Supramolecular microporous structures based on carbon nanotubes and coordinating cumene (C<Subscript>9</Subscript>H<Subscript>12</Subscript>) molecules

A method has been proposed for the formation of self-organized ensembles of carbon nanotubes with the use of coordinating cumene molecules and the development of secondary porosity in the obtained structures. It has been shown that the fraction of nanotubes coordinated into an array grows with increasing molar ratio between cumene molecules and carbon nanotubes upon the synthesis of the supramolecular structures. The secondary porosity develops due to partial desorption of the coordinating molecules from the obtained structure. A supramolecular structure with a residual cumene content of 50 wt % possesses the best adsorption characteristics for the described system. Specific adsorption of nitrogen on the “carbon nanotubes–cumene (50 wt %)” supramolecular structure at 293 K is more than an order of magnitude higher than that on the initial nanotubes. The structure-related energy parameters of the experimentally obtained supramolecular systems have been determined by molecular dynamics methods. The calculation in terms of the theory of volume filling of micropores has shown that the secondary pores of the obtained structures can accumulate methane and hydrogen in amounts as large as 213 nm³/m³ and 4 wt %, respectively.     

Fabrication of carbon nanotube sheets and their bilirubin adsorption capacity

Bilirubin adsorption on carbon nanotube surfaces has been studied to develop a new adsorbent in the plasma apheresis. Powder-like carbon nanotubes were first examined under various adsorption conditions such as temperatures and initial concentrations of bilirubin solutions. The adsorption capacity was measured from the residual concentrations of bilirubin in the solution after the adsorption process using a visible absorption spectroscopy. We found that multi-walled carbon nanotubes (MWCNTs) exhibit greater adsorption capacity for bilirubin molecules than that of single-walled carbon nanotubes (SWCNTs). To guarantee the safety of the adsorbents, we fabricated carbon nanotube sheets in which leakage of CNTs to the plasma is suppressed. Since SWCNTs are more suitable for robust sheets, a complex sheet consisting of SWCNTs as the scaffolds and MWCNTs as the efficient adsorbents. CNT/polyaniline complex sheets were also fabricated. Bilirubin adsorption capacity of CNTs has been found to be much larger than that of the conventional materials because of their large surface areas and large adsorption capability for polycyclic compound molecules due to their surface structure similar to graphite.     

Interaction of water with single-walled carbon nanotubes: reaction and adsorption

The interaction of water vapor with carbon nanotubes at room temperature has been investigated using Fourier transform (FT) IR spectroscopy and density functional theory (DFT) calculations. FTIR data indicate that water molecules adsorb on single-walled carbon nanotubes at room temperature. Comparison to previous studies suggests that the water forms hydrogen-bonded structures inside the nanotubes. Analysis of the FTIR data demonstrates that a small number of water molecules react with the nanotubes, forming C-O bonds, whereas a majority of the water molecules adsorb intact. The DFT calculations show that cleavage of an O-H bond upon adsorption to form adsorbed -H and -OH groups is energetically favorable at defect sites on nanotubes.     

Quantum chemical studies on hydrogen adsorption in carbon-based model systems: role of charged surface and the electronic induction effect

Quantum chemical studies on the molecular hydrogen adsorption in a six-membered carbon ring has been undertaken to mimic the adsorption process in carbon nanotubes, considering the fact that the six-membered carbon ring is found to be one of the basic units of the carbon nanotubes and fullerenes. Our results reveal that the carbon surface as such is not a good candidate for hydrogen adsorption but a charged surface created by doping of an alkali metal atom can play an important role for the improvement in adsorption of molecular hydrogen. The strength of hydrogen interaction as well as the number of hydrogen molecules that can be adsorbed on the system is found to depend on the nature of the cation doped in the system. We have also studied the role of electronic induction by substituting different functional groups in the model system on the hydrogen adsorption energy. The results demonstrate that the binding energy of the cation with the carbon surface as well as the hydrogen adsorption energy can be tuned significantly through the use of suitable substituents. In addition, we have shown that the extended planar or the curved carbon surface of the coronene system alone may not be suitable for an effective molecular hydrogen adsorption. In essence, our results reveal that the ionic surface with a significant degree of curvature will enhance the hydrogen adsorption effectively.     

