Formation Of Multi-walled Carbon Nanotubes Research Articles

Effects of catalysts state on the synthesis of MWCNTs modified expanded graphite through microwave-assisted pyrolysis of ethanol

Multi-walled carbon nanotubes (MWCNTs) were prepared through microwave-assisted pyrolysis of ethanol to enhance the electron transfer of expanded graphite (EG). Three kinds of Fe-containing catalyst precursors (ferric nitrate, citric acid-enhanced ferric nitrate and ferrocene) were designated in the present work to investigate the influences of catalyst states on the formation of MWCNT. The cyclic voltammograms and electrochemical impedance of the prepared specimens were characterized. Furthermore, the microstructures of catalyst-loaded EG before and after microwave treatment were investigated by means of SEM and TEM. The results show that the growth of MWCNT under microwave treatment is determined by the state of catalyst and only nanoscaled Fe-containing catalysts favor the formation of MWCNT. The ferric nitrate tends to grow up into submicron Fe2O3 particles, which impedes the growth of MWCNT. In comparison, citric acid enhances the dispersion of ferric nitrate, allowing the growth of MWCNT. Ferrocene possesses dual functions, namely it assures well distribution of nanoscaled catalysts and is also responsible for the growth of first generation of MWCNT. With the growth of MWCNT on EG, the anodic peak current densities and charge transfer resistance of the MWCNT modified EG outperform those of pure EG remarkably.

Multiwalled carbon nanotube forests grown on the surface of synthetic diamond crystals

Abstract Forests of multiwalled carbon nanotubes (MWCNTs) were grown on the facets of synthetic diamond crystals by treating them in a rotary chemical vapor deposition reactor. Nickelocene was used as a source of both nickel catalyst and carbon for the formation of MWCNTs. The diamond crystals were not subjected to any surface treatment prior to the MWCNT growth. The MWCNTs were anchored in nickel particles adhered to the diamond facets and were partially filled with nickel. These morphological features of the diamond-MWCNT structures together with nanotube branching suggest the formation of MWCNTs by the combined growth mechanism.

Sequestration of Methylene Blue and Lead ions by MWCNT Modified with Polyconducting Polymers

Taking into consideration the perspective of green chemistry, an attempt was made to investigate regenerative material for carbon nanotubes (CNTs) with high productivity. The special properties of CNTs, such as easy functionalization ability, high surface area, and unique thermal, mechanical, and electrical properties, make them an appealing component for composite materials, which resulted in the attraction of great interest on CNT-based composites. This research work focuses mainly on multiwall carbon nanotubes (MWCNTs) synthesized from camphor over iron nitrate catalyst supported on alumina by wet impregnation method. The formation of MWCNTs and incorporation of poly conducting polymers (o- and m-Toluidine) on the surface of MWCNTs was confirmed by high resolution transmission electron microscope (HRTEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR), Ultraviolet visible spectroscopy (UV-Vis), low temperature nitrogen adsorption and Thermo-Gravimetric analysis (TGA). The performance evaluation of as-synthesized MWCNTs and their nanocomposites towards adsorption of methylene blue (MB) dye and Pb(II) was also highlighted. MWCNTs and their nanocomposites pose a great potential as a promising material for application in adsorption from aqueous solution.

Enhanced electromagnetic interference shielding effectiveness of chemical vapor deposited MWCNTs in X-band region

Properties of multi-walled carbon nanotubes (MWCNTs) synthesized by chemical vapor deposition technique using natural precursor camphor have been reported. Peaks observed at 2θ = 25.6° and 43.5° are assigned to the graphitic phase of carbon with (002) and (100) diffraction planes. The morphological investigation demonstrates the formation of MWCNTs at an optimum temperature of 900 °C. After 900 °C temperature, diameter distribution of MWCNTs increases drastically. TEM observations reveal that at 900 °C, the diameter of MWCNTs is minimum and they are contaminated with very few metal catalyst particles. G-band centered at 1594 cm− 1 in the Raman spectrum indicates the presence of a hexagonal graphitic structure of MWCNTs. The microwave properties have been carried out in the 8–12 GHz frequency region. MWCNTs grown at 900 °C shows the high reflection loss (RL) value of −33.56 dB (>99.9% power absorption) at 11.8 GHz however, they exhibit maximum SE value of −65.22 dB at 8 GHz and − 46.27 dB at 10.19 GHz.

