J. Phys. Chem. C 2008, 112, 18783–18786
18783
Carbon Nanotubes Decorated with both Gold Nanoparticles and Polythiophene Marcela M. Oliveira* and Aldo J. G. Zarbin* Department of Chemistry, UniVersidade Federal do Parana´ (UFPR), CP 19081, CEP 81531-990, Curitiba, PR, Brazil ReceiVed: June 14, 2008; ReVised Manuscript ReceiVed: September 25, 2008
This paper reports a simple, one-step procedure to synthesize carbon nanotubes (CNTs) decorated simultaneously with gold nanoparticles and polythiophene, without previous CNT surface functionalization. The method involves the reaction between an aqueous solution of HAuCl4 and thiophene in an aqueous dispersion of carbon nanotubes. CNTs act as seeds for the beginning of the reaction in a heterogeneous nucleation process in which both the Au nanoparticles and polythiophene grow directly on the CNTs surface. Introduction Studies on the synthesis, characterization, properties, and applications of carbon nanotubes (CNTs) have grown exponentially in recent years. The fascinating structural, electronic, mechanical, optical, and thermal properties of CNTs provide potential applications for these materials in a wide range of systems, including electronic and electrochemical devices, gas storage components, field emission sources, catalysis, nanocomposites, and many others.1,2 An exciting and well-developed application for CNTs is in the field of polymer composites.3,4 CNTs are adequate fillers for polymer composites due to their high aspect ratio, low density, strength, stiffness, and high conductivity. Various research groups have observed significantly enhanced conductivity, elastic modulus, and mechanical properties of several polymers upon mixing small portions of CNTs.4,5 In particular, nanocomposites composed of mixtures of CNTs and conducting polymers (such as polyaniline, polypyrrole, and polythiophene) have been developed for a wide range of applications, including active materials for photovoltaic devices.6,7 For example, we recently described a photovoltaic device based on a poly(3hexylthiophene)/CNT nanocomposite as active layer showing a 340% higher efficiency in the conversion of photons to electrons compared to a similar device made of pristine polymer.8 This enhanced efficiency was attributed to the suppression of the recombination of photogenerated electrons and holes (excitons). The dissociation of excitons occurs at the numerous polymer/nanotube interfaces resulting from the homogeneous dispersion of a high aspect ratio of electron acceptors (CNTs) in the active layer of the device (polymer). Different kinds of polymer/CNT nanocomposites are usually prepared by mixing a polymer solution with dispersed nanotubes, or by the in situ polymerization over dispersed CNTs.9 Another very interesting class of material is CNT/metal nanoparticle composites, especially the so-called CNTs decorated with metal nanoparticles.10-14 In these materials, the CNT surface acts as a support for the growth (or adsorption) of metal nanoparticles. The intimate contact between CNTs and supported metal nanoparticles produces synergistic effects, giving materials showing novel and interesting properties with a wide range of potential applications.10-14 Moreover, the one-dimensional as* To whom correspondence should be addressed. E-mail: marcela@ quimica.ufpr.br (M.M.O.);
[email protected] (A.J.G.Z.). Phone: +5541-33613176. Fax:+55-41-33613186.
Figure 1. Raman spectra (laser 632.8 nm) of the samples: (a) neat polythiophene; (b) CNT/Au-NP/PT; (c) CNT.
Figure 2. X-ray diffractogram of the sample CNT/Au-NP/PT.
sembly of nanoparticles is important in catalysis, nanoelectronic circuits, photonics, and several other fields.15-17 The chemical strategies employed in the preparation of carbon nanotubes decorated with metal nanoparticles are based on two approaches. The first approach involves growing nanoparticles directly upon nanotube surfaces, causing adequate precursors to be converted into the metal nanoparticles in the presence of CNTs.10,18 If the reaction is controlled, the growing nanoparticles can be deposited upon the CNT surface through van der Waals
10.1021/jp8052482 CCC: $40.75 2008 American Chemical Society Published on Web 11/08/2008
18784 J. Phys. Chem. C, Vol. 112, No. 48, 2008
Oliveira and Zarbin
Figure 3. TEM images of the sample CNT/Au-NP/PT. The inset in (e) is the diameter histogram to the gold nanoparticles. The arrows in (f) shows the borders of the CNT.
