Do Nanotubes Follow an Amorphization Trajectory as Other Nanocarbons Do?

Various typologies of nanocarbons are considered, including crystalline graphite, amorphous activated carbon, and a large variety of multiwall carbon nanotubes (laboratory-prepared and commercially available, pristine and heat-treated), featured by different purity degree (81.7-99.7 wt %), crystalline quality (amorphous fraction: 0-5 wt %), size (outer diameter: 5-100 nm), and morphology. From the comparative discussion of the results of their systematic investigation by Raman spectroscopy, kinetic thermal analysis, and complementary techniques, the existence of a sharp correlation emerges between the strength of C bonding, as measured by the wavenumber position of the G-band in Raman spectra, and the oxidative resistance of the samples, as monitored by the maximum oxidation-rate temperature. A three-stage model is proposed to account for the effects associated with the increasing localization of states due to the introduction of curvature and lattice defects, accompanying the evolution from crystalline graphite to highly crystallized nanotubes, and from poorly crystallized nanotubes to amorphous activated carbon.