NANO LETTERS
A Convenient Route to Functionalized Carbon Nanotubes
2004 Vol. 4, No. 7 1257-1260
Feng Liang, Anil K. Sadana, Asghar Peera, Jayanta Chattopadhyay, Zhenning Gu, Robert H. Hauge, and W. E. Billups* Department of Chemistry and Center for Nanoscale Science and Technology, Rice UniVersity, 6100 Main Street, Houston, Texas 77005 Received April 16, 2004; Revised Manuscript Received May 14, 2004
ABSTRACT Reductive alkylation of single-walled carbon nanotubes (SWNTs) using lithium and alkyl halides in liquid ammonia yields sidewall functionalized nanotubes that are soluble in common organic solvents. Atomic force microscopy (AFM) and high-resolution tunneling electron microscopy (HRTEM) of dodecylated SWNTs prepared from raw HiPco nanotubes show that extensive debundling has occurred. GC-MS analysis of the byproduct hydrocarbons demonstrates that alkyl radicals are intermediates in the alkylation step.
The extraordinary electronic1 and mechanical properties2,3 exhibited by single-walled carbon nanotubes suggest that important applications await these unique materials.4 However, realistic applications have been hindered by difficulties associated with processing. SWNTs with a high degree of sidewall functionalization can be used to overcome some of these difficulties since the functionalized nanotubes are usually soluble in common organic solvents. The first report of extensive sidewall functionalization by fluorination was reported by Margrave and co-workers in 1998.5 The fluorinated SWNTs could be de-fluorinated by treatment with hydrazine or allowed to react with alkylmagnesium bromides or alkyllithium reagents to yield alkylated SWNTs.6,7 Sidewall functionalization via aryl diazonium salts,8 azomethine ylides,9 carbenes,10 nitrenes,10c,11 organic radicals,12 and the Bingel reaction13 have also been reported. The addition of hydrogen to the sidewalls of SWNTs has been reported to occur under the conditions of the Birch reduction.14 Despite these impressive results, a more accommodating, efficient, and scaleable approach is still desirable. We report here a convenient route to functionalized carbon nanotubes (without sonication) via reductive alkylation15 using lithium and alkyl halides in liquid ammonia. This method is scalable and provides a route to SWNTs functionalized by groups that are suitable for further elaboration. The SWNTs used in this investigation were produced by the HiPco process.16 Studies were carried out using both crude and purified17 material (< ∼1 wt % iron). The functionalization reactions were carried out as follows: 20 mg (1.6 mmol of carbon) of carbon nanotubes were added to a flame-dried 100-mL three-neck round-bottom flask. NH3 (60 mL) was then condensed into the flask * Corresponding author. E-mail:
[email protected]. 10.1021/nl049428c CCC: $27.50 Published on Web 06/03/2004
© 2004 American Chemical Society
followed by the addition of small pieces of lithium metal (231 mg, 33 mmol). The alkyl iodide (6.4 mmol) was then added and the reaction mixture stirred overnight with the slow evaporation of NH3. The flask was then cooled in an ice bath as methanol (10 mL) was added slowly followed by water (20 mL). After acidification (10% HCl), the nanotubes were extracted into hexanes and washed several times with water. The hexane layer was then filtered through a 0.2 µm PTFE membrane filter, washed with ethanol, and dried in a vacuum oven (80 °C) overnight. The alkylated nanotubes exhibit high solubility in chloroform, THF, and DMF. Atomic force microscopy (AFM) images recorded in chloroform for dodecylated SWNTs prepared from purified (left) and raw SWNTs (right) are presented in Figure 1. The average diameter of dodecylated nanotubes obtained from purified SWNTs is around 2 nm as determined by their height, demonstrating that extensive debundling has occurred. The diameter of the dodecylated nanotubes obtained from raw HiPco SWNTs averages only 1.5 nm, which is indicative of complete debundling. The “dots” that are visible in the AFM image from the raw SWNTS are 4-5 nm in height, suggesting that they arise from the carbon encapsulated metal catalyst while the dots that have heights around 1.5 nm may belong to large fullerenes or very short tubes.18 The debundling that is observed when raw SWNTs are used suggests that the purification process enhances the bundling. Additional evidence for extensive debundling is provided by inspection of the HRTEM images presented in Figure 2. The functionalized tubes exhibit a morphology that can be attributed to functionalization by the dodecyl groups. The debundling can be explained in terms of extensive intercalation by the lithium, leading to lithium ions dispersed
Figure 1. Tapping mode AFM images of dodecylated purified SWNTs (left) and raw SWNTs (right) spin-coated onto mica from chloroform.
