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.     

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).     

Fast and clean functionalization of carbon nanotubes by dielectric barrier discharge plasma in air compared to acid treatment

Multi-walled carbon nanotubes (MWCNTs) have been functionalized by a dielectric barrier discharge plasma in air and compared to those functionalized in HNO₃. The MWCNTs were prepared by chemical vapor deposition of xylene using ferrocene as a catalyst at 850 °C. Air oxidation followed by acid treatment was used to purify the MWCNTs, which were then annealed in helium. The MWCNTs were functionalized in air in a plasma reactor at room temperature. Quantitative analyses of gases evolved during the temperature programmed desorption of the functionalized nanotubes were carried out using Fourier transform infrared spectroscopy and gas chromatography. The influence of plasma parameters, including power in the range of 8-90 W and treatment time in the range of 1-9 min, on the number of the functional groups was investigated. It is shown that the extent of functionalization increases with increasing discharge power, provided that the exposure time of the MWCNTs in the plasma atmosphere does not exceed a certain period of time. Compared to acid treatment, plasma functionalization offers the advantages of much shorter treatment time, and produces less damage.     

Nanostructured CNx (0 < x < 0.2) films grown by supersonic cluster beam deposition

Nanostructured CN_x thin films were prepared by supersonic cluster beam deposition (SCBD) and systematically characterized by transmission electron microscopy (TEM), electron energy-loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The incorporation of nitrogen in the films (0 < x < 0.2) and the nanostructure were controlled by using different synthesis routes. Films containing bundles of well-ordered graphene multilayers, onions and nanotubes embedded in an amorphous matrix were grown alongside purely amorphous films by changing the deposition parameters. Graphitic nanostructures were synthesized without using metallic catalysts. The structural and electronic properties of the films have been studied by EELS. The role played by N in the carbon nanostructures has been deduced from XPS line-shape analysis.     

Surface-modified carbons as platinum catalyst support for PEM fuel cells

Carbon forms, such as activated carbon, carbon black, carbon nanofibers and nanotubes, can be used as support materials for precious metal catalysts used in fuel cell electrodes. This work first compares the ability of functionalized high surface area graphitic (carbon nanofibers) and amorphous (activated carbon) carbons to homogeneously support finely divided platinum catalyst particles, then contrasts the performance of platinum/carbon composite electrodes within a hydrogen fuel cell. Functionalization by concentrated acid treatment results in the creation of various oxygen carrying functionalities on the otherwise inert carbon surfaces. The degree of surface functionalization is found to be a function of the functionalization treatment strength. Chemical reduction of the platinum precursor complex using milder reducing agents in the temperature range of 75-85 °C, and using ethylene glycol at 140 °C yields the smallest platinum particle sizes observed in this study, a result confirmed by X-ray diffraction and transmission electron microscopy measurements. X-ray photoelectron spectroscopy measurements confirm the existence of platinum in primarily its metallic state on the functionalized carbon surfaces.     

Prediction of carbon nanotube growth success by the analysis of carbon-catalyst binary phase diagrams

The growth of carbon nanotubes (CNTs) was attempted using a vapor-phase CVD method, and a wide variety of transition metals, in the form of metallocenes and chlorides, were used as potential growth catalysts. Well-aligned mats of multi-walled carbon nanotubes were observed in samples catalyzed with iron, nickel, and cobalt. Nanotube lengths could be varied between 3 μm and 3 mm, and diameters ranged from 20 nm to 60 nm. X-ray diffraction, energy dispersive X-ray spectroscopy, and Mossbauer spectroscopy were used to determine the compositions of the nanotube products. None of the other elements studied, consisting of Cr, Mn, Zn, Cd, Ti, Zr, La, Cu, V, and Gd, were able to successfully catalyze the growth of CNTs. A study was performed of the respective metal-carbon binary phase diagrams to understand why the catalytic ability of different elements varied. Successful catalysts had carbon solubility limits of 0.5 wt.% to 1.5 wt.% carbon, followed closely by nanotube growth through graphite precipitation. Unsuccessful catalysts were found to have either nearly zero carbon solubility, or to form numerous intermediate carbides, making it difficult for the diffusion required for graphite precipitation to occur. These findings were then used to look for other potential CNT catalysts by examining the phase diagrams of a wide variety of metals.     

