Crystallinity Of Multi-walled Carbon Nanotubes Research Articles

Enhancing purity and crystallinity of carbon nanotubes by magnetically assisted arc discharge and thermal purification and their field emission characteristics

This study used a magnetically assisted arc discharge (AD) method and thermal purification to enhance the purity and crystallinity of multi-walled carbon nanotubes (MWCNTs). CNT synthesis in the presence of a magnetic field increased the densities of ionic carbon species and the arc plasma temperature, thereby improving CNT production. AD-MWCNTs produced at an optimal magnetic strength exhibited enhanced purity, crystallinity, and thermal stability. Thermal purification was also conducted to increase the purity of the AD-MWCNTs further. Purity was evaluated using Raman spectroscopy. AD-MWCNTs were homogenized to overcome limitations in light-penetration depths to ensure accurate evaluation. The calculated purities were consistent with the analytical results, confirming the reliability of Raman spectroscopy for purity assessment. The number of working CNT emitters on the film surface was directly related to the purity. Highly pure crystalline AD-MWCNTs exhibited high-performance field-emission properties, namely a high current density of 2.7 A/cm2 and long-term emission stability (12 h at a current density of 500 mA/cm2), with no performance reduction.

Analysis of growth temperature variation and different catalysts on the morphology of synthesized carbon nanotubes

This paper reports the effect of growth temperature (i.e., 750 °C, 800 °C, and 850 °C) and catalysts (iron (Fe) and nickel (Ni)) on the synthesis of carbon nanotubes. X-ray diffraction (XRD) technique, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy is used to study the morphological properties of synthesized carbon-nanotubes. It was observed from SEM results that longer, straight and clean nanotubes with lesser defect density are synthesized when iron is used as catalyst at higher growth temperature. Raman spectroscopy result shows that the defect density decreases with the increase in growth temperature. TEM images revealed that crystalline multi-walled carbon nanotubes were grown at higher growth temperature for iron as catalyst. The electrochemical performance of synthesized thin films of multi-walled carbon nanotubes as active supercapacitor electrodes evaluated using cyclic voltammetry and is 61 F/g.

Pseudo-topotactic growth of diamond nanofibers

We report pseudo-topotactic growth of single-crystal diamond fibers by nanosecond laser melting of amorphous carbon nanofibers (CNFs) and crystalline multi-wall carbon nanotubes (MWCNTs). A rapid laser melting in a super undercooled state and subsequent quenching convert the tips of CNFs and MWCNTs into phase-pure <110> nanodiamonds along the growth directions. Subsequent laser pluses melt regions below <110> nanodiamonds that provide seeds for epitaxial growth. By repeating this process, the length of <110> nanodiamond fibers can be increased, as each pulse results in ∼50 nm nanodiamond region, depending upon the initial size of CNFs and MWCTs. This conversion process can be carried at ambient temperature and pressure in air. The epitaxial nature of <110> nanodiamond fibers has been confirmed by systematic electron-back-scatter-diffraction studies along the fiber in high-resolution scanning electron microscopy, and high-resolution TEM imaging and diffraction. The nature of C–C bonding characteristics was studied by high-resolution electron-energy-loss spectroscopy to establish the formation of diamond phase by the characteristic peak at 292 eV for sp3 bonding (σ∗), and absence of 284 eV peak for sp2 (π∗) graphitic bonding. The characteristic diamond Raman peak at 1332 cm−1 is found to downshift to 1321 cm−1 because of phonon confinement in nanodiamonds associated with nanofibers. These nanodiamond structures can be doped with both n- and p-type dopants with concentrations far higher than thermodynamic solubility limit due to solute trapping during quenching from the liquid phase. Thus, these nanodiamond structures provide ideal platform for nanosensing, computing and communication, including efficient field emitting devices.

One-step path to highly crystalline multi-walled carbon nanotubes with large inner diameters

High crystalline multi-walled carbon nanotubes with large inner diameters have been produced by a one-step synthesis method from ferrocene pyrolysis in a sealed pre-vacuumed quartz tube. Ferrocene serves as both catalyst and carbon source. The diameters of the as-produced carbon nanotubes have been observed by transmission electron microscope to be 40-60 nm with inner diameters of 20-40 t shells for the encapsulation experiments. The high crystallinity of the as-produced carbon nanotubes has also been characterized by Raman scattering and X-ray diffraction (XRD) measurements.

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.

