preparation, characterization and magnetic properties of pani/la-substituted lini ferrite...

Chinese Journal of Chemistry, 2006, 24, 1804—1809 Full Paper

* E-mail: [email protected]; Tel.: 0086-579-2282384; Fax: 0085-579-2282489 Received April 25, 2006; revised July 22, 2006; accepted September 1, 2006. Project supported by Zhejiang Provincial Natural Science Foundation of China (No. Y405038) and Science and Technology Key Project of Zhejiang

Province (No. 2006C21080).

© 2006 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Preparation, Characterization and Magnetic Properties of PANI/La-substituted LiNi Ferrite Nanocomposites

JIANG, Jing(蒋静) LI, Liang-Chao*(李良超) XU, Feng(徐烽)

Institute of Physical Chemistry, Department of Chemistry, Zhejiang Normal University, Jinhua, Zhejiang 321004, China

Magnetic nanocomposites containing polyaniline (PANI)-coated La-substituted LiNi ferrite (LiNi0.5La0.02Fe1.98O4) were synthesized by in situ polymerization in aqueous solution of hydrochloric acid. The nanocomposites exhibited the magnetic hysteresis nature under applied magnetic field. The saturation magnetization (MS) and coercivity (HC) varied with the ferrite content. The obtained nanocomposites were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared spec-troscopy (FT-IR), UV-Visible spectroscopy and vibrating sample magnetometer (VSM). TEM and SEM studies showed that the nanocomposites present the core-shell structure. The results of XRD patterns, FT-IR and UV-Visible spectra indicated the formation of PANI-LiNi0.5La0.02Fe1.98O4 nanocomposites and showed that the in-teraction existed between PANI backbone and ferrite particles in the nanocomposites. The bonding mechanism in the nanocomposites has been proposed.

Keywords nanocomposite, polyaniline, ferrite, magnetic property

Introduction

Conducting polymers have attracted considerable attention for their potential applications to various fields such as electromagnetic interference (EMI) shielding, rechargeable battery, electrodes and sensors, corrosion protection coatings and microwave absorption.1-5 Among the conducting polymers, polyaniline (PANI) has been extensively studied in last two decades due to its unique electrochemical and physicochemical behav-ior, good environment stability, and relatively easy preparation.6,7

Conducting polymer-inorganic composites possess not only the nature of the flexibilities and processability of polymers but also the mechanical strength and hard-ness of inorganic components. Recently, several inter-esting researches have been focused on the PANI-inor-ganic composites to obtain the materials with synergetic or complementary behavior between polyaniline and inorganic nanoparticles.8,9

The soft magnetic LiNi ferrites have been widely used in microwave devices such as isolators, circulators and phase shifters because of their dieletric and mag-netic properties.10,11 It is interesting that the electrical and magnetic properties of ferrites can be tailored by controlling the different type and amount of metal ion substitution. Rare earth ions have unpaired 4f electrons that have a role to originate magnetic anisotropy due to their orbital shape. The magnetocrystalline anisotropy in ferrite is related to 4f-3d couplings between transition

metal and rare earth ions,12 thereby doping rare earth ions into spinel LiNi ferrite can improve their electrical and magnetic properties.

It is known that the conducting materials can effec-tively shield electromagnetic waves generated from an electric source, whereas, electromagnetic waves from a magnetic source, especially at low frequencies, can be effectively shielded only by magnetic materials. Thus, if films having both conducting and magnetic components are used as EMI shielding materials, good shielding ef-fectiveness can be achieved for various electromagnetic sources with a single coating of such materials. Due to these factors, we have the motivation to synthesize conductive and magnetic composites suitable for shielding applications.

Up to date, preparation of polyaniline with ferromag-netic properties has been studied by Wan’s group.13,14 Deng et al.15 have studied the synthesis of magnetic and conducting Fe3O4-cross-linked polyaniline (CLPANI) nanoparticles with core-shell structure by using a pre-cipitation-oxidation technique. Yang et al.16 have re-ported the preparation of conducting and magnetic PAn/γ-Fe2O3 nanocomposite by modification-redoping method. Recently, the fabrication of MnZn or NiZn fer-rite-polyaniline composites has been reported, 17-19 but polyaniline-ferrite systems fabricated by incorporating La-substituted LiNi ferrite into polyaniline have not been reported.

