Features of oxidative functionalization of multiwalled carbon nanotubes

The paper studies the oxidation laws of multiwalled carbon nanotubes (MWCNTs) of various types, which differ in geometric parameters (diameter, length, specific surface), using nitric acid. In different experiments, the duration of oxidation, the concentration of nitric acid, and the consumption of HNO3 per 1 g of MWCNTs were varied. The qualitative composition of the formed oxygen-containing functional groups and the changes occurring with the graphene layers of nanotubes were established by the methods of IR and Raman spectroscopy, and transmission electron microscopy. The total oxygen content in the samples was determined by energy dispersive analysis. The degree of MWCNTs functionalization with acidic functional groups was determined by Boehm titration. An X-ray structural analysis of the obtained samples was carried out and their specific surface area was estimated. It was determined that the most serious changes in the functionalization degree occurred during the first 1–3 hours of oxidation. During this time interval, the achieved values of the functionalization degree were determined mainly by the shape of the MWCNTs graphene layers. At 7–10 hours of treatment, regardless of the initial parameters, MWCNTs have the functionalization degree in the range of 0.5–0.7 mmoLg–1. At the initial stage of oxidation, the amorphous phase is removed. Using dilute solutions of nitric acid does not allow obtaining oxidized nanotubes with the high functionalization degree. It is expedient to reduce the consumption of 65 % HNO3 to 40 mL per 1 g of MWCNTs.