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Synthesis and ferrimagnetic properties of novel Sm-substituted LiNi ferrite–polyaniline nanocomposite
Materials Letters 61 (2007) 1091 – 1096 www.elsevier.com/locate/matlet
Synthesis and ferrimagnetic properties of novel Sm-substituted LiNi ferrite–polyaniline nanocomposite Liangchao Li ⁎, Jing Jiang, Feng Xu Zhejiang Key Laboratory for Reactive Chemistry on Solid Surface, Department of Chemistry, Zhejiang Normal University, Jinhua 321004, China Received 11 January 2006; accepted 22 June 2006 Available online 14 July 2006
Abstract Polyaniline (PANI)–LiNi0.5Sm0.08Fe1.92O4 nanocomposite was synthesized by an in situ polymerization of aniline in the presence of LiNi0.5Sm0.08Fe1.92O4 ferrite. The products were characterized by powder X-ray diffractometer (XRD), Fourier transform infrared (FTIR) and UV–visible absorption spectrometer, thermogravimetric analyser (TGA), atomic force microscope (AFM) and vibrating sample magnetometer (VSM). The results of XRD, FTIR and UV–visible spectra confirmed the formation of PANI–LiNi0.5Sm0.08Fe1.92O4 composite. AFM study showed that ferrite particles had an effect on the morphology of the composite. TGA revealed that the incorporation of ferrite improved the thermal stability of PANI. The nanocomposite under applied magnetic field exhibited the hysteresis loops of ferrimagnetic nature at room temperature. The bonding interaction between ferrite and PANI in the nanocomposite had been studied. © 2006 Elsevier B.V. All rights reserved. Keywords: Magnetic materials; Nanocomposite; Polyaniline; Ferrite; Magnetic properties
1. Introduction Conducting polymers have attracted considerable attention for their potential applications in various fields, such as electromagnetic interference (EMI) shielding [1], rechargeable batteries [2,3], electrodes and sensors [4,5], corrosion protection coatings [6] and microwave absorption [7]. Among the known conducting polymers, polyaniline (PANI) has been extensively studied in the last two decades due to its unique electrochemical and physicochemical behavior, good environment stability and relatively easy preparation [8,9]. Conducting polymer–inorganic composites possess not only the nature of the flexibilities and processability of polymers but also the mechanical strength and hardness of inorganic components. Recently, several interesting research has focused on PANI–inorganic composites to obtain the materials with synergetic or complementary behavior between polyaniline and inorganic nanoparticles [10,11].
⁎ Corresponding author. Tel.: +86 579 228 3088; fax: +86 579 228 2489. E-mail address: [email protected] (L. Li). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.06.061
Polymer–inorganic composites with an organized structure provide a new functional hybrid between organic and inorganic materials [12,13]. Up to date, the preparation of polyaniline with ferromagnetic properties has been studied [14,15]. Deng et al. have studied the synthesis of magnetic and conducting Fe3O4– cross-linked polyaniline (CLPANI) nanoparticles with core– shell structure by using a precipitation–oxidation technique [16]. Yang et al. have reported the preparation of conducting and magnetic PAn/γ-Fe2O3 nanocomposite by modification–redoping method [17]. The soft magnetic spinel ferrites have been widely used in microwave devices [18,19]. The electromagnetic properties of ferrites can be tailored by controlling the different types and amounts of metal ions substitution. Recently, the fabrication of spinel MnZn or NiZn ferrite–polyaniline composites has been reported [20–22], but polyaniline–ferrite systems fabricated by incorporating Sm-substituted LiNi ferrite into polyaniline has not been reported. We attempted to introduce a relatively small amount of rare earth ions into spinel ferrites to improve the electromagnetic properties of spinel ferrites [23,24] by the occurrence of 4f–3d couplings. In the present work, we prepared PANI–LiNi0.5Sm0.08 Fe1.92O4 nanocomposite by an in situ polymerization in aqueous
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solution. The samples were characterized by various experimental techniques, and the magnetization and coercivity of the composite were measured. 2. Experimental
tometer in the range of 300–800 nm. The TG curves of the samples were recorded on a Shimadzu model DT-40 thermal analyser in N2 (flow rate 40 ml/min) at a heating rate of 10 °C/min from room temperature to 800 °C. The magnetic properties of the composite were measured at room temperature by using a vibrating sample magnetometer (VSM, Lakeshore 7403).