In this paper, we prepared PANI-LiNi0.5La0.02Fe1.98O4 nanocomposites by in situ polymerization in aqueous

Nanocomposite Chin. J. Chem., 2006 Vol. 24 No. 12 1805


solution. The samples were characterized by various experimental techniques, and the magnetic properties of nanocomposites were investigated.

Experimental

Materials

Aniline monomer was distilled under reduced pres-sure and stored below 0 ℃. Ammonium peroxydisul-fate (APS), ferric oxide (Fe2O3), lithium carbonate (Li2CO3), nickel sulfate (NiSO4•6H2O), lanthanum ox-ide (La2O3), and oxalic acid (H2C2O4•2H2O) were all of analytically pure grade and used as received. All re-agents were purchased from Shanghai Chemical Agents Ltd Co. in China.

Preparation of LiNi0.5La0.02Fe1.98O4 ferrite (LNLFO)

The La-substituted LiNi ferrites were prepared by a rheological phase reaction method.20 In a typical proce-dure, stoichiometric amounts of Li2CO3 (10 mmol), NiSO4•6H2O (10 mmol), La2O3 (0.2 mmol), Fe2O3 (19.8 mmol) and H2C2O4•2H2O (84 mmol) were thoroughly mixed by grinding in an agate mortar for 30 min, about 35 mL of ethanol were then added to form the mixture in rheological state. The mixture was sealed in a Tef-lon-lined stainless autoclave and maintained at 120 ℃ for 48 h in an oven. The obtained precursors were washed several times with deionized water and ethanol, dried at 60 ℃ for 12 h, and sintered at 1000 ℃ for 2 h in air, followed by cooling in furnace to room tempera-ture with 5 ℃•min－1 cooling rate.

Synthesis of PANI-LiNi0.5La0.02Fe1.98O4 nanocompo-sites

PANI-LiNi0.5La0.02Fe1.98O4 nanocomposites were prepared by in situ polymerization in aqueous solution of hydrochloric acid. In a typical procedure, a certain amount of LiNi0.5La0.02Fe1.98O4 ferrite particles were suspended in 35 mL of 0.1 mol•L－1 HCl solution and stirred for 30 min to get well dispersed. The 1 mL of aniline monomer was then added to the suspension and stirred for 30 min. The 2.49 g of APS in 20 mL of 0.1 mol•L－1 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 nanocomposites were obtained by filtering and washing the suspension with 0.1 mol•L－1 HCl and deionized wa-ter, and dried under vacuum at 60 ℃ for 24 h. The dif-ferent content of PANI-LiNi0.5La0.02Fe1.98O4 nanocompo-sites were synthesized by using 10%, 20%, 30%, 40% (w) of LiNi0.5La0.02Fe1.98O4 ferrite with respect to aniline monomer.

Characterization

X-ray diffraction patterns of samples were recorded on a Philps-Pw3040/60 diffractometer with monochro-matic Cu Kα radiation at a scanning speed of 4 (°)•min－1 in the range of 2θ＝20°—80°. The TEM images and

electron diffraction (ED) patterns were carried out on a Hitachi-800 transmission electron microscope at an ac-celerating voltage of 200 kV. The SEM micrographs were obtained on a Hitachi S4800 scanning electron microscope. The infrared spectra were recorded on a Nicolet Nexus 670 FT-IR spectrophotometer (Nicolet, USA) in the range from 4000 to 400 cm－1 using KBr pellets. UV-Vis spectra of samples dissolved in N, N-dimethylformamide (DMF) were recorded on a Shi-madzu UV-2501PC spectrophotometer in the range of 300—800 nm. The magnetic properties of samples were measured at room temperature by using a Lakeshore 7403 vibrating sample magnetometer (VSM, USA).

