07) 1091–1096www.elsevier.com/locate/matlet

Materials Letters 61 (20

Synthesis and ferrimagnetic properties of novel Sm-substitutedLiNi 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 2006Available online 14 July 2006

Abstract

Polyaniline (PANI)–LiNi0.5Sm0.08Fe1.92O4 nanocomposite was synthesized by an in situ polymerization of aniline in the presence ofLiNi0.5Sm0.08Fe1.92O4 ferrite. The products were characterized by powder X-ray diffractometer (XRD), Fourier transform infrared (FTIR) andUV–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 studyshowed that ferrite particles had an effect on the morphology of the composite. TGA revealed that the incorporation of ferrite improved thethermal stability of PANI. The nanocomposite under applied magnetic field exhibited the hysteresis loops of ferrimagnetic nature at roomtemperature. 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 fortheir potential applications in various fields, such as electromag-netic interference (EMI) shielding [1], rechargeable batteries[2,3], electrodes and sensors [4,5], corrosion protection coatings[6] and microwave absorption [7]. Among the known conductingpolymers, polyaniline (PANI) has been extensively studied in thelast two decades due to its unique electrochemical andphysicochemical behavior, good environment stability andrelatively easy preparation [8,9].

Conducting polymer–inorganic composites possess not onlythe nature of the flexibilities and processability of polymers butalso the mechanical strength and hardness of inorganiccomponents. Recently, several interesting research has focusedon PANI–inorganic composites to obtain the materials withsynergetic or complementary behavior between polyaniline andinorganic nanoparticles [10,11].

⁎ Corresponding author. Tel.: +86 579 228 3088; fax: +86 579 228 2489.E-mail address: sky52@zjnu.cn (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 structureprovide a new functional hybrid between organic and inorganicmaterials [12,13]. Up to date, the preparation of polyaniline withferromagnetic 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 andmagnetic PAn/γ-Fe2O3 nanocomposite by modification–redop-ing method [17].

The soft magnetic spinel ferrites have been widely used inmicrowave devices [18,19]. The electromagnetic properties offerrites can be tailored by controlling the different types andamounts of metal ions substitution. Recently, the fabrication ofspinel MnZn or NiZn ferrite–polyaniline composites has beenreported [20–22], but polyaniline–ferrite systems fabricated byincorporating Sm-substituted LiNi ferrite into polyaniline has notbeen reported.We attempted to introduce a relatively small amountof rare earth ions into spinel ferrites to improve the electromagneticproperties of spinel ferrites [23,24] by the occurrence of 4f–3dcouplings. In the present work, we prepared PANI–LiNi0.5Sm0.08

Fe1.92O4 nanocomposite by an in situ polymerization in aqueous

Fig. 1. XRD patterns of LiNi0.5Sm0.08Fe1.92O4 (a), PANI–LiNi0.5Sm0.08Fe1.92O4

composite (b) and PANI (c).

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solution. The samples were characterized by various experimentaltechniques, and the magnetization and coercivity of the compositewere measured.

2. Experimental

2.1. Materials

Aniline monomer was distilled under reduced pressure andstored below 0 °C. Ammonium peroxydisulfate ((NH4)2S2O8,APS), ferric oxide (Fe2O3), lithium carbonate (Li2CO3), nickelsulfate (NiSO4·6H2O), samarium oxide (Sm2O3) and oxalic acid(H2C2O4·2H2O) were all of analytical reagent grade and used asreceived. All reagents were purchased from Shanghai ChemicalAgents Ltd Co. in China.

2.2. Preparation of LiNi0.5Sm0.08Fe1.92O4 ferrite

Sm-substituted LiNi ferrite LiNi0.5Sm0.08Fe1.92O4 was preparedby a novel rheological phase reaction method [25]. In a typicalprocedure, stoichiometric amounts of Li2CO3 (0.01 mol), NiSO4·6H2O (0.01 mol), Sm2O3 (0.0008 mol), Fe2O3 (0.0192 mol) andH2C2O4·2H2O (0.084 mol) were thoroughly mixed by grinding inan agate mortar for 30 min, about 15 ml anhydrous ethanol wasthen added to form the mixture in rheological state. The mixturewas sealed in a teflonlined stainless-steel autoclave and maintainedat 120 °C for 48 h in an oven. The obtained precursor was washedseveral times with deionized water and ethanol, dried at 60 °C for12 h, and sintered at 1000 °C for 2 h in air, followed by cooling in afurnace 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 anin situ polymerization in aqueous solution. In a typical procedure,a certain amount of LiNi0.5Sm0.08Fe1.92O4 particles was sus-pended in 35 ml 0.1 M HCl solution and stirred for 30 min to getwell dispersed. 1 ml aniline monomer was then added to thesuspension and stirred for 30min. 2.49 g APS in 20ml 0.1MHClsolution was then slowly added dropwise to the suspensionmixture with a constant stirring at room temperature. After 12 h,the polymerization was achieved and the suspension was in darkgreen. The composite was obtained by filtering and washing thesuspension with 0.1 M HCl and deionized water, and dried undervacuum at 60 °C for 24 h.