2.1. Materials 3. Results and discussion
Aniline monomer was distilled under reduced pressure and stored below 0 °C. Ammonium peroxydisulfate ((NH4)2S2O8, APS), ferric oxide (Fe2O3), lithium carbonate (Li2CO3), nickel sulfate (NiSO4·6H2O), samarium oxide (Sm2O3) and oxalic acid (H2C2O4·2H2O) were all of analytical reagent grade and used as received. All reagents were purchased from Shanghai Chemical Agents Ltd Co. in China. 2.2. Preparation of LiNi0.5Sm0.08Fe1.92O4 ferrite Sm-substituted LiNi ferrite LiNi0.5Sm0.08Fe1.92O4 was prepared by a novel rheological phase reaction method [25]. In a typical procedure, stoichiometric amounts of Li2CO3 (0.01 mol), NiSO4·6 H2O (0.01 mol), Sm2O3 (0.0008 mol), Fe2O3 (0.0192 mol) and H2C2O4·2H2O (0.084 mol) were thoroughly mixed by grinding in an agate mortar for 30 min, about 15 ml anhydrous ethanol was then added to form the mixture in rheological state. The mixture was sealed in a teflonlined stainless-steel autoclave and maintained at 120 °C for 48 h in an oven. The obtained precursor was washed several times with deionized water and ethanol, dried at 60 °C for 12 h, and sintered at 1000 °C for 2 h in air, followed by cooling in a furnace to room temperature with 5 °C/min cooling rate. 2.3. Synthesis of PANI–LiNi0.5Sm0.08Fe1.92O4 composite PANI–LiNi0.5Sm0.08Fe1.92O4 composite was prepared by an in situ polymerization in aqueous solution. In a typical procedure, a certain amount of LiNi0.5Sm0.08Fe1.92O4 particles was suspended in 35 ml 0.1 M HCl solution and stirred for 30 min to get well dispersed. 1 ml aniline monomer was then added to the suspension and stirred for 30 min. 2.49 g APS in 20 ml 0.1 M HCl solution was then slowly added dropwise to the suspension mixture with a constant stirring at room temperature. After 12 h, the polymerization was achieved and the suspension was in dark green. The composite was obtained by filtering and washing the suspension with 0.1 M HCl and deionized water, and dried under vacuum at 60 °C for 24 h.
3.1. X-ray diffraction analysis Fig. 1 shows X-ray diffraction patterns of LiNi0.5Sm0.08Fe1.92O4 ferrite, PANI and PANI–LiNi0.5Sm0.08Fe1.92O4 composite. The XRD pattern of LiNi0.5Sm0.08Fe1.92O4 ferrite in Fig. 1(a) shows the single phase spinel structure with the characteristic reflections of the Fd3m cubic spinel group, which confirms the formation of Sm-substituted LiNi ferrite. These results indicate that Fe3+ is replaced by Sm3+ on the octahedral sites in spinel ferrite, and obey the Vegard's law [26]. The typical XRD pattern of PANI (curve c) shows the broad diffraction peaks at about 15.35° and 25.39°, and suggests an amorphous nature, which is consistent with the results obtained by other research groups [27,28]. Fig. 1(b) shows the main diffraction for PANI–LiNi0.5Sm0.08Fe1.92O4 composite, which contains the characteristic peaks of the LiNi0.5Sm0.08 Fe1.92O4 ferrite (curve a) at 2θ= 18.44°, 30.31°, 35.66°, 37.27°, 43.35°, 53.75°, 57.33°, and 62.81°. However, the intensities of the peaks for the composite are weaker than that of the pure ferrite, which reveals that the polyaniline coating layer has an effect on the crystallinity of LiNi0.5Sm0.08 Fe1.92O4 ferrite. In addition, a characteristic amorphous PANI peak can be observed in XRD pattern of the composite (curve b), which indicates the formation of PANI–LiNi0.5Sm0.08Fe1.92O4 composite. The average crystallite size of PANI–LiNi0.5Sm0.08Fe1.92O4 composite can be calculated by the Debye–Scherrer formula [29] b¼
kk Dcosh
ð1Þ
where λ is the wavelength of Cu Kα radiation (0.15418 nm), k is the shape factor taken as 0.9, D is the average crystallite size, θ is the Bragg's angle, and β is the full width at half-maximum (FWHM) of the diffraction peaks. The average crystallite size of PANI–LiNi0.5Sm0.08Fe1.92O4 composite is 87.3 nm, estimated from the XRD peak broadening of the (311) peak.
2.4. Characterization X-ray diffraction patterns of the samples were recorded on a Philips-Pw3040/60 X-ray diffractometer (XRD) with a Nifilter and graphite monochromater, and Cu Kα radiation (λ = 0.15418 nm) at a scanning speed of 4°/min in the range of 2θ =15–80°. The infrared spectra of the products were determined on a Nicolet Nexus 670 Fourier transform infrared spectrometer in the range of 4000–400 cm− 1 using KBr pellets. The UV–vis spectra of the samples dissolved in N,N-dimethylformamide (DMF) were recorded on a Shimadzu UV-2501PC spectropho-
Fig. 1. XRD patterns of LiNi0.5Sm0.08Fe1.92O4 (a), PANI–LiNi0.5Sm0.08Fe1.92O4 composite (b) and PANI (c).