Results and discussion

X-ray diffraction analysis

Figure 1 shows the XRD patterns of LiNi0.5La0.02- Fe1.98O4, HCl-doped PANI and PANI-LiNi0.5La0.02- Fe1.98O4 nanocomposites. The LiNi0.5La0.02Fe1.98O4 par-ticle (curve a) shows the characteristic peaks at 2θ＝18.44°, 30.14°, 35.66°, 37.13°, 43.22°, 53.75°, 57.16° and 62.81° with the reflection of Fd3m cubic spinel group, which indicates the formation of the single-phase spinel. The typical XRD pattern of PANI in Figure 1(f) shows diffraction peaks at 2θ＝25.44°, 43.68°, 51.03° and 72.48°, which is in agreement with the results of Karim et al.21 The broad peak at about 2θ＝25.44° shows amorphous nature ascribed to the periodicity par-allel to the polymer chain. The sharp peaks at 2θ＝43.68°, 51.03° and 72.48° indicate that HCl-doped polyaniline has some degree of crystallinity, which may be attributed to the periodicity perpendicular to the polymer chain.22,23 The diffraction peaks for the nano-composite in Figure 1(b—e) exhibit both the character-istic peaks of LiNi0.5La0.02Fe1.98O4 and PANI. The in-tensity of the diffraction peaks gradually increases with the ferrite content. These results reveal the formation of the PANI-LiNi0.5La0.02Fe1.98O4 nanocomposites.

Figure 1 XRD patterns of PANI-LiNi0.5La0.02Fe1.98O4 nano-composites: (a) 100% LNLFO, (b) 40% LNLFO, (c) 30% LNLFO, (d) 20% LNLFO, (e) 10% LNLFO and (f) HCl doped PANI.

1806 Chin. J. Chem., 2006, Vol. 24, No. 12 JIANG, LI & XU


The average crystallite sizes of PANI-LiNi0.5La0.02- Fe1.98O4 nanocomposites can be calculated by the De-bye-Scherrer formula according to the full width at half maximum (FWHM) of the strongest diffraction peak (2θ＝35.66°):

cos

k

D

λβθ＝ (1)

where λ is the wavelength of Cu target (0.15418 nm), k is the shape factor taken as 0.89, D is the average crys-tallite size, θ is the Bragg’s angle, and β is FWHM of the diffraction peak. The average crystallite sizes of PANI-LiNi0.5La0.02Fe1.98O4 nanocomposites were ob-tained and listed in Table 1.

Table 1 Diffraction data of PANI-LiNi0.5La0.02Fe1.98O4 nano-composites

2θ＝35.66° Content of

ferrite (w) FWHM (2θ) d/nm a/nm D/nm

10% 0.375 0.2524 0.83705 60.6

20% 0.333 0.2520 0.83607 72.6

30% 0.337 0.2518 0.83571 69.3

40% 0.314 0.2518 0.83576 96.4

Morphology

The representative TEM image and SEM micro-graph of PANI-LiNi0.5La0.02Fe1.98O4 nanocomposite containing 20% LiNi0.5La0.02Fe1.98O4 are shown in Fig-ure 2. It is clearly seen that the LiNi0.5La0.02Fe1.98O4 are embedded in the polyaniline matrix forming the core-shell structure of PANI-LiNi0.5La0.02Fe1.98O4 nanocomposite. The black core is magnetic ferrite parti-cles, and the grey shell is PANI in PANI-LiNi0.5- La0.02Fe1.98O4 nanocomposite, due to the different elec-tron penetrability. The inset in Figure 2(a) shows the typical electron diffraction (ED) pattern of PANI- LiNi0.5La0.02Fe1.98O4 nanocomposite. It is indicated that the nanocomposite is composed of polycrystalline fer-rite and amorphous PANI, which is in accordance with the results obtained by XRD analysis. The SEM micro-graph of the nanocomposite is shown in Figure 2(b). It is indicated that the nanocomposite particles are spheri-cal with the average crystallite size in the range of 60—90 nm, which is in agreement with the results of TEM analysis.

FT-IR spectral analysis

Figure 3 shows the IR spectra of HCl-doped PANI and PANI-LiNi0.5La0.02Fe1.98O4 nanocomposites. All spectra exhibit the clear presence of benzenoid and quinoid ring vibrations at 1494 and 1579 cm－1, respec-tively, thereby indicating the oxidation state of emer-aldine salt form of PANI.24 For the HCl-doped PANI, the characteristic peaks are located at 1579, 1494, 1251, 1133 and 800 cm－1. The peaks at 1579 and 1494 cm－1 were attributed to the characteristic C＝C stretching of the

Figure 2 (a) TEM image and ED pattern (inset), (b) SEM mi-crograph of PANI-LiNi0.5La0.02Fe1.98O4 nanocomposite contain-ing 20% LiNi0.5La0.02Fe1.98O4.

quinoid and benzenoid rings, the peaks at 1300 and 1251 cm－1 were assigned to C—N stretching modes of the benzenoid ring, the broad and strong peak around 1133 cm－1 which was described by MacDiarmid et al. as the “electronic-like band” is associated with vibra-tional modes of N＝Q＝N (Q refers to the quinonic- type rings),24 indicating that HCl-doped PANI is formed in our samples. The peak at 800 cm－1 was attributed to the out-of-plane deformation vibration of the p-disubstituted benzene ring. In addition, the peak at about 3446 cm－1 corresponds to N—H stretching mode.