Segregated network formation of multiwalled carbon nanotubes in natural rubber through surfactant assisted latex compounding: A novel technique for multifunctional properties

In the present study Vulcastab VL (polyethylene oxide condensate) has been used as a non-ionic surfactant to get stable aqueous dispersions of multiwalled carbon nanotubes (MWCNTs). Sonication time and surfactant concentration were optimised by UV-vis spectroscopy and dynamic light scattering technique. Homogeneously dispersed surfactant coated individual MWCNTs in water were clearly evident from Transmission electron microscopy (TEM). Natural rubber –Multiwalled carbon nanotube (NR-MWCNT) nanocomposites were prepared by latex stage mixing. Instead of being randomly dispersed, MWCNTs were found to retain at the boundary of rubber particles resulting in a segregated network as evidenced from TEM. Addition of very small amount of MWCNT showed remarkable improvement in mechanical, thermal, electrical and barrier properties of NR. Segregated network of CNTs resulted in very low electrical percolation threshold (0.043 vol%) for the composite with enhanced dielectric properties.

An optimized process for in situ formation of multi-walled carbon nanotubes in templated pores of polymer-derived silicon oxycarbide

A reliable and optimized process to grow carbon nanotubes (CNTs) in templated pores of polymer derived ceramic (PDC) matrix was developed. It is realized through the pyrolysis of a preceramic polymer, i.e., poly (methyl-phenyl-silsesquioxane) (denoted as PMPS), in argon atmosphere at 1000°C together with nickel-catalyst-coated poly-methyl-methacrylate (PMMA) microbeads (denoted as PMMA-Ni). PMPS served as both a precursor for the ceramic matrix and a carbon source for the CNT growth. PMMA microbeads were used as sacrificial pore formers and coated with nickel via an electroless plating method, which provides an improved control of particle size of the catalyst and its distribution in the material. The influence of PMMA-Ni loading on the in situ growth of CNTs and the properties of CNTs/SiOC nanocomposites were studied through thermogravimetric analysis (TGA), scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, and density/porosity measurements. Under optimized conditions, uniform distribution of in situ grown CNTs was observed within the templated pores of the SiOC matrix. The optimized process leads to reproducible high yield of CNTs in the pores. The development of such novel CNT/cellular ceramic nanocomposite materials is of significant interest for a variety of sensor applications.

Catalytic decomposition of methane over Ni/SiO2: influence of Cu addition

The influence of Cu addition to Ni/SiO2 catalysts was studied in the catalytic decomposition of methane. Samples were prepared by impregnation and characterized by SBET, TPR, TPO, XRD and Raman spectroscopy. The activity tests were performed at temperatures ranging from 500 to 700 °C in a fixed bed reactor with in-line gas chromatography analysis. The presence of copper had an effect on the SBET of calcined samples, improving the dispersion and decreasing the reduction temperature of NiO particles. The addition of small amounts of copper facilitates the activation of NiO with methane and allows obtaining small crystallites of Ni°, which prevents the catalyst deactivation by sintering in temperatures above 500 °C. The characterization of carbon deposited on Cu-containing samples indicated the formation of multi-walled carbon nanotubes. The sample containing 1 wt% of Cu has the best performance for the production of hydrogen from methane decomposition.

