A general and efficient route to covalently surface modification of MWCNTs by dopamine and their synergistic reinforcing effects in chitosan films

Multiwalled carbon nanotubes (MWCNTs) were considered as ideal filler in a polymer matrix due to their outstanding mechanical, electrical and thermal properties. This article describes a simple route to preparation of nanocomposite (NC) of chitosan and modified MWCNT by solution casting method. The process involves modification of MWCNT with dopamine and effects of modified MWCNT on dispersion, mechanical, thermal and morphological properties. Atomic force microscopy, Fourier transform infrared spectroscopy, X-ray diffraction (XRD), field emission scanning electron microscopy, transmission electron microscopy, have been used to characterize the NCs. Scanning electron micrographs showed that in all of the NC films, except chitosan/MWCNT-Dop NC 7 wt%, MWCNT were dispersed homogeneously throughout the chitosan matrix. The XRD studies showed that the crystallinity of composite films decreases through hydrogen bonding between chitosan and MWCNT-Dop. The determination of mechanical and thermal properties demonstrated that the NC films exhibited significant enhancement in strength, modulus, and thermal stability compared with the pure chitosan.     

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.     

Synergistic improvement of thermal conductivity of thermoplastic composites with mixed boron nitride and multi-walled carbon nanotube fillers

The thermal conductivity of composites with a polyphenylene sulfide (PPS) matrix and a mixture of boron nitride (BN) power and multi-wall carbon nanotube (MWCNT) fillers was investigated. Synergistic improvement in thermal conductivity of the composite was observed due to the generation of three-dimensional thermal transfer pathways between the BN and MWCNT. The improvement strongly depended on surface treatment of the MWCNTs, such as hydrogen peroxide and acid treatments. The thermal conductivity of the composite was affected by the interaction and interfacial thermal resistance between the PPS matrix and the MWCNTs. The maximum thermal conductivity achieved was 1.74 W/m K for a composite that was pelletizable, injection moldable, and thermally conductive with low electrical conductivity and good mechanical properties.     

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.     

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.     

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.     

Electrical conductivity of carbon nanotube/poly(vinylidene fluoride) composites prepared by high-speed mechanical mixing

Electrically conductive multi-walled nanotube (MWCNT)/poly(vinylidene fluoride) (PVDF) composites with a segregated structure were prepared by high-speed mechanical mixing method. It was found that MWCNTs were uniformly dispersed on polymer particle surfaces. At the MWCNTs composition of 0.1 vol.%, the composites exhibited a dramatic enhancement in electrical conductivity by 11 orders of magnitude. A low percolation threshold was achieved at the CNT concentration of 0.078 vol.%. The mechanical mixing method presented can be adapted to other CNT/polymer composites with a segregated structure.     

An increase in the field emission from vertically aligned multiwalled carbon nanotubes caused by NH3 plasma treatment

We modified vertically aligned multiwalled carbon nanotubes (MWCNTs) grown on a Cu substrate by NH₃ direct current plasma treatment for varying durations and investigated their microstructure and field emission properties. A great improvement on the field emission properties of the MWCNTs was obtained after the plasma treatment. Specifically, the MWCNTs after 1.0 min plasma treatment demonstrated the best field emission performance with the field enhancement factor increasing from 319 to 546, the turn-on electric field decreasing from 11.93 to 8.84 V/μm, and the highest emission current reaching 0.85 mA, as compared to the untreated sample. The improvement was attributed to the morphology change: the tapered structure of the MWCNTs generated by the plasma etching and the increased inter-tip distance due to the cluster structure of the thin tips. The open-ended tips and the increased structural defects introduced by NH₃ plasma also played a role in the enhancement of field emission properties of the MWCNTs.     

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.     

High performance electrochemical H2O2 sensor based on MWCNT thin films fabricated by novel electron beam evaporation

High quality multi-walled carbon nanotubes (MWCNT) were grown by electron beam evaporation (EBE) under a high vacuum of 10⁻⁶ mtorr. The influence of deposition thickness on the orientation, morphology and vibrational bands of MWCNT films fabricated on tantalum (Ta) substrate was discussed. XRD patterns of the film revealed the presence of (002) preferential plane of carbon. Raman spectral analysis show the G-band Raman feature corresponding to high frequency E₂g of first order mode, suggesting that CNTs were composed of crystalline carbon. SEM image of 200 nm thick MWCNT film shows well shaped homogenous fine nanotubes of length ~300 nm and diameter ~70 nm with high purity. The electrochemical performance of the MWCNTs/Ta electrodes was studied by cyclic voltammetry. The sensor prepared with optimum thickness can detect H₂O₂ in the wide range covering 5 µM to 0.025 mM, with the detection limit as low as 0.09 µM. The results demonstrate that the fabrication of MWCNTs/Ta electrode by EBE is a very interesting and useful approach, likely to be a focus of upcoming research efforts in electrochemical sensing.     

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.     

Two-layer materials of polyethylene and a carbon nanotube/cyanate ester composite with high dielectric constant and extremely low dielectric loss

Double-layer materials with one layer being a polyethylene (PE) film and the other layer a multi-wall carbon nanotube (MWCNT)/cyanate ester (CE) resin composite, PE-MWCNT/CE, were prepared. They have high dielectric constant and extremely low dielectric loss. For comparison, MWCNT/CE composites with different contents of MWCNTs were also prepared. Results show that the two kinds of materials have greatly different dielectric properties. With the same content of MWCNTs, the PE-MWCNT/CE material shows a higher dielectric constant and much lower dielectric loss than the MWCNT/CE composite. More specifically, the dielectric constant and loss tangent at 10 Hz of the PE-MWCNT/CE material with 0.5 wt.% MWCNTs are respectively 168 and 0.006, about 1.4 and 2.5 × 10⁻⁵ times the values of the corresponding MWCNT/CE composite. The nature behinds these interesting data was detected from the space charge polarization effect and equivalent circuits. The mechanism for the unique dielectric behavior of the PE-MWCNT/CE materials is that the presence of PE film not only reinforces the space charge polarization, but also subdues the leakage current. On the other hand, based on the discussion on the Cole–Cole plots, an effective method is developed to accurately calculate the relaxation time of space charge polarization in electric conductor/polymer materials.     