Oxygen functionalization of multiwall carbon nanotubes by Ar/H2O plasma treatment

To increase the applicability of multiwall carbon nanotubes (MWCNTs), oxygen-containing functional groups were introduced on the surfaces of MWCNTs by using microwave-excited Ar/H₂O surface-wave plasma. X-ray photoelectron spectroscopy and Raman spectroscopy were used to determine dependencies of Ar/H₂O gas partial pressure, treatment time and microwave power. The oxygen functionalization of MWCNTs by plasma can be achieved very rapidly, about 10 min. The C-O and O-C═O fractions firstly increase and then decrease with increasing Ar partial pressure. The C-O and O-C═O fractions increase with increasing microwave power from 400 W to 700 W. A slight increase of the R (I_D/I_G ratio) value for the treated MWCNTs indicated disordering in the surface microstructure of MWCNTs coincident with the introduction of surface oxygen. The oxygen-containing groups introduced on the surfaces of MWCNTs by plasma treatment are hydrophilic. The dispersion of plasma treated MWCNTs is therefore improved.     

Structural studies of oriented carbon nanotubes in alumina channels using high energy X-ray diffraction

Structural studies of multi-wall carbon nanotubes prepared by template pyrolytic carbon deposition from thermal decomposition of propylene at 800 °C inside channels of an alumina membrane have been performed using X-ray diffraction. The two-dimensional diffraction pattern of the deposited carbon nanotubes, recorded directly within the alumina template using an image plate detector, exhibits two dark arcs corresponding to the (0 0 2) graphitic reflection. The anisotropic scattering distribution indicates alignment of the nanotubes. The diffracted intensity was measured for the powdered samples after removing the alumina membrane using a point detector. A maximum scattering vector of K_max = 20 Å⁻¹ yielded the radial distribution function, providing evidence that the investigated nanotubes form a distorted hexagonal network that implies the presence of five-membered rings.     

Luminescent short thiol-functionalized multi-wall carbon nanotubes

Luminescent short thiol-functionalized multi-wall carbon nanotubes (mean length 100-200 nm) were produced by the reaction between 2-aminoethanethiol molecules and oxidized carbon nanotubes with the aid of a coupling agent in ethanol. After the reactions stop the carbon nanotubes suspension was purified and filtered to separate the shorter carbon nanotubes. The short length carbon nanotubes fraction exhibits an intense luminescence visible to the naked eye. The maximum of the luminescence band and its intensity strongly depends on the excitation wavelength. The sample chemistry and morphology were characterized by means of X-ray photoelectron spectroscopy and scanning electron microscopy.     

Coalescence of single-walled carbon nanotubes and formation of multi-walled carbon nanotubes under high-temperature treatments

Electric arc-discharge single-wall carbon nanotubes are annealed between 1600 and 2800 °C under argon flow. Their stability and evolution are studied by coupling TEM, X-ray diffraction and Raman spectroscopy. The first modifications appear at 1800 °C with a significant decrease of the crystalline order. It is due to SWNTs coalescence leading to smaller bundles but with an increase of the tube diameters from 2 to 4 nm. From 2200 °C, SWNTs progressively disappear to the benefit of MWNTs having at first two to three carbon layers then reaching 7 nm external diameter. The possible mechanisms responsible for the SWNTs coalescence and instability and their transformation in MWNTs are discussed.     

