Multi-walled Carbon Nanotubes Network Research Articles

Reduction of percolation threshold through double percolation in melt‐blended polycarbonate/acrylonitrile butadiene styrene/multiwall carbon nanotubes elastomer nanocomposites

AbstractWe demonstrate a method that involves melt blending of polycarbonate (PC) and melt‐blended acrylonitrile butadiene styrene (ABS) with multiwall carbon nanotubes (MWCNTs) to prepare electrically conducting PC/MWCNT nanocomposites at significantly low MWCNT loading. The partial solubility of ABS in PC led to a selective dispersion of the MWCNTs in the ABS phase after melt‐blending PC and ABS. Thus, a sudden rise in electrical conductivity (∼108 orders of magnitude) of the nanocomposites was found at 0.328 vol% of MWCNT, which was explained in terms of double percolation phenomena. By optimizing the ratio of PC and the ABS–MWCNT mixture, an electrical conductivity of 5.58 × 10−5 and 7.23 × 10−3 S cm−1 was achieved in the nanocomposites with MWCNT loading as low as 0.458 and 1.188 vol%, respectively. Transmission electron microscopy revealed a good dispersion and distribution of the MWCNTs in the ABS phase, leading to the formation of continuous MWCNT network structure throughout the matrix even at very low MWCNT loading. Storage modulus and thermal stability of the PC were also increased by the presence of a small amount of MWCNTs in the nanocomposites.POLYM. COMPOS., 2013. © 2013 Society of Plastics Engineers

Plasma Treatment as a Way of Increasing the Selectivity of Carbon Nanotube Networks for Organic Vapor Sensing Elements

Multi-walled carbon nanotubes networks (MWCNTs) were used as a layer for organic vapor detection. The sensor detects volatile organic compounds (VOC). The gas sensing by MWCNTs is measured by means of macroscopic electrical resistance. The selected solvents had different polarities and volume fractions of saturated vapors. The electrical resistance of MWCNTs increases when exposed to organic solvent vapors, and a reversible reaction is observed when the MWCNT is removed from the vapors. The MWCNTs were modified by means of plasma treatment. For modifications RF plasma in O2 at 50 Pa and an afterglow configuration were used. The modified MWCNTs show an increase in sensitivity caused by creating carboxylic groups on the surface of the carbon nanotubes. It leads, for example, to enhancement of the sensitivity from usual 30 % for heptane at RT to more than 200% after plasma treatment in O2 for 10s.

Temperature Dependence of Electrical Conductivity of Multi-Walled Carbon Nanotube Networks in a Polystyrene Composite

The conductance properties of multi-walled carbon nanotube mats and their polystyrene composite were examined to investigate the mechanism of conduction and the specific role of the supporting polymer. By measuring the temperature dependence of the conductance, it was found that the conduction mechanism in carbon nanotube mat follows the series heterogeneous model when the conductance is described as the sum of metallic and barrier portions of the conduction path. This mechanism is affected by the polymeric portion of the composite, since the temperature dependence of the composite conductance is decreased.

Benzene Sensors Based on Multi-Walled Carbon Nanotube Networks Decorated with Metal Nanoclusters

This paper demonstrates sensors based on multi-walled carbon nanotube (MWNT) networks decorated with metal nanoclusters for benzene vapor detection at room temperature. MWNT networks were deposited between the interdigitated metal electrodes on a Si/SiO2 substrate by AC dielectrophoresis. Metal nanoclusters including Au, Pt, Ti and Ni were deposited by magnetron sputtering onto the MWNT networks with a controlled loading of equivalent thickness of 1.5-1.8 nm as the active sensing layer. The resistances of MWNT networks are reduced after functionalized with metal nanoclusters. Experimental results show that resistances of unmodified and metal-functionalized MWNT networks increase after exposing to benzene, ethanol and water vapor. The MWNT networks decorated with Au, Pt and Ti have more sensitivity than bare MWNT networks while Ni decoration did not enhance the sensitivity of MWNT networks to benzene vapor. The MWNT networks decorated with Pt exhibited highest resistance response to benzene vapor, with a less interference of ethanol vapor and humidity.