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Fig. 2. AFM images of LiNi0.5Sm0.08Fe1.92O4 (a), PANI (b), and PANI–LiNi0.5Sm0.08Fe1.92O4 composite (c–d).
3.2. AFM analysis The morphology and particle size of the samples are observed by the atomic force microscope (AFM). Fig. 2 shows the micrograph of LiNi0.5Sm0.08Fe1.92O4 ferrite, PANI and PANI–LiNi0.5Sm0.08Fe1.92O4 composite. It can be seen from Fig. 2(a) that the ferrite are spherical particles with average size less than 100 nm, but having agglomeration to some extent, due to reducing total surface energy of the system. Fig. 2(b) shows the AFM micrograph of PANI particles with the size in the range of 50–150 nm. In Fig. 2(c–d), PANI–LiNi0.5Sm0.08Fe1.92O4 composite presents some different morphology as compared with that of LiNi0.5 Sm0.08Fe1.92O4. It can be seen that the ferrite particles are surrounded and embedded into the polymer chain of PANI, indicating that the ferrite particles have a core effect on the polymerization of aniline.
like band” is associated with vibrational modes of N_Q_N (Q refers to the quinonic-type rings) [31,32], indicating that PANI is formed in our sample. The peak at 805 cm− 1 is attributed to the out-of-plane deformation vibration of the p-disubstituted benzene ring [34]. In addition, the peak located at about 3447 cm− 1 corresponds to N–H stretching mode [35]. Fig. 3b shows the FTIR spectrum of PANI–LiNi0.5Sm0.08Fe1.92O4 composite, which illustrates several differences from the spectrum of PANI. The N–H bending and asymmetric C–N stretching modes of the
3.3. FTIR spectra analysis Fig. 3 shows the FTIR spectra of PANI and PANI–LiNi0.5Sm0.08 Fe1.92O4 composite. Both spectra exhibit the clear presence of benzenoid and quinoid ring vibrations at near 1494 cm− 1 and 1577 cm− 1, respectively, thereby indicating the oxidation state of emeraldine salt form of PANI [30]. It is observed from Fig. 3a that the characteristic bands of PANI occur at 1577, 1494, 1301, 1240, 1139 and 805 cm− 1. The peaks at 1577 and 1494 cm− 1 are attributed to the characteristic C_C stretching of the quinoid and benzenoid rings [31,32], the peaks at 1301 and 1240 cm− 1 correspond to N–H bending and asymmetric C–N stretching modes of the benzenoid ring, respectively [33]. The strong peak around 1139 cm− 1 which is described by MacDiarmid et al. as the “electronic-
Fig. 3. FTIR spectra of PANI (a) and PANI–LiNi0.5Sm0.08Fe1.92O4 composite (b).
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Fig. 6. Variation of magnetization with the applied field measured at 300 K for the PANI–LiNi0.5Sm0.08Fe1.92O4 composite. Fig. 4. UV–vis spectra of PANI (a) and PANI–LiNi0.5Sm0.08Fe1.92O4 composite (b).
benzenoid ring (1302 and 1245 cm− 1), and vibrational modes of N_Q_N (1147 cm− 1) for the composite shift to higher wavenumber as compared with that of PANI, moreover, their intensity is also weaker than that of the PANI. These results reveal that there exists an interaction between ferrite and PANI chains. 3.4. UV–vis spectra analysis Fig. 4 gives UV–vis absorption spectra of PANI and PANI– LiNi0.5Sm0.08Fe1.92O4 composite. It is observed from Fig. 4a that PANI has two characteristic absorption bands at around 329 nm and 617 nm. The absorption band around 329 nm is attributed to π–π* transition of the benzenoid ring [36,37], while the peak around 617 nm corresponds to the benzenoid-to-quinoid excitonic transition [38,39]. It is found from Fig. 4b that the absorption peaks of PANI–LiNi0.5Sm0.08Fe1.92O4 composite have a red shift of 6 nm and 8 nm, respectively, as compared with that of PANI. These results may indicate the σ–π interaction between ferrite and PANI backbone, which makes the energy of the antibonding orbital decrease, the energy of the π–π* transition of the benzenoid and quinoid ring decreases, so the absorption peaks of the composite exhibit a red shift. 3.5. TGA analysis TG curves of PANI and PANI–LiNi0.5Sm0.08Fe1.92O4 composite are shown in Fig. 5. For PANI, a three-step weight loss can be observed
Fig. 5. TGA curves of PANI (a) and PANI–LiNi0.5Sm0.08Fe1.92O4 composite (b).
