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) KMnO 4 . 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 H 2 SO 4 /HNO 3 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 KMnO 4 in alkali was used as the oxidant, which is relatively mild, different amounts of -OH, -CdO, 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 -CdO. This step depends on the oxidant's ability to generate -C-OH groups and to transform them into -CdO 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 -CdO and their neighborhood groups.

Herein we reported a simple way to enrich the large-diameter single-walled carbon nanotubes (SWNTs) by using a mixed concentrated H 2 SO 4 /HNO 3 treatment. The proportion of the SWNTs with diameter larger than 2 nm measured from transmission electron microscopy was only ca. 3% in the original sample prepared by chemical vapor deposition method. It is found that this proportion becomes remarkably increased with treatment time and reached ca. 20% after 96 h of acid treatment.Carbon nanotubes, in particular single-walled carbon nanotubes (SWNTs) have attracted attentions and interests of researchers in many fields because of their excellent physical and chemical properties. They are expected to be qualified for numerous applications, including flat panel field emission displays, 1-3 nanoelectronic devices, 4,5 chemical sensors, 6,7 hydrogen storages, 8 etc. For most of these applications, it is highly desirable to synthesize the SWNTs with defined properties and controlled structures even if the selective growth of SWNTs is still in its infancy. In view of the difficulties in controlling the diameter of SWNTs, many efforts have been made to push ahead. There are two different approaches for producing largediameter nanotubes. The first approach is the controlled synthesis, achieved by controlling furnace temperature and choosing suitable catalysts or promoters. [9][10][11] The second approach is the postsynthesis treatment either by heating treatment or by electron beam coalescing. 12,13 For example, Nagasawa et al. reported that the SWNTs with diameters of 1.13 and 1.22 nm diameter burn more quickly than the 1.37 nm diameter tubes when heating in oxygen and that a selective oxidation occurred to the thinner SWNTs when refluxing in nitric acid. 14 This suggests a possibility of using suitable chemical oxidation treatment to enrich large-diameter nanotubes. In this work, we show the experimental evidence of such an enrichment of SWNTs in mixed concentrated H 2 SO 4 /HNO 3 , which has been used for shortening the carbon nanotubes. [15][16][17] The SWNTs were produced by the methane chemical vapor deposition (CVD) method using porous MgO supported Fe catalyst. The as-prepared carbon nanotubes were purified by 37% HCl to remove the MgO supports and Fe catalyst first and then filtrated with a 1.2µm diameter pore membrane under vacuum after sonication in 0.2% Triton X-100 solution (pH ) 10). The purified nanotubes (p-SWNTs) were oxidatively treated

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