Purification of multi-walled carbon nanotubes

The discovery of carbon nanotubes (CNTs) has stimulated intensive research to characterize their structure and to determine their physical properties, both by direct measurement and through predictive methods. Many of the fundamental and remarkable properties of CNTs are now well-known, and their exploitation in a wide range of applications forms a large part of research currently in progress. However, the absence of a reliable, large-volume production capacity, simple and efficient purification methods, the high cost of carbon nanotubes and the fact that there is little selectivity in controlling the properties of the product are factors that have principally inhibited the commercialization of CNT technologies. Ever since CNTs were detected, considerable efforts have been directed at their synthesis, characterization and functionalization. Nevertheless, the CNT sample obtained by different techniques has the disadvantage of containing non-CNT impurities, such as graphitic particles, fullerenes, residual catalyst particles and amorphous carbon, which degrade the intrinsic properties of these materials. If the carbon nanotube is ever to accomplish its promise as an industrial material, large and high-quality aliquots, will be required. A number of purification methods involving elimination processes, such as physical separation, gas-phase and liquid-phase oxidation, in combination with chemical treatments, have been developed for nanotube materials. Though the quantitative determination of purity remains controversial, reported yields are best regarded with an appropriate level of scepticism on the method of assay. This review highlights the past and recent developments in the purification of multi-walled carbon nanotubes.