Sustained arc temperature: better marker for phase transformation of carbon black to multiwalled carbon nanotubes in arc discharge method

The present study investigates the role of temperature in the formation of multiwalled carbon nanotubes from carbon black using arc discharge technique. Carbon black has been used as precursor to synthesize carbon nanotubes in argon atmosphere. The arc current has been varied from 25 to 40 A in order to study the morphological changes in carbon black as it gets converted to multi walled carbon nanotubes (MWCNTs). We observed formation of MWCNTs at an arc current of 25 A; however the recorded temperature data suggested correlation of sustained arc temperature with the nanotube formation rather than the magnitude of current in its absoluteness. Interesting to note is that reported current magnitude in published literature are very high (>40 A) for conversion of carbon black but the present investigation shows that it is possible to convert the carbon black to MWCNTs even at lower current values in case the arc temperature is stabilized and sustained for longer period. Detailed investigations suggested that a sustained stable critical temperature of 1400 °C–1600 °C is essential for the growth of nanotubes and an unstable arc causing temperature fluctuation from critical temperature value yields very low or no MWCNTs.

Methane-induced Activation Mechanism of Fused Ferric Oxide-Alumina Catalysts during Methane Decomposition.

Activation of Fe2 O3 -Al2 O3 with CH4 (instead of H2 ) is a meaningful method to achieve catalytic methane decomposition (CMD). This reaction of CMD is more economic and simple against commercial methane steam reforming (MSR) as it produces COx -free H2 . In this study, for the first time, structure changes of the catalyst were screened during CH4 reduction with time on stream. The aim was to optimize the pretreatment conditions through understanding the activation mechanism. Based on results from various characterization techniques, reduction of Fe2 O3 by CH4 proceeds in three steps: Fe2 O3 →Fe3 O4 →FeO→Fe0. Once Fe0 is formed, it decomposes CH4 with formation of Fe3 C, which is the crucial initiation step in the CMD process to initiate formation of multiwall carbon nanotubes.

Yield optimization of nanocarbons prepared via chemical vapor decomposition of carbon dioxide using response surface methodology

In this study, the effect of temperature, carbon source flow rate, and catalyst loading on the yield and morphology of the carbon nanotubes obtained via the direct decomposition of CO2 in the chemical vapor decomposition (CVD) method has been investigated. An experimental design based on response surface methodology (RSM) was employed to investigate and optimize the effect of the reaction parameters on the yield of the obtained nanocarbons, and the most significant factor was found to be the reaction temperature. The carbon deposition amount was measured using the energy-dispersive X-ray spectroscopy (EDS) results. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) micrographs were used to illustrate the structure and morphology of the obtained nanocarbons. The crystallography of the obtained nanocarbons at various temperatures was characterized by the X-ray diffraction (XRD) patterns. Raman spectrum of the obtained CNTs showed that the ratio of the intensity of the G band to D band (IG/ID) for the CNTs obtained at 1100°C is equal to 0.8 which is indicative of a crystalline structure with few defects. Moreover, the formation of multi-walled carbon nanotubes (MWCNTs) was confirmed using thermogravimetric analysis (TGA).

Influence of activated charcoal on the structural and morphological characteristics of ceramic based on silicon oxycarbide (SiOC): A promising approach to obtain a new electrochemical sensing platform

This study deals with the preparation of ceramic materials based on silicon oxycarbide (SiOC), by pyrolysis of polymeric precursors consisting of poly(methylsiloxane) (PMS) and divinylbenzene (DVB), with or without activated charcoal (AC) as additional carbon source. The precursors were obtained by hydrosilylation reaction catalyzed by Pt(0)-complex at 80:20 and 20:80 (PMS:DVB) molar ratios, containing or not 3 wt.% AC. SiOC-based materials were obtained by pyrolysis at 1000 and 1500 °C, under argon atmosphere, and characterized by FT-IR, Raman, 29Si MAS NMR, XRD, elemental analysis, SEM and HRTEM techniques. The presence of AC in PMS-rich ceramic produced carbon long nanowires, and possibly SiC nanowires. The increase of the temperature induced the phase segregation, with the predominance of SiO2 and/or SiC phases, besides having influence on the crystallization of β-SiC and organization of segregated carbon phase (Cfree), with the formation of multiwall carbon nanotubes (MWCNT). The ceramics containing AC were tested as a new electrochemical sensing platform for the detection of acetaminophen by cyclic voltammetry. Higher peak current and lower over potential for acetaminophen, as compared with glassy carbon electrode, were observed by using the ceramic obtained at 1500 °C containing MWCNT as electrodic material, which clearly indicates the great electrocatalytic effect of material.

