Surface structure and adsorption properties of multiwalled carbon nanotubes

The pore structure, sorption parameters, and chemical composition of the surface of multiwalled carbon nanotubes synthesized by catalytic pyrolysis were determined. The dependences of the amount of cholic acid adsorbed by the nanotube surface on time, pH, and concentration of an equilibrium solution were studied. Physical adsorption of cholic acid is mainly the outcome of nonspecific interactions between the acid and the surface of the nanotubes. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1712–1715, October, 2006.     

Nitrogen-doped multiwalled carbon nanotubes and their electrocatalysis towards oxidation of NO

Nitrogen-doped multiwalled carbon nanotubes (N-MWCNTs) were synthesized by thermal decomposition of pyridine and iron phthalocyanine over an iron catalyst in an atmosphere of ammonia. The N-MWCNTs thus obtained were analyzed by X-ray photoelectron spectroscopy. They were found to contain three types of nitrogen (N) atoms, namely pyridine-like, graphite-like, and molecular N. The effect of the pyridine-like N and the graphite-like N was investigated. The pyridine-like N is absorbing nitric oxide (NO) more easily than the graphite-like N. The N-MWCNTs with higher N content (especially the pyridine-like N) have higher catalytic activity (in terms of electrooxidation of NO) than those containing less N. The N-MWCNTs with high levels of pyridine N were incorporated into an electrode which suitable for sensing NO and for removal of NO due to its excellent electrocatalytic activity.     

High-resolution infrared spectroscopy and ab initio studies of the cyclopropane-carbon dioxide interaction

A jet-cooled high-resolution infrared spectrum of the cyclopropane-carbon dioxide complex was detected for the first time, using a rapid scan infrared spectrometer with an astigmatic multipass sample cell. The spectrum was recorded in the vicinity of the CO2 asymmetric stretching band (nu3) and exhibits a b-dipole selection rule. Altogether, over 200 lines were observed, assigned, and fitted to Watson's S-reduction Hamiltonian. Rotational and quartic distortion constants were obtained. The band origin was located at 2347.6263(2) cm(-1), redshifted by 1.5230(2) cm(-1) from the corresponding frequency of the CO2 monomer. The experimentally determined structure shows that CO2 lies next to a C-C bond edge and is perpendicular to the C3 ring, indicating that the interaction is characterized by the bonding between the carbon atom of CO2 and the pseudo-pi system of cyclopropane. The intermolecular distance between the carbon atom of CO2 and the center of mass of cyclopropane was determined to be 3.667(2) A. Complete ab initio geometry optimizations and harmonic frequency calculations were carried out at the level of second-order Moller-Plesset perturbation theory with four different basis sets: cc-pVDZ, 6-311++G(d,p), aug-cc-pVDZ, and aug-cc-pVTZ. The lowest-energy structure identified with the three larger basis sets is in accord with the experimental finding. In addition, a transition state was identified and the tunneling barrier height was computed.     

Nitrogen adsorption characterization of aligned multiwalled carbon nanotubes and their acid modification

Aligned multiwalled carbon nanotube (CNT) arrays were synthesized by using an iron-based sol-gel catalyst and acetylene as the precursor. These CNTs show high purity, uniform diameters and pore-wall thickness. Low temperature nitrogen adsorption was employed to characterize the structural and surface properties of the as-synthesized sample and that modified with boiling concentrated nitric acid. The adsorption characteristics of the as-synthesized and modified CNTs were thoroughly investigated. High-resolution comparative alpha(s)-plot showed that the nitrogen adsorption on CNTs takes place via a multistage mechanism closely related to their structures. It was also found that the acid modification significantly increased the adsorption energy and enhanced the adsorption capacity under low pressures. High-resolution comparative method provided valuable insights about the surface and pore structures of CNTs.     

Selective adsorption and alignment behaviors of double- and multiwalled carbon nanotubes on bare Au and SiO2 surfaces

We present the study of selective adsorption and alignment behaviors of double- and multiwalled carbon nanotubes (dwCNTs and mwCNTs) on self-assembled monolayer (SAM) patterns, bare Au, and SiO2 surfaces. dwCNTs and mwCNTs exhibited stronger affinity to polar SAMs, bare Au, and SiO2 surfaces than to nonpolar SAM surfaces. Furthermore, we found the adsorption probability of smaller carbon nanotubes (CNTs) was higher than that of larger CNTs. As proof of concept, we successfully assembled and aligned dwCNTs and mwCNTs on Au and SiO2 substrates without relying on external forces and demonstrated wafer-scale fabrication of back-gate transistors based on dwCNTs with a high yield.     

Polyurea-functionalized multiwalled carbon nanotubes: synthesis, morphology, and Raman spectroscopy

An in situ polycondensation approach was applied to functionalize multiwalled carbon nanotubes (MWNTs), resulting in various linear or hyperbranched polycondensed polymers [e.g., polyureas, polyurethanes, and poly(urea-urethane)-bonded carbon nanotubes]. The quantity of the grafted polymer can be easily controlled by the feed ratio of monomers. As a typical example, the polyurea-functionalized MWNTs were measured and characterized in detail. The oxidized MWNTs (MWNT-COOH) were converted into acyl chloride-functionalized MWNTs (MWNT-COCl) by reaction with neat thionyl chloride (SOCl2). MWNT-COCl was reacted with excess 1,6-diaminohexane, affording amino-functionalized MWNTs (MWNT-NH2). In the presence of MWNT-NH2, the polyurea was covalently coated onto the surfaces of the nanotube by in situ polycondensation of diisocyanate [e.g., 4,4'-methylenebis(phenylisocyanate)] and 1,6-diaminohexane, followed by the removal of free polymer via repeated filtering and solvent washing. The coated polyurea content can be controlled to some extent by adjusting the feed ratio of the isocyanato and amino groups. The structure and morphology of the resulting nanocomposites were characterized by FTIR, NMR, Raman, confocal Raman, TEM, EDS, and SEM measurements. The polyurea-coated MWNTs showed interesting self-assembled flat- or flowerlike morphologies in the solid state. The signals corresponding to that of the D and G bands of the carbon nanotubes were strongly attenuated after polyurea was chemically tethered to the MWNT surfaces. Comparative experiments showed that the grafted polymer species and structures have a strong effect on the Raman signals of polymer-functionalized MWNTs.     

Conformational studies of Phe-rich foldamers by VCD spectroscopy and ab initio calculations

Employing VCD spectroscopy, we demonstrate that the structural behavior of the oligomers Boc-(L-Phe-L-Oxd)(n)-OBn is similar from n = 2 to n = 6; ab initio calculations for the n = 1 case provide physical insight into the conformational properties. Further information is gained by IR, (1)H NMR, and ECD spectroscopies. ECD spectra suggest the presence of different conformations between n = 1 on one side and longer chain foldamers on the other side. VCD and absorption IR spectra in methanol solutions can be interpreted as indicative of a PPII structure. In the case of Boc-L-Phe-L-Oxd-OBn, VCD spectra in CCl(4) and detailed DFT computational analysis allow one to demonstrate that the most populated conformers exhibit backbone dihedral angles similar to those of a PPII geometry. This is a remarkable outcome, as we had previously demonstrated that the Boc-(L-Ala-D-Oxd)(n)-OBn series folds in a β-band ribbon spiral that is a subtype of the 3(10) helix.     

Ab initio theoretical study of non-covalent adsorption of aromatic molecules on boron nitride nanotubes

We have studied non-covalent functionalization of boron nitride nanotubes (BNNTs) with benzene molecule and with seven other different heterocyclic aromatic rings (furan, thiophene, pyrrole, pyridine, pyrazine, pyrimidine, and pyridazine, respectively). A hybrid density functional theory (DFT) method with the inclusion of dispersion correction is employed. The structural and electronic properties of the functionalized BNNTs are obtained. The DFT calculation shows that upon adsorption to the BNNT, the center of aromatic rings tend to locate on top of the nitrogen site. The trend of adsorption energy for the aromatic rings on the BNNTs shows marked dependence on different intermolecular interactions, including the dispersion interaction (area of the delocalized π bond), the dipole-dipole interaction (polarization), and the electrostatic repulsion (lone pair electrons). The DFT calculation also shows that non-covalent functionalization of BNNTs with aromatic rings can give rise to new impurity states within the band gap of pristine BNNTs, suggesting possible carrier doping of BNNTs via selective adsorption of aromatic rings.     

Adsorption, desorption, and thermodynamic studies of CO2 with high-amine-loaded multiwalled carbon nanotubes

Commercially available multiwalled carbon nanotubes (CNTs) were functionalized with a high mass load of 3-aminopropyltriethoxysilane (APTS) to study their behaviors in the cyclic CO(2) adsorption as well as the associated thermodynamic properties. The breakthrough curve showed a fast kinetics of CO(2) adsorption resulting in percentage ratios of working capacity to equilibrium capacity greater than 80%. The adsorption capacity of CNT(APTS) was significantly influenced by the presence of water vapor and reached a maximum of 2.45 mmol/g at a water vapor of 2.2%. The adsorption capacities and the physicochemical properties of CNT(APTS) were preserved through 100 adsorption-desorption cycles displaying the stability of CNT(APTS) during a prolonged cyclic operation. The heat input required to regenerate spent CNT(APTS) was determined, and the result suggests that adsorption process with solid CNT(APTS) is possibly a promising CO(2) capture technology.     