Soft-x-ray photoemission spectroscopy and ab initio studies on the adsorption of NO2 molecules on defective multiwalled carbon nanotubes

The adsorption of NO(2) molecules on defective multiwalled carbon nanotubes has been studied by soft-x-ray photoemission. The valence band and carbon core-level spectra have been acquired before, during, and after NO(2) exposure. The spectra show a reversible decrease of the density of states at the top of the valence band when NO(2) molecules are adsorbed on the (carbon nanotubes) CNTs. No shift of the C 1s spectra has been observed. Theoretical calculations, using density-functional theory, have been performed on the CNT + NO(2) system, considering semiconducting nanotubes with different diameters and introducing a Stone-Wales [Chem. Phys. Lett. 128, 501 (1986)] defect. The calculation confirms the decrease of the density of states at the top of the valence band in the CNT + NO(2) system, while close to the adsorption site new states appear very close to the Fermi level.     

Adsorption of polyaromatic hydrocarbons on single wall carbon nanotubes of different functionalities and diameters

Adsorption from toluene solution of phenanthrene and tetracene on single wall carbon nanotubes (SWCNT) is measured. Comparison of adsorbents such as laser ablation and HipCO samples reveals multiple factors influencing the adsorption mechanism. Acid functionalized carbon nanotubes have shown markedly increased adsorbability for the polyaromatic molecules. The linear tetracene molecule's adsorption is more promoted on nanotubes with increasing diameter, but also additionally with presence of the carboxylic groups. The adsorption mechanisms on carboxylic sites and on the bold, non-functionalized large-diameter nanotubes are suggested and supported by detailed characterization of the SWCNTs applied.     

Adsorption of CF4 on the internal and external surfaces of opened single-walled carbon nanotubes: a vibrational spectroscopy study

Infrared spectroscopy has been used to make the first experimental discrimination between molecules bound by physisorption on the exterior surface of carbon single-walled nanotubes (SWNTs) and molecules bound in the interior. In addition, the selective displacement of the internally bound molecules has been observed as a second adsorbate is added. SWNTs were opened by oxidative treatment with O(3) at room temperature, followed by heating in a vacuum to 873 K. It was found that, at 133 K and 0.033 Torr, CF(4) adsorbs on closed SWNTs, exhibiting its nu(3) asymmetric stretching mode at 1267 cm(-1) (red shift relative to the gas phase, 15 cm(-1)). Adsorption on the nanotube exterior is accompanied by adsorption in the interior in the case of opened SWNTs. Internally bound CF(4) exhibits its nu(3) mode at 1247 cm(-1) (red shift relative to the gas phase, 35 cm(-1)). It was shown that, at 133 K, Xe preferentially displaces internally bound CF(4) species, and this counterintuitive observation was confirmed by molecular simulations. The confinement of CF(4) inside (10,10) single-walled carbon nanotubes does not result in the production of lattice modes that are observed in large 3D ensembles of CF(4).     

甘氨酸在纳米碳管中的吸附及性质的分子模拟

Molecular Mechanics and Molecular dynamics have been performed to study the adsorption and the diffusion,and optimize the configuration and the energy of glycine molecules in carbon nanotubes. The results of the simulation indicate that the configuration of glycine has been changed,and those varieties will bring on the changes of the biological properties via molecular biology. Carbon nanotube shows relatively strong sorption for glycine molecules,and carbon nanotubes and glycine molecules will produce relatively strong interaction of π-π electrons. The motions between glycine molecules and carbon nanotubes will keep very synergistic status in order to make the system remaining in the state of optimal energy among the simulation.     

Nerve agents interacting with single wall carbon nanotubes: Density functional calculations

First-principles calculations based on density functional theory (DFT) method are used to investigate the adsorption properties of nerve agent DMMP on typical zigzag (semiconducting) and armchair (metallic) single wall carbon nanotubes (SWCNTs). The adsorption energies for DMMP molecule on different adsorption sites on SWCNTs are obtained. The results indicate that DMMP is weakly bound to the outer surface of both the considered SWCNTs and the obtained adsorption energy values and binding distances are typical for the physisorption. We find that DMMP adsorptive capability of metallic CNTs is about twofold that of semiconducting one. The adsorption of DMMP on the higher chiral angle nanotubes was also investigated and the results indicate that nanotube’s chirality increases the adsorption capability of the tube but however the adsorption characteristic is typical for the physisorption. Furthermore, co-adsorption of two DMMP molecules on the SWCNTs as a single-layer/bi-layer of adsorbed molecules as well as the adsorption of one DMMP molecule on the CNT bundles consisting of three SWCNTs has also been examined. The obtained results reveal that for both the considered systems the binding energy was increased for the DMMP adsorption but it’s still typical for the physisorption, consistent with the recent experimental result. The study of the electronic structures and charge analysis indicate that no significant hybridization between the respective orbital takes place and the small interaction obtained quantitatively in terms of binding energies.     

