High Yield Conversion of Carbon Nanotubes to Nanostraws at Mild Conditions
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Kuo Chu Hwang, Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan, R. 0. C. ABSTRACT Several oxidants were examined for their abilities in opening carbon nanotube end caps. Up to 91% of carbon nanotubes were found to have at least one open end when treated by 0.2 M KMnO4/ 10% H 2 0S4 or 0.2 M KMnO4/ 0.2 M Cr03 aqueous solution at 1000 C, 90 min. That is, -82% of carbon nanotubes was converted to "nanostraws". The morphologies of the processed carbon nanotubes reflect the relative strength of these oxidants. The mechanism of carbon nanotube end cap opening processes will be discussed. INTRODUCTION The discovery, 1 and large scale synthesis 2 of carbon nanotubes have initiated many theoretical and experimental studies on the physical and electronic properties of nanotubes. The internal hallow space of carbon nanotubes is expected to have special confinement effects on phase transition and low dimensional chemistry. 4 To fill in substrates, the nanotube end caps have to be opened first. Carbon nanotubes were previously reported to have 90% weight loss, when heated in air at 800' C. 7 The mechanisms of these carbon nanotube cap opening processes are not clear. We report herein that carbon nanotube end caps can be efficiently opened by several oxidants at mild conditions, namely in aqueous solution at 100°C, 90 min. The mechanism of the cap opening processes is most probably the conversion of C-=C double bonds to vicinal diols followed by oxidative cleavage of vicinal diols. EXPERIMENTAL Multilayer carbon nanotubes were purchased from MER Corporation (Tucson, Arizona) with length distribution of 0.1-100 Rim and diameter range of 3-50 nm. In general, 2 mg of nanotubes was heated in 2 ml of 0.2 M oxidant(s)-aqueous solution at 1000 C, 90 min with constant stirring. The processed nanotubes were separated from the solution by centrifugation, and washed by distillated water a few times to remove 75 Mat. Res. Soc. Symp. Proc. Vol. 359 01995 Materials Research Society
residual oxidants. In the case of formation of solid precipitates (such as MnO2 or OsO2) during the heating process, acidic ascorbate solution was added to the final solution to convert these solid oxides to the soluble ionic forms (Mn 2 + or Os3+). The carbon nanotubes was then dispersed in methanol, and a few drop of the nanotube-methanol solution was added to a carbon thin film coated copper grid for transmission electron microscopy (TEM, Hitachi, model H-600-3, operated at 75 KV) measurements. RESULTS AND DISCUSSIONS The basic structure of graphene layers of carbon nanotubes is composed of 5- and 6-member rings and C=C double bonds. 8 To cleave open the carbon nanotube end caps is to break the C=C double bonds. In literature, several oxidants (such as KMnO4 9,10 and OsO4 1 1) are commonly used to convert C=C double bonds to vicinal diols, and others (e.g., MnO2, 12 CrO3,1 3 104-,10 etc.) are known to be able to oxidatively cleave the C-C single bond of vicinal diols to generate two carbonyl groups (see equations I and 2 for examples of
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