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.     

Variations in the microstructure and electrical resistance of the SWCNT films under consecutive photoflash exposures

A film of unpurified single-walled carbon nanotubes (SWCNTs) synthesized by the floating catalyst method using ferrocene as the catalyst precursor was subjected to different numbers of flashes and the products were studied. In addition to the remaining SWCNTs, Fe₂SiO₄ particles covered with amorphous carbon were found to attach on the SWCNTs, and the size increased with flash numbers. Fe₂SiO₄ arose from the oxidation of Fe₃C, a ferrocene-induced catalyst particle embedded in the SWCNTs, where Si provided by SiO₂ released from the mullite tube at 1200 °C during SWCNT growth. The amorphous carbon coating was attributed to insufficient time of the precipitated carbon to crystallize during rapid cooling after the flash. Variation of the Raman I_D/I_G ratio from an initial value of 0.035 to 0.025 after 100 flashes was due to competition between the removal of carbon from the nanotubes and the formation of amorphous carbon on the Fe₂SiO₄ particle surface. The electrical resistance of the SWCNT film increased with the number of flashes but the change became progressively smaller, with the increment decreasing from 17.5% to 0.2%. Similar experiments using purified SWCNTs were performed, and no such particles were observed.     

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.     

The influence of bimetallic catalyst composition on single-walled carbon nanotube yield

We show that the yield of single-walled carbon nanotubes (SWCNTs) grown with bimetallic catalysts is a strong function of their atomic-scale composition. A series of compositionally-tuned Ni_xFe_1?x bimetallic catalysts with a constant mean diameter of 2.0 nm are used to catalyze the growth of nanotubes via a floating catalyst method. Increasing the Fe content in the catalysts is found to lower the fraction of SWCNTs in the collected as-grown product. Based on a simple surface-to-volume model, these results are explained by the higher carbon solubility of Fe compared to Ni which results in a larger amount of carbon precipitation and the formation of multi-walled tubes when the nanotubes are nucleated from catalysts with high Fe content. Overall, our study demonstrates that the size and composition of bimetallic catalysts must be precisely controlled to obtain high yields of SWCNTs for large-scale production.     

Metal catalyst-free mist flow chemical vapor deposition growth of single-wall carbon nanotubes using C60 colloidal solutions

Metal catalyst-free mist flow chemical vapor deposition (CVD) growth of single-walled carbon nanotubes (SWCNTs) with C₆₀ fullerenes has been investigated by using an aqueous colloidal C₆₀ solution. Under the optimum reaction condition, relatively uniform SWCNTs with a mean diameter of 1.28 nm can be synthesized without any treatments of C₆₀ prior to CVD. Cap opening, nucleation and the growth of SWCNTs have been occurring almost simultaneously during the present CVD. C₆₀ can be used as the seeds (i.e., end-caps) of SWCNTs, in which oxygen atoms from water molecules provide etching of C₆₀ into caps. Furthermore, the coalescence of C₆₀ caps into a larger one leads to the growth of SWCNTs with larger diameters.     

Electric field enhancements in In2O3-coated single-walled carbon nanotubes

In₂O₃ nanoparticles are coated on the surfaces of single-walled carbon nanotubes (SWCNTs) by a successive ionic layer adsorption and reaction process. The thickness of the In₂O₃ nanoparticle film is tuned by controlling the number of coating cycles. The electric field around the In₂O₃-coated SWCNTs is compared with that of pristine SWCNTs. Field enhancement of the In₂O₃-coated SWCNTs is confirmed by conductive atomic force microscopy at low electric field (contact mode: 1 V to −1 V) and also field emission (FE) analysis at high electric field (0–4.2 V/μm). The uniformity and emission stability are also measured via FE analysis. Near infrared and X-ray photoemission spectroscopy data are suggested to explain the charge transfer, bandgap change between the In₂O₃ nanoparticles and SWCNTs, and the electric field enhancements in the In₂O₃-coated SWCNTs at both low and high electric field.     

Efficiency of C60 incorporation in and release from single-wall carbon nanotubes depending on their diameters

We show that the efficiency of incorporating C₆₀ in single-wall carbon nanotubes (SWCNTs) and that of the incorporated C₆₀’s release from the SWCNTs depend on the SWCNT diameter. Through transmission electron microscopy, we found that the C₆₀ incorporation efficiency reached its maximum at diameters of 1-2 nm, while the efficiency of C₆₀ release from SWCNTs in toluene was maximized at 3-5 nm. The difficulty of C₆₀ release from SWCNTs with diameters of 5-6 nm might reflect either the effective packing of C₆₀ inside SWCNTs or a flattened SWCNT structure. We occasionally observed C₆₀ molecules arranged in a line along the sidewall inside SWCNTs with large diameters/width (>7 nm), indicating that large diameter SWCNTs were sometimes flattened.     

