The environmental effect on the radial breathing mode of carbon nanotubes. II. Shell model approximation for internally and externally adsorbed fluids

We have previously shown that the upshift in the radial breathing mode (RBM) of closed (or infinite) carbon nanotubes in solution is almost entirely due to coupling of the RBM with an adsorbed layer of fluid on the nanotube surface. The upshift can be modeled analytically by considering the adsorbed fluid as an infinitesimally thin shell, which interacts with the nanotube via a continuum Lennard-Jones potential. Here we extend the model to include internally as well as externally adsorbed waterlike molecules, and find that filling the nanotubes leads to an additional upshift of two to six wave numbers. We show that using molecular dynamics, the RBM can be accurately reproduced by replacing the fluid molecules with a mean field harmonic shell potential, greatly reducing simulation times.     

The effect of carbon nanotubes on epoxy matrix nanocomposites

The paper concerns thermal properties of epoxy/nanotubes composites for aircraft application. In this work, influence of carbon nanotubes on thermal stability, thermal conductivity, and crosslinking density of epoxy matrix was determined. Three kinds of nanotubes were used: non-modified with 1- and 1.5-μm length, and 1-μm length modified with amino groups. Scanning electron microscopy observations were done for examining dispersion of nanotubes in the epoxy matrix. Glass transition temperature (T _g) was readout from differential scanning calorimetry. From dynamic mechanical analysis, crosslinking density was calculated for epoxy and its composites. Also, thermogravimetric analysis was done to determine influence of nanotubes addition on thermal stability and decomposition process of composites. Activation energy was calculated from TGA curves by Flynn–Wall–Ozawa method. Thermal diffusivity was also measured. SEM images proved the uniform dispersion of carbon nanotubes without any agglomerates. It was found that nanotubes modified with amino groups lead to the increase of epoxy matrix crosslinking density. The significant increase in T _g was also observed. On the other hand, addition of carbon nanotubes leads to the decrease of thermal stability of polymer due to the increase of thermal diffusivity.     

The effect of catalyst concentration on the synthesis of single-wall carbon nanotubes

Single wall carbon nanotubes (SWNTs) were synthesized by electric arc discharge method with a mixture of nickel and yttrium as catalysts. The effect of the catalyst concentration on the synthesis of SWNTs was studied. Raman spectra of SWNTs have been recorded with excitation wavelengths from 476.5 to 1064 nm. The Raman peaks of the radial breathing modes (RBM) of SWNTs were assigned. The results indicate that the diameter distribution of SWNTs is in the range of 1.2-1.6 nm, and the SWNTs with diameter 1.43 nm are in the majority. The catalyst concentrations have large effect on the yield of SWNTs and little effect on the diameter distribution of SWNTs.     

The defluoridation of drinking water using multi-walled carbon nanotubes

In this study, the potential sorption capacity of multi-walled carbon nanotubes (MWCNTs) was investigated as a means of removing fluoride from the drinking water of a number of regions in Iran and from experimental solutions. The test was conducted in both batch and continuous operation modes. Batch mode experiments were used to study the effect of parameters such as pH, contact time, ionic strength, adsorbent dose, adsorbent capacity, and the presence of foreign anions on the efficiency of fluoride removal. The results showed that the highest level of sorption occurs at pH 5 (about 94% at 18 min). The ionic strength of the solutions and the presence of co-anions such as chloride, nitrate, sulfate, hydrogen carbonate, perchlorate had a negligible effect on the sorption of F⁻ onto MWCNTs. Sorption capacity measurements revealed that MWCNTs have a saturation capacity of 3.5 mg of F⁻ per gram. Sorption data were best fitted with the Fruendlich sorption isotherm equation, which indicates that F⁻ tends to be adsorbed on MWCNTs in a multilayered manner. Experiments using Kohbanan city drinking water, which contains the highest level of F⁻ among the drinking water samples studied, showed that MWCNTs can remove over 85% of fluoride content.     

Multi-walled carbon nanotubes as a solid-phase extraction adsorbent for the determination of chlorophenols in environmental water samples

Multi-walled carbon nanotubes (MWCNs) are used as adsorbent for solid-phase extraction (SPE) of several chlorophenols (CPs). CPs were adsorbed on MWCNs cartridge, then desorbed with pH 10.0 methanol, finally determined by HPLC. Under the optimized conditions, detection limits of 0.08-0.8 ng mL(-1) were obtained. The method had been applied to analyze the five CPs in tap water and river water.     

