Critical Evaluation of Adsorption–Desorption Hysteresis of Heavy Metal Ions from Carbon Nanotubes: Influence of Wall Number and Surface Functionalization

Single‐, double‐, and multi‐walled carbon nanotubes (SWCNTs, DWCNTs, and MWCNTs), and two oxidized MWCNTs with different oxygen contents (2.51 wt % and 3.5 wt %) were used to study the effect of the wall number and surface functionalization of CNTs on their adsorption capacity and adsorption–desorption hysteresis for heavy metal ions (Ni^II, Cd^II, and Pb^II). Metal ions adsorbed on CNTs could be desorbed by lowering the solution pH. Adsoprtion of heavy metal ions was not completely reversible when the supernatant was replaced with metal ion‐free electrolyte solution. With increasing wall number and amount of surface functional groups, CNTs had more surface defects and exhibited higher adsorption capacity and higher adsorption–desorption hysteresis index (HI) values. The coverage of heavy metal ions on the surface of CNTs, solution pH, and temperature affect the metal ion adsorption–desorption hysteresis. A possible shift in the adsorption mechanism from mainly irreversible to largely reversible processes may take place, as the amount of metal ions adsorbed on CNTs increases. Heavy metal ions may be irreversibly adsorbed on defect sites.     

Adsorption of SO2 by typical carbonaceous material: a comparative study of carbon nanotubes and activated carbons

Applying activated carbons for SO₂ adsorption and conversion to H₂SO₄, as a dry process, has been considered the development direction of desulfurization technology. Coal-based activated carbon, coconut shell activated carbon, single wall carbon nanotube and multi-wall carbon nanotubes were used as typical carbonaceous materials to study the SO₂ adsorption mechanism. SEM, N₂ adsorption, XPS and fixed-bed reaction system were employed to study the morphology, pore structure, surface functional groups and SO₂ adsorption behaviors of the four adsorbents. The fixed-bed experiment was carried out at normal pressure and SO₂ concentration was set 1,000 ppm. According to SEM and N₂ adsorption results, hierarchical pore structure was an important characteristic of activated carbon. Aggregation was an important characteristic of CNTs. Mesopores and macropores took the dominance of pore structure in CNTs. According the SO₂ adsorption data and correlation analysis, it can be concluded that the dominant adsorption type on activated carbons does not alter with adsorption temperature changing. However, the adsorption type of SO₂ adsorption on CNTs changes with adsorption temperature varying. With adsorption temperature increasing, the dominant adsorption type transforms to chemisorption by physisorption. Higher-density π–π* in carbon nanotubes may be the active sites for the SO₂ chemical adsorption. Micropores with the diameter smaller than 0.7 nm were the best SO₂ adsorption place for both activated carbons and carbon nanotubes. The results provided a profound insight into the microstructure and SO₂ adsorption mechanism of the two kinds of carbonaceous materials.     

"Fingertip"-guided noncovalent functionalization of carbon nanotubes by dendrons

Noncovalent functionalization of carbon nanotubes (CNTs) by dendrons was demonstrated. Certain types of dendrons successfully functionalized CNT surfaces through the noncovalent interactions between the peripheries of the dendrons and the sidewalls of CNTs. Dendrons have a unique anisotropic shape and an orthogonal functional group at their apex, and thus can generate a certain spacing between the functional groups upon immobilization on surfaces. Atomic force microscope (AFM) imaging, dispersion experiments, and MicroRaman spectroscopy were employed for the characterization of the functionalization. The binding was found to be governed by the chemical nature of the terminal groups, namely, the "fingertips", through a comparison study on the adsorption efficiency of the dendron analogs. Functional groups such as the carboxylic acid group and the benzyl amide group were effective for the cooperative binding. AFM analysis showed that the average spacing generated by the dendrons was 14-15 nm at a particular adsorption condition. Assembling streptavidin on the tubes through the dendrons and biotin confirmed the realization of the regulated spacing as well as the elimination of unwanted aggregation. The noncovalent functionalization of CNTs by a dendron can be a new approach toward sensible nanobiodevices, not only by introducing biomolecular probes on CNTs without disruption of the electronic network of the tubes, but also by providing the immobilized probe molecules with a space ample enough to minimize steric hindrance for the unhindered interaction with their target species.     