from Fig. 5a. The initial mass loss at lower temperature (less than 120 °C) is due to the residual water and HCl desorption [40]. The second loss in mass is observed from about 200 °C to 450 °C, possibly due to the volatilization of lower weight polyaniline. The final loss in mass at higher temperatures (more than 450 °C) may be due to the thermal degradation of the polyaniline chains. It is seen from Fig. 5b that the thermal stability of the composite is higher than that of pure PANI. This may be caused by the interaction between ferrite particles and PANI chains. 3.6. Magnetization and coercivity Fig. 6 shows the magnetization M versus the applied magnetic field H for the composite at 300 K. The magnetization of PANI– LiNi0.5Sm0.08Fe1.92O4 composite exhibits the hysteresis loops at room temperature, and is weaker than that of the bulk soft magnetic ferrite. The saturation magnetization (MS) and remanent magnetization (Mr) of PANI–LiNi0.5Sm0.08Fe1.92O4 composite is 6.62 emu/g and 1.13 emu/g, respectively. The coercivity (HC) of the composite is small with the value of 102.97 Oe. The magnetic parameters (MS, Mr and HC) determined by the hysteresis loops reveal that PANI– LiNi0.5Sm0.08Fe1.92O4 composite can be used as a soft magnetic material. 3.7. Bonding model The bonding model of PANI–LiNi0.5Sm0.08Fe1.92O4 composite is illustrated in Fig. 7. The charge compensation model for the composite is shown in Fig. 7(a). There is a charge compensation effect between ferrite and PANI chains in the composite. The surface of the ferrite is positively charged due to the polymerization in the acidic environment. Therefore, adsorption of an amount of the Cl− may occur to compensate the positive charges on ferrite surface. In addition, specific adsorption of the Cl− on the ferrite surface may also take place. These specifically adsorbed Cl− would work as the charge compensator for positively charge PANI chain in the formation of PANI–LiNi0.5Sm0.08Fe1.92O4 composite. Moreover, hydrogen bonding between PANI chains also occur in the composite, these interaction will make PANI chains twist to form a network structure. Another probable hydrogen bonding model for the composite is shown in Fig. 7(b). The hydrogen bonding interaction between the polyaniline chains and the oxygen atoms in ferrite occurs in the composite.
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Fig. 7. Bonding model for the composite: (a) charge compensation model and (b) hydrogen bonding model.
The σ–π interaction between metal oxide and PANI may also occur in the composite, which includes (1) the π molecular orbital of PANI overlaps the empty d-orbital of metal ions to form the σ-bond where metallic empty d-orbital is the electron pair acceptor; (2) the π* molecular orbital of PANI overlaps the d-orbital of metal ions to form the π-bond, in which the metal ions is the electron pair donor.
Sm0.08Fe1.92O4 composite. The bonding model including the charge compensation and hydrogen bonding model, and the σ–π interaction between metal oxide and PANI in the composite are investigated. PANI–LiNi0.5Sm0.08Fe1.92O4 composite exhibits the hysteresis loops of ferrimagnetic nature at room temperature, which suggests that PANI–LiNi0.5Sm0.08Fe1.92O4 composite can be used as a soft magnetic material.
4. Conclusions Acknowledgements PANI–LiNi0.5Sm0.08Fe1.92O4 nanocomposite with the ferrimagnetic properties is successfully synthesized by an in situ polymerization. FTIR and UV–vis spectra indicate interaction between PANI and LiNi0.5Sm0.08Fe1.92O4 ferrite in the composite. XRD study demonstrates the formation of PANI–LiNi0.5
This work was supported by the Top Key Discipline of Materials Physics and Chemistry in Zhejiang Provincial Colleges and Zhejiang Provincial Natural Science Foundation of China [Y405038].