Single mode phonon energy transmission in functionalized carbon nanotubes

Although the carbon nanotube (CNT) features superior thermal properties in its pristine form, the chemical functionalization often required for many applications of CNT inevitably degrades the structural integrity and affects the transport of energy carriers. In this article, the effect of the side wall functionalization on the phonon energy transmission along the symmetry axis of CNT is studied using the phonon wave packet method. Three different functional groups are studied: methyl (-CH(3)), vinyl (-C(2)H(3)), and carboxyl (-COOH). We find that, near Γ point of the Brillouin zone, acoustic phonons show ideal transmission, while the transmission of the optical phonons is strongly suppressed. A positive correlation between the energy transmission coefficient and the phonon group velocity is observed for both acoustic and optical phonon modes. On comparing the transmission due to functional groups with equivalent point mass defects on CNT, we find that the chemistry of the functional group, rather than its molecular mass, has a dominant role in determining phonon scattering, hence the transmission, at the defect sites.     

Synthesis and evaluation of molecularly imprinted core-shell carbon nanotubes for the determination of triclosan in environmental water samples

Synthetic core-shell molecularly imprinted polymers (MIPs) were prepared for the extraction of trace triclosan in environmental water samples. The synthesis process combined a surface molecular imprinting technique with a sol-gel process based on carbon nanotubes (CNTs) coated with silica. The morphology and structure of the products were characterized by transmission electron microscopy and Fourier transform infrared spectroscopy. The adsorption properties of the polymers were demonstrated by equilibrium rebinding experiments and Scatchard analysis. The prepared imprinted materials exhibited fast kinetics, high capacity and favorable selectivity. The process of synthesis was quite simple and different batches of MIPs and non-imprinted polymers (NIPs) showed good reproducibility in the template binding. The feasibility of determination of triclosan from real samples was testified using spiked river and lake water samples. The recoveries of river water and lake water samples were ranged from 92.1 to 95.3% and 90.7 to 93.6%, respectively, when the environmental water samples were spiked with 0.1, 0.3, and 0.5 μg L(-1) of TCS. In addition, the reusability of MIPs and NIPs without any deterioration in capacity was demonstrated for at least 10 repeated cycles.     

Curvature effect in the longitudinal unzipping carbon nanotubes

Density functional theory (DFT) calculations are performed to analyze curvature effects in the oxidative longitudinal unzipping of carbon nanotubes (CNTs) of different diameters. The reactions considered involve the adsorption of permanganate, followed by the oxidation of the nanotube, which results in dione and hole formation. The study was performed with armchair CNTs of different diameters and with corrugated graphene layers, which emulate the curvature of CNT of larger radii, with the finding that the curvature and the pyramidalization angle of the these structures strongly affects the stability of the intermediate dione structure formed during the unzipping process. Permanganate adsorption energies increase for more curved surfaces promoting the oxidation reaction in surfaces of small radius, making this reaction spontaneous for small radius. The second permanganate adsorbs on the parallel carbon–carbon bond to first diona formation resulting the longitudinal unzipping of the CNT.     

The effect of atomic hydrogen adsorption on single-walled carbon nanotubes properties

We investigated the adsorption of hydrogen atoms on metallic single-walled carbon nanotubes using ab initio molecular dynamics method. It was found that the geometric structures and the electronic properties of hydrogenated SWNTs can be strongly changed by varying hydrogen coverage. The circular cross sections of the CNTs were changed with different hydrogen coverage. When hydrogen is chemisorbed on the surface of the carbon nanotube, the energy gap will be appeared. This is due to the degree of the sp³ hybridization, and the hydrogen coverage can control the band gap of the carbon nanotube.     

Laponite assisted dispersion of carbon nanotubes in water

The ability of Laponite to stabilize aqueous suspensions of multiwalled carbon nanotubes (MWCNTs) was investigated with the help of analytical centrifugation, microscopic image analysis, and measurements of electrical conductivity of hybrid Laponite+MWCNT suspensions. The impact of nanotube concentration C(n) (0.0025-0.5 wt%) and Laponite/MWCNTs ratio X (varied within 0-1 wt/wt) on the properties of Laponite+MWCNT hybrid suspensions was discussed. It was observed that sonication of MWCNTs at critical minimal concentration of Laponite X(c)≈0.25±0.05 resulted in efficient dispersion and formation of stabilized suspensions of individual nanotubes. The stabilization of nanotubes in the presence of Laponite was explained by adsorption of Laponite particles and formation of a hydrophilic charged shell on the surface of nanotubes. Increase of MWCNT concentration above the critical value resulted in percolation and formation of spatially extended electrically conductive networks of particles.     

