Influence of surface oxidation on the morphological and crystallographic structure of multi-walled carbon nanotubes via different oxidants

In this paper, three kinds of multi-walled carbon nanotubes (MWCNTs) with different diameters (outer diameters: 10 to 20, 30 to 50, and >50 nm) and special surface areas (200, 60, and 40 m2/g, respectively) were oxidized in commonly used liquid oxidizers: (1) concentrated nitric acid, (2) a mixture of nitric + sulfuric acids (V:V, 1:3), (3) hydrogen peroxide, (4) a mixture of hydrogen peroxide + sulfuric acid (V:V, 1:1), and (5) acidic potassium permanganate. Morphology of the pristine and oxidized MWCNTs was characterized by scanning electron microscopy which provides sufficient resolution for direct visualization of their outer diameter distribution. Full width at half maximum (FWHM) in the X-ray diffraction (XRD) investigation of the MWCNT samples before and after the oxidation process was measured. After treatment with oxidants, a clear decrease in nanotube diameters along the tube walls was observed. Decrease in the degree of crystallites started with the FWHM widening of the XRD diffraction peaks. The particle (crystallite) size (d002) calculated by Bragg's law and Scherrer equation increased depending on the kind of oxidants; the procedure can be performed using a mixture of HNO3 + H2SO4 on the surface of the MWCNTs with an outer diameter of 10 to 20nm. However, similar are the diffraction patterns of pristine and oxidized MWCNTs. Therefore, the MWCNTs that underwent oxidation process were able to preserve the first features of their structures, even though some narrowing of outer diameters and decreases in crystallites appeared. Nevertheless, the structure of MWCNTs still remains intact to be used as oxidized nanotubes in most applications.