It is predicted theoretically and understood experimentally that carbon nanotubes (CNTs) possess excellent physical and chemical properties and have wide-range potential applications. However, only some of these properties and applications have been verified or realized. To a great extent, this situation can be ascribed to the difficulties in getting high-purity CNTs. Because as-prepared CNTs are usually accompanied by carbonaceous or metallic impurities, purification is an essential issue to be addressed. Considerable progress in the purification of CNTs has been made and a number of purification methods including chemical oxidation, physical separation, and combinations of chemical and physical techniques have been developed for obtaining CNTs with desired purity. Here we present an up-to-date overview on the purification of CNTs with focus on the principles, the advantages and limitations of different processes. The effects of purification on the structure of CNTs are discussed, and finally the main challenges and developing trends on this subject are considered. This review aims to provide guidance and to stimulate innovative thoughts on the purification of CNTs.
Since their discovery, the possibility of connecting carbon nanotubes together like water pipes has been an intriguing prospect for these hollow nanostructures. The serial joining of carbon nanotubes in a controlled manner offers a promising approach for the bottom-up engineering of nanotube structures-from simply increasing their aspect ratio to making integrated carbon nanotube devices. To date, however, there have been few reports of the joining of two different carbon nanotubes(1-3). Here we demonstrate that a Joule heating process, and associated electro-migration effects, can be used to connect two carbon nanotubes that have the same (or similar) diameters. More generally, with the assistance of a tungsten metal particle, this technique can be used to seamlessly join any two carbon nanotubes-regardless of their diameters-to form new nanotube structures.
Because of their unique physical, chemical, electrical, and mechanical properties, carbon nanotubes (CNTs) have attracted a great deal of research interest and have many potential applications. As large-scale production and application of CNTs increases, the general population is more likely to be exposed to CNTs either directly or indirectly, which has prompted considerable attention about human health and safety issues related to CNTs. Although considerable experimental data related to CNT toxicity at the molecular, cellular, and whole animal levels have been published, the results are often conflicting. Therefore, a systematic understanding of CNT toxicity is needed but has not yet been developed. In this Account, we highlight recent investigations into the basis of CNT toxicity carried out by our team and by other laboratories. We focus on several important factors that explain the disparities in the experimental results of nanotoxicity, such as impurities, amorphous carbon, surface charge, shape, length, agglomeration, and layer numbers. The exposure routes, including inhalation, intravenous injection, or dermal or oral exposure, can also influence the in vivo behavior and fate of CNTs. The underlying mechanisms of CNT toxicity include oxidative stress, inflammatory responses, malignant transformation, DNA damage and mutation (errors in chromosome number as well as disruption of the mitotic spindle), the formation of granulomas, and interstitial fibrosis. These findings provide useful insights for de novo design and safe application of carbon nanotubes and their risk assessment to human health. To obtain reproducible and accurate results, researchers must establish standards and reliable detection methods, use standard CNT samples as a reference control, and study the impact of various factors systematically. In addition, researchers need to examine multiple types of CNTs, different cell lines and animal species, multidimensional evaluation methods, and exposure conditions. To make results comparable among different institutions and countries, researchers need to standardize choices in toxicity testing such as that of cell line, animal species, and exposure conditions. The knowledge presented here should lead to a better understanding of the key factors that can influence CNT toxicity so that their unwanted toxicity might be avoided.
The theoretical maximum tensile strainthat is, elongationof a single-walled carbon nanotube is almost 20%, but in practice only 6% is achieved. Here we show that, at high temperatures, individual single-walled carbon nanotubes can undergo superplastic deformation, becoming nearly 280% longer and 15 times narrower before breaking. This superplastic deformation is the result of the nucleation and motion of kinks in the structure, and could prove useful in helping to strengthen and toughen ceramics and other nanocomposites at high temperatures.