The Reduction Temperature Effect of Fe–Co/MgO Catalyst on Characteristics of Multi-Walled Carbon Nanotubes

Diameters and crystallinity of multi-walled carbon nanotubes (MWCNTs) dependent on reduction temperature of the Fe–Co/MgO catalyst were investigated. MWCNTs were synthesized by catalytic chemical vapor depositing and the Fe–Co/MgO catalyst was fabricated by using a sol-gel method. According to Raman analysis, transmission electron microscopy and thermogravimetric analysis, the diameter distribution of MWCNTs was broadened with increasing reduction temperature of the Fe–Co/MgO catalyst and crystallinity was improved. The above results are attributed to an increased size and enhanced crystallinity of metal catalyst particles by increasing reduction temperature.

Crystalline multiwall carbon nanotubes and their application as a field emission electron source

Using super-aligned carbon nanotube (CNT) film, we have fabricated van der Waals crystalline multiwall CNTs (MWCNT) by adopting high pressure and high temperature processing. The CNTs keep parallel to each other and are distributed uniformly. X-ray diffraction characterization shows peaks at the small angle range, which can be assigned to the spacing of the MWCNT crystals. The mechanical, electrical and thermal properties are all greatly improved compared with the original CNT film. The field emission properties of van der Waals crystalline MWCNTs are tested and they show a better surface morphology stability for the large emission current. We have further fabricated a field emission x-ray tube and demonstrated a precise resolution imaging ability.

Iron catalyzed growth of crystalline multi-walled carbon nanotubes from ambient carbon dioxide mediated by molten carbonates

Until now, research efforts focused on electrochemical conversion of carbon dioxide into stable carbon-based materials have been limited by poor understanding of catalytic effects occurring at surfaces. Here, we demonstrate the capability to simultaneously use atomic layer deposition (ALD), electrode composition control, and current density as a means to direct the formation of iron-based catalysts and grow highly crystalline multi-walled carbon nanotubes at high yields (99%) and with controlled average diameters of 27.5 nm from ambient carbon dioxide captured and dissolved in molten carbonate electrolytes. ALD of passive alumina coatings on a Ni anode prevents electrode corrosion processes and adverse deposition of Ni on the cathode that results in increased CNT diameters, lower CNT quality, and non-CNT products. On the cathode side where CNT growth occurs, our results elucidate the fine balance of iron catalyst accessibility from the cathode interior and the surface chemical properties in order to achieve high yield and high quality CNT growth. Our work provides an intersection between decades of research understanding on catalytic gas-phase growth of CNT materials and the ability to leverage these ideas to sustainably capture ambient carbon dioxide and produce functional CNT materials.

Growth of MWCNTs on Flexible Stainless Steels without Additional Catalysts

Multiwalled carbon nanotubes (MWCNTs) were synthesized on austenitic stainless steel foils (Type 304) using a home-built thermal chemical vapor deposition (CVD) under atmospheric pressure of hydrogen (H2) and acetylene (C2H2). During the growth, the stainless steel substrates were heated at different temperatures of 600, 700, 800, and 900°C. It was found that MWCNTs were grown on the stainless steel substrates heated at 600, 700, and 800°C while amorphous carbon film was grown at 900°C. The diameters of MWCNTs, as identified by scanning electron microscope (SEM) images together with ImageJ software program, were found to be 67.7, 43.0, and 33.1 nm, respectively. The crystallinity of MWCNTs was investigated by an X-ray diffractometer. The number of graphitic walled layers and the inner diameter of MWCNTs were investigated using a transmission electron microscope (TEM). The occurrence of Fe3O4 nanoparticles associated with carbon element can be used to reveal the behavior of Fe in stainless steel as catalyst. Raman spectroscopy was used to confirm the growth and quality of MWCNTs. The results obtained in this work showed that the optimum heated stainless steel substrate temperature for the growth of effective MWCNTs is 700°C. Chemical states of MWCNTs were investigated by X-ray photoelectron spectroscopy (XPS) using synchrotron light.

Effect of Ferrocene Catalyst and Hydrogen Feed on Microstructures of Vertically Aligned MWCNTs

This study investigated the effect of catalyst amount on chemical vapour deposition (CVD) growth of multi-walled carbon nanotubes (MWCNTs) with and without hydrogen feed. The ferrocene weight was varied from 100 mg to 200 mg for CNTs growth over Si/SiO2/Al2O3 substrate. Very few CNTs were seen in micrographs of the samples produced in the absence of the hydrogen feed. Most of the carbon atoms precipitated into amorphous carbon due to existence of inactive catalyst particles. However, CNT structures grown with hydrogen feed were more distinct; the nanotubes were thinner, straight and highly crystalline. MWCNTs arrays/forest length was also increased from 120 µm to 850 µm with hydrogen feed. An increase in catalyst weight significantly affected the diameter, crystallinity, alignment and growth of nanotubes. The lowest inner-shell spacing of 0.348 nm was obtained with 150 mg of ferrocene, which is an indication of growth of relatively pure CNTs. Under the optimum conditions, the areal density of the ferrocene particles was sufficiently increased to get required alignment and crystallinity of MWCNTs.

