A method to increase the energy density of supercapacitor cells by the addition of multiwall carbon nanotubes into activated carbon electrodes

The performance of supercapacitor cells with activated carbon (AC) electrodes was improved by adding a small amount of multiwall carbon nanotubes (MWCNTs). The electrode structure investigated comprised AC, four different types of MWCNTs and two polymer binders, polyvinylidene fluoride or polyvinyl alcohol. All fabricated devices were of the electrochemical double layer capacitor type. The organic electrolyte used was tetraethyl ammonium tetrafluoroborate (TEABF₄) in two different solvents: propylene carbonate or acetonitrile (AN). The electrodes were characterised with scanning electron microscopy and tested for their specific surface area and pore size distribution. The electrode fabrication process was fine-tuned by investigating the effect of the coating thickness on the supercapacitor cell performance. It was established that an AC/MWCNT-based supercapacitor with 30 μm thick roll-coated, composite electrodes of just 0.15%w/w MWCNT content provided superior tested power and energy densities of 38 kW/kg and 28 W h/kg, respectively, compared to 18 kW/kg and 17 W h/kg for AC only–based cells in a 1.5 TEABF₄/AN electrolyte. The increased energy density was attributed to a fine lace of MWCNTs covering the AC microparticles with visible 20–30 nm lace pores and to the high specific area of micropores.     

Field emission enhancement of polypyrrole due to band bending induced tunnelling in polypyrrole-carbon nanotubes nanocomposite

Polypyrrole–multiwalled carbon nanotubes (PPy–MWCNT) nanocomposites with high field emission characteristics have been fabricated by a very simple and effective in situ chemical polymerization technique that bypasses the need of de-agglomerating the MWCNT. Based on structural characterization, we have proposed formation of a three tier structure of PPy–MWCNTs composite with granular PPy on the top. Field emission studies, explained on the basis of band bending at the PPy–MWCNT interface, show drastic increase of field enhancement factor (∼4664) and low turn on field (1.7 V/μm), opening new possibilities of improvement of PPy/PPy–MWCNT composites.     

Direct electron transfer in a mediator-free glucose oxidase-based carbon nanotube-coated biosensor

A novel biosensor was prepared by immobilizing glucose oxidase on multi-walled carbon nanotube (MWCNT)-coated electrospun gold fibers. Homogeneous coating of the electrospun gold fibers by MWCNTs was achieved by electrophoretic deposition at 20 V (40 V cm^?1), a deposition time of 30 s and a solution concentration of 0.25 mg mL^?1. Scanning electron microscopy confirmed the complete coverage of MWCNTs on the fiber surface. The carboxylated MWCNTs on the gold fibers provided an anchor for covalent immobilization of glucose oxidase (GOX). GOX covalently coupled to conductive carbon nanotubes demonstrated direct electron transfer between the enzyme and the electrode surface without the need for a redox active mediator. Electrochemical characterization of the fabricated sensor by cyclic voltammetry revealed that the immobilized GOX exhibited a surface-confined reversible two-electron and two-proton reaction, with an electron transfer rate constant, k_s, of 1.12 s^?1 and a surface coverage of 1.1 × 10^?12 mol cm^?2. The sensor produced a linear response to glucose concentration up to 30.0 mM with a sensitivity of 0.47 μA mM^?1 cm^?2 and a detection limit of 4 μM.     

The influence of nanoscale defects on the fracture of multi-walled carbon nanotubes under tensile loading

We report the mechanical behavior of a unique type of multi-walled carbon nanotube (MWCNT) and an acid-treated version of this MWCNT type that have nanoscale defects on their surfaces from the acid treatment. These defects, from scanning electron microscope (SEM) and transmission electron microscope (TEM) imaging have a ‘channel-like’ appearance, as if a ring of material was cut away from the MWCNT around the circumference. The mechanical properties of individual MWCNTs have been experimentally shown to strongly depend on their structure and structural disorder can drastically reduce the mechanical properties. Tensile-loading experiments using a nanomanipulator tool operated inside a SEM revealed that the tensile strengths of 10 pristine MWCNTs ranged from ～ 2 to ～ 48 GPa (mean 20 GPa). For 10 acid-treated MWCNTs with channel-like defects, tensile strengths ranged from ～ 1 to ～ 18 GPa (mean 6 GPa, thus roughly 70% lower than those of the pristine MWCNTs). Microstructural observations revealed that the fracture of the acid-treated MWCNTs occurred at a channel-like defect region in 8 of the 10 samples. This indicates that the channel-like defects associated with the acid etching are typically going to be the weakest points in the acid-treated MWCNT structure and that stress concentration is present at the defect region.     

