Nanostructured

J. Phys. Chem. B 2006, 110, 14623-14626

14623

Electromagnetic Functionalized and Core-Shell Micro/Nanostructured Polypyrrole Composites Xin Li,† Meixiang Wan,*,† Yen Wei,‡ Jiaoyan Shen,§ and Zhaojia Chen§ Beijing National Laboratory for Molecular Sciences, Organic Solid Laboratory, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China, The Center for AdVanced Polymers and Materials Chemistry, Department of Chemistry, Drexel UniVersity, Philadelphia, PennsylVania 19104, and Laboratory of Extreme Conditions Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, People’s Republic of China ReceiVed: April 15, 2006; In Final Form: June 5, 2006

Core-shell micro/nanostructured and electromagnetic functionalized polypyrrole (PPy) composites were prepared by a self-assembly process associated with the template method in the presence of p-toluenesulfonate acid (p-TSA) as the dopant, in which the spherical hydroxyl iron (Fe[OH], 0.5-5 µm in diameter) functioned not only as the hard template, but also as the “core” of the micro/nanostructure, and the self-assembled PPyp-TSA nanofibers (20-30 nm in diameter) acted as the “shell” (50-100 nm in thickness) of the microspheres. We found that the core-shell micro/nanostructures exhibit controllable electromagnetic properties by adjusting the mass ratio of Fe[OH] to pyrrole monomer. The micelle model was proposed to interpret the self-assembly of the core-shell micro/nanostructured composites.

1. Introduction Nowadays multifunctionalized micro/nanostructures of conducting polymers have received great attention because of their unique properties and technological applications in electrical, optical, and magnetic materials and devices.1 Among those multifunctionalized micro/nanostructures, electromagnetic functionalized micro/nanostructures of conducting polymers are of special interesting due to their potential applications in electromagnetic interference (EMI) shielding and microwaveabsorbing materials.2 Recently, a few articles on the electromagnetic functional micro/nanostructures of conducting polymers have been published.3 For instance, Wan and Zhang4 reported electromagnetic functional nanotubes of polyaniline (PANI) containing magnetic Fe3O4 nanoparticles (d ) 10 nm) by a template-free method associated with a co-structured method. The co-structured method means that the magnetic nanoparticles in the reaction solution co-structure with monomer polymerization to form nanostructured composites containing magnetic material through a self-assembly process. They also reported the conductive PANI-coated γ-Fe2O3 nanoneedle composite by using γ-Fe2O3 nanoneedles as the template.5 These nanostructured PANI composites exhibit electromagnetic properties; however, their conductivity (∼10-2 S/cm) at room temperature is poor. Therefore, improving the electrical properties of the electromagnetic nanostructured composites of conducting polymers is required to satisfy the requirements of technology application. Previous reports have demonstrated that conducting polymers are contributed to their electrical properties, while their magnetic properties are induced by magnetic materials in the composites.6 Thus, molecular structure of the electrical and magnetic * Corresponding author. Telephone: +86-10-62565821. Fax: +86-1062559373. E-mail: [email protected]. † Institute of Chemistry, Chinese Academy of Sciences. ‡ Drexel University. § Institute of Physics, Chinese Academy of Sciences.

Figure 1. SEM images of (a) pure Fe[OH] and (b) PPy-p-TSA nanotube prepared by the self-assembly process in the absence of the Fe[OH] magnet.

