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Fabrication of a Metal Nanoparticles and Polymer Nanofibers Composite Material by an in Situ Chemical Synthetic Route Kaushik Mallick,*,† Mike J. Witcomb,‡ Andy Dinsmore,† and Mike S. Scurrell† Molecular Sciences Institute, School of Chemistry, Electron Microscope Unit, University of the Witwatersrand, Private Bag 3, WITS 2050 South Africa Received February 28, 2005. In Final Form: May 3, 2005 This work demonstrated a facile route for the synthesis of poly(3,5-dimethyl aniline) nanofibers by polymerization of 3,5-dimethyl aniline using Pd-acetate as the oxidant. The reduction of Pd ion is accompanied by oxidative polymerization of 3,5-dimethyl aniline, leading to a metal-polymer composite material. Palladium nanoparticles (∼2 nm) are uniformly distributed throughout the polymer that makes the composite material a unique morphology. The resultant composite material was characterized by means of different techniques. IR and Raman spectra provide the information on the chemical structure of the polymer. TEM images show the morphology of the polymer and size of the metal particles.
Introduction Among the various types of nanomaterials, nanofibers have attracted considerable attention due to their intriguing chemical and physical properties.1 Syntheses of polyaniline nanostructures have been carried out both chemically and electrochemically by polymerizing the monomer with the aid of either a “hard”2 or “soft”3-7 template. Examples of hard templates include zeolite channels, track-etched polycarbonate, and anodized alumina. Soft templates, such as, surfactants,3 micelles,4 liquid crystals,5 thiolated cyclodextrins,6 and polyacids7 are reported to be capable of directing the growth of the polyaniline nanostructure with a diameter of less than 100 nm. Physical methods,8 including electrospinning and mechanical stretching, have also been used to make polyaniline nanofibers. Polyaniline is a conducting polymer that has been widely studied for electronic and optical application.9 The derivatives of polyaniline have attracted a growing scientific attention because their chemical properties are similar to that of polyaniline and, with respect to the parent polymer, they exhibit better solubility in common organic solvents which facilitates easier processability of these materials.10 The incorporation of metal nanoparticles into conducting polymers provides enhanced performance for both the * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +27 (11) 717 6749. † Molecular Sciences Institute, School of Chemistry. ‡ Electron Microscope Unit. (1) (a) Wang, J. F.; Gudiksen, M. S.; Duan, X. F.; Cui, Y.; Lieber, C. M. Science 2001, 293, 1455-1457. (b) Holmes, J. D.; Johnston, K. P.; Doty, R. C.; Korgel, B. A. Science 2000, 287, 1471-1473. (2) (a) Wu, C. G.; Bein, T. Science 1994, 264, 1757-1759. (b) Martin, C. R. Chem. Mater. 1996, 8, 1739-1746. (c) Wang, C. W.; Wang, Z.; Li, M. K.; Li, H. L. Chem. Phys. Lett. 2001, 341, 431-434. (3) Michaelson, J. C.; McEvoy, A. J. Chem. Commun. 1994, 79-80. (4) Yang, Y. S.; Wan, M. X. J. Mater. Chem. 2002, 12, 897-901. (5) Huang, L. M.; Wang, Z. B.; Wang, H. T.; Cheng, C. X.; Mitra, A.; Yan, Y. X. J. Mater. Chem. 2002, 12, 388-391. (6) Choi, S. J.; Park, S. M. Adv. Mater. 2000, 12, 1547-1549. (7) Liu, J. M.; Yang, S. C. Chem. Commun. 1991, 1529-1531. (8) (a) Reneker, D. H.; Chun, I. Nanotechnology 1996, 7, 216-223. (b) He, H. X.; Li, C. Z.; Tao, N. J. Appl. Phys. Lett. 2001, 78, 811-813. (9) Skotheim, T. A.; Elsenbaumer, R. L.; Reynolds, J. R. Handbook of Conducting Polymers; Mercel Dekker: New York, 1997. (10) Hasik, M.; Wenda, E.; Paluszkiewicz, C.; Bernasik, A.; Camra, J. Synth. Met. 2004, 143, 341-49.
