As-grown Multi-walled Carbon Nanotubes Research Articles

Preparation of diameter-controlled free-standing MWCNT membranes and their application for dye adsorption.

The synthesis of multi-walled carbon nanotubes (MWCNTs) was carried out over different Ni-loaded metallic oxide catalyst nanoparticles and under different reduction times to control the outside diameter of the nanotubes. Moreover, high-purity, free-standing membranes were fabricated by a simple filtration of the as-grown MWCNTs. Furthermore, the dye-adsorption properties of the nanotubes depended on the diameter of the carbon nanotubes (CNTs). The adsorption isotherms and kinetics of anionic dyes could be described by Freundlich and pseudo-second-order models, respectively. Thermodynamic studies suggested that the adsorption processes were spontaneous and exothermic. This work provides new insights into the synthesis and application of MWCNTs with the selective adsorption properties of carbon-based materials for the removal of organic dyes.

Metal-organic coordination polymer-derived carbon nanotubes: Preparation and application in detecting small molecules

Two new isostructural metal–organic coordination polymers (MOCPs) based on the bis-pyridyl-bis-amide ligand and thiophene dicarboxylic acid, namely [Cd(3-dpaa)0.5(2,5-TDC)(H2O)2] (1) and [Zn(3-dpaa)0.5(2,5-TDC)(H2O)2] (2) (3-dpaa = N,N′-di(3-pyridyl)adipoamide and 2,5-H2TDC = 2,5-thiophenedicarboxylic acid), have been hydrothermally synthesized and structurally characterized. The fluorescent sensing behaviors of complex 2 have been studied. Moreover, complex 2 was used as a combined catalyst precursor to synthesize multi-walled carbon nanotubes (MWCNTs). The decomposition of ethylene in the presence of the Zn-MOCP by the chemical vapor deposition (CVD) method at 650 °C led to successful obtain as-grown MWCNT products. Interestingly, the as-grown MWCNTs exhibit significant fluorescent sensing activities towards xylene and nitrobenzene.

From coordination polymer to carbon nanotube: Preparation, characterization and rapid adsorption capacity toward organic pollutants

A CuI coordination polymer based on the N,N’-bis(3-pyridinecarboxamide)-1,4-butane (3-dpyb) ligand, namely [Cu(3-dpyb)0.5Cl], is hydrothermally synthesized and structurally characterized, and is used as a catalyst precursor to synthesize multi-walled carbon nanotubes. Interestingly, the as-grown multi-walled carbon nanotubes exhibit high performance in removing dyes from solution and can serve as a low-cost and fast adsorbent. In addition, the adsorption behavior of this new adsorbent fits well with the Freundlich isotherm and the pseudo-second-order kinetic model.

A novel design for mass production of multi-walled carbon nanotubes using Co-Mo/MgO catalyst via pyrolysis of polypropylene waste: effect of operating conditions

A two-stage process was employed for high yield production of multi-walled carbon nanotubes (MWCNTs) via pyrolysis of polypropylene (PP) waste. In the first stage, a new design was used for the pyrolysis of PP waste at 500 °C to form a mixture of condensable hydrocarbons (≥C6) and non-condensable gases (C1–C5) inside a vertical reactor closed from the bottom and connected directly to a vertical condenser at the top. This pyrolysis technique permitted to form a large amount of non-condensable hydrocarbon gases, which were used in the second stage as a carbon source for the production of MWCNTs over Co-Mo/MgO catalyst. The influences of growth temperature (700–850 °C) and carrier gas flow rate of N2 (50–110 sccm) on the yield and morphology of as-deposited MWCNTs have been investigated. The fresh Co-Mo/MgO catalyst and the as-deposited carbon were characterized by XRD, FTIR, TPR, BET surface properties, TEM, Raman spectroscopy and TGA. The results demonstrated that the adjustment of growth temperature and N2 flow-rate caused a marked impact on the yield, type and quality of as-grown MWCNTs. The optimum MWCNTs yields of 32.6 and 38.3 g/gcatalyst have been achieved at the growth temperature of 800 °C and the carrier gas flow rate of 90 sccm, respectively. TEM images illustrated the formation of pure MWCNTs at the growth temperature range of 700–800 °C, whereas mixed materials of MWCNTs and graphene nanosheets (GNSs) were obtained at the growth temperature of 850 °C. Raman spectroscopy illustrated that highly graphitized and crystalline MWCNTs were produced at all operating conditions. TGA proved that all MWCNTs samples exhibited higher thermal stability.

