Removal of Impurities from Carbon Nanotube Materials

1 Removal of Impurities from Carbon Nanotube Materials 

Besides the removal of foreign material, there is another crucial aspect in the purification of carbon nanotubes. Tkny of the procedures presented in Sections 

1 and 3.3.2 yields tubes with different structure indices ( ,m), that is, varying in diameter and electronic projierties. Considerable efforts thus have been made recently to further separate these mixtures and to isolate nanotubes with uniform diameter, or at least with common electronic properties (Section 3.3.6.4). 

Other methods of oxidative purification include the treatment with super-critical water (hydrothermal procedure), thermal oxidation in air and plasma oxidation in the presence of water. All of them make use of the different reactivity of amorphous carbon and nanotubes, with the latter being more stable than the unordered material. The difference, however, is not very big, and so an extensive removal of amorphous carbon inevitably causes massive losses of nanotubes as well. Consequently the application of oxidizing methods always represents a compromise between complete elimination of impurities and retaining the largest possible amount of product. 

The hydrothermal process, that is, the treatment of the sample with water at high pressure and temperature (often in the supercritical range), proved a suitable method for nanotube purification. Water evolves surprising reactivity in redox processes at these conditions. It is then capable of dissolving from the samples both amorphous carbon and metallic impurities (especially in the presence of acids during reaction). Choosing the conditions too harsh, however, will do severe 

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