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J. Phys. Chem. B 2009, 113, 1376–1380
Multifunctional Polypyrrole/Strontium Hexaferrite Composite Microspheres: Preparation, Characterization, and Properties Jing Jiang,*,† Lunhong Ai,† and Liangchao Li‡ Key Laboratory of Applied Chemistry and Pollution Control Technology, College of Chemistry and Chemical Engineering, China West Normal UniVersity, Nanchong 637002, P. R. China, and Zhejiang Key Laboratory for ReactiVe Chemistry on Solid Surface, Department of Chemistry, Zhejiang Normal UniVersity, Jinhua 321004, P. R. China ReceiVed: September 17, 2008; ReVised Manuscript ReceiVed: NoVember 25, 2008
Polypyrrole (PPY)/SrFe12O19 composites with tunable electrical and magnetic properties were synthesized by in situ polymerization of pyrrole in the presence of SrFe12O19 particles. The structure of PPY/SrFe12O19 composites was characterized by means of Fourier transform infrared spectroscopy and X-ray diffraction. Transmission electron microscope and scanning electron microscope images illustrated that the spherical composites consisted of SrFe12O19 hexagonal plates sheathed by PPY. In the electromagnetic measurments, it was found that the ac conductivity of SrFe12O19 particles increased while the saturation magnetization, remanent magnetization, and coercivity decreased after PPY coating; moreover, the desired electrical and magnetic properties of PPY/SrFe12O19 composites can be modulated simply by controlling the contents of SrFe12O19 particles. A possible mechanism was also proposed to interpret the formation of the PPY/SrFe12O19 composites. Introduction Recently, considerable effort has been devoted to the design and preparation of composite particles consisting of core covered by shells of different chemical composition. The interest in such core-shell materials stems from the fact that the properties (mechanical, optical, electrical, magnetic, and catalytic) of core or shell can be tailored by their size, morphology, components, and structure of their shells or cores.1-3 Inherently conducting polymers are attractive materials, as they cover a wide range of functions from insulators to metals and retain the mechanical properties of conventional polymers.4,5 The considerable electrochemical and physicochemical properties hold great promise for various practical applications.6-10 Among the conducting polymer, polypyrrole (PPY) has received a great deal of attention in recent years due to its easy synthesis, good environmental stability, and high electrical conductivity. However, polypyrrole is infusible, insoluble, and suffers from poor processability, mainly because of its rigid, highly conjugated backbone. To overcome these limitations, preparation of polypyrrole/inorganic particles composites has been considered to provide a suitable solution to the processability problem, such as PPY/Fe3O4,11 PPY/TiO2,12 PPY/SiO2,13 PPY/V2O5,14 PPY/ Ag,15 and PPY/PbS,16 etc. These composites have had the ability to enhance their material properties with desirable mechanical and physical characteristics. Among transition metal oxides, strontium hexaferrite (SrFe12O19) has been currently magnetic material with great scientific and technological interest and has been widely used for permanent magnets, magnetic recording media, and microwave absorbers due to its high stability, excellent high-frequency response, large magnetocrystalline anisotropy, and large mag* To whom correspondence should be addressed: e-mail 0826zjjh@ 163.com; Tel +86-817-2568081; Fax +86-817-2224217. † China West Normal University. ‡ Zhejiang Normal University.