Adsorption and diffusion of carbon dioxide and nitrogen through single-walled carbon nanotube membranes

We have used atomically detailed simulations to examine the adsorption and transport diffusion of CO2 and N2 in single-walled carbon nanotubes at room temperature as a function of nanotube diameter. Linear and spherical models for CO2 are compared, showing that representing this species as spherical has only a slight impact in the computed diffusion coefficients. Our results support previous predictions that transport diffusivities of molecules inside carbon nanotubes are extremely rapid when compared with other porous materials. By examining carbon nanotubes as large as the (40,40) nanotube, we are able to compare the transport rates predicted by our calculations with recent experimental measurements. The predicted transport rates are in reasonable agreement with experimental observations.     

Carbon nanotubes as sorbents for the gas phase preconcentration of semivolatile organics in a microtrap

In this paper we present the application of carbon nanotubes as unique sorbents for the fabrication of microtraps for the nanoscale adsorption/desorption of relatively large semivolatile organic molecules. The microtrap application requires high adsorption capacity as well as easy desorbability; the latter being critical for semivolatile compounds. The sorbent characteristics of single and multiwalled carbon nanotubes for gas phase adsorption/desorption of several compounds has been studied. The nonporous nature of carbon nanotubes (CNTs) eliminates the mass transfer resistance related to diffusion into pore structures, thus allowing easy desorbability. At the same time, their high aspects ratios lead to large breakthrough volumes. As compared to a commercial sorbent Carbopack, the breakthrough volume was as much as an order of magnitude higher in the CNTs, while the rate of desorption measured as the peak width at half height of the desorption band was eight times lower. The trapping and desorption characteristics of single and multi walled nanotubes were found to be comparable. We also found that the presence of disordered carbon impurities, which could be removed by controlled oxidative annealing could greatly degrade the performance of CNTs.     

Distribution patterns and controllable transport of water inside and outside charged single-walled carbon nanotubes

The density distribution patterns of water inside and outside neutral and charged single-walled carbon nanotubes (SWNTs) soaked in water have been studied using molecular dynamics simulations based on TIP3P potential and Lennard-Jones parameters of CHARMM force field, in conjunction with ab initio calculations to provide the electron density distributions of the systems. Water molecules show different electropism near positively and negatively charged SWNTs. Different density distribution patterns of water, depending on the diameter and chirality of the SWNTs, are observed inside and outside the tube wall. These special distribution patterns formed can be explained in terms of the van der Waals and electrostatic interactions between the water molecules and the carbon atoms on the hexagonal network of carbon nanotubes. The electric field produced by the highly charged SWNTs leads to high filling speed of water molecules, while it prevents them from flowing out of the nanotube. Water molecules enter the neutral SWNTs slowly and can flow out of the nanotube in a fluctuating manner. It indicates that by adjusting the electric charge on the SWNTs, one can control the adsorption and transport behavior of polar molecules in SWNTs to be used as stable storage medium with template effect or transport channels. The transport rate can be tailored by changing the charge on the SWNTs.     

Carbon nanotubes, new material for purification of water-ethanol mixtures from isomers of propanol

The possibility of purification of water-ethanol mixtures from the unwanted admixtures of higher alcohol, in particular, of the isomeric propanols, with the help of carbon nanomaterial is discussed. Results of quantum-chemical calculations of n-propanol and isopropanol on the surface of the single-walled carbon nanotube of the “armchair” type are presented. Investigations are carried out within the frame of model of molecular cluster using semi-empirical quantum chemical MNDO method. The possibility of adsorption of molecules of propanol isomers on the outer surface of nanotubes of small diameter as well the absence of adsorption of ethanol molecules on them is shown. It indicates the possibility of selective sorption with carbon nanotubes. Main geometric, electronic, and energy characteristics of obtained adsorption complexes are evaluated. A conclusion is made on the possibility to use carbon nanotubes for superfine purification of water-ethanol mixtures from the unwanted admixtures of n- and isopropanol.     

Effects of surface heterogeneity of carbon nanotubes in adsorption of colloid nanoparticles studied by means of computer simulations

This work deals with a construction of an implicit solvent model which can be used in molecular dynamics simulations of systems comprising colloid nanoparticles and carbon nanotubes. Such systems, due to finite sizes of both components, cannot be accurately approximated by a smaller slab geometry and thus represent a particularly difficult case in terms of computer simulations. Adsorption of large colloid nanoparticles on the surfaces of carbon nanotubes were studied and we determined the adsorption energy profiles of the nanoparticles on the carbon nanotubes surfaces. We also determined the adsorption isotherms which help to understand a preferred location of the nanoparticles on the nanotubes surfaces.     

