Influence of Electric Field on Dispersion of Carbon Nanotubes in Liquids

Production processes for carbon nanotubes commonly produce mixtures of solid morphologies that are mechanically entangled or that self‐associate into aggregates. The entangled or aggregated carbon nanotubes often need to be dispersed in corresponding material matrices in order to develop materials that have unique mechanical characteristics or transport properties. The most effective method for dispersion of carbon nanotubes is to prepare fluid suspensions of them in liquid media with applications of surfactant or/and ultrasonication. The authors propose an innovative dispersion method for carbon nanotubes by which an electric field is applied to suspensions of carbon nanotubes in liquids treated by surfactant and ultrasonication. Compared to dispersion without the electric field, the dispersion status of carbon nanotubes in liquid media is evidently improved with the electric field. The results indicate that the electric field conditions are effective for dispersion of carbon nanotubes in liquids and that complex effects of electric field, surfactant, and ultrasonication are beneficial for improvement of dispersion of carbon nanotubes.     

Aqueous dispersion, surface thiolation, and direct self-assembly of carbon nanotubes on gold

We report the efficient aqueous dispersion of pristine HiPco single-walled carbon nanotubes (SWNTs) with ionic liquid (IL)-based surfactants 1-dodecyl-3-methylimidazolium bromide (1) and 1-(12-mercaptododecyl)-3-methylimidazolium bromide (2), the thiolation of nanotube sidewalls with 2, and the controlled self-assembly of positively charged SWNT-1,2 composites on gold. Optical absorption spectra and resonance Raman (RR) data of obtained aqueous SWNT-1,2 dispersions are consistent with debundled and noncovalently functionalized nanotubes whose electronic properties have not been disturbed. Additionally, the dispersion of pristine nanotube material with surfactants 1 and 2 leads to a high degree of purification from carbonaceous particles. The chiralities of the 14 smallest semiconducting HiPco SWNTs in resonance with Raman excitation at 1064 nm (1.165 eV) were determined in SWNT-2 aqueous dispersion using UV-vis-NIR and RR spectra. X-ray photoelectron spectroscopy (XPS) and surface-enhanced resonance Raman scattering (SERRS) spectroscopy of SWNT-2 submonolayers on gold verified the encapsulation of individualized SWNTs with IL surfactants, the cleavage of S-S disulfide bonds formed in aqueous SWNT-2 suspensions, and the direct chemisorption of the SWNT-2 composite on bare gold via the Au-S bond. Aqueous dispersions of SWNTs with IL-based surfactants add biofunctionality to carbon nanotubes by imparting the positive surface charge necessary for interactions with cell membranes. Our technique, which purifies pristine nanotube material and produces water-soluble, positively charged nanotubes with pendent surface-active thiol groups, may also be translated to other carbon nanotubes and carbon nanostructures. Self-assembled, positively charged submonolayers of SWNTs can be further used for applications in cell biology and sensor technology.     

Aggregation behavior of single-walled carbon nanotubes in dilute aqueous suspension

The aggregation behavior of colloidal single-walled carbon nanotubes (SWNT) in dilute aqueous suspensions was investigated using a novel light scattering measurement technique. The aggregation of SWNT in three suspensions was examined: (1) nanotubes after acid treatment; (2) as-received nanotubes stabilized by a nonionic surfactant; and (3) acid-treated nanotubes with nonionic surfactant. Continuous light scattering measurements of the SWNT suspensions (probing the 38-436 nm length scale) made over two weeks showed that the nanotubes in each sample formed networks with fractal-like structures. The as-received nanotubes were stable over the measurement period, while the acid-treated nanotube suspension showed greater dispersion variability over time, yielding looser structures at large length scales and more compact structures at smaller length scales. The addition of surfactant to the acid-treated suspension significantly enhanced nanotube dispersion.     

Improving the effectiveness of interfacial trapping in removing single-walled carbon nanotube bundles

Single-walled carbon nanotube (SWNT) bundles are selectively removed from an aqueous dispersion containing individually suspended carbon nanotubes coated with gum Arabic via interfacial trapping. The suspensions are characterized with absorbance, fluorescence, and Raman spectroscopy as well as atomic force microscopy (AFM) and rheology. The resulting aqueous suspensions have better dispersion quality after interfacial trapping and can be further improved by altering the processing conditions. A two-step extraction process offers a simple and fast approach to preparing high-quality dispersions of individual SWNTs comparable to ultracentrifugation. Partitioning of SWNTs to the liquid-liquid interface is described by free energy changes. SWNT bundles prefer to reside at the interface over individually suspended SWNTs because of greater free energy changes.     

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.     