Multi-walled carbon nanotubes stimulate callus induction,secondary metabolites biosynthesis and antioxidant capacity in medicinal plant Satureja khuzestanica grown in vitro

The present study was carried out to explore the potential effects of multi-walled carbon nanotubes (MWCNTs) on callus induction and secondary metabolism in Satureja khuzestanica. Leaf segments were aseptically cultured in B5 basal medium with different MWCNTs concentrations (0, 25, 50, 100, 250 and 500 μg ml⁻¹). The calli morphogenic responses were measured and the contents of phenolics, flavonoids, rosmarinic acid (RA), caffeic acid (CA), and the activity of polyphenol oxidase (PPO), l-phenylalanine ammonia-lyase (PAL) and peroxidase (POD) were quantified. Moreover, antioxidant activities of calli extract were assayed. Calli growth improved significantly with the increase of MWCNTs concentration, peaked at 50 μg ml⁻¹, and then followed a rapid decrease at 500 μg ml⁻¹. However, metabolic effects observed following exposure to MWCNTs particularly at 100 μg ml⁻¹ tended to be more pronounced than all other treatments, exhibiting significant induction of antioxidant activity with the lowest IC₅₀ value. Maximum oxidative stress index (H₂O₂) and the highest PPO and POD activities were observed on the media treated with 500 μg ml⁻¹ MWCNTs. Our findings suggest for the first time that use of MWCNTs at specific levels could act as a novel elicitor for in vitro biosynthesis of valuable secondary metabolites and antioxidant drugs.     

Fabrication and origin of high-k carbon nanotube/epoxy composites with low dielectric loss through layer-by-layer casting technique

The distribution of polarized space charges and their relaxation behavior in high dielectric constant electric conductor/polymer composites are main factors that determine the frequency-dependent dielectric constant and dielectric loss. However, few reports focus on this motif. We present here the dielectric performance and mechanism of a unique kind of composites with multi-layers (coded as [MWCNT/EP]_x, where x refers to the number of layers), fabricated by using layer-by-layer casting technique. Each composite layer with same thickness was composed of multi-walled carbon nanotubes (MWCNTs) and epoxy (EP) resin. When the loading of MWCNTs is 0.5 wt%, the four-layer [MWCNT0.5/EP]₄ material shows the highest dielectric constant (465 at 1 Hz) and low dielectric loss tangent (0.7 at 1 Hz), about 4 and 2.1 × 10⁻² times the values of traditional MWCNT0.5/EP composite, respectively. By investigating the space charge polarization (SCP), Debye polarization and dielectric moduli in [MWCNT/EP]_x materials, the complex relationships and the origin among dielectric constant, dielectric loss, frequency and the content of filler were clearly elucidated. The SCP within each layer is different from that between layers. The greatly improved dielectric properties of [MWCNT/EP]_x materials are believed to be the reinforced SCP and blocked transport of carriers between every two layers.     

Probing the carbon nanotube-surfactant interaction for the preparation of composites

Adsorption isotherms of four different surfactants, sodium dodecyl sulfate (SDS), sodium dodecyl benzyl sulfonate, benzethonium chloride and Triton X-100 were measured on multi-wall carbon nanotubes (MWCNT) in water. With the surfactant SDS, the isotherms were also measured on single-wall carbon nanotubes (SWCNT) as well as on MWCNT under various ionic strength and temperature conditions. The nature of the polar head had only little influence on adsorption which was mainly driven by hydrophobic interactions. However, the outcome of the dispersion experiment was dependent on the purity of the carbon nanotubes. Using these results, it was possible to prepare concentrated colloidaly stable dispersions of MWCNTs in water (c = 32 g/L). Conducting MWCNT/polymer composite films could then readily be prepared by simple formulation of the MWCNTs with a polymeric dispersion.     

