NANO LETTERS
Unbundled and Highly Functionalized Carbon Nanotubes from Aqueous Reactions
2003 Vol. 3, No. 9 1215-1218
Christopher A. Dyke and James M. Tour* Departments of Chemistry and Mechanical Engineering and Materials Science, and Center for Nanoscale Science and Technology, Rice UniVersity, MS 222, 6100 Main Street, Houston, Texas 77005 Received July 18, 2003
ABSTRACT In this communication, we show that aryldiazonium salts can react efficiently with individual SDS-coated SWNTs in water to form aryl functionalized SWNTs. Remarkably, the resulting SWNTs have up to 1 in 9 carbons along their backbones bearing an organic moiety and they remain unbundled throughout their entire lengths, even with these relatively small functional moieties.
Single-walled carbon nanotubes1 (SWNTs) exhibit a wide range of notable physical2 and electronic properties,3 and have enormous potential in materials applications. The functionalized versions are critical, however, if good dispersion in blends and composites is sought.4 In all reports, bundles of nanotubes are treated with reactive reagents. Accordingly, the outermost nanotubes in a bundle are probably functionalized more than the innermost tubes, and the nanotubes remain predominantly bundled after functionalization.4 It is this bundling (or roping) that contributes, in large part, to their poor solubility5 and poor dispersion in blends and composite materials.6 Exfoliation of the bundles must occur either previous to or during the reaction in order to deliver individually functionalized SWNTs as opposed to functionalized bundles or mixtures of tubes functionalized to grossly different degrees. It is therefore of fundamental interest whether functionalization can ever result in individual nanotubes overcoming the inherent thermodynamic drive toward bundling. Although the electronic properties of nanotubes are compromised via functionalization, their utility for rheological modification of materials blends remains profound.6 Interestingly, reactions of diazonium salts that we employed7 to functionalize SWNTs may be active in aqueous micellar solutions.8 Similarly, the surfactant, sodium dodecyl sulfate (SDS), has recently been used to produce stable suspensions9 of individual HiPco-produced10 SWNTs. In this communication, we show that the aryldiazonium salts can react efficiently with individual SDS-coated SWNTs in water to form aryl functionalized SWNTs. Once the diazonium salt * Corresponding author. E-mail:
[email protected] 10.1021/nl034537x CCC: $25.00 Published on Web 08/19/2003
© 2003 American Chemical Society
is added to the nanotube suspension, it probably migrates to the stern layer of the micelle. As the reagent comes in close proximity to the nanotube, an electron is injected from the nanotube into the diazo functionality, liberating N2 and forming a reactive aryl radical, which reacts with the sidewall of the carbon nanotube. Remarkably, the resulting SWNTs haVe up to 1 in 9 carbons along their backbones bearing an organic moiety and they remain unbundled throughout their entire lengths, eVen with these relatiVely small functional moieties. In a typical experiment (Scheme 1), SDS-coated carbon nanotubes (10 mL, 2.08 µM, 0.02 mmol)9a and diazonium salts (0.32 mmol, 16 equiv per mole of carbon) generated from various 4-substituted anilines7c were added to a flask and stirred at room temperature for 10 min. After that time, Scheme 1.
Functionalization of SWNTs Coated with SDS
absorption spectroscopy was performed on an aliquot to ensure complete reaction, and then the reaction was diluted with acetone (100 mL) and filtered through a PTFE (0.2 µm) membrane. The filtrate was washed with water (100 mL) then acetone (100 mL) (3×) to efficiently remove the SDS
Figure 1. Absorption spectra of (A) SDS-coated SWNTs, (B) aliquot of the reaction mixture (see Scheme 1, product 1) after 10 min, and (C) product 1 after isolation, removal of the SDS, and dispersion in DMF.
