Growth of carbon nanotubes at low powers by impedance-matched microwave plasma enhanced chemical vapor deposition method

A solo carbon nanotube (CNT) was successfully grown on nickel electrodes by a microwave plasma enhanced chemical vapor deposition (MPECVD) method equipped with an impedance-matched substrate holder with the reaction gases composed of hydrogen (H2), carbon dioxide (CO2), and methane (CH4) mixtures. An introduction of carbon dioxide gas before CNTs growth, the substrate temperature can easily be reached above 610 degrees C even heated at a low microwave power. This can be enunciated from fact that carbon dioxide inherits with higher bond energy for molecular dissociation, lower thermal conductivity, and higher heat capacity in comparing to other gases. The electron field emissions for randomly aligned CNTs and well-aligned CNTs grown by MPECVD and by radio frequency assisted hot-filament methods, respectively, are measured and compared. The higher field emission characteristic of the randomly aligned CNTs is presumed to be due to the protruded CNTs, which inheriting with less screening effect and manifesting with defects are crucial to play the effective emission sites.     

The enhanced growth of multi-walled carbon nanotubes using an atmospheric pressure plasma jet

The growth of multi-walled carbon nanotube (MWCNT) forests was investigated using an atmospheric pressure plasma jet (APPJ) system with a mixture of helium and acetylene gases. The MWCNT forests grown on Fe catalyst were compared with those grown on Ni. The growth of MWCNT forests using Fe as the catalyst was better than the growth of MWCNT forests using Ni. The MWCNT forests grown using Fe catalyst and with a plasma power of 30 W were about 17 ± 9% taller than for the plasma off. We were unable to grow MWCNTs using Ni catalyst with the plasma power off; but curly MWCNTs were grown using Ni catalyst if the plasma power was 30 W. It is found that MWCNT growth is enhanced using an APPJ. The height of the forests produced using this APPJ system was also better than that reported by other researchers using either CVD or PECVD systems.     

Biocompatibility of multi-walled carbon nanotubes grown on titanium and silicon surfaces

Cell adhesion and cell viability of aligned multi-walled carbon nanotube (MWCNT) films were verified using Fibroblast L929 mouse cells. The MWCNTs were produced by a microwave plasma chemical vapor deposition (2.45 GHz) on silicon (Si), with a nickel catalyst, and titanium (Ti), with an iron catalyst. MTT assay and cellular adhesion were used for biocompatibility tests (ISO 10993-5). The results show very high cell viability and many layers of cells adhered on the surface formed by the nanotube tips at films grown on silicon surfaces. The MWCNT grown on Ti surfaces presented lower cell viability and a reduced number of cells on the surface formed by the nanotube tips. The different behavior is most probably related to excess iron contamination present in the case of titanium substrate, while nickel catalyst is probably enclosed by the nanotubes.     

Reinforcement effects of MWCNT and VGCF in bulk composites and interlayer of CFRP laminates

The reinforcement effects of two nanofillers, i.e., multi-walled carbon nanotube (MWCNT) and vapor grown carbon fiber (VGCF), which are used at the interface of conventional CFRP laminates, and in epoxy bulk composites, have been investigated. When using the two nanofillers at the interface between two conventional CFRP sublaminates, the Mode-I interlaminar tensile strength and fracture toughness of CFRP laminates are improved significantly. The performance of VGCF is better than that of MWCNT in this case. For epoxy bulk composites, the two nanofillers play a similar role of good reinforcement in Young’s modulus and tensile strength. However, the Mode-I fracture toughness of epoxy/MWCNT is much better than that of epoxy/VGCF.     

Effect of MWCNT Inclusion in TiO2 Nanowire Array Film on the Photoelectrochemical Performance

Rutile TiO2 nanowire array films with multi-walled carbon nanotube(MWCNT) inclusion perpendicularly grown on fluorine-doped tin oxide(FTO) substrate were prepared by a facile hydrothermal method.The absorption edges of the TiO2 nanowire array films are blue-shifted with increasing MWCNT content.The resistance of the TiO2 nanowire array film is decreased by MWCNT inclusion.The optimum TiO2 /MWCNT molar ratio in the feedstock is 1:0.1.For the TiO2 nanowire array film with MWCNT inclusion served as electrode in dye-sensitized solar cell(DSSC),an overall 194% increase of photoelectric conversion efficiency has been achieved.     

