pubs.acs.org/Langmuir © 2009 American Chemical Society
Gram-Scale Preparation of Surfactant-Free, Carboxylic Acid Groups Functionalized, Individual Single-Walled Carbon Nanotubes in Aqueous Solution Aiping Yu,*,† Chen-Chi Lisa Su,† Isaac Roes,† Benson Fan,† and Robert C. Haddon‡ † Department of Chemical Engineering; Nanotechnology Institute, University of Waterloo, 200 University Avenue West, Waterloo, ON, N2L 3G1, Canada and ‡Departments of Chemistry and Chemical & Environmental Engineering, University of California, Riverside, California 92521-0403
Received July 2, 2009. Revised Manuscript Received October 26, 2009 We report a simple method to prepare individual electric arc-produced single-walled carbon nanotubes (SWNTs) in aqueous solution on a large scale through three steps of processing: refluxing in concentrated HNO3, low speed centrifugation, and high speed centrifugation. The bulk production (10 g of starting SWNTs) results in a concentration of 0.2 mg/mL individual SWNTs stably dispersed in DI-H2O without any external protection. The atomic force microscopy images show that the aqueous dispersion contained approximately 80% individual SWNTs with lengths ranging from 500 nm to 1 micrometer. It is found that the stable individual SWNT dispersion has an absolute zeta potential value of ∼72 mV with a concentration of 0.05 mg/mL at pH 5. We believe that it is this high zeta potential resulting from an electrical double layer which produces the repulsion to overcome the van der Waals attraction thereby keeping the SWNTs individually dispersed. The free-standing film prepared from the individual SWNT dispersion exhibits a 4-probe electrical conductivity of ∼2000 S/cm and a transmittance of 60% at 550 nm.
1. Introduction Over the decade, SWNTs have attracted remarkable attention due to their unique electronic, thermal, mechanical and structural properties. Typical SWNT bundles comprise a few to tens of nanotubes, which form a hexagonal close-packed lattice. Although the van der Waals attractive force among these nanotubes is weak on a single carbon atom basis, the interaction of many atoms results in a substantial net binding strength, which is estimated to be about 500 eV/μm of tube-tube interaction using model computations based on the Lennard-Jones potential.1 In some particular areas, such as nanoelectronics, bioelectronics, sensors, and composites, individual SWNTs in aqueous dispersion are highly desired in order to take full advantage of their superior properties. Moreover, future manufacturing of highcurrent, high-speed, and high-density nanotube circuits is very dependent on the separation of metallic and semiconducting SWNTs.2-5 As the first step, bulk production of individual SWNTs is essential toward chirality separation. In 2002, Smalley’s group reported for the first time to have obtained individual nanotubes at a concentration of 20-25 mg/L in D2O (density, 1.10 g/cm-3) by using SDS micelle, PVPwrapping, and ultracentrifugation (122000g for 4 h).6 The mechanism is explained by the thermodynamic drive to eliminate *To whom correspondence should be addressed. Tel.: 519-888-4567 Ext. 38799. Fax: 519-746-4979. E-mail:
[email protected]. (1) Girifalco, L. A.; Hodak, M.; Lee, R. S. Phys. Rev. B 2000, 62, 13104–13110. (2) Wu, Z.; Chen, Z.; Du, X.; Logan, J. M.; Sippel, J.; Nikolou, M.; Kamaras, K.; Reynolds, J. R.; Tanner, D. B.; Hebard, A. F.; Rinzler, A. G. Science 2004, 305, 1273–1276. (3) Guo, J.; Hasan, S.; Javey, A.; Bosman, G.; Lundstrom, M. IEEE Trans. Nanotechnol. 2005, 4, 715–721. (4) Itkis, M. E.; Borondics, F.; Yu, A.; Haddon, R. C. Science 2006, 312, 413– 416. (5) Itkis, M. E.; Yu, A.; Haddon, R. C. Nano. Lett. 2008, 08, 2224–2228. (6) 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, 593–596.
