SWCNTs. Thermogravimetric (TG) analysis, scanning electron microscopy, high resolution transmission electron microscopy, and Raman spectroscopy were used to characterize the samples. The derivative TG (DTG) curves were deconvoluted to quantitatively determine the SWCNT purity of the samples. Our final purified samples showed a yield of ∼26%, a metal impurity of ∼7% and a SWCNT purity of ∼83% as calculated from the deconvoluted DTG curves.     

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.     

Quantitative investigation of mineral impurities of HiPco SWCNT samples: Chemical mechanisms for purification and annealing treatments

HiPco single-walled carbon nanotube samples contain a non-negligible amount (about 30 wt.%) of metal-based impurities. These impurities are prejudicial to the development of CNT-based applications. In the literature, there is no agreement for their nature and their relative content. Consequently, the standard purification treatments could not be selective enough and they lead to damage the carbon nanotubes. In this work, we have deeply investigated the mineral impurities of a raw HiPco SWCNT sample by means of several techniques. We were able to identify and quantify at least six iron-based phases that coexist in the HiPco SWCNT samples. The raw SWCNTs were submitted to a selective purification treatment involving a high temperature treatment under chlorine. The results show that the efficiency of the metal removal is improved by increasing the temperature of treatment leading to a purification yield of 95.4%, while preserving the structural quality of the SWCNTs. The analysis of the mineral residues in the purified and annealed SWCNT samples allows to propose the chemical mechanisms involved during the chemical treatments.     

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     

The influence of single-walled carbon nanotube functionalization on the electronic properties of their polyaniline composites

The “in situ” preparation and characterization of composites of polyaniline (PANI) and single-walled carbon nanotubes (SWCNTs) are reported. To improve the dispersion and compatibility with the polymer matrix the raw SWCNTs were modified following different routes. SWCNTs oxidized by chemical or thermal treatments (nitric acid and air oxidation, respectively) were subjected to covalent functionalization with octadecylamine (ODA). SWCNT/PANI composites were prepared either from just oxidized SWCNTs, or from ODA functionalized SWCNTs. Temperature-programmed desorption, elemental analyses, ultraviolet-visible (UV-vis), UV-vis with near infrared and Raman spectroscopy, X-ray diffraction, scanning and transmission electron microscopy and conductivity measurements were used to characterize the functionalized SWCNT materials, dispersions and composites. The PANI composite prepared from air oxidized SWCNTs showed the best electrical conductivity indicating a better interaction with polyaniline than ODA functionalised SWCNTs. The improvement of conductivity is attributed to the doping effect or charge transfer of quinoide rings from PANI to SWCNTs.     

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.     

Simple purification and selective enrichment of metallic SWCNTs produced using the arc-discharge method

Purified single-walled carbon nanotubes (SWCNTs) were produced using the arc-discharge method with a combination of air oxidation and dispersion–centrifugation processes in a tetrahydrofuran solution containing an amine as a dispersant. Subsequently, SWCNT samples were analyzed using thermo gravimetric analysis, vis-near infrared absorption spectroscopy, scanning electron microscopy, and Raman spectroscopy. Results revealed that metallic SWCNTs were enriched in a supernatant through the dispersion–centrifugation process.     

Preparation and characterization of highly conductive transparent films with single-walled carbon nanotubes for flexible display applications

Dense, aligned single-walled carbon nanotubes (SWCNTs) were obtained by nitric acid treatment and the subsequent removal of metal impurities by HCl. The highly purified SWCNTs were dispersed with sodium dodecyl sulfate in order to obtain a stabilized suspension for spray coating on flexible polyethylene terephthalate (PET) substrate. The low sheet resistance of the resulting thin conductive film on the PET substrate was due to the interconnecting networks of highly purified SWCNT bundles. These bundles formed strong crisscross networks of nanotubes clustered together with well defined channels, thus improving the electrical and optical properties of the film. Its sheet resistance varied from 956 to 472 Ω/square with 85% optical transmittance at a wavelength of 550 nm. The films may be potential candidates for flexible display applications.     

