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Purification and

Separation of Carbon Nanotubes

R.C. Haddon, J. Sippel, A.G. Rinzler, and F. Papadimitrakopoulos Abstract The problems posed by the synthesis and purification of single-walled carbon nanotubes (SWNTs) have inhibited progress in the field. In this article, we review the methods available for measuring the purity of SWNTs and the current status of processes designed to purify them. We emphasize the hierarchy of the purification steps that must be developed in order to obtain high-quality material suitable for the full range of advanced applications that are envisioned for the ultimate carbon nanofiber. We review two strategies for SWNT purification, the assessment of SWNT purity by use of near-IR spectroscopy and its application to the thermal oxidation of thin films of SWNTs, as well as recent advances in the separation of metallic and semiconducting SWNTs. While substantial progress has been made in the purification and separation of SWNTs, we note the need for quality control and quality assurance within the industry. Much work remains before pure SWNTs of specific lengths, diameters, and chirality can be made available for applications. Keywords: carbon nanotubes, near-infrared spectroscopy, purification, purity evaluation, separation, SWNTs.

The structural perfection of singlewalled carbon nanotubes (SWNTs) leads to outstanding materials properties,1 but it also makes them difficult to process and purify.2 Current methods of SWNT production rely on a carbon source exposed to high energies in the presence of a transitionmetal catalyst. Inevitably, as-prepared SWNTs (AP-SWNTs) are contaminated with the catalyst; as a result, the first stage in SWNT purification is generally the removal of this catalyst residue. However, this is often complicated by the presence of carbonaceous impurities, typically amorphous carbon and graphitic nanoparticles, that sometimes enclose the transition-metal catalyst and are quite robust, interfering with the removal of the catalyst. Efforts to purify SWNTs of these residues and impurities have been under way for some time, with important progress demonstrated, but this is just the beginning. The extent to which carbon nanotubes fulfill their technological potential will depend to a large degree on the success achieved 252

in obtaining SWNTs of precisely defined length, diameter, and chirality (see the article by Liu in this issue for a discussion of progress in these areas). This promises to be an even more challenging task that is just now starting to be explored. A hierarchical flow chart for SWNT purification is given in Figure 1. It should be emphasized that the sequence of purification steps for obtaining SWNTs of defined length, diameter, and chiral index (n,m) (where n and m are the integers that define the wrapping vectors describing the geometry of the tube) from AP-SWNTs is little more than a best guess. Much work remains at Stage 0, and the removal of catalyst, amorphous carbon, and graphitic nanoparticles rarely proceeds in a step-bystep manner. Further