Effect of purification of carbon nanotubes on their electrocatalytic properties for oxygen reduction in acid solution

The oxygen reduction reaction has been investigated on acid-treated single-walled (SWCNT) and multi-walled carbon nanotubes (MWCNT) modified glassy carbon (GC) electrodes in acid media using the rotating disk electrode (RDE) method. Different acids were used for the carbon nanotube (CNT) purification. A systematic study was carried out to elucidate whether the metal catalyst impurities of CNTs play a role in the electroreduction of oxygen on the CNT modified GC electrodes. The surface morphology of the carbon nanotube samples was examined by transmission electron microscopy and the concentration of metal catalysts in the CNT materials was determined by energy dispersive X-ray spectroscopy. The acid-treated MWCNTs were also characterised by Raman and X-ray photoelectron spectroscopies. Aqueous suspensions of SWCNTs and MWCNTs used for GC surface modification were prepared in the presence of Nafion. The RDE results indicated that the acid-treated CNT modified GC electrodes are less active catalysts for oxygen reduction than as-received CNTs which could be explained by the absence of metal catalysts on the surface of purified CNTs.     

Improved mechanical properties of carbon nanotube/polymer composites through the use of carboxyl-epoxide functional group linkages

Single-walled and multi-walled carbon nanotubes (CNTs) were functionalized with carboxyl groups and dispersed in a polymer containing an epoxide group. We have then observed experimentally that mutual chemical reaction between the functional groups on the CNTs with the polymer epoxide group can enhance, two-fold, both the tensile strength and elastic modulus, E, of single walled CNT/polymer composites. A simple model was formulated to understand the variation of E with CNT volume fraction, considering agglomeration effects as well. An increase in the work of fracture, obtained from the experimental stress-strain curves, was seen at low nanotube filling fractions and is presumably due to crack bridging of the polymer matrix by CNTs. The influence of CNT length and geometry on mechanical properties, along with the influences of electrical and mechanical percolation thresholds was considered.     

Carbon nanotube-based nanocomposites and their applications

The purpose of the current review article is to present a compherensive understanding regarding pros and cons of carbon nanotube–related nanocomposites and to find ways in order to improve the performance of nanocomposites with new designs. Nanomaterials including carbon nanotubes (CNTs) are employed in industrial applications such as supercapacitors, and biosensors, and etc. The present article has been prepared in three main categories. In the first part, carbon nanotube types have been presented, as single-walled carbon nanotubes, multi-walled carbon nanotubes, and also equivalent circuit models, which have been used to more clarify the experimental measurements of impedance. In the second part, nanocomposites with many carbon, inorganic and polymeric materials such as polymer/CNT, activated carbon/CNT, metal oxide/CNT, and carbon fiber/CNT have been investigated in more detail. In the third part, the focus in on the industrial applications of CNTs. including supercapacitors, biosensors, radar absorbing materials, solar cells, and corrosion protection studies. This review article explains the latest advances in carbon nanotubes and their applications in electrochemical, electrical and optical properties of nanocomposites.     

Carbon nanotube/graphene composite for enhanced capacitive deionization performance

In this study, single-walled carbon nanotubes were combined with graphene oxide nanosheets in aqueous dispersion and then chemically reduced to form the carbon nanotube/graphene (CNT/G) composite as electrodes for capacitive deionization (CDI). The structure of the CNT/G composite was highly porous, with single-walled carbon nanotubes (SWCNTs) sandwiched between graphene sheets that functioned as spacers and provided diffusion paths for smooth and rapid ion conduction. The associated increase in the electrical double-layer capacitance enhanced capacitive deionization performance. The CNT/G composite achieved a specific capacitance of 220 F/g and an electrosorption capacity of 26.42 mg/g with 100% regeneration, showing great potential as a high performance electrode material in CDI applications.     

