NANO LETTERS
Sonication-Assisted Functionalization and Solubilization of Carbon Nanotubes
2002 Vol. 2, No. 3 231-234
Weijie Huang,† Yi Lin,† Shelby Taylor,† Jay Gaillard,‡ Apparao M. Rao,‡ and Ya-Ping Sun*,† Department of Chemistry and Center for AdVanced Engineering Fibers and Films, Howard L. Hunter Chemistry Laboratory, and Department of Physics and Astronomy, Kinard Laboratory, Clemson UniVersity, Clemson, South Carolina 29634 Received October 31, 2001
ABSTRACT Carbon nanotubes were solubilized via functionalization with poly(propionylethylenimine-co-ethylenimine). The diimide-activated amidation reaction for the functionalization was found to be significantly improved in both efficiency and yield by sonication under ambient conditions. It was also found that depending on the duration of sonication the nanotubes were shortened to different lengths in the functionalization reactions. The method allows relatively efficient preparation of solubilized carbon nanotubes of potentially a selectable average length.
The functionalization and solubilization of carbon nanotubes have received much recent attention.1-11 Most of the functionalization reactions reported in the literature are based on the amidation and esterification of nanotube-bound carboxylic acid groups,1,3-5,8,10a which are associated with defects on as-prepared nanotubes and also terminal carbons on shortened nanotubes. In experiments with the functionalization and solubilization of carbon nanotubes via diimideactivated amidation reactions with aminopolymers, we found that sonication during the reactions could significantly increase the reaction efficiency and yield.12 In addition, depending on the duration of sonication, the nanotubes were shortened to different lengths in the functionalization reactions.
Multiple-wall carbon nanotubes (MWNTs) were produced by the chemical vapor deposition method.14,15 Details on the preparation and purification of the nanotube samples are provided in the Supporting Information. Poly(propionylethylenimine-co-ethylenimine) (PPEI-EI) was prepared via a partial hydrolysis of the commercially available poly(propionylethylenimine) (MW ∼ 200 000) under acidic condi† ‡
Howard L. Hunter Chemistry Laboratory. Kinard Laboratory.
10.1021/nl010083x CCC: $22.00 Published on Web 02/02/2002
© 2002 American Chemical Society
tions (see also Supporting Information).16 PPEI-EI copolymer samples of an EI mole fraction around 15% were obtained and used in the functionalization reactions. The amidation of the nanotube-bound carboxylic acids can be accomplished in diimide-activated reactions.4,8 However, it was found that the functionalization reactions can be improved significantly in both efficiency and yield under the condition of continuous sonication. In a typical experiment, a purified MWNT sample (50 mg) was suspended in aqueous KH2PO4 buffer (pH ) 3-4, 15 mL). To the suspension was added PPEI-EI (500 mg) and 1-ethyl-3-(3dimethylaminopropyl)carbodiimide (EDAC, 155 mg). After the reaction at room temperature for 2 or 6 h with continuous sonication (Cole Palmer B3-R, 55 kHz), the reaction mixture was placed in a membrane tubing (cutoff molecular weight 12 000) for dialysis against deionized water for 3 days, followed by high-speed centrifuging (7800 rpm) to separate the residual insoluble nanotubes. According to the weight of the recovered nanotubes, about 10% of the MWNTs remained in solution. Since the residual sample contains some nanotubes that are functionalized but still insoluble,17 such an estimate likely represents the lower limit of the functionalization reaction yield. The dark solution of the PPEI-EIfunctionalized MWNTs was evaporated to remove solvent on a rotary evaporator, yielding glassy black solids. The solid sample is readily soluble in organic solvents such as chloroform and in water, allowing characterizations using solution-based techniques. UV/vis absorption spectra of the PPEI-EI-functionalized MWNTs in room-temperature chloroform are featureless curves, similar to those of other functionalized carbon nanotubes.4,5 The observed absorbances are proportional to
Figure 1. Comparison of the 1H NMR spectra of the MWNTPPEI-EI samples obtained from 2 and 24 h sonication reactions with that of the starting PPEI-EI.
