Field emission properties of N2 and Ar plasma-treated multi-wall carbon nanotubes

We modify multi-wall carbon nanotubes (MWCNTs) by plasma treatment with N₂ and Ar for varying durations and measure their field emission characteristics. The N₂ treated MWCNTs showed significant improvement in field emission properties, while the Ar treated MWCNTs displayed poorer field emission characteristics compared to untreated MWCNTs. X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), Raman spectroscopy and work function measurements are used to investigate the field emission mechanisms after plasma treatments.     

Growth of multiwalled carbon nanotubes during the initial stages of aerosol-assisted CCVD

We report a study of the initial stages of growth of aligned multiwalled carbon nanotubes (MWNT) synthesised by catalytic chemical vapour deposition (CCVD) of liquid aerosol obtained from toluene/ferrocene solution. A special experimental procedure has been developed to stop the process after short durations (30 s to 2 min). Two different pyrolysis temperatures are considered: 800 and 850 °C. Both scanning and transmission electron microscopy (SEM, TEM) coupled to energy-dispersive X-ray (EDX) analyses are used in order to determine the location of catalyst particles and to examine their chemical nature, morphology and size distribution when nanotubes start to grow. During the early stages (30 s), we observe the formation of a layer of catalyst particles on silicon substrates before the growth of nanotubes. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements indicate the occurrence of iron oxide (γ-Fe₂O₃ or Fe₃O₄). In addition, XPS analysis reveals the formation of graphite-like carbon, demonstrating that iron oxide particles catalyse the decomposition of toluene vapour. SEM and TEM observations show that these particles are most often located at the nanotube root, suggesting a base growth mechanism responsible for the formation of aligned nanotube when prolonging growth time (2 min).     

Purification process for single-wall carbon nanotubes

Single-wall carbon nanotubes (SWNTs) have exceptional strength and stiffness and high thermal and electrical conductivity, making them excellent candidates for aerospace structural materials. However, one of the most fundamental challenges is purifying the SWNTs. The purpose of this study was to develop a simple purification process for SWNTs, along with an understanding of the purification process. In addition, uncomplicated analytical methods were sought to screen and compare various purification methods. In this study, we demonstrate an easy method of cleaning SWNTs and evaluating their purity. The cleaning method, which employed oxidative heat treatment followed by acid reflux, was straightforward, inexpensive, and fairly effective. The purification mechanism was determined to be, first, that much of the non-nanotube carbon and iron catalyst was oxidized and, second, that the acid washing removed the iron oxide, leaving relatively pure SWNTs. Also, it was shown that a combination of thermal gravimetric analysis and Raman spectroscopy, both of which take only a few minutes and require little sample preparation, are sufficient as qualitative screening tools to determine the relative purity of SWNTs. Other analytical techniques were used to verify the validity of the screening techniques.     

A novel hybrid of carbon nanotubes/iron nanoparticles: iron-filled nodule-containing carbon nanotubes

A straightforward, one-step method for the preparation of novel carbon nanotube/iron nanoparticle hybrids with some degree of shape control is reported herein. These carbon nanostructures differ from those reported previously: the nanoparticles were not attached to or coated onto the surface of carbon nanotubes but embedded inside the carbon wall. They were synthesized in good yield by thermolysis of ferrocene and thiophene mixtures in a closed steel vessel. The shapes and compositions of these nanostructures can be simply controlled by adjusting the reaction temperature and relative amounts of the precursors. Iron-filled T-junction carbon nanotubes were also obtained easily by this procedure. These iron-filled nodule-containing carbon nanotubes (INCNTs) are either empty or filled with iron or iron carbide (Fe(C)) nanowires. The outer diameters of these nanotubes range from 70 to 150 nm and the lengths reach up to several micrometers. The average size of the Fe(C) nanoparticles (or empty cores) inside the nodules is about 50 nm in diameter. The carbon in the INCNTs is amorphous. Sulfur was found being responsible for the disordered structure and playing a unique role in promoting the growth of INCNTs as well as the formation of T- or Y-junctions.     