Adsorption of benzene,toluene, ethylbenzene and p-xylene by NaOCl-oxidized carbon nanotubes

Multiwalled carbon nanotubes (CNTs) were oxidized by sodium hypochlorite (NaOCl) solution and were employed as adsorbents to study their characterizations and adsorption performance of benzene, toluene, ethylbenzene and p-xylene (abbreviated as BTEX) in an aqueous solution. The physicochemical properties of CNTs such as purity, structure and surface nature were greatly improved after oxidation, which significantly enhanced BTEX adsorption capacity. The adsorption capacity of CNT(NaOCl) increased with contact time and initial adsorbate concentration, but changed insignificantly with solution ionic strength and pH. A comparative study on the BTEX adsorption revealed that the CNT(NaOCl) had better BTEX adsorption as compared to CNTs and granular activated carbon. This suggests that the CNT(NaOCl) are efficient BTEX adsorbents and that they possess good potential for BTEX removal in wastewater treatment.     

Organic Functional Group on Carbon Nanotube Modulates the Maturation of SH-SY5Y Neuronal Models

Carbon nanotubes (CNT) have proven to be excellent substrates for neuronal cultures, showing high affinity and greatly boosting their synaptic functionality. Therefore, growing cells on CNT offers an opportunity to perform a large variety of neuropathology studies in vitro. To date, the interactions between neurons and chemical functional groups have not been studied extensively. To this end, multiwalled CNT (f-CNT) is functionalized with various functional groups, including sulfonic (–SO₃H), nitro (–NO₂), amino (–NH₂), and oxidized moieties. f-CNTs are spray-coated onto untreated glass substrates and are used as substrates for the incubation of neuroblastoma cells (SH-SY5Y). After 7 d, its effect is evaluated in terms of cell attachment, survival, growth, and spontaneous differentiation. Cell viability assays show quite increased proliferation on various f-CNT substrates (CNTs-NO₂ > ox-CNTs ≈ CNTs-SO₃H > CNTs ≈ CNTs-NH₂). Additionally, SH-SY5Y cells show selectively better differentiation and maturation with –SO₃H substrates, where an increased expression of β-III tubulin is seen. In all cases, intricate cell-CNT networks are observed and the morphology of the cells adopts longer and thinner cellular processes, suggesting that the type of functionalization may have an effect of the length and thickness. Finally, a possible correlation is determined between conductivity of f-CNTs and cell-processes lengths.     

Influence of surface functionalization via chemical oxidation on the properties of carbon nanotubes

The surface of carbon nanotubes (CNTs) was functionalized in different chemical oxidants, hydrogen peroxide, mixed concentrated HNO(3)/H(2)SO(4) and acidic KMnO(4) solution. The influences on the properties of CNTs were systematically investigated, such as the structure, the kinds and the contents of the formed surface oxygen-containing functional groups, the pH(PZC) values and the surface hydrophilicity using XRD, HREM, FTIR and chemical titration. The results show that the kinds and the contents of the surface oxygen-containing groups are dependent on the functionalization methods. The formation of the oxygen-containing groups can decrease pH(PZC) values and improve surface hydrophilicity of CNTs. The dispersion of the supported Pd-Pt particles on the functionalized CNTs and their catalytic activity in the profile reaction of naphthalene hydrogenation to tetralin are both promoted due to the presence of these oxygen-containing groups.     

Thermal stability evolution of carbon nanotubes caused by liquid oxidation

Carbon nanotubes (CNTs) grown by chemical vapor decomposition of ethylene on alumina- and silica-supported Fe–Co bimetallic catalysts were examined before and after purification encompassing chemical oxidation treatment in 3 M NaOH and 3 M HNO₃ solutions, sequentially. Thermal properties were investigated and correlated with structural changes followed by TEM, X-ray diffraction and Raman spectroscopy characterization. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TG) were employed simultaneously in the regime of TPO and TPH, in diluted flow of either O₂ or H₂. TG revealed almost complete burning of both refined CNT samples in diluted O₂, indicating the efficiency of the purification method used to remove the catalyst remains. However, different trends and significant magnitudes of changes in the heat of combustion demonstrate changes in CNTs stability after purification as a function of the catalyst support type. This is the consequence of changes in carbon type, CNTs order degree and morphology, as well as the degree of functionalization, which have their own effects on the CNTs thermal stability.     