Figure 3 FT-IR spectra of PANI-LiNi0.5La0.02Fe1.98O4 nano-composites: (a) HCl-doped PANI, (b) 10% LNLFO, (c) 20% LNLFO, (d) 30% LNLFO and (e) 40% LNLFO.

Figure 3(b—e) shows the FT-IR spectra of PANI- LiNi0.5La0.02Fe1.98O4 nanocomposites, which exhibit the similar spectra to the HCl-doped PANI, and the main absorption peaks are tabulated in Table 2. In ferrites the metal ions usually reside at two sublattices designated the



Table 2 FT-IR peaks of PANI-LiNi0.5La0.02Fe1.98O4 nanocomposites with LiNi0.5La0.02Fe1.98O4 content

Wavenumber/cm－1

LNLFO (w)

0% 10% 20% 30% 40%

Peak assignment

3446 3449 3448 3425 3448 ＞N—H stretching vibration

2922 2920 2926 2926 2925 aromatic C—H stretching

1579 1567 1568 1568 1568 C＝C stretching of quinoid rings

1494 1490 1494 1494 1492 C＝C stretching of benzenoid rings

1300, 1251 1301, 1245 1299, 1242 1299, 1244 1302, 1246 C—N stretching of the benzenoid ring

1133 1141 1130 1136 1144 C—H in-plane bending vibration

800 803 800 802 804 para-substituted aromatic rings

697 701 685 693 696 C—H out-of-plane bending vibration

— 585 579 593 594 vibration of the tetrahedral site (ν1)

— 403 413 407 406 vibration of the octahedral site (ν2)

tetrahedral and octahedral sites. Waldron25 and Hafner26 studied the vibrational spectra of ferrites and attributed high frequency band ν1 to the intrinsic vibration of the tetrahedral sites and low frequency band ν2 to the intrinsic vibration of the octahedral site. The ν1 and ν2 lie in the range of 600—580 cm－1 and 440—400 cm－1, respec-tively.27 It was observed from Figure 3(b—e) that the peaks of the nanocomposites located around 596 and 420 cm－1 are intrinsic vibration of the tetrahedral and octahedral sites, respectively, indicating that LiNi0.5- La0.02Fe1.98O4 was doped into the nanocomposites.

UV-Vis spectral analysis

Figure 4 gives UV-Vis absorption spectra of PANI- LiNi0.5La0.02Fe1.98O4 nanocomposites. The HCl-doped PANI has two characteristic absorption bands around 335 and 623 nm. The absorption band around 335 nm was attributed to π-π* transition of the benzenoid ring,28,29 while the peak around 623 nm corresponds to

Figure 4 UV-Vis spectra of PANI-LiNi0.5La0.02Fe1.98O4 nano-composites: (a) HCl doped PANI, (b) 10% LNLFO, (c) 20% LNLFO, (d) 30% LNLFO and (e) 40% LNLFO.

charge transfer from the benzenoid rings to the quinoid rings.30,31 It was found from Figure 4(b—e) that the ab-sorption peaks around 335 nm of PANI-LiNi0.5La0.02- Fe1.98O4 nanocomposites have a red shift as compared to that of HCl-doped PANI, and varying degrees of red shift increase with the ferrite content. This result sug-gests that the interaction between metal ions and PANI chains should make the energy for π-π* transition de-creased.32 The absorption band at 623 nm exhibits gradually blue shift with the ferrite content, implying that ferrite particles tend to diffuse into the PANI chains, which may enhance the interaction between metal ions and PANI chains,33 leading to enlarging the energy of charge transfer from the benzenoid rings to the quinoid rings

Magnetic properties

Figures 5 and 6 show the M-H curves of the as-synthesized samples under applied magnetic field at room temperature. The magnetization of PANI-LiNi0.5- La0.02Fe1.98O4 nanocomposites exhibits a clear hyteretic behavior, and the area within the hyteresis loops was increased with increasing the ferrite content. The pure LiNi0.5La0.02Fe1.98O4 ferrite shows the saturation mag-netization (MS) value of 33.97 emu•g－1, whereas the magnetization of PANI-LiNi0.5La0.02Fe1.98O4 nanocom-posites decreases with increasing the content of PANI coating layer, attributed to the diamagnetic contribution of PANI.

