Fabrication of high strength PVA/SWCNT composite fibers by gel spinning

High-strength composite fibers were prepared from polyvinyl alcohol (PVA) (Degree of polymerization: 1500) reinforced by single-walled carbon nanotubes (SWCNTs) containing few defects. The SWCNTs were dispersed in a 10 wt.% PVA/dimethylsulfoxide solution using a mechanical homogenizer that reduced the size of SWCNT aggregations to smaller bundles. The macroscopically homogeneous dispersion was extruded into cold methanol to form fibers by gel spinning followed by a hot-drawing. The tensile strength of the well-oriented composite fibers with 0.3 wt.% SWCNTs was 2.2 GPa which is extremely high value among PVA composite fibers ever reported using a commercial grade PVA. The strength of neat PVA fibers prepared by the same procedure was 1.7 GPa. Structural analysis showed that the PVA component in the composite fibers possessed almost the same structure as that of neat PVA fibers. Hence a small amount of SWCNTs straightforward enhanced by 0.5 GPa the tensile strength of PVA fibers. The results of mechanical properties and Raman spectra for the SWCNT composites suggest the relatively good interfacial adhesion of the nanotubes and PVA that improves the load transfer from the polymer matrix to the reinforcing phase.     

Temperature and time dependence study of single-walled carbon nanotube growth by catalytic chemical vapor deposition

The temperature and time dependence of single-walled carbon nanotube (SWCNT) growth by chemical vapor deposition of ethanol on Fe₂O₃/MgO catalyst are compared at both low (∼27 Pa) and atmospheric pressure limits. SWCNTs are synthesized in two reactors with different geometries and operating pressures and are characterized by Raman spectroscopy. Both reactors show SWCNT growth within a relatively narrow temperature window of 700-850 °C, with an optimum growth time of 35 min for the cold wall reactor and 75 min for the quartz tube reactor. A kinetic model comprising of ethanol decomposition, SWCNT formation, and water etching is developed to better understand the growth mechanism. The existence of a temperature window and an optimum growth time in both reactors can be well described by the kinetic model. Simulation results suggest that the temperature and time dependence can be explained by the competition between the growth of SWCNTs and that of amorphous carbon.     

Surfactant-free assembling of functionalized single-walled carbon nanotube buckypapers

The electrical and textural properties of single-walled carbon nanotube buckypapers were tunned through chemical functionalization processes. Single-walled carbon nanotubes (SWCNTs) were covalently functionalized with three different chemical groups: Carboxylic acids (-COOH), benzylamine (-Ph-CH₂-NH₂), and perfluorooctylaniline (-Ph-(CF₂)₇-CF₃). Functionalized SWCNTs were dispersed in water or dimethylformamide (DMF) by sonication treatments without the addition of surfactants or polymers. Carbon nanotube sheets (buckypapers) were prepared by vacuum filtration of the functionalized SWCNT dispersions. The electrical conductivity, textural properties, and processability of the functionalized buckypapers were studied in terms of SWCNT purity, functionalization, and assembling conditions. Carboxylated buckypapers demonstrated very low specific surface areas (< 1 m²/g) and roughness factor (R_a = 14 nm), while aminated and fluorinated buckypapers exhibited roughness factors of around 70 nm and specific surface areas of 160-180 m²/g. Electrical conductivity for carboxylated buckypapers was higher than for as-grown SWCNTs, but for aminated and fluorinated SWCNTs it was lower than for as-grown SWCNTs. This could be interpreted as a chemical inhibition of metallic SWCNTs due to the specificity of the diazonium salts reaction used to prepare the aminated and fluorinated SWCNTs. The utilization of high purity as-grown SWCNTs positively influenced the mechanical characteristics and the electrical conductivity of functionalized buckypapers.     

Influence of shear in the crystallization of polyethylene in the presence of SWCNTs

It is well accepted that due to epitaxy matching, carbon nanotubes are good nucleating agent for linear polyethylene. We demonstrate that not only in the quiescent conditions but also at the relatively low shear rates the presence of single-walled carbon nanotube (SWCNT) accelerates the crystallization kinetics of polyethylene (PE). The influence of SWCNTs on the crystallization kinetics in the quiescent condition is followed with the help of rheological and differential scanning calorimetry studies. The influence of flow on the stretch of the polymer chain is probed using time-resolved small-angle X-ray scattering (SAXS) and is verified with the Deborah number. SAXS data indicates that the strong shearing conditions (shear rate > 50/s for 1 s) are requisite to form shish-kebab structure in the neat polymer. However, for the low shear (shear rate < 50/s for 1 s), the shish-kebab structure that arises due to chain orientation is enhanced in the presence of SWCNTs. The development of oriented structures in SWCNT/PE composites and their absence in the neat polymer under low shear rate indicates that the presence of SWCNTs plays a significant role in the chain orientation. Overall, the results manifest the influence of SWCNTs on chain relaxation of the polymer.     

