The bulk polymerisation of N-vinylcarbazole in the presence of both multi- and single-walled carbon nanotubes: A comparative study

The bulk polymerisation of N-vinylcarbazole (NVC) at an elevated temperature in the presence of both multi- and single-walled carbon nanotubes (CNTs) leads to the formation of two different types of composite materials, the morphology and properties of which were characterised by a field emission scanning electron microscopy (FE-SEM), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis, and electrical property measurements. The efficiency of CNTs to initiate the NVC polymerisation was investigated using both multi-walled CNTs (MWCNTs) and single-walled CNTs (SWCNTs). The focus was on three major aspects: the degree of polymerisation, the morphology and the properties of the resulting nanocomposite materials. Results showed that SWCNTs were more efficient in initiating NVC polymerisation than MWCNTs, and the morphology of resultant nanocomposites revealed wrapping and grafting of some poly(N-vinylcarbazole) (PNVC) chains on the SWCNT surfaces. The morphology of the PNVC/MWCNT nanocomposites showed only homogeneous wrapping of the outer surfaces of MWCNTs by PNVC chains. The direct current (dc) electrical conductivity of pure PNVC improved dramatically in the presence of both MWCNTs and SWCNTs, however, the extent of improvement is higher in the case of PNVC/MWCNT nanocomposites.     

Growth of carbon hybrid materials by grafting on pre-grown carbon nanotube surfaces

Carbon hybrids consisting of carbon nanotubes (CNTs) and a different type of carbon nanomaterial grafted on them. Single-walled CNTs (SWCNTs) used as the scaffold were grown by thermal chemical vapor deposition (CVD) using Fe nanoparticles as the catalysts. Carbon nanowalls (CNWs) were grown on the pre-grown SWCNT surfaces by hot-filament CVD. When the pre-grown SWCNT surfaces were oxidized using an acid, carbon nanoballoons were grown. These results indicate that a process including different types of growth combined with surface modification of the pre-grown nanomaterials is promising for creation of new hybrids.     

Comparative study of fluorinated single- and few-wall carbon nanotubes by X-ray photoelectron and X-ray absorption spectroscopy

Pristine and ball-milled samples containing single-wall carbon nanotubes (SWCNTs) and few-wall carbon nanotubes (FWCNTs) have been fluorinated at room temperature using gaseous BrF₃ as a fluorinating agent. X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy were used to estimate the chemical composition and to probe the electronic structure of the fluorinated CNTs. Analysis of the XPS C 1s spectra revealed that fluorinated carbon atoms in SWCNTs are bounded with one CF-group at least while most of the fluorinated carbon atoms in FWCNTs are surrounded by bare carbon atoms only. The ball-milling of the samples during 1 hour has insignificant effect on CNT length and more likely produces defects in CNT surface layers. These defects increase fluorination ability of CNTs and provide access for fluorine atoms to the subsurface layers of FWCNTs. NEXAFS investigation revealed that some of CNTs, which probably constitute interior of FWCNTs or CNT ropes, are not fluorinated during the conditions used and the fluorine atoms interact more strongly with CNT surfaces having a larger curvature.     