Sorption of Bisphenol A in Aqueous Solutions on Irradiated and as-Grown Multiwalled Carbon Nanotubes

In this study, non-irradiated and weathered multiwalled carbon nanotubes (MWCNTs) obtained through irradiation, were studied as adsorbents for BPA, both nanomaterials being characterized before and after the adsorption process. The objectives of our investigation were to compare the characteristics of non-irradiated and irradiated MWCNTs, to evaluate the adsorption capacity of BPA by pristine and irradiated MWCNTs and to determine the variation of the kinetic, sorption and thermodynamic parameters during sorption process using both sorbents.

Work function and electrical properties of individual multiwalled carbon nanotube: influenced by nature of catalyst and substrate

To employ the multiwalled carbon nanotubes (MWCNTs) in electronic devices, their growth on conductive substrates is a prerequisite. Since electronic properties of MWCNT might be influenced by catalyst and substrate, therefore, to understand their influence on the work function and electrical resistance, individual MWCNT needs to be studied. In this report, we have demonstrated the growth of MWCNTs on conducting substrates such as copper, stainless steel, and insulating SiO2 using thermal CVD technique. First, as-grown MWCNTs on different substrates have been characterized with scanning electron microscopy and Raman spectroscopy. Furthermore, electrical measurements on as-grown individual MWCNT has been performed using electrostatic force microscopy and conductive atomic force microscopy. Performed measurements reveal that work function and electrical resistance of MWCNT vary from 4.32 to 5.32 eV and 4–498 kΩ, respectively, depending on the nature of catalyst as well as substrates. We envision that performed study might boost the on-going research activity in the area of MWCNT-based nanoelectronic devices.

Effect of Gas Flowrate on Nucleation Mechanism of MWCNTs for a Compound Catalyst

Activation of the catalyst particles during a CVD process can be anticipated from the carbon feeding rate. In this study, Fe2O3/Al2O3 catalyst was synthesized with uniformly dispersed iron over alumina support for onward production of multiwalled carbon nanotubes (MWCNTs) in a fluidized bed chemical CVD reactor. The effect of the ethylene flowrate on catalytic activity of the compound catalyst and morphology of the as-grown MWCNTs was also investigated in this study. The dispersed active phases of the catalyst and optimized gas flowrate helped in improving the tube morphology and prevented the aggregation of the as-grown MWCNTs. The flowrates, below 100 sccm, did not provide sufficient reactants to interact with the catalyst for production of defect-free CNT structures. Above 100 sccm, concentration of the carbon precursor did not show notable influence on decomposition rate of the gas molecules. The most promising results on growth and structural properties of MWCNTs were gained at ethylene flowrate of 100 sccm. At this flowrate, the ratio of G and D intensity peaks (IG/ID) was deliberated about 1.40, which indicates the growth of graphitic structures of MWCNTs.

Trimetallic catalyst synthesized multi-walled carbon nanotubes and their application for hydrogen storage

Multi-walled carbon nanotubes (MWCNTs) were synthesized by rapid thermal decomposition method using trimetallic catalyst supported MgO. MWCNTs prepared via trimetallic catalyst shows much higher BET specific surface area compared to current monometallic and bimetallic catalyst. As-grown and pristine MWCNTs were found to adsorb nitrogen reversibly and their adsorption uptake exhibits type-II BET isotherm. Existence of small impurities, such as metal and metal oxides present in the MWCNTs, was confirmed by thermogravimetric analysis as well as via energy-dispersive X-ray spectroscopy. An over 10 wt% enhancement of hydrogen storage capacity of as-grown MWCNTs compared to pristine was found to be due to the presence of impurities. Fast kinetics and complete reversibility gives indication that the process responsible for hydrogen adsorption uptake in MWCNTs is physisorption. A linear relation between hydrogen uptake (~0.22 and 0.20 wt%) and equilibrium hydrogen pressure was obtained for both as-grown and pristine MWCNTs.