netization as well.17 It is well-known that conducting polymers can effectively shield electromagnetic waves generated from an electric source, whereas electromagnetic waves from a magnetic source can be effectively shielded only by magnetic materials.18 Thus, the incorporation of magnetic constituents and conducting polymeric materials into multifunctional composites opens new possibilities for the achievement of good shielding effectiveness for various electromagnetic sources. To the best of our knowledge, little work has been done on the preparation and electromagnetic properties of PPY/SrFe12O19 composites. In the present study herein, we employed a simple one-pot in situ polymerization method to synthesize PPY/ SrFe12O19 composite microspheres with electromagnetic behaviors where SrFe12O19 was used as magnetic constituent. The desired electrical and magnetic properties of PPY/SrFe12O19 composites can be modulated simply by controlling the contents of SrFe12O19 particles. Experimental Section Pyrrole was distilled twice under reduced pressure and stored below 0 °C. Citric acid, ammonia, Fe(NO3)3 · 9H2O, Sr(NO3)2, and (NH4)2S2O8 (APS) were all of analytical purity and used without further purification. Synthesis. SrFe12O19 hexaferrite particles were prepared by a citrate sol-gel combustion process. Stoichiometric amounts of Fe(NO3)3 · 9H2O and Sr(NO3)2 were dissolved in a minimum amount of deionized water by stirring on a hot plate at ca. 50 °C with the ratio of iron to strontium being set at 11.5. Citric acid was then added to the mixture solution to chelate Sr2+ and Fe3+. The molar ratios of citric acid to metal ions used were 1:1. An ammonia solution was added to adjust the pH value to 7. The clear solution was slowly evaporated at 80 °C with constant stirring, and then the viscous gels were formed. By increasing the temperature up to 200 °C, the gel precursors were combusted to form brown loose powders. Finally, the as-burnt
10.1021/jp808270n CCC: $40.75 2009 American Chemical Society Published on Web 01/09/2009
PPY/SrFe12O19 Composite Microspheres
J. Phys. Chem. B, Vol. 113, No. 5, 2009 1377
Figure 1. XRD patterns of (a) SrFe12O19, (b) PPY/SrFe12O19 composite (PPY/SF2), and (c) PPY.
powders were calcined at 1100 °C for 1 h. The hexaferrite SrFe12O19 particles were obtained. PPY/SrFe12O19 composites was prepared by in situ polymerization of pyrrole in the presence of SrFe12O19 particles. The whole experiment was conducted in an ultrasonic apparatus (model KQ-250DB, Kunshan Ultrasonic Instrument Co. Ltd.), using a power of 100 W and operated at 50 kHz. In a typical procedure, a certain amount of SrFe12O19 hexaferrite particles was added to 35 mL of 0.5 M HCl solution contaning 1 mL of pyrrole monomer under ultrasonic stirring for 30 min. 3.51 g of APS in 20 mL of 0.5 M HCl solution was then slowly added dropwise to the above mixture. The polymerization was carried out by ultrasonic stirring for 8 h at room temperature. The composites were obtained by filtering and washing the reaction mixture with deionized water and ethanol and dried under vacuum at 60 °C for 24 h. The composites with different content of SrFe12O19 particles were synthesized by modulating mass ratios of pyrrole monomer to SrFe12O19 particles at 9:1 (PPY/ SF1), 4:1 (PPY/SF2), and 7:3 (PPY/SF3). Characterization. The XRD patterns of the samples were collected on a Philips X’pert Pro MPD diffractometer with Cu KR radiation (λ ) 0.154 18 nm). Scanning electron microscopy (SEM) investigations were carried out on a Hitachi S4800 scanning electron microscope. Transmission electron microscopy (TEM) measurments were carried out on a JEOL JEM-2010 transmission electron microscope at an accelerating voltage of 200 kV. Infrared spectra were recorded on a Nicolet Avatar 360 spectrometer in the range of 400-2000 cm-1 using KBr pellets. The thermogravimetry analysis of composites was examined by using a Netzsch STA 449 thermogravimetric analyzer at a heating rate of 10 °C/min in air. Magnetic measurements were carried out at room temperature using a vibrating sample magnetometer (VSM, Lakeshore 7404) with a maximum magnetic field of 15 kOe. The ac conductivity of samples at room temperature were performed on an Agilent E4991A RF impedance/material analyzer in the frequency range from 1 MHz to 1 GHz. Results and Discussion Figure 1 shows the X-ray diffraction (XRD) curves of SrFe12O19, PPY, and PPY/SrFe12O19 composite (PPY/SF2). It can be observed that the broad amorphous diffraction peak appearing at 2θ ) 15-28° range (centered at 2θ ) 23°) in the XRD curves of PPY and PPY/SrFe12O19 composite, which is characteristic of the doped PPY,19 and this broad diffraction
Figure 2. FTIR spectra of (a) PPY, (b) PPY/SrFe12O19 composite (PPY/SF2), and (c) SrFe12O19.