Polypropylene/carbon nanotube nano/microcellular structures with high dielectric permittivity,low dielectric loss,and low percolation threshold

Nano/microcellular polypropylene/multiwalled carbon nanotube (MWCNT) composites exhibiting higher electrical conductivity, lower electrical percolation, higher dielectric permittivity, and lower dielectric loss are reported. Nanocomposite foams with relative densities (ρ_R) of 1.0–0.1, cell sizes of 70 nm–70 μm, and cell densities of 3 × 10⁷–2 × 10¹⁴ cells cm⁻³ are achieved, providing a platform to assess the evolution of electrical properties with foaming degree. The electrical percolation threshold decreases more than fivefold, from 0.50 down to 0.09 vol.%, as the volume expansion increases through foaming. The electrical conductivity increases up to two orders of magnitude in the nanocellular nanocomposites (1.0 > ρ_R > ∼0.6). In the proper microcellular range (ρ_R ≈ 0.45), the introduction of cellular structure decreases the dielectric loss up to five orders of magnitude, while the decrease in dielectric permittivity is only 2–4 times. Thus, microcellular composites containing only ∼0.34 vol.% MWCNT present a frequency-independent high dielectric permittivity (∼30) and very low dielectric loss (∼0.06). The improvements in such properties are correlated to the microstructural evolution caused by foaming action (biaxial stretching) and volume exclusion. High conductivity foams have applications in electromagnetic shielding and high dielectric foams can be developed for charge storage applications.     

A study on rheological behavior of MWCNTs/epoxy composites

Rheological behaviors of multiwalled carbon nanotubes (MWCNTs)/epoxy composites with various MWCNT contents were investigated by using a time sweep and frequency sweep experiment with oscillatory rheometry. The functional groups on the acid-treated MWCNTs were investigated by fourier transfer-infrared spectroscopy (FT-IR). The composites containing acid-treated MWCNTs exhibited faster gel time than pure epoxy resins. The storage (G′) and loss (G″) moduli of the composites showed solid-like behavior owing to interaction between the MWCNTs and the epoxy resins. The 1.0 wt% MWCNT composites had the highest crosslinking activation energy (E_c) due to good dispersion and strong interfacial bonding. These results imply that three-dimensional crosslinking might take place among the hydroxyl group in epoxy resins and the carbonyl or hydroxyl group in acid-treated MWCNTs.     

Upscaling potential of the CVD stacking growth method to produce dimensionally-controlled and catalyst-free multi-walled carbon nanotubes

Multi-walled CNTs (MWCNTs) with structural characteristics optimised for bio-applications have been produced using a catalyst-supported chemical vapour deposition (CVD) method. The upscale potential of the process was demonstrated by combining classical semi-continuous and stacked-growth modes. The vertically aligned MWCNT films thus obtained were multi-layered with five continuous strata of well-structured nanotubes. Following gentle disentanglement, the stacks were converted to individual MWCNTs with short dimensions (a final length and diameter of ～1.2 μm and ～12 nm) and almost catalyst-free (<0.04%). Overall, our process produces dispersed, bio-tailored MWCNTs with an output growth-yield 20 times higher than a standard CVD setup and exempt of complex or destructive post-growth steps of purification and separation. These constitute key steps towards the mass production of MWCNTs with low toxicological risks, an essential prerequisite for biomedical applications.     

Excellent electromagnetic interference shielding and mechanical properties of high loading carbon-nanotubes/polymer composites designed using melt recirculation equipped twin-screw extruder

This paper describes for the first time a facile, scalable and commercially viable melt blending approach involving use of twin-screw extruder with melt recirculation provision, for uniform dispersion of up to 4.6 vol% multiwall carbon nanotubes (MWCNTs) within polypropylene random copolymer (PPCP). Morphological characterization of PPCP/MWCNT nanoscale composites (NCs) was done using scanning electron microscopy and transmission electron microscopy, which show good dispersion of MWCNTs in the PPCP matrix even at high loadings and confirm the formation of true NCs. The improved dispersion leads to the formation of electrically conducting three dimensional networks of MWCNTs within PPCP matrix at very low percolation threshold (∼0.19 vol%). The attainment of dc conductivity value of ∼10⁻³ S/cm, tensile strength of ∼42 MPa and good thermal stability for 4.6 vol% MWCNTs loading NC along with electromagnetic interference (EMI) shielding effectiveness (SE) value of −47 dB (>99.99% attenuation), demonstrate its potential for making light weight, mechanically strong and thermally stable EMI shields. These NCs also display specific SE value of ∼−51 dB cm³/g which is highest among unfoamed polymer NCs.     

