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Functionalized Multiwall Carbon Nanotube/Gold Nanoparticle Composites Bumsu Kim* and Wolfgang M. Sigmund Department of Materials Science & Engineering, University of Florida, 225 Rhines Hall, P.O. Box 116400, Gainesville, Florida 32611 Received March 4, 2004. In Final Form: June 26, 2004
Multiwall carbon nanotubes (MWCNTs) were chemically oxidized in a mixture of sulfuric acid and nitric acid (3:1) while being ultrasonicated. The effect of oxidative ultrasonication at room temperature on development of functional groups on the carbon nanotubes was investigated. The dispersability and the carboxylic acid group concentration of functionalized MWCNTs (fMWNTs) varied with reaction time. The concentration of carboxylic acid groups on fMWNTs increased from 4 × 10-4 mol/g of fMWNTs to 1.1 × 10-3 mol/g by doubling the treatment period from 4 to 8 h. The colloidal stability of aqueous fMWCNTs dispersions was enhanced through elongated oxidation. fMWCNTs that were reacted longer than 4 h did not precipitate in aqueous media for at least 24 h. The layer-by-layer self-assembly of polyelectrolytes on fMWCNTs was characterized by zeta potential measurements. The zeta potential of fMWCNTs changed from negative charge to positive charge when cationic polyelectrolytes were self-assembled on their surface. With addition of anionic polyelectrolytes, cationic polyelectrolyte coated fMWCNTs showed the expected charge reversal as expected for multilayer self-assembly. Complex formation of positively charged gold nanoparticles and negatively charged fMWCNTs was achieved with and without polyelectrolyte coatings by electrostatic interaction. The complex formation was characterized by high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy. The here found complex formation of positively charged colloidal gold and defect sites on fMWNTs indicates the location of functional groups on carbon nanotubes. It is suggested that positively charged colloids such as gold nanoparticles could be used for detection of defect sites on carbon nanotubes.
Introduction Carbon nanotubes (CNTs) have been intensively studied for potential applications due to their outstanding physical properties.1-4 Functionalized CNTs have been of interest for dispersion enhancement in processing or for chemical modifications.5,6 Shortening of carbon nanotubes (sCNTs) by ultrasonication with oxidizing acid mixtures is frequently used to functionalize CNTs.7 CNT applications in devices such as fuel cells or sensors are expected to be enhanced by CNT/nanoparticle complexes. Additionally, templates for nanotrees based on CNT/nanoparticle complexes are of recent interest.8-10 Various approaches for CNT/nanoparticle complexes were suggested, such as, physical evaporation, chemical reaction with functionalized CNTs, and electroless deposition method.7-8,11-13 Recently, functionalized gold * To whom correspondence should be addressed: tel, +1-352846-3343; fax, +1-352-392-7219; e-mail,
[email protected]. (1) De Heer, W. A.; Chatelain, A.; Ugarte, E. Science 1995, 270, 1179. (2) Ajayan, P. M.; Stephan, O.; Colliex, C.; Trauth, D. Science 1994, 265, 1212. (3) Chen, P.; Wu, X.; Lin, J.; Tan, K. L. Science 1999, 285, 91. (4) Kong, J.; Franklin, N. R.; Zhou, C. W.; Chapline, M. G.; Peng, S.; Cho, K. J.; Dai, H. J. Science 2000, 287, 622. (5) Fu, K. F.; Sun, Y. P. J. Nanosci. Nanotechnol. 2003, 3, 351. (6) Sun, Y. P.; Fu, K. F.; Lin, Y.; Huang, W. J. Acc. Chem. Res. 2002, 35, 1096. (7) 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. (8) Li, W. Z.; Liang, C. H.; Zhou, W. J.; Qiu, J. S.; Zhou, Z. H.; Sun, G. Q.; Xin. Q. J. Phys. Chem. B 2003, 107, 6292. (9) Kong, J.; Chapline, M. G.; Dai, H. J. Adv. Mater. 2001, 13, 1384. (10) Bezryadin, A.; Lau, C. N.; Tinkham, M. Nature 2000, 404, 971. (11) Xue, B.; Chen, P.; Hong, Q.; Lin, J. Y.; Tan, K. L. J. Mater. Chem. 2001, 11, 2378. (12) Li, J.; Moskovits, M.; Haslett, T. L. Chem. Mater. 1998, 10, 1963.
