Qualitative and quantitative study of carboxylic functional group creating on the surface of multiwall carbon nanotubes through mild oxidation process

Abstract

In present study, the effect of five different common oxidants, under mild oxidation, on the surface of multiwall carbon nanotubes (MWCNTs) with different diameters (10–20, 30–50, >50nm) for creating an oxygen-containing groups, especially carboxylic functional groups (COOH), qualitatively and quantitatively was investigated. MWCNTs were treated with acids and mixtures thereof: 1 – concentrated nitric acid, 2 – mixture of nitric and sulfuric acids (V:V/1:3), 3 – hydrogen peroxide, 4 – mixture of hydrogen peroxide and sulfuric acids (V:V/1:1), and 5 – acidic potassium permanganate in reflux and mild condition according to boiling point of oxidants. First, the presence of carboxylic functional groups on the surface of MWCNTs was proved using of Fourier transform infrared (FT-IR) spectroscopy and thermo-gravimetric analysis (TGA). The microstructure and crystallites were determined by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. Surface chemistry was evaluated in terms of the carboxylic groups determined by acid–base titration method proposed by Boehm. Also electrical conductivity and zeta potential of initial and oxidized MWCNTs were investigated. It was found that ratio of carboxylic-containing group is dependent on the type of oxidizers and diameters of MWCNTs. FT-IR and TGA results were confirmed presence of carboxylic groups of the MWCNTs surface. Decrease in the degree of crystallites started with widening full width of half maximum (FWHM) of XRD peak. SEM images show the morphology of the MWCNTs after oxidation remain intact. The most undamaging oxidation procedure with reasonable amount of carboxyl-containing groups can be performed using mixture of nitric and sulfuric acids on the surface of 10–20nm diameters MWCNTs according to titration data. Electrical conductivity and zeta potential of all oxidized samples were increased as compared with pristine MWCNTs with carboxylic anion group. Now, the oxidized MWCNTs were more useful for dispersing in materials and production conductive composites.