VOLUME 3, NUMBER 3, MARCH 2003 © Copyright 2003 by the American Chemical Society
Selective Attachment of Gold Nanoparticles to Nitrogen-Doped Carbon Nanotubes Kuiyang Jiang,* Ami Eitan, Linda S. Schadler, Pulickel M. Ajayan, and Richard W. Siegel Department of Materials Science and Engineering and Rensselaer Nanotechnology Center, Rensselaer Polytechnic Institute, Troy, New York 12180
Nicole Grobert Max-Planck-Institut fu¨ r Metallforschung, Heisenbergstrasse 3, D-70569 Stuttgart, Germany
Martine Mayne CEA Saclay, DRECAM, SPAM, F-91191 Gif Sur YVette, France
Marisol Reyes-Reyes, Humberto Terrones, and Mauricio Terrones AdVanced Materials Department, IPICyT, AV. Venustiano Carranza 2425-A, Colonia Bellas Lomas, 78210 San Luis Potosi, SLP, Mexico Received November 26, 2002; Revised Manuscript Received January 24, 2003
ABSTRACT Gold nanoparticles were selectively attached to chemically functionalized surface sites on nitrogen-doped carbon (CNx) nanotubes. A cationic polyelectrolyte was adsorbed on the surface of the nanotubes by electrostatic interaction between carboxyl groups on the chemically oxidized nanotube surface and polyelectrolyte chains. Negatively charged 10 nm gold nanoparticles from a gold colloid suspension were subsequently anchored to the surface of the nanotubes through the electrostatic interaction between the polyelectrolyte and the nanoparticles. This approach provides an efficient method to attach other nanostructures to carbon nanotubes and can be used as an illustrative detection of the functional groups on carbon nanotube surfaces.
Since their discovery in 1991,1 carbon nanotubes have been of great interest because of their unique structural, electrical, * Corresponding author. E-mail:
[email protected]. 10.1021/nl025914t CCC: $25.00 Published on Web 02/13/2003
© 2003 American Chemical Society
and mechanical properties. Their potential applications include nanodevices, quantum wires, ultrahigh-strength engineering fibers, sensors, and catalyst supports.2-6 To optimize the use of nanotubes in many of these applications,
Figure 1. FT-IR spectra of (a) raw CNx nanotubes and (b) acidtreated CNx nanotubes.
there is a need to attach functional groups to their surface and then assemble the nanotubes into structures or attach other nanostructures to the nanotubes.7-13 In the present study, a new and simple method to attach gold nanoparticles to the surface of carbon nanotubes is introduced. Nitrogendoped multiwalled carbon nanotubes (CNx MWNT) processed using a pyrolysis method were initially chemically modified with an H2SO4-HNO3 treatment. Subsequent treatment with a cationic polyelectrolyte and exposure to negatively charged gold nanoparticles showed that the functional groups were present at the ends of the nanotubes and along their length. Novel hybrid nanostructures with homogeneously distributed gold nanoparticles on nitrogendoped carbon nanotubes resulted. CNx MWNT were synthesized by pyrolyzing ferrocene/ melamine mixtures at 1050 °C in an Ar atmosphere.14-16 The as-produced material consists of carpet-like structures containing highly oriented nanotubes of uniform diameter (ca. 10-40 nm o.d.) and length (ca. 50-60 µm). Previously reported work on this material suggests that N-doping of the carbon nanotube lattice, introduced through catalytic decomposition, results in substitutional introduction of nitrogen into the nanotube lattice (2-7% as determined by electron energy loss spectroscopy).16 The electronic structure of these nanotubes has been modified to include electron donor states near the conduction band edge.14 For large N content, the N
