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Electrostatic Interactions between Shortened Multiwall Carbon Nanotubes and Polyelectrolytes Bumsu Kim, Hyun Park, and Wolfgang M. Sigmund* Department of Materials Science & Engineering, University of Florida, 225 Rhines Hall, P.O. Box 116400, Gainesville, Florida 32611 Received October 24, 2002. In Final Form: December 10, 2002
Introduction Carbon nanotubes (CNTs) have been intensively investigated since Iijima’s discovery1 due to their outstanding physical properties:2-5 especially electrical, optical, mechanical, and structural properties. Recently, attention has been paid to shortened carbon nanotubes (sCNTs) due to potential applications such as nanotube arrays,6 nanotube probes,7 thin nanotube films,8 and optical limit materials.9 sCNTs were reported to be produced by chemical oxidation methods.6,10 sCNTs are carboxylated during the oxidative shortening reaction, and carboxyl groups on the sCNTs and functionalization of carboxyl groups on sCNTs have been verified by IR spectroscopy.6,11,12 Prepared sCNTs were found to be well dispersed in water, ethanol, and other solvents without surfactants.6,8 The importance of interactions between CNTs and polymers has been increasingly recognized, since complexes with CNTs and polymers can be formed. Single wall carbon nanotubes (SWCNTs) were found to be wrapped with polymers bearing polar side chains, such as polyvinylpyrrolidone or polystyrenesulfonate.13 These nanotube/polymer complexes are stable in water. Polymer wrapped SWCNT structures were also synthesized with poly(m-phenylene-co-2,5-dioctoxy-p-phenylenevinylene).14 These supramolecular structures exhibit excellent * To whom correspondence should be addressed. Telephone: +1-352-846-3343. Fax: +1-352-392-7219. E-mail: wsigm@ mse.ufl.edu. (1) Iijima, S. Nature 1991, 354, 56. (2) Ajayan, P. M. Chem. Rev. 1999, 99, 1787. (3) Xie, S. S.; Chang, B. H.; Li, W. Z.; Pan, Z. W.; Sun, L. F.; Mao, J. M.; Chen, X. H.; Qian, L. X.; Zhou, W. Y. Adv. Mater. 1999, 11, 1135. (4) Dai, L.; Mau, A. W. H. Adv. Mater. 2001, 13, 899. (5) Rao, C. N. R.; Satishkumar, B. C.; Govindaraj, A.; Nath, M. ChemPhysChem 2001, 2, 78. (6) Liu, Z. F.; Shen, Z. Y.; Zhu, T.; Hou, S. F.; Ying, L. Z.; Shi, Z. J.; Gu. Z. N. Langmuir 2000, 16, 3569. (7) Yang, Y. L.; Zhang, J.; Nan, X. L.; Liu, Z. F. J. Phys. Chem. B 2002, 106, 4139. (8) Shimoda, H.; Oh, S. J.; Geng, H. Z.; Walker, R. J.; Zhang, Z. B.; McNeil, L. E.; Zhou, O. Adv. Mater. 2002, 14, 899. (9) Jin, Z. X.; Haung, L.; Goh, S. H.; Xu, G. Q.; Ji, W. Chem. Phys. Lett. 2002, 352, 328. (10) Liu. J.; Rinzler, A. G.; Dai, H. J.; Hafner, J. H.; Bradley, R. K.; Boul, P. J.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.; RodriguezMacias, F.; Shon, Y. S.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998, 280, 1253. (11) Hamon, M. A.; Chen, J.; Hu, H.; Chen, Y.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Adv. Mater. 1999, 11, 834. (12) Mawhinney, D. B.; Naumenko, V.; Kuzenetsova, A.; Yates, J. T. J.; Liu, J.; Smalley, R. E. J. Am. Chem. Soc. 2000, 122, 2382. (13) O’Connell, M. J.; Boul, P.; Ericson, L. M.; Huffman, C.; Wang, Y. H.; Haroz, E.; Kuper, C.; Tour, J.; Ausman, K. D.; Smalley, R. E. Chem. Phys. Lett. 2001, 342, 265. (14) Curran, S. A.; Ajayan, P. M.; Blau, W. J.; Carroll, D. L.; Coleman, J. N.; Dalton, A. B.; Davey, A. P.; Drury, A.; Mccarthy, B.; Maier, S.; Strevens, A. Adv. Mater. 1998, 10, 1091.