Amine basicity (pKb) controls the analyte binding energy on single walled carbon nanotube electronic sensor arrays

A wide range of analytes adsorb irreversibly to the surfaces of single walled carbon nanotube electronic networks typically used as sensors or thin-film transistors, although to date, the mechanism is not understood. Using thionyl chloride as a model electron-withdrawing adsorbate, we show that reversible adsorption sites can be created on the nanotube array via noncovalent functionalization with amine-terminated molecules of pKa < 8.8. A nanotube network comprising single, largely unbundled nanotubes, near the electronic percolation threshold is required for the effective conversion to a reversibly binding array. By examining 11 types of amine-containing molecules, we show that analyte adsorption is largely affected by the basicity (pKb) of surface groups. The binding energy of the analyte is apparently reduced by its adsorption on the surface chemical groups instead of directly on the SWNT array itself. This mediated adsorption mechanism is supported by X-ray photoelectron spectroscopy (XPS) and molecular potential calculations. Reversible detection with no active regeneration at the parts-per-trillion level is demonstrated for the first time by creating a higher adsorption site density with a polymer amine, such as polyethyleneimine (PEI). Last, we demonstrate that this transition to reversibility upon surface functionalization is a general phenomenon.     

A Review Featuring Fabrication,Properties and Applications of Carbon Nanotubes (CNTs) Reinforced Polymer and Epoxy Nanocomposites

Carbon nanotubes (CNTs) have long been recognized as the stiffest and strongest man-made material known to date.In addition,their high electrical conductivity has roused interest in the areas of electrical appliances and communication related applications.However,due to their miniature size,the excellent properties of these nanostructures can only be exploited if they are homogeneously embedded into light-weight matrices as those offered by a whole series of engineering polymers.In order to enhance their chemical affinity to engineering polymer matrices,chemical modification of the graphitic sidewalls and tips is necessary.The mechanical and electrical properties to date of a whole range of nanocomposites of various carbon nanotube contents are also reviewed in this attempt to facilitate progress in this emerging area.Recently,carbonaceous nano-fillers such as graphene and carbon nanotubes (CNTs) play a promising role due to their better structural and functional properties and broad range of applications in every field.Since CNTs usually form stabilized bundles due to van der Waals interactions,they are extremely difficult to disperse and align in a polymer matrix.The biggest issues in the preparation of CNTs reinforced composites reside in efficient dispersion of CNTs into a polymer matrix,the assessment of the dispersion,and the alignment and control of the CNTs in the matrix.An overview of various CNT functionalization methods is given.In particular,CNT functionalization using click chemistry and the preparation of CNT composites employing hyperbranched polymers are stressed as potential techniques to achieve good CNT dispersion.In addition,discussions on mechanical,thermal,electrical,electrochemical and applications ofpolymer/CNT composites are also included.     

Covalent Functionalization of Single-Walled Carbon Nanotubes Through the Fluorination Stage for Integration into an Epoxy Composite

The upper limit of the elastic modulus has been estimated for a polymer–carbon nanotube–epoxy matrix nanocomposite. This limit can be achieved if the nanotubes are integrated into the matrix, i.e., they form a continuous reinforcing network inside the matrix, and if the nanotubes are single-walled or double-walled carbon nanotubes. A technique for carbon nanotube functionalization via fluorination and fluorine substitution and a technique for calculating the degree of nanotube functionalization based on reaction yield measurements are proposed. For fluorine substitution by epoxy-diane resin and diaminodiphenylmethane, the degree of functionalization is С–(FG)_x, x ~ 0.011–0.013 and the FG-molecular fragment containing the epoxy (amino) group corresponding to functionalization of ~5% of the surface С atoms of nanotubes. The control reaction showed that the epoxy groups preserve the chemical activity, while part of the amino groups are deactivated. The grafted epoxy(amino) groups ensure nanotube surface lyophilicity in epoxy composites and integrate the nanotubes into the epoxy matrix owing to the chemical bonds.     