Assembly of untreated single-walled carbon nanotubes at a liquid-liquid interface

Untreated single-walled carbon nanotubes (SWCNTs) were assembled at a liquid-liquid interface to form an ultrathin film. The SWCNTs were dispersed into water using sodium dodecyl sulfate (SDS) as a solubilizing agent. Then, hexane was added to the dispersion to create a liquid-liquid interface. The SWCNTs were assembled at the interface to form a smooth ultrathin film when ethanol was added to the SWCNT water dispersion/hexane solution. The assembly mechanism was considered to be caused by the decreased wettability of SDS-coated SWCNT during the addition of ethanol because of desorption of SDS from the SWCNT surface. The assembly was remarkably robust and easily transferable to substrates. An AFM image of the film transferred onto a silicon substrate shows a closely packed uniform film of 3-8 nm thickness. The SWCNT ultrathin film showed high transparency of ca. 97% with an electrical conductivity of 71.4 S/cm. Fabrication processing was carried out in ambient conditions, thereby making it an attractive application for use in flexible electric devices.     

Production of single-walled carbon nanotubes with narrow diameter distribution using iron nanoparticles derived from DNA-binding proteins from starved cells

Iron nanoparticles derived from DNA-binding proteins from starved cells (Dps) were used to grow single-walled carbon nanotubes (SWCNTs) with narrow diameter distribution. An atomic force microscopy, Raman spectroscopy, and photoluminescence were used for evaluation of diameter or chirality distribution of the SWCNTs. We found that thin SWCNTs (1.1 nm diameter) were grown from the large Dps-derived nanoparticles (2.4 nm diameter) on and above the substrates. From the size comparison with ferritins and Co-filled apoferritins, we also found that SWCNTs become thinner as the catalyst becomes smaller. The synthesis of smaller catalysts (ca. 1 nm diameter) and their use for growth becomes crucial for the control of SWCNT diameter.     

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.     

Synthesis of radially aligned single-walled carbon nanotubes on a SiO2/Si substrate by introducing sodium chloride

Radially aligned single-walled carbon nanotubes (SWCNTs) were synthesized on a SiO₂/Si substrate with thermal chemical vapor deposition by introducing sodium chloride (NaCl) onto the substrate surface. The growth of such SWCNTs was sensitive to the thickness of the SiO₂ layer on the Si substrate and the speed of the reactive gas flow. Cristobalite crystals were found to be formed on the substrate after the SWCNT growth process and were significant for the growth of radially aligned SWCNTs. The SWCNTs were assumed to be directed by the cristobalite crystals along a certain crystal direction on the (1 0 1) crystal face.     

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.     

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.     

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.     

The growth of single-walled carbon nanotubes on a silica substrate without using a metal catalyst

Single-walled carbon nanotubes (SWCNTs) have been directly grown on a SiO₂ substrate using the chemical vapor deposition (CVD) of ethanol without a catalyst. Care was taken to exclude the possibility that the SWCNT growth was induced by conventional metal catalysts such as Fe, Co and Ni resulting from the contamination. Pretreatment of the SiO₂ at 950 °C or a higher temperature in H₂ before CVD was critical for the synthesis of SWCNTs. After CVD process, nano-scale carbon particles were produced besides SWCNTs. Based on these results, we propose that the annealing of SiO₂ substrates in H₂ at high temperature generates defects on their surfaces, and these defects provide nucleation sites for the formation of carbon nanoparticles and assist the formation of carbon nanocaps, thus leading to the SWCNT growth.     

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.     

