J. Phys. Chem. C 2008, 112, 3519-3524
3519
High Quality Dispersions of Functionalized Single Walled Nanotubes at High Concentration Johnny Amiran, Valeria Nicolosi, Shane D. Bergin, Umar Khan, Philip E. Lyons, and Jonathan N. Coleman* School of Physics and the Centre for Research on AdaptiVe Nanostructures and NanodeVices (CRANN), Trinity College Dublin, Dublin 2, Ireland ReceiVed: September 19, 2007; In Final Form: NoVember 27, 2007
Single walled nanotubes are difficult to disperse in solvents, with dispersion quality limited by nanotube bundling at high concentration. We quantitatively study dispersions of singlewall nanotubes, functionalized with the bulky molecules PABS, PEG, and ODA, in common solvents. TGA measurements coupled with AFM analysis of deposited nanotubes shows almost complete coverage of the functionalities along the nanotube body. The best solvents are characterized by Hildebrand solubility parameters that are close to those of the functional groups. At low concentration, the dispersions contain predominately individual functionalized SWNTs as evidenced by root-mean-square bundle diameters of ∼3-4 nm. This can be compared with the measured diameter of individual functionalized nanotubes of ∼3 nm. These nanotubes display very weak concentration dependent aggregation when dispersed in common solvents. Root-mean-square bundle diameters of only ∼5-6 nm were observed at concentrations as high as 1 mg/mL. This translates into >100 bundles per cubic micron of solvent, much higher than observed in other systems. These results have practical implications for the production of well dispersed polymer-nanotube composites that would be expected to display high interfacial stress transfer.
Introduction Many of the processing routes or characterization methods commonly used in carbon nanotube research require the dispersion of nanotubes in liquid media. To this end large numbers of papers have appeared describing the dispersion of nanotubes using physisorbed third phase dispersants such as surfactants,1-7 polymers,8,9 or biomolecules.10,11 In addition, recent work has shown that nanotubes can be effectively dispersed and exfoliated in certain solvents.12-15 A third strategy for nanotube dispersion, described in a wide range of papers and reviews,16-18 is to covalently functionalize the nanotubes with bulky molecules. The functionalized nanotubes tend to form a (meta)stable colloidal suspension, sterically stabilized by the osmotic pressure induced when functional groups from adjacent nanotubes move into the same region of space.19 Additionally, in some cases stabilization is achieved by electrostatic interactions between the functional groups.20 Furthermore, the functional group-solvent interaction lowers the enthalpy of mixing and can increase the entropy of mixing (solvent configurational effects) resulting in a lower free energy of mixing. In some cases the free energy of mixing may even be negative resulting in a true solution.21 However, very little work has been done to quantitatively study the interaction of the functionalized nanotube and the solvent. Here we show that the interaction is dominated by the properties of the functional group such that a functionalized nanotube behaves like an object with Hildebrand solubility parameter identical to that of its functional groups. In general, much of the characterization of dispersions such as these, while quantitative, has been carried out at a single concentration. It has been shown recently that the state of exfoliation of both nanotubes and nanowires is strongly concentration dependent.13,22-26 This has serious implications, as bundling of * To whom correspondence should be addressed. E-mail:
[email protected].