X-ray absorption near-edge structure and photoelectron spectroscopy of single-walled carbon nanotubes modified by a HBr solution

X-ray absorption near-edge structure (XANES) spectroscopy and photoelectron spectroscopy (PES) have been used to investigate single-walled carbon nanotubes (SWNTs) modified by immersion in a HBr solution at room temperature. After treatment XANES spectra of SWNTs show a new pronounced feature, which has been assigned to new bonds between the sidewall of the SWNTs and Br atoms. This investigation demonstrates the unique capabilities of the XANES spectroscopy as a tool to achieve structural and bonding information of carbon nanotubes induced by chemical processes.     

Encapsulating C59N azafullerene derivatives inside single-wall carbon nanotubes

Filling of single-wall carbon nanotubes with C₅₉N azafullerene derivatives is reported from toluene solvent at ambient temperature. The filling is characterized by high-resolution transmission electron microscopy and Raman spectroscopy. The filling efficiency is the same as for C₆₀ fullerenes and the tube-azafullerene interaction is similar to the tube-C₆₀ interaction. Vacuum annealing of the encapsulated azafullerene results in the growth of inner tubes, however no spectroscopic signature of nitrogen built in the inner walls is detected.     

Oxidative functionalization of carbon nanotubes in atmospheric pressure filamentary dielectric barrier discharge (APDBD)

The surface compositional and any structural changes that occur on carbon nanotubes using air-atmospheric pressure dielectric barrier discharge (APDBD) for functionalization are investigated employing Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), thermal gravimetric analysis (TGA), X-ray diffraction (XRD), and neutron diffraction techniques. Atmospheric pressure plasmas (APP) are suggested to be particularly suitable for functionalization of aligned nanotubes, where wet chemical manipulation could damage or even destroy the highly desirable vertical alignment. In this work a detailed experimental study elucidating the effects of APDBD plasma treatment parameters (e.g. power density, discharge composition, inter-electrode gap and treatment time) on the electronic structure, physical, and chemical behaviour of carbon nanotubes has been conducted. In an atmospheric air we find an optimal oxidative functionalization of CNTs in our DBD system within few seconds (<5 s) at a discharge power of ∼0.5 kW. This investigation may find useful application as functionalization technique for CNT engineered devices and sensors.     

Structure of carbon nanotubes probed by local and global probes

A review is proposed of different techniques available today for the characterization of the atomic structure of carbon nanotubes. This review covers the electron microscopies, various diffraction techniques, scanning probe microscopies, and optical spectroscopies, including Raman scattering. The advantages and limitations of the characterization techniques are discussed.     

Etching effects of ethanol on multi-walled carbon nanotubes

The morphology and microstructure of multi-walled carbon nanotubes (MWCNTs) were modified using ethanol as a mild gas reactant. The etching by OH radicals and deposition of C radicals on the carbon nanotubes were considered to be responsible for the modification of the MWCNT structures and the formation of new carbon nanostructures. The effects of etching and deposition on the MWCNTs were confirmed by using methanol as another gas reactant; this molecule has a higher ratio of hydroxyl radicals to carbon atoms than ethanol. In addition, water vapor, containing no carbon atoms in the molecule, was also applied to etch the MWCNTs as a weak oxidant which resulted in stronger etching effects on the MWCNTs than methanol and ethanol.     

Reduction of solubilized multi-walled carbon nanotubes

In this paper, the chemical reduction of solubilized multi-walled carbon nanotubes by LiAlH₄ was investigated. The amide groups on the nanotubes could be reduced to hydroxyl groups, which was confirmed by FTIR and XPS studies. The Raman spectroscopic investigation showed that the morphology of the nanotubes did not change after the reduction.     

Functionalization of single-walled carbon nanotubes by using alkyl-halides

In this paper we demonstrate the functionalization of single-walled carbon nanotubes prepared by chemical vapor deposition. The chosen functionalization agents were alkyl-halides such as trifluoromethane (TFM) and trichloromethane (TCM); or double bond containing alkyl-halides as tetrachloroethylene (TCE) and hexafluoropropene (HFP) that can easily form radicals. Functionalization of samples was carried out under mild conditions, by ball milling of nanotubes in an atmosphere of functionalization agent, at room temperature. For the sake of comparison, chlorination was also performed by chlorine gas. In this process the cleavage of nanotube C-C bonds results in active sites, which can activate molecules in gas phase or adsorbed on the surface of carbon nanotubes. Halogenated samples were characterized by means of particle induced γ-ray emission, transmission electron microscopy, thermogravimetry, and X-ray photoelectron spectroscopy. We concluded that this method gives functionalized single-walled carbon nanotubes in the range of 0.3-3.5 wt.% of fluorine and 5.5-17.5 wt.% of chlorine.     