Figure 2. Typical SEM (a and b, the inset of b is SEM of a single sphere surface at high multiple) and TEM (c) images of PPy-p-TSA/ Fe[OH] composite at [Fe[OH]]/[Py] ) 0.5:1.

component in micro/nanostructured composites will affect the electromagnetic properties of the micro/nanostructured composites. Among those conducting polymers, polypyrrole (PPy) stands out for its high conductivity, stability in air, and promising applications.7 In particular, the room conductivity of PPy doped with p-toluenesulfonate acid (p-TSA) by an electrochemical method could reach 200 S/cm.8 Thus, it is interesting to select PPy as a conducting polymer to prepare electromagnetic micro/ nanostructures via a self-assembly process. In this article, novel core-shell micro/nanostructured and electromagnetic functionalized composites of PPy were prepared by a self-assembly process in the presence of p-TSA as the dopant, in which the spherical hydroxyl iron (Fe[OH], 0.5-5 µm in diameter) acted as the “core” and the self-assembled PPy

10.1021/jp062339z CCC: $33.50 © 2006 American Chemical Society Published on Web 07/13/2006

14624 J. Phys. Chem. B, Vol. 110, No. 30, 2006

Li et al.

Figure 3. FTIR spectrum. (a) PPy-p-TSA nanotubes and (b) PPy-pTSA/Fe[OH] composite at [Fe[OH]]/[Py] ) 1:1.

Figure 4. XRD spectrums. (a) Fe[OH], (b) PPy-p-TSA nanotubes, and (c) PPy-p-TSA/Fe[OH] composite at [Fe[OH]]/[Py] ) 1:1.

nanofibers (20-30 nm in diameter) served as the “shell” (50100 nm in thickness). We found that the micro/nanostructured composites exhibit high conductivity (∼σmax ) 50.6 S/cm) and superparamagnetic properties. The effect of the mass ratio of Fe[OH] to pyrrole monomer (written as [Fe[OH]]/[Py]) on the electromagnetic properties was investigated, and the selfassembly mechanism of the micro/nanostructures is discussed.

mL of FeCl3 aqueous solution (1.0 M) was then added to the mixed solution, and the dripping time was kept as long as 3 h. Next, the mixture was allowed to react for another 7 h with mechanical stirring under 0-5 °C. The product was washed with deionized water, ethanol, and then ethyl ether several times, and finally dried at 40-60 °C under vacuum for 24 h to obtain a dark-green powder of PPy-p-TSA/Fe[OH] composite. The PPy-p-TSA nanotubes were also prepared by the same process in the absence of Fe[OH]. To investigate the effect of Fe[OH] content on the structure, morphology, and electromagnetic properties, Fe[OH] in the range of 0.05-0.6 g was used to prepare the composites by a same synthesis procedure. 2.3. Measurements and Instruments. Field emitting scanning electron microscope (SEM, JSM-6700F) and transmission electron microscope (TEM, JEM-200CX) were used to measure the morphology of the PPy-p-TSA nanotubes and micro/ nanostructured composite. FTIR spectra (KBr pellet, PerkinElmer System) and X-ray diffraction (XRD, RINT2000 wideangle goniometer) were used to characterize the micro/ nanostructure. The magnetization curves at different magnetic field (-20 to 20 kOe) were measured at room temperature by an extracting sample magnetometer (Neel Laboratory CF-1).

2. Experiment 2.1. Materials. Pyrrole monomer (A. R., Beijing Mashi Fine Chem. Co.) was distilled under reduced pressure. Hydroxyl iron powder (Fe[OH], Nanjing Institute of Technology), ferric chloride hexahydrate (FeCl3‚6H2O, Tianjin Shuangchuan Chem. Reagent Factory), p-toluenesulfonic acid (C6H4CH3SO3H, pTSA, Beijing Chem. Co.) and other reagents were all A. R. grade and used as received without further treatment. 2.2. Synthesis Procedure. p-TSA (6.6 × 10-3 mol) was dissolved in 10.0 mL of deionized water, and then pyrrole (7.0 × 10-3 mol) and the precalculated amount of Fe[OH] were added to the above mixture. After the mixture was mechanically stirred for 20 min at 0-5 °C using the ice-water bath, 16.3

SCHEME 1: Schematic Diagram of the Formation Mechanism for PPy-p-TSA Nanostructures and PPy-p-TSA/Fe[OH] Compositea

a (a) Synthesis of micro/nanostructured PPy composites by Fe[OH] acting as the hard template through a self-assembly process. (b) Synthesis of the PPy nanotube by a self-assembly process.