Enhanced mechanical performance of Al2O3-C refractories with nano carbon black and in-situ formed multi-walled carbon nanotubes (MWCNTs)

The mechanical performance of low-carbon Al2O3-C refractories can be further enhanced using the in-situ formation approach of MWCNTs. Evidently, the nickel nitrate catalyst in the matrix benefit the improved mechanical properties of Al2O3-C refractories after coking at elevated temperatures. The in-situ formation of MWCNTs and SiC whiskers contributes to their higher cold modulus of rupture. Moreover, dispersing catalyst into the binder solution promotes a fabric-like structure interlaced through MWCNTs and SiC whiskers in the binder system and thus a better bonding strength can be achieved. Compared to the Al2O3-C refractories with a close starting carbon content fabricated by adding as-received MWCNTs or the combined carbon sources of nano carbon black and flake graphite, superior thermal shock resistances were attained with the approaches presented in this paper. Additionally, MWCNTs are more influential to the toughening and SiC whiskers are more responsible for the strengthening behaviors.

A contribution from dielectric analysis to the study of the formation of multi-wall carbon nanotubes percolated networks in epoxy resin under an electric field

The formation of percolation networks in epoxy matrix nanocomposites reinforced with multi-wall carbon nanotubes (MWNT) during the curing process, at different MWNT contents, was studied by using a parallel plate cell subjected to a 300 V/cm AC electric field at 1 kHz. The percolation was verified by the electrical current output measured during and after the resin curing. The behavior of electric dipoles was characterized by impedance spectroscopy and followed the Debye first order dispersion model, by which an average relaxation time of 6.0 × 10−4 s and a cut-off frequency of 1.7 kHz were experimentally found. By applying the theory of percolation, a critical probability, pc, equal to 0.038 vol% and an exponent of conductivity of 2.0 were found. Both aligned and random samples showed dipole relaxation times typical of interfacial and/or charge-hopping polarization, while the permittivity exhibited an exponential decrease with frequency. This behavior can be related to the increased ability to trap electrical charges due to the formation of the carbon nanotubes network. Optical and electron microscopies confirm the theoretical prediction that the application of an electric field during cure helps the process of MWNT debundling in epoxy resin.

Temperature effect on the synthesis of carbon nanotubes and core–shell Ni nanoparticle by thermal CVD

The synthesis of carbon nanostructures has been studied for the past two decades, not only for their excellent electrical, physical and thermal properties but also due to their potential applications on different kinds of sciences, including medical and biological ones. In this sense, the CVD technique has shown very good results in terms of quality and quantity in the synthesis of these nanostructures. Unfortunately, some problems have been presented in the successful fabrication of carbon nanotubes and Ni core–shell nanoparticles in terms of quality, quantity and separation of both nanostructures. For this reason, in this work carbon nanostructures were synthetized by CVD utilizing CH4 as the carbon precursor gas, on Ni/SiO2 catalytic substrates with different nickel thicknesses (15, 10, 5, 2nm), at different temperatures (900°C, 800°C, 700°C). The samples were analyzed by Raman spectroscopy, SEM and TEM; the results indicate a majority formation of multi-walled carbon nanotubes (MWCNT) at lower temperatures and Ni thickness, and a formation of Ni core–shell nanoparticles at higher temperatures, leaving a total control on the process in order to obtain both nanostructures only with a few variation of the temperature and avoiding the separation post-process.