A pressure-dependent selective growth of single-walled and multi-walled carbon nanotubes using plasma enhanced chemical vapor deposition

Pressure-induced transition (5−100 kPa) of carbon nanotube (CNT) morphology in plasma enhanced chemical vapor deposition (PECVD) is presented. High-purity, vertically-aligned single-walled CNTs (SWCNTs) were synthesized only when PECVD was used at atmospheric pressure, while multi-walled CNTs were preferentially synthesized when the total pressure was lower than 20 kPa. In the reduced pressure range, nanostructured catalysts were easily coagulated at the initial stage of CNT nucleation even if an excess supply of reactive species and high-energy ion bombardment were absent. If catalyst coagulation was avoided at the moment of CNT nucleation, SWCNTs were grown in the root growth regime even at 5 kPa; however, the top CNT layer was severely contaminated by amorphous carbon, produced as a result of excess supply of reactive species.     

Electrical and mechanical characteristics of buckypapers and evaporative cast films prepared using single and multi-walled carbon nanotubes and the biopolymer carrageenan

The electrical and mechanical characteristics of composite materials prepared using evaporative casting and vacuum filtration of carbon nanotubes (CNTs) dispersed in the biopolymer τ-carrageenan (IC) are reported. It is demonstrated that the contact angle of water with films is proportional to the CNT mass and volume fraction, which is used to compare the properties of buckypapers with those of evaporative cast films. Multi-walled carbon nanotube films were found to exhibit higher conductivity values compared to those observed for single-walled carbon nanotubes composites at comparable contact angle values up to true nanotube volume fraction of 0.12. Buckypapers prepared by varying the absolute amount of CNT mass while keeping the IC amount of mass constant, were found to be more robust and conducting compared to evaporative cast films. In contrast, buckypapers prepared by changing the amount of IC mass while keeping the CNT amount of mass constant were found to be more conducting, but less robust compared to evaporative cast films. It is suggested that the electrical characteristics of these gel-carbon nanotube materials are determined by the relative amounts of mass (or volume) of CNTs and polymer, while the mechanical characteristics are governed by the absolute amounts of mass (or volume).     

Enhanced device performance using different carbon nanotube types in polymer photovoltaic devices

The effects of incorporating different types of carbon nanotubes (single-walled, SWCNT; double-walled, DWCNT; and multi-walled carbon nanotubes, MWCNT) in organic solar cells were studied. Different compositions of carbon nanotubes in poly(3-hexiltiophene) (P3HT) were investigated. Absorption spectroscopy, photoluminescence and current–voltage measurements using 10 mW/cm² white light illumination in air, were carried out to determine the photovoltaic behaviour of the organic solar cells prepared. The addition of carbon nanotubes (CNTs) to the polymer increases the power conversion efficiency by three orders of magnitude compared with the device without CNTs, but there is no clear connection with the number of walls or diameter of the nanotubes. A critical concentration value of 5 wt.% of CNT is found to achieve the highest open-circuit voltage and the best performance for the majority of the CNTs studied.     

Carbon nanotube sheet electrodes for anisotropic actuation of dielectric elastomers

The performance of dielectric electroactive polymer (D-EAP) based actuators depends critically on the electrode characteristics. Among the most challenging issues in the application of D-EAPs is the device-level complexity in producing sufficient directional actuation at acceptably low electric fields. In this work, a simple carbon nanotube (CNT) based electrode for D-EAP actuators is demonstrated that vastly improves directional strain response originating from the mechanical anisotropy of the electrode material. In this novel approach, highly aligned carbon nanotube (CNT) sheet electrodes are applied on acrylate adhesive films show high directed linear actuation strain of greater than 40% at a relatively low electric field (100 V μm⁻¹). The fiber-oriented CNT sheet applied around the D-EAP film, exhibits strong interaction between CNT fibers in the electrode and the D-EAP film to produce a robust conductive-nanolayer at the interface, on actuation cycling. The design paradigm provides a great potential for the fabrication of soft linear actuators.     