Direct conversion of natural gas into COx-free hydrogen and MWCNTs over commercial Ni–Mo/Al2O3 catalyst: Effect of reaction parameters

A commercial hydrotreating nickel molybdate/alumina catalyst was used for the direct conversion of natural gas (NG) into COx-free hydrogen and a co-valuable product of multi-walled carbon nanotubes (MWCNTs). The catalytic runs were carried out atmospherically in a fixed-bed flow reactor. The effect of reaction temperature between 600 and 800°C, and dilution of the NG feed with nitrogen as well as pretreatment of the catalyst with hydrogen were investigated. At a reaction temperature of 700°C and dilution ratio of NG/N2=20/30, the optimum yield of H2 (∼80%) was obtained with higher longevity. However, using the feed ratio of NG/N2=30/20, the optimum yield of MWCNTs was obtained (669%). X-ray diffraction pattern for the catalyst after the reaction showed that the MWCNTs were grown on the catalyst at all reaction temperatures under study. TEM pictures revealed that the as-grown MWCNTs at 600, 650 and 800°C are short and long with a low graphitization degree. At 700°C a forest of condensed CNTs is formed, whereas both carbon nanofibers and CNTs were formed at 750°C.

Synthesis of Multi-walled Carbon Nanotubes from Glycine Max Oil and Their Potential Applications

The discovery of carbon nanotubes has created new era in the field of nanotechnology. Spectacular properties of these nanostructured materials, stimulating scientists to peep into this tiny tube with ever increasing curiosity. The immediate challenge is to produce desired structural and characteristic featured carbon nanotubes in large quantities. Chemical vapor deposition is the most popular method of producing carbon nanotubes and it is of low-cost and highly useful technique for mass production of carbon nanotubes. These efforts requires not only chosen technique but also based on the precursor and the catalytic support. Glycine max oil a botanical hydrocarbon, has been found to be effective precursor for the synthesis of multi-walled carbon nanotubes (MWNTs) by Spray Pyrolysis over well dispersed Fe /Mo catalyst supported on silica at 650 °C under Ar atmosphere. As-grown MWNTs were characterized by SEM, HRTEM, Raman spectroscopy and Nitrogen adsorption studies. Raman spectroscopy revels that MWNTs are well graphitized. Dynamic and equilibrium studies of adsorption of Basic brown-4 on MWNTs were also reported.

Re-Grown Aligned Carbon Nanotubes with Improved Field Emission

In this work, a simple technique to improve the field emission property of multi-walled carbon nanotubes is presented. Re-grown multi-walled carbon nanotubes are grown on the same substrates after the as-grown multi-walled carbon nanotubes are transferred to other substrates using polydimethylsiloxane as intermediation. For the duration of the synthesis of the re-grown multi-walled carbon nanotubes, similar synthesis parameters used in growing the as-grown multi-walled carbon nanotubes are utilized. As a form of possible application, field emission studies show -2.6 times improvement in field enhancement factor and more uniform emission for the re-grown multi-walled carbon nanotubes. In addition, the turn-on field is reduced from 2.85 V/microm to 1.40 V/microm. Such significant improvements are attributed to new emission sites comprising of sharp carbonaceous impurities encompassing both tip and upper portion of the multi-walled carbon nanotubes. As such, this technique presents a viable route for the production of multi-walled carbon nanotubes with better field emission quality.

Optimized network of multi-walled carbon nanotubes for chemical sensing

This work reports the design of a resistive gas sensor based on 2D mats of multi-walled carbonnanotubes (MWCNTs) grown by aerosol-assisted chemical vapour deposition. The sensorsensitivity was optimized using chlorine as analyte by tuning both CNT network morphologyand CNT electronic properties. Optimized devices, operating at room temperature, havebeen calibrated over a large range of concentration and are shown to be sensitive down to27 ppb of chlorine. The as-grown MWCNT response is compared with responses of2000 °C annealed CNTs, as well as of nitrogen-doped CNTs and CNTs functionalized withpolyethyleneimine (PEI). Under chlorine exposure, the resistance decrease ofas-grown and annealed CNTs is attributed to charge transfer from chlorine toCNTs and demonstrates their p-type semiconductor behaviour. XPS analysis ofCNTs exposed to chlorine shows the presence of chloride species that confirmselectron charge transfer from chlorine to CNTs. By contrast, the resistance ofnitrogen-doped and PEI functionalized CNTs exposed to chlorine increases, inagreement with their n-type semiconductor nature. The best response is obtainedusing annealed CNTs and is attributed to their higher degree of crystallinity.