Percolated pore networks of oxygen plasma-activated multi-walled carbon nanotubes for fast response, high sensitivity capacitive humidity sensors

We report on the preparation of capacitive-type relative humidity sensors incorporating plasma-activated multi-wall carbon nanotube (p-MWCNT) electrodes and on their performance compared with existing commercial technology. Highly open porous conductive electrodes, which are almost impossible to obtain with conventional metal electrodes, are fabricated by spray-depositing MWCNT networks on a polyimide layer. Oxygen plasma activation of the MWCNTs is also explored to improve the water adsorption of the MWCNT films, by introducing oxygen-containing functional groups on the CNT surface. Polyimide humidity sensors with optimized p-MWCNT network electrodes exhibit exceptionally fast response times (1.5 for adsorption and 2 s for desorption) and high sensitivity (0.75 pF/% RH). These results may be partially due to their percolated pore structure being more accessible for water molecules, expending the diffusion of moisture to the polyimide sensing film, and partially due to the oxygenated surface of p-MWCNT films, allocating more locations for adsorption or attraction of water molecules to contribute to the sensitivity.

Rheologic and mechanical properties of multiwalled carbon nanotubes-reinforced poly(trimethylene terephthalate) composites

This paper investigates the rheologic and mechanical properties of melt-blended poly(trimethylene terephthalate) (PTT)/multiwalled carbon nanotube (MWCNT) composites and the effect of acid treatment of MWCNT on these properties. The microstructure of the composites was studied by SEM and TEM in terms of the dispersion state of the nanotubes and the polymer–nanotube interaction. Incorporation of MWCNTs into PTT matrix resulted in an increase in both complex viscosity and moduli than those of neat PTT. A dramatic increase in the melt viscosity of composites observed with loading of MWCNT in the range of 0.5 and 2 wt% showed the formation of interconnected network of MWCNT in the polymer matrix at a rheologic percolation threshold. Acid treatment of MWCNT showed significant effect on the rheologic properties of PTT and led to the enhancement of both complex viscosity and moduli due to strong interfacial interaction between acid-treated MWCNT and PTT matrix. The effect of acid treatment was also evident by mechanical properties of the PTT/MWCNT composites. The untreated MWCNT showed only increase in modulus of PTT matrix; whereas, after acid treatment, both tensile strength and modulus of PTT matrix enhanced significantly.

Heterojunction photodiode fabricated from multiwalled carbon nanotube/ZnO nanowire/p-silicon composite structure

A heterojunction photodiode was fabricated from multiwalled carbon nanotubes (MWCNTs)/ZnO nanowires/p-Si (100) substrate composite structure. The heterojunction photodiode demonstrated a faster transient response and higher responsivity than the reference sample without deposition of MWCNTs, which is attributed to improved carrier collection and transport efficiency through the MWCNTs network. The high photoresponsivities of the devices are explained in terms of operation as a hybrid of photodiode and photoconductor modes. The spectral response of the devices showed dependence on voltage polarity and is attributed to the high valance band offset in the interfacial region of ZnO and p-Si substrate.

Facile synthesis of MoS3/carbon nanotube nanocomposite with high catalytic activity toward hydrogen evolution reaction

Hydrogen has been proposed as a future energy carrier in the transition from the current hydrocarbon economy. Exploring advanced materials for electrocatalytic and photoelectrochemical water splitting has become one of the most important issues for bulk and inexpensive hydrogen production. In this study, the nanocomposite of MoS3 and multi-walled carbon nanotubes (MWCNTs) with the high catalytic activity toward hydrogen evolution reaction (HER) was easily synthesized using wet chemistry process. With the aid of functional groups present in MWCNTs, amorphous MoS3 nanoparticles were highly dispersed over MWCNT surface. It was found that MoS3 on the MWCNTs was electrochemically reduced to MoS2 before HER and thus the amorphous MoS2 was identified as the actual catalyst for HER. Furthermore, MoS2 with amorphous structure exhibited the higher HER activity than crystalline MoS2 due to the fact that the former had a higher number of exposed edges. In addition, the catalytic activity of nanocomposite of MoS3 and MWCNTs was increased with decreasing the loading amount of MoS3 on MWCNTs and the optimal MoS3 loading on MWCNTs was 33wt%. Based on the extensive transmission electron microscopy analysis and capacitance measurements, the catalytic activity of the nanocomposite was highly correlated to its active surface area which was controlled by MoS3 morphology on MWCNT surface. The nanocomposite of MoS3 and MWCNTs exhibited excellent HER activity with a small overpotential of ∼0.13V, large cathodic currents and a Tafel slope as small as 40mV/decade. The impedance measurements suggested that the high catalytic activity of nanocomposite of MoS3 and MWCNTs was stemmed from the synergistic effect from the highly exposed edges of amorphous MoS3 nanoparticles and the excellent electrical coupling to the conductive MWCNT network. Furthermore, it was found that the current density of this hybrid catalyst was decreased to 88% of the initial value after the continuous 500 cycling, which showed the reasonable stability in the long-term operation. The present work suggested that the highly active and stable nanocomposite of MoS3 and MWCNTs showed a great potential as a low cost alternative to Pt in water splitting.