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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.matlet.2006.06.061. References [1] T. Mäkelä, S. Pienimaa, T. Taka, S. Jussila, H. Isotalo, Synth. Met. 85 (1997) 1335. [2] S. Kuwabata, S. Masui, H. Yoneyama, Electrochim. Acta 44 (1999) 4593. [3] J. Qiao, T. Hamaya, T. Okada, Polymer 46 (2005) 10809. [4] J.Q. Kan, X.H. Pan, C. Chen, Biosens. Bioelectron. 19 (2004) 1635. [5] J.A. Cobos-Murcia, L. Galicia, A. Rojas-Hernández, M.T. Ramírez-Silva, R. Álvarez-Bustamante, M. Romero-Romo, G. Rosquete-Pina, M. Palomar-Pardavé, Polymer 46 (2005) 9053. [6] N. Ahmad, A.G. MacDiarmid, Synth. Met. 78 (1996) 103. [7] T.L. Rose, S.D. Antonio, M.H. Jillson, A.B. Kron, R. Suresh, F. Wang, Synth. Met. 85 (1997) 1439. [8] N. Gospodinova, L. Terlemezyan, Prog. Polym. Sci. 23 (1998) 1443. [9] R.J. Mathew, D.L. Yang, B.R. Mattes, Macromolecules 35 (2002) 7575. [10] C. Danielle, S. Michelle, A. Ivo, Z. Aldo, Chem. Mater. 15 (2003) 4658. [11] Y. Qiu, L. Gao, J. Phys. Chem., B 109 (2005) 19732. [12] S. Yoshimoto, F. Ohashi, T. Kameyama, Macromol. Rapid Commun. 25 (2004) 1687. [13] M.S. Silverstein, H. Tai, A. Sergienko, Y. Lumelsky, S. Pavlovsky, Polymer 46 (2005) 6682. [14] M.X. Wan, W.C. Li, J. Polym. Sci., A, Polym. Chem. 34 (1997) 2129. [15] M.X. Wan, J.H. Fan, J. Polym. Sci., A, Polym. Chem. 36 (1998) 2749. [16] J. Deng, X. Ding, W. Zhang, Y. Peng, J. Wang, X. Long, P. Li, A.S.C. Chan, Polymer 43 (2002) 2179. [17] Q.L. Yang, J. Zhai, L. Feng, Y.L. Song, M.X. Wan, L. Jiang, W.G. Xu, Q.S. Li, Synth. Met. 135–136 (2003) 819. [18] E. Schloeman, J. Magn. Magn. Mater. 209 (2000) 15. [19] M. Pardavi-Horvath, J. Magn. Magn. Mater. 215–216 (2000) 171.
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Synthesis and ferrimagnetic properties of novel Sm-substituted LiNi ferrite–polyaniline nanocomposite Liangchao Li ⁎, Jing Jiang, Feng Xu Zhejiang Key Laboratory for Reactive Chemistry on Solid Surface, Department of Chemistry, Zhejiang Normal University, Jinhua 321004, China Received 11 January 2006; accepted 22 June 2006 Available online 14 July 2006
Abstract Polyaniline (PANI)–LiNi0.5Sm0.08Fe1.92O4 nanocomposite was synthesized by an in situ polymerization of aniline in the presence of LiNi0.5Sm0.08Fe1.92O4 ferrite. The products were characterized by powder X-ray diffractometer (XRD), Fourier transform infrared (FTIR) and UV–visible absorption spectrometer, thermogravimetric analyser (TGA), atomic force microscope (AFM) and vibrating sample magnetometer (VSM). The results of XRD, FTIR and UV–visible spectra confirmed the formation of PANI–LiNi0.5Sm0.08Fe1.92O4 composite. AFM study showed that ferrite particles had an effect on the morphology of the composite. TGA revealed that the incorporation of ferrite improved the thermal stability of PANI. The nanocomposite under applied magnetic field exhibited the hysteresis loops of ferrimagnetic nature at room temperature. The bonding interaction between ferrite and PANI in the nanocomposite had been studied. © 2006 Elsevier B.V. All rights reserved. Keywords: Magnetic materials; Nanocomposite; Polyaniline; Ferrite; Magnetic properties
1. Introduction Conducting polymers have attracted considerable attention for their potential applications in various fields, such as electromagnetic interference (EMI) shielding [1], rechargeable batteries [2,3], electrodes and sensors [4,5], corrosion protection coatings [6] and microwave absorption [7]. Among the known conducting polymers, polyaniline (PANI) has been extensively studied in the last two decades due to its unique electrochemical and physicochemical behavior, good environment stability and relatively easy preparation [8,9]. Conducting polymer–inorganic composites possess not only the nature of the flexibilities and processability of polymers but also the mechanical strength and hardness of inorganic components. Recently, several interesting research has focused on PANI–inorganic composites to obtain the materials with synergetic or complementary behavior between polyaniline and inorganic nanoparticles [10,11].
⁎ Corresponding author. Tel.: +86 579 228 3088; fax: +86 579 228 2489. E-mail address: [email protected] (L. Li). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.06.061
Polymer–inorganic composites with an organized structure provide a new functional hybrid between organic and inorganic materials [12,13]. Up to date, the preparation of polyaniline with ferromagnetic properties has been studied [14,15]. Deng et al. have studied the synthesis of magnetic and conducting Fe3O4– cross-linked polyaniline (CLPANI) nanoparticles with core– shell structure by using a precipitation–oxidation technique [16]. Yang et al. have reported the preparation of conducting and magnetic PAn/γ-Fe2O3 nanocomposite by modification–redoping method [17]. The soft magnetic spinel ferrites have been widely used in microwave devices [18,19]. The electromagnetic properties of ferrites can be tailored by controlling the different types and amounts of metal ions substitution. Recently, the fabrication of spinel MnZn or NiZn ferrite–polyaniline composites has been reported [20–22], but polyaniline–ferrite systems fabricated by incorporating Sm-substituted LiNi ferrite into polyaniline has not been reported. We attempted to introduce a relatively small amount of rare earth ions into spinel ferrites to improve the electromagnetic properties of spinel ferrites [23,24] by the occurrence of 4f–3d couplings. In the present work, we prepared PANI–LiNi0.5Sm0.08 Fe1.92O4 nanocomposite by an in situ polymerization in aqueous
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solution. The samples were characterized by various experimental techniques, and the magnetization and coercivity of the composite were measured. 2. Experimental
tometer in the range of 300–800 nm. The TG curves of the samples were recorded on a Shimadzu model DT-40 thermal analyser in N2 (flow rate 40 ml/min) at a heating rate of 10 °C/min from room temperature to 800 °C. The magnetic properties of the composite were measured at room temperature by using a vibrating sample magnetometer (VSM, Lakeshore 7403).