solution concentrations, obeying the Lambert-Beer’s law. The 1H NMR spectra of the functionalized nanotube samples are compared with that of the parent PPEI-EI in Figure 1, where the disappearance of the ethylene proton signals (2.9 ppm) for the EI moieties in the copolymer may be attributed to the functionalization. Upon the formation of amide linkages in the functionalization, the environment of these protons might become considerably more inhomogeneous (significant signal broadening) and their chemical shifts might be closer to those of the ethylene protons that are associated with the PPEI amide moieties (signal shifts). More direct evidences for the soluble sample to contain carbon nanotubes include the results from transmission electron microcopy (TEM) and Raman analyses. The sample for TEM measurements was prepared by evaporating a drop of the nanotube solution on a carbon-coated copper grid. Shown in Figure 2 is a typical TEM image for the PPEIEI-functionalized MWNT samples described above. These solubilized MWNTs are apparently of different lengths and diameters and are dispersed at the individual nanotube level. As compared in Figure 3, the Raman spectra of the functionalized MWNTs are similar to that of the pristine nanotubes, showing the characteristic tangential-mode and defect peaks.18 The yield of the functionalization reaction is dependent on reaction time. When the EDAC-activated reaction with sonication was for 24 h instead of 2 or 6 h, for example, the amount of solubilized nanotubes increased significantly to ∼27% of the starting nanotube sample, estimated from the weight of the residual insoluble nanotubes. Again this likely represents the lower limit of the functionalization reaction yield because the recovered sample contains nanotubes that are functionalized but still insoluble, as discussed above. The sonication apparently promotes the EDAC-activated amidation reaction of the nanotube-bound carboxylic acids with the polymer-bound amino groups. However, it may also be able to facilitate other minor solubilization pathways. For 232
Figure 2. TEM image of the soluble PPEI-EI-functionalized MWNTs obtained from a 6 h sonication reaction. The scale bar represents 300 nm.
Figure 3. Raman spectra of the pristine MWNT sample (- - -) and the functionalized MWNT samples obtained with 2 h (s) and 24 h (- ‚ - ‚ -) sonication reactions (after correction for the luminescence background).
example, when the same reaction mixture without EDAC was sonicated under the same experimental conditions for 24 h, the amount of nanotubes in solution was ∼3% of the starting nanotube sample. However, this is considerably less than the amount (∼27%) achieved in the presence of EDAC. Since the EDAC-activated reaction is highly specific,19 these results may be considered as important evidence for the amidation of the nanotube-bound carboxylic acids being the primary mode of functionalization by PPEI-EI.4 The enhancement in the reaction yield may be attributed to the sonication-induced activation of the carboxylic acids by EDAC, as rationalized for the sonication enhancements in other diimide-activated reactions.20 The lengthening of reaction time with continuous sonication has other effects in addition to improving the reaction yield. The sonication under EDAC-activated amidation reaction conditions apparently results in not only the funcNano Lett., Vol. 2, No. 3, 2002
Figure 5. Dependence of the average nanotube length on the sonication reaction time for soluble PPEI-EI-functionalized MWNTs.
Figure 4. TEM images of the MWNT samples upon continuous sonication for 24 h with (upper) and without (bottom) PPEI-EI and EDAC. Both scale bars represent 300 nm.
tionalization of the nanotubes by PPEI-EI but also the shortening of the nanotubes. The latter is dependent on the duration of the sonication, shortening more significantly with the lengthening of the sonication-assisted reaction time. Shown in Figure 4 is a TEM image of the PPEI-EIfunctionalized MWNT sample obtained with 24 h sonication reaction. These functionalized nanotubes are considerably shorter than those obtained with 6 h sonication reaction under otherwise the same experimental conditions (Figure 2). In a control experiment for comparison, a suspension of MWNTs only (without PPEI-EI and EDAC) was sonicated for 24 h. The TEM results of the sample upon the sonication show no significant shortening of the nanotubes (Figure 4). The reactant and catalyst of the reaction (PPEI and EDAC) obviously play a significant role in the shortening of the nanotubes. This is, to the best of our knowledge, the first report on simultaneous functionalization and shortening of carbon nanotubes. Of particular importance is the fact that here MWNTs are shortened via sonication in neutral to slightly acidic solution under ambient reaction conditions, in contrast to the typically strongly acidic and oxidative conditions reported in the literature for the sonochemical shortening of carbon nanotubes.21 In addition, because a longer sonication reaction time corresponds to a higher Nano Lett., Vol. 2, No. 3, 2002
