Vertically Aligned Multi-walled Carbon Nanotubes Research Articles

Growth mechanism of multilayer-graphene-capped, vertically aligned multiwalled carbon nanotube arrays

The authors describe a rigorous investigation of the growth mechanism of composite structures consisting of graphene multilayers supported by vertically aligned multiwalled carbon nanotubes (VA-MWCNTs). The synthesis was performed via chemical vapor deposition with ethanol as a carbon source and iron films ranging in thickness from 1 to 9 nm as the catalyst. The morphology of grown films was investigated using scanning electron microscopy and transmission electron microscopy (TEM), and the crystallinity was studied using TEM and Raman spectroscopy. Thicker Fe films (8 or 9 nm) yielded composite structures, thin Fe films (1 to 4 nm) produced pure VA-MWCNTs, and Fe layers between 5 and 7 nm produced an intermediate structure composed of bundles of VA-MWCNTs fused together at their tips. The authors present growth mechanisms for all three structures. The authors attribute the change from VA-MWCNT to intermediate/composite with higher Fe film thicknesses to the formation of graphitic layers at the initial growth stage.

Reproducible Fabrication of Robust, Renewable Vertically Aligned Multiwalled Carbon Nanotube/Epoxy Composite Electrodes

We describe the reproducible fabrication of robust, vertically aligned multiwalled carbon nanotube (VACNT)/epoxy composite electrodes. The electrodes are characterized by cyclic voltammetry, impedance spectroscopy, and scanning electron and atomic force microscopies. Low background currents are obtained at the electrodes, and common redox probe molecules and NADH show excellent voltammetric behavior. When electrode performance deteriorates due to fouling, the electrode surfaces can be reproducibly renewed by mechanical polishing followed by O(2) plasma treatment. The electrochemical performance of the electrodes is maintained after more than 100 cycles of use and renewal.

Terahertz time-domain spectroscopy characterization of vertically aligned carbon nanotube films

Terahertz time-domain spectroscopy measurements were made for vertically aligned multi-walled carbon nanotube (VACNT) films. We obtained the frequency dependent complex permittivity and conductivity (on the assumption that permeability μ = 1) of several samples exhibiting Drude behaviour for lossy metals. The obtained material properties of VACNT films provide information for potential microwave and terahertz applications.

Synthesis of Carbon Nanotubes, Carbon Spheres and Slices of Vertically Aligned Multi-Walled Carbon Nanotubes

Carbon nanotubes, carbon spheres and slices of vertically aligned multiwalled carbon nanotubes (MWNTs) were synthesized simultaneously by chemical vapour deposition (CVD). Electron microscopy image showed carbon nanotubes were multiwalled carbon nanotubes, several micrometers in length. Carbon spheres were of uniform diameter (about 1 μm). Slices of vertically aligned multiwalled carbon nanotubes were about 5 mm in length, 3 mm in width and 8 μm in thickness. The interior of the slice was composed of densely packed, vertical aligned MWNTs, and faceted carbon beads. They were stable and easily separated from each other. The formation mechanism of slice was also discussed.

Micro-Patterning of Vertically-Aligned Multi-Walled Carbon Nanotube Supercapacitor Electrodes to Improve Energy Density

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Modification of Vertically-Aligned Multi-Walled Carbon Nanotube Arrays with Nanoparticles using Electrodeposition and Applications in Sensing

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Proposed model for growth preference of plate-like nanohydroxyapatite crystals on superhydrophilic vertically aligned carbon nanotubes by electrodeposition

For the first time, the mechanism of growth of plate-like n-HA electrodeposited on superhydrophilic vertically aligned multi-walled carbon nanotubes (VACNT) is presented and a model for the specific growth preference is discussed. Results show that the carboxyl (carboxylic acid)/carboxylate functional groups directly attached on VACNT tips after oxygen plasma treatment were essential for the acceleration of the OH− formation and the deposition of plate-like HA crystals. FEG-SEM, EDX and XRD showed that a homogeneous highly crystalline HA film, stoichiometric and with preferential growth in the (002) plane direction, was formed.