Facile route to multi-walled carbon nanotubes under ambient conditions

A facile and practical route is demonstrated to prepare multi-walled carbon nanotubes (MWCNTs) by a sonochemical method. By applying ultrasonic irradiation to the mixture of ferrocene and xylene with small amount of water, crystalline MWCNTs were selectively synthesized under ambient conditions, allowing MWCNTs with diameter of 25-40 nm to be obtained. A control experiment revealed that a small amount of water was essential for producing MWCNTs. The analysis of Raman spectra of MWCNTs showed that the intensity of the D-band relative to the G-band was 0.98, indicating relatively good crystallinity of these MWCNTs. This result was consistent with that of TEM observation.

Controlled surface damage of amorphous and crystalline carbon nanotubes for enhanced field emission

The effect of argon (Ar) ion treatment on the field-emission (FE) properties of amorphous carbon nanotubes (aCNTs) and crystalline multiwall carbon nanotubes (MWCNTs) is reported in this article. Under optimum Ar-ion irradiation conditions, the FE properties of carbon nanotubes (CNTs) have been found to improve. The turn-on field of MWCNTs downshifted from 2.81 to 0.85 V μm−1 and from 9.17 to 3.27 V μm−1 for aCNTs. Improvement of the FE behavior is associated with the morphological modifications of CNTs, as a result of Ar-ion irradiation. Ar-ion bombardment creates defect sites on the CNT surface as confirmed by various techniques such as field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM) and Fourier transform infrared spectroscopy (FTIR). We have proposed that the defects caused modifications of the CNT surfaces resulting in an effective increment of the field-enhancement factor β.

A reversible strain-induced electrical conductivity in cup-stacked carbon nanotubes

We have used in situ current-voltage measurements of cup-stacked carbon nanotubes (CSCNTs) to establish reversible strain induced (compressive bending) semiconducting to metallic behavior. The corresponding electrical resistance decreases by two orders of magnitude during the process, and reaches values comparable to those of highly crystalline multi-walled carbon nanotubes (MWCNTs) and graphite. Joule heating experiments on the same CSCNTs showed that the edges of individual cups merge to form "loops" induced by the heating process. The resistance of these looped CSCNTs was close to that of highly deformed CSCNTs (and crystalline MWCNTs), thus suggesting that a similar conduction mechanism took place in both cases. Using a combination of molecular dynamics and first-principles calculations based on density functional theory, we conclude that an edge-to-edge interlayer transport mechanism results in conduction channels at the compressed side of the CSCNTs due to electronic density overlap between individual cups, thus making CSCNTs more conducting. This strain-induced CSCNT semiconductor to metal transition could potentially be applied to enable functional composite materials (e.g. mechanical sensors) with enhanced and tunable conducting properties upon compression.

Field emission characteristics of carbon nanotube films fabricated on a metal mesh by filtration

A novel-structured field emitter was fabricated by vacuum-filtrating in sequence an aqueous suspension of thin, highly crystalline multiwalled carbon nanotubes (MWCNTs) and an aqueous suspension of thick, defective MWCNTs through a metal mesh placed on a polymer membrane. We could form a mechanically strong CNT film by weaving carbon nanotubes (CNTs) through the metal mesh residing inside the film. On drying at 70°C, the polymer membrane was spontaneously separated from the CNT-mesh hybrid structure due to a large difference between their thermal expansion coefficients, producing vertically aligned bushes of thin CNTs on the CNT film. On top of the bushes, sharp CNT tips were well developed and worked excellently as field emitters. This structure was designed in such a way that the thin CNTs worked as field emitters while the thick CNTs supported the emitter CNTs. Our CNT emitters showed a high emission current density of 220mA/cm2 at 4.3V/μm, and its emission life span, measured at 40mA/cm2 in a DC bias mode, was ∼171h. We expect that the CNT emitters with a novel structure are promising for the applications to field emission sources requiring small area but high current, for example, X-ray generators, microwave amplifiers, etc.