Interplay of wall number and diameter on the electrical conductivity of carbon nanotube thin films

We report the interplay between the carbon nanotube (CNT) structure (wall number and diameter) and assembly structure (packing density) on the electrical conductivity of CNT thin films. By controlling the CNT average wall number from 1.0 to 5.5 (and inevitably changing of the diameter from 3.0 to 8.7 nm), the electrical conductivity of CNT films showed a unique and unexpected phenomenon, i.e. peaking for films made from an average wall number of ∼2.7 that was ∼3-times higher than that from single-walled CNTs and ∼1.6-times higher than that from 5.5-walled CNTs. By developing a first-order model, the individual contributions of individual CNT structure and assembly structure were estimated, and we found that the peak arose from offsetting factors: increase in the effective CNT electrical conductivity and decrease in the packing density with increased wall number. The synergetic effect between the CNT structure and the assembly structure would provide a scientific framework to deeply understand CNT assemblies.     

Conducting polymer/carbon nanotube composite films made by in situ electropolymerization using an ionic surfactant as the supporting electrolyte

We have demonstrated a simple and general strategy, namely in situ electropolymerization by using an ionic surfactant as the electrolyte, for alignment of disordered CNTs within conducting polymer/carbon nanotube composite films. The single- or multi-walled CNTs were first dispersed in an aqueous solution containing SDS (sodium dodecyl sulfate), then electroactive monomer pyrrole or N-methylpyrrole was added into the above mixture, finally electrochemical reaction was proceeded at the surface of the Au electrode and correspondingly a series of conducting polymer/carbon nanotube composite films with the orientation of carbon nanotubes were obtained.     

Preparation of a HDPE/Carbon Nanotube Composite

This experiment adopted mixed acid (H₂SO₄:HNO₃ = 3:1) to purify multi-walled carbon nanotubes, and used a silane coupling agent n-octyltriethoxysilane (OTES) to modify carbon nanotubes, respectively. Then we mixed OTES-modified carbon nanotubes (CNTs) with high-density polyethylene (HDPE) resin to make a composite. TGA analysis results revealed that as the CNTs'content increased, the Td₁₀ tended to rise. The amount of composite residual at 500°C also increased, as well as the composite electrical conductivity. When a concentration of 5% OTES was used to modify the CNT, the resultant composite exhibited better electrical conductivity.     

Horizontally aligned carbon nanotube field emitters having a long term stability

Horizontally aligned carbon nanotube (CNT) field emitters fabricated by electrophoresis deposition and fissure formation techniques show good field emission properties such as high current density, low turn-on voltage and long-term stability. Horizontally aligned multi-walled carbon nanotube (MWCNT) field emitters show an unusual very long-term stability, much better stability than the single-walled carbon nanotube (SWCNT) ones. The cause of the degradation is due to the heat generated by the resistance of CNTs. We were able to prevent effectively the degradation of the horizontally aligned field emitters by using MWCNTs and an additional deposition of aluminum on the CNT films, and the required time for 10% degradation is very long, 121 h.     

Multifunctional materials based on iron/iron oxide-filled carbon nanotubes/natural rubber composites

This work describes the synthesis, characterization and study of properties of novel multifunctional composite materials obtained by natural-rubber latex and a special kind of multi-walled carbon nanotube (CNT), in which the cavities are filled by magnetic species. To prepare stable aqueous dispersions of CNTs, two approaches have been employed, based on a mixture of the surfactant sodium dodecyl sulfate or a previous treatment with a mixture of acid solutions. The composites have been characterized by infrared and Raman spectroscopy, X-ray micro-tomography, transmission and scanning electron microscopy, and atomic and magnetic force microscopy. A good adhesion between the CNTs and the polymer matrices was obtained. Several samples have been prepared containing different amounts of CNTs (from 0.01 to 10 wt.%), and the effect of carbon nanotubes in the electrical, mechanical, chemical and thermal properties of the composites was evaluated by thermogravimetric analysis, resistivity measurements, stress–strain mechanical tests and solvent sorption measurements. The occurrence of networks of CNTs in the polymeric matrices provided significant increases in all of these properties. Additionally, because of the species encapsulated in CNT, the composites exhibit magnetic behavior, confirmed by magnetic force microscopy. This approach results in novel multifunctional material with great potential for further applications.     