Dispersion of pristine single-walled carbon nanotubes using pyrene-capped polystyrene and its application for preparation of polystyrene matrix composites

A pyrene-capped polystyrene (PyPS) is synthesized by an anionic polymerization method and acts as dispersant for dispersion of pristine single-walled carbon nanotubes (SWCNTs). Through a well-known π-stacking interaction confirmed qualitatively by proton nuclear magnetic resonance and fluoroscopic analyses, PyPS is strongly but noncovalently adsorbed onto the nanotube surface, affording highly uniform and stable SWCNT dispersion in chloroform with the nanotube content as high as 250 ± 30 mg L⁻¹. Since no direct chemical reaction takes place on the nanotubes, their intrinsic electronic structure is maintained, thus ensuring them as functional fillers for application in conductive polymer composites. The so-obtained dispersion is subsequently used to prepare polystyrene matrix composites. A solution-based process adopted here preserves the good nanotube dispersing state in dispersion into the composites. Hence, the resultant composites show good optical transmittance and a low electrical percolation threshold of 0.095 wt.% SWCNTs. In comparison, the composites with absence of PyPS prepared by the same process have a relatively high percolation threshold of 0.28 wt.% SWCNTs.     

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.     

Mechanistic investigations of single-walled carbon nanotube synthesis by ferrocene vapor decomposition in carbon monoxide

The single-walled carbon nanotubes (SWCNTs) were synthesized by the carbon monoxide disproportionation reaction on Fe catalyst particles formed by ferrocene vapor decomposition in a laminar flow aerosol (floating catalyst) reactor. On the basis of in situ sampling of the product collected at different locations in the reactor, kinetics of the SWCNT growth and catalyst particle crystallinity were studied. Catalyst particles captured before SWCNT nucleation as well as inactive particles were determined to have cementite (Fe₃C) phase, while particles with γ- and α-Fe phases were found to be embedded in the SCWNTs. The growth rate in the temperature range from 804 to 915 °C was respectively varied from 0.67 to 2.7 μm/s. The growth rate constant can be described by an Arrhenius dependence with an activation energy of E_a = 1.39 eV, which was attributed to the carbon diffusion in solid iron particles. CNT growth termination was explained by solid-liquid phase transition in the catalyst particles. A high temperature gradient in the reactor was found to not have any effect on the diameter during the SWCNT growth and as a result on the chirality of the growing SWCNTs.     

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.     

Comparison of single-walled carbon nanotube growth from Fe and Ni nanoparticles using quantum chemical molecular dynamics methods

Metal-catalyzed SWCNT growth has been modeled using quantum chemical molecular dynamics (QM/MD) in conjunction with feeding of carbon atoms to C₄₀-Fe₅₅ and C₄₀-Ni₅₅ model complexes at 1500 K. The rate of Fe₅₅-catalyzed SWCNT growth determined in this work was 19% slower than the Fe₃₈-catalyzed growth rate. Conversely, Ni₅₅-catalyzed SWCNT growth exhibited a growth rate 69% larger than of Fe₅₅-catalyzed SWCNT growth, a fact consistent with excellent performance of Ni in laser evaporation and carbon-arc experiments. Ni₅₅-catalyzed growth was preceded by the formation of extended polyyne chains at the base of the SWCNT, and so differed fundamentally from Fe₅₅-catalyzed growth. These polyyne chains usually persisted for 10-30 ps. Subsequent polyyne ring condensation resulted in carbon polygon addition at the SWCNT base. The relative stabilities of the C_n carbon cluster moieties on the Fe₅₅ and Ni₅₅ surfaces were consistent with the relative strengths of the Fe-C, Ni-C and C-C interactions. The presence of smaller carbon moieties on the Fe₅₅ surface led to the dissemination of surface iron atoms, and subsequent diffusion of short C_n units through the subsurface region of the catalyst particle. Conversely, the Ni₅₅ catalyst particle was observed to be more stable, remaining intact to a greater extent.     

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.     

Near-infrared photoresponse in single-walled carbon nanotube/polymer composite films

We present a near-infrared photoresponse study of single-walled carbon nanotube/poly(3-hexylthiophene)-block-polystyrene polymer (SWCNT/P3HT-b-PS) composite films for different loading ratios of SWCNT in the polymer matrix. Compared to the pure SWCNT film, the photoresponse [(light current − dark current)/dark current] is much larger in the SWCNT/polymer composite films. The photoresponse is up to 157% when SWCNTs are embedded in P3HT-b-PS while for a pure SWCNT film it is only 40%. We also show that the photocurrent strongly depends on the position of the laser spot with maximum photocurrent occurring at the metal-film interface. We explain the photoresponse due to exciton dissociations and charge carrier separation caused by a Schottky barrier at the metallic electrode-SWCNT interface.