ARTICLES Characterization of Fractions from Repeated Functionalization Reactions of Carbon Nanotubes Yi Lin, Shelby Taylor, Weijie Huang, and Ya-Ping Sun* Department of Chemistry and Center for AdVanced Engineering Fibers and Films, Howard L. Hunter Chemistry Laboratory, Clemson UniVersity, Clemson, South Carolina 29634-0973 ReceiVed: March 13, 2002; In Final Form: September 13, 2002 The fractionation of a purified pristine multiple-walled carbon nanotube (MWNT) sample was achieved via repeated functionalization reactions to produce several soluble nanotube fractions. The reactions were based on the esterification of the nanotube-bound carboxylic acids. The soluble nanotube fractions and the insoluble solid residue were characterized by using optical spectroscopy, electron microscopy, and other techniques. The results show that all of these soluble samples contain primarily functionalized MWNTs and that there is little difference among these soluble nanotube fractions, either in the degree of nanotube dispersion or in the nanotube length and diameter distributions. There are apparently no preferential solubilization of carbon nanoparticles or other carbon impurities present in the starting nanotube sample and also no selectivity in the solubilization of MWNTs of different sizes (lengths and diameters) in the functionalization reactions. Mechanistic implications of the results are discussed. Introduction The chemical modification and solubilization of carbon nanotubes represent an emerging area in the research on nanotube-based materials. Several research groups have reported successful functionalization reactions for single-walled (SWNT) and multiple-walled (MWNT) carbon nanotubes. 1-14 These reactions may roughly be divided into two categories: the functionalization via a direct attachment to the graphitic surface and the functionalization via the use of intrinsic or induced (due to the shortening of the nanotubes, 15,16 for example) defects. For the former approach, Margrave, Smalley, and co-workers reported the fluorination of SWNTs. 2 In various alcohol solvents, these functionalized SWNTs were solvated as individual tubes, making it possible to carry out further solution chemistry. Pekker and co-workers reported the hydrogenation of carbon nanotubes via the Birch reduction in ammonia. 12 Other examples for the direct attachment of functional groups to the nanotube graphitic surface include the reports of Tour and co-workers on the derivatization of small-diameter (ca. 0.7 nm) SWNTs, 11 and the report of Wilson and co-workers on the functionalization and solubilization of SWNTs in reactions with anilines. 10 The other category for the nanotube functionalization includes the reactions with the nanotube-bound carboxylic acids. Haddon and co-workers first reported the use of these acid groups for attaching long alkyl chains to SWNTs via amide linkages 1 or via carboxylate-ammonium salt ionic interactions. 8 Sun and co-workers showed that the esterification of the carboxylic acids can also be used to functionalize and solubilize nanotubes of any lengths. 4-6 An advantage with the ester linkages is that they can be easily defunctionalized via acid- or base-catalyzed hydrolysis, allowing the recovery of carbon nanotubes from the soluble samples. 6 In addition to long alkyl chains, polymeric systems have also been employed for the solubilization or dispersion of carbon nanotubes. 4,13,14,17-19 In particular, polymer- bound amino or hydroxy moieties have been used in the amidation and esterification reactions to attach the polymers to carbon nanotubes. Recently, Sun and co-workers reported the functionalization of both SWNTs and MWNTs with lipophilic and hydrophilic dendron species, which are more effective than simple long-chain alkyl functionalities for the solubilization of the nanotubes in organic solvents and water. 6a The solubilized carbon nanotube samples have allowed the fractionation and purification of the nanotubes in solution via techniques such as chromatography. 9 On the other hand, the functionalization itself may be used to fractionate nanotube samples. The results from such fractionation are particularly valuable to an evaluation of several important issues, including the selectivity of the functionalization reaction with respect to carbon nanotubes vs carbon impurities in the purified sample and the selectivity toward nanotubes of different sizes. Here we report the fractionation of a purified MWNT sample via repeated functionalization reactions. The results from charac- terization and analysis of the soluble fractions and the solid residue using spectroscopic, microscopic, and other techniques are presented and discussed. Experimental Section Materials. Methyl 3,5-dihydroxybenzoate (98%) was pur- chased from Avocado Research Chemicals, 1-bromohexadecane (99%) and thionyl chloride (99.5+%) from Acros, and lithium aluminum hydride (LiAlH 4 ) from Alfa Aesar. Solvent grade THF was first dried and distilled over molecular sieves, and * Corresponding author. 914 J. Phys. Chem. B 2003, 107, 914-919 10.1021/jp020701d CCC: $25.00 © 2003 American Chemical Society Published on Web 01/07/2003