2.4. Characterization

X-ray diffraction patterns of the samples were recorded ona Philips-Pw3040/60 X-ray diffractometer (XRD) with a Ni-filter and graphite monochromater, and Cu Kα radiation(λ=0.15418 nm) at a scanning speed of 4°/min in the range of2θ=15–80°. The infrared spectra of the products were determinedon a Nicolet Nexus 670 Fourier transform infrared spectrometer inthe range of 4000–400 cm−1 using KBr pellets. The UV–visspectra of the samples dissolved in N,N-dimethylformamide(DMF) were recorded on a Shimadzu UV-2501PC spectropho-

tometer in the range of 300–800 nm. The TG curves of the sampleswere 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 roomtemperature to 800 °C. The magnetic properties of the compositewere measured at room temperature by using a vibrating samplemagnetometer (VSM, Lakeshore 7403).

3. Results and discussion

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 XRDpattern of LiNi0.5Sm0.08Fe1.92O4 ferrite in Fig. 1(a) shows the singlephase spinel structure with the characteristic reflections of the Fd3mcubic spinel group, which confirms the formation of Sm-substitutedLiNi ferrite. These results indicate that Fe3+ is replaced by Sm3+ on theoctahedral sites in spinel ferrite, and obey the Vegard's law [26]. Thetypical XRD pattern of PANI (curve c) shows the broad diffractionpeaks 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 thecomposite are weaker than that of the pure ferrite, which reveals that thepolyaniline coating layer has an effect on the crystallinity of LiNi0.5Sm0.08

Fe1.92O4 ferrite. In addition, a characteristic amorphous PANI peak can beobserved in XRD pattern of the composite (curve b), which indicates theformation 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]

b ¼ kkDcosh

ð1Þ

where λ is the wavelength of Cu Kα radiation (0.15418 nm), k is theshape factor taken as 0.9,D is the average crystallite size, θ is the Bragg'sangle, and β is the full width at half-maximum (FWHM) of the diffractionpeaks. 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.

Fig. 3. FTIR spectra of PANI (a) and PANI–LiNi0.5Sm0.08Fe1.92O4 composite (b).

Fig. 2. AFM images of LiNi0.5Sm0.08Fe1.92O4 (a), PANI (b), and PANI–LiNi0.5Sm0.08Fe1.92O4 composite (c–d).

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3.2. AFM analysis

The morphology and particle size of the samples are observed by theatomic force microscope (AFM). Fig. 2 shows the micrograph ofLiNi0.5Sm0.08Fe1.92O4 ferrite, PANI and PANI–LiNi0.5Sm0.08Fe1.92O4

composite. It can be seen from Fig. 2(a) that the ferrite are sphericalparticles with average size less than 100 nm, but having agglomeration tosome extent, due to reducing total surface energy of the system. Fig. 2(b)shows theAFMmicrograph of PANI particles with the size in the range of50–150 nm. In Fig. 2(c–d), PANI–LiNi0.5Sm0.08Fe1.92O4 compositepresents some different morphology as compared with that of LiNi0.5Sm0.08Fe1.92O4. It can be seen that the ferrite particles are surrounded andembedded into the polymer chain of PANI, indicating that the ferriteparticles have a core effect on the polymerization of aniline.

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 benzenoidand quinoid ring vibrations at near 1494 cm−1 and 1577 cm−1,respectively, thereby indicating the oxidation state of emeraldine saltform of PANI [30]. It is observed fromFig. 3a that the characteristic bandsof PANI occur at 1577, 1494, 1301, 1240, 1139 and 805 cm−1. The peaksat 1577 and 1494 cm−1 are attributed to the characteristic C_C stretch-ing of the quinoid and benzenoid rings [31,32], the peaks at 1301 and1240 cm−1 correspond to N–H bending and asymmetric C–N stretchingmodes of the benzenoid ring, respectively [33]. The strong peak around1139 cm−1 which is described by MacDiarmid et al. as the “electronic-

like band” is associated with vibrational modes of N_Q_N (Q refers tothe quinonic-type rings) [31,32], indicating that PANI is formed in oursample. The peak at 805 cm−1 is attributed to the out-of-plane defor-mation vibration of the p-disubstituted benzene ring [34]. In addition,the peak located at about 3447 cm−1 corresponds to N–H stretchingmode [35].

Fig. 3b shows the FTIR spectrum of PANI–LiNi0.5Sm0.08Fe1.92O4

composite, which illustrates several differences from the spectrum ofPANI. The N–H bending and asymmetric C–N stretching modes of the

Fig. 6. Variation of magnetization with the applied field measured at 300 K forthe PANI–LiNi0.5Sm0.08Fe1.92O4 composite.