Figure 3. TGA and DTGA curves of PPY/SrFe12O19 composite (PPY/ SF2).
peak of PPY in the PPY/SrFe12O19 composite is weak, indicating that the crystallinity of PPY in the PPY/SrFe12O19 composite is much lower than that of pristine PPY. Eight main diffraction peaks at 2θ ) 30.4°, 32.4°, 34.2°, 37.2°, 40.4°, 55.2°, 56.9°, and 63.2° of SrFe12O19 hexaferrite assigned to scattering from (110), (107), (114), (203), (205), (217), (2011), and (220) planes of the hexagonal SrFe12O19 (JCPDS Card No. 84-1531) are observed in Figure 1a,b, showing the existence of SrFe12O19 in the PPY/SrFe12O19 composites.20 Figure 2 shows the FTIR spectra of the PPY, PPY/SrFe12O19 composite (PPY/SF2), and SrFe12O19 particles. For the polypyrrole (Figure 2a), the characteristic peaks appear at 1545 and 1472 cm-1 (the fundamental vibrations of the pyrrole rings), 1305 cm-1 ()C-H in-plane vibration), 1190 cm-1 (C-N stretching vibration), 1102 cm-1 (the in-plane deformation vibration of NH+, which is formed on the polymer chains by protonation), 1045 cm-1 (the C-H and N-H in-plane deformation vibrations), 917 cm-1 (C-C out-of-plane ring deformation vibration), and 805 cm-1 (the C-H out-of-plane ring deformation vibration). The FTIR spectrum of PPY/SrFe12O19 composite (Figure 2b) is almost identical to that of polypyrrole; however, the characteristic peak for SrFe12O19 at around 591 and 435 cm-1
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Figure 4. SEM micrographs of (a) SrFe12O19 particles and (b) PPY/SrFe12O19 composite (PPY/SF2); TEM image (c) of PPY/SrFe12O19 composite (PPY/SF2).
Figure 5. Hysteresis loops of SrFe12O19 particles and PPY/SrFe12O19 composites.
in Figure 2b reveals the presence of SrFe12O19 in the PPY/ SrFe12O19 composite.21 Figure 3 shows the thermogravimetric analysis (TGA) and differential TGA (DTGA) curves of PPY/SrFe12O19 composite (PPY/SF2). It can be seen that the first small fraction of weight loss from room temperature to about 100 °C is mainly due to the expulsion of absorbed water in the sample. Obvious two weight loss of the composite has been found in the temperature ranges 180-370 and 380-500 °C because of decomposition of polypyrrole chains.22 On the basis of the TGA curve, the weight fraction of SrFe12O19 in PPY/SrFe12O19 composite (PPY/ SF2) was estimated to be ∼18%, which is clsoe to the designed value. The morphology of the obtained PPY/SrFe12O19 composite (PPY/SF2) has been studied by SEM and TEM, as shown in Figure 4. It is indicated that the SrFe12O19 particles appear the hexagonal platelike shape with the random grain orientation. As shown in Figure 4b, PPY has enwrapped the SrFe12O19 particles effectively, and composite particles are spherical in shape with rough surface. The TEM image of PPY/SrFe12O19 composite displayed in Figure 4c indicates that the SrFe12O19 particles are embedded in the PPY matrix, forming the core-shell structure. The black core is magnetic hexaferrite SrFe12O19 particle with the diameter in the range of 100-180 nm, and the gray shell is PPY in the composite due to the different electron penetrability. Figure 5 presents the field-dependent magnetization of SrFe12O19 and PPY/SrFe12O19 composites at room temperature, which indicates the hysteretic characteristic. The magnetic parameters such as saturation magnetization (MS), remanent magnetization (Mr), and coercivity (HC) of PPY/SrFe12O19 composites determined by hysteresis loops varied with the SrFe12O19 content. The ratio of remanent magnetization and saturation magnetization (Mr/MS) for SrFe12O19 and PPY/ SrFe12O19 composites is very close to 0.5 (Figure 6), revealing
Figure 6. Variation of MS and HC as a function of SrFe12O19 content for PPY/SrFe12O19 composites. The inset of the figure shows variation of Mr/MS as a function of SrFe12O19 content.