surfactant9a and excess unreacted diazonium salt. The solids (functionalized nanotubes 1-6) were then collected and dried overnight in a vacuum oven at 65 °C and characterized. Figure 1A is the absorption spectra of micelle-coated unreacted SWNTs showing the distinctive van Hove singularities that are characteristic of the SDS-coated individual SWNTs.9a If the SDS-coating had been lost, the van Hove singularities would remain, however, they would not be as well defined.4a,9a Figure 1B is the spectrum of an aliquot (functionalized nanotube 1) of the reaction mixture after 10 min showing that all of the van Hove transitions disappeared. The collected, SDS-free, dried product, 1, was dispersed in DMF, and the spectrum, Figure 1C, confirmed the loss of the van Hove singularities. The loss of transitions in the absorption spectra is indicative of covalent functionalization,4 and in all cases (products 1-6), this complete loss was observed. The Raman spectrum of the unreacted SDS-coated SWNTs (Figure 2A) was obtained using the literature protocol9a by filtering 10 mL of the SDS-coated SWNTs through a PTFE (0.2 µm) membrane, thereby confirming the recovery of pristine SWNTs. The Raman spectrum of the SDS-free 1 (Figure 2B), isolated as described above, had the disorder mode even higher in intensity than the tangential breathing mode, and the radial breathing modes are nearly unobservable. This is clearly the most significant level of functionalization that we are aware of based upon Raman analysis. Upon heat treating 1 in a thermogravimetric analyzer (TGA) (vide infra), the aryl groups are removed and we see the restoration of the pristine SWNT signals (Figure 2C).4 The syntheses of 1, 2, and 3 were scaled-up to obtain accurate TGA data. Instead of using 10 mL of the micellecoated SWNTs, 100 mL was reacted. The SDS-free, dried product 1 weighed 5 mg, product 2 weighed 3 mg, and product 3 weighed 5 mg. TGA of 1 (10 °C/min to 750 °C, Ar) showed 49 wt % loss (Figure 3), 2 had 40 wt % loss, and 3 had 45 wt % loss, corresponding to 1 in 9, 1 in 16, 1216
Figure 2. Raman spectra (780.6 nm excitation) of (A) filtered SWNT/SDS, (B) aryl chloride functionalized nanotubes 1, and (C) functionalized nanotube 1 after TGA showing the recovery of the pristine SWNTs.
Figure 3. TGA of aryl chloride functionalized nanotubes 1.
and 1 in 13 SWNT carbons being aryl substituted, respectively. This is strikingly the highest degree of substitution that we have obtained to date using any protocol. Atomic force microscopy (AFM) analysis (Figure 4) of the starting nanotubes and the products revealed that the functionalized nanotubes are either easily exfoliated in organic solvents or incapable of bundling. Once the surfactant was removed from the unreacted, micelle-coated SWNTs (vide supra), they were resuspended in DMF via sonication (1 min) and spin-coated onto freshly cleaved mica. AFM analysis showed that the smooth-sided, highly polarizable unfunctionalized nanotubes rebundled as expected.9a However, analysis of the functionalized SWNTs (treated precisely as the unreacted nanotube sample) showed little propensity to rebundle, throughout their entire lengths, as seen is Figure 4A. Interestingly, products 1, 2, and 6 had increased overall Nano Lett., Vol. 3, No. 9, 2003
Figure 4. AFM analysis of washed and SDS-free unfunctionalized, pristine SWNTs and functionalized product, 2, both obtained by spincoating a DMF solution onto mica: (A) height image and (B) amplitude image of the unfunctionalized SWNTs, and (C) height image and (D) amplitude image of 2. Section analysis of 2 showed that the diameter of the nanotubes are 1.5 nm. Grid A and grid B are each 2×2 µm.