Facile synthesis of CuO mesocrystal/MWCNT composites as anode materials for high areal capacity lithium ion batteries

CuO mesocrystal entangled with multi-wall carbon nanotube (MWCNT) composites are synthesized through a facile scalable precipitation and a followed oriented aggregation process. When evaluated as anode materials for lithium ion batteries, the CuO-MWCNT composites exhibit high areal capacity and good cycling stability (1.11 mA h cm⁻² after 400 cycles at the current density of 0.39 mA cm⁻²). The excellent electrochemical performance can be ascribed to the synergy effect of the unique structure of defect-rich CuO mesocrystals and the flexible conductive MWCNTs. The assembled architecture of CuO mesocrystals can favor the Li-ion transport and accommodate the volume change effectively, as well as possess the structural and chemical stability of bulk materials, while the entangled MWCNTs can maintain the structural and electrical integrity of the electrode during the cycles.     

Electrochemical detection of catechol at integrated carbon nanotubes electrodes

The mechanical stability of the electrode plays a very important role in the long-term stability of electrochemical behavior. In this paper, multi-wall carbon nanotubes (MWCNTs) electrodes were prepared in the holes of glass directly by microwave plasma chemical vapor deposition and the electrochemical behavior of catechol at the integrated MWCNT electrodes was investigated. The oxygen plasma treated CNTs had excellent electrochemical behavior for the analysis of catechol. The catechol was detected in the linear concentration range of 1.0 × 10^− 6 mol L^− 1–1.0 × 10^− 3 mol L^− 1. And because CNTs were integrated directly on the substrate, the stable response to catechol solution showed that the carbon nanotubes electrodes had long-term stability.     

Highly stable field emission properties from well-crystalline 6-Fold symmetrical hierarchical ZnO nanostructures

Herein, we report the growth, characterization and field emission application of well-crystalline 6-fold symmetrical hierarchical ZnO nanostructures grown on silicon substrate by thermal evaporation process. The detailed morphological characterizations revealed that the prepared material possess six-fold structures in which ZnO nanoneedles are symmetrically grown on each facets of core hexagonal ZnO nanorods in such a manner that they made beautiful 6-fold symmetrical hierarchical structure. The detailed structural studies confirmed that the grown hierarchical structures possess well-crystallinity with wurtzite hexagonal phase. The room-temperature photoluminescence (PL) spectrum exhibited a strong UV emission confirming good optical properties. The Raman-scattering revealed the wurtzite hexagonal phase for as-grown hierarchical structures. The field emission properties of the 6-fold symmetrical hierarchical ZnO nanostructures were tested and a turn-on voltage equal to 2.8 kV, corresponds to emission current of 65 nA, was observed. A threshold voltage of 4.6 kV with a maximum emission current of about 9.36 µA was also recorded. A high emission current stability profile over a period of ~7000 s was noted for the fabricated FE device.     

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.     

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.     

High-performance field emission from a carbon nanotube carpet

We have created a field emitter composed of a carbon nanotube (CNT) yarn, which was prepared by direct spinning through chemical vapor deposition and then formed into a carpet structure by tying the yarn to a conductive substrate before cutting it. The structure of the carpet is arranged to induce the tips of the CNT yarn to protrude toward the anode for maximum electron emission. The turn-on field, threshold field, and field enhancement factor of the device are 0.33, 0.48 V/μm, and 19,141, respectively. Extremely low operating electric fields and a high field enhancement factor result from the high density of CNT emitters with high crystallinity, the electrically good contact between the emitters and the substrate, and the effects of the multistage structure. The emission is stable even at a high current density of 2.13 mA/cm², attributed to the strong adhesion between the emitters and the substrate. The emission performance is found to be customizable by adjusting the structure, for example, the CNT pile density. These results are relevant for practical applications, such as large-area flat-panel displays, large-area low-voltage lamps, and X-ray sources.     

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.     

Time dependent growth of ZnO nanoflowers with enhanced field emission properties

Herein, we report the facile growth of ZnO nanoflowers composed of nanorods on silicon substrate by non-catalytic thermal evaporation process. The grown nanoflowers were examined in terms of their morphological, structural, optical and field emission properties. The detailed characterizations revealed that the nanoflowers are grown in high density, possessing well-crystallinity and exhibiting wurtzite hexagonal phase. The Raman-scattering spectrum shows a sharp optical-phonon E₂ mode at 437 cm⁻¹ which confirmed the wurtzite hexagonal phase for the grown nanoflowers. The room-temperature PL spectrum depict a strong ultraviolet emission at 381 nm, revealed good optical properties for the ZnO nanoflowers. The field emission studies revealed that a turn-on field for the ZnO nanoflowers based field emission device was 4.3 V/μm and the emission current density reached to 0.075 mA/cm² at an applied electric field of 7.2 V/μm and exhibit no saturation. The field enhancement factor ‘β’ for the fabricated device was estimated from the F-N plot and found to be ~2.75×10³. Finally, systematic time-dependent experiments were performed to determine the growth process for the formation of ZnO nanoflowers composed of nanorods.