Facile synthesis of shell–core polyaniline/SrFe₁₂O₁₉ composites and magnetic properties

Abstract

Polyaniline (PANI)/SrFe₁₂O₁₉ (SrM) composites were synthesized by in situ polymerization and characterized by XRD, FT-IR, SEM, TEM and VSM. The results showed that SrFe₁₂O₁₉ particles obtained by a hydrothermal method were of uniform hexagonal plate-like structures with diameters ranging from 2–5 μm. PANI/SrM composites with 15 wt% PANI displayed a shell–core structure. The SrFe₁₂O₁₉ particles were embedded in PANI. Magnetic property results showed that, due to the presence of a nonmagnetic PANI layer, the values of M_s, M_r, H_c were all lower than those of the pure SrFe₁₂O₁₉ particle, and indicated the information of a “shell–core” structure composite.

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1. Introduction

Polyaniline (PANI) has been intensively investigated as an electromagnetic interference (EMI) shielding material and broadband microwave adsorbing material, due to some significant characteristics such as very light weight, flexibility and reasonably facile processibility.^1–4 However, PANI as a microwave-absorbing material has only electrical loss, which is not of any help in improving the microwave absorption property and widening the absorption bandwidth.^5,6 To compensate for this defect, the integration of magnetic materials and conducing polymers has attracted increased interest. Recently, several interesting research projects have focused on the preparation of PANI composites with magnetic and conducting properties. T. H. Ting et al.⁷ synthesized PANI/BaFe₁₂O₁₉ composite by in situ polymerization with different Ba ferrite content. S. Sultana et al. reported PANI/CuFe₂O₄ composites via in situ polymerization and discussed the photocatalytic properties of PANI/CuFe₂O₄ composites.⁸ Yavuz et al. reported a novel approach for synthesizing the PANI–ferrite particles with a hybrid structure via an oxidative electrochemical polymerization of aniline.⁹

As is well known, M-type ferrites with hexagonal structure can strongly absorb electromagnetic waves by the mechanism of moment precession resonance (MPR) and display a promising application in microwave absorption due to their permittivity and permeability losses in the microwave frequency band.^10,11 In particular, strontium hexaferrite (SrM) has been widely used in magnetic recording and microwave absorption, due to its high stability, excellent high-frequency response, large magneto-crystalline anisotropy, and large magnetization as well.¹² In previous studies, Hessien et al.¹³ have synthesized the SrFe₁₂O₁₉ nanoparticles from the natural sample. Jean et al.¹⁴ have synthesized SrFe₁₂O₁₉ nanoparticles by hydrothermal method accompanied by calcination. Up to now, a great deal of studies has been devoted to choosing the hexaferrite as the magnetic component in polyaniline-based composites. The previous work reported that the morphology and crystalline size have a significant influence on the composite materials' properties and application.¹⁵ Therefore, we attempt to control SrM ferrite crystalline morphology and size to achieve good shielding and wave adsorbing properties.

Herein, the high-purity strontium ferrite particles were synthesized firstly by hydrothermal method, and then the polyaniline (PANI)/strontium ferrite (SrM) composites were synthesized by in situ polymerization under mechanical stirring after the ultrasonic dispersion of the mixture. Structure, morphology and magnetic properties of the composite materials were characterized by various instruments and investigated. And a possible polymerization mechanism for PANI/SrFe₁₂O₁₉ composites was also discussed.

2. Experimental

2.1 Synthesis of SrFe₁₂O₁₉ ferrite particles

The SrFe₁₂O₁₉ particles were prepared by hydrothermal process using analytical Fe(NO₃)₃·9H₂O and Sr(NO₃)₂ as starting materials. Firstly, aqueous solutions of Fe(NO₃)₃·9H₂O and Sr(NO₃)₂ were coprecipitated by NaOH. In this process, the molar ratio of OH⁻/NO₃⁻ (R_O/N) and the atomic ratio Fe/Sr (R_F/S) were set as 2 and 5, respectively. This mixture was then poured into a Teflon lined autoclave. The autoclave was closed and heated at 240 °C for 5 h. After allowing the autoclave to cool down to room temperature, the precipitate was separated from solution by centrifugation, washed with deionized water, absolute alcohol and dried at 60 °C for 24 h.

2.2 Synthesis of PANI/SrM composite

0.228 ml aniline (An) and 0.03 g SrFe₁₂O₁₉ were added in 20 ml HCl solution (1 mol l⁻¹) and homogeneously dispersed under 30 min ultrasonic dispersion and mechanical stirring. 0.625 g of peroxydisulfate (APS) was dissolved in 20 ml HCl solution, which was used as an oxidizing agent. The APS solution was then slowly added dropwise to initiate the polymerization. The polymerization was allowed to proceed for 24 h, with mechanical stirring. PANI/SrM composite was obtained by filtering and washing the suspension with methanol and deionized water several times, dried under vacuum at 70 °C for 24 h.

