Effect of Chemical Oxidation on the Structure of Single-Walled Carbon Nanotubes

In the present study, we report the systematic investigation of the effect of chemical oxidation on the structure of single-walled carbon nanotubes (SWNTs) by using different oxidants. The oxidation procedure was characterized by using infrared spectroscopy and transmission electron microscopy (TEM). The SWNTs were produced by chemical vapor deposition (CVD) and oxidized with three kinds of oxidants:  (1) nitric acid (2.6 M), (2) a mixture of concentrated sulfuric acid (98 wt %) and concentrated nitric acid (16 M) (v/v = 3/1) and (3) KMnO₄. The results reveal that the different functional groups can be introduced when the SWNTs are treated with different oxidants. Refluxing in dilute nitric acid can be considered as a mild oxidation for SWNTs, introducing the carboxylic acid groups only at those initial defects that already exist. The abundance of the carboxylic acid groups generated with this oxidant remained constant along with the treating time. In contrast, sonication of SWNTs in H₂SO₄/HNO₃ increased the incidence of carboxylic acid groups not only at initial defect sites but also at newly created defect sites along the walls of SWNTs. Compared to the two oxidants above, when KMnO₄ in alkali was used as the oxidant, which is relatively mild, different amounts of −OH, −CO, and −COOH groups were introduced. The oxidation processes begin mainly with the oxidation of the initial defects that arise during the CVD growth of the SWNTs and are accompanied by processes that can be roughly divided into two steps:  (1) the defect-generating step and (2) the defect-consuming step. Specifically, during the defect-generating step, the oxidants attack the graphene structure by electrophilic reactions and generate active sites such as −OH and −CO. This step depends on the oxidant's ability to generate −C−OH groups and to transform them into −CO groups. During the defect-consuming step, the graphene structure of the tube was destroyed by the oxidation of the generated active sites in step 1. The defect-consuming step mostly counts on the ability of the oxidant to etch/destroy the graphite-like structure around the already generated −CO and their neighborhood groups.