Bimetallic Fe/Mo–SiO2 aerogel catalysts for catalytic carbon vapour deposition production of carbon nanotubes

Highly porous nanocomposite aerogels based on bimetallic Fe and Mo nanoparticles with a variable Fe:Mo weight ratio ranging from 5:1; 3:1; and 0.7:1 dispersed on amorphous silica were obtained. N2 physisorption, X-ray diffraction, and transmission electron microscopy indicate that the Fe/Mo–SiO2 nanocomposite aerogels as obtained by a co-gelation sol–gel route followed by supercritical drying and reduction treatment under H2 exhibit Fe and Mo nanocrystals with size in the range 4–10 and 15 nm, respectively, supported on highly porous silica. The catalytic performance of the Fe/Mo–SiO2 aerogels for the synthesis of multi wall carbon nanotubes (MWCNT) by catalytic chemical vapour deposition (CCVD) was evaluated in terms of amount and quality of the produced CNTs as assessed by gravimetric results, thermal analysis, and TEM. The effect of catalyst composition and CCVD temperature was investigated, pointing out that high reaction temperatures (800 °C) favor the formation of MWCNTs with high quality in elevated yield, the highest C uptake value being >400 %. Catalyst composition and CCVD temperature were also found to affect the homogeneity of CNT morphology, the best MWCNT quality (with outer diameter 23–25 nm) being achieved at 800 °C with the catalyst having the largest Mo content.

Multiwall Carbon Nanotubes and Nanofibers in Gallionella

This is a report of microbial formation of multiwall carbon nanotubes (MWCNT) and nanofibers at normal pressure and temperature. Our results demonstrate a single cell organism's ability to form complicated material of high industrial interest. The microorganism, Gallionella, is classified as autotrophic and dysoxic. It uses CO2 for its carbon source and grows in environments with low concentrations of free oxygen. The organisms were taken from a deep bedrock tunnel where water leaking from cracks in the rock formed a precipitate of iron as a bacterial slime on the rock wall. Detailed investigations of the samples by transmission electron microscopy (TEM) revealed several types of MWCNT. The stalk single MWCNT was observed with a diameter of about 10 nm and with an inner diameter of 1.35 nm. The wall of the nanotube is built by graphite layers. The 10 to 20 sheets are used to form the tubes. The measured spacing between the lines is 0.34 nm, which is an average value of interlayer spacing, close to the graphitic distance (0.335 nm). HRTEM images reveal a two-dimensional lattice with a spacing of 0.24 nm, indicating the presence of graphene.

Microwave plasma CVD-grown graphene–CNT hybrids for enhanced electron field emission applications

The growth and electron emission characteristics were investigated from a hybrid structure of multiwalled carbon nanotubes (MWCNTs) and multilayer layer graphene (MLG) deposited on silicon substrate coated with iron catalyst and an interlayer of aluminium. The hybrid structures were synthesized in a two-step process by microwave plasma-enhanced chemical vapour deposition technique. The formation of MWCNTs takes place by absorption and precipitation of carbon radicals into the catalyst particles. Thereafter, ample carbon forms MLG on tip of the MWCNTs resulting in a MLG-MWCNTs hybrid nanostructure. MLG was observed to grow branching out of the tips and sidewalls of the MWCNTs and is expected to attach by Van der Walls bonds. Transmission electron microscopy and micro-Raman spectroscopy confirmed the crystalline nature of the hybrid structures. Electron emission studies were carried out using a diode-type field emission setup. The enhancement factor was found to be ~3,500 for bare MWCNTs, ~4,070 to ~5,000 for hybrid structures and ~6,500 for N-doped MLG-MWCNTs hybrid structures. Modification in the defects structure and enhancement of emission sites are suggested to be responsible for the increase of the field emission characteristics.