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the hydrophobic interface between the tubes and their aqueous medium. Since that, continuation of the effort has always been focused on surfactant protection, polymer wrapping, or nonaqueous medium to prepare individual SWNTs.7-24 Bandyopadhyaya and coauthors had dispersed full-length, individual tubes in a gum Arabic aqueous solution by physical adsorption of polymers which exert steric repulsion among the polymer-coated tubes, allowing the disassembly of SWNT ropes into individual (7) Bandyopadhyaya, R.; Nativ-Roth, E.; Regev, O.; Yerushalmi-Rozen, R. Nano Lett. 2002, 2, 25–28. (8) Moore, V. C.; Strano, M. S.; Haroz, E. H.; Hauge, R. H.; Smalley, R. E.; Schmidt, J.; Talmon, Y. Nano Lett 2003, 3, 1379–1382. (9) 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, 338–342. (10) Dyke, C. A.; Tour, J. M. Nano Lett 2003, 3, 1215–1218. (11) Kovtyukhova, N. I.; Mallouk, T. E.; Pan, L.; Dickey, E. C. J. Am. Chem. Soc. 2003, 125, 9761–9769. (12) Paredes, J. I.; Burghard, M. Langmuir 2004, 20, 5149–5152. (13) Kim, K. K.; Bae, D. J.; Yang, C. M.; An, K. H.; Lee, J. Y.; Lee, Y. H. J. Nanosci. Nanotechnol. 2005, 5, 1055–1059. (14) Giordani, S.; Bergin, S.; Nicolosi, V.; Lebedkin, S.; Blau, W. J.; Coleman Phys. Stat. Sol. (b) 2006, 243, 3058–3062. (15) Nakamura, G.; Narimatsu, K.; Niidome, Y.; Nakashima, N. Chem. Lett. 2007, 36, 1140–1141. (16) Schwamb, T.; Choi, T.; Schirmer, N.; Bieri, N.; Burg, B.; Tharian, J.; Sennhauser, U.; Poulikakos, D. Nano. Lett. 2007, 7, 3633–3638. (17) Jeng, E. S.; Barone, P. W.; Nelson, J. D.; Strano, M. S. Small 2007, 3, 1602– 1609. (18) White, B.; Banerjee, S.; O’Brien, S.; Turro, N. J.; Herman, I. P. J. Phys. Chem. C 2007, 111, 13684–13690. (19) Forment-Aliaga, A.; Weitz, R. T.; Sagar, A. S.; Lee, E.; Konuma, M.; Burghard, M.; Kern, K. Small 2008, 4, 1671–1675. (20) Tsyboulski, D. A.; Bachilo, S. M.; Kolomeisky, A. B.; Weisman, R. B. ACS Nano 2008, 2, 1770–1776. (21) Okamoto, M.; Fujigaya, T.; Nakashima, N. Adv. Func. Mater. 2008, 18, 1776–1782. (22) Duque, J. G.; Cognet, L.; Parra-Vasquez, A. N. G.; Nicholas, N.; Schmidt, H. K.; Pasquali, M. J. Am. Chem. Soc. 2008, 130, 2626–2633. (23) Jeong, S. Y.; Jeon, S. H.; Han, G. H.; An, K. H.; Bae, D. J.; Lim, S. C.; Hwang, H. R.; Han, C. S.; Yun, M.; Lee, Y. H. J. Nanosci. Nanotechnol. 2008, 8, 329–334. (24) Yan, L. Y.; Poon, Y. F.; Chan-Park, M. B.; Chen, Y.; Zhang, Q. J. Phys. Chem. C 2008, 112, 7579–7587.