Assembly and Electrical Characterization of Multiwall Carbon Nanotube Interconnects

We demonstrate a general method to assemble and contact arrays of individual multiwall carbon nanotube (MWCNT) interconnects between electrodes in one batch. We have also collected about 200 resistance measurements to compare four different contact metals: Al, Au, Ti, and Pd. We have also measured the radio frequency characteristics of the assembled MWCNTs up to 15 GHz. High-resolution transmission electron microscopy analysis has been used to correlate the electrical and physical characteristics of the MWCNTs.      

Synthesis of polyaniline/multi-walled carbon nanotube nanocomposites in water/oil microemulsion

A water/oil microemulsion system having been successfully used for synthesizing polyaniline(PANi) nanoparticles, was employed for preparing PANi/multi-walled carbon nanotube (MWCNT) nanocomposites via in situ chemical oxidative polymerization. The structures and the electrical property of PANi/MWCNT nanocomposites were also studied. The studies showed that PANi could coat MWCNTs to form nanocables with core-shell structure, and the backbone structure of PANi was not damaged by the introduction of MWCNTs. The conductivities of PANi/MWCNT nanocomposites were higher than that of PANi. Moreover, a model was supposed to be used for describing a PANi/MWCNT nanocable formation by in situ microemulsion polymerization.     

Ballistic-performance optimization of a hybrid carbon-nanotube/E-glass reinforced poly-vinyl-ester-epoxy-matrix composite armor

The material model for a multi-walled carbon nanotube (MWCNT) reinforced poly-vinyl-ester-epoxy matrix composite material (carbon nanotube reinforced composite mats, in the following) developed in our recent work (M. Grujicic et al. submitted), has been used in the present work within a transient non-linear dynamics analysis to carry out design optimization of a hybrid polymer-matrix composite armor for the ballistic performance with respect to the impact by a fragment simulating projectile (FSP). The armor is constructed from E-glass continuous-fiber poly-vinyl-ester-epoxy matrix composite laminas interlaced with the carbon nanotube reinforced composite mats. Different designs of the hybrid armor are obtained by varying the location and the thickness of the carbon nanotube reinforced composite mats. The results obtained indicate that at a fixed thickness of the armor, both the position and the thickness of the carbon nanotube reinforced composite mats affect the ballistic performance of the armor. Specifically, it is found that the best performance of the armor is obtained when thicker carbon nanotube reinforced composite mats are placed near the front armor face, the face which is struck by the projectile. The results obtained are rationalized using an analysis of the elastic wave reflection and transmission behavior at the lamina/met and laminate/air interfaces.     

Fabrication of multi-walled carbon nanotube–carbon fiber hybrid material via electrophoretic deposition followed by pyrolysis process

An integrated multi-walled carbon nanotube (MWCNT)–carbon fiber (CF) hybrid material has been fabricated by electrophoretic deposition of acid-functionalized MWCNTs on CF surface followed by soaking in a 10% solution of petroleum pitch in toluene, followed by pyrolysis in a nitrogen atmosphere. It has been revealed that MWCNTs entirely covered the CF surface. Mechanical properties of composites reinforced by MWCNT–CF hybrids were considerably enhanced (up to 120% in tensile strength and 100% in elastic modulus) compared to composites reinforce by as-received CFs. According to fractography observations, robust interlocking occurred between epoxy matrix and MWCNT–CF hybrids.     

Growth of carbon nanotubes on glass substrate by MPECVD

We have grown carbon nanotubes (CNTs) with a microwave plasma-enhanced chemical vapor deposition (MPECVD) method, which has been regarded as one of the most promising candidates for the synthesis of CNTs due to the vertical alignment, the low temperature and the large area growth. We had used methane (CH₄) and hydrogen (H₂) gas for the growth of CNTs. 10-nm-thick Ni catalytic layer were deposited on the Ti-coated glass substrate by RF magnetron sputtering method. In this work, we report the low-temperature growing properties of the CNTs on glass substrate with a MPECVD. We have pretreated the Ni/Ti/glass catalytic layer in different microwave power (600, 700, 800, and 900 W) and grown the CNTs in the same microwave power (800 W). SEM (Scanning electron microscopy) images of the Ni catalytic layer shows the diameter and density variation to be dependent with the pretreatment conditions. Raman spectroscopy of CNTs shows that the synthesized CNTs were multi-wall CNTs.     