A study of the stability of aqueous suspensions of functionalized carbon nanotubes

The properties of aqueous suspensions of carbon nanotubes have been studied as depending on the conditions of their functionalization in a mixture of sulfuric and nitric acids. The elemental composition and contents of carboxyl, lactone, and hydroxyl groups in carbon nanotubes have been determined at different durations and temperatures of functionalization. The influence of functionalization conditions on the value of the electrokinetic potential of carbon nanotubes in aqueous suspensions and the nanotube solubility in water has been investigated. It has been found that the absolute value of the electrokinetic potential of nanotubes and their solubility in water increase with both the duration and temperature of functionalization due to a rise in the number of functional groups on their surface. The optimal regimes of functionalization of carbon nanotubes have been determined from the point of view of preserving their structure and stability in aqueous dispersions.     

Dispersion of single-walled carbon nanotubes in dimethylacetamide and a dimethylacetamide–cholic acid mixture

A way of dispersing single-walled carbon nanotubes in preparing stable suspensions with high concentrations of individual nanotubes in amide solvents is described. The obtained suspensions are studied via Raman spectroscopy. The dependence of the degree of single-walled carbon nanotube (SWNT) dispersion in individual and mixed amide solvents on the type of solvent, the mass of nanotubes, and the concentration of cholic acid is established. A technique for processing spectral data to estimate the diameters and chiralities of individual nanotubes in suspension is described in detail.     

Dispersion and orientation of single-walled carbon nanotubes in a chromonic liquid crystal

A post-synthesis alignment of individual single-walled carbon nanotubes (SWCNTs) is desirable for translating their unique anisotropic properties to a macroscopic scale. Here, we demonstrate excellent dispersion, orientation and concomitant-polarised photoluminescence of SWCNTs in a nematic chromonic liquid crystal. The methods to obtain stable suspension are described, and order parameters of the liquid crystal matrix and of the nanotubes are measured independently.     

The effect of oxidation on physicochemical properties and aqueous stabilization of multiwalled carbon nanotubes: comparison of multiple analysis methods

Surface oxidation can alter physicochemical properties of multiwalled carbon nanotubes (MWCNTs) and influence their aqueous stabilization. Many techniques have been used to characterize the physicochemical properties and aqueous stabilization of MWCNTs. However, the relationship between the change in physicochemical property and the aqueous stabilization of MWCNTs merits more studies, and the multiple characterization techniques have not been well compared. This study systematically and comparatively investigated the effect of oxidation on the physicochemical properties and aqueous stabilization of MWCNTs using multiple analysis methods. Increased surface area, disclosed tube ends, defects on the sidewalls, disruption of the electronic structure, and removal of metal catalysts and amorphous carbon were observed for the oxidized MWCNTs (o-MWCNTs) using the multipoint Brunauer-Emmett-Teller (BET) method, transmission electron microscope observation, Raman spectroscopy, UV-Vis spectroscopy, and thermogravimetric analysis. An oxidation-time-dependent increase in oxygen content of the MWCNTs was verified by the methods of elemental analysis, mass difference calculation, and X-ray photoelectron spectroscopy (XPS). Fourier transform infrared spectroscopy, XPS, and the Boehm titration were employed to study the functionalities on the MWCNT surfaces. Despite the limitations of these techniques, the results indicated that the dramatic increase in carboxyl groups was mainly responsible for the significant increase in oxygen content after the oxidation. The dissociation of the grafted functional groups increased electronegativity of the o-MWCNTs and facilitated the aqueous stabilization of o-MWCNTs through electrostatic repulsions. The oxidation affected the UV-Vis absorbance of MWCNT suspensions. The absorbances at 800 nm of the stabilized MWCNT suspensions had a good correlation with the MWCNT concentrations and could be used to quantify the MWCNT suspensions. The findings of this work are expected to boost the research on carbon nanotubes and their environmental behaviors.     

Raman spectroscopy of individual single-walled carbon nanotubes from various sources

Resonance Raman spectroscopy/microscopy was used to study individualized single-walled carbon nanotubes (SWNTs) both in aqueous suspensions as well as after spin-coating onto Si/SiO2 surfaces. Four different SWNT materials containing nanotubes with diameters ranging from 0.7 to 1.6 nm were used. Comparison with Raman data obtained for suspensions shows that the surface does not dramatically affect the electronic properties of the deposited tubes. Raman features observed for deposited SWNTs are similar to what was measured for nanotubes directly fabricated on surfaces using chemical vapor deposition (CVD) methods. In particular, individual semiconducting tubes could be distinguished from metallic tubes by their different G-mode line shapes. It could also be shown that the high-power, short-time sonication used to generate individualized SWNT suspensions does not induce defects in great quantities. However, (additional) defects can be generated by laser irradiation of deposited SWNTs in air, thus giving rise to an increase of the D-mode intensity for even quite low power densities (approximately 10(4) W/cm2).     