The effect of gas adsorption on the field emission mechanism of carbon nanotubes

We have investigated systematically the effects of various gas adsorbates (H2, N2, O2, and H2O) on the electronic structures and the field emission properties of open edges of single-walled carbon nanotubes by density functional calculations. All of the molecules, except N2, dissociate and chemisorb on open nanotube edges with large adsorption energies. The Fermi levels are moved toward the valence (conduction) bands for O2 (H2, H2O) adsorption induced by the Mulliken charge transfer on the tube edge. The Fermi level shift for N2 adsorption is negligible. Adsorption of H2O enhances the field emission current, whereas H2 adsorption does not affect the field emission current much because of the absence of the density of states near the Fermi level. The correlation of the electronic structures and the field emission current is further discussed.     

The effect of arylferrocene ring substituents on the synthesis of multi-walled carbon nanotubes

The synthesis of shaped carbon nanomaterials (SCNMs) such as carbon nanotubes (CNTs), amorphous carbon, carbon fibres (CFs) and carbon spheres (CSs) was achieved using para-substituted arylferrocenes, FcPhX (X = H, OH, Br, COCH₃) or a mixture of ferrocene (FcH) and substituted benzenes (PhX; X = H, OH, Br, COCH₃). The reactions were carried out by an injection chemical vapour deposition (CVD) method using toluene solutions (carrier gas: 5% H₂ in Ar at a flow rate of 100 ml/min) in the temperature range of 800-1000 °C. In most instances multi-walled CNTs (MWCNTs) were produced. Variations in the concentrations of precursor catalysts, the injection rate and temperature affected the type, distribution and dimensions of the SCNMs produced. The overall finding is that the presence of Br and O in these studies significantly reduces CNT growth. A comparative study on the effect of FcPhX versus FcH/PhX mixtures was investigated. The SCNMs were characterized by transmission electron microscopy (TEM), Raman spectroscopy and thermal gravimetric analysis (TGA).     

The adsorption of resorcinol from water using multi-walled carbon nanotubes

Adsorption of resorcinol and other phenolic derivatives on pristine multi-walled carbon nanotubes (MWCNTs) and HNO₃ treated MWCNTs has been investigated in attempt to explore the possibility to use MWCNTs as efficient adsorbents for pollutants. MWCNTs showed higher adsorption ability in a rather wide pH range of 4–8 for resorcinol, while decreased uptake capacity was found for acid-treated MWCNTs. Other phenolic derivatives such as phenol, catechol, hydroquinone and pyrogallol were employed to study the influence of the number and position of hydroxyl groups on the adsorption capacity. The amounts adsorbed by MWCNTs increased with the increasing number of hydroxyl. The substitution of phenol with a hydroxyl in meta-position leads to a much higher absorption ability than substitution in ortho- or para-position, which suggested that MWCNTs possess a great potential in removal of resorcinol from water, as well as the other phenolic derivatives.     

Functionalization of single-walled carbon nanotubes "on water"

Single-walled carbon nanotubes (SWNTs) are exfoliated and functionalized into small bundles and individuals by vigorous stirring "on water" in the presence of a substituted aniline and an oxidizing agent. This is an example of an "on water" reaction that leads to functionalized SWNTs, and it represents a "green", or environmentally friendly, process. A variety of reaction conditions were explored. The products were analyzed with Raman, UV-vis-NIR, and X-ray photoelectron spectroscopies, atomic force and transmission electron microscopies, and thermogravimetric analysis.     

Joint adsorption of benzene and water on carbon nanotubes

The joint adsorption of water and benzene on nanosized carbon tubes (NCTs) (with a specific surface area of 413 m²/g) synthesized by carbonizing methylene chloride in cylindrical pores of an Al₂O₃ matrix was studied. ¹H NMR spectroscopy with layer-by-layer freezing of the liquid phase was used to characterize the water bound in pores at various contents of benzene and water. Due to its higher energy of interaction with carbon surfaces, benzene was demonstrated to decrease the energy of interaction of water with the surface of the NCT sample from 43 to 15 J/g. It was suggested that, in the presence of benzene, H-bonded water clusters only weakly bound to the surface are formed in the cylindrical cavities of the NCTs.     