Catalytic growth of carbon nanotubes through CHNO explosive detonation

Multi-walled carbon nanotubes (CNTs) have been efficiently synthesized by a self-heating detonation process, operated at low loading densities of picric acid (PA), which acts as the explosive to provide needed high temperatures and parts of carbon sources. Paraffin or benzene provides additional carbon source for tube assembling and hydrogen source to capture oxygen in PA to form H₂O and thus to survive some carbon species from oxidation. Cobalt nanoparticles, in situ formed from a detonation-assisted decomposition and reduction of cobalt acetate, show good catalytic activity for nanotube nucleation and growth and for disproportionation reaction of CO generated from the PA detonation. The nanotubes and catalyst particles are characterized by SEM, TEM, EDX, SAED, XRD, and Raman spectroscopy techniques. Some tubes are well crystallized but others have lots of structural defects, especially for the tubes with thin walls and bamboo-like shapes. The catalyst particles show conical shapes and exhibit a fcc crystalline structure of parent cobalt. These data also experimentally show that tube growth is at a very high rate and suggest that it is possible for a large-scale synthesis of CNTs under high-density and high-pressure conditions.     

Structure modification of single-wall carbon nanotubes

Various purification processes were applied to single-wall carbon nanotubes synthesized by metal catalyzed laser ablation. Structure modifications introduced by these processes were investigated by high-resolution transmission electron microscopy and Raman spectroscopy. An apparent structure modification after purification was the increase of bundle size although breaking of nanotubes and a change of nanotube diameter distribution were also observed. More vigorous attacking of single-wall carbon nanotube structure was identified by a strong mixed-acid treatment.     

Preparation and characterisation of single-walled carbon nanotubes functionalised with amines

The amines octadecylamine, 2-aminoanthracene, 1-H,1-H-pentadecafluorooctylamine, 4-perfluorooctylaniline and 2,4-bis(perfluorooctyl)aniline, have been reacted with chemically modified single-walled carbon nanotubes (SWCNTs). The chemical modification consisted on a thermal treatment in air of arc discharge-grown SWCNTs with an optional purification with HNO₃. The selected amines have been allowed to interact with the obtained materials and with those resulting from an additional treatment with thionyl chloride. The resulting materials were characterised with different spectroscopic techniques such as X-ray photoelectron spectroscopy, attenuated total reflectance Fourier transform infrared spectroscopy, Raman spectroscopy, electron energy loss spectroscopy and fluorescence spectroscopy complemented with the thermogravimetric analysis and transmission electron microscopy. The chemically modified SWCNTs exhibit different solubility properties depending on the amines and the solvents used.     

Fabrication of superhydrophobic surfaces with non-aligned alkyl-modified multi-wall carbon nanotubes

Films of superhydrophobic multi-wall carbon nanotubes (MWCNTs) have been obtained by using alkyl-modified MWCNTs (MWCNT(COOC₁₈H₃₇)_n) and a simple and effective preparation method. The films show both a high contact angle and a small sliding angle for water droplets. A particular characteristic is that on the superhydrophobic surface the alkyl-modified MWCNTs are not intentionally aligned, thus avoiding the preparation techniques using aligned carbon nanotubes to produce the same effect.     

Biomolecules as selective dispersants for carbon nanotubes

Both single-wall or multi-wall nanotube growth result in a soot-like material that contains the desired product. The selective recovery of the nanotube product from this soot still represents a challenge. Methods to date either result in chemical modification of the nanotubes themselves, are time consuming or use expensive to produce polymers. Using a variety of biomolecules, we have been able to selectively suspend multi-wall carbon nanotubes in aqueous solutions while leaving behind the extraneous by-products in the precipitate. At comparable biomolecule to soot ratios, all biomolecules selectively retained more nanotubes than an organic polymer previously used to purify multi-wall nanotubes (PmPV—poly m-phenylene-co-2,5-dioctoxy-p-phenylenevinylene). The highest recovery as determined by Electron Paramagnetic Resonance was 56% of the available nanotubes. The method provides a simple, one-step, non-destructive purification process that facilitates the formation of pure multi-wall carbon nanotube containing biodispersions.