single walled nanotubes (SWNT) at high concentrations reduces the usefulness of SWNT dispersions for applications such as composite formation. In this work, we show that SWNT functionalized with bulky molecules display far weaker concentration dependent bundling than has been observed for Hipco SWNTs. This results in stable dispersions at concentrations of at least 1 mg/mL that display root-mean-square bundle diameters of 30 mg/mL. In general, for electrical or mechanical applications, a nanotube mass fraction of ∼1% is used. This means that nanotube concentrations of >0.3 mg/mL are required. Thus, for effective composites, good dispersion must be maintained at concentrations up to ∼0.3 mg/mL. In the past, this has been problematic for SWNT dispersions. For example, comparative studies show that SWNT based composites have disappointing mechanical properties due to SWNT aggregation at high concentration.34 Recently, this problem was circumvented by preparing composites by vacuum filtration from very low concentration dispersions.35 Unfortunately, this method is impractical for many applications. However, the demonstration of high quality dispersions of functionalized nanotubes at concentrations of up to 1 mg/mL means that high quality composites can be produced by drop casting. In addition, for structural composites, these nanotubes fulfill the twin conditions of good dispersion coupled with potential nanotube-matrix stress transfer via the functional groups.31 We would expect the long-chain functional groups to spontaneously entangle with the polymer matrix for any polymer whose Hildebrand parameter matches that of the functional group. This should result in very effective stress transfer. We predict that the availability of such nanotubes commercially will instigate an upsurge in research into SWNT-polymer composites for mechanical applications. Conclusion In conclusion, we have examined the dispersion of functionalized nanotubes in common solvents. We find the properties of the functional group mainly determine solvent choice. In addition we have demonstrated weak concentration dependent bundling of functionalized SWNT. This allows the fabrication of stable dispersions at reasonably high concentrations containing bundles with Drms ∼ 6 nm and populations of up to 106 rods per cubic micron. The latter value is ∼75 times higher than that obtained on dispersing Hipco SWNT in NMP. These dispersions are good enough at high enough concentration that we can expect to use them to make much improved composites. Acknowledgment. We acknowledge the Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN) for financial support. Supporting Information Available: Description of the procedure used to estimate the Hildebrand parameter of ODA
3524 J. Phys. Chem. C, Vol. 112, No. 10, 2008 (including Tables S1 and S2), a calculation of the density of the functionalized SWNTs, and Figures S1-5. These figures relate to the thermogravimetry curves for the functionalized SWNTs, the optical absorption spectra, bundle diameter distributions and the data relating to the populations of individual SWNTs. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) McDonald, T. J.; Engtrakul, C.; Jones, M.; Rumbles, G.; Heben, M. J. J. Phys. Chem. B 2006, 110, 25339. (2) Moore, V. C.; Strano, M. S.; Haroz, E. H.; Hauge, R. H.; Smalley, R. E.; Schmidt, J.; Talmon, Y. Nano Lett. 2003, 3, 1379. (3) 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. P.; Hauge, R. H.; Weisman, R. B.; Smalley, R. E. Science 2002, 297, 593. (4) Strano, M. S.; Moore, V. C.; Miller, M. K.; Allen, M. J.; Haroz, E. H.; Kittrell, C.; Hauge, R. H.; Smalley, R. E. J. Nanosci. Nanotechnol. 2003, 3, 81. (5) Vaisman, L.; Wagner, H. D.; Marom, G. AdV. Colloid Interface Sci. 2006, 128, 37. (6) Vigolo, B.; Coulon, C.; Maugey, M.; Zakri, C.; Poulin, P. Science 2005, 309, 920. (7) Zhou, W.; Islam, M. F.; Wang, H.; Ho, D. L.; Yodh, A. G.; Winey, K. I.; Fischer, J. E. Chem. Phys. Lett. 2004, 384, 185. (8) Coleman, J. N.; Dalton, A. B.; Curran, S.; Rubio, A.; Davey, A. P.; Drury, A.; McCarthy, B.; Lahr, B.; Ajayan, P. M.; Roth, S.; Barklie, R. C.; Blau, W. J. AdV. Mater. 2000, 12, 213. (9) Murphy, R.; Coleman, J. N.; Cadek, M.; McCarthy, B.; Bent, M.; Drury, A.; Barklie, R. C.; Blau, W. J. J. Phys. Chem. B 2002, 106, 3087. (10) Zheng, M.; Jagota, A.; Semke, E. D.; Diner, B. A.; McLean, R. S.; Lustig, S. R.; Richardson, R. E.; Tassi, N. G. Nat. Mater. 2003, 2, 338. (11) Zheng, M.; Jagota, A.; Strano, M. S.; Santos, A. P.; Barone, P.; Chou, S. G.; Diner, B. A.; Dresselhaus, M. S.; McLean, R. S.; Onoa, G. B.; Samsonidze, G. G.; Semke, E. D.; Usrey, M.; Walls, D. J. Science 2003, 302, 1545. (12) Furtado, C. A.; Kim, U. J.; Gutierrez, H. R.; Pan, L.; Dickey, E. C.; Eklund, P. C. J. Am. Chem. Soc. 2004, 126, 6095. (13) Giordani, S.; Bergin, S. D.; Nicolosi, V.; Lebedkin, S.; Kappes, M. M.; Blau, W. J.; Coleman, J. N. J. Phys. Chem. B 2006, 110, 15708. (14) Landi, B. J.; Ruf, H. J.; Worman, J. J.; Raffaelle, R. P. J. Phys. Chem. B 2004, 108, 17089.