Percolation and gel-like behavior of multiwalled carbon nanotube/polypropylene composites influenced by nanotube aspect ratio

The influence of aspect ratio of multiwalled carbon nanotube (MWCNTs) on rheological and electrical properties of MWCNT/polypropylene (PP) composites has been studied. Owing to the pre-grinding for MWCNTs and the efficient twin-extruder mixing, the dispersion of MWCNTs of different aspect ratios in the PP matrix is all uniform and homogeneous. With incorporation of 10 wt% MWCNTs with an aspect ratio of 167, an electrical conductivity of 1.2 S/cm could be achieved. A concept of “gel-like behavior” is introduced to investigate the MWCNT network formation within the composites. The rheological percolation threshold (cpG') and the gel content (cg) of different MWCNT/PP composites are derived by applying the power law scaling and the Winter–Chambon method, respectively. It is revealed that the composites containing MWCNTs with the largest aspect ratio have the lowest cpG' and cg values. Furthermore, the electrical conductivities of composites are measured and the electrical percolation thresholds (cpσ) are obtained. The difference in gel content among different aspect ratios as well as the difference between rheological and electrical percolation thresholds is discussed.

Clusters of α-LiFeO2 nanoparticles incorporated into multi-walled carbon nanotubes: a lithium-ion battery cathode with enhanced lithium storage properties

We report the preparation of a novel nanocomposite architecture of α-LiFeO2-MWCNT based on clusters of α-LiFeO2 nanoparticles incorporated into multiwalled carbon nanotubes (MWCNTs). The composite represents a promising cathode material for lithium-ion batteries. The preparation of the nanocomposite is achieved by combining a molten salt precipitation process and a radio frequency oxygen plasma for the first time. We demonstrate that clusters of α-LiFeO2 nanoparticles incorporated into MWCNTs are capable of delivering a stable and high reversible capacity of 147 mA h g(-1) at 1 C after 100 cycles with the first cycle Coulombic efficiency of ~95%. The rate capability of the composite is significantly improved and its reversible capacity is measured to be 101 mA h g(-1) at a high current rate of 10 C. Both rate capability and cycling stability are not simply a result of introduction of functionalized MWCNTs but most likely originate from the unique composite structure of clusters of α-LiFeO2 nanoparticles integrated into a network of MWCNTs. The excellent electrochemical performance of this new nanocomposite opens up new opportunities in the development of high-performance electrode materials for energy storage application using the radio frequency oxygen plasma technique.

Fabrication of Metal Alloy‐Deposited Flexible MWCNT Buckypaper for Thermoelectric Applications

We report the fabrication of a flexible network of multiwall carbon nanotubes (MWCNTs) known as buckypaper (BP) for thermoelectric (TE) applications. A thermal evaporation method was used to deposit TE metal alloys onto the BP. The TE properties were improved primarily by increasing the Seebeck coefficient values (50 and 75 μV/K) and the electrical conductivity by approximately 10 000 S/m. High‐temperature resistivity studies were performed to confirm the semiconductivity of buckypaper. Variations in resistivity were observed to be the result of the metal alloys coated on the BP surface. We also demonstrated that a substantial increase in the Seebeck coefficient values can be obtained by connecting 3 and 5 layers of metal‐deposited BP in series, thereby enhancing the TE efficiency of MWCNT‐based BP for application in thermoelectric devices.

Individual dispersion of carbon nanotubes in epoxy via a novel dispersion–curing approach using ionic liquids

The effective dispersion of carbon nanotubes (CNTs) in a thermoset was achieved using ionic liquid as the dispersion-curing agent. We preferentially dispersed multiwalled carbon nanotubes (MWCNTs) down to individual tube levels in epoxy resin. Here the dispersion is ruled by the depletion of physical bundles within the MWCNT networks, for which molecular ordering of ionic liquids is considered responsible. The quantitative analyses using ultra small angle X-ray scattering (USAXS) confirmed the dispersion of individual MWCNTs in the matrix. The distance between the dispersed nanotubes was calculated at different nanotube loadings using the power law fitting of the USAXS data. The fine dispersion and subsequent curing, both controlled by ionic liquid, lead to composites with substantially enhanced fracture mechanical and thermomechanical properties with no reduction in thermal properties. Merging processing techniques of nanocomposites with ionic liquid for efficient dispersion of nanotubes and preferential curing of thermosets facilitates the development of new, high performance materials.

