http://journals.cambridge.org Downloaded: 04 Feb 2013 IP address: 212.14.22.114 Simultaneous purification and functionalization of carbon nanotubes using chlorination Iwona Pelech, a) Urszula Narkiewicz, and Dariusz Moszy nski Faculty of Chemical Engineering, Institute of Chemical and Environmental Engineering, West Pomeranian University of Technology in Szczecin, 70-310 Szczecin, Poland Robert Pelech Faculty of Chemical Engineering, Institute of Organic Chemical Technology, West Pomeranian University of Technology in Szczecin, 70-310 Szczecin, Poland (Received 6 March 2012; accepted 25 June 2012) Multiwalled carbon nanotubes (MWCNTs) obtained using ethylene as a carbon source and nanocrystalline iron as a catalyst were used as the initial material. The functionalization of MWCNTs was carried out using chlorine in the liquid and gas phase. In the second case, the reaction was conducted in the temperature range from 50 to 450 °C for 2 h. The presence of chlorine species on the surface of chlorinated samples was confirmed by x-ray photoelectron spectroscopy (XPS). A quantitative analysis of metal impurity content was validated by means of thermogravimetric analysis. Better results of metal removal were achieved when the chlorination process was conducted in the gas phase and the ratio of metal in samples amounted from 2.3% to 5.1%. I. INTRODUCTION Carbon nanotubes (CNTs) have potential applications in many fields, e.g., as electron emitters in the field of emission display, scanning probe microscopy tips, sensors, etc. 1–4 CNTs can also be applied as fillers for polymer composites, because an addition of CNTs to polymer matrix improves mechanical properties of composite. As pristine nanotubes are insoluble in many liquids such as water and most solvents, it is difficult to disperse them in a polymer matrix. To make nanotubes more easily dis- persible in liquids, functionalization processes are carried out. Functionalization results in the attachment of mole- cules or functional groups to CNTs’ sidewalls. For the oxidative modification of CNTs, a gas- or liquid-phase oxidation method can be applied. Various acid agents, such as nitric acid (HNO 3 ), HNO 3 and sulfuric acid (H 2 SO 4 ), perchloric acid (HClO 4 ), H 2 SO 4 and potassium dichromate (K 2 CrO 7 ), H 2 SO 4 and potas- sium permanganate (KMnO 4 ) are used as oxidants in the liquid phase. Oxidation reactions generate functional groups, such as: ÀCOOH, ÀOH, ÀC5O, and eliminate metal particles. 5–8 Another method of CNTs’ functionalization is based on a modification of CNTs’ surfaces using halogens: fluorine or chlorine. Barthos et al. 9 proposed the functionalization of single-walled carbon nanotubes (SWCNTs) in agate ball mill using alkyl halides, such as trifluoromethane, trichloro- methane, tetrachloroethylene, and hexafluoropropene. For the sake of comparison, chlorination was also performed using chlorine gas. It was concluded that this method gives functionalized SWCNTs in the range of 0.3–3.5% wt% of fluorine and 5.5–17.5 wt% of chlorine and it was proved that liquid materials containing halogen are more effective as functionalizing agents than gases. Direct fluorination of CNTs was carried out using hydrofluoric acid 10 or fluorine gas. 11–13 However, this process can lead to products that hardly allow any further reactions. 11 Chlorine addition to MWCNTs was performed by Kónya et al. 14 The authors reported that ball milling induces functionalization of MWCNTs in reactive atmospheres giving rise to formation of short CNTs containing different chemical functional groups, including chlorine. A method to purify MWCNTs involving chlorine water and ammonia water treatment was described in. 15 The authors indicated that catalyst particles, amorphous carbon and carbon particles were almost com- pletely eliminated and some carboxyl group and C–Cl bonds were introduced. Functionalized CNTs with dichlor- ocarbene (CCl 2 ) 16 and thionyl chloride 17,18 can be con- verted with other chemical agents to add the corresponding functional group. Tang et al. 19 used CNTs together with nickel oxide (Ni 2 O 3 ) to reduce flammability of propylene. In this article, we present a functionalization of CNTs’ surface using chlorination method, which enables a simul- taneous purification of MWCNTs from metal impurities. Details regarding the influence of experimental conditions (liquid or gas phase) and in the case of the gas phase—the effect of temperature, on the removal degree of metal par- ticles and the amount of chlorine is presented. A quanti- tative analysis of metal impurity content was validated by means of thermogravimetric analysis (TGA). The pre- sence of chlorine species on the surface of chlorinated a) Address all correspondence to this author. e-mail: ipelech@zut.edu.pl DOI: 10.1557/jmr.2012.243 J. Mater. Res., Vol. 27, No. 18, Sep 28, 2012 Ó Materials Research Society 2012 2368