Ammonia borane confined by poly(methyl methacrylate)/multiwall carbon nanotube nanofiber composite,as a polymeric hydrogen storage material

In this work, poly(methyl methacrylate)/ammonia borane/multiwall carbon nanotube (PMMA/AB/MWCNT) nanofiber composites have been fabricated and the synergetic nanoconfinement effect of nanofiber and CNT components on dehydrogenation temperature and liberating unwanted byproducts of AB (NH₃BH₃) have been studied. The results of dehydrogenation of PMMA/AB and PMMA/AB/MWCNT samples show 112 and 85 °C exothermic reaction temperatures, which are dramatically lower than pure AB (120 °C). Furthermore, by capture and interaction of AB molecules in the MWCNT and PMMA nanofiber structures, the enthalpy of exothermic decomposition decreases from ?21.00 to ?1.83 kJ mol^?1 H₂, suggesting that this type of AB nanofiber composite can provide a convenient reversible hydrogen storage material. The utilization of MWCNT as carbon catalyst and confining of AB result in a decrease of ammonia borane weight loss from 60.00 to 2.88 wt% which in turn can vigorously decline the emission of byproduct impurities. The synthesis process of PMMA/AB/MWCNT nanofiber composites causes the crystal structure of AB particles changed to the amorphous structure which has been clearly confirmed by X-ray diffraction analyses. The strategy of combining nanofiber structure and MWCNT as carbon catalyst with AB particles can be presented as a practicable solution to reach lower operational temperature and to decline undesirable volatile products.     

Sn/SnO2/Mwcnt composite anode and electrochemical impedance spectroscopy studies for Li-ion batteries

Tin/tinoxide/multi-walled carbon nanotube (Sn/SnO₂/MWCNT) core-shell structure nanocomposite anode is produced by thermal evaporation and subsequent plasma oxidation with using MWCNT buckypaper. Metallic tin is evaporated onto free-standing and flexible MWCNT buckypaper having controlled porosity and subsequent RF plasma oxidized in Ar:O₂(1:1) gas mixture. X-ray diffraction and scanning electron microscopy are used to determine the structure and morphology of the obtained nanocomposite. The electrochemical characteristics of the nanocomposite anode are examined by using electrochemical impedance spectroscopy and galvanostatic charge–discharge experiments. Family of Nyquist plots during first discharge process are obtained and studied at different voltage values.     

Arc process parameters for single-walled carbon nanotube growth and production: experiments and modeling

Collarets rich in single-walled carbon nanotubes (SWCNTs) have been grown using a direct current arc method. Arc process parameters such as current, pressure, and anode to cathode distance were varied experimentally and by modeling to provide an optimal working window. The best collaret yields were obtained when helium was used as a buffer gas. Mixing helium with argon in the buffer permits controlling nanotube diameters. In addition to an experimental study, a modeling approach was developed assuming local thermal equilibrium and homogenous and heterogeneous neutral chemistry. The gas-phase chemical model involves 81 neutral carbon species (C1, C2, . . ., C79, C60F, C70F) and 554 reactions with rates taken from data of Krestinin and Moravsky. Axial profiles of temperature, C atom, C2 radical, and fullerene distributions in the reactor are predicted as a function of process parameters. Carbon nanotube growth is considered by a set of surface reactions simulating open nanotube growth. Because nanotube surface chemistry is controlled by the local terminated bond and not by the bulk nanotube bond, a mechanistic approach based on the formal resemblance between the bonding and the structure of open nanotube and other carbon surfaces is proposed to explain nanotube growth. Predicted growth rates are in the range of 100 to 1000 microm/min.     