Comparison of the efficiency of various substrates in growing vertically aligned carbon nanotube carpets

Vertically aligned multi-walled carbon nanotube (VACNT) carpets have been synthesized on metal and isolating thin films without the use of a pre-deposited catalyst. We used ferrocene as precursor powder catalyst and acetylene as a carbon source to grow VACNT carpets using three-temperature-zone chemical vapor deposition (TTZ-CVD). Tuning the carrier gas Ar flow rate and the sublimation time of ferrocene in the TTZ-CVD extended the lifetime of the catalyst, increasing the efficiency of growth. Increasing the number of ferrocene loads increased the growth height of VACNT carpets on metal and isolating thin films. The effective growth rates of VACNT carpets were 10.4, 7.13, 4.8, 3, 2, 9.55, 7.6, 13.3, and 32.32 μm/min, respectively, on Cu, Ti, Au, Pt, Mo, Cr, Ag, Ni, and Si. VACNT carpets such as SiO 2 and Al 2O 3 also grew on the isolating thin films with growth rates of 37.28 and 23.8 μm/min, respectively. By controlling the growth parameters, the growth of VACNT carpets on selective substrates can be controlled.

Influence of polar groups on the wetting properties of vertically aligned multiwalled carbon nanotube surfaces

In this work, we have studied superhydrophilic and superhydrophobic transitions on the vertically aligned multiwalled carbon nanotube (VACNT) surfaces. As-grown, the VACNT surfaces were superhydrophobic. Pure oxygen plasma etching modified the VACNT surfaces to generate superhydrophilic behavior. Irradiating the superhydrophilic VACNT surfaces with a CO2 laser (up to 50 kW cm−2) restored the superhydrophobicity to a level that depended on the laser intensity. Contact angle and surface energy measurements by the sessile drop method were used to examine the VACNT surface wetting. X-ray photoelectron spectroscopy (XPS) showed heavy grafting of the oxygen groups onto the VACNT surfaces after oxygen plasma etching and their gradual removal, which also depended on the CO2 laser intensity. These results show the great influence of polar groups on the wetting behavior, with a strong correlation between the polar part of the surface energy and the oxygen content on the VACNT surfaces. In addition, the CO2 laser treatment created an interesting cage-like structure that may be responsible for the permanent superhydrophobic behavior observed on these samples.

Confinement effect and spreading of water into microchannels fabricated on the VACNT surfaces

An easy and fast method to produce microchannels on the vertically aligned multiwalled carbon nanotube (VACNT) surfaces is described. Alternating superhydrophilic and superhydrophobic channels were built on VACNT films, using oxygen plasma functionalization and CO 2 laser treatment, respectively. A combined effect of wettability and capillary forces promotes an effective spreading, and confinement of water microdroplets on the superhydrophilic channels.

Microwave characterization of vertically aligned multiwalled carbon nanotube arrays

Vertically aligned multiwalled carbon nanotube (VACNT) films have been characterized by rectangular waveguide measurements. The complex scattering parameters (S-parameters) are measured by a vector network analyzer at X-band frequencies. The effective complex permittivity and permeability of the VACNT films have been extracted. The extracted parameters are verified by full wave simulations and very good agreement has been obtained. The results of the systematic error analysis are presented and the errors are within the acceptable range. The performance of VACNT films as an absorber is examined, and comparison with the conventional carbon loaded materials shows that a 90% size reduction is possible while maintaining the same absorption level.

A low-potential, H 2O 2-assisted electrodeposition of cobalt oxide/hydroxide nanostructures onto vertically-aligned multi-walled carbon nanotube arrays for glucose sensing

A novel nanocomposite was synthesized using a cathodic, low-potential, electrochemical reduction of H 2O 2 to homogeneously deposit cobalt oxide/hydroxide (denoted as CoOx·nH 2O) nanostructures onto vertically well-aligned multi-walled carbon nanotube arrays (MWCNTs), while the MWCNTs were prepared by catalytic chemical vapor deposition (CVD) on a tantalum (Ta) substrate. The CoOx· nH 2O–MWCNTs nanocomposite exhibits much higher electrocatalytic activity towards glucose (Glc) after modification with CoOx· nH 2O than before. This non-enzymatic Glc sensor has a high sensitivity (162.8 μA mM −1 cm −2), fast response time (<4 s) and low detection limit (2.0 μM at signal/noise ratio = 3), and a linear dynamic range up to 4.5 mM. The sensor output is stable over 30 days and unaffected by common interferents that co-exist with Glc in analytical samples; it is also resistant to chloride poisoning. These features make the CoOx·nH 2O–MWCNTs nanocomposite a promising electrode material for non-enzymatic Glc sensing in routine analysis.