Figure 2. (A,B) HRTEM images of dodecylated SWNTs.
between the negatively charged tubes. The intense blue color associated with solvated electrons disappears rapidly as the lithium is added to the suspension of nanotubes in liquid ammonia, suggesting that electron transfer to the SWNTs is a facile process. Preliminary experiments using Raman spectroscopy suggest that the carbon/lithium ratio is 2:3. Addition of the alkyl halide would lead to the formation of a radical anion that would dissociate readily to yield halide and the alkyl radical. It is well established that radicals add readily to nanotubes.12 A GC-MS analysis of the filtrate provides confirmation of the radical pathway. Thus n-C12H26, C12H24, and n-C24H50 are formed as major byproducts when n-dodecyl iodide is used as the alkylating reagent (Figure S1, Supporting Information). n-C12H26 and C12H24 would 1258
Scheme 1
arise from disproportionation of the dodecyl radical, whereas n-C24H50 is formed by dimerization of dodecyl radicals. Functionalized SWNTs were characterized by Raman specNano Lett., Vol. 4, No. 7, 2004
Figure 3. Raman spectra (780 nm excitation) of (A) purified SWNTs, (B) dodecylated product 2a, and (C) product 2a after TGA analysis (30 min hold at 80 °C, ramp 10 °C min-1 to 800 °C, 20 min hold at 800 °C) in argon showing the recovery of the pristine SWNTs.
Figure 5. TGA-MS of the products evolved from n-butylated SWNTs. Ion current vs time plots for m/z ions (A) 57 CH3CH2CH2CH2, (B) 56 CH3CH2CHdCH2, (C) 58 CH3CH2CH2CH3. Table 1. Weight loss and estimated carbon/alkyl group ratio from TGA at 800 °C in argon
Figure 4. FTIR spectrum of dodecylated SWNTs obtained from purified material.
troscopy, infrared spectroscopy, and thermogravimetric analysis. Direct evidence for covalent sidewall functionalization is found in the Raman spectra of dodecylated (2a) SWNTs (Figure 3). Pristine SWNTs exhibit two strong bands: a diameter-dependent radial breathing mode at ∼230 cm-1 and a tangential mode at ∼1590 cm-1. The weak band centered at 1290 cm-1, the disorder mode, is attributed to sp3-hybridized carbon in the hexagonal framework of the nanotube walls. Raman spectra of dodecylated SWNTs prepared from purified nanotubes exhibit a disorder peak with higher intensity than the tangential mode. This indicates a high degree of covalent functionalization. Figure 3C was recorded using SWNTs in which the dodecyl groups had been removed thermally under an atmosphere of argon. The FTIR spectrum of dodecylated SWNTs from purified material is shown in Figure 4. Strong peaks at 2919 and 2845 cm-1 can be assigned to aliphatic C-H stretching frequencies. The mode centered at 1529 cm-1 is assigned to the activated CdC double bonds in the dodecylated nanotube structure.19 Further evidence for covalent functionalization of the SWNTs has been provided by thermogravimetric analysis (TGA) of 2b in the 100-800 °C range connected with online monitoring of the volatile products by a mass spectrometer (MS) operating in electron impact ionization mode. The evolution of butyl groups at about 200 °C is shown by major Nano Lett., Vol. 4, No. 7, 2004
compound
weight loss (%) observed
Carbon/alkyl group ratio
2a 2b 2c 2d 2e 2f
41 22 16 16 25 15
20 17 25 26 35 28