2.3 Characterization

The crystal structures of the samples (SrM, PANI, PANI/SrM) were performed by X-ray diffraction analysis (XRD, Rigaku D/Mmax-2500 PC, CuKα). The Fourier transform infrared spectroscopy (FTIR) spectra were recorded using Nexu670 spectrometer. The surface morphology and particle size were observed using field emission scanning electron microscopy (FE-SEM, JSM-2800LV) and transmission electron microscopy (TEM, JEM-2100). The magnetic properties of the samples were measured by vibrating sample magnetometer (VSM, HH-15) at room temperature.

3. Result and discussion

3.1 X-ray diffraction

Fig. 1 shows the XRD patterns of SrM ferrite, PANI and PANI/SrM composite. It is found that for strontium ferrite (Fig. 1a), diffraction peaks are observed at 2θ = 22.99°, 30.83°, 32.19°, 34.11°, 40.31°, 42.42°, 55.05°, 56.59°, 63.05°, 72.58°, 75.46°, which correspond to the typical M type SrFe₁₂O₁₉ (P63/mmc PDF#33-1340), furthermore, no any secondary phase is detected, which demonstrates the single phase SrM ferrites are synthesized by hydrothermal method. XRD pattern of PANI (Fig. 1b) shows one broad diffraction peak centered at 2θ = 25.15°, which can be ascribed to the periodicity parallel and perpendicular to polymer chains of PANI.¹⁶ As seen Fig. 1c, diffraction peak positions of PANI/SrM composite are basically similar to the as-prepared SrFe₁₂O₁₉ ferrite particles. Simultaneously, there is a wide peak in the range of 15–30° and the center at 25.15°, which corresponds to characteristic peak of PANI. Furthermore, it is also observed that the intensity of peaks for PANI/SrM composite are weaker than that of pure SrFe₁₂O₁₉ ferrite, which reveals that presence of PANI coatings layer. These results indicate that PANI/SrM core–shell composite are obtained.

Fig. 1 XRD patterns of (a) SrFe₁₂O₁₉, (b) PANI, and (c) PANI/SrFe₁₂O₁₉ composite.

3.2 FTIR spectra

The FTIR spectra of the SrM, PANI and PANI/SrM composite are shown in Fig. 2. The FTIR spectrum of SrM has been well reported in the literature.¹⁷ In this case, the FTIR spectrum of SrM (Fig. 2a) reveals that the peaks at 400–600 cm⁻¹ are intrinsic vibrations of the tetrahedral and octahedral sites for the SrFe₁₂O₁₉, respectively. The FTIR spectrum of PANI (Fig. 2b) shows that it contains all of the characteristic peaks of PANI. The peaks appeared at 806 cm⁻¹, 1120 cm⁻¹, 1254 cm⁻¹, 1486 cm⁻¹ and 1541 cm⁻¹, which represent the C–H out of plane bending vibration of a 1,4-disubstituted aromatic ring, C–H bending mode of a quinoid ring, C–N and C–N⁺ stretching vibration mode or C–H bending mode of a benzenoid ring, C=N stretching mode of a benzenoid ring and the C=C stretching mode of a quinoid ring, respectively.¹⁸ As show in Fig. 2c, the FTIR spectrum of PANI/SrM composite is almost identical to that of PANI. However, it is also observed that the characteristic peaks of PANI/SrM composite are slightly shifted towards a higher wave number (1578 cm⁻¹, 1301 cm⁻¹, 1160 cm⁻¹) compared to those of the pure PANI. Hence, these results reveal that there exists an interaction between SrFe₁₂O₁₉ particles and PANI chains, and suggests well wrapping of SrM ferrite particles with PANI in PANI/SrM composites.

Fig. 2 FT-IR spectra of (a) SrFe₁₂O₁₉, (b) PANI and (c) PANI/SrFe₁₂O₁₉ composite.