Large scale synthesis of bundles of aligned carbon nanotubes using a natural precursor: turpentine oil

Bundles of aligned carbon nanotubes (ACNTs) have been synthesised by spray pyrolysis of turpentine oil (inexpensive precursor) and ferrocene mixture at 800°C. Turpentine oil (C10H16), a plant-based precursor was used as a source of carbon and argon as a carrier gas. The bundles of ACNTs have been grown directly inside the quartz tube. The as-grown ACNTs have been characterised through X-ray diffraction, Raman spectroscopy, scanning and transmission electron microscopic techniques. Scanning electron microscope images reveal that the bundles of ACNTs are densely packed and are of ∼70–130 µm in length. High-resolution transmission electron microscopy and Raman spectroscopy observations indicate that as-grown multi-walled carbon nanotubes (CNTs) are well graphitised. These CNTs have been found to have outer diameters between ∼15 and 40 nm. This technique suggests a low-cost route for the large-scale formation of ACNTs bundles.

Carbon Nanotube: The Inside Story

Carbon nanotubes (CNTs) were serendipitously discovered as a byproduct of fullerenes by direct current (DC) arc discharge; and today this is the most-wanted material in the nanotechnology research. In this brief review, I begin with the history of the discovery of CNTs and focus on CNTs produced by arc discharge in hydrogen atmosphere, which is little explored outside my laboratory. DC arc discharge evaporation of pure graphite rod in pure hydrogen gas results in multi-walled carbon nanotubes (MWCNTs) of high crystallinity in the cathode deposit. As-grown MWCNTs have very narrow inner diameter. Raman spectra of these MWCNTs show high-intensity G-band, unusual high-frequency radial breathing mode at 570 cm(-1), and a new characteristic peak near 1850 cm(-1). Exciting carbon nanowires (CNWs), consisting of a linear carbon chain in the center of MWCNTs are also produced. Arc evaporation of graphite rod containing metal catalysts results in single-wall carbon nanotubes (SWCNTs) in the whole chamber like macroscopic webs. Two kinds of arc method have been developed to produce SWCNTs: Arc plasma jet (APJ) and Ferrum-Hydrogen (FH) arc methods. Some new purification methods for as-produced SWCNTs are reviewed. Finally, double-walled carbon nanotubes (DWCNTs) are also described.

Synthesis of carbon nanotubes on single crystal diamond

Vertically aligned multiwalled carbon nanotubes (MWCNTs) have been synthesised on Ni coated single crystal diamond substrates using a glow-discharge technique. A mixture of gases including CH 4, H 2 and N 2 has been used for growth. The effect of the CH 4/H 2 gas mixture and growth temperature on the structure and yield of the MWCNTs has been studied. Atomic force microscopy is used to characterise the annealed Ni film prior to growth. Scanning electron microscope studies have also been carried out to observe the yield, height and diameter of MWCNTs produced under various experimental conditions. Raman spectroscopy has been performed to provide quantitive information on the crystallinity of our as-grown MWCNTs. It has been shown that highly adherent, vertically aligned MWCNTs can be grown on type Ib diamond (100) substrates with an interface free from metal catalyst.

Thermal Analysis of Multi-walled Carbon Nanotubes by Kissinger’s Corrected Kinetic Equation

ABSTRACTCarbon nanotubes (CNTs) have been a popular material in recent years, but their thermal characteristics have not been understood completely. We investigated the unique thermal stability of multi-walled carbon nanotubes (MWCNTs) and used nitric acid (HNO3) to purify MWCNTs to promote its activation energy (Ea). The study used differential scanning calorimetry (DSC), thermogravimetric analyzer (TGA), and Fourier transform infrared (FTIR) spectrometer to analyze as-grown MWCNTs and modified MWCNTs. For DSC, the heating rate was chosen to be 0.25 to 2.0 °C/min. From DSC results, Ea and exothermic onset temperature (T0) of the modified MWCNTs increased with increasing HNO3 concentration. The TGA results showed that both as-grown and modified MWCNTs’ decomposition temperatures were higher than 500°C in air. The infrared spectra of as-grown MWCNTs and modified MWCNTs have shown that the gas phase composition is CO2 after TGA linked with FTIR. By Kissinger’s corrected kinetic equation, Ea increased with increasing HNO3 concentration. Through this study, we realized that as-grown MWCNTs and modified MWCNTs are thermally hazardous materials with high potential heat of decomposition, especially under fire exposure. Thus, it is important to know the thermal hazard characteristics of material with a measure to prevent its thermal damage during perturbed situations.

