Purification of carbon nanotubes produced by the electric arc-discharge method

As-grown carbon nanotubes (CNTs) samples produced by electric arc-discharge method have shown several undesired impurities, such as amorphous carbon, metallic particles and different graphitized carbon materials that prevents their potential use. Therefore, due to the wide variety of impurities present in these samples, the purification process should be carried out in specific steps. For instance, the effective residual catalyst metal nanoparticles removal is a big challenge, since these materials are coated with a graphitic shell which prevents acid treatments from attacking the metals. In this work two different purification protocols based on the combination of gas-phase treatments, followed by the liquid-phase oxidation steps of CNTs with high degree of impurities were investigated. A significant removal of carbon impurities with minimal introduction of defects in the CNTs walls was evaluated, however the presence of active encapsulated metallic particles was still observed. Through TEM, TGA, EDS and XPS analysis, it was possible showed that these metals are encapsulated in different levels of metallic carbides (such as Ni3C), with very difficult removal. Even though the catalytic metals were not completely removed by the protocols used, the combination of the alternating gas phase with the acid treatments ensured CNTs samples with structural integrity and impurities removal of up to 75%.