Comparative study of the covalent diazotization of graphene and carbon nanotubes using thermogravimetric and spectroscopic techniques

The direct diazotization of the sp(2) carbon network of graphene and multi-walled carbon nanotubes (MWNTs) is one of the many methods employed to introduce functional groups into these nanostructures. Herein, a methodical study on solvent-free diazotization with ethynylaniline in the presence of isoamyl nitrite is reported. Thermogravimetric analysis and UV-visible, infrared and Raman spectroscopies are used to precisely determine the degree of modification, confirm the presence of physisorption and describe the mechanism of elimination of the modifying groups. The results suggest that the same synthetic protocol in both cases leads to a certain degree of covalent modification, whilst a proportion of the modifying groups remains adsorbed to the carbon nanostructure. A higher level of global modification was observed for MWNTs. It was found that the elimination mechanism of the covalently-linked modifiers is identical for both nanostructures and involves two steps; acetylenic-aromatic bond rupture in the modifier followed by modifier-carbon nanostructure cleavage.

Enhanced Strain‐Dependent Electrical Resistance of Polyurethane Composites with Embedded Oxidized Multiwalled Carbon Nanotube Networks

The effect of different chemical oxidation of multiwalled carbon nanotubes with H2O2, HNO3, and KMnO4 on the change of electrical resistance of polyurethane composites with embedded oxidized nanotube networks subjected to elongation and bending has been studied. The testing has shown about twenty‐fold increase in the electrical resistance for the composite prepared from KMnO4 oxidized nanotubes in comparison to the composites prepared from the pristine and other oxidized nanotubes. The evaluated sensitivity of KMnO4 treated composite in terms of the gauge factor increases with strain to nearly 175 at the strain 11%. This is a substantial increase, which ranks the composite prepared from KMnO4 oxidized nanotubes among materials as strain gauges with the highest electromechanical sensitivity. The observed differences in electromechanical properties of the composites are discussed on basis of their structure which is examined by the measurements of Fourier transform infrared spectroscopy, X‐ray photoelectron spectroscopy, and scanning electron microscope. The possible practical use of the composites is demonstrated by monitoring of elbow joint flexion during two different physical exercises.

Fabrication of high-performance supercapacitors based on transversely oriented carbon nanotubes

High-performance supercapacitors with organic electrolyte 1 M TEABF4 (tetraethyl ammonium tetrafluoroborate) in PC (propylene carbonate) were fabricated and tested, based on multiwall carbon nanotubes (MWNTs) deposited by electrophoresis on three types of alternative substrates: aluminium foil, ITO (indium tin oxide) coated PET (polyethylene terephthalate) film and PET film. In all cases, SEM (scanning electron microscopy) and STEM (scanning transmission electron microscopy) micrographs demonstrated that protruding, transversely oriented MWNT structures were formed, which should increase the transverse conductivity of these MWNT electrodes. The best supercapacitor cell of MWNT electrodes deposited on aluminium foil displayed good transverse orientation of the MWNT structures as well as an in-plane MWNT network at the feet of the protruding structures, which ensured good in-plane conductivity. Capacitor cells with MWNT electrodes deposited either on ITO-coated PET film or on PET film demonstrated lower but still very good performance due to the high density of transversely oriented MWNT structures (good transverse conductivity) but some in-plane inhomogeneities. Capacitor cells with drop-printed MWNTs on aluminium foil, without any transverse orientation, had 16–30 times lower specific capacitance and 5–40 times lower power density than the capacitor cells with the electrophoretically deposited MWNT electrodes.

