Evaluation of crystalline perfection degree of multi-walled carbon nanotubes: correlations between thermal kinetic analysis and micro-Raman spectroscopy

Commercially available and laboratory-prepared multi-walled carbon nanotubes (MWCNTs) are systematically investigated by the use of micro-Raman spectroscopy (MRS), thermogravimetric analysis (TGA) and complementary techniques (scanning electron microscopy (SEM) and transmission electron microscopy (TEM)) with the aim of establishing a standardised post-growth diagnostic protocol for the assessment of their overall crystalline quality. By studying a set of 'reference' samples, clear correlations are evidenced between the Raman graphitisation indexes (D/G, G′/G and G′/D intensity ratios) commonly adopted to describe the crystalline arrangement of nanotubes, and their reactivity towards oxygen, as measured by the apparent activation energy needed for their oxidation, inferred from the kinetic analysis in quasi-isothermal conditions. The higher the crystalline perfection degree, the higher the energy needed for oxidising them. The efficacy of the found correlations in indirectly assessing the reactivity of nanotubes prepared under different conditions is successfully demonstrated by the use of a second set of samples. The physical meaning and range of validity of the shown correlations are further discussed. Aiming to establish a standardised post-growth diagnostic protocol for the assessment of the crystalline perfection degree of MWCNTs, clear correlations are evidenced between the Raman graphitisation indexes (D/G, G′/G and G′/D intensity ratios) and the MWCNT reactivity towards oxygen, as measured by the apparent activation energy needed for their oxidation, inferred from the kinetic analysis in quasi-isothermal conditions.