Mechanism study on adsorption of acidified multiwalled carbon nanotubes to Pb(II)

Adsorption of acidified multiwalled carbon nanotubes (MWCNTs) to heavy metal using Pb(II) as a model was investigated and characterized by many techniques. The main adsorption mechanism of acidified MWCNTs to Pb(II) is proposed on the basis of adequate analysis. The results show that the oxygenous functional groups can be formed on MWCNTs after MWCNTs were treated by concentrated nitric acid. The oxygenous functional groups play an important role in Pb(II) adsorption to form chemical complex adsorption, which accounts for 75.3% of all the Pb(II) adsorption capacity. The Pb(II) in the form of PbO, Pb(OH)(2), and PbCO(3) adsorbed on the surface of the acidified MWCNTs is only 3.4% of the total Pb(II) adsorption capacity. The Pb(II) species adsorbed on acidified MWCNTs mainly aggregate on the ends and at the defects sites on the acidified MWCNTs.     

Prediction of adsorption of small molecules in porous materials based on ab initio force field method

Computational prediction of adsorption of small molecules in porous materials has great impact on the basic and applied research in chemical engineering and material sciences. In this work,we report an approach based on grand canonical ensemble Monte Carlo(GCMC) simulations and ab initio force fields. We calculated the adsorption curves of ammonia in ZSM-5 zeolite and hydrogen in MOF-5(a metal-organic-framework material). The predictions agree well with experimental data. Because the predictions are based on the first principle force fields,this approach can be used for the adsorption prediction of new molecules or materials without experimental data as guidance.     

Dispersing and functionalizing multiwalled carbon nanotubes in TiO2 sol

We report that oxidized multiwalled carbon nanotubes (MWCNTs) can be synchronously dispersed and functionalized in TiO2 sol via an in situ sol-gel process. Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and atomic force microscopy (AFM) were used to characterize the functionalized MWCNTs. The results revealed that the hydrolysis and condensation originated from Ti(OC4H9)4 molecules favor the dispersion of MWCNTs in as-prepared TiO2 sol. Based on the strong interaction between the oxidized MWCNTs and TiO2 sol during the in situ sol-gel process, MWCNT (core)-TiOx (shell) tubular composites and TiO2 nanotubes can be obtained through filtrating, washing, and annealing of this kind of TiO2 sol containing functionalized MWCNTs, as revealed by TEM, XPS, Raman spectroscopy, and redispersion experiment. By casting the dilute dispersion of functionalized MWCNTs onto a hydrophilic Si surface, discrete and individual nanotubes can be observed by AFM.     

Heterogeneity of multiwalled carbon nanotubes based on adsorption of simple aromatic compounds from aqueous solutions

The surface heterogeneity of multiwalled carbon nanotubes (MWCNTs) is studied on the basis of adsorption isotherms from dilute aqueous phenol and dopamine solutions at various pH values. The generalized Langmuir–Freundlich isotherm equation was applied to investigate the cooperative effect of the surface heterogeneity and the lateral interactions between the adsorbates. The theoretical isosteric heats of adsorption were obtained assuming that the heat of adsorption profile reveals both the energetic heterogeneity of the adsorption system and the strength of the interactions between the neighboring molecules. The adsorption energy distribution functions were calculated by using algorithm based on a regularization method. The great advantage of this method is that the regularization makes no assumption about the shape of the obtained energy distribution functions. Analysis of the isosteric heats of adsorption for MWCNTs showed that the influence of the surface heterogeneity is much stronger than the role of the lateral interactions. The most typical adsorption heat is 20–22 kJ/mol for both phenol and dopamine. After purification of nanotubes, heat value for phenol dropped to 16–17 kJ/mol. The range of the energy distribution is only slightly influenced by the surface chemistry of the nanotubes in the aqueous conditions.     