Adsorption of single-ringed N- and S-heterocyclic aromatics on carbon nanotubes

Adsorption of single-ringed N- and S-heterocyclic aromatics on single-walled carbon nanotubes (SWCNTs) was examined to explore the potential of using carbon nanotubes (CNTs) as drug carriers and environmental adsorbents. Adsorbates included pyrimidine, 2-aminopyrimidine, 4,6-diaminopyrimidine, thiophene, benzene and aniline. Adsorbents included pristine SWCNTs, oxidized SWCNTs, and nonporous graphite. Adsorption of N- and S-heterocyclic aromatics was significantly enhanced by non-hydrophobic interactions. Particularly, the -NH₂-substituted compounds exhibited much stronger (up to 2 orders of magnitude) adsorption affinities to oxidized SWCNTs than benzene, even though they are much less hydrophobic. The π-π coupling or electron donor-acceptor (EDA) interactions are likely adsorption-enhancement mechanisms for all six compounds. The lone-pair electrons of the N heteroatoms and the -NH₂ group can enable n-π EDA interactions with SWCNT surfaces. Lewis acid-base interactions are another significant adsorption-enhancement mechanism for the -NH₂-substituted compounds (and possibly for pyrimidine) on SWCNTs. For the N-heterocyclic aromatics, adsorption affinity is highly dependent on the O-functionality of the SWCNT surface and on solution pH, due to the speciation reactions of both adsorbates and SWCNT surface O-functional groups, indicating that selective adsorption of N-heterocyclic aromatics is possible by combining the surface functionality of CNTs and solution chemistry.     

Simultaneous dispersion and alignment of carbon nanotubes in epoxy resin through chronoamperometry

A novel technique of dispersion, stretching and alignment of carbon nanotubes (CNTs) in an epoxy resin has been developed based on chronoamperometry. The procedure is to apply an electrical field in non-cured CNT/epoxy mixtures, using an electrode of large surface. CNT movements can be promoted by their intrinsic polarisation or by the introduction of electrical charges over their surfaces. The efficiency of the process can be monitored through the electrical current passing through the system as a function of time. Different experimental conditions, such as the applied voltage, have been analysed. The composites have been characterised through their thermal, mechanical and electrical behaviour. They show high dispersion degree of nanotubes, which are found to be stretched and untangled. CNTs show alignment along a preferred direction in the epoxy matrix, which affects to their electrical conductivity.     

High-crystallization polyoxymethylene modification on carbon nanotubes with assistance of supercritical carbon dioxide: Molecular interactions and their thermal stability

As a typical engineering plastic and high-crystallization polymer, polyoxymethylene (POM) has been successfully wrapped on single-walled carbon nanotubes (SWCNTs) using a simple supercritical carbon dioxide (SC CO₂) antisolvent-induced polymer epitaxy method. The characterization results of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) reveal that the SWCNTs are coated by laminar POM with the thicknesses of a few nanometers. The polymer adsorption on CNTs via multiple weak molecular interactions of CH groups with CNTs has been identified with FTIR and Raman spectroscopy. The experimental results indicate that the decorating degree of POM on the surface of CNTs increases significantly with the increase of SC CO₂ pressure, and accordingly the dispersion of SWCNT modified by POM at higher pressure are more excellent than that of obtained at lower pressure. Further the processing stability of POM/CNTs composites are investigated by differential scanning calorimetry and thermogravimetric analysis. The experimental results obtained show that their thermal stability behavior is closely related to surface properties of CNTs. Apparently, the composites with POM-decorating SWCNTs as the filler shows higher melting points compared to the POM composites with pristine SWCNTs as the filler. Therefore, we anticipate this work may lead to a controllable method making use of peculiar properties of SC CO₂ to help to fabricate the functional CNTs-based nanocomposites containing highly crystalline thermoplastic materials such as POM.     

Catalytic activity of cobalt and iron phthalocyanines or porphyrins supported on different carbon nanotubes towards oxygen reduction reaction

Non-noble metal catalysts for O₂ reduction were prepared by dispersing iron(II) phthalocyanine, cobalt(II) tetra-tert-butylphthalocyanine, cobalt(II) 2,3,7,8,12,13,17,18-octaethyl-porphine, and cobalt(II) 5,10,15,20-tetrakis(4-tert-butylphenyl)-porphyrine on carbon nanotubes (CNTs) used as high surface area support. Different types of CNTs (SWCNTs, DWCNTs and MWCNTs) were investigated as an effective substitute for commonly used carbon black in carbon-supported phthalocyanines and porphyrins. The oxygen reduction reaction (ORR) activity of those CNT-supported catalysts in alkaline and acidic solutions was studied. The results show that: (i) all catalytic systems including MWCNTs are more efficient for O₂ reduction than those with SWCNTs and DWCNTs, (ii) the oxidative chemical treatment of the CNTs increases the electrocatalytic performance of the corresponding CNT-supported catalysts, (iii) similarly to Vulcan-supported catalysts, iron(II) phthalocyanine gives the best electroactivity among the investigated CNT-supported materials and (iv) finally, the MWCNT-supported iron(II) phthalocyanine catalyst chemically treated in oxidative conditions shows an ORR catalytic activity comparable to a commonly used Pt/C catalyst with similar current densities and a very low overpotential (60 mV).     