diameters of 1.2, 1.5 (Figure 4B), and 2.1 nm, respectively, which can be attributed to the increased lengths of the organic moieties appended to the sidewall of the SWNTs. TEM analysis of product 1 also confirmed the predominance of individual SWNTs. Figure 5 is a comparison of the originally SDS-treated unfunctionalized SWNTs (that were then washed to make them SDS-free) that rebundle (Figure 5A) versus functionalized SWNTs 3 that remain predominantly unbundled (Figure 5B) even in this SDS-free state. TEM analysis of 3 noticeably exhibits “roughened” Nano Lett., Vol. 3, No. 9, 2003
sidewalls that would be expected by heavy degrees of functionalization. As anticipated, the functionalized SWNTs were more soluble than aryl functionalized versions that we prepared by other methods.7 There is wide discrepancy between solubility numbers in the literature due to explicable variations in filtration methods. Our protocol for solubility determination (which includes two filtrations using 0.2 µm PTFE filters) has been published5 and the values are internally consistent with our former reports; however, they 1217
of 0.7 mg/mL, DMF (0.8 mg/mL), chloroform (0.6 mg/mL), and THF (0.6 mg/mL) compared to 0.03 and 0.05 mg/mL solubility in THF for the solvent-free7a and electrochemical7c generated materials, respectively.11 The saturated solutions of 3 remained for months without precipitation or flocculation. The mechanism for the arylation has been discussed previously. Since we see no biphenyl generated in the process, it is likely that the diazonium salt receives an electron from the nanotube, followed by ejection of N2 to form the aryl radical in close proximity to the tube, followed by nanotube-aryl radical coupling.5 Acknowledgment. Financial support from the following is gratefully acknowledged: NASA, JSC-NCC-9-77 and URETI NCC-01-0203; NSF, NSR-DMR-0073046; and AFOSR, F49620-01-1-0364. We thank W. Guo for aid with the TEM. References
Figure 5. TEM of (A) washed, filtered SDS-free SWNTs, and (B) SDS-free product 3.
are understandably difficult to compare to the values obtained by others.4b,11 Product 3 had a solubility in o-dichlorobenzene
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(1) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603. (b) Dresselhaus, M. S.; Dresselhaus, G.; Avouris, Ph. Carbon Nanotubes: Synthesis, Structure, Properties, and Applications; Springer: Berlin, 2001. (2) Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Physical Properties of Carbon Nanotubes; Imperial College Press: London, 1998. (3) Dai, H. Acc. Chem. Res. 2002, 35, 1035. (4) Bahr, J. L.; Tour, J. M. J. Mater. Chem. 2002, 12, 1952. (b) Banerjee, S.; Kahn, M. G. C.; Wong, S. S. Chem. Eur. J. 2003, 9, 1898. (5) Bahr, J. L.; Mickelson, E. T.; Bronikowski, M. J.; Smalley, R. E.; Tour, J. M. Chem. Commun. 2001, 193. (6) Mitchell, C. A.; Bahr, J. L.; Arepalli, S.; Tour, J. M.; Krishnamoorti, R. Macromolecules 2002, 35, 8825. (7) Dyke, C. A.; Tour, J. M. J. Am. Chem. Soc. 2003, 125, 1156. (b) Bahr, J. L.; Tour, J. M. Chem. Mater. 2001, 13, 3823. (c) Bahr, J. L.; Yang, J.; Kosynkin, D. V.; Bronikowski, M. J.; Smalley, R. E.; Tour, J. M. J. Am. Chem. Soc. 2001, 123, 6536. (8) Bravo-Diaz, C.; Soengas-Fernandez, M.; Rodriquez-Sarabia, M. J.; Gonzalez-Romero, E. Langmuir 1998, 14, 5098. (b) Moss, R. A.; Dix, F. M.; Romsted, L. J. Am. Chem. Soc. 1982, 104, 5048. (9) O’Connell, M. J.; Bachilo, S. M.; Huffman, C. B.; Moore, V. C.; Strano, M. S.; Haroz, E. H.; Rialon, K. L.; Boul, P. J.; Noon, W. H.; Kittrell, C.; Ma, J.; Hauge, R. H.; Weisman, R. B.; Smalley, R. E. Science 2002, 297, 593. (b) Islam, M. F.; Rojas, E.; Bergey, D. M.; Johnson, A. T.; Yodh, A. G. Nano Lett. 2003, 3, 269. (c) Shim, M.; Shi Kam, N. W.; Chen, R. J.; Li, Y.; Dai, H. Nano Lett. 2002, 2, 285. (10) 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. (11) For far higher reported solubility, see: Georgakilas, V.; Kordatos, K.; Prato, M.; Guldi, D. M.; Holzinger, M.; Hirsch, A. J. Am. Chem. Soc. 2002, 124, 760.
NL034537X
Nano Lett., Vol. 3, No. 9, 2003