Dispersion of multi-wall carbon nanotubes by an ionic liquid-based polyether in aqueous solution

Multi-wall carbon nanotubes (MWCNTs) can be effectively dispersed by an ionic liquid-based polyether, poly(1-glycidyl-3-methylimidazolium chloride) (PGMIC) in aqueous solution. The amount of dispersed MWCNTs increases with the increasing of PGMIC concentration, and then decreases. Reaggregation of MWCNTs is observed when PGMIC exceeded the optimal concentration, which may be due to the conformational change of PGMIC molecules around MWCNT. The ultrasonic dispersion method is better than stirring method in the PGMIC solution. Furthermore, the acidic solution is convenient to prepare stable MWCNTs suspensions. Through the characterizations of ultraviolet–visible–near infrared, thermogravimetric analysis and Fourier transform infrared, it can be concluded that electrostatic repulsions, hydrophobic effect, n–π, and cation–π interactions played important roles in the dispersion of MWCNTs.     

Effects of surfactant and boron doping on the BWF feature in the Raman spectrum of single-wall carbon nanotube aqueous dispersions

We examine the Breit-Wigner-Fano (BWF) line shape in the Raman spectra of carbon single-wall nanotubes (SWNTs) dispersed in aqueous suspensions. Bundling and electronic effects are studied by comparing undoped SWNTs (C-SWNTs) to boron-doped nanotubes (B-SWNTs) in a variety of different surfactant solutions. For SWNTs dispersed with nonionic surfactants that are less effective in debundling than ionic surfactants, the Raman spectra retain a large BWF feature. However, we demonstrate that even for SWNTs dispersed as isolated nanotubes by ionic surfactants the BWF feature may be present and that the intensity of the BWF is highly sensitive to the specific surfactant. In particular, surfactants with electron-donating groups tend to enhance the BWF feature. Also, modification of the SWNT electronic properties by boron doping leads to enhanced surfactant dispersion relative to undoped C-SWNTs and also to modification of the BWF feature. These observations are in agreement with reports demonstrating an enhancement of the BWF by bundling but also agree with reports that suggest electron donation can enhance the BWF feature even for isolated SWNTs. Importantly, these results serve to caution against using the lack or presence of a BWF feature as an independent measure of SWNT aggregation in surfactant dispersions.     

Dispergation and modification of multi-walled carbon nanotubes in aqueous solution

Multi-walled carbon nanotubes (MWCNTs) are widely applied in development of composite materials. However, their properties are directly influenced by the degree of uniformity of dispersion of MWCNTs in the material’s matrix. In this paper, the dispersing of raw MWCNTs (R-MWCNTs) and decorated MWCNTs (D-MWCNTs) was studied in aqueous solution. The D-MWCNTs were obtained by chemical modification method by treatment of initial MWCNTs with the mixture of concentrated nitric and sulfuric acids (3: 1 vol/vol). To achieve a good dispersion of the MWCNTs, a method utilizing ultrasonic processing and surfactant (polyvinylpyrrolidone, PVP) was employed. MWCNTs were characterized by Fourier transform infrared spectroscopy (FT–IR) and X-ray diffraction (XRD). The prepared MWCNTs suspensions were investigated by UV spectroscopy, zeta potential measurements, surface tension and transmission electron microscopy (TEM). The D-MWCNTs have better dispersibility in aqueous solution; this attributed to the functional groups formed on their surface during chemical modification. The PVP surfactant in a certain concentration of 0.6 g/L has the maximum dispersing effect on MWCNTs in aqueous solution, the optimum concentration ratio of PVP and MWCNTs was 3: 1.     

Characterization of Carbon Based Nanoparticles Dispersion in Aqueous Solution Using Dynamic Light Scattering Technique

Characterization of physicochemical properties of nanoparticles in aqueous environment prior to conducting hazard studies is strongly recommended by many scientific organizations. In this work we studied the dissolution behaviour and physicochemical properties of carbon based nanoparticles in aqueous solution. The time evolution of the size distribution and the state of dispersion of carbon black and carbon nanotubes in physiological solution have been investigated by means of Dynamic Light Scattering technique. The influence of mechanical agitation such as sonication and stirring on the agglomeration state and particle size distribution has been investigated. However, such processes seem to have little or no effect as far as agglomeration is concerned.     

Stability of the aqueous suspensions of nanotubes in the presence of nonionic surfactant

Stability of aqueous suspensions of multiwalled carbon nanotubes (MWNTs) and their percolation behavior are investigated. Nanotubes of aqueous suspensions show a strong tendency to aggregation and networking into electroconductive clusters. The percolation threshold of the electrical conductivity is rather low and of order phi approximately 0.01 (where phi is the volume fraction), which can be explained by the high aspect ratio of MWNTs. Strong influence of the nonionic surfactant Triton X-305 on the colloidal stability of aqueous suspensions of MWNTs is observed. Addition of surfactant exerts a stabilizing effect at surfactant concentration C(s) proportional to the weight concentration C of MWNTs, C(s) approximately C mol/dm3. The transient behavior of electrical conductivity in the aqueous suspensions is explained by fractal aggregation processes. The fractal dimension is shown to be sensitive to the surfactant concentration C(s).     