Influence of wall number and surface functionalization of carbon nanotubes on their antioxidant behavior in high density polyethylene

Carbon nanotubes (CNTs) are extensively incorporated as reinforcement into polymeric materials due to their extraordinary properties. The antioxidant ability of CNTs in high density polyethylene (HDPE) was studied. Single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), and hydroxylated multi-walled carbon nanotubes (MWCNTs-OH) were involved to investigate the influence of wall number and surface functionalization of CNTs on their antioxidant behavior in HDPE. Based on measurements of the oxidation induction temperature and oxidation induction time of CNT/HDPE composites, it is found that the antioxidant ability of the three kinds of CNTs is in the following order: MWCNTs-OH > MWCNTs > SWCNTs. The antioxidant ability and mechanism of CNTs are further examined by electron spin resonance spectra and Raman spectra. It is observed that the antioxidant behavior of CNTs obeys a free radical scavenging mechanism. The order of the radical scavenging efficiency and the defect concentration for CNTs are in good agreement with that of their antioxidant ability in HDPE. With more walls and surface hydroxyl groups, the CNTs have more structural defects and exhibit higher antioxidant ability. The study raises the possibility that CNTs can improve antioxidant properties as well as mechanical properties of polymer matrix.     

Structural changes of carbon nanotubes in their macroscopic films and fibers by electric sparking processing

Macroscopic films and fibers of chemical vapor deposition (CVD) grown carbon nanotubes (CNTs) were electrically sparked between the nanotube assemblies and the electrode of a direct current power source. Microstructural analyses showed that the electric sparking ablated the ‘hard’ amorphous shells from the CNTs in the film and reshaped them into nanosize particles. This led to the formation of the unique heterostructure of nanoparticle-bonded single-walled carbon nanotubes (SWCNTs) in the sparked film. The ablation of the amorphous shell revealed the inner core of the CVD as-grown nanofiber, which was SWCNT bundles. In case of treating a fiber assembly of purer SWCNT bundles, the electric discharge densified the SWCNTs in the fibers and enhanced their alignment. The selective removal of the amorphous phase from the CNTs in the film is believed to be due to the localized heating generated by sparking. The electric sparking processing could be useful to engineer the structures of CNTs in their macroscopic assemblies, and to reveal the internal structures of the CNTs.     

New carbon nanotube–conducting polymer composite electrodes for drug delivery applications

A novel drug delivery system (DDS) based on a carbon nanotube (CNT)–poly(3,4‐ethylenedioxythiophene) (PEDOT) composite was constructed via a layering method. Single‐walled CNTs (SWNTs) were immobilized on a gold electrode using a layer‐by‐layer technique. In particular, cysteamine (Cys) was firstly bonded to the gold surface through the strong S? Au association and SWNTs were subsequently linked onto the Cys layer through condensation reaction of ? NH₂ and carboxyl groups by 1‐ethyl‐3‐(3‐dimethylaminopropyl)carbodiimide/N‐hydroxysuccinimide coupling. X‐ray photoelectron spectroscopy and Raman spectroscopy demonstrate that this is a facile route for immobilizing CNTs on gold electrodes. Finally PEDOT was electropolymerized on the SWNT‐functionalized electrode to make a SWNT–PEDOT composite, and the modified electrode was applied as a DDS. Dexamethasone, as a model drug, was incorporated into PEDOT in the electropolymerization. Investigations of the electrochemical properties of SWNT–PEDOT demonstrate that SWNTs greatly improve the conductivity and increase the charge capacity of PEDOT. The composite exhibits a petal‐like surface structure, 20–30 nm thick and 100–200 nm wide. Compared to a DDS based on pure PEDOT synthesized under the same conditions, SWNT–PEDOT has the merits of higher drug release rate and larger release amount. The average mass release for every five voltammetry cycles increases from 1.4126 to 1.8864 mg cm^?2. Copyright © 2011 Society of Chemical Industry     