“host” and the “guest”, and this can lead to interesting physical properties and important potential applications11 such as electrodes of batteries, display devices, immunodiagnostic assay, etc. To exploit the full potential of the technological application of the composite material, it is important to characterize the nature of the association between the different components. Although the incorporation of gold,12 copper,13 platinum,14 and palladium10,15 nanoparticles in the conducting polymers has been reported by using chemical or electrochemical techniques, the synthesis of conducting polymer-metal nanoparticle composite materials having a nanofiberlike morphology has not yet been undertaken. Here we report on a novel synthetic protocol for the fabrication of a poly(3,5-dimethyl aniline) nanofiber and palladium nanoparticle (polymer-metal) composite material in which nanoparticles are highly dispersed in a polymer fiber matrix. In this paper, 3,5-dimethyl aniline and Pd-acetate were used as the precursors of the polymer fibers and Pd particles, respectively. During the reaction, 3,5-dimethyl aniline undergoes oxidation and forms poly(3,5-dimethyl aniline), whereas the reduction of Pd-acetate results in the formation of Pd nanoparticles, the combining of the two processes ultimately forming a “polymer-metal” composite material. The major advantage of the in situ protocol is that both the polymer and the nanosized metal particles are produced simultaneously, and this is expected to yield a highly intimate contact between them. (11) Gangopadhyay, R.; De, A. Chem. Mater. 2000, 12, 608-622. (12) (a) Dai, X.; Tan, Y.; Xu, J. Langmuir 2002, 18, 9010-9016. (b) Kinyanjui, J. M.; Hatchett, D. W.; Smith, J. A.; Josowicz, M. Chem. Mater. 2004, 16, 3390-3398. (c) Sarma, T. K.; Chattopadhyay, A. Langmuir 2004, 20, 4733-4737. (d) Mallick, K.; Witcomb, M. J.; Dinsmore, A.; Scurrell, M. S. Macromol. Rapid Commun. 2005, 26, 232. (e) Liu, Y.-C.; Chuang, T. C. J. Phys. Chem. B 2003, 107, 12383-12386. (f) Wang, Y.; Liu, Z.; Han, B.; Sun, Z.; Huang, Y.; Yang, G. Langmuir 2005, 21, 833. (13) (a) Ma, Z. H.; Tan, K. L.; Kang, E. T. Synth. Met. 2000, 14, 17-25. (b) Vijaya Kumar, R.; Mastai, Y.; Diamant, Y.; Gedanken, A. J. Mater. Chem. 2001, 11, 1209-1213. (14) (a) Yoda, S.; Hasegawa, A.; Suda, H.; Uchimaru, Y.; Haraya, K.; Tsuji, T.; Otake, K. Chem. Mater. 2004, 16, 2363-2368. (b) O’Mullane, A. P.; Dale, S. E.; Macpherson, J. V.; Unwin, P. R. Chem. Commun. 2004, 1606-1607. (15) (a) Huang, S. W.; Neoh, K. G.; Kang, E. T.; Tan, K. L. J. Mater. Chem. 1998, 8, 1743-1748. (b) Hasik, M.; Wenda, E.; Bernasik, A.; Kowalski, K.; Sobczak, J. W.; Sobczak, E.; Bielan˜ska, E. Polymer 2003, 44, 7809-7819.