Enhanced Electron Field Emission from Carbon Nanotube Matrices

ABSTRACTField emission from as-grown carbon nanotube (CNTs) films often suffered from high threshold electric field, and low emission site density due to screening effects. These problems can be resolved by patterned growth of CNTs on lithographically prepared catalyst films. However, these approaches are expensive and not applicable for future emitting devices with large display areas. Here we show that as-grown CNTs films can have low emission threshold field and high emission density without using any lithography processes. We have reduced screening effects and work function of as-grown CNTs films and created the novel CNT matrices by addition of vapor- and/or liquid- phase deposition. Furthermore, these CNT matrices can continuous emit electrons for 40 hours without significant degradation. The fabrication of our CNT matrices is described as follows. First, CNT films were grown by plasma-enhanced chemical vapor deposition. These vertically-aligned multiwalled carbon nanotubes (VA-MWCNTs) are having typical length and diameter of 4 microns and 40 nm, respectively. Spacing between these CNTs is ~80 nm in average, leading to poor emission properties due to the screening effect. These as-grown samples were then subjected to the deposition of strontium titanate (SrTiO3) by pulsed-laser deposition to reduce both the work function and screening effect of CNTs. The emission properties of these coated samples can be further improved by fully filled the spaces between VA-MWCNTs by poly-methyl metha acrylate (PMMA). The field emission threshold electric field was decreased from 4.22 V/μm for as-grown VA-MWCNTs to 1.7 V/μm for SrTiO3 coated VA-MWCNTs. The addition filling with PMMA and mechanical polishing can further reduce the threshold to 0.78V/μm for the so called PMMA-STO-CNT matrices. Long term emission stability and emission site density were also enhanced.

Metal free, end-opened, selective nitrogen-doped vertically aligned carbon nanotubes by a single step in situ low energy plasma process

We report a novel single step in situ process of growth and nitrogen-electron cyclotron resonance plasma treatment of vertically aligned multi walled carbon nanotubes (VA-MWCNTs) that leads to concurrent end opening by metal cap removal, nitrogen incorporation and intercalation along with substitution at graphitic sites resulting in n-type electronic doping. Microscopic and spectroscopic evaluations of the nitrogen treated MWCNTs reveal negligible iron content with significant conservation of both structure and alignment. The nitrogen induced electronic change increases distinct π* states as evidenced by Near Edge X-ray Absorption Fine Structure (NEXAFS) and 5 cm−1 downshift of G-band, as observed from Raman spectroscopy, confirm n-type doping. The combined effect of plasma activation (both cavities and surface of the end opened VA-MWCNTs) and n-type doping enhances the field emission performance of the CNTs resulting in high current density (15 mA cm−2) at low applied voltage of 1.5 V μm−1 with low turn on and threshold electric fields (Eto-0.52 and Eth-0.76 V μm−1). This low energy highly controllable plasma has great implications not only in the fabrication of various n-type materials and bio related application but also many other interesting areas for cost effective energy related applications.

The performance of superhydrophobic and superoleophilic carbon nanotube meshes in water–oil filtration

Vertically-aligned multi-walled carbon nanotubes (CNTs) were grown on stainless steel (SS) mesh by thermal chemical vapor deposition with a diffusion barrier of Al 2O 3 film. These three-dimensional porous structures (SS-CNT meshes) were found to be superhydrophobic and superoleophilic. Water advancing contact angles of 145–150° were determined for these SS-CNT meshes in air and oil (gasoline, isooctane). Oil, on the other hand, completely wet the SS-CNT meshes. This combined superhydrophobic and superoleophilic property repelled water while allowed the permeation of oil. Filtration tests demonstrated efficiencies better than 80% of these SS-CNT meshes as the filtration membranes of the water-in-oil emulsions. We have conducted quantitative analysis on the diameters of the oil droplets in both the feed emulsion and the filtrate. Then, we have evaluated the issue of water blockage and possible way to improve the filtration efficiency. Finally, the filtration and blockage mechanisms are proposed.