Carbon Nanotubes-Based Label-Free Affinity Sensors for Environmental Monitoring

Nanostructures, such as nanowires, nanobelts, nanosprings, and nanotubes, are receiving growing interest as transducer elements of bio/chemical sensors as they provide high sensitivity, multiplexing, small size, and portability. Single-walled carbon nanotubes (SWNTs) are one such class of nanostructure materials that exhibit superior sensing behavior due to its large-surface carbon atoms that are highly responsive to surface adsorption events. Further, their compatibility with modern microfabrication technologies and facile functionalization with molecular recognition elements make them promising candidates for bio/chemical sensors applications. Here, we review recent results on nanosensors based on SWNTs modified with biological receptors such as aptamers, antibodies, and binding proteins, to develop highly sensitive, selective, rapid, and cost-effective label-free chemiresistor/field-effect transistor nanobiosensors for applications in environmental monitoring.     

DFT Theoretical Calculation of the Site Selectivity of Dihydroxylated (5, 0) Zigzag Carbon Nanotube

Functionalization is an important method to change electrical and thermodynamic properties of carbon nanotubes. In this study, the effect of functionalization of a single-walled carbon nanotube (SWCNT) was investigated with the aid of density functional theory. For this case, a (5, 0) zigzag SWCNT model containing 60 C atoms with 10 hydrogen atoms added to the dangling bonds of the perimeter carbons was used. To model hydroxyl CNT two terminal H atoms were replaced by two –OH groups. All the functionalized CNTs are thermodynamically more stable and have higher dipole moment with respect to the pristine CNT. Depending on the positions of hydroxyl groups on CNT five isomers of C₆₀H₈(OH)₂ were obtained. The structure of these five isomers and molecular properties such as the HOMO–LUMO gaps, the dipole moments, and the density of state were calculated. Our results indicate that the HOMO–LUMO gap strongly depends on the placement of the hydroxyl groups on the nanotubes. The isomers were hydroxyl groups locate on the anti-position show the highest distortions in the structure of the CNT.

     

Dispersible Carbon Nanotubes

A method is proposed to produce nanoparticles dispersible and recyclable in any class of solvents, and the concept is illustrated with the carbon nanotubes. Classically, dispersions of CNTs can be achieved through steric stabilization induced by adsorbed or grafted polymer chains. Yet, the surface modification of CNTs surfaces is irreversible, and the chemical nature of the polymer chains imposes the range of solvents in which CNTs can be dispersed. To address this limitation, supramolecular bonds can be used to attach and to detach polymer chains from the surface of CNTs. The reversibility of supramolecular bonds offers an easy way to recycle CNTs as well as the possibility to disperse the same functional CNTs in any type of solvent, by simply adapting the chemical nature of the stabilizing chains to the dispersing medium. The concept of supramolecular functionalization can be applied to other particles, for example, silica or metal oxides, as well as to dispersing in polymer melts, films or coatings.     

Electrochemistry and electroanalytical applications of carbon nanotubes: a review.

This review addresses recent developments in electrochemistry and electroanalytical chemistry of carbon nanotubes (CNTs). CNTs have been proved to possess unique electronic, chemical and structural features that make them very attractive for electrochemical studies and electrochemical applications. For example, the structural and electronic properties of the CNTs endow them with distinct electrocatalytic activities and capabilities for facilitating direct electrochemistry of proteins and enzymes from other kinds of carbon materials. These striking electrochemical properties of the CNTs pave the way to CNT-based bioelectrochemistry and to bioelectronic nanodevices, such as electrochemical sensors and biosensors. The electrochemistry and bioelectrochemistry of the CNTs are summarized and discussed, along with some common methods for CNT electrode preparation and some recent advances in the rational functionalization of the CNTs for electroanalytical applications.     