peaks at m/z 57 {CH3CH2CH2CH2} and 56 {CH3CH2CHd CH2} as well as a smaller peak at m/z 58 {CH3CH2CH2CH3}. These results are presented in Figure 5. TGA also provides a measure of the degree of functionalization. Weight loss and carbon/alkyl group ratios are presented in Table 1. The reductive alkylation of other forms of carbon including fullerenes, MWNTs, carbon black, and diamond is under investigation currently. Acknowledgment. We gratefully acknowledge financial support from the Robert A. Welch Foundation, the National Science Foundation, and Carbon Nanotechnologies, Inc. Supporting Information Available: GC-MS of byproducts from the synthesis of 2a. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) (a) Trans, S. J.; Verschueren, A. R. M.; Dekker, C. Nature 1998, 393, 49. (b) Dai, H.; Wong, E. W.; Lieber, C. M. Science 1996, 272, 523. (2) (a) Traecy, M. M. J.; Ebbesen, T. W.; Gibson, J. M. Nature 1996, 381, 678. (b) Wong, E. W.; Sheehan, P. E.; Lieber, C. M. Science 1997, 277, 1971. (c) Poncharal, P.; Wang, Z. L.; Ugarte, D.; de Heer, W. A. Science 1999, 283, 1513. (3) (a) Yakobson, B. I.; Smalley, R. E. Am. Sci. 1997, 85, 324. (b) Calvert, P. Nature 1999, 399, 210. (c) Ajayan, P. M. Chem. ReV. 1999, 99, 1787. (4) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787. 1259
(5) Mickelson, E. T.; Huffman, C. B.; Rinzler, A. G.; Smalley, R. E.; Hauge, R. H.; Margrave, J. L. Chem. Phys. Lett. 1998, 296, 188. (6) Boul, P. J.; Liu, J.; Mickelson, E. T.; Huffman, C. B.; Ericson, L. M.; Chiang, I. W.; Smith, K. A.; Colbert, D. T.; Hauge, R. H.; Margrave, J. L.; Smalley, R. E. Chem. Phys. Lett. 1999, 310, 367. (7) Mickelson, E. T.; Chiang, I. W.; Zimmerman, J. L.; Boul, P. J.; Lozano, J.; Liu, J.; Smalley, R. E.; Hauge, R. H.; Margrave, J. L. J. Phys. Chem. B 1999, 103, 4318. (8) Bahr, J. L.; Yang, J.; Kosynkin, D. V.; Bronikowski, M. J.; Smalley, R. E.; Tour, J. M. J. Am. Chem. Soc. 2001, 123, 6536. (9) (a) Georgakilas, V.; Kordatos, K.; Prato, M.; Guldi, D. M.; Holzinger, M.; Hirsh, A. J. Am. Chem. Soc. 2002, 124, 760. (b) Georgakilas, V.; Tagmatarchis, N.; Pantarotto, D.; Bianco, A.; Briand, J.-P.; Prato, M. Chem. Commun. 2002, 3050. (c) Pantarotto, D.; Partidos, C. D.; Graff, R.; Hoebeke, J.; Briand, J.-P.; Prato, M.; Bianco, A. J. Am. Chem. Soc. 2003, 125, 6160. (10) (a) Chen, Y.; Haddon, R. C.; Fang, S.; Rao, A. M.; Eklund, P. C.; Lee, W. H.; Dickey, E. C.; Grulke, E. A.; Pendergrass, J. C.; Chavan, A.; Haley, B. E.; Smalley, R. E. J. Mater. Res. 1998, 13, 2423. (b) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Science 1998, 282, 95. (c) Holzinger, M.; Vostrowsky, O.; Hirsch, A.; Hennrich, F.; Kappes, M.; Weiss, R.; Jellen, F. Angew. Chem., Int. Ed. 2001, 40, 4002. (d) Hu, H.; Zhao, B.; Hamon, M. A.; Kamaras, K.; Itkis, M. E.; Haddon, R. C. J. Am. Chem. Soc. 2003, 125, 14893.
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(11) Holzinger, M.; Abraham, J.; Whelan, P.; Graupner, R.; Ley, L.; Hennrich, F.; Kappes, M.; Hirsch, A. J. Am. Chem. Soc. 2003, 125, 8566. (12) (a) Peng, H.; Reverdy, P.; Khabashesku, V. N.; Margrave, J. L. Chem. Commun. 2003, 362. (b) Ying, Y.; Saini, R. K.; Liang, F.; Sadana, A. K. Org. Lett. 2003, 5, 1471. (c) Peng, H.; Alemany, L. B.; Margrave, J. M.; Khabashesku, V. N. J. Am. Chem. Soc. 2003, 125, 15174. (13) Coleman, K. S.; Bailey, S. R.; Fogden, S.; Green, M. L. H. J. Am. Chem. Soc. 2003, 125, 8722. (14) Pekker, S.; Salvetat, J.-P.; Jakab, E.; Bonard, J.-M.; Forro, L. J. Phys. Chem. B 2001, 105, 7938. (15) Rabideau, P. W. Tetrahedron 1989, 45, 1579. (16) Nikolaev, P.; Bronikowski, M. J.; Bradley, R. K.; Rohmund, F.; Colbert, D. T.; Smith, K. A.; Smalley, R. E. Chem. Phys. Lett. 1999, 313, 91. (17) Xu, Y.; Peng, H.; Hauge, R. H.; Smalley, R. E., unpublished results. (18) Ramesh, S.; Brinson, B.; Johnson, M. P.; Gu, Z.; Saini, R. K.; Willis, P.; Marriott, T.; Billups, W. E.; Margrave, J. L.; Hauge, R. H.; Smalley, R. E. J. Phys. Chem. B 2003, 107, 1360. (19) Gu, Z.; Peng, H.; Hauge, R. H.; Smalley, R. E.; Margrave, J. L. Nano Lett. 2002, 2, 1009.
NL049428C
Nano Lett., Vol. 4, No. 7, 2004