Catalytic hydrogen transfer of ketones over atomic layer deposited highly-dispersed platinum nanoparticles supported on multi-walled carbon nanotubes

Hydrogen transfer of ketones, catalyzed by highly-dispersed platinum nanoparticles supported on multi-walled carbon nanotubes (MWCNTs), was studied. Pt nanoparticles were deposited on gram quantities of non-functionalized MWCNTs by atomic layer deposition (ALD) at 300 °C, using (methylcyclopentadienyl) trimethylplatinum and oxygen as precursors. TEM analysis showed that ~ 1.4 nm Pt nanoparticles were highly dispersed on MWCNTs. The heterogeneous hydrogen-transfer reactions of acetophenone indicated that an acetophenone conversion of 100% and a 1-phenylethanol selectivity of 99.0% could be obtained with a ketone to Pt mass ratio of 24,690 and a ketone to KOH mass ratio of 22 at 150 °C for 5 h. The selectivity of the Pt/MWCNT catalyst was higher than that of the commercial Pt/C catalyst, due to the highly-dispersed, uniform Pt nanoparticles and the unique porous structures of the Pt/MWCNT catalyst. The high stability of the Pt/MWCNT catalyst was demonstrated by reutilization of the catalyst. The high reactivity and selectivity of this catalyst for hydrogen transfer reduction were also demonstrated for other ketones.     

The electrical conductivity of carbon nanotube/carbon black/polypropylene composites prepared through multistage stretching extrusion

We reported the design of a grape-cluster-like conductive network in a polypropylene (PP) matrix, where oriented multi-walled carbon nanotubes (MWCNTs) served as branches and provided charge transport over large distances while grape-like carbon black (CB) aggregates enriched around MWCNTs and linked these conductive tubes through charge transport over small distances. The key for construction of this grape-cluster-like conductive network was the extension and orientation of MWCNTs, which was achieved in this work by multistage stretching extrusion with an assembly of laminating-multiplying elements (LMEs, which divide and recombine polymer melts). The highest efficient grape-cluster-like conductive network was obtained at a CB:MWCNT weight ratio of 6. The experimental results showed that this novel grape-cluster-like conductive network provided a low percolation threshold for PP/CB/MWCNT composites due to the synergistic effect of CB and oriented MWCNTs. When the combined CB and MWCNT content was about 6.9 vol%, the electrical resistivity of PP/CB/MWCNT composites prepared by multistage stretching extrusion with 6 LMEs decreased to only 0.63 Ω cm.     

Carbon nanotube growth at 420 °C using nickel/carbon composite thin films as catalyst supports

Nickel/carbon composite (Ni/C) thin films were used as catalyst supports for the growth of vertically aligned multiwalled carbon nanotubes (MWCNTs) at temperature as low as 420 °C. Nickel nanoparticles embedded within the carbon matrix of Ni/C films have served as catalysts for the synthesis of nanotubes by PECVD using acetylene/ammonia plasma. Two different nickel contents (40 at.% and 60 at.%) in the films were used. Analysis indicated a diffusion of nickel atoms in the form of nanoparticles to the film surface upon annealing. This diffusion depends on both annealing temperature and nickel concentration in the films and affects the MWCNT growth at low temperature. The MWCNT synthesis was tested at growth temperature ranging between 335 and 520 °C. The growth of MWCNTs at 420 °C was only achieved by using Ni/C films with a high nickel content (60 at.%). These MWCNTs did not present considerable loss in their growth rate and structural quality compared to MWCNTs grown on classical substrates (Ni catalysts deposited on TiN), at higher temperature (520–600 °C). The results suggest that carbon saturation at the surface and subsurface of nickel catalysts of the Ni/C films is responsible for the improvement of MWCNT growth at low temperature.     