10.1021/la050534j CCC: $30.25 © 2005 American Chemical Society Published on Web 07/12/2005
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Experimental Section Materials. Unless otherwise indicated, all chemicals used were of analytical grade and were used as received. Toluene was from Merck, and Pd-acetate was from Next Chemica. A stock solution of Pd-acetate (10-2 mol dm-3) was prepared in toluene. 3,5-Dimethyl aniline was purchased form BDH (London) and distilled before use it. Procedure. In a typical experiment, 0.06 g of 3,5-dimethyl aniline was dissolved in 10 mL of toluene in a 25 mL conical flask. To the stock solution was added slowly drop by drop 0.5 mL of Pd-acetate. Initially, a yellow colorization was observed followed by a yellow cloudy precipitation. The entire procedure was carried out at room temperature and under continuous stirring conditions. Subsequently, the solution was allowed to stand at rest for 5 min during which time all the precipitated material deposited at the bottom of the flask. TEM specimens were prepared by pipetting 2µL of the deposited material onto a lacey carbon coated copper grid. The rest of the solution was then filtered, and the solid mass was used for both IR and Raman analyses. Instrumentation. Transmission electron microscopy (TEM) studies of the particles were carried out at an accelerated voltage of 200 kV using a Philips CM200 TEM equipped with a LaB6 source. An ultrathin windowed energy-dispersive X-ray spectrometer (EDS) and a Gatan Imaging Filter (GIF) attached to the TEM were used to determine the chemical composition of the samples. Raman spectra were acquired using the green (514.5 nm) line of an argon ion laser as the excitation source. Light dispersion was carried out using the single spectrograph stage of a JobinYvon T64000 Raman spectrometer. Power at the sample was kept very low (0.73mW), and the laser beam diameter at the sample was ∼1 µm. Infrared spectra, in the region 4000-700 cm-1, were obtained from a Perkin-Elmer 2000 FT-IR spectrometer operating at a resolution of 4 cm-1. The sample was deposited in the form of thin film on a sodium chloride disk.
Figure 1. (A) TEM image showing an example of the various sizes of polymer nanofibers produced; (B) TEM image of a single polymer fiber having a diameter ca. 300 nm. The diffraction pattern, inset in Figure 1b, reveals diffuse scattering from the amorphous polymer and diffraction rings from the Pd nanoparticles.
Result and Discussion Transmission Electron Microscopy Image of the Polymer Composite. The TEM images in Figure 1 illustrate the fiberlike morphology of the polymer. Figure 1A is an example of the large population of polymer fibers with different sizes, whereas Figure 1B images part of a single fiber, which is about 300 nm in diameter. The inset in Figure 1B is the diffraction pattern from the fiber which shows both diffuse scattering from the amorphous polymer and diffraction rings that dark field imaging confirms are from the Pd nanoparticles. Figure 2, panels A and B, shows TEM images of the surface morphology and internal microstructure of the polymer. It is clear that the surface is not smooth. Both on the rough surfaces and in the interior of the polymer as shown by these and stereo images, there are highly distributed dark regions of diameter about 2 nm. The EDS spectrum (Figure 3B) obtained from the area of the composite shown in Figure 3A as well as when the electron beam was positioned on a single particle indicates the presence of palladium. In addition to this, EELS mapping for the Pd distribution has provided unambiguous confirmation that the dark spots are palladium. This is most clearly illustrated in Figure 4 where a fine strand of polymer composite was isolated such that the particles were not overlapping. Figure 4A is a zero-loss image of the strand. This is an energy filtered image such that it is only derived from electrons which have retained the beam energy when passing through a thin sample. This image contains no useful microanalytical information. In contrast, Figure 4B is a palladium map from the same region. This Pd jump-ratio image was obtained by dividing the Pd-N2,3 postedge loss image (an image derived from the signal from an energy window placed just above the ionization
Figure 2. (A) and (B) show the surface morphology and internal microstructure of the fiber. The ∼2 nm sized dark spots are the palladium nanoparticles, which show a high dispersion throughout the polymer.
energy of Pd N shell X-rays, the signal being the sum of the background signal at this position which contains no microanalytical information, and the signal resulting from
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Figure 3. (B) EDS spectrum from the area shown in (A). The palladium peaks are derived from the nanoparticles, whereas the copper peak comes from the TEM copper mesh support grid.