3.3 Morphology and EDS analysis

The morphology and particle size of strontium ferrite and PANI/SrM composites are determined by SEM and TEM. The SEM and TEM micrographs of strontium ferrite (Fig. 3a and c) reveal that the particles are approximately hexagonal in shape with diameter ranging from 2 to 5 μm. A slight agglomeration is also observed, which is due to the magneto dipole interactions among the magnetic SrFe₁₂O₁₉ particles.¹⁹ The EDS spectrum (Fig. 3b) of SrFe₁₂O₁₉ particle shows that the ratio about Fe, Sr and O was corresponded with the chemical formula of SrFe₁₂O₁₉, considering the reasonable experimental error. The result confirms that the composition is in agreement with the experiment design. Fig. 3d shows the TEM image of the PANI/SrM composite. It is clearly seen that a net-like structure is formed. The black region is SrFe₁₂O₁₉ particles, and the gray colored shell is PANI in the composite. The color difference is attributed to the different electron penetrability. These results indicate the ferrite particles are wrapped by PANI. In addition, it also can be observed that the PANI can minimize the aggregation of ferrite particles, which is due to the repulsive forces between magnetic particles and PANI. Hence, from the results of SEM and TEM, it is clear that synthetic method in this study is applicable to the synthesis of core shell PANI/SrM composites. The results obtained are quite similar to other systems reported elsewhere.^20,21

Fig. 3 SEM image of (a) SrFe₁₂O₁₉, TEM images of (b) SrFe₁₂O₁₉ (c) PANI/SrFe₁₂O₁₉ composite.

3.4 Magnetic properties

Fig. 4 shows the magnetic hysteresis loops of the pure PANI, SrFe₁₂O₁₉ particles and PANI/SrM composites at room temperature. The magnetic parameters such as saturation magnetization (M_s), remanence magnetization (M_r) and coercivity (H_c) of the composites determined by the hysteresis loops are given in Table 1. It can be seen that the M_s value of PANI and PANI/SrM composites are much lower than that of the pure SrFe₁₂O₁₉ ferrite. The result is obvious, because PANI is nonmagnetic. The existence of nonmagnetic PANI can be envisaged as a magnetic dead layer on the surface, thus affecting the magnitude of magnetization due to quenching of the surface moment. The M_r and H_c are also less than those of SrFe₁₂O₁₉ ferrite. Generally, the coercivity of a material depends upon many factors, such as microstructure, grain shape, composition, magnetic anisotropy etc.²² For the PANI/SrM composites, in the polymerization process, PANI is deposited on the ferrite surface and crystallite boundaries, which has a healing effect to cover the ferrite surface defects, such as pores and cracks, leading to a decrease in magnetic surface anisotropy of ferrite particles. Consequently, the PANI/SrM composites show lower coercivity value compared to that of strontium ferrite.

Fig. 4 Magnetic hysteresis loops of (a) PANI, (b) PANI/SrFe₁₂O₁₉ (c) SrFe₁₂O₁₉ particles.

Table 1 Saturation magnetization (M_s), remnant magnetization (M_r) and coercivity (H_c) of samples

Sample	Ms/emu g^−1	Mr/emu g^−1	Hc/kA m^−1
PANI	4.95	2.25	96.42
PANI/SrM	46.91	22.35	84.35
SrFe12O19	58.63	29.31	98.12

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3.5 Mechanism of polymerization

The polymerization mechanism of PANI/SrM composites are proposed, as shown in Fig. 5. According to charge compensation mechanism, due to the polymerization in the acidic conditions, the surface of the ferrite is positively charged. Therefore, adsorption of an amount of anions such as Cl⁻ may compensate the positive charges on the ferrite surface. Moreover, in this charge compensation system, extra adsorption of the Cl⁻ on the ferrite surface would work as the charge compensator for positively charged PANI chains in the formation of PANI/SrM composites. As aniline monomers are shifted to cationic anilinium ions under the acidic condition, the electrostatic interactions occur between anions absorbed on ferrite surface and cationic anilinium ions. In addition, it is also probable that there is hydrogen bonding between the polyaniline chains and the oxygen atoms on the ferrite surface in the core–shell composites. The interactions can ensure ferrite particles to be embedded into the polymer chains and form ‘core–shell’ structural composites.

Fig. 5 The simple synthesis procedure mechanism of core–shell material of PANI/SrFe₁₂O₁₉.

4. Conclusion

The shell–core structure PANI/SrFe₁₂O₁₉ composites are successfully prepared via in situ polymerization. The synthetic procedure is simple and feasible. The crystallite size of the synthesized SrFe₁₂O₁₉ particles can be as fine as 2–5 μm. SEM and TEM results reveal that the SrFe₁₂O₁₉ ferrite particles are embedded in to the PANI matrix forming the core–shell structure. The intrinsic magnetic hysteresis loop measurements indicate that the M_s, M_r and H_c are all less than those of pure SrFe₁₂O₁₉ ferrite due to the presence of nonmagnetic PANI layer.

Acknowledgements

This work was supported by National Natural Science Foundation of China (No. 51202091), Natural science foundation of Jiangsu Province (No. BK20141300) and Postdoctoral Science Foundation of China (No. 2013M541612).

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