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PANI nanocomposites
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 314 (2007) 93–99 www.elsevier.com/locate/jmmm
Effect of ferrofluid concentration on electrical and magnetic properties of the Fe3O4/PANI nanocomposites Javed Alam, Ufana Riaz, Sharif Ahmad Materials Research Laboratory, Department of Chemistry, Jamia Millia Islamia, New Delhi 110025, India Received 4 January 2007; received in revised form 5 February 2007 Available online 7 March 2007
Abstract Magnetic nanocomposites can be controlled and tailored to provide the desired mechanical, physical, chemical, and biomedical properties depending on the final applications. The coating of ferrite nanoparticles with polymers affords the possibility of minimizing agglomeration in large-scale commercial synthesis of nanocomposite materials. The process of coating not only provides effective encapsulation of individual nanoparticles, but also controls the growth in size, thus, yielding a better overall size distribution. In this paper, in-situ polymerization of aniline was carried out in different concentration of the ferrofluid with the aim to obtain agglomerate free nanocomposites. The role of the ferrite concentration was investigated by the spectral, morphological, conductivity, and magnetic properties of Fe3O4/polyaniline (PANI) nanocomposites. XRD revealed the presence of spinel phase of Fe3O4 and the particle size was calculated to be 14.3 nm. Spectral analysis confirmed the formation of PANI encapsulated Fe3O4 nanocomposite. Conductivity of the nanocomposites was found to be in the range of 0.001–0.003 S/cm. Higher saturation magnetization of 3.2 emu/g was observed at 300 K, revealing a super paramagnetic behavior of this nanocomposite. r 2007 Elsevier B.V. All rights reserved. Keywords: Polyaniline; Nanocomposite; Super paramagnetism; Conductivity
1. Introduction Magnetic polymer nanocomposites represent a class of functional materials, where magnetic nanoparticles are embedded in polymer matrices. These nanocomposites hold immense potential for application in cell separation, enzyme immunoassay, drug targeting, electromagnetic device applications, and electromagnetic interference suppression [1–4]. Multi-component conducting polymer systems with nanoparticles of metal oxides can be tailored to obtain desired mechanical properties besides novel electrical, magnetic, and optical properties. A number of articles have been published on the magnetic and conducting polymeric nanocomposites of polyaniline (PANI) as well as polypyrrole composites containing nanoparticles such as TiO2 [5], ZrO2 [6], g-Fe2O3 [7], Fe3O4 [8], and SnO2 [9]. The properties of these systems are sensitive to the particle size, inter particle interaction, and temperature Corresponding author. Tel.: +91 26982815.
E-mail address: [email protected] (S. Ahmad). 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.02.195
[10]. Till date, the most commonly studied magnetic nanomaterials are the oxides used in the form of ferrites and substituted ferrites in stable colloidal suspensions known as ferrofluids. Ferrofluids are the colloidal dispersion of nanosize 2–10 nm magnetic particles in some suitable carrier medium. Ferro-fluid-polymer composites materials are formed by the dispersion of ferro-ferrimagnetic materials with other non-magnetic particles in a host matrix. Although, it is an established fact that nanoparticles exhibit novel magnetic properties, the ideal response in general is associated with isolated particles. A critical obstacle in assembling and maintaining a nanoscale magnetic material is usually its tendency to agglomerate, which is a deterrent to its applications [10]. In practical applications, one invariably has to consider a collection or aggregate of particles forming nanopowders. Further, several applications require consolidation and sintering of nanophase materials into solid blocks or thin films. These processing routes often lead to unavoidable formation of agglomerations and larger grains that effectively prevent the materials from attaining
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their full potential in terms of the desirable magnetic response associated with nanoparticles [11]. Polymer encapsulated magnetic nanoparticles are of great technological interest as the coating provides a matrix for binding of the particles and also prevents grain growth and agglomeration. The process of coating not only provides effective encapsulation of individual nanoparticles, but also controls the growth in size thus, yielding uniform overall size distribution. Encapsulation of magnetic particles with preformed natural or synthetic polymers is the simple and classical method to prepare magnetic polymeric particles [12–15]. Ugelstad et al. [16] developed an effective method through direct precipitation of iron salt inside the porous polystyrene particles. The development of emulsion polymerization, such as the conventional emulsion polymerization, soap-free emulsion polymerization, miniemulsion polymerization, and microemulsion polymerization, was led to new synthesis methods [17,18]. However, the control over the dispersion of the nanoparticle phase embedded in a polymer matrix is still in its embryonic stage at the research level and requires extensive investigation. To achieve excellent dispersion, competition between polymer–polymer and polymer–particle interactions have to be balanced to avoid clustering of particles in polymer nanocomposites. Hence, to exploit the full potential of the technological applications of these materials, it is important to address the above problems. In the present work, the effect of ferrofluid concentration on the spectral, morphological, conductivity, and magnetic properties of the Fe3O4/PANI nanocomposite has been investigated. Our aim was to obtain the ferrite/polyaniline nanocomposites with less agglomeration along with promising magnetic properties. The Fe3O4/PANI nanocomposites were prepared by in-situ polymerization of aniline in ferric hydroxide sol of 2%, 6% and 8% concentrations, respectively. The resulting nanocomposites were analyzed by FT-IR, UV–vis, XRD, conductivity, and SEM as well as VSM studies.