electrons that lost energy by generating Pd N shell X-rays) by the preedge image (an image derived from the signal from an energy window placed just before the ionization energy of Pd N shell X-rays, this image being a background image). From comparing the zero-loss and jump-ratio image, it can be seen that the dark areas in Figure 4A correspond to all of the Pd particles mapped in Figure 4B. Infrared Spectra of the Polymer. The IR spectrum, Figure 5, reveals the presence of the different species, which are involved in the fabrication of the composite polymer. The bands at 3439, 3356, and 3222 cm-1 correspond to the N-H stretching vibration, whereas the bands at 3022, 2913, and 2855 cm-1 result from the aromatic C-H stretching vibration. The peaks at 1602 and 1485 cm-1 are due to the stretching deformation of the quinone and the benzene rings, respectively. The 1318 cm-1 band is assigned to C-N stretching in a secondary aromatic amine, whereas the peaks at 1025 and 1170 cm-1 represent the aromatic C-H in-plane bending modes. The out-of-plane deformation of C-H in the 1,4-disubstituted benzene ring is located at 833 cm-1. Raman Spectra Analysis of the Polymer. The Raman spectrum shows the presence of the charge carrier species, which are responsible for the conducting nature of the polymer. In a typical Raman spectra of the synthesized composite polymer material (Figure 6), the bands between 1100 and 1700 cm-1 are sensitive to the poly(3,5-dimethyl aniline) nanofiber oxidation state. The band at 1160 cm-1 corresponds to the CH benzene deformation mode indicating the presence of quinoid rings. A broad band between about 1445 and 1475 cm-1, derived from two overlapping peaks, corresponds to the CdN
Figure 4. (A) Zero-loss image of a polymer composite strand about 10 nm in diameter; (B) Pd-N2,3 edge jump-ratio image of the area shown in 4A. All of the dark regions in 4A can clearly be identified as palladium nanoparticles.
Figure 5. IR spectrum of the composite material. The peaks at 1602 and 1485 cm-1 are due to the stretching deformation of quinone and benzene rings, respectively.
stretching of the quinoid units. A band between 1200 and 1250 cm-1, with a peak at 1220 cm-1, can be assigned to the C-N stretching mode of the polaronic units. Three
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Scheme 1. Mechanism for the Radical Cation Coupling of 3,5-dimethyl Aniline Leading to the Formation of Poly(3,5-dimethyl aniline).
peaks at 1300, 1325, and 1375 cm-1 correspond to the C-N•+ stretching modes of the delocalized polaronic charge carriers,16 the high intensity of these peaks indicating the richness of the sample in these charge carrier species. The C-C stretching of the benzenoid ring was evidenced by the peak at 1600 cm-1. Mechanism. Poly(3,5-dimethyl aniline) is a polyaniline derivative that contains the -CH3 group in positions 3 and 5 with respect to the nitrogen atoms. The in situ co-
formation of palladium nanoparticles and poly(3,5-dimethyl aniline) suggests the following formation mechanism and sequence. The presence of an electron-donating group (-CH3) in the aniline facilitates the relay of electrons through the -N••H2 group to form an electrostatic bond with the Pd2+ cations. During the electrostatic attachment, [(CH3)2Ph-N•+H2] like species are formed which at acidic condition undergo polymerization, an oxidation process. Each step of the polymerization process is associated with the release of one electron (scheme 1), which is utilized to reduce the palladium ions to form palladium atoms. These atoms subsequently coalesce to form clusters, which then are stabilized within the growing polymer. The sequence of steps occurring at the nanoscale level suggests that an intimate contact between the metal and the polymer is likely to develop. Conclusion In this work, we demonstrate a facile synthesis route to prepare poly(3,5-dimethyl aniline) nanofibers using Pd-acetate as oxidant. During the reaction, palladium ions are reduced by an in situ process and form Pd nanoparticles. The generated Pd nanoparticles are uniformly dispersed in the polymer with a uniform size distribution (∼2 nm). A single step, one-pot method such as that now described for the preparation of “polymer-metal” intimate composite material (ICM) could well be of importance in the field of sensors, catalysts, and electronics with improved performance.
Figure 6. Raman spectra of the poly(3,5-dimethyl aniline)metal composite material. The peaks at 1300, 1325, and 1375 cm-1 correspond to the C-N•+ stretching modes of the delocalized polaronic charge carriers, whereas their high intensity indicates that the sample is rich in these charge carrier species. It is these species that are responsible for the conductivity of the polymer.
Acknowledgment. K.M. is grateful to the University of the Witwatersrand for the award of a Postdoctoral fellowship. LA050534J (16) Tagowska, M.; Palys, B.; Jackowska, K. Synth. Met. 2004, 143, 223-229.