Formation of carbon nanotubes in water by the electric-arc technique

A simplified arc discharge apparatus was used for growing carbon nanotubes, required only water (solution) in a glass container with no need for vacuum, water-cooled chamber. Carbon nanotubes with highest purity (20%) and highest yield (7 mg/min) were obtained when using salt solution as the medium. Resonance Raman spectrum of multi-walled carbon nanotubes (MWNTs) presented in as-grown materials was measured and RBM peaks originating from very thin core nanotubes were observed. The results show that high-quality MWNTs can be effectively prepared in water-arcing process.     

Structures and electronic transport of water molecular nanotubes embedded in carbon nanotubes

In this paper, ice nanotubes confined in carbon nanotubes are investigated by molecular dynamics. The trigonal, square, pentagonal, and hexagonal water tubes are obtained, respectively. The current-voltage (I-V) curves of water nanotubes are found to be nonlinear, and fluctuations of conductance spectra of these ice nanotubes show that the transport properties of ice nanotubes are quite different from those of bulk materials. Our studies indicate that the conductance gap of ice nanotube is related to the difference value from the Fermi energy EF to the nearest molecular energy level E0. Increasing the diameter of a water molecular nanostructure results in the increase of the conductance.     

Simulated water adsorption in chemically heterogeneous carbon nanotubes

Grand canonical Monte Carlo simulations are used to study the adsorption of water in single-walled (10:10), (12:12), and (20:20) carbon nanotubes at 298 K. Water is represented by the extended simple point charge model and the carbon atoms as Lennard-Jones spheres. The nanotubes are decorated with different amounts of oxygenated sites, represented as carbonyl groups. In the absence of carbonyl groups the simulated isotherms are characterized by negligible amounts of water uptake at low pressures, sudden and complete pore filling once a threshold pressure is reached, and wide adsorption-desorption hysteresis loops. In the presence of a few carbonyl groups the simulated adsorption isotherms are characterized by pore filling at lower pressures and by narrower adsorption-desorption hysteresis loops compared to the results obtained in the absence of carbonyl groups. Our results show that the distribution of the carbonyl groups has a strong effect on the adsorption isotherms. For carbonyl groups localized in a narrow section the adsorption of water may be gradual because a cluster of adsorbed water forms at low pressures and grows as the pressure increases. For carbonyl groups distributed along the nanotube the adsorption isotherm is of type V.     

Deuterium attachment to carbon nanotubes in deuterated water

A systematic investigation on the unusual attachment of labile deuterium to carbon nanotubes in deuterated water and alcohols is reported. The carbon nanotubes were solubilized through the established functionalization of the nanotube-bound carboxylic acids to allow solution-phase reaction and characterization. The deuterium attachment was found under several experimental conditions, including the use of deuterated ethanol as a co-reactant in the nanotube functionalization reaction and the refluxing of functionalized or simply purified carbon nanotubes in deuterated water and alcohols. The solubility of the functionalized carbon nanotube samples in common organic solvents and water allowed unambiguous (2)H NMR characterization. The reproducible broad (2)H NMR signal at approximately 6.5 ppm is assigned to carbon nanotube-attached deuterium species. The assignment is supported by the results from FT-IR measurements. The carbon-deuterium interaction is so strong that the corresponding vibration resembles the typical C-D stretching mode in the characteristic frequency region. The FT-IR peak intensities also correlate well with the (2)H NMR signal integrations in a series of samples. Mechanistic implications of the results are discussed.     

Modification and dispersion of multi-walled carbon nanotubes in water

In the present work, the decorated purified raw multi-walled carbon nanotubes (R-MWCNTs) were obtained by chemical modification (CM) by treatment with concentrated sulfuric acid and concentrated nitric acid mixture with a certain volume ratio of 1: 3. The R-MWCNTs and CM-MWCNTs samples were investigated by X-ray Diffraction (XRD) analysis and Fourier transform infrared spectroscopy (FT-IR). The prepared MWCNTs were homogeneously dispersed in water using a commercial surfactant (Polyvinyl pyrrolidone, (PVP)) and ultra-sonication. The dispersion of MWCNTs was obtained by UV-Vis analysis. The results show that chemical modification purified MWCNTs and more effective functional groups were attached on the surface of MWCNTs. Meanwhile, R-MWCNTs and CM-MWCNTs were uniformly distributed in aqueous PVP solution and the dispersion of CM-MWCNTs in water was better.