that SrFe12O19 particles exhibit the single magnetic domain characteristic.23 As seen from Figure 6, the MS and HC of PPY/ SrFe12O19 composites are lower than that of SrFe12O19 and decrease with decreasing the SrFe12O19 content. The observed decrease in MS of PPY/SrFe12O19 composites with decreasing the SrFe12O19 content shows the SrFe12O19 particles are responsible for the magnetic behavior of the composites. According to the Stoner-Wohlfarth theory, magnetic anisotropy (EA) energy for single-domain particles can be expressed as
EA ) KV sin2 θ
(1)
where K is the magnetic anisotropy constant, V is the volume of the particle, and θ is the angle between magnetization direction and the easy axis of the nanoparticle.24 The coercivity represents the strength of the magnetic field that is necessary to surpass the anisotropy barrier and allow the magnetization of the particle following the magnetic field orientation. From eq 1, lowering the anisotropy of a particle will lower the activation energy barrier and results in a lower applied field required for spin reversal, i.e., a lower coercivity.25 Magnetic properties observed for fine particles are a combination of many anisotropy mechanisms. An effective anisotropy constant (K) could be obtained by adding the bulk anisotropy and surface contributions. For a spherical particle the following expression has been used to account for K:26
K ) Kb + (6/d)KS
(2)
where Kb is the bulk magnetocrystalline anisotropy, KS is the surface anisotropy, and d is the particle diameter. KS is usually
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J. Phys. Chem. B, Vol. 113, No. 5, 2009 1379
Figure 7. Variation of ac conductivity of PPY and PPY/SrFe12O19 composites as a function of frequency.
maximum for free surfaces and is reduced by solid coverage. The decrease in KS resulting from the particle coverage by the PPY reduces the effective magnetocrystalline anisotropy (K) and therefore decreases HC. On the basis of the above discussion, the magnetic properties of PPY/SrFe12O19 composites can be tailored by controlling the different amounts of magnetic SrFe12O19 particles. Electrical conductivity measurements are known to be very sensitive for the study of electronic properties of materials. In amorphous systems, ac conductivity measurements provide useful information concerning various relaxation phenomenon related to the electrical polarization process. Figure 7 shows the variation of ac conductivity (σac) of PPY and PPY/SrFe12O19 composites as a function of frequency. Within the measurable frequency range, σac tends to remain constant for all samples up to about 107 Hz and thereafter increases with frequency. This is characteristic of disordered material where conductivity is due to hopping of charge carriers between localized states.27 In addition, as shown in Figure 7, the variation of σac as a function of SrFe12O19 content at different frequency reveals the tunable conductivity of the composites. It is well-known fact that the conductivity of composite depends, apart from frequency and temperature, on degree of protonation, percent crystallinity, crystalline domain size, and order in crystalline and amorphous regions which have a relationship with the delocalization length. A observed decrease in σac of the PPY/SrFe12O19 composites with increasing the content of SrFe12O19 could be due to an increase in the disorderliness of composites, leading to a reduction in the delocalization length and particle blockage the conduction path of PPY.28,29 Also, increasing the SrFe12O19 contents leads to a larger number of polarons where the interpolaron coupling gets progressively stronger even though disorder present, resulting in severe pinning of polarons, thus restricting their contribution at higher frequencies and hence in the reduction of conductivity.30 Figure 8 illustrates the polymerization process and probable interaction in the PPY/SrFe12O19 composites. The surface charge of metal oxide is positive below the pH of the point of zero charge (PZC), while it is negative above it. Since the surface of SrFe12O19 has PZC of pH ) 4.0,31 it is positively charged in the acidic polymerization conditions. Thus, adsorption of an amount of anions such as Cl- seems likely to occur, which may compensate the positive charges on the SrFe12O19 surface. Besides the charge compensation, specific adsorption of Clon the SrFe12O19 surface may take place. These specifically adsorbed anions would work as the charge compensator for positively charge PPY chain in the formation of PPY/SrFe12O19
Figure 8. Schematic illustration of the polymerization process for PPY/ SrFe12O19 composites.