Vapor sensing performances of PVDF nanocomposites containing titanium dioxide nanotubes decorated multi-walled carbon nanotubes

Inorganic nanocarbon hybrid materials are good alternatives for superior electrochemical performance and specific capacitance to their traditional counterparts. Nanocarbons act as a good template for the growth of metal nanoparticles on it and their hybrid combinations enhance the charge transport and rate capability of electrochemical materials without sacrificing the specific capacity. In this study, titanium dioxide nanotubes (TNT) are synthesized hydrothermally in the presence of multi-walled carbon nanotubes (MWCNT) where the latter acts as base template material for the metal oxide nanotube growth. The MWCNT–TNT hybrid material possesses very high dielectric strength and this is used to enhance the dielectric property of the polymer polyvinyledene fluoride (PVDF). Solution mixing was used to prepare the PVDF/MWCNT–TNT nanocomposites by varying the filler concentrations from 0.5 to 2.5 wt%. Excellent vapor sensing was noticed for the PVDF nanocomposites with different rate of response towards commonly used laboratory solvents. The composites and the fillers were characterized for its morphology and structural properties using scanning and transmission electron microscopy, X-ray diffraction studies and infrared spectroscopy. Vapor sensing was measured as relative resistance variations against the solvent vapors, and the dielectric properties of the composites were measured at room temperature during the frequency 10²–10⁷ Hz. Experimental results revealed the influence of filler synergy on the properties of PVDF and the enhancement in the solvent vapor detectability and dielectric properties reflects the ability of these composite films in flexible vapor sensors and in energy storage.     

Ternary nanocomposite designed by MWCNT backbone PPy/Pd for efficient catalytic approach toward reduction and oxidation reactions

An induced active palladium nanoparticle (Pd NP) on multiwall carbon nanotube with polypyrrole matrix (MWCNT/PPy/Pd) was prepared and their physiochemical properties were investigated. The XPS (X-ray photoelectron spectroscopy) analysis showed the important amine and imine linkage at 400.3?eV and 398.8?eV for the interaction of the polymer matrix. FTIR spectroscopy proved the polypyrrole stretching frequencies and palladium nitrogen interaction (Pd-N at 1618?cm^?1). TGA-DSC analysis showed that 51% of polymer composite remained up to 1000?°C. The developed nanocomposite (NC) catalytic ability was tested in p-nitrophenol reduction and methanol oxidation studies. The kinetic rate constant (k) was 0.193?min^?1 and NC exhibited good reusability up to 10 cycles for p-nitrophenol reduction. The electrochemical measurements indicate that MWCNT/PPy/Pd has a significantly high active surface area (3.13?cm^?2), low onset potential and high current density. MWCNT/PPy/Pd ternary NC is a capable candidate for both electro-oxidation of methanol and a reduction of 4-nitrophenol.     

Effects of feed gas composition and catalyst thickness on carbon nanotube and nanofiber synthesis by plasma enhanced chemical vapor deposition

Many engineering applications require carbon nanotubes with specific characteristics such as wall structure, chirality and alignment. However, precise control of nanotube properties grown to application specifications remains a significant challenge. Plasma-enhanced chemical vapor deposition (PECVD) offers a variety of advantages in the synthesis of carbon nanotubes in that several important synthesis parameters can be controlled independently. This paper reports an experimental study of the effects of reacting gas composition (percentage methane in hydrogen) and catalyst film thickness on carbon nanotube (CNT) growth and a computational study of gas-phase composition for the inlet conditions of experimentally observed carbon nanotube growth using different chemical reaction mechanisms. The simulations seek to explain the observed effects of reacting gas composition and to identify the precursors for CNT formation. The experimental results indicate that gas-phase composition significantly affects the synthesized material, which is shown to be randomly aligned nanotube and nanofiber mats for relatively methane-rich inlet gas mixtures and non-tubular carbon for methane-lean incoming mixtures. The simulation results suggest that inlet methane-hydrogen mixture coverts to an acetylene-methane-hydrogen mixture with minor amounts of ethylene, hydrogen atom, and methyl radical. Acetylene appears to be the indicator species for solid carbon formation. The simulations also show that inlet methane-hydrogen mixture does not produce enough gas-phase precursors needed to form quality CNTs below 5% CH4 concentrations in the inlet stream.     