coordination within the carbon lattice is of the pyridine-type (two bonded C atoms to each N atom), which is different from the direct substitution of 3-fold coordinated carbon, and results in a subtle rehybridization between the dopant atoms and the carbon lattice. High-resolution TEM also indicates a bamboo-like structure for the inner core of the nanotubes, but the surface morphology is close to that observed for CVD grown multiwalled carbon nanotubes, except for the chemical composition. The as-produced CNx MWNT were suspended in a concentrated sulfuric acid/nitric acid mixture (3:1 v/v) and sonicated in a sonic bath for 2 h. A nanotube mat was obtained after filtration and was then thoroughly washed with a dilute sodium hydroxide aqueous solution. In previous research on undoped MWNT,17 this acid-treatment was reported to result in carboxyl, carbonyl, hydroxyl, and sulfate groups on the carbon nanotubes, but their locations were unknown. In the present work, FT-IR studies of raw and acid-treated CNx MWNT (as shown in Figure 1) indicated that this acid-treatment also generated these four types of functional groups on the CNx MWNT: hydroxyl groups (3424 cm-1), carboxyl groups (1719 cm-1), carbonyl groups (1626 cm-1), and sulfate groups (1384 cm-1). These functional groups are hydrophilic and the nanotubes were dispersed easily in water. The nanotube suspension was then mixed with a cationic polyelectrolyte, poly(diallyldimethylammonium)chloride (PDADMAC), Mw ∼ 100,000-200,000, and NaCl aqueous solution for 30 min. PDADMAC was adsorbed to the surface of the nanotubes because of the electrostatic interaction between the carboxyl groups and the polyelectrolyte. After filtration and thorough washing with a NaCl aqueous solution and deionized water, the nanotubes were dispersed in water again, and mixed with a gold colloid (10 nm) for 30 min. The negatively charged gold nanoparticles were anchored to the surface of the nanotubes through the electrostatic interaction between the polyelectrolyte and the nanoparticles. After filtration and thorough washing with deionized water, TEM samples were prepared by dispersing the nanotubes in water. The complete scheme for the nanotube surface modification and the gold nanoparticle attachment on the nanotube surfaces is shown in Figure 2.
Figure 2. Schematic view of the process for anchoring gold nanoparticles to CNx nanotubes. 276
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Acknowledgment. This work was supported by the U.S. Army SBCCOM, Natick Soldier Center. We also thank CONACYT-Me´xico grants: W-8001-millennium initiative (H.T., M.T.), G-25851 (H.T., M.T.), 36365-E (H.T.), 37589-U (M.T.), and the EU; CNT-NET project contract No. G5RTCT2001-05026 (M.T.), for financial support. N.G. acknowledges the DFG grant Ru342/11-2 and the EU NANOCOMP project contract No. HPRN-CT-2000-00037 for financial support. References Figure 3. TEM photograph of gold nanoparticle-CNx nanotube hybrid structures.
TEM studies confirmed the success of the attachment of gold nanoparticles to CNx MWNT, as shown in Figure 3. Well-dispersed gold nanoparticles decorate the walls and ends of the nanotubes quite uniformly. Since the gold nanoparticles are attached through a well-defined scheme that involves interaction of the sites on the nanotube walls decorated by carboxyl functional groups, our decoration technique presents an excellent method for monitoring the presence of these chemical groups on the surfaces of nanotubes. It is also noted that the interaction between the gold nanoparticles and nanotubes is quite strong, because thorough washing does not remove them. As a control, acid-treated CNx MWNT (without PDADMAC treatment) were mixed with gold colloid, and almost no gold nanoparticles were found on the nanotubes. This indicates that PDADMAC plays a key role in the attachment; it acts as a bridge to connect gold nanoparticles with CNx nanotubes. In summary, gold nanoparticles were anchored to the surfaces of CNx nanotubes by a simple and versatile scheme of electrostatic adsorption. By choosing different kinds of polyelectrolytes, the surfaces of carbon nanotubes can be tailored to be negatively or positively charged, so many other nanoparticles (e.g., semiconductor nanocrystals, magnetic nanoparticles, etc.) can be selectively attached to the surfaces of nanotubes. In addition, this method of decorating nanotubes can be used to identify the location of functional groups. These nanoparticle-decorated nanotube heterostructures could be used in catalytic, electronic, optical, and magnetic applications.
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