Cathodoluminescence of fluorine doped amorphous carbon nanoparticles deposited by a filtered cathodic arc plasma system

The fluorine doped amorphous carbon nanoparticles (a-C:F NPs) films with sizes 50-100 nm were deposited on polyethylene terephthalate in an atmosphere of CF₄ by a 90°-bend magnetic filtered cathodic arc plasma system. The surface morphology of a-C:F NPs films was observed by field emission scanning electron microscope and atomic force microscope. The microstructure and chemical bonding nature of the a-C:F NPs films were investigated by Raman, X-ray diffraction and X-ray photoelectron spectroscopy. This work presents cathodoluminescence (CL) spectra of a-C:F NPs films obtained at 1.9-2.4 eV and verifies luminescence from a-C:F NPs films in the visible region. The incorporation of fluorine into the carbon network results in orange emission (∼2.03 eV) due to the transitions between fluorine-related electron levels and σ* states, and the red emission (∼1.97 eV) results from the recombination of carriers in the valence π and conduction π* states. The peak at ∼2.10 eV may result from the defects of the structures in a-C:F NPs films.     

A spectro-microscopic investigation of Fe-Co bimetallic catalysts supported on MgO for the production of thin carbon nanotubes

Fe-Co bimetallic catalysts supported on MgO were studied for the catalytic chemical vapor deposition growth of carbon nanotubes (CNTs). Different wt.% metal loadings were investigated at various deposition temperatures and times. Characterization of the products involved thermal analysis (DTA-TGA), X-ray diffraction, spectroscopy (Raman, UPS, EELS and STS) and microscopy (SEM, TEM and STM) techniques. It was found that the metal content is critical, not only to the yield and the structural quality of the synthesized carbon nanotubes, but it can be also used to tune the desired type of synthesized nanotubes. Lower (2 wt.%) loadings of Fe-Co catalysts favor the formation of single- and/or double-wall CNTs for deposition time and temperature 30 min and 800 °C, respectively. Thermal analysis and Raman measurements showed that these thin CNTs were synthesized at high amounts (CNT-per-catalyst wt.% of more than 100%), exhibiting high graphitization degree with only traces of by-products (mainly amorphous carbon) among them. Microscopy results revealed the formation of CNTs bundles, consisting of individual nanotubes with less than 2 nm outer diameter, while additional energy loss measurements pointed out that the deposited CNTs are mainly single wall. Higher (10 wt.%) Fe-Co loadings resulted to the formation of multi-wall CNTs.     

Synthesis, characterization, and electrochemical testing of carbon nanotubes derivatized with azobenzene and anthraquinone

Multi-walled and single-walled carbon nanotubes were side-wall functionalized with azobenzene and anthraquinone residues, i.e., chemical groups possessing redox activity, for potential utilization in functional catalysis and memory storage devices. Solvent-free synthesis was performed with diazonium salts generated in situ where it was found that it was simple and effective method. Nanotube functionalization was confirmed and characterized by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). It is worth noting, that single-walled carbon nanotubes (SWCNTs) functionalized with azobenzene produced Raman modes typical of substituted azobenzenes with spectral peaks at ∼1137, 1412, and 1447 cm⁻¹. The nanotubes containing electroactive substituents were transferred onto electrode substrates using the Langmuir-Blodgett approach and characterized by cyclic voltammetry. The amount of electroactive groups per mg of nanotubes was calculated based on the peak of cathodic current. A highly reproducible voltammetric response was obtained with a single nanotube layer or multiple nanotube/octadecanol layers. It is believed that devices such as these will be invaluable for future high-performance electrodes.     

Synthesis of novel Y-junction hollow carbon nanotrees

A simple catalyst-dwindling route without additional templates has been developed, for the first time, to synthesize novel Y-junction hollow carbon nanotrees in a high-yield of 90% by the reaction of ferrocene with 1,2-dichlorobenzene at 180 °C. The nanotrees have an average length about 5 μm, an average external trunk diameter of about 500 nm, branch diameters ranging from 70 to 100 nm, and wall thickness ranging from 24 to 30 nm. Unlike traditional carbon nanotubes, these nanotrees have close-ended branches but open-ended roots. X-ray powder diffraction, field emission scanning electron microscopy, transmission electron microscope, high-resolution transmission electron microscopy, electron diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy were used to characterize the products. It is also found that proper reaction temperatures and appropriate proportion of ferrocene and 1,2-dichlorobenzene play important roles in this route. The formation mechanism of treelike carbon nanotubes was simply proposed.     