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conductivity without any restriction of luminescence properties.14 In this study, we focus on the ζ potential of shortened multiwall carbon nanotubes (sMWCNTs) and the interactions of sMWCNTs with polyelectrolytes. The aim of this work is to build an experimental foundation for sMWCNT suspensions’ colloidal stability in aqueous media (pH 6.5). The control of charge and steric effects for colloidal particles is crucial for their processing and applications. Polyelectrolytes are chosen, since they allow for exciting surface patterning structures using nanolithography or stamping techniques and in combination with similar and oppositely charged nano-objects could allow for selfassembly of novel nanostructures. Experimental Section Multiwall carbon nanotubes (MWCNTs, CVD method, 95%) were obtained from Iljin Nanotech Inc. All following materials were obtained from Aldrich and used as received: poly(diallyldimethylammonium chloride) (PDAC, 20 wt % in water, molecular weight 400 000-500 000), poly(sodium 4-styrenesulfonate) (PSS, Mw ) 70 000), sodium chloride (NaCl, 99+%), potassium chloride (KCl, 99%), potassium hydroxide (KOH, 1 M), hydrochloric acid (HCl, 1 M). pH 7 (potassium phosphate) and pH 10 (potassium carbonate, potassium borate) buffer solutions were purchased from Fisher scientific. MWCNT raw soot was heated in air at 600 °C for 2 h and then soaked in hydrochloric acid for 24 h and centrifuged. The precipitate was rinsed with deionized water three times and dried under nitrogen gas. MWCNTs were chemically shortened by sonification in a mixture of sulfuric acid and nitric acid (3:1) for 8 h. The resulting sMWCNTs were washed with deionized water and separated by centrifuging three times. After being dried in a nitrogen stream, sMWCNTs were dispersed in deionized water at a solid loading of 30 mg/50 mL. PDAC (10-5 M) and PSS (10-4 M) solutions were prepared by dissolving them in deionized water containing the desired amount of NaCl (0 M ∼ 1 M). Electrophoretic measurements were carried out with a Zetaplus analyzer (Zetaplus, Brookhaven, USA). The ionic strength of a dilute suspension was maintained at 10-3 M using KCl. The sample was ultrasonicated for 5 min before taking the measurements. The viscosity measurements were done using a MCR 300 Modular compact rheometer (Paar-Physica). The bob-cylinder geometry was used in our experiments. The measurements were carried out at 25 °C. The shear rate was ramped from 0.001 to 1100 s-1. The solution was presheared at 800 s-1 for 1 min. The slurry was then kept stationary for 1 min to equilibrate. This was followed by the measurement. The time duration between two measurements was 15 s. The relative viscosity was calculated as a ratio of the viscosity of the sMWCNTs’ added polyelectrolyte solution to the viscosity of the polyelectrolyte solution at the same temperature. All viscosity data are taken at the shear rate 100 s-1. sMWCNTs were observed with field-emission scanning microscopy (FE-SEM, 6335F, JEOL) and atomic force microscopy (AFM, DIMENSION 3100, Digital instrument) by tapping mode measurement with silicon cantilevers in ambient conditions.
Results and Discussion The carboxylic groups of our sMWCNTs were verified by FT-IR with strong stretching bands of carboxylic groups at 1710 cm-1. This is in accordance with findings of other groups.6,15 Liu et al. reported sMWCNTs prepared by chemical oxidation to form carboxylic acid groups mainly at their ends.6 The side wall defects of SWCNTs were (15) Mawhinney, D. B.; Naumenko, V.; Kuznetsova, A.; Yates, J. T.; Liu, J.; Smalley, R. E. Chem. Phys. Lett. 2000, 324, 213.
10.1021/la026746n CCC: $25.00 © 2003 American Chemical Society Published on Web 02/08/2003
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Langmuir, Vol. 19, No. 6, 2003
Notes
Figure 1. FE-SEM and AFM images of shortened multiwall carbon nanotubes (a) and the structural formulas of the polyelectrolytes (b).
reported to be 5.5% ( 2.5% under harsh condition treatment.15 For long SWCNTs, the fraction of defect sites attributable to the end caps is much smaller than 5.5%. Figure 1a shows FE-SEM and AFM images of sMWCNTs. The average diameter of sMWCNTs is 10-20 nm. The sMWCNTs show a variation in length measuring several hundreds of nanometers. Figure 1b shows the structure of polyelectrolytes used in this experiment. sMWCNT suspensions are stabilized in water and did not flocculate within the time frame (weeks) of the experiments. It can be expected that they are colloidally stable for months. The ζ potentials of sMWCNTs as a function of pH are given in Figure 2. The isoelectric point of sMWCNTs is around pH 2. Therefore, sMWCNTs will be charged negatively when dispersed in deionized water. These negative charges explain the colloidal stability of sMWCNTs in aqueous media. The decrease of the absolute value of the ζ potential at pH 10 might be due to a higher ionic strength of the buffer solution. Though sMWCNTs were found to have a broad length distribution, ζ potential data are reproducible. Figure 3 plots the viscosity of polyelectrolyte solutions on dilution with a sMWCNT suspension (30 mg/50 mL of deionized water) and water. It is found that, in 10-5 M PDAC solution, the viscosity of the PDAC solution decreases with increasing dilution with water, which is to be expected. Surprisingly, the dilution based on a sMWCNT suspension causes a stronger decrease in viscosity. It is assumed that this is caused by PDAC
Figure 2. ζ potential of sMWCNTs as a function of pH at 25 °C.