Effect of bundling on the π plasmon energy in sub-nanometer single wall carbon nanotubes

Due to their unusual electronic and vibrational properties, single walled carbon nanotubes (SWCNTs) with sub-nanometer diameters d ∼ 0.5–0.9 nm have recently gained interest in the carbon community. Using UV–Vis–NIR spectroscopy and ultra-centrifugation, we have conducted a detailed study of the π plasmon energy (present at∼5–7 eV) in sub-nm SWCNTs as a function of the size of the bundle. We find that the energy of the π plasmon peak E varies with the bundle diameter D_h as E = (-0.023 eV)^∗ln(D_h/d_o) + 5.37 eV, where d_o = 0.5 nm and corresponds to the smallest tube diameter.¹ This is compared with the same data for HiPCo and Carbolex SWCNTs of larger diameter (1–1.4 nm) confirming a clear dependence of E on the bundle size, which is present in addition to the previously reported dependence of E on SWCNT diameter d.     

Parametric study of synthesis conditions in plasma-enhanced CVD of high-quality single-walled carbon nanotubes

High-quality single-walled carbon nanotubes (SWCNTs) have been synthesized from H₂-CH₄ mixtures on a MgO-supported bimetallic Mo/Co catalyst using microwave plasma-enhanced chemical vapor deposition (PECVD). Reaction parameters including temperature, H₂:CH₄ ratio, plasma power, and synthesis time have been examined to assess their influence on SWCNT synthesis. Raman spectroscopy and high-resolution field emission scanning electron microscopy reveal that the quality, selectivity, density and predominant diameter of SWCNTs depend on the varied synthesis parameters. Results of this study can be used to optimize SWCNT synthesis conditions and products and to improve understanding of the growth of SWCNTs by PECVD.     

Highly compressible carbon nanowires synthesized by coating single-walled carbon nanotubes

Carbon nanowires with a diameter of 40–60 nm were synthesized by coating single-walled carbon nanotubes (SWCNTs) in an intermittent, one-stage DC arc discharge process. Transmission electron microscopy shows that these nanowires consist of a SWCNT with an amorphous carbon coating, whose thickness depends on the time of arc discharge. The mechanical properties of blocks of these nanowires were tested by load–unload cyclic compression and static force thermomechanical experiments. The results show that carbon nanowire blocks exhibit better compressive behavior than pure SWCNTs blocks, and carbon nanowires show a typical nonlinear strain–temperature response due to the amorphous carbon layer. A mechanism of adsorption-controlled growth of amorphous carbon on the SWCNTs in the vapor phase is proposed for the formation of the nanowires.     

Synthesis and properties of sulfonated biphenyl poly(ether sulfone) and its mixed‐matrix membranes containing carbon nanotubes for gas separation

We prepared mixed‐matrix membranes (MMMs) composed of carboxylated single‐walled carbon nanotubes (f‐SWCNTs) and a sulfonated biphenyl poly(ether sulfone) (S‐PPSU) polymer matrix. The thermal stability and properties of the pores of the S‐PPSU and f‐SWCNTs were characterized by thermogravimetric analysis and sorption isotherm curves, respectively; these showed that the surface and pore diameter decreased after the introduction of carboxyl groups to the single‐walled carbon nanotubes (SWCNTs), and the pore properties did not restore original values even when the f‐SWCNTs were preheated to 350 °C to remove carboxyl groups. The gas‐separation measurement showed that the MMMs comprised of the S‐PPSU and f‐SWCNTs possessed better gas‐separation properties than the ones composed of biphenyl poly(ether sulfone) and SWCNTs. The permeability for N₂, O₂, He, and CO₂ and the selectivity for O₂/N₂ and O₂/CO₂ were enhanced simultaneously because of the good dispersion of f‐SWCNTs and the improved interaction between the two phases. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44995.     

Synthesis of single-walled carbon nanotubes by an arc-discharge method using selenium as a promoter

Single-walled carbon nanotubes (SWCNTs) with high purity and very narrow diameter distribution have been synthesized using the dc arc-discharge method with Y–Ni alloy as catalyst and selenium (Se) as promoter. The SWCNTs show a very narrow diameter distribution mainly at about 1.5 nm, and can further be readily purified up to >99% purity with traditional purification including HNO₃ reflux and air oxidation. The key factor of the wetting effect of Se in the SWCNTs growth improvement process is proposed and discussed. Moreover, a new less-destructive purification method including electrolysis, air-oxidation and centrifugation has been introduced, and SWCNTs with semiconducting content up to 94% have been produced through density gradient ultracentrifugation method.     