Biofunctionalizable flexible bucky paper by combination of multi-walled carbon nanotubes and polynorbornene-pyrene – Application to the bioelectrocatalytic reduction of oxygen

Owing to their conductivity, carbon nanotubes (CNTs) coatings are widely used for modifying electrode surfaces. In particular, the formation of bucky papers (BP) based on CNT assemblies obtained by filtration, represents an attractive way of creating a new kind of electrode. However, these BP are brittle and difficult to manipulate, hence their applications remain limited due to their fragility and lack of flexibility. Notably, their use for “in vivo” experiments is markedly hampered by the potential release of CNTs. The strengthening of the mechanical stability of BP has hereby been explored by the combination of BP and linear polymeric chains displaying non-covalent π-interactions with CNTs. These organic polymers act as a cross-linking agent throughout the BP assembly thus conferring stability and flexibility. Flexible BP electrodes were produced by simply mixing poly(norbornene) bearing pyrene groups and multi-walled CNTs, both dispersed in dimethylformamide followed by filtration through a PTFE membrane. The immobilization of laccase as a model protein was achieved on the resulting BP electrodes by adsorption or chemical grafting. These biocathodes displayed a direct electrical communication with laccase, allowing the reduction of oxygen in water (pH 5) with maximum current densities of ca 1.1 mA cm⁻² at 0.4 V vs. SCE.     

Predicting mechanical properties of multiscale composites

Abstract

The effect of carbon nanotube (CNT) integration in polymer matrixes (two-phase) and fibre reinforced composites (three-phase) was studied. Simulations for CNT/polymer composites (nanocomposites) and CNT/fibre/polymer composites (multiscale) were carried out by combining micromechanical theories applied to nanoscale and woven fibre micromechanic theories. The mechanical properties (Young’s modulus, Poisson’s ratio and shear modulus) of a multiscale composite were predicted. The relationships between the mechanical properties of nano- and multiscale composite systems for various CNT aspect ratios were studied. A comparison was made between a multiscale system with CNTs infused throughout and one with nanotubes excluded from the fabric tows. The mechanical properties of the composites improved with increased CNT loading. The influence of CNT aspect ratio on the mechanical properties was more pronounced in the nanocomposites than in the multiscale composites. Composites with CNTs in the fibre strands generated more desirable mechanical properties than those with no CNTs in the fibre strands.     

Carbon nanotube induced polymer crystallization: The formation of nanohybrid shish-kebabs

Carbon nanotubes (CNTs) have attracted tremendous attention in recent years because of their superb optical, electronic and mechanical properties. In this article, we aim to discuss CNT-induced polymer crystallization with the focus on the newly discovered nanohybrid shish-kebab (NHSK) structure, wherein the CNT serves as the shish and polymer crystals are the kebabs. Polyethylene (PE) and Nylon 6,6 were successfully decorated on single-walled carbon nanotubes (SWNTs), multi-walled carbon nanotubes (MWNTs), and vapor grown carbon nanofibers (CNFs). The formation mechanism was attributed to “size-dependent soft epitaxy”. Polymer CNT nanocomposites (PCNs) containing PE, Nylon 6,6 were prepared using a solution blending technique. Both pristine CNTs and NHSKs were used as the precursors for the PCN preparation. The impact of CNTs on the polymer crystallization behavior will be discussed. Furthermore, four different polymers were decorated on CNTs using the physical vapor deposition method, forming a two-dimensional NHSK structure. These NHSKs represent a new type of nanoscale architecture. A variety of possible applications will be discussed.     