purification. Aniline (Merck, India) was double distilled and was stored in refrigerator prior to use. All chemicals were of AR grade. 2.1. Preparation of ferrofluid The preparation of ferrofluid was carried out at room temperature (25 1C) by the addition of NH4OH solution (5 N) to aqueous solution of different concentrations of ferric chloride (2, 6 and 8 wt%) with slow stirring. The pH of the solution was maintained between 4 and 5. Ammonium hydroxide solution was added until a red cherry color appeared indicating formation of ferric hydroxide sol of pH ¼ 6, with inherent viscosity Z ¼ 0.466, and specific gravity 1.0642. The 2, 6 and 8 wt% ferrofluids obtained were then used as oxidants for the polymerization of PANI. 2.2. Preparation of polyaniline–ferrite nanocomposite A fixed volume of ferrofluid was added drop wise to a 250 ml round bottom flask containing HCl (8 ml, 1 N) and double distilled aniline (5 ml) with slow and continuous stirring maintained at a constant temperature and pH, as shown in Table 1. After an induction period of 30 min, the color of the sol changed from red to green, which confirmed the polymerization of aniline in conducting form. Polymerization was further continued for 12 h at room temperature. The green precipitate of Fe3O4/PANI composite obtained was filtered, washed several times with distilled water, methanol, and then dried in vacuum for 72 h at 60 1C. The nanocomposites were designated as 2Fe3O4/PANI, 6-Fe3O4/PANI and 8-Fe3O4/PANI, according to the concentration of the ferrofluid used in the preparation. 3. Characterization 3.1. Spectral analysis
2. Experimental Ammonium hydroxide solution (25%) and ferric chloride (MW 162.21) (Merck, India) were used without further
FT-IR spectra of the composites were taken in dried KBr powder on Perkin-Elmer spectrometer model 1750. UV–vis spectra were taken on Perkin-Elmer lambda EZ-221.
Table 1 Characteristics of Fe3O4/PANI nanocomposites Nanocomposite
Sol concentration (M)
pH
Temperature (1C)
Yield (%)
Reaction time
Total time (days)
Conductivity (S/cm)
2-Fe3O4/PANI 2-Fe3O4/PANI 2-Fe3O4/PANI 6-Fe3O4/PANI 6-Fe3O4/PANI 6-Fe3O4/PANI 8-Fe3O4/PANI 8-Fe3O4/PANI 8-Fe3O4/PANI
0.49 0.612 0.612 0.073 0.073 0.073 0.307 0.615 0.615
1 3 5 1 3 5 1 3 5
6 10 15 6 10 15 6 10 15
13 14 16 12 21 23 18 21 25
3 6 12 3 6 12 3 6 12
7 7 7 3 3 3 3 3 3
— — 0.003 — — 0.0018 — — 0.001
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3.2. Morphological analysis
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X-ray diffractograms were recorded on X-ray diffractometer model Philips PW3710 using copper Ka radiations. Scanning electron micrographs were taken on JEOL JSM840 scanning electron microscope under thin gold film.
nanocomposite was obtained (Table 1) owing to the formation of excess of ferrite nanoparticles. Maximum yield was obtained when polymerization was carried out at room temperature and pH ¼ 5, while the percent yield was found to vary slightly with the increase in the polymerization time after 12 h.