Elucidating the effect of heating induced structural change on electrical and thermal property improvement of single wall carbon nanotube

We have clarified that the electrical and thermal properties of single-walled carbon nanotubes (SWCNTs) are improved by multiple structural changes (wall number, diameter, and crystallinity) induced by high temperature treatment. Focusing on the relationship between structural change and electrical and thermal properties, high-purity SWCNTs were fabricated using the water-assisted CVD method and treated at high temperatures (1500–2000 °C) in an argon atmosphere. We showed that the electrical and thermal properties of the SWCNTs were improved by ∼2.9 and 3.0, respectively, which required lower treatment temperatures than for multi-walled CNTs (MWCNTs). In addition to the crystallinity improvement, the wall number and diameter increased with treatment temperature. When compared to as-grown SWCNTs of similar wall number and diameter, the heat treated SWCNTs exhibited higher electrical and thermal properties, which suggested that the property improvements could be attributed to not only to the wall number and diameter but also to the improvement in crystallinity.     

Dispersion and thermal conductivity of carbon nanotube composites

A mechanical method was used to shorten carbon nanotubes (CNTs) for improving dispersion without reducing their thermal conductivity. Single walled carbon nanotubes (SWCNTs) were mechanically cut to produce short and open-ended fullerene pipes. These shortened SWCNTs were then used in polymer composites. Both atomic force microscopy and scanning electron microscopy characterizations suggested that nanotube shortening significantly improved CNT dispersion. Thermal conductivity of composites containing short CNTs were found to be much better than those containing pristine CNTs.     

Ion-exchange adsorption of copper(II) ions on functionalized single-wall carbon nanotubes immobilized on a glassy carbon electrode

The cation-exchange property of oxidatively treated carbon nanotubes (CNTs) is newly reported. Single-wall carbon nanotubes (SWNTs), which were oxidatively treated, were immobilized on a glassy carbon surface and, on this CNT-modified electrode, Cu(II) ions were spontaneously adsorbed and their redox waves electrochemically measured. It is suggested that the adsorption of the cationic Cu(II) ions occurs by their electrostatic interaction with the negatively charged carboxylic anions on the CNTs after the ion-exchange with protons. The surface coverage of the adsorbed Cu(II) ions depending on the dipping time, the amount of immobilized CNTs, and the Cu(II) concentration was estimated from the electrochemical chronocoulometric measurements. The effect of the ionic strength on the adsorption of the Cu(II) ions was investigated and the adsorption strengths of various alkali metal cations and protons were compared. It is hoped that this new cation-exchange property of CNT-modified electrodes may extend their range of electrochemical applications.     

Growth of dense single-walled carbon nanotubes in nano-sized silicon dioxide holes for future microelectronics

Dense and aligned single-walled carbon nanotubes (SWCNTs) were synthesised in nano-sized silicon dioxide holes patterned using electron beam lithography for microelectronics applications. Carbon nanotubes are new materials with potential uses for interconnects and field effect transistors (FETs) of LSI. As single-walled carbon nanotubes have lower resistance than multi-walled carbon nanotubes in close-packed arrangements and show both metallic and semiconducting behaviour, there is a great deal of interest in using dense SWCNTs for low resistive interconnects and high current transistors. Here, we report not only a method for fabrication of SWCNTs in nano-sized holes, but also differences in growth rate and Raman spectroscopy of CNTs in holes of various sizes. The growth rate of CNTs in the holes decreased as the hole size was reduced, due to the amount of carbon radicals diffusing to the catalyst particles at the bottom of the holes.     