Conformational studies of n-propylamine by combined ab initio MO calculations and Raman spectroscopy

The results of ab initio SCF-MO calculations performed with a 3-21G(N*) basis set, for fully optimized geometries of five conformations of n-propylamine, are presented. The calculated relative order of total energies for these conformers is TT≈GG′>TG>GT>GG. At 300 K, the Boltzmann distribution of populations is 18, 37, 20, 19 and 7%, respectively.Raman spectra of n-propylamine and n-propylamine-N-d₂ in the liquid phase exhibit a number of bands whose temperature-dependent intensities clearly suggest the occurrence of different conformers in simultaneous equilibria. Deuteration of the amine group originates pairs of Raman bands at 428 and 440 cm⁻¹ and at 863 and 885 cm⁻¹. The bands at 428 and 885 cm⁻¹ are favoured by reduction of temperature. Normal coordinate calculations permit the assignment of the Raman and i.r. spectra in good agreement with experimental evidence. Among the five possible conformers of n-propylamine, it is possible to detect the presence of at least three conformations in the liquid phase, corresponding to the skeletal trans (TT and GT) and at least one of the skeletal gauche (TG, GG or GG′) forms. In the solid phase, only the bands ascribed to the TT form were observed.The ab initio results for the isolated molecule show that the all-trans conformation, TT, and the conformation GG′ have the smallest energies. On the other hand, the vibrational results for the liquid and solid phases indicate that the all-trans conformation, TT, is the more populated form. In addition, this conformer presents the highest calculated dipole moment, in good agreement with the liquid phase Raman spectroscopic results which point out that this conformation is favoured by polar solvents. Intermolecular interactions operating in the liquid n-propylamine, possibly of the hydrogen bonding type, are responsible for altering the relative order of conformational stability as predicted by the ab initio SCF-MO results for the isolated molecule.     

壳聚糖－戊二醛的量子化学从头算

采用量子化学从头算方法研究了壳聚糖-戊二醛交联膜的稳定结构,研究了壳聚糖和戊二醛通过氨醛缩合形成阳离子膜的几何构型、键能、键序和电子迁移,讨论了膜的成键形式和稳定性．     

Influence of pH and surface oxygen-containing groups on multiwalled carbon nanotubes on the transformation and adsorption of 1-naphthol

The pH-dependent behavior, including the transformation of 1-naphthol by oxidative polymerization to form precipitates in solution and the adsorption of 1-naphthol onto carbon nanotubes (CNTs), was examined. Neglecting the precipitate loss of 1-naphthol and possibly of similar chemicals may result in the overestimation of their adsorption and inadequate interpretation of the underlying adsorption mechanisms. Surface oxygen-containing groups on CNTs and the dissociated species of these groups can interact with the dissociated and neutral species of 1-naphthol in a way similar to polymerization, thus promoting the adsorption of 1-naphthol onto CNTs. Adsorption onto CNTs may reduce the polymeric precipitates of 1-naphthol in solution by possibly decreasing aqueous 1-naphthol concentrations. These observations and the underlying mechanisms are important for predicting the fate and risks of naphthalene and carbaryl in the environment because 1-naphthol is a primary metabolite of naphthalene and carbaryl. In addition, it is possible to enhance the removal of 1-naphthol and similar chemicals by controlling the pH and designing specific surface functional groups for CNTs.     

Structure of protonated carbon dioxide clusters: infrared photodissociation spectroscopy and ab initio calculations

The infrared photodissociation spectra (IRPD) in the 700 to 4000 cm(-1) region are reported for H+ (CO2)n clusters (n = 1-4) and their complexes with argon. Weakly bound Ar atoms are attached to each complex upon cluster formation in a pulsed electric discharge/supersonic expansion cluster source. An expanded IRPD spectrum of the H+ (CO2)Ar complex, previously reported in the 2600-3000 cm(-1) range [Dopfer, O.; Olkhov, R.V.; Roth, D.; Maier, J.P. Chem. Phys. Lett. 1998, 296, 585-591] reveals new vibrational resonances. For n = 2 to 4, the vibrational resonances involving the motion of the proton are observed in the 750 to 1500 cm(-1) region of the spectrum, and by comparison to the predictions of theory, the structure of the small clusters are revealed. The monomer species has a nonlinear structure, with the proton binding to the lone pair of an oxygen. In the dimer, this nonlinear configuration is preserved, with the two CO2 units in a trans configuration about the central proton. Upon formation of the trimer, the core CO2 dimer ion undergoes a rearrangement, producing a structure with near C2v symmetry, which is preserved upon successive CO2 solvation. While the higher frequency asymmetric CO2 stretch vibrations are unaffected by the presence of the weakly attached Ar atom, the dynamics of the shared proton motions are substantially altered, largely due to the reduction in symmetry of each complex. For n = 2 to 4, the perturbation due to Ar leads to blue shifts of proton stretching vibrations that involve motion of the proton mostly parallel to the O-H+-O axis of the core ion. Moreover, proton stretching motions perpendicular to this axis exhibit smaller shifts, largely to the red. Ab initio (MP2) calculations of the structures, complexation energies, and harmonic vibrational frequencies are also presented, which support the assignments of the experimental spectra.