A method for the coating of silica spheres with an ultrathin layer of pristine single-walled carbon nanotubes

We describe a method to fabricate silica gel particles coated with a monolayer of pristine single-walled carbon nanotubes (SWCNTs). SWCNTs dissolved in 1-methyl-2-pyrrolidinone (NMP) are mixed with amino-functionalized silica gels having different diameters. Strong interaction between the amino group and the SWCNT surfaces induces the adsorption of the SWCNTs on the silica, while the stable solvation in NMP hampers further adsorption of the tubes. This approach enables the production of a homogeneous, nondestructive and high-yield coating of the SWCNTs onto the silica surfaces, especially for larger sphere with a diameter over 1 μm. The density and bundling degree of the SWCNTs on the silica gel surfaces are finely controlled by simply changing the ratios of the SWCNTs to the silica gels as well as the SWCNT concentrations. We also describe the coating of the silica gels with metallic SWCNTs. The SWCNT-coated silica gels are useful for a wide range of materials, such as the stationary phase for liquid chromatography and catalyst supporting materials.     

Spectral properties of single-walled carbon nanotubes encapsulating fullerenes

Single-walled carbon nanotubes (SWCNTs) with diameter ranged from 1.22 to 1.6 nm filled with C₆₀, C₇₀ and C₆₀H₂₈ molecules (peapods), as well as double-walled carbon nanotubes (DWCNTs) derived from peapods, were studied by HRTEM, UV-vis-NIR and Raman spectroscopy. Suspensions with accurate concentration were used for spectroscopic studies to enable quantitative comparison of different substances. Filling of the SWCNTs with C₇₀ molecules resulted in a reduced van der Waals interaction between the tubes in a bundle. The DWCNTs have lower intensity of the van Hove bands and weaker photoluminescence. Raman spectra at 633 and 1064 nm excitation wavelengths reveal that RBM frequencies of C₆₀ and C₇₀ peapods are equally downshifted compared to empty tubes. It was found that filling of the nanotubes with C₆₀ and C₇₀ caused spectral shifts of absorption bands: thin tubes display red shifts, while thick ones show blue shifts. DWCNTs and C₆₀H₂₈@SWCNTs do not show any shifts. All the results suggest that the filling of nanotubes with fullerenes alters the average diameter of the electron cloud around SWCNT framework; namely, it increases for thin SWCNTs, and decreases for thick ones. Our attempts to structurally assign thick nanotubes using reported extrapolations from data for thin tubes were unsuccessful.     

A comparison of three carbon nanoforms as catalyst supports for the oxygen reduction reaction

Defective graphene nanosheets (GNSs), single-walled carbon nanotubes (SWCNTs), and herringbone graphite nanofibres (GNFs) were used as Pd₃Pt₁ catalyst supports for an oxygen reduction reaction (ORR). Raman spectroscopy and cyclic voltammetry analyses revealed oxygen-containing functional groups and physical defects on the surfaces of the SWCNTs, GNFs, and synthesised GNSs. Mass-transfer-corrected Tafel diagrams obtained in an O₂-saturated electrolyte showed that the SWCNTs with a high curvature allowed for more surface Pt atoms; thus, these Pd₃Pt₁ catalysts are the first SWCNT system to promote the ORR. These catalysts, however, were slower than the GNS-supported catalysts after 0.875 V (vs. SCE; saturated calomel electrode). In terms of the kinetic current density, the highest mass activity was found for the Pd₃Pt₁/GNS composites. Additionally, according to rotating-ring disk electrode (RRDE) measurements, the H₂O production efficiencies for the Pd₃Pt₁/GNS, Pd₃Pt₁/SWCNT, and Pd₃Pt₁/GNF systems were 70.35%, 66.7%, and 9.58%, respectively. Among these carbon supports, Pd₃Pt₁ on GNS showed the greatest efficiency and durability for producing H₂O via an approximate four-electron pathway; this efficiency was ascribed to metal-support interaction.     