Fig. 4. UV–vis spectra of PANI (a) andPANI–LiNi0.5Sm0.08Fe1.92O4 composite (b).

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benzenoid ring (1302 and 1245 cm−1), and vibrational modes ofN_Q_N (1147 cm−1) for the composite shift to higher wavenumberas compared with that of PANI, moreover, their intensity is also weakerthan that of the PANI. These results reveal that there exists aninteraction 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 PANIhas two characteristic absorption bands at around 329 nm and 617 nm.The absorption band around 329 nm is attributed to π–π* transition ofthe benzenoid ring [36,37], while the peak around 617 nm correspondsto the benzenoid-to-quinoid excitonic transition [38,39]. It is foundfrom 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 comparedwith that of PANI. These results may indicate the σ–π interactionbetween ferrite and PANI backbone, which makes the energy of theantibonding orbital decrease, the energy of the π–π* transition of thebenzenoid and quinoid ring decreases, so the absorption peaks of thecomposite exhibit a red shift.

3.5. TGA analysis

TG curves of PANI and PANI–LiNi0.5Sm0.08Fe1.92O4 compositeare 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 than120 °C) is due to the residual water and HCl desorption [40]. Thesecond loss in mass is observed from about 200 °C to 450 °C, possiblydue to the volatilization of lower weight polyaniline. The final loss inmass at higher temperatures (more than 450 °C) may be due to thethermal degradation of the polyaniline chains. It is seen from Fig. 5bthat the thermal stability of the composite is higher than that of purePANI. This may be caused by the interaction between ferrite particlesand PANI chains.

3.6. Magnetization and coercivity

Fig. 6 shows the magnetization M versus the applied magneticfield H for the composite at 300 K. The magnetization of PANI–LiNi0.5Sm0.08Fe1.92O4 composite exhibits the hysteresis loops atroom temperature, and is weaker than that of the bulk soft magneticferrite. The saturation magnetization (MS) and remanent magnetiza-tion (Mr) of PANI–LiNi0.5Sm0.08Fe1.92O4 composite is 6.62 emu/gand 1.13 emu/g, respectively. The coercivity (HC) of the composite issmall 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 magneticmaterial.

3.7. Bonding model

The bonding model of PANI–LiNi0.5Sm0.08Fe1.92O4 composite isillustrated in Fig. 7. The charge compensation model for thecomposite is shown in Fig. 7(a). There is a charge compensationeffect between ferrite and PANI chains in the composite. The surfaceof the ferrite is positively charged due to the polymerization in theacidic environment. Therefore, adsorption of an amount of the Cl−

may occur to compensate the positive charges on ferrite surface. Inaddition, specific adsorption of the Cl− on the ferrite surface may alsotake place. These specifically adsorbed Cl− would work as the chargecompensator for positively charge PANI chain in the formation ofPANI–LiNi0.5Sm0.08Fe1.92O4 composite. Moreover, hydrogen bond-ing between PANI chains also occur in the composite, theseinteraction will make PANI chains twist to form a network structure.Another probable hydrogen bonding model for the composite isshown in Fig. 7(b). The hydrogen bonding interaction between thepolyaniline chains and the oxygen atoms in ferrite occurs in thecomposite.

Fig. 7. Bonding model for the composite: (a) charge compensation model and (b) hydrogen bonding model.

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The σ–π interaction between metal oxide and PANI may also occurin the composite, which includes (1) the π molecular orbital of PANIoverlaps the empty d-orbital of metal ions to form the σ-bond wheremetallic empty d-orbital is the electron pair acceptor; (2) the π*molecular orbital of PANI overlaps the d-orbital of metal ions to formthe π-bond, in which the metal ions is the electron pair donor.

4. Conclusions

PANI–LiNi0.5Sm0.08Fe1.92O4 nanocomposite with the ferri-magnetic properties is successfully synthesized by an in situpolymerization. FTIR and UV–vis spectra indicate interactionbetween PANI and LiNi0.5Sm0.08Fe1.92O4 ferrite in the compos-ite. XRD study demonstrates the formation of PANI–LiNi0.5

Sm0.08Fe1.92O4 composite. The bonding model includingthe charge compensation and hydrogen bonding model, and theσ–π interaction between metal oxide and PANI in the compositeare investigated. PANI–LiNi0.5Sm0.08Fe1.92O4 composite exhi-bits the hysteresis loops of ferrimagnetic nature at room tem-perature, which suggests that PANI–LiNi0.5Sm0.08Fe1.92O4

composite can be used as a soft magnetic material.

Acknowledgements

This work was supported by the Top Key Discipline ofMaterials Physics and Chemistry in Zhejiang ProvincialColleges and Zhejiang Provincial Natural Science Foundationof China [Y405038].

1096 L. Li et al. / Materials Letters 61 (2007) 1091–1096

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.

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