nanoparticles were attached on noncovalently functionalized CNTs.14 Gold and platinum nanoparticles were synthesized on the side walls of CNTs by the spontaneous reduction of metal ions.15 Oxidized CNTs coated with poly(diallyl dimethylammonium) chloride were used as a template for gold nanoparticle self-assembly.16 Furthermore, the polymer wrapping around CNTs was reported as an efficient method to stabilize CNTs in solutions.17 The multilayer polymer coating on CNT by noncovalent attachment and electrostatic reaction was reported as a template for nanoparticle assembly.18 Though functionalization of CNTs by a chemical oxidation method to enhance the solubility of CNTs has been reported, the effect of functionalization time on functionalized multiwall carbon nanotubes (fMWCNTs) has not been reported yet. For further chemical reactions and many industrial applications with functionalized CNTs, it is crucial to understand functionalized CNTs more quantitatively. Therefore, we report in this study the dispersability of fMWCNTs in aqueous media depending on reaction time. The concentration of carboxylic acid groups was measured by potentiometric titration. The layer-by-layer coating with polyelectrolytes on fMWCNTs was observed by zeta potential measurement at pH 7. (13) Azamian, B. R.; Coleman, K. S.; Davis, J. J.; Hanson, N.; Green, M. L. H. Chem. Commun. 2002, 366. (14) Chen, R. J.; Zhan, Y. G.; Wang, D. W.; Dai, H. J. J. Am. Chem. Soc. 2001, 123, 3838. (15) Choi, H. C.; Shim, M.; Bangsaruntip, S.; Dai, H. J.J. Am. Chem. Soc. 2002, 124, 9058. (16) Jiang, K. Y.; Eitan, A.; Schadler, L. S.; Ajayan, P. M.; Siegel, R. W.; Grobert, N.; Mayne, M.; Reyes-Reyes, M.; Terrones, H.; Terrones. M. Nano Lett. 2003, 3, 275. (17) O’Connell, M.; Boul, P.; Ericson, L. M.; Huffman, C.; Wang, Y.; Haroz, E.; Kuper, C.; Tour, J.; Ausman, K. D.; Smalley, R. E. Chem. Phys. Lett. 2001, 342, 265. (18) Carrillo, A.; Swartz, J. A.; Gamba, J. M.; Kane, R. S.; Chakrapani, N.; Wei, B. Q.; Ajayan, P. M. Nano Lett. 2003, 3, 1437.
10.1021/la049424n CCC: $27.50 © 2004 American Chemical Society Published on Web 08/18/2004
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Figure 1. Schematic illustration of the experimental procedure.
Figure 2. Stability of fMWCNTs dispersions prepared with different functionalization times.
Positively charged gold nanoparticles were anchored on negatively charged fMWCNTs prepared by polyelectrolyte layer-by-layer self-assembly. Positively charged gold nanoparticles were also attached on raw fMWCNTs because of negatively charged carboxylic acid groups produced by the chemical oxidation process followed by deprotonation on immersion in water. Experimental Section Multiwall carbon nanotubes (MWCNTs, CVD method, 95%) were purchased from Iljin Nanotech Inc. All following materials were obtained from Aldrich and used as received: poly(diallyldimethylammonium chloride) (PDAC, 20 wt % in water, Mw )
100000-200000), poly(sodium 4-styrenesulfonate) (PSS, Mw ) 70000), HAuCl4‚3H2O (99%), tetraoctylammonium bromide (98%), toluene (95%), sodium borohydrate (NaBH4) (99%), 4-(dimethylamino)pyridine (99%), sodium chloride (NaCl, 99+%), hydrochloric acid (HCl, 36%), sulfuric acid (H2SO4, 98%), and nitric acid (HNO3, 70%). 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 and dried under air. MWCNTs were chemically functionalized by ultrasonification in a mixture of sulfuric acid and nitric acid (3:1) for 8 h. fMWCNTs were washed with deionized water and separated by centrifuging three times. After being dried, fMWCNTs were dispersed in deionized water. PDAC and PSS were dissolved in deionized
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Table 1. Zeta Potentials of Used Materials at pH 7 zeta potential (mV) MWCNTs fMWCNTs PDAC coated fMWCNTs PDAC/PSS coated fMWCNTs gold nanoparticles
3.2 -55.2 60.91 -46.2 32.4
water at a concentration of 0.1 mg/1 mL, containing 0.05 M NaCl for layer-by-layer assembly. Positively charged gold nanoparticles were prepared by the phase transfer approach following the procedure of Gittins and colleagues.19 fMWCNTs were coated with PDAC by dispersing fMWCNTs in PDAC solutions (0.1 mg/ 1 mL) for 3 h with sonification. To obtain a PDAC/PSS bilayer coating, fMWCNTs coated with PDACs were mixed with PSS solutions for 1 h with sonification. Modified fMWCNTs were centrifuged three times with deionized water. Finally, positively charged nanoparticles were added in polyelectrolyte-coated fMWCNT dispersions. The experimental procedure is shown in Figure 1. Electrophoretic measurements were carried out with a zeta potential analyzer (Zetaplus, USA). Samples were ultrasonicated for 30 min before taking measurements. The zeta potential of prepared materials at pH 7 is summarized in Table 1. The amount of carboxylic acid groups on fMWCNTs was determined by potentiometric titration. fMWCNT dispersions (10 mg/ 40 mL) were titrated with 0.005 M NaOH solution. fMWCNTs and gold nanoparticles were characterized by highresolution transmission electron microscopy (HR-TEM, 2010F, JEOL).