interactions. In the second approach, metal nanoparticles are previously prepared, modified with a suitable surface functional group, and then bonded to the CNTs by covalent or ionic linking.10,19 Both strategies require surface modifications of CNTs, which involve previous chemical treatment that may alter some of the CNT properties. In addition to CNT/conducting polymers or CNT/metal nanoparticle nanocomposites, the recent literature contains few reports about the preparation of nanocomposites formed by the mixture of these three components: carbon nanotubes, metal nanoparticles, and conducting polymers, in which several CNTs and metal nanoparticles are dispersed upon polymer bulk.20,21 However, to the best of our knowledge, there are no reports about a material composed of carbon nanotubes decorated with both metal nanoparticles and conducting polymer. This special type of nanocomposite should present very interesting properties due the intimate contact between the components, with potential applications in photovoltaic and electrochromic devices, catalysis, organic light-emitting diodes (OLEDs), and nanoelectronic components, etc. In this paper we report a simple, onestep procedure to synthesize carbon nanotubes decorated simultaneously with gold nanoparticles and polythiophene, without previous CNT surface functionalization. Experimental Methods HAuCl4 (Across), FeCl3 (Across), chloroform (Merck), and HCl 37% (w/w) solution (Merck) were used as received. Ferrocene (Fluka) was purified by sublimation, and thiophene (Merck) was distilled before the use. All aqueous solutions were prepared using Milli-Q deionized water. The multiwalled carbon nanotubes (MWCNTs) used in this work were obtained through the pyrolysis of pure ferrocene, according our previous report.22 Because of the preparation route, these MWCNTs have their cavities filled with nanowires of iron and/or iron oxide.22 The synthesis of the carbon nanotubes/gold nanoparticles/ polythiophene material was based on a procedure whereby the polymer and the metal nanoparticles are formed together in a CNT-containing medium. In a typical procedure, 6.9 × 10-3 g of multiwalled CNTs (MWCNTs) were mixed in 10.0 mL of an aqueous solution of HCl 0.5 mol L-1, and this mixture was
subjected to ultrasonication for 1 h. The dispersion was kept under magnetic stirring, and 4.4 mL of a 1.23 × 10-2 mol L-1 aqueous solution of HAuCl4 (corresponding to 5.4 × 10-5 mol), followed by 43.6 µL of freshly distilled thiophene (5.4 × 10-7 mol), were then added. The reaction was magnetically stirred for 24 h. After this, the suspended solid was separated by centrifugation, washed several times with deionized water, and dried at room atmosphere. The solid material obtained by this procedure is referred to herein as CNT/Au-NP/PT (carbon nanotubes/gold nanoparticles/polythiophene). Several different reactions were carried out employing the same procedure described above and varying the ratio of the precursors. These experiments led us to conclude that the key factor for controlling the morphology of the final material is the CNT/gold precursor/ monomer ratio. The pristine polythiophene was synthesized by chemical polymerization of thiophene from iron (III) chloride solution.23 In a typical procedure 0.30 g of FeCl3 (1.85 × 10-3 mol) was dissolved in 50.0 mL of chloroform, and the mixture was heated at the reflux temperature. To the mixture was added 250 µL of thiophene (3.12 × 10-4 mol), and the system was maintained under reflux for 24 h. After this the heating was turned off, and the system was cooled to the room temperature. The resulting suspended solid (polythiophene) was separated by centrifugation, washed several times with deionized water and ethanol, and dried at 40 °C. The Raman spectra were obtained in a Renishaw Raman Image spectrophotometer, coupled to an optical microscope that focuses the incident radiation down to an approximately 1 µm spot. A He-Ne laser (emitting at 632.8 nm) was used, with incidence potency of 0.2 mW over 3000-200 cm-1. X-ray diffraction measurements were done in a Shimadzu XRD-3A diffractometer using Cu KR radiation, with 40 KV and 40 mA, at 0.02° scan rate (in 2θ). Powder silicon reflections were used for 2θ calibration. TEM measurements were done in a JEOL 120 KV instrument or in a JEOL 300 KV JEM 3010 microscope (HRTEM mode). The samples for observation were suspended in water and allowed to settle for 15 min. Then a drop of the supernatant dispersion was placed onto a carbon film supported by a copper grid.