reaction yield, there is a possibility for a direct relationship between the shortening of the nanotubes and the improvement in the observed functionalization reaction yield. The simultaneous functionalization and shortening of MWNTs are dependent on the sonication reaction time in a monotonic fashion. Shown in Figure 5 are results from a more systematic investigation concerning the effect of sonication reaction time on the average length of the solubilized nanotubes. Despite the uncertainty associated with the determination of nanotube sizes from TEM images, the results show a clear trend of decreasing average nanotube length with increasing sonication reaction time. In summary, the diimide-activated amidation reaction for attaching polymers to carbon nanotubes can be improved in both efficiency and yield via sonication under ambient conditions. The sonication-assisted reaction offers a relatively convenient method for the functionalization and solubilization of carbon nanotubes. A higher reaction yield can be obtained with a longer sonication-reaction time, though the shortening of nanotubes due to the prolonged sonication needs to be taken into consideration in the functionalization for a specific application. Nevertheless, it is an interesting and important finding that the nanotubes can in fact be simultaneously functionalized and shortened via sonication under ambient reaction conditions. The method may potentially be used to prepare solubilized carbon nanotubes of a selectable average length. The application of the same method to the functionalization and solubilization of single-wall carbon nanotubes (SWNTs) is in progress. Preliminary results for SWNTs show many similarities to those of MWNTs presented and discussed above. Acknowledgment. We thank B. Martin and D. Hill for experimental assistance. Y-P.S. acknowledges NSF (CHE9727506 and, in part, EPS-9977797), NASA (NCC1-01036, NGT1-52238, and NAT1-01036), the South Carolina 233
Space Grant Consortium, and the Center for Advanced Engineering Fibers and Films (NSF-ERC at Clemson University) for financial support. A.M.R. acknowledges financial support from the NASA Ames Research Center (NCC25421). The support of NSF (CHE-9700278) for the acquisition of the 500-MHz NMR instrument is also acknowledged. Supporting Information Available: Details on the preparation and purification of the nanotube samples. This material is available free of charge via the Internet at http:// pubs.acs.org. References (1) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Science 1998, 282, 95. (2) (a) 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. (b) 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. (3) (a) Hamon, M. A.; Chen, J.; Hu, H.; Chen, Y.; Itkis, M. E.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. AdV. Mater. 1999, 11, 834. (b) Niyogi, S.; Hu, H.; Hamon, M. A.; Bhowmik, P.; Zhao, B.; Rozenzhak, S. M.; Chen, J.; Itkis, M. E.; Meier, M. S.; Haddon, R. C. J. Am. Chem. Soc. 2001, 123, 733. (c) Chen, J.; Rao, A. M.; Lyuksyutov, S.; Itkis, M. E.; Hamon, M. A.; Hu, H.; Cohn, R. W.; Eklund, P. C.; Colbert, D. T.; Smalley, R. E.; Haddon, R. C. J. Phys. Chem. B 2001, 105, 2525. (4) (a) Riggs, J. E.; Guo, Z.; Carroll, D. L.; Sun, Y.-P. J. Am. Chem. Soc. 2000, 122, 5879. (b) Riggs, J. E.; Walker, D. B.; Carroll, D. L.; Sun, Y.-P. J. Phys. Chem. B 2000, 104, 7071. (c) Czerw, R.; Guo, Z.; Ajayan, P. M.; Sun, Y.-P.; Carroll, D. L. Nano Lett. 2001, 1, 423. (5) (a) Sun, Y.-P.; Huang, W.; Lin, Y.; Fu, K.; Kitaygorodskiy, A.; Riddle, L. A.; Yu, Y. J.; Carroll, D. L. Chem. Mater. 2001, 13, 2864. (b) Fu, K.; Huang, W.; Lin, Y.; Riddle, L. A.; Carroll, D. L.; Sun, Y.-P. Nano Lett. 2001, 1, 439. (6) Sun, Y.; Wilson, S. R.; Schuster, D. I. J. Am. Chem. Soc. 2001, 123, 5348. (7) Bahr, J. L.; Yang, J. P.; Kosynkin, D. V.; Bronikowski, M. J.; Smalley, R. E.; Tour, J. M. J. Am. Chem. Soc. 2001, 123, 6536. (8) (a) Wong, S. S.; Joselevich, E.; Woolley, A. T.; Cheung, C. L.; Lieber, C. M. Nature 1998, 394, 52. (b) Wong, S. S.; Woolley, A. T.; Joselevich, E.; Cheung, C. L.; Lieber, C. M. J. Am. Chem. Soc. 1998, 120, 8557.
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Nano Lett., Vol. 2, No. 3, 2002
