Evaluation of the overall crystalline quality of amorphous carbon containing multi-walled nanotubes

A set of nanotubes with moderate contents of amorphous phases (0.4–5.1 wt %) are systematically investigated by Raman spectroscopy, kinetic thermal analysis, and complementary techniques. Results show that, in spite of their small amount, the presence of amorphous phases has a strong impact on the oxidation path of nanotubes. The apparent activation energy for oxidizing them, as inferred from thermal kinetic analysis, decreases linearly with increasing the content of amorphous species. The presence of non-sp2 defects causes the failure of the commonly adopted Raman graphitization indexes, with consequent breakdown of the correlations between crystalline quality and activation energy evidenced in the absence of the amorphous phases. This drawback is overcome by introducing a generalized Raman indicator of the crystalline quality, whose value increases with increasing energy needed for oxidizing nanotubes independently on the typology (sp2 or non-sp2) of defects present in their walls.