CO 2 laser treatment for stabilization of the superhydrophobicity of carbon nanotube surfaces

In this work, the authors demonstrate the formation of stable superhydrophobic vertically aligned multiwalled carbon nanotube (VACNT) surfaces through CO2 laser irradiance, in which the contact angle value reached 161°. VACNT arrays were synthesized by microwave plasma chemical vapor deposition using N2/H2/CH4 [10/90/14 SCCM (SCCM denotes cubic centimeter per minute at STP)]. CO2 laser technique was applied on VACNT surfaces with irradiance at different laser powers to promote the great stability of superhydrophobic surfaces. Contact angle measurement reveals that irradiated VACNT surface is superhydrophobic at all irradiances tested. Unlike as-grown VACNT, the samples treated with CO2 laser show no sign of water seepage even after a prolonged period of time (∼24 h). This characteristic is very interesting and has various possible functional applications in micro- and nanomaterials and devices.

Stable Electron Field Emission from PMMA−CNT Matrices

We have created PMMA-CNT matrices by embedding opened-tip vertically aligned multiwalled carbon nanotubes (VA-MWCNTs) with poly(methyl methacrylate) (PMMA). These PMMA-CNT matrices are excellent electron field emitters with an emission threshold field of 1.675 V/μm, more than 2-fold lower that that of the as-grown sample. In addition, the emission site density from these matrices is high, merely filling up the entire sample surface. Emission stability test at ∼1.35 mA/cm(2) was performed continuously for 40 h with no significant degradation. On the basis of our theoretical simulation and hypothetical modeling, we attribute these performances to the reduced screening effect and fewer Joule heatings due to the shorter effective transport distance of the electrons in MWCNTs.

Cytocompatibility studies of vertically-aligned multi-walled carbon nanotubes: Raw material and functionalized by oxygen plasma

It was presented a strong difference on cell adhesion and proliferation of functionalized vertically-aligned multi-walled carbon nanotube (VACNT) scaffolds compared to raw-VACNT. Biocompatibility in vitro tests were performed on raw-VACNT after superficial modification by oxygen plasma, which changes its superhydrophobic character to superhydrophilic. Two cytocompatibility tests were applied: 1) total lactate dehydrogenase colorimetric assay for the study of proliferating cells; and 2) cellular adhesion by scanning electron microscopy. Results showed that superhydrophilic VACNT scaffolds stimulate cell growth with proliferation up to 70% higher than normal growth of cell culture.

Magnetic behaviour of non‐contacting Ni nanoparticles encapsulated in vertically aligned carbon nanotubes

AbstractMagnetic properties of carbon nanotubes (CNT) obtained by plasma‐enhanced chemical vapour deposition (PECVD) have been studied. The growth of these nanotubes has been activated from Ni catalyst nanoparticles. Samples consist of Ni nanoparticles encapsulated at the tip of vertically aligned multiwalled carbon nanotubes (VACNTs) forming an homogeneous and dense large area monolayer of isolated (non‐contacting) nanoparticles. The magnetic characterisation has been performed in the temperature range of 5‐300 K with magnetic fields up to 9 T. The results show that the wide size range (30‐180 nm) of the particles originates the coexistence of blocked and superparamagnetic particles and leads to the strong ferromagnetic behaviour of the whole assembly. The coercivity decreases monotonically with increasing temperature and the value for the intrinsic coercivity is 225 Oe. The encapsulation of Ni nanoparticles by VACNTs preserves them from aggregation. This makes possible to tune the coercivity by controlling size distribution of particle monolayers. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Vertically aligned carbon nanotube electrodes for lithium-ion batteries

As portable electronics become more advanced and alternative energy demands become more prevalent, the development of advanced energy storage technologies is becoming ever more critical in today's society. In order to develop higher power and energy density batteries, innovative electrode materials that provide increased storage capacity, greater rate capabilities, and good cyclability must be developed. Nanostructured materials are gaining increased attention because of their potential to mitigate current electrode limitations. Here we report on the use of vertically aligned multi-walled carbon nanotubes (VA-MWNTs) as the active electrode material in lithium-ion batteries. At low specific currents, these VA-MWNTs have shown high reversible specific capacities (up to 782 mAh g −1 at 57 mA g −1). This value is twice that of the theoretical maximum for graphite and ten times more than their non-aligned equivalent. Interestingly, at very high discharge rates, the VA-MWNT electrodes retain a moderate specific capacity due to their aligned nature (166 mAh g −1 at 26 A g −1). These results suggest that VA-MWNTs are good candidates for lithium-ion battery electrodes which require high rate capability and capacity.