Effect of Mild Nitric Acid Oxidation on Dispersability, Size, and Structure of Single-Walled Carbon Nanotubes

Abstract

Oxidation of single-walled carbon nanotubes (SWNTs) with nitric acid increases their dispersability in water, methanol, and N,N-dimethylformamide. Two oxidation protocols, sonication in 8 M HNO3 at 40 °C and reflux in 2.6 M HNO3, have been examined using SWNTs produced by the CoMoCat, HiPco, and pulsed laser vaporization (PLV) methods. The dispersability of all types of nanotubes increased substantially after 1 h of sonication and after 2−4 h of reflux. Longer treatments resulted in little further improvement in dispersability and at reflux degraded the SWNTs. Stable dispersions of CoMoCat SWNTs in DMF at concentrations as high as 0.4 g/L were achieved without the use of surfactants or polymers. Raman spectroscopy showed greater covalent functionalization of the SWNTs by the reflux procedure than by the sonication procedure. Concurrent with improved dispersability, oxidation resulted in smaller diameters and shorter lengths as determined from AFM images, which show mostly bundles rather than individual tubes. The lengths of SWNTs after oxidation decreased in the order PLV > HiPco > CoMoCat. Recommendations for the method of conditioning of the various types of SWNTs depend on their intended use.