Dissolution and alkylation of industrially produced multi-walled carbon nanotubes Damien Voiry a,b , Cristina Valle ´s a,b,1 , Olivier Roubeau c , Alain Pe ´nicaud a,b, * a CNRS, UPR 8641, Centre de Recherche Paul Pascal, 115 avenue du Dr A. Schweitzer, 33600 Pessac, France b Universite ´ de Bordeaux, UPR 8641 Pessac, F-33600, France c Instituto de Ciencia de Materiales de Arago ´n (ICMA), CSIC and Universidad de Zaragoza, Plaza San Francisco s/n, 50009 Zaragoza, Spain ARTICLE INFO Article history: Received 9 July 2010 Accepted 31 August 2010 Available online 7 September 2010 ABSTRACT Industrial, CVD multi-walled carbon nanotubes are shown to form true solutions by reduc- tion with alkali metals and subsequent exposure to polar solvents such as DMSO. Addition of an electrophile such as bromo-decanoic acid allows functionalization of the tubes. The whole process, dissolution and functionalization includes no harsh treatment such as acid treatment and/or sonication and allows to saturate the outer surface of the MWCNTs with functional groups. Comparison is made with previous work on single-walled carbon nanotubes. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Since the beginning of the carbon nanotubes era, [1] intense research has been carried on worldwide to produce carbon nanotubes in large scale [2]. Although single-walled carbon nanotubes (SWCNTs) remain the material of choice for academic laboratories, multi-walled carbon nanotubes (MWCNTs) are undoubtedly the next large scale commercially available carbonaceous material. Several worldwide chemical companies produce MWCNTs and make possible various applications for such exciting materials. Indeed, MWCNTs are much cheaper than their single-walled analogs with a price announced to be competitive with that of high-grade carbon black, such as Vulcan XC 72 (ca 10 $/kg). Large volumes applications such as smart composites, antistatic paintings and so on are thus realistically envisioned for MWCNTs. Addi- tionally, intense toxicology research is now beginning to as- sess under what conditions and in which state can carbon nanotubes be considered risk-free for industrial usage [3,4]. Furthermore, contrary to SWCNTs, MWCNTs are electroni- cally homogeneous, being all metallic conductors. Covalent functionalization of carbon nanotubes, either single or mul- ti-walled, is used extensively to solubilize or disperse and therefore manipulate them [5–7]. With respect to SWCNTs, the studies regarding covalent functionalization of MWCNTs have though been somewhat less numerous and varied in terms of the reaction used. Among the different reactions known, reductive alkylation has been reported to be an effi- cient way to functionalize SWCNTs [7–11]. Recently, stoichi- ometric control of single-walled carbon nanotubes functionalization has been demonstrated [12]. This was achieved by addition of an electrophile R-X to a solution [13] of carbon nanotubes alkali metal salts of controlled stoichi- ometry in dimethylsulfoxide (DMSO). Herein we demonstrate that this process can be applied to multi-walled nanotubes. Indeed, in a manner similar to their single-walled counter- parts, [13] multi-walled carbon nanotubes are rendered truly soluble in polar aprotic organic solvents by reduction with polyaromatic anion radical salts of alkali metals. Subsequent addition of electrophiles to these MWCNT salts of varied charges leads to functionalized MWCNTs in a straightforward manner. In view of the advantages of this method, – (i) no 0008-6223/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.08.057 * Corresponding author at: CNRS, UPR 8641, Centre de Recherche Paul Pascal, 115 avenue du Dr A. Schweitzer, 33600 Pessac, France. Fax: +33 556845600. E-mail address: penicaud@crpp-bordeaux.cnrs.fr (A. Pe ´ nicaud). 1 Dept. of Nanotechnology, Instituto de Carboquimica, CSIC, C/Miguel Luesma Castan, 4, E-50018 Zaragoza, Spain. CARBON 49 (2011) 170 – 175 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon