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maghemite nanoparticles
European Polymer Journal 45 (2009) 613–620
Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Macromolecular Nanotechnolgy
Synthesis and electromagnetic properties of polyaniline-coated silica/maghemite nanoparticles Tar-Hwa Hsieh a,*, Ko-Shan Ho a, Xiaotao Bi b, Yu-Kai Han a, Zhi-Long Chen a, Chia-Hao Hsu a, Yu-Chen Chang c a
Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, 415 Chien-Kung Road, Kaohsiung 807, Taiwan Department of Chemical and Biological Engineering, The University of British Columbia, Vancouver, Canada c Institute of Materials Science and Engineering, National Sun Yat-Sen University, Kaohsiung, 70 Lian-Hai Road, Kaohsiung, Taiwan
a r t i c l e
i n f o
Article history: Received 25 July 2008 Received in revised form 15 December 2008 Accepted 20 December 2008 Available online 31 December 2008
Keywords: PANI/SiO2/c-Fe2O3 nanocomposites Multilayer core-shell structure Electromagnetic properties Superparamagnetic behavior
a b s t r a c t Polyaniline coated silica/maghemite nanoparticles (PANI/SiO2/c-Fe2O3 composites) were synthesized by the combination of a sol–gel process and an in-situ polymerization method, in which ferrous and ferric salts as well as tetraethyl orthosilica (TEOS) acted as the precursor for c-Fe2O3 and silica, respectively. As a result, the SiO2/c-Fe2O3 particle showed a coreshell structure, with c-Fe2O3 as the magnetic core and silica as the shell of the particle. The shell thickness can be controlled by changing the TEOS concentration. The PANI/SiO2/cFe2O3 composites revealed a multilayer core-shell structure, where PANI is the outer shell of the composite. The doping level and the conductivity of PANI/SiO2/c-Fe2O3 composites decreased with increasing the TEOS content due to the presence of the less coated PANI on the SiO2/c-Fe2O3 core at higher TEOS content. For a SQUID analysis at room temperature, all c-Fe2O3 containing composites showed a typical superparamagnetic behavior. The saturation magnetization of SiO2/c-Fe2O3 nanoparticles decreased with increasing the TEOS content due to the increase in silica shell thickness, while the saturation magnetization of PANI/SiO2/c-Fe2O3 composites also decreased with increasing the TEOS content, which is attributed to the lower conductivity of PANI in the composites at higher TEOS content. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction In the past decade, polyaniline (PANI) containing iron oxides with both electrical and magnetic properties have been studied extensively. The preparation of PANI composites with magnetic and conductive properties has been mostly studied by Wan’s group by (1) blending the PANI in N-methyl-2-pyrrolidone (NMP) with FeSO4 aqueous solution, and precipitating Fe2+ into c-Fe2O3 [1] and (2) reacting FeCl24H2O and FeCl36H2O with aniline followed by treatment with KOH aqueous solution [2]. A poly (aniline-co-aminobenzenesulfonic acid) copolymer containing
* Corresponding author. Tel.: +886 7 3814526x5117; fax: +886 7 3830674. E-mail address: [email protected] (T.-H. Hsieh). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.12.039
c-Fe2O3 magnetic particles has also been prepared with the first method [3]. Tang et al. [4] have developed a versatile process employing anionic surfactants to prepare processable free standing films of PANI containing up to 50% c-Fe2O3 nanomagnets. Mallikarjuna et al. [5] reported the preparation of novel nanocomposites of polyaniline dispersed with c-Fe2O3 nanoparticles by an in-situ polymerization method. These composites exhibited high dielectric constants in proportion to the amount of c-Fe2O3. Sharma et al. [6] also reported the magnetic properties of iron oxide-PANI nanoclusters prepared at various aniline concentrations. Moreover, Bao and Jiang [7] prepared PANI/ Fe3O4 nanocomposites by the high-energy ball milling method. Recently, PANI containing iron oxides composites with core-shell structure have been extensively investigated since they exhibited dual properties of core and shell
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materials, leading to potential applications in electromagnetic shielding, batteries, electrochromic device, molecular electronic, non-linear optics, sensors, and microwaveabsorbing materials [8–11]. Deng et al. [9] have reported the synthesis of core-shell PANI/Fe3O4 composites with an average diameter of 80 nm by in-situ emulsion polymerization. These composites exhibited high electrical and ferromagnetic properties in proportion to the percentage of Fe3O4 and degree of doping. Long et al. [10] suggested a template-free method to prepare PANI/Fe3O4 nanorods with a core-shell structure. The saturation magnetization (3.45 emu/g at 300 K) was lower than the bulk magnetite (Ms = 84 emu/g) and pure Fe3O4 nanoparticles (Ms = 65 emu/g) due to the Fe3O4–PANI interactions. Lu et al. [11] have developed a method to synthesize well-dispersed PANI/Fe3O4 nanoparticles with aniline dimer-COOH polymerization. The core-shelled PANI/Fe3O4 nanoparticles exhibited typical superparamagnetic characteristics with a relatively high saturation magnetization (Ms = 21 emu/g) and a low coercive force (lc = 0). Recently, we [12] successfully synthesized PANI/c-Fe2O3 nanocomposites in the ternary system of toluene/SDS/water by using a reverse micelle method, with the product exhibiting typical superparamagnetic characteristics. The dispersed c-Fe2O3 phase was non-uniformly distributed in the PANI matrix, and exhibited a wide size distribution caused by PANI–cFe2O3 interactions, which led to the resultant composites possessing a low conductivity (ca. 103 S/cm) and low saturation magnetization (2.05–1.08 emu/g). Despite the effect of PANI–c-Fe2O3 interactions on the structure, morphology and electromagnetic properties of PANI/cFe2O3 composites in the reverse micelle polymerization has been examined, it is highly desirable to further develop the synthetic method to prevent the agglomeration of the c-Fe2O3 nanoparticles from PANI–c-Fe2O3 interactions, achieving a chemically stability composite with high conductivity and saturation magnetization. Isolating the magnetic particle and conductive PANI by silica could possibly generate novel composites with well-designed multilayer core-shell structures, and intriguing electronic and magnetic properties. A number of reports have described the preparation of the silica-coated iron oxide to reduce the agglomeration of iron oxide nanoclusters. Zhang et al. [13] reported the synthesis of core-shell SiO2/c-Fe2O3 nanoparticles by coprecipitation of ferrous and ferric salts encapsulated within sol–gel derived SiO2. The nonmagnetic SiO2 coating formed by hydrolysis and polycondensation of tetraethoxysilane on the surface of c-Fe2O3 nanoclusters provides a mean for thermally stable dispersion of c-Fe2O3 clusters. Haddad et al. [14] prepared silica-coated magnetite nanoparticles with a diameter of 20–30 nm and a shell thickness of 5–7 nm. The resulting particles showed superparamagnetic characteristics with a decrease in the saturation magnetization by about 40%. Rosenzweig et al. [15] reported the preparation of coreshell SiO2/c-Fe2O3 composites with a diameter of about 250 nm by a modified Stöber method, in which magnetite nanocrystals were separated by a thin rigid silica shell. The resulting particles maintained their superparamagnetic properties in the silica composites. Recently, the preparation of silica nanoparticles coated with a thin layer of con-
ductive PANI attracted particular interest, because the surface of the nanoparticles can easily be altered to meet the specific catalytic, magnetic, electronic, optical, or optoelectronic requirements. Xia and Wang [16] prepared PANI/silica nanoparticle composites through ultrasonic irradiation. The formed PANI was deposited on the surface of the nano-SiO2, which led to a core-shell structure with a strong interaction between PANI and nano-SiO2. Ruckenstein et al. [17] also reported that PANI grafted silica nanoparticles could be prepared facilely by the chemical oxidative polymerization of aniline from the amino groups on the surface of aminopropyl silica nanoparticles. Jang et al. [18] developed a method for the fabrication of coreshell PANI/silica nanoparticles by in-situ polymerization of aniline monomers adsorbed on the silica surface through electrostatic interactions. Although the core-shell PANI/iron oxide composites have been widely studied either by coating iron oxide with silica or by coating conductive PANI onto the silica surface, no work has been reported on the preparation of multilayer core-shell PANI/SiO2/c-Fe2O3 composites and their electromagnetic characteristics. In the present study, PANI coated SiO2/c-Fe2O3 nanoparticles were synthesized by combining the sol–gel process and the in-situ polymerization method. At a molar ratio of Fe3+/Fe2+ = 2, the effect of TEOS content on the structure, morphology and electromagnetic properties of the PANI/SiO2/c-Fe2O3 composites was investigated by Fourier transform infrared spectroscopy (FT-IR), Ultraviolet–visible spectroscopy (UV–vis), wide angle X-ray diffraction (WAXD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), micro-ohmetry and superconductor quantum interference device (SQUID), respectively. The origin of the electrical and magnetic properties of the nanocomposite is also discussed.
2. Experimental 2.1. Preparation
c-Fe2O3 magnetic nanoparticles were prepared by the precipitation–oxidation method [19] described in our previous study [12]. The c-Fe2O3 particles were found to exist in the form of spherical clusters, with a diameter of 10– 20 nm estimated from the WAXD patterns by applying the Scheere’s equation [20]. SiO2/c-Fe2O3 composites were prepared by a modified Stöber method [21]. The c-Fe2O3 particle containing alcohol aqueous solution was prepared by dispersing 0.5 g c-Fe2O3 into a mixture of 240 mL alcohol and 60 mL deionized water in a water bath under ultrasonic vibration for 30 min. Then, NH4OH (28–30 wt.%) solution was added into the above-prepared solution stirring at room temperature until the pH reached 11, followed by gradual addition of different amounts of TEOS (1–5 mL) into this solution to initiate the formation of SiO2/c-Fe2O3 nanoparticles. The synthesis process lasted for 12 h at 50 °C with stirring in order to form the nanoparticles of SiO2/c-Fe2O3. The prepared SiO2/c-Fe2O3 nanoparticles are designated here as SF-1, SF-3 and SF-5, respectively, where SF and the number indicate the pres-
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The composite powder was pressed at room temperature into tablets of approximately 1 mm in thickness and 13 mm in diameter and 0.3 0.3 mm2 for the X-ray and the XPS analysis, respectively. The X-ray diffraction patterns of the tablet samples were obtained with a Rugaku Denki D/max-2200 diffractometer operated at 45 kV and 40 mA using Cu Ka radiation (k = 0.154 nm), with the diffraction angles (2h) ranging from 20° to 70° at 2°/min. The weight loss of samples was conducted in a Du Pont TA Instrument SDT Q600 thermogravimetric analysis (TGA) unit with about 4–8 mg samples at a heating rate of 20 °C/min under air atmosphere. The scanning temperature ranged from room temperature to 700 °C. Samples were embedded in the epoxy resin, and sectioned into about 70 nm in thickness using a Reichert microtone at room temperature. Samples were then corrected in a though filled with methanol, and placed on a 200 mesh copper grid for the TEM observation. The TEM were obtained with a JEOL II-1200EX apparatus running at an acceleration voltage of 80 kV. Both SiO2/c-Fe2O3 nanoparticles and EB form composites were examined with 32 scans and 2 cm1 resolution on a BIO-RAD FTS-165 FT-IR using KBr pellets. The XPS of tablets was carried out on a VG Scientific ESCALAB-250 with Mg Ka (1253.6 eV) radiation. The conductivity of tablet samples (D = 1.295 cm, thickness = 1 mm) at room temperature was measured with Agilent HP4338A micro-ohmetry using 4-probes method with silver electrode. The magnetization of the samples was characterized using a SQUID magnetometer (Quantum
3. Results and discussion 3.1. Structure and magnetic properties of SiO2/c-Fe2O3 nanoparticles WAXD patterns of synthesized SiO2/c-Fe2O3 particles obtained using various TEOS contents are shown in Fig. 1. It can be seen that WAXD patterns of SiO2/c-Fe2O3 particles exhibit diffraction peaks at 2h = 20–30°, corresponding to the characteristic peaks of the silica, and the peak at 2h = 35.5° is the characteristic peak of the (3 1 1) crystal plane of c-Fe2O3 [5,12]. When the content of TEOS is increased, the characteristic peak of c-Fe2O3 becomes smaller and boarder, while the characteristic peaks of silica grow and become clearer. A smaller characteristic peak of c-Fe2O3 and clearer characteristic peaks of SiO2 in nanoparticles at higher TEOS content indicate that the crystallinity of SiO2/c-Fe2O3 particles decreases with the increase in TEOS content, which suggests that c-Fe2O3 nanoclusters are in the amorphous silica matrix. However, this trend becomes more noticeable at higher TEOS content. Fig. 2(a)–(c) display the morphology of SiO2/c-Fe2O3 particles by TEM. It is found that the outer shell of the particle exhibits a fine increment in brightness compared with the dark inner core, which confirms the well-known coreshell structure of SiO2/c-Fe2O3 particles, where c-Fe2O3 is the magnetic core embedded within the silica matrix and the silica shell. It is noted that the core-shell structured SiO2/c-Fe2O3 particles are rather monodispersed, although most silica matrices have trapped more than one magnetic core because of the aggregation of the iron oxide nanoparticles prior to or during the coating process [22]. The particle size and the silica shell thickness of the SiO2/c-Fe2O3 nanoparticles determined by TEM are 52, 95 and 195 nm, as well as 10, 33 and 55 nm, respectively, for the three tested samples, indicating that both particle size and silica shell thickness of the nanoparticles increase with increasing the TEOS content. This observation clearly demon-
(a)
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(c)
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2θ (deg.) Fig. 1. WAXD patterns of SiO2/c-Fe2O3 particles synthesized at various TEOS contents: (a) c-Fe2O3; (b) SF-1; (c) SF-3; (d) SF-5.
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2.2. Characterization
Drsign, MPMS) at 300 and 5 K, respectively, measured in the applied field at ±1.5 T.
Intensity
ence of SiO2/c-Fe2O3 particles and the content (in mL) of TEOS used in the preparation, respectively. PANI coated SiO2/c-Fe2O3 composites were synthesized via an in-situ polymerization of aniline monomer in an aniline/silica solution, which contains well-dispersed SiO2/c-Fe2O3 nanoparticles. The aniline/silica solution was prepared by adding 0.3 g SiO2/c-Fe2O3 nanoparticles into 50 mL deionized water, followed by the addition of 0.2 g aniline and HCl aqueous solution (35 wt.%) to the above-prepared solution under vigorous stirring for 1 h until the pH reached 3. Then, the ammonium persulfate (APS) aqueous solution (0.26 g APS dissolved in 2 mL distilled water) was added gradually into the aniline/silica solution to start the in-situ polymerization of aniline monomers. The polymerization continued under magnetic stirring for 6 h at 0 °C, the emeraldine salt form of PANI/SiO2/c-Fe2O3 composites (ES form) was obtained. The PANI/SiO2/cFe2O3 composites in the emeraldine base form (EB form) were prepared by deprotonation of the obtained ES form composites, in which the PANI/SiO2/c-Fe2O3 composites (ES form) was mixed with an excess amount of 1 M NaOH solution under stirring for 24 h, then washed with distilled water and methanol several times, followed by drying in vacuum at 60 °C for 2 h. In the present study, the ES or EB form composites synthesized at various TEOS contents are designated as ES-SF-1, ES-SF-3 and ES-SF-5, or EB-SF1, EB-SF-3 and EB-SF-5, where ES, EB and the number indicate the ES, EB form composites and the content of TEOS, respectively.
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Fig. 2. TEM micrographs of (a) SF-1, (b) SF-3, (c) SF-5, (d) ES-SF-1, (e) ES-SF-3 and (f) ES-SF-5. The average diameter of SiO2/c-Fe2O3 particles is 52, 92 and 195 nm, respectively.
strates that the nanoparticle characteristics of SiO2/cFe2O3 nanoparticles can be easily controlled by simply varying the initial amount of TEOS [22,23]. The FT-IR spectra of synthesized SiO2/c-Fe2O3 nanoparticles at various TEOS contents are shown in Fig. 3. The characteristic peaks of the SiO2/c-Fe2O3 particles at 634, 567 and 451 cm1 are due to the stretching vibrations of FeAO [24], the peaks at 799 and 471 cm1 as well as the strong peak at 1082 cm1 and the shoulder at 1188 cm1 are associated with the bending vibration of SiAOASi and OASiAO bond, and the overlap peaks of the SiAOASi symmetric stretching mode and the CH stretching mode in plane bend, respectively. When compared with c-Fe2O3, the characteristic peaks of SiO2/c-Fe2O3 nanoparticles are
shifted to higher frequencies from 634 to 640 cm1 and 567 to 582 cm1, respectively, indicating the presence of some interactions between the c-Fe2O3 and the silica. Such interactions are likely caused by the van der Waals attraction between uncharged silica spheres with the c-Fe2O3 magnetic core [25]. When TEOS content is increased, the core size of the embedded c-Fe2O3 particles apparently increases from ca. 28 to 85 nm, exhibiting a more expansive core morphology, which can be attributed to the decrease of van der Waals force at high TEOS contents, as seen in Fig. 2. The result demonstrates that the iron oxide surface has a strong affinity to silica. Hence, no primer is required to promote the deposition and adhesion of silica, and the core-shell SiO2/c-Fe2O3 nanoparticles can be successfully
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(a)
Transmittance
(b)
(c)
Table 1 Values of saturation magnetization (Ms) for SiO2/c-Fe2O3 nanoparticles synthesized at various TEOS contents. Sample
Particle size (nm)
Silica shell thickness (nm)
Ms (emu/g)
c-Fe2O3
10–20 52 95 195
– 10 30 55
60.1 43.1 26.8 12.7
SF-1 SF-3 SF-5
(d)
4000
3500
3000
2500 2000 1500 -1 Wavenumber (cm )
1000
500
Fig. 3. FT-IR spectra of SiO2/c-Fe2O3 nanoparticles synthesized with various TEOS contents; (a) c-Fe2O3; (b) SF-1; (c) SF-3; (d) SF-5.
synthesized via a sol–gel process by simply varying the initial amount of TEOS. The hysteresis loops of the c-Fe2O3 and SiO2/c-Fe2O3 nanoparticles measured at 300 K are shown in Fig. 4. It can be seen that, both c-Fe2O3 and SiO2/c-Fe2O3 nanoparticles exhibit negligible coercivity and remanence, indicating that the products possess the typical superparamagnetic characteristics. Values of the saturation magnetization of SiO2/c-Fe2O3 nanoparticles are shown in Table 1. It can be seen that the saturation magnetization of the SiO2/c-Fe2O3 is smaller than c-Fe2O3 (Ms is 60.0 emu/g) and decreases from 43.1 to 12.7 emu/g with increasing the TEOS content, which is attributed to the increased nonmagnetic silica shell thickness at high TEOS contents [14,26]. The result demonstrates that the magnetic properties of the SiO2/c-Fe2O3 nanoparticles are strongly influenced by the silica shell thickness. On the other hand, the magnetic properties of the SiO2/c-Fe2O3 nanoparticles can be easily controlled by varying the starting c-Fe2O3 and TEOS contents during fabrication.
In order to identify the existence of PANI in the composite, the EB form composites were first prepared by dissolving in NMP, which is a good solvent for the emeraldine base form of PANI (EB), and then characterized by FT-IR. Fig. 5 shows the FT-IR spectra of EB form composites. In Fig. 5(a), the characteristic peaks at 1497, 1590 and 1310 cm1 are associated with the stretching vibration of benzenoid ring and quinoid ring, as well as CAN stretching of the secondary aromatic amine, respectively, of PANI (EB). Fig. 5(b)–(d) show the characteristic peaks of benzenoid ring and quinoid ring of the EB form composites at 1494 and 1581 cm1, respectively, which are shifted to lower frequency from 1494 to 1483 cm1 and 1581 to 1572 cm1, respectively, with increasing TEOS content, suggesting that PANI is exactly coated on the surface of SiO2/c-Fe2O3 nanoparticles owing to the electrostatic attraction between cationic anilinium ions with negatively charged surface of the silica core [25]. The result indicated that PANI can be successfully polymerized onto the surface of SiO2/c-Fe2O3 core via in-situ polymerization. The possible mechanism for the formation of SiO2/c-Fe2O3 nanoparticles and PANI/SiO2/c-Fe2O3 composites is shown in Fig. 6. In the first step shown in Fig. 6(a), c-Fe2O3 nanoclusters are prepared by precipitation–oxidation method and the magnetic particles are trapped within silica spheres via a sol– gel process based on the hydrolysis of TEOS, forming uniform core-shell SiO2/c-Fe2O3 nanoparticles. In the second step shown in Fig. 6(b), aniline monomers are first con-
(a) (b) (c) (d)
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Transmittance
M (emu/g)
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-15000
-10000
-5000
0
5000
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15000
H (Oe) Fig. 4. Saturation magnetization vs. applied magnetic field at 300 K for SiO2/c-Fe2O3 nanoparticles synthesized at various TEOS contents; (a) cFe2O3; (b) SF-1; (c) SF-3; (d) SF-5.
4000
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3000
2500
2000
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-1
Wavenumber (cm ) Fig. 5. FT-IR spectra of PANI/SiO2/c-Fe2O3 (EB form) composites synthesized at various TEOS contents; (a) EB-0; (b) EB-SF-1; (c) EB-SF-3; (d) EBSF-5.
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3.2. Structure and electromagnetic properties of PANI/SiO2/cFe2O3 composites
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Fe2+
In water
OC2H5 (TEOS)
OC2H5
OH In ethanol and water
- Fe2O3 20 ~ 30 nm
b
Si
Si
HO
Fe3+
Excess of OH Coprecipitation
Si
HO
OC2H5 C2H5O
HO
a
Si
- Fe2O3 Silica layer ~ 55 nm
NH2
HCl APS
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+
NH3
SiO2/ -Fe2O3 nanoparticle
+
In-situ polymerization
PANI/SiO2/ composite
-Fe2O3
Fig. 6. Schematic diagram illustrating the synthetic procedure of PANI/SiO2/c-Fe2O3 composites (a) formation of core-shell SiO2/c-Fe2O3 nanoparticle; (b) formation of multilayer core-shell PANI/SiO2/c-Fe2O3 composite.
verted to cationic anilinium ions in acidic conditions at a pH of 3 and then adsorbed onto the negatively charged surface of silica nanoparticles (to the isoelectric point of silica at pHs greater than 2) through electrostatic attractions [18], further in-situ polymerized along the silica shell by the addition of the oxidant, resulting in the formation of PANI/SiO2/c-Fe2O3 capsules with a multilayer core-shell structure. The TGA data of PANI/SiO2/c-Fe2O3 are summarized in Table 2. It can be seen that the PANI content slightly decreases with the increase in the initial TEOS content. The result suggests that the effective concentration of aniline monomer on the SiO2/c-Fe2O3 core decreases with increasing the core size due to the decrease of the numbers of active sites on the negatively charged silica surface, leading to less PANI being coated onto the SiO2/c-Fe2O3 core at higher TEOS content. The typical N (1s) XPS core-level spectrum of the PANI/ SiO2/c-Fe2O3 composite can be deconvoluted into three component peaks centered at 398.5 ± 0.3, 399.5 ± 0.2 and 401.8 ± 0.3 eV, representing the nitrogen atoms of imine, Table 2 TGA data of PANI/SiO2/c-Fe2O3 composites synthesized at various TEOS contents. Sample
Residual weight at 700 °C (%)
Total PANI content (%)a
SF-1 SF-3 SF-5 ES-0 ES-SF-1 ES-SF-3 ES-SF-5
91.8 92.7 93.0 0 19.1 20.9 21.8
– – – 0 72.7 71.8 71.2
a
Absorbed PANI content on the surface of SiO2/c-Fe2O3 core.
amine units as well as positively charged nitrogen atom, respectively [27,28]. The area fractions of each peak are summarized in Table 3. The doping level of PANI in the composite, estimated from the N+/(N+ + ANHA + AN@) ratio, reveals that the doping level of PANI/SiO2/c-Fe2O3 composites decreases from 0.14 to 0.08 as the TEOS content increases from 1 to 5 ml. In comparison to ES-0, the doping level of PANI/SiO2/c-Fe2O3 composites is apparently lower than that of ES-0, suggesting that the short PANI chains are coated on the SiO2/c-Fe2O3 core, resulting in the lower doping level of PANI in the composites. Moreover, the ratio of oxidation unit to reduction unit of PANI, as indicated by the (N+ + AN@)/ANHA ratio, shows that the (N++AN@)/ANHA ratio decreases from 1.10 to 0.96 with increasing the TEOS content from 1 to 4 ml. The (N+ + AN@)/ANHA ratio for samples with 1–5 ml TEOS (ES-1 to ES-5) is smaller than that of ES-0, indicating that the growth of quinoid ring is markedly retarded, which may lead to the lowered doping level and smaller (N+ + AN@)/ANHA ratio in PANI/SiO2/c-Fe2O3 composites when compared with ES-0 [12]. WAXD patterns of synthesized PANI/SiO2/c-Fe2O3 nanocomposites obtained using various TEOS contents are shown in Fig. 7. Diffraction peaks at 2h = 9.2°, 15.1°, 21.3°and 25.5° are believed to correspond to the characteristic peaks of the PANI doped by HCl, while the peaks at 2h = 20–30° and 35.5° are corresponding to the characteristic peak of silica and the (3 1 1) crystal plane of c-Fe2O3, respectively. As the content of TEOS increases, both characteristic peaks of the c-Fe2O3 and doped-PANI (2h = 25.5°) become smaller and boarder, indicating the increased amorphous silica content and shortened PANI chains coated onto the surface of the c-Fe2O3 and SiO2/c-Fe2O3
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Sample
ANHA
Area
ANH@
Area
N+
Area
Doped level
(N+ + AN@)/ANHA
ES-0 ES-SF-1 ES-SF-3 ES-SF-5
399.7 399.5 399.3 399.2
503.3 554.3 554.3 508.8
398.8 398.5 398.4 398.2
426.3 456.7 471.3 407.1
401.3 401.3 400.7 400.7
290.1 158.6 132.8 79.7
0.238 0.136 0.115 0.080
1.42 1.11 1.09 0.96
core, respectively, at higher TEOS content, leading to the significant reduction in the crystallinity of c-Fe2O3 particles and doped-PANI in the composites. For a better comparison, TEM micrographs of PANI/SiO2/c-Fe2O3 composites in Fig. 2(d)–(f) were used to show their morphologies. It can be observed that, as compared to SiO2/c-Fe2O3 nanoparticles, the outer layer exhibits an irregular brightness compared with the dark inner core, which confirms the presence of the multilayer core-shell structure of PANI/SiO2/c-Fe2O3 nanocomposites, with SiO2/c-Fe2O3 as the core and covered by PANI as the shell. It is also noted that the coverage of PANI chains on the SiO2/c-Fe2O3 core is very fragmented in shape at high TEOS content. The conductivity and the saturation magnetization of PANI/SiO2/c-Fe2O3 composites synthesized with various TEOS contents are shown in Table 4. The conductivity of PANI/SiO2/c-Fe2O3 composites mainly results from the conductive channels in the composite, which consists of conductive PANI and nonconductive silica [29]. It is found that the conductivity of PANI/SiO2/c-Fe2O3 composites de-
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Intensity
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creases from 0.062 to 0.024 S/cm with increasing the TEOS content from 1 to 5 ml, likely due to less PANI content and/ or shorter PANI chains on the SiO2/c-Fe2O3 core at higher TEOS content, which leads to the lower conductivity of PANI in the composites than that without TEOS (i.e., sample ES-0). As revealed by the TEM observation, the better coverage of PANI polymers on the SiO2/c-Fe2O3 core may provide more continuous conductive channels for electron transfer. The result suggests that the conductivity of PANI/ SiO2/c-Fe2O3 composites is strongly related to the chains length of the coated PANI and its coverage morphology. Fig. 8 shows the magnetization hysteresis of PANI/SiO2/cFe2O3 composites with various compositions. It can be seen that PANI/SiO2/c-Fe2O3 composites exhibit negligible coercivity and remanence, indicating that the composites have the typical superparamagnetic characteristics. Values of the saturation magnetization are also shown in Table 4. It is noted that, as compared to SiO2/c-Fe2O3 nanoparticles, the saturation magnetization of PANI/SiO2/c-Fe2O3 composites is apparently less than that of SiO2/c-Fe2O3 nanoparticles. This may be attributed to the shielding effect of conductive PANI chains under the magnetic field. As for PANI/SiO2/c-Fe2O3 composites, the saturation magnetization of PANI/SiO2/c-Fe2O3 composites significantly decreases from 7.4 to 3.5 emu/g with the increase in TEOS content, contributing from the decrease in conductivity of PANI in the composites. The loss tangent (tan d = e00 /e0 , where e00 and e0 are permittivity and loss factor, respectively) is also reduced, leading to the decrease in the absorption effect under the magnetic field [30,31]. The re-
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Table 4 Values of conductivity and saturation magnetization (Ms) for PANI/SiO2/cFe2O3 composites synthesized at various TEOS contents. Sample
Conductivity (S/cm)
Silica shell thickness (nm)
Ms (emu/g)a
Ms (emu/g)
c-Fe2O3
– 0.570 0.062 0.027 0.024
– – 10 30 55
60.0 – 43.1 26.8 12.7
60.0 – 7.4 5.0 3.5
ES-0 ES-SF-1 ES-SF-3 ES-SF-5 a
Data obtained from Table 1.
M (emu/g)
2θ Fig. 7. WAXD patterns of PANI/SiO2/c-Fe2O3 composites synthesized at various TEOS contents; (a) c-Fe2O3; (b) ES-SF-1; (c) ES-SF-3; (d) ES-SF-5; (e) ES-0.
(a) (b) (c)
2 0 -2 -4 -6 -8 -15000
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0
5000
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H (Oe) Fig. 8. Saturation magnetization vs. applied magnetic field at 300 K for PANI/SiO2/c-Fe2O3 composites synthesized at various TEOS contents: (a) ES-SF-1; (b) ES-SF-3; (c) ES-SF-5.
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Table 3 XPS analysis results for PANI/SiO2/c-Fe2O3 composites synthesized at various TEOS contents.
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sult suggests that the saturation magnetization of PANI/ SiO2/c-Fe2O3 composites is strongly affected by the conductivity of the outer PANI shell.
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4. Conclusions In the present study, we developed a novel synthesis process for the preparation of PANI/SiO2/c-Fe2O3 nanocomposites with a multilayer core-shell structure. The TEM observation showed that the SiO2/c-Fe2O3 particles exhibited a well-defined core-shell structure, with cFe2O3 as the magnetic core and silica as the shell. The particle size of SiO2/c-Fe2O3 ranged from 52 to 195 nm in diameter, which increased with increasing the TEOS content, in which the c-Fe2O3 core exhibited more expanded morphology as TEOS content increased. The PANI/SiO2/cFe2O3 composites revealed a multilayer core-shell structure, where PANI was the outer shell of the composite with an irregular morphology. The doping level and the conductivity of PANI/SiO2/c-Fe2O3 composites decreased with increasing the TEOS content because of less PANI coated onto the SiO2/c-Fe2O3 core at higher TEOS content. For SQUID analysis at room temperature, all c-Fe2O3 containing composites showed the typical superparamagnetic behavior. The saturation magnetization of SiO2/c-Fe2O3 nanoparticles decreased with increasing the TEOS content due to the increase of silica shell thickness at high TEOS contents. Moreover, the saturation magnetization of PANI/SiO2/c-Fe2O3 nanoparticles decreased with increasing the TEOS content, which may result from the lower conductivity of PANI in the composites at higher TEOS content.
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19] [20] [21]
[22]
[23]
References [1] Wan M, Fan J. Synthesis and ferromagnetic properties of composites of a water-soluble polyaniline copolymer containing iron oxide. J Polym Sci A Polym Chem 1998;36(15):2749–55. [2] Liu G, Freund MS. New approach for the controlled cross-linking of polyaniline: synthesis and characterization. Macromolecules 1997;30(19):5660–5. [3] Wan M, Li J. Synthesis and electrical-magnetic properties of polyaniline composites. J Polym Sci A Polym Chem 1998;36(15):2799–805. [4] Tang BZ, Geng YH, Sun QH, Zhang XX, Jing X. Processible nanomaterials with high conductivity and magnetizability. Preparation and properties of maghemite/polyaniline nanocomposite films. Pure Appl Chem 2000;72(1-2):157–62. [5] Mallikarjuna NN, Manohar SK, Kulkarni PV, Venkataraman A, Aminabhavi TM. Novel high dielectric constant nanocomposites of polyaniline dispersed with c-Fe2O3 nanoparticles. J Appl Polym Sci 2005;97(5):1868–74. [6] Raksha S, Subhalakshmi L, Annapoorni S, Parmanand S, Akihisa I. Composition dependent magnetic properties of iron oxidepolyaniline nanoclusters. J Appl Phys 2005;97(1):014311–6. [7] Bao L, Jiang JS. Evolution of microstructure and phase of Fe3O4 in system of Fe3O4–polyaniline during high-energy ball milling. Phys B Condens Matter 2005;367(1-4):182–7. [8] Riskin M, Basnar B, Huang Y, Willner I. Magnetoswitchable charge transport and bioelectrocatalysis using maghemite–Au core-shell
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
nanoparticle/polyaniline composites. Adv Mater 2007;19(18): 2691–5. Denga J, Hea CL, Peng Y, Wang J, Longa X, Li P, et al. Magnetic and conductive Fe3O4–polyaniline nanoparticles with core-shell structure. Synth Met 2003;139(2):295–301. Long Y, Chen Z, Duvail JL, Zhang Z, Wan M. Electrical and magnetic properties of polyaniline/Fe3O4 nanostructures. Phys B Condens Matter 2005;370(1–4):121–30. Lu X, Yu Y, Chen L, Mao H, Gao H, Wang J, et al. Aniline dimer-COOH assisted preparation of well-dispersed polyaniline–Fe3O4 nanoparticles. Nanotechnology 2005;16(9):1660–5. Hsieh TH, Ho KS, Huang CH, Wang YZ, Chen ZL. Electromagnetic properties of polyaniline/maghemite nanocomposites. I. The effect of re-doping time on the electromagnetic properties. Synth Met 2006;156(21–24):1355–61. Zhang L, Papaefthymiou GC, Ying JY. Size quantization and interfacial effects on a novel c-Fe2O3/SiO2 magnetic nanocomposite via sol–gel matrix-mediated synthesis. J Appl Phys 1997;81(10):6892–900. Haddad PS, Duarte EL, Baptista MS, Goya GF, Leite CAP, Itri R. Synthesis and characterization of silica-coated magnetic nanoparticles. Prog Colloid Polym Sci 2004;128:232–8. Barnakov YA, Yu MH, Rosenzweig Z. Manipulation of the magnetic properties of magnetite–silica nanocomposite materials by controlled stober synthesis. Langmuir 2005;21(16):7524–7. Xia H, Wang Qi. Preparation of conductive polyaniline/nanosilica particle composites through ultrasonic irradiation. J Appl Polym Sci 2003;87(11):1811–7. Li ZF, Swihart MT, Ruckenstein E. Luminescent silicon nanoparticles capped by conductive polyaniline through the self-assembly method. Langmuir 2004;20(5):1963–71. Jang J, Ha J, Lim B. Synthesis and characterization of monodisperse silica–polyaniline core-shell nanoparticles. Chem Commun 2006:1622–4. Massart R. Preparation of aqueous magnetic liquids in alkaline and acidic media. IEEE Trans Magn 1981;17(2):1247–8. Klong HP, Alexander LF. X-ray diffraction procedures for crystalline and amorphous materials. New York: John Wiley and Sons; 1954. Stöber W, Fink A, Bohn E. Controlled growth of monodisperse silica spheres in the micron size range. J Colloid Interface Sci 1968;26(1): 62–9. Salgueiriño-Maceira V, Correa-Duarte MA, Spasova M, Liz-Marzán LM, Farle M. Composite silica spheres with magnetic and luminescent functionalities. Adv Funct Mater 2006;16(4):509–14. Deng YH, Wang CC, Hu JH, Yang WL, Fu SK. Investigation of formation of silica-coated magnetite nanoparticles via sol–gel approach. Colloids Surf A Physicochem Eng Aspects 2005;262(1–3):87–93. Chaneac C, Tronc E, Jolivet JP. Magnetic iron oxide–silica nanocomposites. Synthesis and characterization. J Mater Chem 1996;6(12):1905–11. Donselaar LN, Philipse AP. Interactions between silica colloids with magnetite cores: diffusion, sedimentation and light scattering. J Colloid Interface Sci 1999;212(1):14–23. Ma D, Guan J, Normandin F, De´nomme´e S, Enright G, Veres T, et al. Multifunctional nano-architecture for biomedical applications. Chem Mater 2006;18(7):1920–7. Chen SA, Lin LC. Polyaniline doped by the new class of dopant, ionic salt: structure and properties. Macromolecules 1995;28(4): 1239–45. Neoh KG, Pun MY, Kang ET, Tan KL. Polyaniline treated with organic acids: doping characteristics and stability. Synth Met 1995;73(3): 209–15. Feng X, Yang G, Liu Y, Hou WH, Zhu JJ. Synthesis of polyaniline/ MCM-41 composite through surface polymerization of aniline. J Appl Polym Sci 2006;101(3):2088–94. Chandrasekhar P, Naishadham K. Broadband microwave absorption and shielding properties of a poly(aniline). Synth Met 1999;105(2): 115–20. Sun GC, Yao KL, Liao HX, Niu ZC, Liu ZL. Microwave absorption characteristics of chiral materials with Fe3O4–polyaniline composite matrix. J Electron 2000;87(6):735–40.
Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Macromolecular Nanotechnolgy
Synthesis and electromagnetic properties of polyaniline-coated silica/maghemite nanoparticles Tar-Hwa Hsieh a,*, Ko-Shan Ho a, Xiaotao Bi b, Yu-Kai Han a, Zhi-Long Chen a, Chia-Hao Hsu a, Yu-Chen Chang c a
Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, 415 Chien-Kung Road, Kaohsiung 807, Taiwan Department of Chemical and Biological Engineering, The University of British Columbia, Vancouver, Canada c Institute of Materials Science and Engineering, National Sun Yat-Sen University, Kaohsiung, 70 Lian-Hai Road, Kaohsiung, Taiwan
a r t i c l e
i n f o
Article history: Received 25 July 2008 Received in revised form 15 December 2008 Accepted 20 December 2008 Available online 31 December 2008
Keywords: PANI/SiO2/c-Fe2O3 nanocomposites Multilayer core-shell structure Electromagnetic properties Superparamagnetic behavior
a b s t r a c t Polyaniline coated silica/maghemite nanoparticles (PANI/SiO2/c-Fe2O3 composites) were synthesized by the combination of a sol–gel process and an in-situ polymerization method, in which ferrous and ferric salts as well as tetraethyl orthosilica (TEOS) acted as the precursor for c-Fe2O3 and silica, respectively. As a result, the SiO2/c-Fe2O3 particle showed a coreshell structure, with c-Fe2O3 as the magnetic core and silica as the shell of the particle. The shell thickness can be controlled by changing the TEOS concentration. The PANI/SiO2/cFe2O3 composites revealed a multilayer core-shell structure, where PANI is the outer shell of the composite. The doping level and the conductivity of PANI/SiO2/c-Fe2O3 composites decreased with increasing the TEOS content due to the presence of the less coated PANI on the SiO2/c-Fe2O3 core at higher TEOS content. For a SQUID analysis at room temperature, all c-Fe2O3 containing composites showed a typical superparamagnetic behavior. The saturation magnetization of SiO2/c-Fe2O3 nanoparticles decreased with increasing the TEOS content due to the increase in silica shell thickness, while the saturation magnetization of PANI/SiO2/c-Fe2O3 composites also decreased with increasing the TEOS content, which is attributed to the lower conductivity of PANI in the composites at higher TEOS content. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction In the past decade, polyaniline (PANI) containing iron oxides with both electrical and magnetic properties have been studied extensively. The preparation of PANI composites with magnetic and conductive properties has been mostly studied by Wan’s group by (1) blending the PANI in N-methyl-2-pyrrolidone (NMP) with FeSO4 aqueous solution, and precipitating Fe2+ into c-Fe2O3 [1] and (2) reacting FeCl24H2O and FeCl36H2O with aniline followed by treatment with KOH aqueous solution [2]. A poly (aniline-co-aminobenzenesulfonic acid) copolymer containing
* Corresponding author. Tel.: +886 7 3814526x5117; fax: +886 7 3830674. E-mail address: [email protected] (T.-H. Hsieh). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.12.039
c-Fe2O3 magnetic particles has also been prepared with the first method [3]. Tang et al. [4] have developed a versatile process employing anionic surfactants to prepare processable free standing films of PANI containing up to 50% c-Fe2O3 nanomagnets. Mallikarjuna et al. [5] reported the preparation of novel nanocomposites of polyaniline dispersed with c-Fe2O3 nanoparticles by an in-situ polymerization method. These composites exhibited high dielectric constants in proportion to the amount of c-Fe2O3. Sharma et al. [6] also reported the magnetic properties of iron oxide-PANI nanoclusters prepared at various aniline concentrations. Moreover, Bao and Jiang [7] prepared PANI/ Fe3O4 nanocomposites by the high-energy ball milling method. Recently, PANI containing iron oxides composites with core-shell structure have been extensively investigated since they exhibited dual properties of core and shell
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materials, leading to potential applications in electromagnetic shielding, batteries, electrochromic device, molecular electronic, non-linear optics, sensors, and microwaveabsorbing materials [8–11]. Deng et al. [9] have reported the synthesis of core-shell PANI/Fe3O4 composites with an average diameter of 80 nm by in-situ emulsion polymerization. These composites exhibited high electrical and ferromagnetic properties in proportion to the percentage of Fe3O4 and degree of doping. Long et al. [10] suggested a template-free method to prepare PANI/Fe3O4 nanorods with a core-shell structure. The saturation magnetization (3.45 emu/g at 300 K) was lower than the bulk magnetite (Ms = 84 emu/g) and pure Fe3O4 nanoparticles (Ms = 65 emu/g) due to the Fe3O4–PANI interactions. Lu et al. [11] have developed a method to synthesize well-dispersed PANI/Fe3O4 nanoparticles with aniline dimer-COOH polymerization. The core-shelled PANI/Fe3O4 nanoparticles exhibited typical superparamagnetic characteristics with a relatively high saturation magnetization (Ms = 21 emu/g) and a low coercive force (lc = 0). Recently, we [12] successfully synthesized PANI/c-Fe2O3 nanocomposites in the ternary system of toluene/SDS/water by using a reverse micelle method, with the product exhibiting typical superparamagnetic characteristics. The dispersed c-Fe2O3 phase was non-uniformly distributed in the PANI matrix, and exhibited a wide size distribution caused by PANI–cFe2O3 interactions, which led to the resultant composites possessing a low conductivity (ca. 103 S/cm) and low saturation magnetization (2.05–1.08 emu/g). Despite the effect of PANI–c-Fe2O3 interactions on the structure, morphology and electromagnetic properties of PANI/cFe2O3 composites in the reverse micelle polymerization has been examined, it is highly desirable to further develop the synthetic method to prevent the agglomeration of the c-Fe2O3 nanoparticles from PANI–c-Fe2O3 interactions, achieving a chemically stability composite with high conductivity and saturation magnetization. Isolating the magnetic particle and conductive PANI by silica could possibly generate novel composites with well-designed multilayer core-shell structures, and intriguing electronic and magnetic properties. A number of reports have described the preparation of the silica-coated iron oxide to reduce the agglomeration of iron oxide nanoclusters. Zhang et al. [13] reported the synthesis of core-shell SiO2/c-Fe2O3 nanoparticles by coprecipitation of ferrous and ferric salts encapsulated within sol–gel derived SiO2. The nonmagnetic SiO2 coating formed by hydrolysis and polycondensation of tetraethoxysilane on the surface of c-Fe2O3 nanoclusters provides a mean for thermally stable dispersion of c-Fe2O3 clusters. Haddad et al. [14] prepared silica-coated magnetite nanoparticles with a diameter of 20–30 nm and a shell thickness of 5–7 nm. The resulting particles showed superparamagnetic characteristics with a decrease in the saturation magnetization by about 40%. Rosenzweig et al. [15] reported the preparation of coreshell SiO2/c-Fe2O3 composites with a diameter of about 250 nm by a modified Stöber method, in which magnetite nanocrystals were separated by a thin rigid silica shell. The resulting particles maintained their superparamagnetic properties in the silica composites. Recently, the preparation of silica nanoparticles coated with a thin layer of con-
ductive PANI attracted particular interest, because the surface of the nanoparticles can easily be altered to meet the specific catalytic, magnetic, electronic, optical, or optoelectronic requirements. Xia and Wang [16] prepared PANI/silica nanoparticle composites through ultrasonic irradiation. The formed PANI was deposited on the surface of the nano-SiO2, which led to a core-shell structure with a strong interaction between PANI and nano-SiO2. Ruckenstein et al. [17] also reported that PANI grafted silica nanoparticles could be prepared facilely by the chemical oxidative polymerization of aniline from the amino groups on the surface of aminopropyl silica nanoparticles. Jang et al. [18] developed a method for the fabrication of coreshell PANI/silica nanoparticles by in-situ polymerization of aniline monomers adsorbed on the silica surface through electrostatic interactions. Although the core-shell PANI/iron oxide composites have been widely studied either by coating iron oxide with silica or by coating conductive PANI onto the silica surface, no work has been reported on the preparation of multilayer core-shell PANI/SiO2/c-Fe2O3 composites and their electromagnetic characteristics. In the present study, PANI coated SiO2/c-Fe2O3 nanoparticles were synthesized by combining the sol–gel process and the in-situ polymerization method. At a molar ratio of Fe3+/Fe2+ = 2, the effect of TEOS content on the structure, morphology and electromagnetic properties of the PANI/SiO2/c-Fe2O3 composites was investigated by Fourier transform infrared spectroscopy (FT-IR), Ultraviolet–visible spectroscopy (UV–vis), wide angle X-ray diffraction (WAXD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), micro-ohmetry and superconductor quantum interference device (SQUID), respectively. The origin of the electrical and magnetic properties of the nanocomposite is also discussed.
2. Experimental 2.1. Preparation
c-Fe2O3 magnetic nanoparticles were prepared by the precipitation–oxidation method [19] described in our previous study [12]. The c-Fe2O3 particles were found to exist in the form of spherical clusters, with a diameter of 10– 20 nm estimated from the WAXD patterns by applying the Scheere’s equation [20]. SiO2/c-Fe2O3 composites were prepared by a modified Stöber method [21]. The c-Fe2O3 particle containing alcohol aqueous solution was prepared by dispersing 0.5 g c-Fe2O3 into a mixture of 240 mL alcohol and 60 mL deionized water in a water bath under ultrasonic vibration for 30 min. Then, NH4OH (28–30 wt.%) solution was added into the above-prepared solution stirring at room temperature until the pH reached 11, followed by gradual addition of different amounts of TEOS (1–5 mL) into this solution to initiate the formation of SiO2/c-Fe2O3 nanoparticles. The synthesis process lasted for 12 h at 50 °C with stirring in order to form the nanoparticles of SiO2/c-Fe2O3. The prepared SiO2/c-Fe2O3 nanoparticles are designated here as SF-1, SF-3 and SF-5, respectively, where SF and the number indicate the pres-
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The composite powder was pressed at room temperature into tablets of approximately 1 mm in thickness and 13 mm in diameter and 0.3 0.3 mm2 for the X-ray and the XPS analysis, respectively. The X-ray diffraction patterns of the tablet samples were obtained with a Rugaku Denki D/max-2200 diffractometer operated at 45 kV and 40 mA using Cu Ka radiation (k = 0.154 nm), with the diffraction angles (2h) ranging from 20° to 70° at 2°/min. The weight loss of samples was conducted in a Du Pont TA Instrument SDT Q600 thermogravimetric analysis (TGA) unit with about 4–8 mg samples at a heating rate of 20 °C/min under air atmosphere. The scanning temperature ranged from room temperature to 700 °C. Samples were embedded in the epoxy resin, and sectioned into about 70 nm in thickness using a Reichert microtone at room temperature. Samples were then corrected in a though filled with methanol, and placed on a 200 mesh copper grid for the TEM observation. The TEM were obtained with a JEOL II-1200EX apparatus running at an acceleration voltage of 80 kV. Both SiO2/c-Fe2O3 nanoparticles and EB form composites were examined with 32 scans and 2 cm1 resolution on a BIO-RAD FTS-165 FT-IR using KBr pellets. The XPS of tablets was carried out on a VG Scientific ESCALAB-250 with Mg Ka (1253.6 eV) radiation. The conductivity of tablet samples (D = 1.295 cm, thickness = 1 mm) at room temperature was measured with Agilent HP4338A micro-ohmetry using 4-probes method with silver electrode. The magnetization of the samples was characterized using a SQUID magnetometer (Quantum
3. Results and discussion 3.1. Structure and magnetic properties of SiO2/c-Fe2O3 nanoparticles WAXD patterns of synthesized SiO2/c-Fe2O3 particles obtained using various TEOS contents are shown in Fig. 1. It can be seen that WAXD patterns of SiO2/c-Fe2O3 particles exhibit diffraction peaks at 2h = 20–30°, corresponding to the characteristic peaks of the silica, and the peak at 2h = 35.5° is the characteristic peak of the (3 1 1) crystal plane of c-Fe2O3 [5,12]. When the content of TEOS is increased, the characteristic peak of c-Fe2O3 becomes smaller and boarder, while the characteristic peaks of silica grow and become clearer. A smaller characteristic peak of c-Fe2O3 and clearer characteristic peaks of SiO2 in nanoparticles at higher TEOS content indicate that the crystallinity of SiO2/c-Fe2O3 particles decreases with the increase in TEOS content, which suggests that c-Fe2O3 nanoclusters are in the amorphous silica matrix. However, this trend becomes more noticeable at higher TEOS content. Fig. 2(a)–(c) display the morphology of SiO2/c-Fe2O3 particles by TEM. It is found that the outer shell of the particle exhibits a fine increment in brightness compared with the dark inner core, which confirms the well-known coreshell structure of SiO2/c-Fe2O3 particles, where c-Fe2O3 is the magnetic core embedded within the silica matrix and the silica shell. It is noted that the core-shell structured SiO2/c-Fe2O3 particles are rather monodispersed, although most silica matrices have trapped more than one magnetic core because of the aggregation of the iron oxide nanoparticles prior to or during the coating process [22]. The particle size and the silica shell thickness of the SiO2/c-Fe2O3 nanoparticles determined by TEM are 52, 95 and 195 nm, as well as 10, 33 and 55 nm, respectively, for the three tested samples, indicating that both particle size and silica shell thickness of the nanoparticles increase with increasing the TEOS content. This observation clearly demon-
(a)
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2θ (deg.) Fig. 1. WAXD patterns of SiO2/c-Fe2O3 particles synthesized at various TEOS contents: (a) c-Fe2O3; (b) SF-1; (c) SF-3; (d) SF-5.
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2.2. Characterization
Drsign, MPMS) at 300 and 5 K, respectively, measured in the applied field at ±1.5 T.
Intensity
ence of SiO2/c-Fe2O3 particles and the content (in mL) of TEOS used in the preparation, respectively. PANI coated SiO2/c-Fe2O3 composites were synthesized via an in-situ polymerization of aniline monomer in an aniline/silica solution, which contains well-dispersed SiO2/c-Fe2O3 nanoparticles. The aniline/silica solution was prepared by adding 0.3 g SiO2/c-Fe2O3 nanoparticles into 50 mL deionized water, followed by the addition of 0.2 g aniline and HCl aqueous solution (35 wt.%) to the above-prepared solution under vigorous stirring for 1 h until the pH reached 3. Then, the ammonium persulfate (APS) aqueous solution (0.26 g APS dissolved in 2 mL distilled water) was added gradually into the aniline/silica solution to start the in-situ polymerization of aniline monomers. The polymerization continued under magnetic stirring for 6 h at 0 °C, the emeraldine salt form of PANI/SiO2/c-Fe2O3 composites (ES form) was obtained. The PANI/SiO2/cFe2O3 composites in the emeraldine base form (EB form) were prepared by deprotonation of the obtained ES form composites, in which the PANI/SiO2/c-Fe2O3 composites (ES form) was mixed with an excess amount of 1 M NaOH solution under stirring for 24 h, then washed with distilled water and methanol several times, followed by drying in vacuum at 60 °C for 2 h. In the present study, the ES or EB form composites synthesized at various TEOS contents are designated as ES-SF-1, ES-SF-3 and ES-SF-5, or EB-SF1, EB-SF-3 and EB-SF-5, where ES, EB and the number indicate the ES, EB form composites and the content of TEOS, respectively.
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Fig. 2. TEM micrographs of (a) SF-1, (b) SF-3, (c) SF-5, (d) ES-SF-1, (e) ES-SF-3 and (f) ES-SF-5. The average diameter of SiO2/c-Fe2O3 particles is 52, 92 and 195 nm, respectively.
strates that the nanoparticle characteristics of SiO2/cFe2O3 nanoparticles can be easily controlled by simply varying the initial amount of TEOS [22,23]. The FT-IR spectra of synthesized SiO2/c-Fe2O3 nanoparticles at various TEOS contents are shown in Fig. 3. The characteristic peaks of the SiO2/c-Fe2O3 particles at 634, 567 and 451 cm1 are due to the stretching vibrations of FeAO [24], the peaks at 799 and 471 cm1 as well as the strong peak at 1082 cm1 and the shoulder at 1188 cm1 are associated with the bending vibration of SiAOASi and OASiAO bond, and the overlap peaks of the SiAOASi symmetric stretching mode and the CH stretching mode in plane bend, respectively. When compared with c-Fe2O3, the characteristic peaks of SiO2/c-Fe2O3 nanoparticles are
shifted to higher frequencies from 634 to 640 cm1 and 567 to 582 cm1, respectively, indicating the presence of some interactions between the c-Fe2O3 and the silica. Such interactions are likely caused by the van der Waals attraction between uncharged silica spheres with the c-Fe2O3 magnetic core [25]. When TEOS content is increased, the core size of the embedded c-Fe2O3 particles apparently increases from ca. 28 to 85 nm, exhibiting a more expansive core morphology, which can be attributed to the decrease of van der Waals force at high TEOS contents, as seen in Fig. 2. The result demonstrates that the iron oxide surface has a strong affinity to silica. Hence, no primer is required to promote the deposition and adhesion of silica, and the core-shell SiO2/c-Fe2O3 nanoparticles can be successfully
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(a)
Transmittance
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Table 1 Values of saturation magnetization (Ms) for SiO2/c-Fe2O3 nanoparticles synthesized at various TEOS contents. Sample
Particle size (nm)
Silica shell thickness (nm)
Ms (emu/g)
c-Fe2O3
10–20 52 95 195
– 10 30 55
60.1 43.1 26.8 12.7
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Fig. 3. FT-IR spectra of SiO2/c-Fe2O3 nanoparticles synthesized with various TEOS contents; (a) c-Fe2O3; (b) SF-1; (c) SF-3; (d) SF-5.
synthesized via a sol–gel process by simply varying the initial amount of TEOS. The hysteresis loops of the c-Fe2O3 and SiO2/c-Fe2O3 nanoparticles measured at 300 K are shown in Fig. 4. It can be seen that, both c-Fe2O3 and SiO2/c-Fe2O3 nanoparticles exhibit negligible coercivity and remanence, indicating that the products possess the typical superparamagnetic characteristics. Values of the saturation magnetization of SiO2/c-Fe2O3 nanoparticles are shown in Table 1. It can be seen that the saturation magnetization of the SiO2/c-Fe2O3 is smaller than c-Fe2O3 (Ms is 60.0 emu/g) and decreases from 43.1 to 12.7 emu/g with increasing the TEOS content, which is attributed to the increased nonmagnetic silica shell thickness at high TEOS contents [14,26]. The result demonstrates that the magnetic properties of the SiO2/c-Fe2O3 nanoparticles are strongly influenced by the silica shell thickness. On the other hand, the magnetic properties of the SiO2/c-Fe2O3 nanoparticles can be easily controlled by varying the starting c-Fe2O3 and TEOS contents during fabrication.
In order to identify the existence of PANI in the composite, the EB form composites were first prepared by dissolving in NMP, which is a good solvent for the emeraldine base form of PANI (EB), and then characterized by FT-IR. Fig. 5 shows the FT-IR spectra of EB form composites. In Fig. 5(a), the characteristic peaks at 1497, 1590 and 1310 cm1 are associated with the stretching vibration of benzenoid ring and quinoid ring, as well as CAN stretching of the secondary aromatic amine, respectively, of PANI (EB). Fig. 5(b)–(d) show the characteristic peaks of benzenoid ring and quinoid ring of the EB form composites at 1494 and 1581 cm1, respectively, which are shifted to lower frequency from 1494 to 1483 cm1 and 1581 to 1572 cm1, respectively, with increasing TEOS content, suggesting that PANI is exactly coated on the surface of SiO2/c-Fe2O3 nanoparticles owing to the electrostatic attraction between cationic anilinium ions with negatively charged surface of the silica core [25]. The result indicated that PANI can be successfully polymerized onto the surface of SiO2/c-Fe2O3 core via in-situ polymerization. The possible mechanism for the formation of SiO2/c-Fe2O3 nanoparticles and PANI/SiO2/c-Fe2O3 composites is shown in Fig. 6. In the first step shown in Fig. 6(a), c-Fe2O3 nanoclusters are prepared by precipitation–oxidation method and the magnetic particles are trapped within silica spheres via a sol– gel process based on the hydrolysis of TEOS, forming uniform core-shell SiO2/c-Fe2O3 nanoparticles. In the second step shown in Fig. 6(b), aniline monomers are first con-
(a) (b) (c) (d)
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M (emu/g)
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H (Oe) Fig. 4. Saturation magnetization vs. applied magnetic field at 300 K for SiO2/c-Fe2O3 nanoparticles synthesized at various TEOS contents; (a) cFe2O3; (b) SF-1; (c) SF-3; (d) SF-5.
4000
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-1
Wavenumber (cm ) Fig. 5. FT-IR spectra of PANI/SiO2/c-Fe2O3 (EB form) composites synthesized at various TEOS contents; (a) EB-0; (b) EB-SF-1; (c) EB-SF-3; (d) EBSF-5.
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3.2. Structure and electromagnetic properties of PANI/SiO2/cFe2O3 composites
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Fe2+
In water
OC2H5 (TEOS)
OC2H5
OH In ethanol and water
- Fe2O3 20 ~ 30 nm
b
Si
Si
HO
Fe3+
Excess of OH Coprecipitation
Si
HO
OC2H5 C2H5O
HO
a
Si
- Fe2O3 Silica layer ~ 55 nm
NH2
HCl APS
MACROMOLECULAR NANOTECHNOLOGY
+
NH3
SiO2/ -Fe2O3 nanoparticle
+
In-situ polymerization
PANI/SiO2/ composite
-Fe2O3
Fig. 6. Schematic diagram illustrating the synthetic procedure of PANI/SiO2/c-Fe2O3 composites (a) formation of core-shell SiO2/c-Fe2O3 nanoparticle; (b) formation of multilayer core-shell PANI/SiO2/c-Fe2O3 composite.
verted to cationic anilinium ions in acidic conditions at a pH of 3 and then adsorbed onto the negatively charged surface of silica nanoparticles (to the isoelectric point of silica at pHs greater than 2) through electrostatic attractions [18], further in-situ polymerized along the silica shell by the addition of the oxidant, resulting in the formation of PANI/SiO2/c-Fe2O3 capsules with a multilayer core-shell structure. The TGA data of PANI/SiO2/c-Fe2O3 are summarized in Table 2. It can be seen that the PANI content slightly decreases with the increase in the initial TEOS content. The result suggests that the effective concentration of aniline monomer on the SiO2/c-Fe2O3 core decreases with increasing the core size due to the decrease of the numbers of active sites on the negatively charged silica surface, leading to less PANI being coated onto the SiO2/c-Fe2O3 core at higher TEOS content. The typical N (1s) XPS core-level spectrum of the PANI/ SiO2/c-Fe2O3 composite can be deconvoluted into three component peaks centered at 398.5 ± 0.3, 399.5 ± 0.2 and 401.8 ± 0.3 eV, representing the nitrogen atoms of imine, Table 2 TGA data of PANI/SiO2/c-Fe2O3 composites synthesized at various TEOS contents. Sample
Residual weight at 700 °C (%)
Total PANI content (%)a
SF-1 SF-3 SF-5 ES-0 ES-SF-1 ES-SF-3 ES-SF-5
91.8 92.7 93.0 0 19.1 20.9 21.8
– – – 0 72.7 71.8 71.2
a
Absorbed PANI content on the surface of SiO2/c-Fe2O3 core.
amine units as well as positively charged nitrogen atom, respectively [27,28]. The area fractions of each peak are summarized in Table 3. The doping level of PANI in the composite, estimated from the N+/(N+ + ANHA + AN@) ratio, reveals that the doping level of PANI/SiO2/c-Fe2O3 composites decreases from 0.14 to 0.08 as the TEOS content increases from 1 to 5 ml. In comparison to ES-0, the doping level of PANI/SiO2/c-Fe2O3 composites is apparently lower than that of ES-0, suggesting that the short PANI chains are coated on the SiO2/c-Fe2O3 core, resulting in the lower doping level of PANI in the composites. Moreover, the ratio of oxidation unit to reduction unit of PANI, as indicated by the (N+ + AN@)/ANHA ratio, shows that the (N++AN@)/ANHA ratio decreases from 1.10 to 0.96 with increasing the TEOS content from 1 to 4 ml. The (N+ + AN@)/ANHA ratio for samples with 1–5 ml TEOS (ES-1 to ES-5) is smaller than that of ES-0, indicating that the growth of quinoid ring is markedly retarded, which may lead to the lowered doping level and smaller (N+ + AN@)/ANHA ratio in PANI/SiO2/c-Fe2O3 composites when compared with ES-0 [12]. WAXD patterns of synthesized PANI/SiO2/c-Fe2O3 nanocomposites obtained using various TEOS contents are shown in Fig. 7. Diffraction peaks at 2h = 9.2°, 15.1°, 21.3°and 25.5° are believed to correspond to the characteristic peaks of the PANI doped by HCl, while the peaks at 2h = 20–30° and 35.5° are corresponding to the characteristic peak of silica and the (3 1 1) crystal plane of c-Fe2O3, respectively. As the content of TEOS increases, both characteristic peaks of the c-Fe2O3 and doped-PANI (2h = 25.5°) become smaller and boarder, indicating the increased amorphous silica content and shortened PANI chains coated onto the surface of the c-Fe2O3 and SiO2/c-Fe2O3
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Sample
ANHA
Area
ANH@
Area
N+
Area
Doped level
(N+ + AN@)/ANHA
ES-0 ES-SF-1 ES-SF-3 ES-SF-5
399.7 399.5 399.3 399.2
503.3 554.3 554.3 508.8
398.8 398.5 398.4 398.2
426.3 456.7 471.3 407.1
401.3 401.3 400.7 400.7
290.1 158.6 132.8 79.7
0.238 0.136 0.115 0.080
1.42 1.11 1.09 0.96
core, respectively, at higher TEOS content, leading to the significant reduction in the crystallinity of c-Fe2O3 particles and doped-PANI in the composites. For a better comparison, TEM micrographs of PANI/SiO2/c-Fe2O3 composites in Fig. 2(d)–(f) were used to show their morphologies. It can be observed that, as compared to SiO2/c-Fe2O3 nanoparticles, the outer layer exhibits an irregular brightness compared with the dark inner core, which confirms the presence of the multilayer core-shell structure of PANI/SiO2/c-Fe2O3 nanocomposites, with SiO2/c-Fe2O3 as the core and covered by PANI as the shell. It is also noted that the coverage of PANI chains on the SiO2/c-Fe2O3 core is very fragmented in shape at high TEOS content. The conductivity and the saturation magnetization of PANI/SiO2/c-Fe2O3 composites synthesized with various TEOS contents are shown in Table 4. The conductivity of PANI/SiO2/c-Fe2O3 composites mainly results from the conductive channels in the composite, which consists of conductive PANI and nonconductive silica [29]. It is found that the conductivity of PANI/SiO2/c-Fe2O3 composites de-
(a)
Intensity
(b)
(c)
creases from 0.062 to 0.024 S/cm with increasing the TEOS content from 1 to 5 ml, likely due to less PANI content and/ or shorter PANI chains on the SiO2/c-Fe2O3 core at higher TEOS content, which leads to the lower conductivity of PANI in the composites than that without TEOS (i.e., sample ES-0). As revealed by the TEM observation, the better coverage of PANI polymers on the SiO2/c-Fe2O3 core may provide more continuous conductive channels for electron transfer. The result suggests that the conductivity of PANI/ SiO2/c-Fe2O3 composites is strongly related to the chains length of the coated PANI and its coverage morphology. Fig. 8 shows the magnetization hysteresis of PANI/SiO2/cFe2O3 composites with various compositions. It can be seen that PANI/SiO2/c-Fe2O3 composites exhibit negligible coercivity and remanence, indicating that the composites have the typical superparamagnetic characteristics. Values of the saturation magnetization are also shown in Table 4. It is noted that, as compared to SiO2/c-Fe2O3 nanoparticles, the saturation magnetization of PANI/SiO2/c-Fe2O3 composites is apparently less than that of SiO2/c-Fe2O3 nanoparticles. This may be attributed to the shielding effect of conductive PANI chains under the magnetic field. As for PANI/SiO2/c-Fe2O3 composites, the saturation magnetization of PANI/SiO2/c-Fe2O3 composites significantly decreases from 7.4 to 3.5 emu/g with the increase in TEOS content, contributing from the decrease in conductivity of PANI in the composites. The loss tangent (tan d = e00 /e0 , where e00 and e0 are permittivity and loss factor, respectively) is also reduced, leading to the decrease in the absorption effect under the magnetic field [30,31]. The re-
(d)
8 (e)
6 20
30
40
50
60
70
4
Table 4 Values of conductivity and saturation magnetization (Ms) for PANI/SiO2/cFe2O3 composites synthesized at various TEOS contents. Sample
Conductivity (S/cm)
Silica shell thickness (nm)
Ms (emu/g)a
Ms (emu/g)
c-Fe2O3
– 0.570 0.062 0.027 0.024
– – 10 30 55
60.0 – 43.1 26.8 12.7
60.0 – 7.4 5.0 3.5
ES-0 ES-SF-1 ES-SF-3 ES-SF-5 a
Data obtained from Table 1.
M (emu/g)
2θ Fig. 7. WAXD patterns of PANI/SiO2/c-Fe2O3 composites synthesized at various TEOS contents; (a) c-Fe2O3; (b) ES-SF-1; (c) ES-SF-3; (d) ES-SF-5; (e) ES-0.
(a) (b) (c)
2 0 -2 -4 -6 -8 -15000
-10000
-5000
0
5000
10000
15000
H (Oe) Fig. 8. Saturation magnetization vs. applied magnetic field at 300 K for PANI/SiO2/c-Fe2O3 composites synthesized at various TEOS contents: (a) ES-SF-1; (b) ES-SF-3; (c) ES-SF-5.
MACROMOLECULAR NANOTECHNOLOGY
Table 3 XPS analysis results for PANI/SiO2/c-Fe2O3 composites synthesized at various TEOS contents.
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sult suggests that the saturation magnetization of PANI/ SiO2/c-Fe2O3 composites is strongly affected by the conductivity of the outer PANI shell.
MACROMOLECULAR NANOTECHNOLOGY
4. Conclusions In the present study, we developed a novel synthesis process for the preparation of PANI/SiO2/c-Fe2O3 nanocomposites with a multilayer core-shell structure. The TEM observation showed that the SiO2/c-Fe2O3 particles exhibited a well-defined core-shell structure, with cFe2O3 as the magnetic core and silica as the shell. The particle size of SiO2/c-Fe2O3 ranged from 52 to 195 nm in diameter, which increased with increasing the TEOS content, in which the c-Fe2O3 core exhibited more expanded morphology as TEOS content increased. The PANI/SiO2/cFe2O3 composites revealed a multilayer core-shell structure, where PANI was the outer shell of the composite with an irregular morphology. The doping level and the conductivity of PANI/SiO2/c-Fe2O3 composites decreased with increasing the TEOS content because of less PANI coated onto the SiO2/c-Fe2O3 core at higher TEOS content. For SQUID analysis at room temperature, all c-Fe2O3 containing composites showed the typical superparamagnetic behavior. The saturation magnetization of SiO2/c-Fe2O3 nanoparticles decreased with increasing the TEOS content due to the increase of silica shell thickness at high TEOS contents. Moreover, the saturation magnetization of PANI/SiO2/c-Fe2O3 nanoparticles decreased with increasing the TEOS content, which may result from the lower conductivity of PANI in the composites at higher TEOS content.
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19] [20] [21]
[22]
[23]
References [1] Wan M, Fan J. Synthesis and ferromagnetic properties of composites of a water-soluble polyaniline copolymer containing iron oxide. J Polym Sci A Polym Chem 1998;36(15):2749–55. [2] Liu G, Freund MS. New approach for the controlled cross-linking of polyaniline: synthesis and characterization. Macromolecules 1997;30(19):5660–5. [3] Wan M, Li J. Synthesis and electrical-magnetic properties of polyaniline composites. J Polym Sci A Polym Chem 1998;36(15):2799–805. [4] Tang BZ, Geng YH, Sun QH, Zhang XX, Jing X. Processible nanomaterials with high conductivity and magnetizability. Preparation and properties of maghemite/polyaniline nanocomposite films. Pure Appl Chem 2000;72(1-2):157–62. [5] Mallikarjuna NN, Manohar SK, Kulkarni PV, Venkataraman A, Aminabhavi TM. Novel high dielectric constant nanocomposites of polyaniline dispersed with c-Fe2O3 nanoparticles. J Appl Polym Sci 2005;97(5):1868–74. [6] Raksha S, Subhalakshmi L, Annapoorni S, Parmanand S, Akihisa I. Composition dependent magnetic properties of iron oxidepolyaniline nanoclusters. J Appl Phys 2005;97(1):014311–6. [7] Bao L, Jiang JS. Evolution of microstructure and phase of Fe3O4 in system of Fe3O4–polyaniline during high-energy ball milling. Phys B Condens Matter 2005;367(1-4):182–7. [8] Riskin M, Basnar B, Huang Y, Willner I. Magnetoswitchable charge transport and bioelectrocatalysis using maghemite–Au core-shell
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
nanoparticle/polyaniline composites. Adv Mater 2007;19(18): 2691–5. Denga J, Hea CL, Peng Y, Wang J, Longa X, Li P, et al. Magnetic and conductive Fe3O4–polyaniline nanoparticles with core-shell structure. Synth Met 2003;139(2):295–301. Long Y, Chen Z, Duvail JL, Zhang Z, Wan M. Electrical and magnetic properties of polyaniline/Fe3O4 nanostructures. Phys B Condens Matter 2005;370(1–4):121–30. Lu X, Yu Y, Chen L, Mao H, Gao H, Wang J, et al. Aniline dimer-COOH assisted preparation of well-dispersed polyaniline–Fe3O4 nanoparticles. Nanotechnology 2005;16(9):1660–5. Hsieh TH, Ho KS, Huang CH, Wang YZ, Chen ZL. Electromagnetic properties of polyaniline/maghemite nanocomposites. I. The effect of re-doping time on the electromagnetic properties. Synth Met 2006;156(21–24):1355–61. Zhang L, Papaefthymiou GC, Ying JY. Size quantization and interfacial effects on a novel c-Fe2O3/SiO2 magnetic nanocomposite via sol–gel matrix-mediated synthesis. J Appl Phys 1997;81(10):6892–900. Haddad PS, Duarte EL, Baptista MS, Goya GF, Leite CAP, Itri R. Synthesis and characterization of silica-coated magnetic nanoparticles. Prog Colloid Polym Sci 2004;128:232–8. Barnakov YA, Yu MH, Rosenzweig Z. Manipulation of the magnetic properties of magnetite–silica nanocomposite materials by controlled stober synthesis. Langmuir 2005;21(16):7524–7. Xia H, Wang Qi. Preparation of conductive polyaniline/nanosilica particle composites through ultrasonic irradiation. J Appl Polym Sci 2003;87(11):1811–7. Li ZF, Swihart MT, Ruckenstein E. Luminescent silicon nanoparticles capped by conductive polyaniline through the self-assembly method. Langmuir 2004;20(5):1963–71. Jang J, Ha J, Lim B. Synthesis and characterization of monodisperse silica–polyaniline core-shell nanoparticles. Chem Commun 2006:1622–4. Massart R. Preparation of aqueous magnetic liquids in alkaline and acidic media. IEEE Trans Magn 1981;17(2):1247–8. Klong HP, Alexander LF. X-ray diffraction procedures for crystalline and amorphous materials. New York: John Wiley and Sons; 1954. Stöber W, Fink A, Bohn E. Controlled growth of monodisperse silica spheres in the micron size range. J Colloid Interface Sci 1968;26(1): 62–9. Salgueiriño-Maceira V, Correa-Duarte MA, Spasova M, Liz-Marzán LM, Farle M. Composite silica spheres with magnetic and luminescent functionalities. Adv Funct Mater 2006;16(4):509–14. Deng YH, Wang CC, Hu JH, Yang WL, Fu SK. Investigation of formation of silica-coated magnetite nanoparticles via sol–gel approach. Colloids Surf A Physicochem Eng Aspects 2005;262(1–3):87–93. Chaneac C, Tronc E, Jolivet JP. Magnetic iron oxide–silica nanocomposites. Synthesis and characterization. J Mater Chem 1996;6(12):1905–11. Donselaar LN, Philipse AP. Interactions between silica colloids with magnetite cores: diffusion, sedimentation and light scattering. J Colloid Interface Sci 1999;212(1):14–23. Ma D, Guan J, Normandin F, De´nomme´e S, Enright G, Veres T, et al. Multifunctional nano-architecture for biomedical applications. Chem Mater 2006;18(7):1920–7. Chen SA, Lin LC. Polyaniline doped by the new class of dopant, ionic salt: structure and properties. Macromolecules 1995;28(4): 1239–45. Neoh KG, Pun MY, Kang ET, Tan KL. Polyaniline treated with organic acids: doping characteristics and stability. Synth Met 1995;73(3): 209–15. Feng X, Yang G, Liu Y, Hou WH, Zhu JJ. Synthesis of polyaniline/ MCM-41 composite through surface polymerization of aniline. J Appl Polym Sci 2006;101(3):2088–94. Chandrasekhar P, Naishadham K. Broadband microwave absorption and shielding properties of a poly(aniline). Synth Met 1999;105(2): 115–20. Sun GC, Yao KL, Liao HX, Niu ZC, Liu ZL. Microwave absorption characteristics of chiral materials with Fe3O4–polyaniline composite matrix. J Electron 2000;87(6):735–40.