CNTs with both Gold Nanoparticles and Polythiophene
Figure 4. HRTEM images of the sample CNT/Au-NP/PT.
Results and Discussion Figure 1 shows the Raman spectra of the pristine polythiophene (Figure 1a), the resulting CNT/Au-NP/PT material (Figure 1b), and the pristine CNTs (Figure 1c). The spectrum in Figure 1a shows well-known polythiophene bands at 1495 (ring stretching antisymmetric CRdCβ) 1450 (ring stretching symmetric CRdCβ), 1420 (symmetric stretching CdC of radical cations) 1368 (ring symmetric stretching CβsCβ), 1218 (symmetric stretching CRsCR), 1155 (antisymmetric stretching CRsCR), 1054 (bending CβsH), and 693 (ring deformation CsSsC), which were attributed to the vibrational modes of the pristine polymer.24 The spectrum of the pristine CNTs (Figure 1c) is characteristic of multiwalled carbon nanotubes,25 with the so-called D band at 1332 cm-1, G band at 1579 cm-1, and the second order G′ band at 2663 cm-1. The spectrum of the CNT/Au-NP/PT (Figure 1b) clearly shows the superposition of the two aforementioned spectra, indicating that the polythiophene was formed through the reaction of HAuCl4 solution and thiophene and that the CNTs initially available in the reaction medium were incorporated into the resulting material. It is important to mention that we have collected several Raman
J. Phys. Chem. C, Vol. 112, No. 48, 2008 18785 spectra of the CNT/Au-NP/PT sample, focusing the laser spot in different regions and different batches of the sample, and the results were exactly the same that showed in the Figure 1b. Figure 2 shows the X-ray diffractograms of the CNT/AuNP/PT material. Besides the amorphous halo (characteristic of polythiophene) and the (002) peak located at 0.34 nm (attributed to the distance between the concentric layers in the MWCNTs),1 the CNT/Au-NP/PT diffractogram presents the well-defined peaks characteristics of gold with a fcc structure. Figure 3 shows TEM images of the samples. The pristine CNT morphology depicted in Figure 3a consists of MWCNTs with part of their cavities filled with iron or iron oxide (recognized by the darker contrasting regions). Parts b-f of Figure 3 present images of the CNT/Au-NP/PT samples. A uniform, well-distributed and nonagglomerated dispersion of gold nanoparticles is clearly visible on the CNT surfaces (average diameter of the gold nanoparticles ) 4.3 nm; see histogram in the inset of Figure 3c). However, a close examination of the images in Figure 3 revealed a shell of another type of material, attributed to the polythiophene, growing perfectly around the edges of the nanotubes. In fact, the gold nanoparticles were actually embedded in this polymeric shell, distributed randomly from the surface of the CNT to the surface of the polymeric shell. The more magnified image in Figure 3f (which shows only one decorated CNT) clearly reveals the spatial borders of the different materials: the high contrast wire visible in the center of the image is due to iron compounds encapsulated in the CNT cavity (the borders of the CNT are indicated by arrows). Around the CNT is a clear polymeric shell with a thickness of approximately 16 nm, and embedded in this structure are several gold nanoparticles decorating the CNTs. The structure of the CNT/Au-NP/PT material is more clearly visible in the high-resolution TEM images in Figure 4. The high crystallinity of the MWCNT, the MWCNT-filling, and