Oxidative heat treatment of 316L stainless steel for effective catalytic growth of carbon nanotubes

Abstract Substrates consisting of stainless steel (SS) type 316L have been typically considered as non-responsive to catalytic formation of multi-walled carbon nanotubes (MW-CNTs) because of the protective chromium oxide layers on their surfaces. In current practices lengthy multi-step pretreatments are needed to break down these protective layers. This study oxidized 316L SS wire mesh in air, at 800 °C, for time periods ranging from 1 to 20 min prior to its use as a substrate catalyst for CNT synthesis from pyrolyzate gases of polyethylene, i.e., a mixture of hydrocarbons and hydrogen. The shortest pretreatment of the substrate (1 min) produced the maximum yield of CNTs (0.0145 g/g of catalyst) in these experiments, whereas longer pretreatments resulted in progressively lower yields. The 10 min pretreatment produced MWCNT forests vertically growing in planes perpendicular to the substrate surface (the periphery of the cylindrical wires of the mesh), 20 μm long and 21 nm in diameter on the average. Shorter (1–9 min) or longer (11–20 min) substrate heat treatments did not produce aligned CNTs at this temperature. Oxidative heat treatments of the substrate lengthier than 20 min did not produced any nanotubes. The results demonstrated an expedient and cost-effective method to activate the 316L SS substrate, and resulted in controllable CNT growth.

Dispersion, migration, and ‘network-like’ structure formation of multiwall carbon nanotubes in co-continuous, binary immiscible blends of polyamide 6 and acrylonitrile-butadiene-styrene copolymer during simultaneous melt-mixing

Multiwall carbon nanotubes (MWNTs) were melt-mixed in polyamide 6 (PA6) and acrylonitrile–butadiene–styrene (ABS) copolymer blends using a simultaneous mixing protocol in order to investigate the state of dispersion of MWNTs in PA6/ABS blends. The blend composition was varied from 40/60 (wt/wt) to 60/40 (wt/wt) in PA6/ABS blends, which showed ‘co-continuous’ morphology in the presence of MWNTs. State of dispersion of MWNTs in these blends was assessed through bulk electrical conductivity measurements, morphological analysis, solution experiments, and UV-vis spectroscopic analysis. MWNTs were subsequently modified with a novel organic modifier, sodium salt of 6-aminohexanoic acid (Na-AHA), to improve the state of dispersion of MWNTs. Blends with unmodified MWNTs exhibited the DC electrical conductivity in the range ∼10−11 to ∼10−5 S/cm, whereas blends with Na-AHA-modified MWNTs exhibited DC electrical conductivity in the range ∼10−7 to ∼10−5 S/cm. The reduction in MWNTs ‘agglomerate’ size (∼73.7 μm for 40/60 blend with unmodified MWNTs to ∼59.9 μm in the corresponding blend with Na-AHA-modified MWNTs) was observed through morphological analysis. The rheological studies showed increased complex viscosity and storage moduli in lower frequency region in case of blends with Na-AHA-modified MWNTs confirming a refined ‘network-like’ structure of MWNTs. POLYM. ENG. SCI., 55:443–456, 2015. © 2014 Society of Plastics Engineers

Electrically conducting polyolefin composites containing electric field-aligned multiwall carbon nanotube structures: The effects of process parameters and filler loading

The characteristics of network formation of multiwall carbon nanotubes (MWCNTs) inside ethylene–octene copolymer (EOC) melts under an alternating current (AC) electric field and the resulting electrical conductivity improvements are studied by combining dynamic and steady state resistivity measurements. Fine MWCNT dispersion during melt compounding of the samples is accomplished by means of a novel non-specific, non-covalent functionalization method. It is found that the electrified composite films exhibit nanotube assembly into columnar structures parallel to the electric field, accompanied by dramatic increases in electrical conductivity up to eight orders of magnitude. Experimentally acquired resistivity data are used to derive correlations between the characteristic insulator-to-conductor transition times of the composites and process parameters, such as electric field strength (E), polymer viscosity (η) and nanotube volume fraction (ϕ). Finally, a criterion for the selection of (η,E,C) conditions that enable MWCNT assembly under an electric field controlled regime (i.e., minimal Brownian motion-driven aggregation effects) is developed. The correlations presented herein not only provide insights in the MWCNT assembly process, but can also guide the experimental design in future studies on electrified composites or assist in the selection of process parameters in composites manufacturing.