2.1. Materials 3. Results and discussion
Aniline monomer was distilled under reduced pressure and stored below 0 °C. Ammonium peroxydisulfate ((NH4)2S2O8, APS), ferric oxide (Fe2O3), lithium carbonate (Li2CO3), nickel sulfate (NiSO4·6H2O), samarium oxide (Sm2O3) and oxalic acid (H2C2O4·2H2O) were all of analytical reagent grade and used as received. All reagents were purchased from Shanghai Chemical Agents Ltd Co. in China. 2.2. Preparation of LiNi0.5Sm0.08Fe1.92O4 ferrite Sm-substituted LiNi ferrite LiNi0.5Sm0.08Fe1.92O4 was prepared by a novel rheological phase reaction method [25]. In a typical procedure, stoichiometric amounts of Li2CO3 (0.01 mol), NiSO4·6 H2O (0.01 mol), Sm2O3 (0.0008 mol), Fe2O3 (0.0192 mol) and H2C2O4·2H2O (0.084 mol) were thoroughly mixed by grinding in an agate mortar for 30 min, about 15 ml anhydrous ethanol was then added to form the mixture in rheological state. The mixture was sealed in a teflonlined stainless-steel autoclave and maintained at 120 °C for 48 h in an oven. The obtained precursor was washed several times with deionized water and ethanol, dried at 60 °C for 12 h, and sintered at 1000 °C for 2 h in air, followed by cooling in a furnace to room temperature with 5 °C/min cooling rate. 2.3. Synthesis of PANI–LiNi0.5Sm0.08Fe1.92O4 composite PANI–LiNi0.5Sm0.08Fe1.92O4 composite was prepared by an in situ polymerization in aqueous solution. In a typical procedure, a certain amount of LiNi0.5Sm0.08Fe1.92O4 particles was suspended in 35 ml 0.1 M HCl solution and stirred for 30 min to get well dispersed. 1 ml aniline monomer was then added to the suspension and stirred for 30 min. 2.49 g APS in 20 ml 0.1 M HCl solution was then slowly added dropwise to the suspension mixture with a constant stirring at room temperature. After 12 h, the polymerization was achieved and the suspension was in dark green. The composite was obtained by filtering and washing the suspension with 0.1 M HCl and deionized water, and dried under vacuum at 60 °C for 24 h.
3.1. X-ray diffraction analysis Fig. 1 shows X-ray diffraction patterns of LiNi0.5Sm0.08Fe1.92O4 ferrite, PANI and PANI–LiNi0.5Sm0.08Fe1.92O4 composite. The XRD pattern of LiNi0.5Sm0.08Fe1.92O4 ferrite in Fig. 1(a) shows the single phase spinel structure with the characteristic reflections of the Fd3m cubic spinel group, which confirms the formation of Sm-substituted LiNi ferrite. These results indicate that Fe3+ is replaced by Sm3+ on the octahedral sites in spinel ferrite, and obey the Vegard's law [26]. The typical XRD pattern of PANI (curve c) shows the broad diffraction peaks at about 15.35° and 25.39°, and suggests an amorphous nature, which is consistent with the results obtained by other research groups [27,28]. Fig. 1(b) shows the main diffraction for PANI–LiNi0.5Sm0.08Fe1.92O4 composite, which contains the characteristic peaks of the LiNi0.5Sm0.08 Fe1.92O4 ferrite (curve a) at 2θ= 18.44°, 30.31°, 35.66°, 37.27°, 43.35°, 53.75°, 57.33°, and 62.81°. However, the intensities of the peaks for the composite are weaker than that of the pure ferrite, which reveals that the polyaniline coating layer has an effect on the crystallinity of LiNi0.5Sm0.08 Fe1.92O4 ferrite. In addition, a characteristic amorphous PANI peak can be observed in XRD pattern of the composite (curve b), which indicates the formation of PANI–LiNi0.5Sm0.08Fe1.92O4 composite. The average crystallite size of PANI–LiNi0.5Sm0.08Fe1.92O4 composite can be calculated by the Debye–Scherrer formula [29] b¼
kk Dcosh
ð1Þ
where λ is the wavelength of Cu Kα radiation (0.15418 nm), k is the shape factor taken as 0.9, D is the average crystallite size, θ is the Bragg's angle, and β is the full width at half-maximum (FWHM) of the diffraction peaks. The average crystallite size of PANI–LiNi0.5Sm0.08Fe1.92O4 composite is 87.3 nm, estimated from the XRD peak broadening of the (311) peak.