Critical aspects related to processing of carbon nanotube/unsaturated thermoset polyester nanocomposites

Carbon nanotubes (CNTs) have outstanding mechanical, thermal and electrical properties. As a result, particular interest has been recently given in exploiting these properties by incorporating carbon nanotubes into some form of matrix. Although unsaturated polyesters with styrene have widespread use in the industrial applications, surprisingly there is no study in the literature about CNT/thermoset polyester nanocomposite systems. In the present paper, we underline some important issues and limitations during the processing of unsaturated polyester resins with different types of carbon nanotubes. In that manner, 3-roll mill and sonication techniques were comparatively evaluated to process nanocomposites made of CNTs with and without amine (NH₂) functional groups and polyesters. It was found that styrene evaporation from the polyester resin system was a critical issue for nanocomposite processing. Rheological behaviour of the suspensions containing CNTs and tensile strengths of their resulting nanocomposites were characterized. CNT/polyester suspensions exhibited a shear thinning behaviour, while polyester resin blends act as a Newtonian fluid. It was also found that nanotubes with amine functional groups have better tensile strength, as compared to those with untreated CNTs. Transmission electron microscopy (TEM) was also employed to reveal the degree of dispersion of CNTs in the matrix.     

碳纳米管在水中的分散性

以超声波为辅助工具,分别研究阳离子、阴离子型等表面活性剂在水中对碳纳米管分散性的影响,进而研究两种表面活性剂复配后对碳纳米管分散性的影响。通过记录分散的碳纳米管溶液的保存时间和SEM对分散效果进行分析和观察。     

Non‐Covalent Functionalization of Carbon Nanotubes with Surfactants and Polymers

Carbon nanotubes (CNTs) possessing unique structure and properties are attractive building blocks for novel materials and devices of important practical interest. However, the insolubility or poor dispersibility of pristine CNTs in common solvents poses a serious obstacle to their further development. To effectively utilize CNTs as building blocks for nanotechnology, CNTs have been covalently and noncovalently functionalized in a number of ways to render them soluble in aqueous or organic solutions. Here, we review recent progress and advances that have been made on dispersion of carbon nanotubes in aqueous and organic media by non‐covalent functionalization with surfactants and polymers.     

Purification and dispersibility of multi-walled carbon nanotubes in aqueous solution

In this paper, the pristine multi-walled carbon nanotubes (P-MWCNTs) were purified either by the high temperature treatment (HT-MWCNTs) or by concentrated acid treatment (CA-MWCNTs). The HT-MWCNTs were prepared by heating at 500°C, while the CA-MWCNTs were treated by the mixture of concentrated nitric and sulfuric acids taken in a volume ratio of 3: 1. Ultrasonic processing and surfactants were utilized to achieve homogenous MWCNTs suspensions. The HT-MWCNTs and CA-MWCNTs were characterized by thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FT-IR). Among these three MWCNTs, the prepared homogeneously dispersed MWCNTs suspensions were characterized by UV–Vis absorbency and transmission electron microscopy (TEM). Finally, the dispersion mechanism was discussed. The results showed that both high temperature treatment and concentrated acid treatment can be used for purification of the P-MWCNTs, removing the amorphous carbon and other impurities. In these suspensions, the purified MWCNTs showed a better dispersibility in aqueous solution. The high temperature treatment was a kind of physical purification treatment method and it just burned the amorphous carbon away and strengthened the structure of MWCNTs, while the concentrated acid treatment was a chemical purification treatment method and this chemical treatment method grafted more effective groups to improve the dispersibility of MWCNTs.     

Spectroscopic study of suspensions of single-wall carbon nanotubes in polyaniline solutions in <Emphasis Type="Italic">N</Emphasis>-methylpyrrolidone in UV—Vis—NIR regions

Stable suspensions of carbon nanoparticles in polyaniline solutions in N-methylpyrrolidone were compared with the suspensions in aqueous solutions of cetyltrimethylammonium bromide using spectrophotometry in UV— Vis—NIR regions. Polyaniline in low concentrations in N-methylpyrrolidone was found to be a more efficient surfactant than cetyltrimethylammonium bromide in water. Analysis of the optical spectra of suspensions of carbon soots in solutions of the polymer made it possible to reveal a noticeable change in the spectra of the starting polyaniline and nanotubes, indicating the chemical interaction of polyaniline with the carbon nanotubes.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2584–2588, December, 2004.