Published on Web 11/16/2009
DOI: 10.1021/la902341w
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tubes.7 Hwang and Smalley had also reported the preparation of aligned, individual SWNTs in superacid (fuming sulfuric acid), which are surrounded by a finite number of sulfuric acid anions; the ordered dispersion was extruded via solution spinning into continuous lengths of macroscopic neat SWNT fibers.25White and coauthors had obtained very highly charged individual SWNTs by dispersing the nanotubes in high concentrations of anionic and cationic surfactants, whereas almost neutral SWNTs were obtained by using nonionic surfactants (100000g for 4.5 h). They had also done systematic studies of the zeta (ζ)-potential distributions of surfactant-wrapped individual SWNTs.18 Zheng’s9 and other groups17 reported that DNAs are also able to efficiently bind to SWNTs through π-stacking interaction, resulting in helical wrapping on to the SWNT surface (1 mg SWNT in 1 mg/mL DNA, centrifuged at 16000g). Zheng’s work further led to separation of metallic and semiconducting SWNTs by the implementation of ion-exchange chromatography. Other than that, Kim and coauthors had introduced a multiple-step process to disperse individual SWNTs in nonaqueous solvent (dichloroethane) by purification and sonication of SWNTs in dichloroethane for ∼15 h, followed by centrifugation at 17 000 rpm for ∼3 h.13 Ashcroft and coauthors had reported the preparation of the individual ultrashort (20-80 nm lengths) SWNTs in THF via fluorination and Birch reduction.26 The residual surfactant, biopolymer, or superacid requires extra effort to remove, and in some particular systems, an aqueous dispersion is highly desired. Meanwhile, the majority of the effort has been focused on HiPCO tubes (0.7-1.3 nm),27-29 while the electric arc-discharge produced SWNTs (EAC-SWNTs) have been explored less. Being another important source for SWNTs, EAC-SWNTs have a larger mean diameter of 1.38 nm, thus these tubes are more robust and can sustain more harsh treatment. Here we demonstrate that individual EAC-SWNTs can be obtained in DI-H2O by a three-step approach on the gram scale. To the best of our knowledge, this is the first time in literature showing that individual SWNTs can stay stable in aqueous solution without any external protection on a large scale.
2. Experimental Section Preparation of Individual SWNTs. As-prepared SWNTs (AP-SWNTs) (containing ca. 30-40 wt % metal catalyst, using TGA analysis) were supplied by Carbon Solutions, Inc. Alumina membrane with a pore size of 20 nm was purchased from Whatman Inc., and all the other chemicals were purchased from Aldrich. AP-SWNTs were refluxed in concentrated HNO3 for 5 h, which removes metal impurities as well as part of the amorphous carbon, partially exfoliates the nanotube bundles, and attacks the SWNTs at the caps and defect sites thereby leaving the nanotubes functionalized with carboxylic acid groups.30 It is noted here that when adding concentrated HNO3 to AP-SWNTs, caution needs to be taken to avoid the mixture catching fire. It is suggested to use N2 protection or use a very small amount of DI-H2O to wet the AP-SWNTs first. (25) Ericson, L. M.; Fan, H.; Peng, H. Q.; Davis, V. A.; Zhou, W.; Sulpizio, J.; Wang, Y. H.; Booker, R.; Vavro, J.; Guthy, C.; Parra-Vasquez, A. N. G.; Kim, M. J.; Ramesh, S.; Saini, R. K.; Kittrell, C.; Lavin, G.; Schmidt, H.; Adams, W. W.; Billups, W. E.; Pasquali, M.; Hwang, W. F.; Hauge, R. H.; Fischer, J. E.; Smalley, R. E. Science 2004, 305, 1447–1450. (26) Ashcroft, J. M.; Hartman, K. B.; Mackeyev, Y.; Hofmann, C.; Pheasant, S.; Alemany, L. B.; Wilson, L. J. Nanotechnology 2006, 17, 5033–5037. (27) Zhou, W.; Ooi, Y. H.; Russo, R.; Papanek, P.; Luzzi, D. E.; Fischer, J. E.; Bronikowski, M. J.; Willis, P. A.; Smalley, R. E. Chem. Phys. Lett. 2001, 350, 6–14. (28) Nikolaev, P. J. Nanosci. Nanotechnol. 2004, 4, 307–316. (29) Elliott, J. A.; Sandler, J. K. W.; Windle, A. H.; Young, R. J.; Shaffer, M. S. P. Phys. Rev. Lett. 2004, 92. (30) Hu, H.; Zhao, B.; Itkis, M. E.; Haddon, R. C. J. Phys. Chem. B 2003, 107, 13838–13842.