2.4. Characterization X-ray diffraction patterns of the samples were recorded on a Philips-Pw3040/60 X-ray diffractometer (XRD) with a Nifilter and graphite monochromater, and Cu Kα radiation (λ = 0.15418 nm) at a scanning speed of 4°/min in the range of 2θ =15–80°. The infrared spectra of the products were determined on a Nicolet Nexus 670 Fourier transform infrared spectrometer in the range of 4000–400 cm− 1 using KBr pellets. The UV–vis spectra of the samples dissolved in N,N-dimethylformamide (DMF) were recorded on a Shimadzu UV-2501PC spectropho-
Fig. 1. XRD patterns of LiNi0.5Sm0.08Fe1.92O4 (a), PANI–LiNi0.5Sm0.08Fe1.92O4 composite (b) and PANI (c).
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Fig. 2. AFM images of LiNi0.5Sm0.08Fe1.92O4 (a), PANI (b), and PANI–LiNi0.5Sm0.08Fe1.92O4 composite (c–d).
3.2. AFM analysis The morphology and particle size of the samples are observed by the atomic force microscope (AFM). Fig. 2 shows the micrograph of LiNi0.5Sm0.08Fe1.92O4 ferrite, PANI and PANI–LiNi0.5Sm0.08Fe1.92O4 composite. It can be seen from Fig. 2(a) that the ferrite are spherical particles with average size less than 100 nm, but having agglomeration to some extent, due to reducing total surface energy of the system. Fig. 2(b) shows the AFM micrograph of PANI particles with the size in the range of 50–150 nm. In Fig. 2(c–d), PANI–LiNi0.5Sm0.08Fe1.92O4 composite presents some different morphology as compared with that of LiNi0.5 Sm0.08Fe1.92O4. It can be seen that the ferrite particles are surrounded and embedded into the polymer chain of PANI, indicating that the ferrite particles have a core effect on the polymerization of aniline.
like band” is associated with vibrational modes of N_Q_N (Q refers to the quinonic-type rings) [31,32], indicating that PANI is formed in our sample. The peak at 805 cm− 1 is attributed to the out-of-plane deformation vibration of the p-disubstituted benzene ring [34]. In addition, the peak located at about 3447 cm− 1 corresponds to N–H stretching mode [35]. Fig. 3b shows the FTIR spectrum of PANI–LiNi0.5Sm0.08Fe1.92O4 composite, which illustrates several differences from the spectrum of PANI. The N–H bending and asymmetric C–N stretching modes of the
3.3. FTIR spectra analysis Fig. 3 shows the FTIR spectra of PANI and PANI–LiNi0.5Sm0.08 Fe1.92O4 composite. Both spectra exhibit the clear presence of benzenoid and quinoid ring vibrations at near 1494 cm− 1 and 1577 cm− 1, respectively, thereby indicating the oxidation state of emeraldine salt form of PANI [30]. It is observed from Fig. 3a that the characteristic bands of PANI occur at 1577, 1494, 1301, 1240, 1139 and 805 cm− 1. The peaks at 1577 and 1494 cm− 1 are attributed to the characteristic C_C stretching of the quinoid and benzenoid rings [31,32], the peaks at 1301 and 1240 cm− 1 correspond to N–H bending and asymmetric C–N stretching modes of the benzenoid ring, respectively [33]. The strong peak around 1139 cm− 1 which is described by MacDiarmid et al. as the “electronic-
Fig. 3. FTIR spectra of PANI (a) and PANI–LiNi0.5Sm0.08Fe1.92O4 composite (b).
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Fig. 6. Variation of magnetization with the applied field measured at 300 K for the PANI–LiNi0.5Sm0.08Fe1.92O4 composite. Fig. 4. UV–vis spectra of PANI (a) and PANI–LiNi0.5Sm0.08Fe1.92O4 composite (b).
