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.     

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.     

有机硅聚醚共聚物功能化处理制备多壁碳纳米管悬浊液

以有机硅聚醚共聚物(PSPEO)为分散剂, 水为溶剂, 超声波作用下对硝酸纯化的多壁碳纳米管、浓硫酸与浓硝酸组成的混酸剪切的多壁碳纳米管功能化处理, 分别得到1～2.5 mg/mL和3～5 mg/mL的多壁碳纳米管悬浊液. 所得悬浊液有较好的稳定性, 这得益于有机硅聚醚共聚物独特的结构与性能. 用TEM, HRTEM, UV-vis, Raman光谱等技术对多壁碳纳米管悬浊液进行表征, 结果表明5～10 nm的PSPEO覆盖在碳纳米管表面并与碳纳米管强相互作用, 实现了碳纳米管的分散.     

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.     

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.     

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).     

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.     

Weak polyelectrolyte control of carbon nanotube dispersion in water

The dispersion of nanotubes by pH-responsive polymers (i.e., weak polyelectrolytes) enables the macroscopic properties of aqueous suspensions to be tuned. Microstructural changes were achieved as a function of pH in aqueous suspensions containing single-walled carbon nanotubes and imaged by cryogenic-TEM. Clear evidence of pH-sensitive nanotube dispersion is shown. We expect that many useful properties of these nanotube-polymer systems could be sensitive to microstructure, making this technique important for aqueous processing of carbon nanotubes and macroscopic tailoring of solid polymer nanocomposite behavior.     

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.     

The stability of suspensions of multiwalled carbon nanotubes in organic solvents in the presence of triton X-165

The stability of suspensions of multiwalled carbon nanotubes in isopropanol, toluene, and p-xylene has been studied. Suspensions of multiwalled carbon nanotubes in isopropanol (unlike those in toluene and p-xylene) display high aggregation stability, which is explained by the formation of repulsive solvation layers as a result of interactions between the polar solvent and nanotube surface. The addition of a nonionic surfactant, Triton X-165, noticeably increases aggregate sizes and accelerates sedimentation processes in nanotube suspensions in isopropanol, while, in toluene and p-xylene, the aggregate sizes decrease and the nanotube suspensions are stabilized.     

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.     

非离子表面活性剂对多壁碳纳米管在乙醇中高浓度分散的作用

如果能在无需聚合物或共价官能团辅助的条件下, 将多壁碳纳米管(MWNTs)高浓度地分散在乙醇中, 那么向各种复合材料引入MWNTs的过程就会更加便捷. 为此, 制备了多种含有表面活性剂的多壁碳纳米管乙醇悬浮液并对比考查了它们的稳定性. 非离子表面活性剂Triton X-100 和Tween 65显示出了在乙醇中分散悬浮高浓度MWNTs的能力, 能够使1.0 g·L-1 MWNTs乙醇悬浮液的上层清液经240 h后浓度仍分别在0.50和0.35 g·L-1以上. 这样长时间稳定的、没有聚合物或共价官能团辅助的MWNTs乙醇悬浮液, 其浓度比文献报道的值高. 进一步探讨了这些非离子表面活性剂分子结构对于分散MWNTs的优势, 并直观给出了其吸附于碳纳米管表面的可能形式. X射线光电子能谱和透射电子显微镜的表征结果都证实了表面活性剂分子吸附于碳纳米管表面.     

An unexpected new optimum in the structure space of DNA solubilizing single-walled carbon nanotubes

Here we report quantitative data on the amount of single-walled carbon nanotubes that can be suspended with oligodeoxynucleotides in aqueous buffer, together with rate constants for the thermal denaturation of the resulting DNA-nanotube complexes at elevated temperatures. Sequence motifs d(GT)n and d(AC)n with n=2, 3, 5, 10, 20, or 40 were employed, both individually and as equimolar mixtures of the complementary strands. Unexpectedly, the greatest suspending efficiency was found for the mixture of short, complementary oligonucleotides d(GT)3 and d(AC)3. Unlike the suspending efficiency, the kinetic stability of the nanotube suspensions increases with increasing chain length of the DNA, with half life times of >25 h at 90 degrees C for the complexes of the longest strands. Our results identify a new, unexpected optimum in DNA sequence space for suspending carbon nanotubes. They also demonstrate that suspending power depends on the presence of complementary strands. Exploratory assays suggest that nanotubes can be deposited site-selectively from suspensions formed with short DNA sequences.     

Rheological and dynamic-mechanical behavior of carbon nanotube/vinyl ester-polyester suspensions and their nanocomposites