composites; moreover, the electrostatic interactions between anions adsorbed on the SrFe12O19 surface and the protonated PPY chains may appear. In addition, in the polymerization process, hydrogen bonding may exist between the oxygen atoms located at hexaferrite sublattices exposed on the SrFe12O19 surface and the protonated PPY chains in the nanocomposites. These interactions may ensure the encapsulation of SrFe12O19 particles in the polymer chains and improve the stability of PPY/ SrFe12O19 composites. Conclusions In summary, the electrical-magnetic multifunctional PPY/ SrFe12O19 composite microspheres were successfully synthesized by a facile in situ polymerization of pyrrole in the presence of SrFe12O19 particles. The structure and morphologies of PPY/ SrFe12O19 composites were characterized by FTIR, XRD, TGA, SEM, and TEM. The electromagnetic measurments revealed that the MS, Mr, HC, and σac of composites decreased with decreasing the SrFe12O19 content, indicating that the desired electrical and magnetic properties of polypyrrole/SrFe12O19 composites can be modulated simply by controlling the contents of SrFe12O19 particles. A possible mechanism was proposed to interpret the formation of the PPY/SrFe12O19 composites. Acknowledgment. This work was supported by Scientific Research Foundation of China West Normal University (07B008, 07B005). References and Notes (1) Liu, Q.; Xu, Z.; Finch, J. A.; Egerton, R. Chem. Mater. 1998, 10, 3936. (2) Vero´nica, S. M.; Marina, S.; Michael, F. AdV. Funct. Mater. 2005, 15, 1036. (3) Berndt, I.; Pedersen, J. S.; Richtering, W. Angew. Chem., Int. Ed. 2006, 45, 1737. (4) Kang, E. T.; Neoh, K. G.; Tan, K. L. Prog. Polym. Sci. 1998, 23, 77. (5) Heeger, A. J. J. Phys. Chem. B 2001, 105, 8475. (6) Choi, Y. S.; Joo, S. H.; Lee, S. A.; You, D. J.; Kim, H.; Pak, C.; Chang, H.; Seung, D. Macromolecules 2006, 39, 3275. (7) Yang, C. H.; Chih, Y. K. J. Phys. Chem. B 2006, 110, 19412. (8) Fujii, S.; Aichi, A.; Akamatsu, K.; Nawafune, H.; Nakamura, Y. J. Mater. Chem. 2007, 17, 3777. (9) Liu, Z.; Wang, J.; Xie, D. H.; Chen, G. Small 2008, 4, 462. (10) Sutar, D. S.; Padma, N.; Aswal, D. K.; Deshpande, S. K.; Gupta, S. K.; Yakhmi, J. V. Sens. Actuators, B 2007, 128, 286.
1380 J. Phys. Chem. B, Vol. 113, No. 5, 2009 (11) Wu, T.-Z.; Yen, S.-J.; Chen, E.-C.; Sung, T.-W.; Chiang, R.-K. J. Polym. Sci., Polym. Chem. 2007, 45, 4647. (12) Roux, S.; Soler-Illia, G. J. de A. A.; Demoustier-Champagne, S.; Audebert, P.; Sanchez, C. AdV. Mater. 2003, 15, 217. (13) Wang, D.; Wang, Y.; Li, X.; Luo, Q.; An, J.; Yue, J. Catal. Commun. 2008, 9, 1162. (14) Huguenin, F.; Torresi, R. M. J. Phys. Chem. C 2008, 112, 2202. (15) Pinter, E.; Patakfalvi, R.; Fulei, T.; Gingl, Z.; Dekany, I.; Visy, C. J. Phys. Chem. B 2005, 109, 17474. (16) Jing, S.; Xing, S.; Yu, L.; Zhao, C. Mater. Lett. 2008, 62, 41. (17) Kubo, O.; Ido, T.; Yok, H. IEEE Trans. Magn. 1982, 18, 1122. (18) Yavuz, O.; Ram, M. K.; Aldissi, M.; Poddar, P.; Hariharan, D. J. Mater. Chem. 2005, 15, 810. (19) Kojima, Y.; Usuki, A. A.; Kawasumi, M.; Okada, A.; Kurauchi, T.; Kamigaito, O. J. Polym. Sci., Polym. Chem. 1993, 31, 983. (20) Feng, X.; Huang, H.; Ye, Q.; Zhu, J. J.; Hou, W. J. Phys. Chem. C 2007, 111, 8463. (21) Wuang, S. C.; Neoh, K. G.; Kang, E. T.; Pack, D. W.; Leckband, D. E. J. Mater. Chem. 2007, 17, 3354.
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