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Scheme 1. The Schematic Illustration of the Procedure
After the nitric acid treatment, the nanotubes (a-SWNTs) were washed with DI-H2O until the light brown filtrate turned clear, redispersed in DI-H2O again by stir-bar mixing (caution: no sonication at this step), and centrifuged at 4000 rpm for 30 min. The supernatant was removed, and the sediment was collected and redispersed in DI-H2O for another cycle of centrifugation. The whole low speed centrifugation process was repeated three times, and the sediment was collected for further treatment. The low speed centrifuge aims to remove most of the amorphous carbon generated in the nitric acid treatment. Without this step, the amorphous carbon, which has a very high stability in water, will not be able to be separated by the next step of centrifugation. The sediment (b-SWNTs) collected from low speed centrifugation was subsequently processed by bath sonication for 60 min and centrifugation at 20000 rpm for 50 min. The high speed centrifugation removes bundles of SWNTs, graphitic nanoparticles, and graphitic-layer-encapsulated catalyst nanoparticles. The supernatant, containing approximate 80% of individual SWNTs (i-SWNTs) was collected for use and characterization. The sediment was collected and sonicated for another 60 min and followed by another cycle of high speed centrifugation. The high speed centrifugation was also repeated three times. Starting from 10 g of AP-SWNTs, the total i-SWNTs collected from supernatant of high speed centrifugation was estimated to be about 1-1.5 g, varying by batches. The concentration of i-SWNTs dispersed in DI-H2O was approximately 0.2 mg/mL, estimated by filtering of 1/10 of the i-SWNT dispersion and drying the obtained cake in an oven. The whole procedure is illustrated in Scheme 1. Titration was performed to determine the percentage of carboxylic acid groups using NaOH and NaHCO3 according to the literature.31 The result shows that there are 6-7% of carboxylic acid groups attached to i-SWNTs by assuming that the SWNTs are solely composed of carbon. The transparent SWNT thin film was prepared using a vacuum filtration apparatus (diameter 47 mm, resulting in a film of 40 mm in diameter), an alumina membrane, and a leveler for horizontal alignment of the filtration membrane to control the uniformity of the film. For preparing a 100 nm thick film, 30 mL of DI-H2O was added to the freshly prepared i-SWNT dispersion (0.2 mg/mL) to dilute to 0.05 mg/mL. A 3.5 mL portion of the diluted dispersion was added to a clean container with an additional 100 mL of DIH2O. The thickness of the film was calculated from the volume of 2 tF ¼ V Cwhere D is the diameter of the the dispersion using πD 4 film (4 cm); t is the thickness of the film; F is the density of SWNTs (1.4 g/cm3); V is the volume of dispersion (3.5 mL) and C refers to be the concentration of the SWNT dispersion (0.05 mg/mL). Before filtering the solution, the filtration membrane was aligned with a leveler. The solution was filtered, and the SWNT film on the alumina membrane was dried at 90 °C for about 30 min. To transfer the SWNT film to a glass substrate, a 0.05 M NaOH solution bath was employed to etch away the alumina membrane. The SWNT thin film floating in the bath was collected on a glass substrate and transferred to a clean DI-H2O bath, where it was left for 10-20 min to remove any absorbed NaOH. The film on the glass substrate was then allowed to dry at 90 °C for about 30 min. The annealing of the SWNT film was carried out by heating the film in a furnace under vacuum at 350 °C for 6 h. Instruments and Measurement. AFM images of the SWNTs exfoliated by the above procedure were taken in tapping mode (Veeco, Inc.). The ultraviolet, visible, near-infrared (UV-vis-NIR) spectra were recorded with a Varian Cary 500 (31) Hu, H.; Bhowmik, P.; Zhao, B.; Hamon, M. A.; Itkis, M. E.; Haddon, R. C. Chem. Phys. Lett. 2001, 345, 25–28.
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Table 1. Mass and Metal Residue of SWNTs after Each Step of Processing materials AP-SWNTs a-SWNTs b-SWNTs i-SWNTs
mass (g)
metal residue from TGA analysis (wt %)
10 5-6 2-3 1-1.5
30 5 5