adsorption on sMWCNTs in the solution due to electrostatic attraction of opposite charges. An increase in viscosity due to network formation between sMWCNTs and PDAC had been expected. The viscosity decrease might be best explained by PDAC helically wrapped
Notes
Figure 3. Viscosity of polyelectrolyte solutions on dilution: (9) PDAC solution on dilution with sMWCNT suspensions (solids loading of 30 mg/50 mL); (b) PDAC solution on dilution with water; (2) PSS solution on dilution with sMWCNT suspensions (solids loading of 30 mg/50 mL); (1) PSS solution on dilution with water.
sMWCNTs, as has been reported for PDAC/SWCNT solutions.13 In PSS solution, no differences in viscosity are observed for diluting with either water or sMWCNTs suspension. The general trend of decreasing viscosity on dilution is to be expected. Furthermore, since no sediments are produced during and after the experiments, it can be assumed that no special PSS sMWCNT supramolecular structures formed. An expected thermodynamic drive to remove a hydrophobic interface13 from the CNTs by wrapping with PSS is believed not to be crucial due to an increased hydrophilicity of sMWCNTs and the electrostatic repulsion between similarly charged PSS and sMWCNTs. The shortening process for MWCNTs lessens their hydrophobic properties by adding alcohol, carbonyl, and carboxyl groups at the ends and the sidewalls of MWCNTs. Both sMWCNTs and PSS are negatively charged in deionized water at pH 6.5. Figure 4 shows the effect of salt on viscosity for a 10-5 M PDAC solution on dilution with a sMWCNTs suspension. For the investigated 10-5 M PDAC starting solution with salt concentrations of 0, 0.01, 0.1, and 1 M, it is found that dilution causes the expected decrease in viscosity. As can be seen in Figure 4, there is an additional impact besides dilution coming from the salt concentration. The higher the starting ionic strength, the less is the reduction in viscosity on dilution with a sMWCNT suspension. This is in accordance with the findings from Clark et al., who reported on the ionic effects of NaCl on interactions between PDAC and carboxylic acid functionalized flat substrates.16 They report that for salt concentrations above 0.4 M NaCl the adsorption of PDAC on carboxylic functionalized surfaces is sharply decreased, and no adsorption of PDAC was detected in 1 M NaCl solutions. This behavior is attributed to the screening adsorption reduction described by van De steeg17 and others.18 In (16) Clark, S. L.; Montague, M. F.; Hammond, P. T. Macromolecules 1997, 30, 7237.
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Figure 4. Effect of ionic strength on the viscosity of a 10-5 M PDAC solution at 25 °C: (9) 0 M NaCl; (b) 0.01 M NaCl; (2) 0.1 M NaCl; (1) 1 M NaCl on dilution with sMWCNT suspensions (solids loading of 30 mg/50 mL).
brief, at very high salt concentrations, the Na+ ions compete with highly shielded PDAC for adsorption on the carboxylated surface and the adsorbed amount of PDAC decreases rapidly. In our system, the difference in viscosities, which indicates the adsorption, follows similar trends but at lower salt concentrations. The biggest change in viscosity is found from 0.01 to 0.1 M NaCl. No difference is found for 0.1 and 1 M. Our data show an impact of ionic strength at lower salt concentrations than those of Hammond’s group.16 This is believed to be due to a lower surface density of carboxylic acid groups in our case. In summary, the stability of sMWCNTs in water can be explained by the development of electrostatic charges stemming from the carboxylic acid groups on sMWCNTs. The rheology of sMWCNTs and polyelectrolytes in solution indicates interactions of the charged species. According to the here reported ζ potential of sMWCNTs and the known protolytic behavior of polyelectrolytes at pH 6.5, the interactions of these compounds can be assumed to be dominated by Coulomb interactions. Furthermore, the impact of ionic strength on the viscosity confirms the Coulomb nature of the interactions. The impact of the ionic strength on sMWCNTs’ and polycations’ viscosities can be explained by adsorption competition of Na+ ions and polycations. These results broaden routes to understand the functionalization of sMWCNTs, to manipulate sMWCNT/polyelectrolyte composites for various applications, and to control the stability and electrostatic interactions of sMWCNTs in water. Acknowledgment. This work was supported by DARPA/Army Research Office under Grant No. DAAD1900-1-0002 through the center for materials in sensors and actuators (MINSA). LA026746N (17) van de Steeg, B. H. M.; Cohen Stuart, M. A.; de Keizer, A.; Bijsterbosch, B. H. Langmuir 1992, 8, 2538. (18) Bo¨hmer, M. R.; Evers, O. A.; Scheutjens, J. M. H. M. Macromolecules 1990, 23, 2288.