Carbon nanotubes have attracted great interest in multidisciplinary study since their discovery. Herein, radionuclide Am-243(III) sorption to uncapped multiwall carbon nanotubes (MWCNTs) was carried out at 20 +/- 2 degrees C in 0.01 and 0.1 M NaClO4 solutions. Effects of Am-243(III) solution concentration, ionic strength, and pH on Am-243(III)sorption to MWCNTs were also investigated. The sorption is strongly dependent on pH values and weakly dependent on the ionic strength in the experimental conditions. The results show that MWCNTs can adsorb Am-243(III) with extraordinarily high efficiency by forming very stable complexes. Chemisorption or chemicomplexation is the main mechanism of Am-243(III) sorption on the surface of MWCNTs. MWCNTs can be a promising candidate for the preconcentration and solidification of Am-243(III) or its analogue lanthanides and actinides from large volumes of aqueous solution, as required for remediation purposes, and perhaps also as a sorbent for the removal of heavy metal ions from the industry wastewater.
Graphene nanoribbons (GNRs) are materials with properties distinct from those of other carbon allotropes(1-5). The all-semiconducting nature of sub-10-nm GNRs could bypass the problem of the extreme chirality dependence of the metal or semiconductor nature of carbon nanotubes (CNTs) in future electronics(1,2). Currently, making GNRs using lithographic(3,4,6), chemical(7-9) or sonochemical(1) methods is challenging. It is difficult to obtain GNRs with smooth edges and controllable widths at high yields. Here we show an approach to making GNRs by unzipping multi-walled carbon nanotubes by plasma etching of nanotubes partly embedded in a polymer film. The GNRs have smooth edges and a narrow width distribution (10-20 nm). Raman spectroscopy and electrical transport measurements reveal the high quality of the GNRs. Unzipping CNTs with well-defined structures in an array will allow the production of GNRs with controlled widths, edge structures, placement and alignment in a scalable fashion for device integration.
Carbon nanotubes have attracted great interdisciplinary interest because of their unique structure and properties. However, carbon‐nanotube research is challenged by several problems, such as: i) mass production of material, ii) control of length, diameter, and chirality, and iii) manipulation for use in diverse technological fields. Issues regarding the synthesis and purification as well as the functionalization and solubilization of carbon nanotubes are relevant topics in this rapidly growing field. In this paper, covalent and noncovalent approaches to functionalized and solubilized nanotubes are examined in detail, with particular emphasis on the change of properties that accompany the chemical modification. Following the discovery of fullerenes, carbon nanotubes have been the focus of great scientific interest within the last decade. Various issues relating to the synthesis, purification, and solubilization of carbon nanotubes have become of paramount importance because of their role in practical applications. In this article, we focus on the main functionalization efforts that have been produced recently, since the study of these huge molecules in solution (see figure) can shed further light on their intriguing properties.
Common‐or‐garden starch can render single‐walled carbon nanotubes (SWNTs) readily soluble in water. The secret is to preorganize the linear amylose component in the starch into a helix with iodine prior to bringing the SWNTs on the scene. The SWNTs displace the iodine molecules in a “pea‐shooting” type of mechanism (see scheme). After some physical cajoling of the aqueous solution containing the starch–SWNT complex, a fine “bucky paper” is formed. Spitting in the aqueous solution, followed by sitting around for a few hours, also enables equally fine “bucky paper” to be harvested.
The effect of oxidation on the structural integrity of multiwalled carbon nanotubes through acidic (nitric acid and a mixture of sulfuric acid and hydrogen peroxide) and basic (ammonium hydroxide/hydrogen peroxide) agents has been studied. In order to purify the as-received material, a non-oxidative treatment (with hydrochloric acid) was also applied. Electron microscopy and thermogravimetric analysis clearly revealed that the nitric acid-treated material under reflux conditions suffered the highest degree of degradation, such as, nanotube shortening and additional defect generation in the graphitic network. Basic oxidative treatment led to the complete removal of amorphous carbon and metal oxide impurities but the structural integrity was found to be intact. X-ray photoelectron spectroscopy was employed to confirm the different functionalities produced for each oxidation agent, whereas titration measurements determined the relative concentration of carboxylic functions onto the graphitic surface. Moreover, a general relationship between the chemical treatment and the amount of non-graphitic carbon was established by means of Raman spectroscopy measurements. The possibility of controlling the required amount of functionality, carboxylic and hydroxyl, via these oxidation procedures is discussed.