Surface Modification of Multiwalled Carbon Nanotubes to Impart Technological Properties

Abstract

The work considers the processes of modifying the surface of multiwalled carbon nanotubes with various polar groups: carboxyl, alcoholic hydroxyl, and quaternary ammonium salt. A procedure is developed using an oxidizing mixture of hydrogen peroxide, nitric acid, and sulfuric acid for the carboxylation of carbon nanotubes at a temperature of 70°C, which minimizes the formation of amorphous carbon and increases the concentration of carboxyl groups on the surface to 5.5 wt %. Tubes with a surface modified with a quaternary ammonium salt are obtained from carboxylated carbon nanotubes by reaction with triethanolamine. The modification of carbon nanotubes with ethyl hydroxyl groups is carried out with the participation of a free-radical initiator in an ethanol medium. The initial defects of nanotubes determine the number and nature of the defects at which chemical bonding occurs during functionalization, regardless of the method of nanocarbon material processing. The thermal–oxidative stability of the initial and modified carbon nanotubes in air is explored. The initial nanotubes are the most stable; they maintain their properties even when heated up to 520°C in air. For modified nanotubes, the thermal stability decreases in the series multiwalled nanotubes with carboxyl groups on the surface, multiwalled nanotubes with alcoholic hydroxyl groups, and multiwalled nanotubes modified with a quaternary ammonium salt, the oxidation of which begins at 400°C. The grafting of alcohol hydroxyl groups onto the surface of multiwalled carbon nanotubes is of considerable interest for obtaining an adsorption material with a developed surface, capable of covalently binding metal ions due to hydroxyl groups, similarly to complexing agents such as ethylene glycol or pyrocatechol. Carbon nanomaterials modified with polar groups exhibit good adsorption properties with respect to heavy metal ions. The degree of extraction of zinc and copper ions in the case of carboxylated nanotubes reaches 98%.