Results and Discussion Figure 2 shows fMWCNT dispersions with different functionalization times in deionized water without surfactants. All samples were prepared by dispersing 10 mg of fMWCNTs in deionized water and 15 min of sonification. fMWCNTs prepared by less than 4 h of functionalization (sonification) settled within 30 min. fMWCNTs prepared with more than 4 h of functionalization were colloidally stable for at least 24 h in aqueous medium. The carboxylic groups of fMWCNTs were confirmed by FT-IR with stretching bands of carboxylic acid groups at 1710 cm-1 as shown in Figure 3a. Figure 3b shows the carboxylic group concentration of fMWCNTs in dependence of chemical reaction times. Due to the stability of fMWCNTs dispersions in deionized water, potentiometric titration could be performed with fMWCNTs that had been treated oxidatively for more than 4 h. The concentration of carboxylic acid groups on fMWCNTs increased from 4 × 10-4 to 1.1 × 10-3 mol/g with extending functionalization times from 4 to 8 h. After 7 h of chemical attack, the number of carboxyl groups did not increase any more. Figure 4a shows the HR-TEM image of fMWCNTs after 8 h of treatment. The typical lengths of fMWCNTs was about a few hundred nanometers. The typical diameters of fMWCNTs ranged from 10 to 20 nm. Catalyst nanoparticles could not be detected after treatment since the functionalization process also purifies the CNTs. In the next experiments positively charged gold nanoparticles (zeta potential: 32.4 mV at pH 7) were selfassembled on fMWCNTs without polyelectrolyte coatings by electrostatic interactions as shown in Figure 4b. fMWCNTs were negatively charged by deprotonation of the carboxylic acid groups produced during chemical oxidation. The zeta potential measurement for fMWCNTs yielded -55.2 mV at pH 7. (19) Gittins, D. I.; Caruso, F. Angew. Chem., Int. Ed. 2001, 40, 3001.
Figure 3. FTIR spectrum of 8 h functionalized fMWCNTs (a) and the amount of carboxylic acid groups (mol/g) on sMWCNTs depending on reaction time (b). All concentrations were measured by potentiometry using 0.005 M NaOH solution.
Positively charged gold nanoparticles were anchored on PDAC/PSS bilayer coated fMWCNTs (zeta potential: -46.2 mV at pH 7) by electrostatic interactions as shown in Figure 4c. Energy dispersive X-ray (EDX) characterization of the samples showed a weak sulfur peak for the PDAC/PSS bilayer coated fMWCNTs. The sulfur peak could be attributed to the sulfur of the PSS coating. However, it was difficult to detect the nitrogen peak from the PDAC coating because of the high concentration of and the overlapping with carbon and oxygen peaks. The gold peak was detected on the large spot that contains immobilized gold nanoparticles. Here again, no trace of catalyst particles was detected. For samples with PDAC-coated fMWCNTs (zeta potential: 60.91 mV at pH 7), no gold nanoparticle attachments could be observed. This is attributed to the electric double layer repulsion coming from the large zeta potential of both similarly charged surfaces at low salt concentrations. The density of positively charged gold nanoparticles self-assembled on raw fMWCNTs was lower than those anchored on PDAC/PSS bilayer coated fMWCNTs. The density difference may be caused by enlarged reactive sites (negatively charged sites) from polyelectrolyte coatings. These results suggested that positively charged gold nanoparticles can be used to detect defect (or charged) sites on CNTs.
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Figure 4. HR-TEM images of (a) fMWCNTs and (b) gold nanoparticles electrostatically attached to fMWCNTs. (c) HR-TEM images and corresponding EDX diagrams showing peaks for gold nanoparticles attached to PDAC/PSS coated fMWCNTs on copper grids.
Conclusions fMWCNTs were synthesized by chemical oxidation at room temperature while ultrasonicating. This treatment yielded oxygen-containing functional groups. The colloidal properties of fMWCNTs depending on functionalization times were studied. Increasing functionalization times enhanced the stability of fMWCNTs dispersions in aqueous media. fMWCNTs treated longer than 4 h were colloidally stable in aqueous media for at least 24 h. The concentration of carboxylic acid groups on fMWCNTs was measured quantitatively by potentiometric titration. The concentration of carboxylic acid groups on sMWCNTs increased from 4 × 10-4 to 1.1 × 10-3 mol/g with increasing functionalization time. Positively charged gold nano-
particles were immobilized on negatively charged fMWCNTs prepared by layer-by-layer self-assembly due to opposite charged surfaces. Positively charged gold nanoparticles also interacted with raw fMWCNTs because of carboxylic acid groups on fMWCNTs. It is suggested that positively charged gold nanoparticles could be used to detect negatively charged defect sites on CNTs. 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). LA049424N