Figure 7 shows the saturation magnetization (MS) versus the ferrite contents in PANI-LiNi0.5La0.02Fe1.98O4 nanocomposites. In a ferrimagnet, due to antiferromag-netic coupling, the magnetic moments of A and B sites are aligned anti-parallel and do not cancel each other, resulting in a net magnetization. It was observed that the saturation magnetization of nanocomposites was in-creased with the ferrite content, and was smaller than that of the pure ferrite. According to the equation Ms＝

1808 Chin. J. Chem., 2006, Vol. 24, No. 12 JIANG, LI & XU


Figure 5 Hysteresis loops of LiNi0.5La0.02Fe1.98O4 ferrite.

Figure 6 Hysteresis loops of PANI-LiNi0.5La0.02Fe1.98O4 nano-composites: (a) 10% LNLFO, (b) 20% LNLFO, (c) 30% LNLFO and (d) 40% LNLFO.

φmS, MS is related to the volume fraction of the particles (φ) and the saturation moment of a single particle (mS).34 It may be considered that the saturation magnetization of the nanocomposites depends on the volume fraction of the magnetic ferrite particles, due to the diamagnetic PANI coating layer contribution to the total magnetiza-tion, resulting in the changes of the saturation magneti-zation.

Figure 7 Saturation magnetization (MS) versus the contents of ferrite in nanocomposites.

Figure 8 shows that the coercivity (HC) increases with the ferrite content for PANI-LiNi0.5La0.02Fe1.98O4 nanocomposites. The coercivity is related to the micro-

structure of the nanocomposites. Polycrystalline ferrites have an irregular structure, geometric and crystallo-graphic nature, such as pores, cracks, surface roughness and impurities. In the polymerization process, oli-gomeric aniline intermediates are adsorbed on the ferrite surface and crystallite boundaries to form the primary nucleation centers, and PANI chains subsequently grow from these centers in an oxidized pernigraniline form, this process may have a healing effect to cover the fer-rite surface defects, such as pores and cracks, but this healing effect is weakened with increasing of the ferrite loading,17 leading to increasing of the coercivity for the nanocomposites.

Figure 8 Coercivity (HC) versus the contents of ferrite in nano-composites.

Bonding interactions

The bonding interaction between ferrite particles and PANI backbone in the nanocomposites is illustrated in Figure 9. We propose the probably bonding mechanism,

Figure 9 Bonding mechanism in the nanocomposites.



i.e. charge compensation mechanism, in the nanocom-posite according to the results of spectral analysis. 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 compensate the positive charges on ferrite surface. Beside the charge compensation process, specific adsorption of the Cl－ on the ferrite surface would work as the charge compensa-tor for protonated PANI chain in the formation of PANI-ferrite nanocomposites. There is the charge com-pensation effect between ferrites and PANI chains in the nanocomposites. In addition, the hydrogen bonding be-tween PANI chains may also exist in the nanocompo-sites.

Conclusion

PANI-LiNi0.5La0.02Fe1.98O4 nanocomposites with the magnetic behavior are successfully synthesized by in situ polymerization in aqueous solution of hydrochloric acid. TEM and SEM studies show that the nanocompo-sites present the core-shell structure. FT-IR and UV-Vis spectra indicate the interaction between PANI chains and ferrite particles in the nanocomposites. XRD dif-fraction patterns demonstrate the formation of PANI-LiNi0.5La0.02Fe1.98O4 nanocomposites. The nano-composites exhibit the intrinsic magnetic hysteresis loops, and the magnetic properties depend on the ferrite content. The bonding mechanism in the nanocomposites was proposed.

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(E0604258 ZHAO, X. J.; ZHENG, G. C.)

preparation, characterization and magnetic properties of pani/la-substituted lini ferrite...

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