Effect of carbon nanotube purification on the electrical and mechanical properties of poly(ethylene terephthalate) composites with carbon nanotubes in low concentration

Surface properties of carbon nanotubes (CNTs) were altered by purification with nitric acid, sulfuric acid, ammonium hydroxide, and hydrogen peroxide. As‐received and purified CNT‐based conductive poly (ethylene terephthalate) composites were prepared with a twin‐screw extruder. The effects of CNT purification on the surface properties of the CNTs and on the morphology and electrical and mechanical properties of CNT‐based composites were investigated. Surface energy measurements showed that the acidic component of the surface energies of the CNTs increased after purification. According to Fourier transform infrared (FTIR) spectroscopy, the purification resulted in the formation of oxygen‐containing functional groups on the surfaces of the CNTs. Electron spectroscopy for chemical analysis results indicate the removal of the metallic catalyst residues and an increase in the oxygen content of the CNT surfaces as a result of the purification procedure. X‐ray diffraction analyses revealed a change in the crystalline structure of the CNTs after purification. All of the composites prepared with the purified CNTs had higher electrical resistivities and tensile and impact strength values than the composites based on the as‐received CNTs because of the functional groups and defect sites formed on the surfaces of the CNTs during purification. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Effects of the composition and molecular weight of maleimide polymers on the dispersibility of carbon nanotubes in chloroform

We synthesized maleimide polymers (MIPs) as effective carbon nanotube (CNT) dispersants for stable CNT-dispersed solutions. The MIPs were random copolymers which consist of N-phenyl maleimide (PhMI) units that imparted strong physical adsorption to the CNT surfaces and methyl methacrylate units; we evaluated the effects of the PhMI content and molecular weight in the MIPs on the dispersibility of the CNTs in chloroform. Increasing the PhMI content in the MIPs increased the grafting amount (GA) (g m⁻²) of MIPs on the CNT surfaces and led to a large increase of CNT dispersion. In particular, when single-walled carbon nanotubes (SWCNTs) were used, MIPs having 100 mol% PhMI units (MIP100s) were imperative for a drastic increase in the GA value. Increasing the weight average molecular weight (M_w) of the MIPs resulted in no significant effect on CNT dispersibility; however, the particularly low M_w (?10,000) of the MIPs dramatically improved CNT dispersion. Therefore, MIP100s with particularly low M_w’s were extremely effective for dispersing various CNTs with different diameters. In the case of SWCNTs, much more effective exfoliation of SWCNT bundles was achieved by MIP100s with particularly low M_w’s than MIP100s with higher M_w’s.     

Improvement of carbon nanotube dispersion in electrospun polyacrylonitrile fiber through plasma surface modification

In this study, surfaces of multiwalled carbon nanotubes (CNTs) were functionalized with poly(hexafluorobutyl acrylate) (PHFBA) thin film using a rotating-bed plasma-enhanced chemical vapor deposition (PECVD) method without imparting any defects on their surfaces. Polyacrylonitrile (PAN) electrospun polymer fiber mats and composite fiber mats with CNTs and functionalized CNTs (f-CNTs) were prepared. The wettability and chemical and morphological properties of the synthesized fiber mats were investigated, and the dispersion of CNTs and f-CNTs in the polymer matrix was compared according to the contact angle results of electrospun polymer mats. According to the chemical and morphological characterization results, PHFBA-coated CNTs were dispersed more uniformly in the polymer matrix than the uncoated CNTs. The f-CNTs/PAN composite fiber mat exhibits a lower surface energy than the pristine CNTs/PAN fiber mat. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47768.     