Core-Shell Micro/Nanostructured PPy Composites

Figure 5. Effect of the [Fe[OH]]/[Py] ratios on the conductivity of the PPy-p-TSA/Fe[OH] composites.

The conductivity at room temperature was measured by a fourprobe method with a Keithley 196 SYSTEM DMM digital multimeter and an ADVANTEST R6142 programmable DC voltage/current generator as the current source. 3. Results and Discussion 3.1. Morphology and Self-Assembly Mechanism. Figure 1 gives typical SEM images of pure Fe[OH] and PPy-p-TSA prepared by a template-free method in the absence of Fe[OH]. It is clear that the Fe[OH] is a microsphere in shape and 0.5-5 µm in diameter, whereas PPy-p-TSA is shaped as nanotubes and 200-800 nm in diameter, which is consistent with our previous report.9 Typical SEM and TEM images of PPy-p-TSA/Fe[OH] composites prepared by a self-assembly process associated with Fe[OH] as the template are shown in Figure 2. It shows that the PPy-p-TSA/Fe[OH] has a unique core-shell micro/nanostructure, in which the spherical Fe[OH] magnet acts as the “core”, while the self-assembled PPy-p-TSA nanofibers with 20-30 nm in diameter formed on the surface of the Fe[OH] magnet serves as the “shell” (50-100 nm) of the composite microspheres.

J. Phys. Chem. B, Vol. 110, No. 30, 2006 14625 The formation mechanism of the PPy-p-TSA nanotubes and PPy-p-TSA/Fe[OH] micro/nanostructures is shown in Scheme 1. In our previous reports,9b we proposed that the micelles composed of p-TSA served as the “soft-templates” for forming PPy-p-TSA nanostructures in the absence of the Fe[OH] magnet due to the surfactant function of the -HSO3 group in p-TSA molecular. The polymerization took place at the micelle/water interface because of hydrophilic FeCl3 as the oxidant and growth of the PPy-p-TSA nanostructures is controlled by the polymerization and elongation process.10 The above-described selfassembly process for the PPy-p-TSA nanotubes is illustrated in Scheme 1b. When the Fe[OH] magnet is used, it serves as the hard template responsible for the formation of the composite microspheres as shown in Scheme 1a. Obviously, the micelles formed in the reaction solution can be adsorbed on the surface of the Fe[OH] microspheres through a static interaction. Once FeCl3 was added, the polymerization of PPy-p-TSA nanofibers on the surface of the Fe[OH] spheres took place according to the above-mentioned self-assembly process. Therefore, special core-shell and micro/nanostructured PPy-p-TSA/Fe[OH] composites are expected, where Fe[OH] magnetic microsphere and the self-assembled PPy-p-TSA nanofibers act as the “core” and “shell” of the micro/ nanostructured composites, respectively. On the basis of the above discussion, we expected a critical mass ratio of Fe[OH] to pyrrole monomer represented by [Fe[OH]]/[Py] for the formation of the special micro/nanostructures and the optimum [Fe[OH]]/[Py] ratios are estimated to be in the range of 0.5-1.5. 3.2. Structure Characterization. FTIR spectra of the PPyp-TSA nanotubes and PPy-p-TSA/Fe[OH] micro/nanostructure composites are shown in Figure 3. It is clear that the two spectra are similar to each other, indicating the main polymer chains of the PPy-p-TSA/Fe[OH] composites are similar to that of PPyp-TSA nanotubes. For example, the CdC, C-N stretching vibration peak at 1550 and 1453 cm-1, the C-H in-plane vibration at 1313 and 1164 cm-1, the C-H in-plane bending at 1041 cm-1, and the ring deformation at 894 cm-1 can all be observed.11 In addition, the strong absorption at 3418 cm-1 and weak absorption at 2919 cm-1, assigned as N-H and C-H stretching, respectively, are also observed.12 Furthermore, the

Figure 6. Magnetization versus the applied field at T ) 300 K for Fe[OH] (see inset). (a) PPy-p-TSA nanotubes and (b) PPy-p-TSA/Fe[OH] composites at [Fe[OH]]/[Py] ) 0.5:1 and (c) PPy-p-TSA/Fe[OH] composite at [Fe[OH]]/[Py] ) 2:1 and (d) PPy-p-TSA/Fe[OH] composites at [Fe[OH]]/[Py] ) 1:1.