Dynamics of carbon nanotube alignment by electric fields

The dynamics of multiwall carbon nanotube (MWCNT) alignment inside viscous media using electric fields is investigated. Electrical current measurements were performed in situ during the application of an electric field to liquid solutions of deionized water or dissolved polymer containing MWCNTs. The variation of electrical current over time was associated to the dynamics of the MWCNT network formation. The influence of the electric field magnitude and frequency on the MWCNT network formation was studied. MWCNT migration towards the negative electrode was observed when a direct current electric field was applied, whereas formation of an aligned MWCNT network was achieved for an alternating current electric field. The increase of the electric field frequency promotes a faster formation of an aligned MWCNT network and thinner MWCNT bundles. A higher viscosity of the liquid medium yields slower MWCNT alignment evidenced by a slower change of electrical current through the viscous system. An analytical model based on the dielectrophoresis-induced torque, which considers the viscosity of the medium, is also proposed to explain the dynamics of MWCNT alignment. Furthermore, aligned MWCNT/polysulfone solid composites were fabricated and electrically characterized. The solid composites presented anisotropic electrical conductivity, which was more evident for low MWCNT concentrations (0.1–0.2 wt%).

Superior long-term cycling stability of SnO2 nanoparticle/multiwalled carbon nanotube heterostructured electrodes for Li-ion rechargeable batteries

We demonstrate the fabrication of hybrid nanocomposite electrodes with a combination of SnO2 nanoparticles (NPs) and conducting multiwalled carbon nanotube (MWCNT) anodes (SnO2@CNT) through the direct anchoring of SnO2 NPs on the surface of electrophoretically pre-deposited MWCNT (EPD-CNT) networks via a metal–organic chemical vapor deposition process. This SnO2@CNT nanocomposite displays large reversible capacities of over 780, 510, and 470 mA h g−1 at 1 C after 100, 500, and 1000 cycles, respectively. This outstanding long-term cycling stability is a result of the uniform distribution of SnO2 NPs (∼8.5 nm), a nanoscale EPD-CNT network with good electrical conductivity, and the creation of open spaces that buffer a large volume change during the Li-alloying/dealloying reaction of SnO2.

Controlling the dynamic percolation of carbon nanotube based conductive polymer composites by addition of secondary nanofillers: The effect on electrical conductivity and tuneable sensing behaviour

Abstract In this paper, the electrical properties of ternary nanocomposites based on thermoplastic polyurethane (TPU) and multi-walled carbon nanotubes (MWCNTs) are studied. In particular two nanofillers – differing in shape and electrical properties – are used in conjunction with MWCNTs: an electrically conductive CB and an insulating needle-like nanoclay, sepiolite. The ternary nanocomposites were manufactured in a number of forms (extruded pellets, filaments and compression moulded films) and their morphological and electrical properties characterised as function of time and temperature. The presence of both secondary nanofillers is found to affect the formation of a percolating network of MWCNTs in TPU, inducing a reduced percolation threshold and tuneable strain sensing ability. These ternary nanocomposites can find application as conductive and multi-functional materials for flexible electronics, sensing films and fibres in smart textiles.

Electro-optical memory of a nematic liquid crystal doped by multi-walled carbon nanotubes

A pronounced irreversible electro-optical response (memory effect) has been recently observed for nematic liquid crystal (LC) EBBA doped by multi-walled carbon nanotubes (MWCNTs) near the percolation threshold of the MWCNTs (0.02-0.05 wt. %). It is caused by irreversible homeotropic-to-planar reorientation of LC in an electric field. This feature is explained by electro-hydrodynamically stimulated dispergation of MWCNTs in LC and by the formation of a percolation MWCNT network which acts as a spatially distributed surface stabilizing the planar state of the LC. This mechanism is confirmed by the absence of memory in the EBBA/MWCNT composites, whose original structure is fixed by a polymer. The observed effect suggests new operation modes for the memory type and bistable LC devices, as well as a method for \textit{in situ} dispergation of carbon nanotubes in LC cells.

Improvement of Superhydrophobicity of Multi-Walled Carbon Nanotubes Produced by Fluorination

In this work, we synthesized superhydrophobic coatings by chemical surface functionalization of multi-walled carbon nanotubes (MWCNTs). This was accomplished through the radical polymerization of 3-(trimethoxysilyl) propyl methacrylate modified MWCNTs and fluoro acrylate/methyl methacrylate. The chemical compositions and microstructures of the prepared MWCNT surface were investigated using X-ray photoelectron spectroscopy, Fourier transform infrared spectrometry, and scanning electron microscopy, respectively. The wettability of the MWCNTs surface was determined through contact angle assessments in different liquids. The resulting surface exhibited a water contact angle of <TEX>$157.7^{\circ}$</TEX>, which is clear evidence of its superhydrophobicity. The 3D MWCNT networks and the low surface energy of the -C-C- and -C-F- groups play important roles in creating the superhydrophobic surface of the MWCNTs.