Theoretical and Experimental Studies on the Carbon‐Nanotube Surface Oxidation by Nitric Acid: Interplay between Functionalization and Vacancy Enlargement

Abstract

The nitric acid oxidation of multiwalled carbon nanotubes leading to surface carboxylic groups has been investigated both experimentally and theoretically. The experimental results show that such a reaction involves the initial rapid formation of carbonyl groups, which are then transformed into phenol or carboxylic groups. At room temperature, this reaction takes place on the most reactive carbon atoms. At higher temperatures a different mechanism would operate, as evidenced by the difference in activation energies. Experimental data can be partially related to first-principles calculations, showing a multistep functionalization mechanism. The theoretical aspects of the present article have led us to propose the most efficient pathway leading to carboxylic acid functional groups on the surface. Starting from mono-vacancies, it ends up with the synergistic formation of dangling -COOH groups and the enlargement of the vacancies.

Revisiting oxygen reduction reaction on oxidized and unzipped carbon nanotubes

“…This indicates that the 532.0 eV peak for SOCNT-48h was mainly attributable to thermally unstable carboxyl groups [26]. This agrees with the result of oxidized carbons after treatment with nitric acid [18]. Since the same peak remains for OCNT-H2.0-800, the oxygen groups on OCNT-H2.0 should be dominated by carbonyl and hydroxyl groups as they have much better thermal stability [26].…”

“…Starting with purified CNTs, we have selectively impaired the nanotube by either piercing through the CNT walls with aggressive oxidation [17], or caustically etching from the surface [18]. By such methods, oxygen groups can either be spread throughout the carbon framework (unzipped and oxidized) or confined at the surface (surface etched).…”

Tuning CNT Properties for Metal-Free Environmental Catalytic Applications

“…Oxidation of CNTs is frequently performed using nitric acid as reagent; in addition, to removing amorphous carbon and metal impurities, abundant oxygenated groups are generated on the carbon surface [28]. Although a non-selective functionalization is achieved, the O-containing groups can be anchored on the defect sites of sidewall and open caps of CNTs or on the defect sites and edges [25].…”

Properties of Carbon Nanotubes

“…Removing a carbon atom leaves a vacant space with three dangling carbon bonds in the nanotube network [210]. The triplet state is slightly more stable than the singlet state [211]. These three dangling bonds are unstable and undergo recombination to make a chemical bond between two of them forming a pentagon ring and one remaining dangling bond À in a nonagon ring [212].…”