the gold nanoparticles is evident from the lattice fringes of these structures, and the amorphous nature of the polymer shell around the CNT is perfectly clear. The interlayer spacing for the MWCNT was 0.34 nm, which was consistent with the values expected for the separation between the concentric walls of this material. The lattice fringes of several nanoparticles should be measured, allowing them to be attributed to planes of facecentered cubic gold. It is important to note that variations in the CNT/HAuCl4/ thiophene ratios produced inhomogeneous samples, showing not only CNTs decorated with both gold nanoparticles and polythiophene but also other morphologies such as isolated gold nanoparticles, isolated polythiophene, and polythiophene mixed with gold nanoparticles but without CNTs (data not shown). In the reaction involved in the synthesis of polymer and gold nanoparticles, the gold precursor (HAuCl4) acts as the oxidizing agent that triggers the oxidative polymerization of thiophene (which results in the formation of polymer) and is concomitantly reduced to metallic gold nanoparticles. When this reaction is conducted in an aqueous CNT dispersion, the CNTs act as seeds for the beginning of the reaction in a heterogeneous nucleation process. If the CNT/HAuCl4/thiophene ratio is adequate, homogeneous nucleation does not occur, and the entire reaction takes place only on the CNT surface, resulting in CNTs decorated with both polythiophene and gold nanoparticles. If the CNT/HAuCl4/thiophene ratio is far from the ideal, homogeneous nucleation may occur, and the resulting material is a mixture of the products obtained by the heterogeneous nucleation (CNTs decorated with both polythiophene and gold
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Figure 5. Schematic representation of the proposed model for the formation of CNTs decorated by both polythiophene and gold nanoparticles.
nanoparticles) and those resulting from the homogeneous nucleation (gold/polythiophene mixtures, for example). Figure 5 illustrates this proposed mechanism, which is consistent with the experimental results reported above. The HAuCl4/thiophene reaction conducted in the absence of CNTs leads to the formation of polythiophene/gold nanoparticle nanocomposites, and the ratio of the precursors is the determining factor of the size, shape, and distribution of the resulting gold nanoparticles (data to be published). Conclusions In summary, we described here a very simple and efficient route to prepare a novel type of nanocomposite in which carbon nanotubes are decorated with both metal nanoparticles and a conducting polymer. Another advantage of this method besides its simplicity is the fact that no CNT surface functionalization is required. However, the dispersion of the CNTs and the stability of this dispersion are crucial to the success of the route. Previous treatment of CNTs aimed at adding surface modifications to allow for the dispersion of large amounts of CNTs and keep the dispersion stable for longer periods should be interesting to increase the amount of material that can be produced. Our research group is currently engaged in studying the effects of previous chemical treatments of CNTs on the product obtained by this synthesis route. Unlike recently reported methods for decorating CNTs with inorganic nanoparticles, the driving force of our