Selective growth of single-walled carbon nanotubes over Co–MgO catalyst by chemical vapor deposition of methane

The selective synthesis of SWCNTs with narrow chirality and diameter distribution by methane decomposition over a Co–MgO catalyst is reported. Raman spectroscopy, temperature programmed oxidation (TPO), UV–Vis–NIR absorption spectroscopy, and nitrogen physisorption were used to probe SWCNTs morphology, reaction selectivity, SWCNTs chirality and diameter distribution, and carbon yield. The catalyst was examined by nitrogen physisorption, X-ray diffraction (XRD), temperature programmed reduction (TPR), and UV–Vis-diffuse reflectance spectroscopy to elucidate the structure and chemical state of the species responsible for SWCNT growth. The results established a clear link between the degree of dispersion of Co species inside the MgO lattice and the catalyst activity and selectivity for SWCNT growth. High dispersion and stabilization of Co species influenced catalytic activity for methane decomposition and the high SWCNT selectivity. The yield of carbon and SWCNT selectivity increased with an increase in temperature, however, SWCNTs diameter distribution shifts to larger diameter tubes as synthesis temperature was increased.     

Evidence for large hydrogen capacity in single-walled carbon nanotubes encapsulated by thin Pd layers onto a Pd substrate

We report a study of hydrogen storage and its mechanism in a novel material, representing single-walled carbon nanotubes (SWCNTs) encapsulated by thin Pd layers onto a Pd substrate. A synergetic effect resulting in combination of the Pd and the SWCNT properties with regard to hydrogen has been achieved. We showed that adding SWCNTs increases the H₂-capacity of the Pd–SWCNT composite under electrochemical loading only by up to 25% relative to the Pd metal alone. At the same time, with regard to the added SWCNTs, such synergetic approach (providing high H₂ pressure from highly H-loaded massive Pd substrate into a small fraction of deposited SWCNT) allowed us to achieve a net capacity of 8–12 wt.%. H₂, thus, bringing a unique chance to study hydrogen storage mechanism in highly H-loaded SWCNT. Using ESR technique it was established that the Pd–C_x π-complexes forming at the openings of SWCNTs could be considered as hydrogen adsorption sites, providing both high gravimetric capacity (H/C > 1) and low hydrogen binding energy in the Pd encapsulated SWCNT.     

The purification of HiPco SWCNTs with liquid bromine at room temperature

A new procedure to purify HiPco single-walled carbon nanotubes (SWCNTs) from iron catalyst impurity is introduced. The protocol, which uses liquid bromine at room temperature (RT) as an oxidant, improves nanotube purity from iron by a factor of approximately 10, while maintaining good nanotube integrity as demonstrated by near infrared (NIR) luminescence and absorbance measurements. When HiPco SWCNTs are dissolved in RT Br₂(l) (free of O₂ and H₂O), the metallic iron impurity is quickly oxidized to its bromide salt and easily removed by aqueous washing or by washing with dilute acid. The iron content (by ICP-AE) for the purified SWCNT material was 2.8-3.6% by weight (for three different samples) for a single purification step, but could be lowered to 1.6-1.8% with an additional purification cycle. Characterization of the resulting purified SWCNT material has been achieved by TEM imaging, XPS, ICP-AE analysis, Raman spectroscopy, electronic absorption spectroscopy, and by NIR photoluminescence measurements. Finally, the new Br₂(l) purification procedure has been compared to and contrasted with other established purification procedures for HiPco SWCNTs and found to be a highly desirable alternative.     

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.     

DFT and tight binding Monte Carlo calculations related to single-walled carbon nanotube nucleation and growth

Density-functional theory (DFT) calculations for idealized nucleation processes of (5, 5) and (10, 0) single-walled carbon nanotubes (SWCNTs) on a 55 atom nickel cluster (Ni₅₅) showed that it requires a larger chemical potential to grow a carbon island (which is the simplest structure that can lead to formation of the SWCNTs) on the cluster than to extend the island into a SWCNT or to have the carbon atoms dispersed on the cluster surface. Hence, in the thermodynamic limit the island will only form once the (surface of the) cluster is saturated with carbon, and the island will spontaneously form a SWCNT at the chemical potentials required to create the island. The DFT (zero Kelvin) and tight binding Monte Carlo (1000 K) also show that there is a minimum cluster size required to support SWCNT growth, and that this cluster size can be used to control the diameter, but probably not the chirality, of the SWCNT at temperatures relevant to carbon nanotube growth. It also imposes a minimum size of clusters that are used for SWCNT regrowth.     

Characterization of SWCNT and PAN/SWCNT films

Three different films, poly(acrylonitrile-co-methylacrylate)/single wall carbon nanotubes (PAN-MA/SWCNT), poly(acrylonitrile-co-methylacrylate)/carbon black (PAN-MA/CB) and pure functionalized SWCNT, are analyzed. The diffuse reflectance and transmittance of the films from 2 μm to 18 μm are characterized with an integrating-sphere Fourier transform spectrophotometer system. The SWCNT film shows high reflectance and low emissivity. Surface roughness characterization by laser scanning confocal microscopy confirms that the low emissivity is not due to a highly polished surface and is therefore more likely due to the metallic behavior of the SWCNTs. Characterization using infra red thermography highlighted the thermal protective behavior of the SWCNT film; the maximum temperature obtained from a 5.2 kW/m² heat flux exposure was 50 °C lower than that for the two (SWCNT, CB) PAN-MA based films.     