Electrochemical characteristics of the immobilization of calf thymus DNA molecules on multi-walled carbon nanotubes

Immobilization of DNA on carbon nanotubes plays an important role in the development of new types of miniature DNA biosensors. Electrochemical characteristics of the immobilization of calf thymus DNA molecules on the surfaces of multi-walled carbon nanotubes (MWNTs) have been investigated by cyclic voltammetry and electrochemical impedance analysis. The peak currents for Fe(CN)(6)(3-)/Fe(CN)(6)(4-) redox couple observed in the cyclic voltammograms decrease and the electron-transfer resistance (R(et)) obtained from the Nyquist plots increase due to the immobilization of DNA molecules (dsDNA or ssDNA) on the surfaces of MWNTs. Most of calf thymus DNA are covalently immobilized on MWNTs via diimide-activated amidation between the carboxylic acid groups on the carbon nanotubes and the amino groups on DNA bases, though the direct adsorption of the DNA molecules on MWNTs can be observed. Additionally, the interaction between DNA molecules immobilized on MWNTs and small biomolecules (ethidium bromide) can be observed obviously by cyclic voltammetry and electrochemical impedance analysis. This implies that the DNA molecules immobilized at the surface of MWNTs, with little structure change, still has the ability to interact with small biomolecules.     

Can chiral single walled carbon nanotubes be used as enantiospecific adsorbents?

Single-walled carbon nanotubes can exist in chiral forms and can adsorb a range of molecules. We use atomistic simulations to consider whether enantiopure carbon nanotubes might be effective enantiospecific adsorbents for chiral molecules. We examine the adsorption of both enantiomers of trans-1,2-dimethylcyclopropane and trans-1,2-dimethylcyclohexane in a range of chiral nanotubes. Our simulations indicate that these molecules are strongly adsorbed in nanotubes, that is, they have large heats of adsorption, but the energy differences between adsorbed enantiomers are negligible. We argue that this result is generic for chiral organic molecules adsorbed in carbon nanotubes, suggesting that these materials will not be effective enantiospecific adsorbents.     

Stockage de l’hydrogène dans les nanotubes de carbone à paroi unique : nouvelle méthode de préparation par excitation laser

A new mode for hydrogen adsorption and storage in single-wall carbon nanotubes is used, on the basis of laser excitation. Remember that this method has been useful to obtain, in the case of the fullerene C₆₀, many complex C₆₀-atoms or C₆₀-molecules, where atoms or molecular particles are trapped inside the C₆₀-molecules. We think this method might be important to store many hydrogen molecules inside carbon nanotubes.     

Axially Chiral Facial Amphiphiles with a Dihydronaphthopentaphene Structure as Molecular Tweezers for SWNTs

Syntheses of chiral 6,15‐dihydronaphtho[2,3‐c]pentaphene derivatives of opposite configurations are reported. Starting from anthracene, the strategy involves two key steps: a Diels–Alder reaction on a prochiral dianthraquinone, and an enantiomeric resolution using (?)‐menthol. The final molecules exhibit very strong optical activity, as shown by their circular dichroism spectra, and are examples of chiral facial amphiphiles. Their adsorption at the surface of single‐walled carbon nanotubes (SWNTs) has also been studied, and has been found to occur preferentially on 0.8–1.0 nm diameter nanotubes among the population of a high‐pressure CO conversion (HiPco) SWNT sample (0.8–1.2 nm). The synthesised facial amphiphiles act as nano‐tweezers for the diameter‐selective solubilisation of SWNTs in water. The expected optical activities of the SWNT samples solubilised by each of the chiral amphiphiles have been studied by circular dichroism spectroscopy, but the results are not yet conclusive.     

Adsorption activity of carbon nanotubes formed from brown sphagnum moss in mechanical activation

Carbon nanotubes have been formed as a result of mechanical activation of amorphous carbon obtained after the pyrolytic treatment of brown sphagnum moss. The yield of nanotubes grows from 8 to 75 wt % with increasing the time of mechanical activation from 4 to 27 h. An integrated study of the adsorption properties of carbon materials obtained from sphagnum moss is presented. It is shown that the capability of the oxidative modification of carbon nanotubes depends on the parameters of the formation process, and the efficiency of adsorption capacity is a function of the storage time of the carbon material. The potential of carbon materials obtained from sphagnum moss as enterosorbents is considered.