Surface modification of poly-l-lactic acid fibrous scaffolds by a molecular-designed multi-walled carbon nanotube multilayer for enhancing cell interactions

We presented a molecular-designed multi-walled carbon nanotube (MWCNT) layer-by-layer (LbL) multilayer on poly-l-lactic acid (PLLA) electrospun fibers for engineering cell/CNT interfaces. A stable, positively charged monolayer was created on the fiber surface by the aminolysis reaction of poly(ethylene imine) (PEI) with PLLA, followed by alternate deposition in negatively charged MWCNT and positively charged chitosan (CS). Thermogravimetric analysis indicated a sustained growth of the MWCNT during the self-assembly process. The interactions between MWCNT and polycation crucially affected the specific structure and properties of the MWCNT multilayer. MWCNT/PEI electrostatic interactions reduced the gap between MWCNTs and improved the π → π¹ transitions. However, the CS chains tended to be more serpentine than the chains of PEI molecules, which might have hindered the π → π¹ transitions. On the other hand, the electrostatic interactions might have enhanced the disorder grade of the MWCNT structure, as indicated by Raman analysis. The scaffolds maintained their fibrous and porous structure after MWCNT multilayer modification and supported fibroblast growth. The MWCNT multilayer induced cell migration toward the interior of the scaffolds. Therefore, we created a simple yet efficient method of building a CNT multilayer on three-dimensional (3D) fibrous scaffolds for enhancing cell-matrix interactions.     

Stabilization and dispersion of PtRu and Pt nanoparticles on multiwalled carbon nanotubes using phosphomolybdic acid,and the use of the resulting materials in a direct methanol fuel cell

PtRu and Pt nanoparticles were deposited on the surface of multiwalled carbon nanotubes (MWCNTs) with the assistance of phosphomolybdic acid (PMo) by a one-pot hydrothermal reduction strategy. Transmission electron microscopy shows a high-density PtRu (or Pt) nanoparticles uniformly dispersed on the surface of the MWCNTs with an average diameter of 1.8 nm for PtRu nanoparticles and 2.4 nm for Pt nanoparticles. Moreover, the as-prepared PMo/PtRu/MWCNT and PMo/Pt/MWCNT electrocatalysts are highly electroactive for the electrochemical oxidation of methanol. Cyclic voltammograms show a high electrochemical surface area (ESA) and a large current density for methanol oxidation at the modified electrode by PMo/PtRu/MWCNT and PMo/Pt/MWCNT electrocatalysts. Electrochemical impedance spectroscopy reveals a high CO tolerance for PMo/PtRu/MWCNT and PMo/Pt/MWCNT electrocatalysts in the electrochemical catalysis of methanol oxidation. For comparison, PtRu/MWCNT and Pt/MWCNT electrocatalysts were prepared in control experiments without PMo. The results demonstrate that PtRu and Pt nanoparticles deposited on MWCNTs in the presence of PMo were superior to those on MWCNTs without PMo in several respects including: (1) a smaller size and a higher dispersion; (2) a higher ESA; (3) a larger current density for methanol oxidation; (4) a higher tolerance for CO poisoning.     

Multiwalled carbon nanotubes–BaTiO3/silica composites with high complex permittivity and improved electromagnetic interference shielding at elevated temperature

Composites with silica matrix and mixed filler of multiwalled carbon nanotubes (MWCNTs) and BaTiO₃ powder were fabricated. Excellent uniform dispersion of MWCNTs can be obtained using a two-step mixing method. Both of the real and imaginary parts of complex permittivity increased with increasing MWCNT content and measured temperature. The electromagnetic interference (EMI) shielding results showed that the absorption mechanism is the main contribution to the total EMI shielding effectiveness (SE). Compared with the EMI SE resulting from reflection, the absorption showed more dependence on the MWCNT content, measured temperature and frequency. The total EMI SE is greater than 20 dB at 25 °C and 50 dB at 600 °C in the whole frequency range of 12.4–18 GHz with a 1.5 mm composite thickness, which suggests that the MWCNT–BaTiO₃/silica composites could be good candidates for the EMI shielding materials in the measured frequency and temperature region.     

