Influence of surface oxidation on the morphological and crystallographic structure of multi-walled carbon nanotubes via
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Influence of surface oxidation on the morphological and crystallographic structure of multi-walled carbon nanotubes via different oxidants Hassan Khani* and Omid Moradi*
Abstract 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. Keywords: Multi-walled carbon nanotube; Surface functionalization; Morphology; Crystallographic structure
Background Carbon nanotubes (CNTs) are divided into two types, namely single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs). SWCNTs are a graphite sheet rolled into a cylinder of a few micrometers in length and a few nanometers in diameter. MWCNTs consist of several such cylinders nested inside each other. MWCNTs have attracted great attention as a new kind of nanomaterial since their discovery in 1991. Carbon nanotubes have unique mechanical, electrical, magnetic, optical, and thermal properties. Owing to their inert nature, the nanotubes tend to form bundles with each other and * Correspondence: [email protected]; [email protected] Department of Chemistry, Shahre Qods Branch, Islamic Azad University, Tehran, Iran
thus do not disperse well in organic matrices in their pristine state. Suitable enhancement of the surface of MWCNTs is thus required in order to optimize their dispersion in the organic matrices. Out of various possible ways to achieve surface functionalization, chemical oxidation means of surface modification are quite common [1-4]. In this c
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