Proteomics-based safety evaluation of multi-walled carbon nanotubes

This study evaluated the biological responses to multi-walled carbon nanotubes (MWCNTs). Human monoblastic leukemia cells (U937) were exposed to As-grown MWCNTs and MWCNTs that were thermally treated at 1800 degrees C (HTT1800) and 2800 degrees C (HTT2800). Cell proliferation was highly inhibited by As-grown but not HTT2800. However, both As-grown and HTT1800, which include some impurities, were cytotoxic. Proteomics analysis of MWCNT-exposed cells revealed 37 protein spots on 2-dimensional electrophoresis gels that significantly changed (p<0.05) after exposure to HTT1800 with a little iron and 20 spots that changed after exposure to HTT2800. Peptide mass fingerprinting identified 45 proteins that included heat shock protein beta-1, neutral alpha-glucosidase AB, and DNA mismatch repair protein Msh2. These altered proteins play roles in metabolism, biosynthesis, response to stress, and cell differentiation. Although HTT2800 did not inhibit cell proliferation or cause cytotoxicity in vitro, some proteins related to the response to stress were changed. Moreover, DJ-1 protein, which is a biomarker of Parkinson's disease and is related to cancer, was identified after exposure to both MWCNTs. These results show that the cytotoxicity of MWCNTs depends on their impurities, such as iron, while MWCNTs themselves cause some biological responses directly and/or indirectly in vitro. Our proteomics-based approach for detecting biological responses to nanomaterials is a promising new method for detailed safety evaluations.

Experimental Demonstration of Hydrophobicity Variation in Carbon Nanotubes by Surface Modifications

Surface modification of the carbon nanotubes is important for their application in various fields. In this study, multiwalled carbon nanotubes (MWCNTs) were grown on cobalt catalyst layer (3 nm thickness) sputtered on a silicon substrate using Microwave Plasma Enhanced Chemical Vapor Deposition (MPECVD) system. The as-grown MWCNT layer was found to have a thickness of ∼5 μm and an average nanotube diameter ∼70 nm. The surface of as-grown nanotubes was modified by polyvinyl alcohol (PVA). The surface hydrophobicity has been estimated by measuring the static contact angles of water on nanotube surfaces before and after the modification by varying PVA concentration in the range of 1:10,000-5:10,000 (w/v). This functionalisation showed a dramatic change of the surface of nanotubes from highly hydrophobic to hydrophilic. The fictionalization of MWCNTs was confirmed further with electrochemical measurements. [DOI: 10.1380/ejssnt.2009.334]

An EELS study on an individual boron-doped multi-walled carbon nanotube

A pellet of as-grown multi-walled carbon nanotubes (MWNTs) prepared by compression was heat-treated at 3000°C for 30 min to eliminate metal impurities and a piece of the 3000°C-treated pellet was boron doped with a diffusion method. Electron energy loss (EEL) spectrum measurements for an individual boron-doped MWNT (B-MWNT) selected from the boron doped pellet were taken along the central line of the high resolution transmission electron microscope image, i.e. the line of the MWNT axis of the individual B-MWNT. Along the central line, a K-shell excitation EEL spectrum composed of two spectra, a weak boron K-shell excitation EEL spectrum and a strong carbon K-shell excitation EEL spectrum, was obtained at some positions, while at other positions only a carbon K-shell excitation EEL spectrum was recorded. Semi quantitative analysis of the energy loss near the edge structure of the boron K-edge revealed that boron doping had taken place quite locally in the individual B-MWNT.

Synthesis of carbon nanotubes at low temperature by filament assisted atmospheric CVD and their field emission characteristics

Multiwalled carbon nanotubes (MWNTs) were synthesized using a hot filament assisted chemical vapor deposition (CVD) at the atmospheric pressure at a substrate temperature of 550 °C. The size of nanotubes was controlled by changing the size of catalyst particles. The structure and composition of these nanotubes were investigated using scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. The electron field emission current of MWNTs was also measured. It was found that the nanotubes with smaller the diameter had higher the emission current levels though synthesis conditions except catalyst particles were the same. These as-grown MWNTs had emission current densities of 6.5 mA/cm 2 and 2.5 mA/cm 2 at 1 V/μm for 5–8 nm and 20 nm size carbon nanotube samples, respectively. The results indicated that the MWNTs synthesized had low emission threshold voltages and high emission current levels that are favorable properties for field emission-based display device applications.