Amiran et al. (15) Bergin, S. D.; Nicolosi, V.; Giordani, S.; de Gromard, A.; Carpenter, L.; Blau, W. J.; Coleman, J. N. Nanotechnology 2007, 18, 455705. (16) Banerjee, S.; Hemraj-Benny, T.; Wong, S. S. AdV. Mater. 2005, 17, 17. (17) Niyogi, S.; Hamon, M. A.; Hu, H.; Zhao, B.; Bhowmik, P.; Sen, R.; Itkis, M. E.; Haddon, R. C. Acc. Chem. Res. 2002, 35, 1105. (18) Xiao, Q.; Wang, P. H.; Si, Z. C. Prog. Chem. 2007, 19, 101. (19) Jones, R. A. L. Soft Condensed Matter, 1st ed.; Oxofrd University Press: Oxford, U.K., 2002. (20) Zhao, B.; Hu, H.; Yu, A. P.; Perea, D.; Haddon, R. C. J. Am. Chem. Soc. 2005, 127, 8197. (21) Penicaud, A.; Poulin, P.; Derre, A.; Anglaret, E.; Petit, P. J. Am. Chem. Soc. 2005, 127, 8. (22) Coleman, J. N.; Fleming, A.; Maier, S.; O’Flaherty, S.; Minett, A. I.; Ferreira, M. S.; Hutzler, S.; Blau, W. J. J. Phys. Chem. B 2004, 108, 3446. (23) McCarthy, D. N.; Nicolosi, V.; Vengust, D.; Mihailovic, D.; Compagnini, G.; Blau, W. J.; Coleman, J. N. J. Appl. Phys. 2007, 101. (24) Nicolosi, V.; Vengust, D.; Mihailovic, D.; Blau, W. J.; Coleman, J. N. Chem. Phys. Lett. 2006, 425, 89. (25) Nicolosi, V.; Vrbanic, D.; Mrzel, A.; McCauley, J.; O’Flaherty, S.; McGuinness, C.; Compagnini, G.; Mihailovic, D.; Blau, W. J.; Coleman, J. N. J. Phys. Chem. B 2005, 109, 7124. (26) Cathcart, H.; Quinn, S.; Nicolosi, V.; Kelly, J. M.; Blau, W. J.; Coleman, J. N. J. Phys. Chem. C 2007, 111, 66. (27) Liu, J.; Casavant, M. J.; Cox, M.; Walters, D. A.; Boul, P.; Lu, W.; Rimberg, A. J.; Smith, K. A.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 1999, 303, 125. (28) Rubinstein, M.; Colby, R. H. Polymer Physics, 1st ed.; Oxford University Press: Oxford, U.K., 2003. (29) Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd ed.; Wiley: New York, 1989. (30) Hildebrand, J. H.; Prausnitz, J. M.; Scott, R. L. Regular and related solutions, 1st ed.; Van Nostrand Reinhold Company: New York, 1970. (31) Coleman, J. N.; Khan, U.; Blau, W. J.; Gun’ko, Y. K. Carbon 2006, 44, 1624. (32) Nicolosi, V.; McCarthy, D. N.; Vengust, D.; Mihailovic, D.; Blau, W. J.; Coleman, J. N. Eur. Phys. J.-Appl. Phys. 2007, 37, 149. (33) Bergin, S. D.; Nicolosi, V.; Cathcart, H.; Lotya, M.; Rickard, D.; Sun, Z.; Blau, W. J.; Coleman, J. N. J. Phys. Chem. C 2008, 112, 972977. (34) Cadek, M.; Coleman, J. N.; Ryan, K. P.; Nicolosi, V.; Bister, G.; Fonseca, A.; Nagy, J. B.; Szostak, K.; Beguin, F.; Blau, W. J. Nano Lett. 2004, 4, 353. (35) Blighe, F.; Hernandez, Y.; Blau, W. J.; Coleman, J. N. AdV. Mater. 2007, 19, 4443.