An efficient,reusable copper-oxide/carbon-nanotube catalyst for N-arylation of imidazole

Copper oxide nanoparticles (CuONPs) were successfully decorated on acid treated multi-wall carbon nanotubes (f-MWCNTs) using copper acetate precursor by a very simple “mix and heat” method, and was used as a heterogeneous nanocatalyst for the N-arylation of imidazole for the first time. The transmission electron microscopic (TEM) images of the prepared nanocatalyst (CuO/MWCNT) showed a good adhesion of CuONPs on anchoring sites of the MWCNTs. The factual loading of Cu in CuO/MWCNT was 7.64 wt% as confirmed by inductively coupled plasma-mass spectrometry (ICP-MS). The chemical state of Cu on MWCNTs was +2 as revealed by wide angle X-ray diffraction (WAXD), X-ray photoelectron spectroscopic (XPS) and temperature programmed reduction (TPR) techniques. Initially the reaction conditions were optimized and then the scope of the catalytic system was extended with different aryl halides. 0.98 mol% (5 mg) of the catalyst was sufficient for N-arylation of imidazole. A literature survey showed that this is the smallest amount of catalyst used for this reaction in research reported to date. The CuO/MWCNT is chemically as well as physically very stable, heterogeneous in nature and reusable. After the catalytic reaction, MWCNTs were successfully separated from the used CuO/MWCNT and characterized by Raman, TEM and WAXD.     

Self-assembled natural rubber/multi-walled carbon nanotube composites using latex compounding techniques

Functionalization of multi-walled carbon nanotubes (MWCNTs) plays an important role in eliminating nanotube aggregation for reinforcing polymeric materials. We prepared a new class of natural rubber (NR)/MWCNT composites by using latex compounding and self-assembly technique. The MWCNTs were functionalized with mixed acids (H2SO₄/HNO₃ = 3:1, volume ratio) and then assembled with poly (diallyldimethylammonium chloride) and latex particles. The Fourier transform infrared spectroscopy, transmission electron microscopy, and scanning electron microscopy were used to investigate the assembling mechanism between latex particles and MWCNTs. It is found that MWCNTs are homogenously dispersed in the natural rubber (NR) latex as individual nanotubes since strong self-aggregation of MWCNTs has been greatly depressed with their surface functionalization. The well-dispersed MWCNTs produce a remarkable increase in the tensile strength of NR even when the amount of MWCNTs is only 1 wt.%. Dynamic mechanical analysis shows that the glass transition temperature of composites is higher and the inner-thermogenesis and thermal stability of NR/MWCNT composites are better, when compared to those of the pure NR. The marked improvement in these properties is largely due to the strong interfacial adhesion between the NR phase and MWCNTs. Functionalization of MWCNTs represents a potentially powerful technology for significant reinforcement of natural rubber materials.     

Spontaneous polymerization of 2-ethynylpyridine with acylated multi-walled carbon nanotubes in supercritical carbon dioxide and their optical and electrochemical performance

A facile and green approach was proposed for the synthesis of multi-walled carbon nanotubes (MWCNTs) covalently functionalized with poly (2-ethynylpyridine) (MWCNT/P2EP) in supercritical carbon dioxide as a reaction medium. The oxidized MWCNTs were refluxed with thionyl chloride to yield COCl terminated MWCNTs, which were subsequently used as an initiator for the spontaneous polymerization of 2-ethynylpyridine to produce the MWCNT/P2EP hybrid. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction confirmed the formation of amorphous MWCNT/P2EP with a large surface area of 38 m² g⁻¹ and high nitrogen content (up to 8%). Microscopic results revealed that the MWCNTs were well embedded in the polymer matrix and the P2EP chains were wrapped around the carbon nanotube wall. The strong covalent coupling at the interface of the MWCNT/P2EP resulted in high electrical conductivity and enhanced thermal stability. Furthermore, the optical and electrochemical properties were investigated. The hybrid exhibited a photoluminescence peak at 510 nm corresponding to the photon energy of 2.44 eV.