Preparation and characterization of carbon membranes made from poly(phthalazinone ether sulfone ketone)

Carbon membranes were prepared from a novel polymeric precursor of poly(phthalazinone ether sulfone ketone) (PPESK), of which the changes of microstructure and chemical compositions during pyrolysis from 500 °C to 950 °C were monitored by thermal gravimetric analysis, X-ray diffraction, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. It has been found that the weight loss of the PPESK precursor up to 800 °C is about 43.0 wt%. After the heat treatment, the typical chemical structure of the PPESK precursor disappears, at the same time a graphite-like structure with more aromatic rings is formed. The interlayer spacing (i.e., d value) decreases from 0.471 nm to 0.365 nm as the pyrolysis temperature increases. The gas permeation performance of carbon membranes has been tested using pure single gases including H₂, CO₂, O₂ and N₂. For the carbon membrane obtained by carbonizing the PPESK precursor at 800 °C, the maximum ideal permselectivities for H₂/N₂, CO₂/N₂ and O₂/N₂ gas pairs could reach 278.5, 213.8 and 27.5, respectively.     

Mass production of high-quality multi-walled carbon nanotube bundles on a Ni/Mo/MgO catalyst

A new catalyst (Ni/Mo/MgO) is reported, with which one can synthesize multi-walled carbon nanotube (MWNT) bundles with a yield of more than 45 times the amount of the pristine catalyst, using a methane-hydrogen mixture as precursor. Powder X-ray diffraction, Raman spectroscopy and thermal gravimetric analysis show that the purity of the as-prepared MWNTs is over 97%. The diameter of the carbon nanotubes is 9-20 nm, measured by high-resolution electron microscopy on 421 individual MWNTs. The high purity of the as-prepared MWNTs allows us to omit the usual complex purification process required for carbon nanotubes synthesized by chemical vapor deposition. Because of its durable high activity, the Ni/Mo/MgO catalyst in its pristine state is ideal for mass production of high-quality MWNTs. The synergism of nickel and molybdenum is considered the main reason for the high yield of carbon nanotubes.     

SiN/bamboo like carbon nanotube composite electrodes for lithium ion rechargeable batteries

A dual stage technique employing hot filament chemical vapor deposition (HFCVD) and radio frequency sputtering was used to synthesize SiN/BCNTs (bamboo like carbon nanotubes) on copper substrates. The films were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Electron field emission studies (EFE), charge-discharge, and cyclic voltammetry. The comprehensive characterization is consistent with a nanolayer of amorphous SiN on BCNTs. Field emission experiments confirm the excellent contact of the SiN nanolayer with the surface of the BCNTs necessary for fabrication of a coin cell. Electrochemical testing shows that SiN/BCNT electrode can deliver an initial discharge capacity of 2000 mAh g⁻¹ which is higher than the capacity of graphite and the reversible capacity after ten cycles is 300 mAh g⁻¹. The cyclic voltammetry results suggest good reversibility with Li during cycling.     

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.     

Preparation and mechanical properties of highly-aligned carbon micro-trees

Carbon micro-trees with a diameter of 15-25 μm were synthesized in an acetylene atmosphere by arc-plasma assisted chemical vapor deposition from a coal-based carbon anode using iron as catalyst. The as-obtained carbon micro-trees were studied by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, polarized light microscopy and Raman spectroscopy. It is found that the carbon micro-trees grow on the anode surface as highly-oriented arrays and have an anisotropic yet highly graphitized structure with a solid inner core. The Young’s modulus of the micro-trees was determined to be 0.4 TPa on average, which is comparable to that of carbon nanotubes reported in literature.     

Synthesis of helically coiled carbon nanotubes by reducing ethyl ether with metallic zinc

Helically coiled carbon nanotubes (HCNTs) with diameter of 50-100 nm were obtained on a large scale by reducing ethyl ether with metallic zinc at 700 °C. As the temperature was raised to 750 °C, most of the products were solid carbon spheres besides HCNTs. X-ray diffraction pattern and the Raman spectroscopy indicate that the product is graphite phase. HCNTs with diameter of 50-100 nm in large quantity were found in the products.