Growth of single-walled carbon nanotubes with controlled diameters and lengths by an aerosol method

On the basis of combined study of the transmission electron microscopy, scanning electron microscopy, Raman spectroscopy and ultraviolet–visible–near infrared absorption spectroscopy, the properties of the single-walled carbon nanotubes (SWCNTs), synthesized by aerosol (floating catalyst) chemical vapor deposition method by ferrocene vapor decomposition in the presence of carbon monoxide, are studied in details. The results show that increasing the temperature gives rise to the formation of high quality and large diameter SWCNTs. By monitoring the water-cooled probe position, both the bundle length and the diameter of the SWCNTs are effectively tuned due to the variation of the residence time and temperature profile in the reactor. An introduction of a small amount of CO₂ suppresses the growth of small diameter nanotubes and enlarges the mean diameter of SWCNT samples. The mean diameter of SWCNTs could be easily altered in a broad range from 1.1 to 1.9 nm during growth, which is essential for the SWCNT applications in optical and electronic devices.     

Evidence of sidewall covalent functionalization of single-walled carbon nanotubes and its advantages for composite processing

Single-walled carbon nanotubes (SWCNTs) were functionalized in a three-step procedure. The first step is a radical reaction creating a covalent bond between the carbon nanotube surface and grafted p-methoxyphenyl functional groups. In a second step, a deprotection of the methoxy functions generates free alcohol groups and in the final step an esterification is done in order to install a double bond for further polymerization. Evidence that functionalization has actually occurred on the SWCNT sidewalls is furnished through investigations involving several complementary techniques (visual dispersion tests, transmission electron microscopy, thermal gravimetric analysis and adsorption volumetry). We show that surface properties of SWCNTs are changed throughout the chemical treatments and that the obtained level of functionalization is low. Incorporation of functionalized SWCNTs in a polymer (poly(methyl methacrylate)) matrix was done through an in situ polymerization process. Observations of the obtained composites using scanning and transmission electron microscopy illustrate that interactions between the SWCNT surface and the polymer matrix are improved.     

Controlled attachment of metal nanoparticles to single walled carbon nanotubes as a key step in their seeded growth and lengthening

The present study demonstrates the viability of the reductive attachment step of the single walled carbon nanotube (SWCNT) lengthening process in which long SWCNTs are grown from short nanotube seeds. Aryl sulfonate sidewall-functionalized, carboxylate end-functionalized SWCNTs are attached to an inorganic cluster pro-catalyst (FeMoC) via ligand exchange. The SWCNT-FeMoC complex was electrodeposited onto highly ordered pyrolytic graphite (HOPG), heated and exposed to etching conditions. Pre- and post-treatment AFM imaging shows that controlled reductive etching of the SWCNTs is attainable at a variety of pressures and temperatures in hot surface/cold gas and hot surface/hot gas systems.     

High-performance biosensors based on enzyme precipitate coating in gold nanoparticle-conjugated single-walled carbon nanotube network films

Single-walled carbon nanotube (SWCNT) network films with high network density were prepared by vacuum-filtering a suspension of SWCNTs, and used as a host of enzyme precipitate coating of glucose oxidase (EPC-GOx). EPC-GOx was fabricated into the SWCNT network films in a two-step process of enzyme precipitation and crosslinking. High GOx loading in a form of EPC expedited the generation of electrons while the good connectivity of conductive SWCNTs in the network structure increased the electron transfer rate. According to amperometric measurements, the sensitivities of GOx/SWCNT electrodes, governed by both generation and transfer of electrons, were significantly enhanced by filling up the open pores of SWCNT films with the EPC-GOx when compared to the approaches of covalent-attachment (CA) and enzyme coating (EC) with no step of enzyme precipitation. For example, the sensitivities of CA, EC and EPC-GOx were 0.039, 0.140, and 5.72 μA mM⁻¹, respectively. High sensitivity of EPC-GOx was maintained under iterative uses for 10 days. The deposition of gold nanoparticles into SWCNT films has resulted in high-performance glucose sensors with a remarkable sensitivity of 24.5 μA mM⁻¹, which can be explained by further expedited electron transfer due to deposited gold nanoparticles.