14626 J. Phys. Chem. B, Vol. 110, No. 30, 2006 weak band at 1731 cm-1 is attributed to the carbonyl group, which indicates that PPy is somewhat over-oxidized during the growth process.13 However, some spectroscopic differences are also observed, indicating the interaction of iron in Fe[OH] with nitrogen in the PPy polymer chain.14 Figure 4 gives the XRD spectra of pure Fe[OH], PPy-p-TSA nanotubes, and PPy-p-TSA/Fe[OH] composite at [Fe[OH]]/[Py] ) 1:1. It is clear that the PPy-p-TSA/Fe[OH] composite combined both the diffraction peaks from Fe[OH] at 2θ ) 44.52° (sharp peak, d ) 2.03 Å) and 2θ ) 65.26° (weak peak, d ) 1.43 Å) and the broad peak from PPy15 with somewhat of a change in intensity and position. This indicates that there is no obvious chemical interaction between Fe[OH] and PPy-pTSA in the composites. 3.3. Electromagnetic Properties. We found that the electrical and magnetic properties of the PPy-p-TSA/Fe[OH] micro/ nanostructures are strongly affected by the [Fe[OH]]/[Py] ratios. As shown in Figure 5, the maximum conductivity of the composites at [Fe[OH]]/[Py] ) 1:1 is as high as 53.6 S/cm, which is increased by more than two times compared with that of PPy-p-TSA nanotubes and is consistent with our previous results.9 Here, it is interesting to note that the variation tendency of the conductivity of the PPy-p-TSA/Fe[OH] micro/nanostructures versus the content of the magnetic materials shown in Figure 5 is significantly different from our previous results obtained from PANI-Fe3O4 nanotubes4 or PANI-γ-Fe2O35 nanoneedles, in which the conductivity of the composites decreases with the increase of the content of magnetic nanoparticles in the composites due to the insulating property of the magnetic particles. As mentioned before, the conductive PPyp-TSA nanofibers formed on the Fe[OH] magnetic spheres (Figure 2) result in decrease of the intercontact resistance between composites that leads to enhancement of the conductivity at a lower [Fe[OH]]/[Py] ratio. On the other hand, the conductivity will be decreased due to the large quantum of insulating magnet in the composites disrupting the PPy polymer chain. Furthermore, when pure PPy-p-TSA nanotube and Fe[OH] microsphere are only mixed together, the conductivity of the mixture decreases with the increase of Fe[OH] content in the mixture and no maximum conductivity is observed, which indicate that interaction between the components undoubtedly exists during the in situ polymerization process. As shown in Figure 6, Fe[OH] magnetic spheres are superparamagnetic, and the saturated magnetization (Ms), the remnant magnetization (Mr), and coercive force (Hc) are estimated to be 180.4 emu/g, 0, and 0, respectively. On the other hand, the PPyp-TSA nanotubes are antimagnetic. Obviously, the superparamagnetic behavior observed in PPy-p-TSA/Fe[OH] composites is induced by the superparamagnetic Fe[OH] spheres. Moreover, the Ms of the PPy-p-TSA/Fe[OH] composite is also dependent on the [Fe[OH]]/[Py], and the maximum Ms of the PPy-p-TSA/ Fe[OH] composite is observed at [Fe[OH]]/[Py] ) 1:1, which is consistent with the phenomena in conductivity. To summarize, electrical and magnetic measurements show that the core-shell micro/nanostructures of the PPy-p-TSA/Fe[OH] composite exhibit controllable electromagnetic properties when only the [Fe[OH]]/[Py] ratio is adjusted. 4. Conclusions We proposed a simple route to prepare core-shell structured and electromagnetic functionalized PPy-p-TSA/Fe[OH] micro/ nanostructures with high conductivity (σmax ) 53.6 S/cm), in which the spherical hydroxyl iron functioned as the “core” of the micro/nanostructure, and the self-assembled PPy-p-TSA nanofibers acted as the “shell”. In particularly, the electromag-