Calculations of the energy of mixing carbon nanotubes with polymers

A method for calculating the energy of mixing carbon nanotubes (CNTs) with polymers is presented. The formation of the nanocomposite is analyzed in terms of a simple path in which the nanotubes are exfoliated from a bundle and dispersed in a distorted polymer with cylindrical cavities to accommodate the nanotubes. From this perspective, the energy of mixing is the difference between the energy required to exfoliate the nanotubes from a bundle and the energy needed to extract the nanotubes from the polymer matrix relative to the relaxed polymer without any nanotubes. These energy components are evaluated by performing molecular mechanics calculations on individual, localized models representing the polymer, nanotube bundles, and polymer/CNT agglomerates. This method is applied to polystyrene/CNT composites and the factors that determine their thermodynamic stability are identified. To a first approximation, the interaction energies (per unit surface area of the nanotubes) are independent of the lengths and chiral indices, but dependent on the diameters of the component nanotubes. By the application of this method, we show that the energy of mixing CNTs with PS is endothermic until the diameters of the component nanotubes exceed about 2.2 nm; at diameters greater than this value the energy of mixing becomes exothermic. This may explain why it is so difficult to obtain good dispersion of single-walled CNTs (SWCNTs) in PS, since they rarely grow to have diameters greater than about 1.4 nm. On the other hand, since the diameters of multi-walled CNTs typically exceed 10 nm, we would expect them to disperse much better than SWCNTs in polystyrene.     

NaCl adsorption in multi-walled carbon nanotube/active carbon combination electrode

In this paper, we proposed a new process for fabricating electrochemical double layer capacitors employing active carbon and multi-walled carbon nanotubes to adsorb Na⁺ and Cl^- from NaCl solution. Due to their unique mesoporosity, active carbons have high ability to desalt NaCl solution. But they have many defects such as high electrical resistance, high-energy consumption and low intensity. Since carbon nanotube is a new material which has high intensity and low resistance, we can composite the merits of active carbon and carbon nanotube and develop carbon nanotube/active carbon materials combination electrode. It was tested that when carbon nanotube content in carbon materials is 10%, the characteristics of combination electrode is the best for us to desalt brackish water because of their high desalination characterization and low energy consumption. Though there are a few technical problems to be solved, our results show a promising technique for desalting salt water.     

Synthesis,characterizations, and physical properties of carbon nanotubes coated by conducting polypyrrole

A new type of carbon nanotube (CNT) (diameter of <100 nm) coated by conducting polypyrrole (PPY) was synthesized by in situ polymerization on CNTs. The structure of the resulting complex nanotubes (CNT‐PPY) was characterized by elemental analysis, X‐ray photoelectron spectroscopy, Raman spectra, and X‐ray diffraction. These indicated no significant chemical interaction between PPY and the CNT. The electrical, magnetic, and thermal properties of the complex nanotubes were measured and showed the physical properties of the CNTs were modified by conducting PPY. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2605–2610, 1999     

Chemically immobilised carbon nanotubes on silicon: Stable surfaces for aqueous electrochemistry

Diazonium ion chemistry has been used to electrochemically graft aminophenyl layers onto p-type silicon (1 0 0) substrates. A condensation reaction was used to immobilise single-walled carbon nanotubes with high carboxylic acid functionality directly to this layer. Electrochemical monitoring of the aminophenyl groups confirmed the formation of an amide linkage between the single-walled carbon nanotubes and the aminophenyl layer. The carbon nanotube electrode showed high stability and good electrochemical performance in aqueous solution. At moderate scan rates the Ru(NH₃)₆^+3/+2 couple exhibited quasi-reversible electron transfer kinetics with a standard heterogenous rate constant of 1.2 × 10⁻³ cm s⁻¹ at the covalently-linked carbon nanotube surface. The electrode thus combines the advantages of a silicon substrate for easy integration into sophisticated electrical and electronic devices, carbon nanotubes for desirable electrochemical properties, and stability in aqueous medium for future applications in environmental sensing.