Electrical conductivity and shielding effectiveness of poly(trimethylene terephthalate)/multiwalled carbon nanotube composites

Poly(trimethylene terephthalate) (PTT)/multiwalled carbon nanotube (MWCNT) composites have been fabricated to evaluate the potential of PTT composites as electromagnetic interference (EMI) shielding material. The room temperature electrical conductivity, complex permittivity, and shielding effectiveness (SE) of PTT/MWCNT composites were studied in the frequency range of 8.2–12.4 GHz (X-band). The dc conductivity (σ) of composites increased with increasing MWCNT loading and a typical percolation behavior was observed at 0.48 vol% MWCNT loading. The highest EMI SE of PTT/MWCNT composites was ~23 decibel (dB) at 4.76 vol% MWCNT loading which suggest that these composites can be used as light weight EMI shielding materials. The correlation among the SE, complex permittivity, and electrical conductivity was also studied. The EMI shielding mechanism of PTT/MWCNT composites was studied by resolving the total EMI SE into absorption and reflection loss.     

CoS-carbon nanotube heterostructure: one-step synthesis and optical properties

This article reports on the synthesis, characterization, and optical properties of a cobalt sulphide (CoS) quantum dot (QD)-decorated multi-walled carbon nanotube (MWCNT) heterostructure. A novel one-pot chemical-solution route has been used for the in situ synthesis of a CoS-decorated MWCNT heterostructure without disturbing the inherent structure of the MWCNTs. The synthesized heterostructure has been extensively characterized by scanning electron microscopy, scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and Fourier transform infrared spectrometry. The UV-absorption and fluorescence spectral properties of pristine MWCNTs are significantly improved after heterostructure formation with CoS-QDs.     

Conducting polymer‐coated, palladium‐functionalized multi‐walled carbon nanotubes for the electrochemical sensing of hydroxylamine

Electrochemical sensors of hydroxylamine were fabricated on glassy carbon electrodes (GCEs) by the electropolymerization of 3,4‐ethylenedioxypyrrole (EDOP) and 3,4‐ethylenedioxythiophene (EDOT) on palladium (Pd) nanoparticles attached to thiolated multi‐walled carbon nanotubes (MWCNTs), denoted as PEDOP/MWCNT‐Pd/GCE and PEDOT/MWCNT‐Pd/GCE. The sensors were characterized by field emission scanning electron microscopy and electrochemical impedance spectroscopy. They showed strong catalytic activity toward the oxidation of hydroxylamine. Cyclic voltammetry and amperometry were used to characterize the sensors' performances. The detection limits of hydroxylamine by PEDOP/MWCNT‐Pd/GCE and PEDOT/MWCNT‐Pd/GCE were 0.22 and 0.24 μM (S/N = 3), respectively. The sensors' sensitivity, selectivity, and stability were also investigated.     

Growth of metal-free carbon nanotubes on glass substrate with an amorphous carbon catalyst layer

We have investigated the direct growth of metal-free carbon nanotubes (CNTs) on glass substrates with microwave-plasma enhanced chemical vapor deposition (MPECVD). Amorphous carbon (a-C) films were used as a catalyst layer to grow metal-free CNTs. The a-C films were deposited on Corning glass substrates using RF magnetron sputtering with the use of a carbon target (99.99%) at room temperature. They were pretreated with hydrogen plasma using a microwave PECVD at 600 degrees C. Then, CNTs were prepared using microwave PECVD with a mixture of methane (CH4) and hydrogen (H2) gases. The CNTs were grown at different substrate temperatures (400 degrees C, 500 degrees C, and 600 degrees C) for 30 minutes. Other conditions were fixed. The growth trends of CNTs against substrate temperature were observed by field emission scanning electron microscopy (FE-SEM). The structure of a-C catalyst layer and grown CNTs were measured by Raman spectroscopy. High-resolution transmission electron microscopy (HR-TEM) images showed that the CNTs had bamboo-like multi-walled structures. Energy dispersive spectroscopy (EDS) measurements confirmed that the CNTs consisted of only carbon.