Li et al. netic properties can be controlled by changing the mass ratio of Fe[OH] to pyrrole monomer. The electrical properties of the composite micro/nanostructures result from the conductive PPy nanotubes, whereas the superparamagnetic behavior is induced by the Fe[OH] spherical magnet. Moreover, highly conductive PPy-p-TSA nanofibers formed on the Fe[OH] spheres reduced the intercontact resistance that leads to a maximum conductivity in the curve of the conductivity versus Fe[OH] content. Acknowledgment. This project was supported by the National Nature Sciences Foundation of China (No. 50533030), Grant of Oversea Outstanding Scientists of the Chinese Academy of Sciences, and The Cooperation Lab of National Center for Nanoscience and Technology. References and Notes (1) (a) Wan, M. X. Conducting Polymer Nanofibers. In Encyclopedia of Nanoscience and Nanotechnology; Nalwa, H. S., Ed.; American Scientific Publishers: Stevenson Ranch, CA, 2004; Vol. 2, pp 153-169. (b) Scott, J. C. Science 1997, 278, 2071. (c) McFarland, E. W.; Tang, J. Nature 2003, 421, 616. (d) Coronado, E.; Gala´n-Mascaro´s, J. R.; Go´mez-Garcı´a, C. J.; Laukhin, V. Nature 2000, 408, 447. (2) (a) Chandrasekhar, P.; Naishadham, K. Synth. Met. 1999, 105, 115. (b) Krinichnyi, V. I. Synth. Met. 2000, 108, 173. (c) Wang, Y. Y.; Jing, X. L. Polym. AdV. Technol. 2005, 16, 344. (d) Lee, C. Y.; Lee, D. E.; Jeong, C. K.; Hong, Y. K.; Shim, J. H.; Joo, J.; Kim, M. S.; Lee, J. Y.; Jeong, S. H.; Byun, S. W.; Zang, D. S.; Yang, H. G. Polym. AdV. Technol. 2002, 13, 577. (e) Maiti, S. N.; Mahapatro, P. K. Polym. Compos. 1992, 13, 47. (f) Tan, S. T.; Zhang, M. Q.; Rong, M. Zh.; Zeng, H. M. Polym. Compos. 1999, 20, 406. (g) Koul, S.; Chandra, R.; Dhawan, S. K. Polymer 2000, 41, 9305. (3) (a) Kim, H. S.; Sohn, B. H.; Lee, W.; Lee, J. K.; Choi, S. J.; Kwon, S. J. Thin Solid Films 2002, 419, 173. (b) Long, Y. Z.; Chen, Zh. J.; Duvail, J. L.; Zhang, Zh. M.; Wan, M. X. Physica B 2005, 370, 121. (c) Yoon, M.; Kim, Y.; Kim, Y. M.; Yoon, H.; Volkov, V.; Avilov, A.; Park, Y. J.; Park, I. W. J. Magn. Magn. Mater. 2004, 272-276, 1259. (d) Wan, M. X.; Fan, J. H. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 2749. (4) Zhang, Zh. M.; Wan, M. X. Synth. Met. 2003, 132, 205. (5) Zhang, Zh. M.; Wan, M. X.; Wei, Y. Nanotechnology 2005, 16, 2827. (6) (a) Wan, M. X.; Li, J. C. Synth. Met. 1999, 101, 844. (b) Deng, J. G.; Peng, Y. X.; He, Ch. L.; Long, X. P.; Li, P.; Chan, A. S. C. Polym. Int. 2003, 52, 1182. (7) (a) Tanikaw, K.; Okuao, Z.; Iwaoka, T.; Hataso, M. J. Appl. Phys. 1997, 48, 2424. (b) Lee, Y.; Kim, C. Y. Synth. Met. 1995, 74, 103. (c) Li, J.; Wang, E.; Green, M.; West, P. E. Synth. Met. 1995, 74, 127. (d) Satoch, M.; Iskikawa, H.; Amane, K.; Hasegawa, E.; Yoshino, K. Synth. Met. 1994, 65, 39. (e) Kaynak, A.; Unsworth, J.; Clout, R.; Mohan, A. S.; Beard, G. E. J. Appl. Polym. Sci. 1994, 54, 219. (8) (a) Salm, M.; Diaz, A. F.; Logan, A. J. Mol. Cryst. Liq. Cryst. 1982, 83, 265. (b) Qian, R. Y.; Qiu, J. J. Polym. J. 1987, 19, 157. (c) Saton, M.; Kaneto, K.; Yoshino, K. J. Appl. Phys. 1985, 24, 423. (d) Ko, J. M.; Park, D. Y.; Myung, N. V.; Chung, J. S.; Nobe, K. Synth. Met. 2002, 128, 47. (e) Suri, K.; Annapoorni, S.; Tandon, R. P.; Rath, C.; Aggrawal, V. K. Curr. Appl. Phys. 2003, 3, 209. (f) Liu, J.; Wan, M. X. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 2734. (9) (a) Wan, M. X.; Huang, K.; Zhang, L. J.; Zhang, Zh. M.; Wei, Zh. X.; Yang, Y. Sh. Int. J. Nonlinear Sci. Numer. Simul. 2002, 3, 465. (b) Yang, Y. Sh.; Wan, M. X. Nanotechnology 2002, 13, 771. (10) Zhang, Zh. M.; Wei, Zh. X.; Zhang, L. J.; Wan, M. X. Acta Mater. 2005, 53, 1373. (11) (a) Chowdhury, D.; Paul, A.; Chattopadhyay, A. Langmuir 2005, 21, 4123. (b) Maeda, S.; Corradi, R.; Armes, S. P. Macromolecules 1995, 28, 2905. (c) Zaid, B.; Aeiyach, S.; Lacaze, P. C. Synth. Met. 1994, 65, 27. (d) Lan, Y, Wang, E. B.; Song, Y. H.; Song, Y. L.; Kang, Zh. H.; Xu, L.; Li, Zh. Polymer 2006, 47, 1480. (12) (a) Ferna´ndez Romero, A. J.; Lo´pez Cascales, J. J.; Otero, T. F. J. Phys. Chem. B 2005, 109, 21078. (b) Jeong, R. A.; Lee, G. J.; Kim, H. S.; Ahn, K.; Lee, K.; Kim, K. H. Synth. Met. 1998, 98, 9. (c) Fusalba, F.; Be´langer, D. J. Phys. Chem. B 1999, 103, 9044. (13) Lu, G. W.; Li, Ch.; Shi, G. Q. Polymer 2006, 47, 1778. (14) Tandon, R. P.; Tripathy, M. R.; Arora, A. K.; Hotchandani, S. Sens. Actuators, B 2006, 114, 768. (15) (a) Liu, Y. Ch.; Chuang, T. C. J. Phys. Chem. B 2003, 107, 12383. (b) Mitchell, G. R. Polym. Commun. 1986, 27, 346. (c) Zhang, X.; Bai, R. B. Langmuir 2003, 19, 10703. (d) Bhat, N. V.; Gadre, A. P.; Bambole, V. A. J. Appl. Polym. Sci. 2001, 80, 2511.