Electrocatalytic oxygen reduction on single-walled carbon nanotubes supported Pt alloys nanoparticles in acidic and alkaline conditions

The objective of this study is to improve the catalytic activity of platinum by alloying with transition metal (Pd) in gas diffusion electrodes (GDEs) by oxygen reduction reaction (ORR) at cathode site and comparison of the acidic and alkaline electrolytes. The high porosity of single-walled carbon nanotubes (SWCNTs) facilitates diffusion of the reactant and facilitates interaction with the Pt surface. It is also evident that SWCNTs enhance the stability of the electrocatalyst. Functionalized SWCNTs are used as a means to facilitate the uniform deposition of Pt on the SWCNT surface. The structure of SWCNTs is nearly perfect, even after functionalization, while other types of CNTs contain a significant concentration of structural defects in their walls. So catalysts supported on SWCNTs are studied in this research. The electrocatalytic properties of ORR were evaluated by cyclic voltammetry, polarization experiments, and chronoamperometry. The morphology and elemental composition of Pt alloys were characterized by X-ray diffraction (XRD) analysis and inductively coupled plasma atomic emission spectroscopy (ICP-AES) system. The catalytic activities of the bimetallic catalysts in GDEs have been shown to be not only dependent on the composition, but also on the nature of the electrolytes. The GDEs have shown a transition from the slow ORR kinetics in alkaline electrolyte to the fast ORR kinetics in the acidic electrolyte. The results also show that introduction of Pd as transition metal in the Pt alloys provides fast ORR kinetics in both acidic and alkaline electrolytes. The performance of GDEs with Pt–Pd alloy surfaces towards the ORR as a function of the alloy’s overall composition and their behavior in acidic electrolyte was also studied. These results show that the alloy’s overall composition and also the nature of the electrolytes have a large effect on the performance of GDEs for ORR.     

A study of the electrical properties of carbon nanotube-NiFe2O4 composites: Effect of the surface treatment of the carbon nanotubes

Novel carbon nanotube-NiFe₂O₄ composite materials have been prepared for the first time by in situ chemical precipitation of metal hydroxides in ethanol in the presence of carbon nanotubes (CNTs) and followed by hydrothermal processing. The obtained composite powders were characterized using XRD, TEM and EDS. The effect of surface oxidation treatment of CNTs on their properties was investigated by FTIR, zeta potential and hydrodynamic radius distribution characterization. Electrical conductivity measurements show that surface oxidation treatment of CNTs can improve the electrical conductivity of the composites more pronouncedly than pristine CNTs do. With 10 wt.% addition of surface treated CNTs, the electrical conductivity is increased by 5 orders of magnitude. The surface oxidized CNTs are crucial for this significant increase in electrical conductivity, which provides strong adhesion between the nanotubes and the matrix to give a homogeneous carbon nanotube-NiFe₂O₄ composite.     

Removal of nickel(II) from aqueous solution by carbon nanotubes

Single‐walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs) were oxidized by NaClO solutions and were employed as sorbents to study sorption characteristics of nickel(II) from aqueous solution. The surface properties of CNTs such as functional groups, total acidic sites and negatively charged carbons were greatly improved after oxidation, which made CNTs become more hydrophilic and resulted in sorption of more Ni²⁺. The amount of Ni²⁺ sorbed onto oxidized CNTs increased with a rise in agitation speed, initial Ni²⁺ concentration and solution pH in the range 1–8, but decreased with a rise in CNT mass and solution ionic strength. The sorption mechanisms are complicated and appear attributable to electrostatic forces and chemical interactions between the Ni²⁺ and the surface functional groups of the CNTs. The oxidized SWCNTs and MWCNTs have shorter equilibrium time and better Ni²⁺ sorption performance than the oxidized granular activated carbon, suggesting that both NaClO oxidized CNTs are efficient Ni²⁺ sorbents and that they possess good potential applications in water treatment. Copyright © 2006 Society of Chemical Industry