Fabrication,adsorption and photocatalytic properties of ZnTi0.6Fe1.4O4/Carbon nanotubes composites

ZnTi_xFe_(2–x)O_4 and ZnTi_(0.6)Fe_(1.4)O_4/Carbon nanotubes(ZT_(0.6)F_(1.4)/CNTs) composites were prepared by chemical co-precipitation method. The composition, microstructure, magnetic property, adsorption and photocatalytic activity of the prepared samples were characterized by means of modern analytical techniques. The results indicated that ZT_(0.6)F_(1.4)/CNTs composites not only held the original special structure and excellent adsorption properties of CNTs, but also had suitable magnetic property and excellent photocatalytic activity. The removal rate of the samples on Rhodamine B(RhB) depended on the adsorption of CNTs and the photocatalytic degradation of ZT_(0.6)F_(1.4) in the composites. The maximum adsorption amount(q_m) of ZT_(0.6)F_(1.4)/CNTs with the mass ratios of ZT_(0.6)F_(1.4) to CNTs(mZ/C)=1 was up to 17.153 mg g~(–1) for RhB, its adsorption behavior was in accord with Langmuir model, and its photocatalytic degradation activity on RhB had a positive correlation with the content of ZT_(0.6)F_(1.4) in the sample. The experimental results indicate that the total removal rate of composite with mZ/C=1 on RhB was more than 95% and the composite had good decontamination capability on industrial dye wastewater. In addition, the samples can be recovered conveniently, activated easily and had good performance for recycling.     

Oxygen adsorption characteristics on hybrid carbon and boron‐nitride nanotubes

In this work, first‐principles density functional theory (DFT) is used to predict oxygen adsorption on two types of hybrid carbon and boron‐nitride nanotubes (CBNNTs), zigzag (8,0), and armchair (6,6). Although the chemisorption of O₂ on CBNNT(6,6) is calculated to be a thermodynamically unfavorable process, the binding of O₂ on CBNNT(8,0) is found to be an exothermic process and can form both chemisorbed and physisorbed complexes. The CBNNT(8,0) has very different O₂ adsorption properties compared with pristine carbon nanotubes (CNTs) and boron‐nitride nanotube (BNNTs). For example, O₂ chemisorption is significantly enhanced on CBNNTs, and O₂ physisorption complexes also show stronger binding, as compared to pristine CNTs or BNNTs. Furthermore, it is found that the O₂ adsorption is able to increase the conductivity of CBNNTs. Overall, these properties suggest that the CBNNT hybrid nanotubes may be useful as a gas sensor or as a catalyst for the oxygen reduction reaction. © 2014 Wiley Periodicals, Inc.     

Removal of radiocobalt from aqueous solution by different sized carbon nanotubes

Batch adsorption technique was applied to study the adsorption of radiocobalt on multiwalled carbon nanotubes (CNTs) with deferent sizes. The aim of this work was to examine the effect of contact time, pH, solid content, foreign ions and CNT particle sizes on the removal of Co(II) ions from aqueous solutions by CNTs. The results indicated that the adsorption of Co(II) was strongly dependent on pH and the adsorption capacity was in inverse proportion to the particle sizes of CNTs. The adsorption of Co(II) was weakly affected by ionic strength and foreign ions. Ion exchange and surface complexation were the main adsorption mechanisms. The kinetics of Co(II) adsorption on CNTs was described well by pseudo-second-order model. The Langmuir and Freundlich models were applied to interpret the adsorption data. The results are important to understand the physicochemical behavior of Co(II) with CNTs, and for the application of CNTs in the preconcentration of radiocobalt from large volumes of aqueous solutions.     

Comparative study of Cr(III) adsorption by carbon nanotubes and active carbons

The adsorption of trivalent chromium ions from aqueous solutions on the surface of carbon materials, namely, multiwall carbon nanotubes (NTs) and two samples of active carbon, is studied depending on pH and adsorbate concentration in the system. Isotherms of Cr(III) adsorption by the aforementioned materials are obtained. It is shown that chromium ions are predominantly bound by surface carboxyl groups. The adsorption of chromium ions reduces the electrokinetic potential of NTs and, at chromium concentrations C _Cr(III) > 10^–5 M, leads to the reversal of the surface charge. The adsorption value decreases in the series NT > Merck carbon > Norit carbon, in contrast to an increase in the adsorbate affinity to the adsorbent in this series, as determined from the slope of the initial section of the Langmuir isotherms. Small amounts of chromium ions sorbed at low concentrations in solution (C _Cr(III) ≤ 10^–5 M) are comparable with the concentration of hydrogen ions displaced from the surface, thus making it possible to suppose the existence of an ionexchange adsorption mechanism. As the concentration of Cr(III) increases, the equivalent displacement of H⁺ is violated, thereby indicating the development of other adsorption mechanisms (complexation).