Radial followed by longitudinal unzipping of multiwalled carbon nanotubes

A hybrid structure of graphene nanoleaves attached on a carbon nanotube and rectangular graphene nanosheets (GNSs) were fabricated by means of radial followed by longitudinal unzipping of crystalline multiwalled carbon nanotubes (MWCNTs) grown by catalytic chemical vapor deposition, using a chemical method containing sulfuric acid and nitric acid. Consecutive unzipping in the radial direction and then in the longitudinal direction of a nanotube has never been reported. Our simple chemical approach will open the path to fabrication for rectangular GNSs in the hundred nanometers by few hundred nanometers scale and a broad application of MWCNTs to the novel carbon nanostructure.

Effect of Fe 3+ concentration on MWCNTs formation in liquid arcing method

The formation of multi-walled carbon nanotubes (MWCNTs) during arc discharge in aqueous solutions of Fe 2(SO 4) 3 and FeCl 3 was studied. The concentration of iron ions changed from zero (deionized water) to 0.25 M and the cathodic products were examined by transmission electron microscopy, Raman spectrometry, and thermal gravimetric analysis. The experimental results showed that the crystallinity of MWCNTs is improved by increasing the concentration of iron ions. Nevertheless, the process yield and overall quality of the produced CNTs were significantly affected by iron concentration in the aqueous solution. This observation suggested that there should be an optimum iron concentration at which the formation of MWCNTs is favored. As compared with the sulfate solution, a higher process yield is obtained in the presence of chloride ions in agreement with previous reports.

Efficient field emission from coiled carbon nano/microfiber on copper substrate by dc-PECVD

Crystalline coiled carbon nano/micro fibers in thin film form have been synthesized via direct current plasma enhanced chemical vapor deposition (PECVD) on copper substrates with acetylene as a carbon precursor at 10 mbar pressure and 750 °C substrate temperature. The as-prepared samples were characterized by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). XRD pattern as well as selected area electron diffraction (SAED) pattern showed that the samples were crystalline in nature. SEM and HRTEM studies showed that as synthesized coiled carbon fibers are having average diameter ∼100 nm and are several micrometers in length. The as-prepared samples showed moderately good electron field emission properties with a turn-on field as low as 1.96 V/μm for an inter-electrode distance 220 μm. The variation of field emission properties with inter-electrode distance has been studied in detail. The field emission properties of the coiled carbon fibrous thin films are compared with that of crystalline multiwalled carbon nanotubes and other carbon nanostructures.

New Raman-Peak at 1850 cm&lt;SUP&gt;−1&lt;/SUP&gt; Observed in Multiwalled Carbon Nanotubes Produced by Hydrogen Arc Discharge

The new peak (near 1850 cm(-1)) assigned to carbon linear chain included in the centre of very thin innermost multiwalled carbon nanotubes (MWNTs) has been verified by Raman spectroscopy. These MWNTs were produced by dc arc discharge of pure graphite rods in pure hydrogen gas and existed in the cathode deposit. In this paper, we clarified that the new Raman-peaks could also be observed in the cathode deposit including MWNTs produced by hydrogen dc arc discharge using graphite electrode with added Y or La. By changing the quantity of addition (Y or La), dc arc current and pressure of ambient hydrogen gas, the optimum condition to get maximum intensity of the new Raman-peaks was obtained. For the case of 1 wt% La, dc 50 A, H2 pressure of 50 Torr was found to be optimum, and the intensity of new Raman-peak was even higher than the G-band peak. For the case of 1 wt% Y, dc 50 A, H2 pressure of 50 Torr was optimum, but the intensity of new Raman-peak was weaker than the G-band peak. Transmission electron microscopy observation revealed that the crystallinity of MWNTs produced with pure graphite rod was better than those produced with added Y or La.

Synthesis of carbon nanotubes from acetylene on the FeCoMgO catalytic system obtained by ball milling

Highly crystalline multi wall carbon nanotubes have been synthesized by RF-CVD from acetylene at 850°C over a Fe:Co:MgO catalyst. The catalytic system was obtained by mixing for 100 h Fe, Co and MgO powders in a ball milling device under petroleum ether environment, followed by oxidation in air at 500°C for 24 h. Most of the nanotubes had external diameters in order of dozens of nm and lengths of microns, resulting in an aspect ration of over 1000. Their external to internal diameter ratio varied between 2.5 and 3.

Multi-walled carbon nanotube-reinforced magnesium alloy composites

This study demonstrates the ability to fabricate lightweight, ductile but mechanically strong magnesium alloy (AZ91D) composites by introducing a small number of high crystalline multi-walled carbon nanotubes. It is demonstrated that 1% of relatively short and straight carbon nanotubes distributed homogeneously on the outer surface of magnesium powders act as an effective reinforcing filler to prevent deformation, thereby contributing to the enhanced tensile strength of magnesium alloy composites (e.g., from 315 to 388 MPa).