benzenoid ring (1302 and 1245 cm− 1), and vibrational modes of N_Q_N (1147 cm− 1) for the composite shift to higher wavenumber as compared with that of PANI, moreover, their intensity is also weaker than that of the PANI. These results reveal that there exists an interaction between ferrite and PANI chains. 3.4. UV–vis spectra analysis Fig. 4 gives UV–vis absorption spectra of PANI and PANI– LiNi0.5Sm0.08Fe1.92O4 composite. It is observed from Fig. 4a that PANI has two characteristic absorption bands at around 329 nm and 617 nm. The absorption band around 329 nm is attributed to π–π* transition of the benzenoid ring [36,37], while the peak around 617 nm corresponds to the benzenoid-to-quinoid excitonic transition [38,39]. It is found from Fig. 4b that the absorption peaks of PANI–LiNi0.5Sm0.08Fe1.92O4 composite have a red shift of 6 nm and 8 nm, respectively, as compared with that of PANI. These results may indicate the σ–π interaction between ferrite and PANI backbone, which makes the energy of the antibonding orbital decrease, the energy of the π–π* transition of the benzenoid and quinoid ring decreases, so the absorption peaks of the composite exhibit a red shift. 3.5. TGA analysis TG curves of PANI and PANI–LiNi0.5Sm0.08Fe1.92O4 composite are shown in Fig. 5. For PANI, a three-step weight loss can be observed
Fig. 5. TGA curves of PANI (a) and PANI–LiNi0.5Sm0.08Fe1.92O4 composite (b).
from Fig. 5a. The initial mass loss at lower temperature (less than 120 °C) is due to the residual water and HCl desorption [40]. The second loss in mass is observed from about 200 °C to 450 °C, possibly due to the volatilization of lower weight polyaniline. The final loss in mass at higher temperatures (more than 450 °C) may be due to the thermal degradation of the polyaniline chains. It is seen from Fig. 5b that the thermal stability of the composite is higher than that of pure PANI. This may be caused by the interaction between ferrite particles and PANI chains. 3.6. Magnetization and coercivity Fig. 6 shows the magnetization M versus the applied magnetic field H for the composite at 300 K. The magnetization of PANI– LiNi0.5Sm0.08Fe1.92O4 composite exhibits the hysteresis loops at room temperature, and is weaker than that of the bulk soft magnetic ferrite. The saturation magnetization (MS) and remanent magnetization (Mr) of PANI–LiNi0.5Sm0.08Fe1.92O4 composite is 6.62 emu/g and 1.13 emu/g, respectively. The coercivity (HC) of the composite is small with the value of 102.97 Oe. The magnetic parameters (MS, Mr and HC) determined by the hysteresis loops reveal that PANI– LiNi0.5Sm0.08Fe1.92O4 composite can be used as a soft magnetic material. 3.7. Bonding model The bonding model of PANI–LiNi0.5Sm0.08Fe1.92O4 composite is illustrated in Fig. 7. The charge compensation model for the composite is shown in Fig. 7(a). There is a charge compensation effect between ferrite and PANI chains in the composite. The surface of the ferrite is positively charged due to the polymerization in the acidic environment. Therefore, adsorption of an amount of the Cl− may occur to compensate the positive charges on ferrite surface. In addition, specific adsorption of the Cl− on the ferrite surface may also take place. These specifically adsorbed Cl− would work as the charge compensator for positively charge PANI chain in the formation of PANI–LiNi0.5Sm0.08Fe1.92O4 composite. Moreover, hydrogen bonding between PANI chains also occur in the composite, these interaction will make PANI chains twist to form a network structure. Another probable hydrogen bonding model for the composite is shown in Fig. 7(b). The hydrogen bonding interaction between the polyaniline chains and the oxygen atoms in ferrite occurs in the composite.
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Fig. 7. Bonding model for the composite: (a) charge compensation model and (b) hydrogen bonding model.
The σ–π interaction between metal oxide and PANI may also occur in the composite, which includes (1) the π molecular orbital of PANI overlaps the empty d-orbital of metal ions to form the σ-bond where metallic empty d-orbital is the electron pair acceptor; (2) the π* molecular orbital of PANI overlaps the d-orbital of metal ions to form the π-bond, in which the metal ions is the electron pair donor.
Sm0.08Fe1.92O4 composite. The bonding model including the charge compensation and hydrogen bonding model, and the σ–π interaction between metal oxide and PANI in the composite are investigated. PANI–LiNi0.5Sm0.08Fe1.92O4 composite exhibits the hysteresis loops of ferrimagnetic nature at room temperature, which suggests that PANI–LiNi0.5Sm0.08Fe1.92O4 composite can be used as a soft magnetic material.
4. Conclusions Acknowledgements PANI–LiNi0.5Sm0.08Fe1.92O4 nanocomposite with the ferrimagnetic properties is successfully synthesized by an in situ polymerization. FTIR and UV–vis spectra indicate interaction between PANI and LiNi0.5Sm0.08Fe1.92O4 ferrite in the composite. XRD study demonstrates the formation of PANI–LiNi0.5
This work was supported by the Top Key Discipline of Materials Physics and Chemistry in Zhejiang Provincial Colleges and Zhejiang Provincial Natural Science Foundation of China [Y405038].
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