method is a heterogeneous nucleation process, in which the concentration of the reagents is kept below the limit of homogeneous nucleation. The material presented in this work has a novel morphology and is expected to possess several interesting properties. We believe that the method can be extended to other metals, other conducting polymers, and other types of CNTs (e.g., single-walled CNTs). In our laboratory, efforts are being aimed in this direction. Acknowledgment. Authors acknowledge the financial support by CNPq (Processes 550.394/2007-9 477.859/2006-2 and 304.344/2007-9) and CAPES/PROCAD and Brazilian Network on Carbon Nanotubes Research (MCT/CNPq). We also acknowledge CME-UFPR for the TEM images and LNLS-
National Synchrotron Light Laboratory, Brazil (project HRTEM 6777) for the HRTEM images. References and Notes (1) Terrones, M. Annu. ReV. Mater. Res. 2003, 33, 419. (2) Charlier, J.-C.; Blase´, X.; Roche, S. ReV. Mod. Phys. 2007, 79, 677. (3) Grunlan, J. C.; Liu, L.; Kim, Y. S. Nano Lett. 2006, 6, 911. (4) Wang, T.; Lei, C-H.; Dalton, A. B.; Creton, C.; Lin, Y.; Shiral Fernando, K. A.; Sun, Y.-P.; Manea, M.; Assua, J. M.; Keddie, J. L. AdV. Mater. 2006, 18, 2730. (5) Grunlan, J. C.; Kim, Y.-S.; Ziaee, S.; Wei, X.; Abdel-Magid, B.; Tao, K. Macromol. Mater. Eng. 2006, 291, 1035. (6) Panhuis, M.; Doherty, K. J.; Sainz, R.; Benito, A. M.; Maser, W. K. J. Phys. Chem. C 2008, 112, 1441. (7) Musumeci, A. W.; Silva, G. G.; Liu, J.-W.; Martens, W. N.; Waclawik, E. R. Polymer 2007, 48, 1667. (8) Canestraro, C. D.; Schnitzler, M. C.; Zarbin, A. J. G.; Luz, M. G. E.; Roman, L. S. Appl. Surf. Sci. 2006, 252, 5575. (9) Hatchett, D. W.; Josowicz, M. Chem. ReV. 2008, 108, 746. (10) Georgakilas, V.; Gournis, D.; Tzitzios, V.; Pasquato, L.; Guldi, D. M.; Prato, M. J. Mater. Chem. 2007, 17, 2679. (11) Correa-Duarte, M.; Liz-Marza´n, L. M. J. Mater. Chem. 2006, 16, 22. (12) Endo, M.; Kim, Y. A.; Ezaka, M.; Osada, K.; Yanagisawa, T.; Hayashi, T.; Terrones, M.; Dresselhaus, M. S. Nano Lett. 2003, 3, 723. (13) Day, T.; Unwin, P. R.; Macpherson, J. V. Nano Lett. 2007, 7, 51. (14) Ren, G.; Xing, Y. Nanotechnology 2006, 17, 5596. (15) Mubeen, S.; Zhang, T.; Yoo, B.; Deshusses, M. A.; Myung, N. V. J. Phys. Chem. C 2007, 111, 6321. (16) Rakhi, R. B.; Reddy, A. L. M.; Shaijumon, M. M.; Sethupathi, K.; Ramaprabhu, S. J. Nanopart. Res. 2008, 10, 179. (17) Zhang, R.; Wang, X. Chem. Mater. 2007, 19, 976. (18) Wang, Z.; Liu, Q.; Zhu, H.; Liu, H.; Chen, Y.; Yang, M. Carbon 2007, 45, 285. (19) Ellis, A. V.; Vijayamohanan, K.; Goswami, R.; Chakrapani, N.; Romanathan, L. S.; Ajayan, P. M.; Ramanath, G. Nano Lett 2003, 3, 279. (20) Feng, H.; Wang, H.; Zhang, Y.; Yan, B.; Shen, G.; Yu, R. Anal. Sci. 2007, 23, 235. (21) Reddy, K. R.; Lee, K. P.; Gopalan, A; Kim, M. S.; Showkat, A. M.; Nho, Y. C. J. Polym. Sci. A 2006, 44, 3355. (22) Schnitzler, M. C.; Oliveira, M. M.; Ugarte, D.; Zarbin, A. J. G. Chem. Phys. Lett. 2003, 381, 541. (23) Andersson, M. R.; Selse, D.; Berggren, M.; Ja¨rvinen, H.; Hjertberg, ¨ sterholm, J.-E. Macromolecules 1994, T.; Ingana¨s, O.; Wennersto¨m, O.; O 27, 6503. (24) Chen, F.; Shi, G.; Zhang, J.; Fu, M. Thin Solid Films 2003, 424, 283. (25) Sveningsson, M.; Morjan, R.-E.; Nerushev, O. A.; Ba¨ckstro¨m, J.; Campbell, E. E. B.; Rohmund, F. Appl. Phys. A: Mater. Sci. Process. 2001, 73, 409.
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