Rheological properties of vinyl ester-polyester resin suspensions containing various amounts (0.05, 0.1 and 0.3 wt.%) of multi walled carbon nanotubes (MWCNT) with and without amine functional groups (-NH₂) were investigated by utilization of oscillatory rheometer with parallel plate geometry. Dispersion of corresponding carbon nanotubes within the resin blend was accomplished employing high shear mixing technique (3-roll milling). Based on the dynamic viscoelastic measurements, it was observed that at 0.3 wt.% of CNT loadings, storage modulus (G′) values of suspensions containing MWCNTs and MWCNT-NH₂ exhibited frequency-independent pseudo solid like behavior especially at lower frequencies. Moreover, the loss modulus (G″) values of the resin suspensions with respect to frequency were observed to increase with an increase in contents of CNTs within the resin blend. In addition, steady shear viscosity measurements implied that at each given loading rate, the resin suspensions demonstrated shear thinning behavior regardless of amine functional groups, while the neat resin blend was almost the Newtonian fluid. Furthermore, dynamic mechanical behavior of the nanocomposites achieved by polymerizing the resin blend suspensions with MWCNTs and MWCNT-NH₂ was investigated through dynamic mechanical thermal analyzer (DMTA). It was revealed that storage modulus (E′) and the loss modulus (E″) values of the resulting nanocomposites increased with regard to carbon nanotubes incorporated into the resin blend. In addition, at each given loading rate, nanocomposites containing MWCNT-NH₂ possessed larger loss and storage modulus values as well as higher glass transition temperatures (T_g) as compared to those with MWCNTs. These findings were attributed to evidences for contribution of amine functional groups to chemical interactions at the interface between CNTs and the resin blend matrix. Transmission electron microscopy (TEM) studies performed on the cured resin samples approved that the dispersion state of carbon nanotubes with and without amine functional groups within the matrix resin blend was adequate. This implies that 3-roll milling process described herein is very appropriate technique for blending of carbon nanotubes with a liquid thermoset resin to manufacture nanocomposites with enhanced final properties.     

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.     

Effect of electrooptical memory in suspensions of carbon nanotubes in liquid crystals

Electrooptical response and microstructure of dispersions of multiwall carbon nanotubes in N-(4-ethoxybenzilidene)-4-n-butyl aniline nematic liquid crystal (LC) are studied. Irreversible response on the applied electric field (electrooptical memory) was revealed in oriented layers of such suspensions. The essence of this effect consists in the fact that, after the switch-on and subsequent switch-off of the field, the optical transmittance of suspension layer placed between two crossed polarizers substantially increases compared to the initial value which is typical for homeotropic orientation of LC. The efficiency of electrooptical memory nonmonotonically depends on the concentration of nanotubes in suspension, c _CNT, reaching its maximum at c _CNT = 0.02–0.05 wt %. It is shown that the memory of suspensions is caused by the incomplete relaxation of LC molecules from planar to initial homeotropic state after the electric field switch-off. The model is proposed and substantiated, according to which the planar state of LC is stabilized by the network of nanotubes formed upon the disintegration of aggregates under the action of electrohydrodynamic flows. The disclosed memory effect is rather common; it is brought about in the suspensions of carbon nanotubes based on other LCs in which electrohydrodynamic instabilities are developed.     

Influence of polyvinyl pyrrolidone on the dispersion of multi-walled carbon nanotubes in aqueous solution

The sonication-driven suspensions of multi-walled carbon nanotubes (MWNTs) were prepared using the polyvinyl pyrrolidone as a dispersant. UV-visible absorbency, surface tension, zeta potential, adsorption isotherm and transmission electron microscopy, were used to characterize the homogeneity and stability of MWNTs suspensions with different concentrations of polyvinyl pyrrolidone. All results show that the optimum concentration of polyvinyl pyrrolidone in suspension is 0.5 g/L.     

Layer-by-layer assembled multilayer films of titanate nanotubes, Ag- or Au-loaded nanotubes, and nanotubes/nanosheets with polycations

Aqueous suspensions of hydrothermally synthesized titanate nanotubes and poly(diallyldimethylammonium chloride) (PDDA) have been employed to fabricate multilayer films on various substrates in a layer-by-layer fashion. Atomic force microscopy displays the dense coverage of the substrate surface by the nanotubes. UV-vis absorption spectroscopy confirms the consecutive growth of PDDA/nanotube layer pairs. Single crystalline Ag and Au nanoparticles with narrow size distribution spatially correlating with the nanotubes have been obtained by treating the nanotubes with AgNO(3) or HAuCl(4) aqueous solution followed by chemical reduction. The noble metal nanoparticles show a strong surface plasmon absorption band. A multilayer film construction of the noble-metal-loaded nanotubes has also been achieved. This process has been further extended to the heteroassembly of nanotubes/nanosheets in different layer sequences.     

Dispersion of Carbon Nanotubes with “Green” Detergents

Solubilization of carbon nanotubes (CNTs) is a fundamental technique for the use of CNTs and their conjugates as nanodevices and nanobiodevices. In this work, we demonstrate the preparation of CNT suspensions with “green” detergents made from coconuts and bamboo as fundamental research in CNT nanotechnology. Single-walled CNTs (SWNTs) with a few carboxylic acid groups (3–5%) and pristine multi-walled CNTs (MWNTs) were mixed in each detergent solution and sonicated with a bath-type sonicator. The prepared suspensions were characterized using absorbance spectroscopy, scanning electron microscopy, and Raman spectroscopy. Among the eight combinations of CNTs and detergents (two types of CNTs and four detergents, including sodium dodecyl sulfate (SDS) as the standard), SWNTs/MWNTs were well dispersed in all combinations except the combination of the MWNTs and the bamboo detergent. The stability of the suspensions prepared with coconut detergents was better than that prepared with SDS. Because the efficiency of the bamboo detergents against the MWNTs differed significantly from that against the SWNTs, the natural detergent might be useful for separating CNTs. Our results revealed that the use of the “green” detergents had the advantage of dispersing CNTs as well as SDS.