Computational study of B- or N-doped single-walled carbon nanotubes as NH3 and NO2 sensors

The adsorption of NH₃ and NO₂ in B- or N-doped (10, 0) single-walled carbon nanotubes (SWCNTs) was investigated by using density functional computations to exploit their potential applications as gas sensors. NH₃ can be chemisorbed only in B-doped SWCNTs with apparent charge transfer, so B-doped SWCNTs can be used as NH₃ sensors. Both B- and N-doping make NO₂ chemisorption feasible in SWCNTs, but the binding of NO₂ with B is too strong, indicating an impractical recovery time as gas sensors. Due to the medium (optimal) adsorption energy and the conductance reduction accompanied with the charge transfer between SWCNTs and gas molecules, N-doped SWCNTs are potentially good NO₂ sensors.     

Soft Immobilization of Proteins onto Single-Walled Carbon Nanotubes through Nickel Complexed Nitrilotriacetic Acid-End Functionalized Polystyrenes

We describe the immobilization of histidine-tagged green fluorescent protein (His₆-GFP) specifically and reversibly onto single-walled carbon nanotubes (SWCNTs) using nickel complexed nitrilotriacetic acid-end functionalized polystyrene (Ni-NTA-PS) by conjugation between the protein and the polymer in phosphate buffer solution (PBS). We synthesized Ni-NTA-PS by atom transfer radical polymerization (ATRP) using a nitrilotriacetic acid (NTA) initiator. The SWCNTs were dispersed in PBS along with Ni-NTA-PS and His₆-GFP aided by sonication. The diameter of the dispersed SWCNTs was measured by transmission electron microscope (TEM). The immobilization of His₆-GFP and dispersion of SWCNTs were controlled by addition of excess imidazole. It was found that the immobilized His₆-GFP was more stable than free His₆-GFP in organic solvent (PBS/DMF, 1/1, v/v).     

Synthesis of single-walled carbon nanotubes using hemoglobin-based iron catalyst

Hemoglobin (Hb) was used as a catalyst for the growth of single-walled carbon nanotubes (SWCNTs). Hb was deposited onto a hydrophilic treated substrate by spin coating method. After oxidation at 800 °C, protein chains were decomposed and iron oxide nanoparticles remained with an average diameter of 2.29 nm. High quality SWCNTs were synthesized with an average diameter of 1.22 nm. The protein chains prevent iron atoms aggregation and so the size of the nanoparticles is smaller than that from ferritin-like proteins.     

Single-chirality separation of ultra-thin semiconducting arc discharge single-walled carbon nanotubes

Ultra-thin single-walled carbon nanotubes (SWCNTs) with a narrow diameter distribution are prepared with a Fe catalyst under an optimized H₂/He atmosphere by arc discharge first time. These SWCNTs were enriched with a pH–temperature controlled gel chromatography single-chirality separation process. The separation process uses the electrostatic interaction between the gel and the functional groups on the sidewalls of nanotubes, which is defined by the curvature of the C–C bond and the chiral vector (n, m). Tuning the interaction through varying the pH and temperature enables the separation of five species from the sodium dodecyl sulfate (SDS) wrapped SWCNTs in aqueous solution, namely (7, 5), (9, 4), (7, 6), (8, 6), and (8, 7), as evidenced by several optical spectroscopy techniques.     

Grafting of an aminated poly(phenylene sulphide) derivative to functionalized single-walled carbon nanotubes

An aminated poly(phenylene sulphide) derivative (PPS-NH₂) has been covalently anchored to the surface of epoxy and acid-functionalized single-walled carbon nanotubes (SWCNTs). The characterisation through Fourier transform infrared spectroscopy, nuclear magnetic resonance, thermogravimetric analysis and Kaiser test corroborated the success of the grafting reactions, and allowed the identification and quantification of the covalent moieties. Scanning and transmission electron microscopy indicated an increase in the bundle diameter of the SWCNTs upon anchoring of the polymer chains. The results showed that the storage modulus, glass transition temperature and electrical conductivity of the polymer were exceptionally enhanced by the attachment to the SWCNTs. In contrast, the crystallization and melting temperature, degree of crystallinity and crystal size considerably decreased, as revealed by differential scanning calorimetry and X-ray diffraction experiments, due to the inactive nucleating role of these SWCNTs and the intense restrictions on chain mobility imposed by the SWCNT–polymer interactions. Acid-functionalized SWCNTs were more effective for reinforcing PPS-NH₂ than epoxy-functionalized SWCNTs, attributed to the formation of a larger number of covalent bonds, albeit led to a smaller increase in the electrical conductivity of the polymer. The results herein offer useful insights into the development of multifunctional CNT-reinforced thermoplastic composites for a wide variety of applications.