3.3. Conductivity studies
4.2. X-ray diffraction analysis
Conductivity was measured by standard two-probe method using Keithley multimeter models 2001 at 30 1C.
The X-ray diffractogram of Fe3O4 (Fig. 1) shows the characteristic diffraction peaks of Fe3O4 (2 2 0), Fe3O4 (3 1 1), Fe3O4 (4 0 0), Fe3O4 (4 2 2), Fe3O4 (5 1 1) and Fe3O4 (4 4 0). The size D of the Fe2O3 particles was calculated by Scherrer’s equation [24]
3.4. Magnetic measurements The magnetic measurements were performed using Vibrating Sample Magnetometer Model EG & G Princeton at 300 K by applying a field of 1000 A/m and at a frequency of 16 Hz. 4. Results and discussion 4.1. Effect of ferrofluid concentration on the properties of the nanocomposite The Fe3O4/PANI composite was obtained and a possible mechanism of its formation is predicted in Scheme 1. The ferrite particles in the stable ferrofluid dispersion interact with the conducting polymer via hydrogen bonding, which gets adsorbed to the surface of the ferrite resulting in encapsulation of the ferrite nanoparticles by PANI. The nanocomposites were found to move in the direction of applied magnetic field, which indicates their magnetic behavior. This property can be utilized in the magnetic separation of materials from suspension. To reduce the formation of agglomerates of ferrite nanoparticles during the preparation of polyaniline–ferrite nanocomposites, we adopted the in-situ polymerization technique for the synthesis of Fe3O4/PANI nanocomposites. The color of the solution was found to change from red to green upon polymerization of aniline in the presence of ferric hydroxide sol, which confirmed the polymerization of aniline in the former. It was also observed that with the increase in the ferrite concentration, higher yield of the PANI-Fe3O4
Scheme 1. Mechanism of formation of Fe3O4/PANI nanocomposite.
D¼
0:89l , b cos y
where l is the X-ray wavelength, b is the full-width at halfmaxima (FWHM) of the X-ray diffraction peak, and y is the Bragg angle. According to the b values of Fe3O4 (3 1 1), the particle size was calculated to be about 14.3 nm, which confirms the nanosize of ferrite. The Fe3O4/PANI nanocomposites however, reveal broadening of the ferrite peaks. It is interesting to note that the amorphous nature of the nanocomposite increases with the increase in the concentration of the ferrite sol. This can be attributed to the presence of higher amount of PANI in the composite. The presence of diffused broad peaks indicates lower crystalline order owing to the formation of larger fraction of PANI [20]. The preponderance of amorphous peaks of PANI indicates that the crystalline behavior of ferrite is suppressed as a result of its encapsulation by PANI, as PANI displays a diffuse broad peak ranging from 201 to 351.The results obtained are consistent with the ones obtained by other research groups [21,22]. This observation is further corroborated by the spectral as well as SEM studies discussed in the proceeding sections. 4.3. Spectral analysis The FT-IR spectra of pristine ferrite (Fig. 2) exhibit a small peak at 3000 cm1, which can be associated with the O–H stretching vibrations in physically adsorbed hydrogen-bonded water molecule. The small peaks in the region 1500–1400 cm1 can also be correlated to the different modes of bonded water molecules existing in the ferrofluid while the prominent peak at 510 cm1, is attributed to the stretching and torsional vibration modes of the Fe–O bonds in tetrahedral and octahedral sites, respectively [23]. The spectra of 2-Fe3O4/PANI (Fig. 2) reveals the N–H stretching vibration peak corresponding to PANI at 3394 cm1 while the peaks corresponding to benzenoid (NH–B–NH) and quinonoid (NH–Q–NH) rings are observed at 1590, 1498, and 1305 cm1, respectively. The CN vibration peak is observed at 1166 cm1 while the characteristic peak of ferrite is observed at around
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Fig. 1. X-ray diffractograms of Fe3O4/PANI nanocomposites.
previous works, the suppression of –OH vibrational modes in the 3000 cm1 region has been related to evidence of host–guest interaction as a consequence of complete water release upon inclusion of ferrite in the polymer [27]. In our case, the increase in the broadness and intensity in O–H stretching vibration peak is observed incase of 6-Fe3O4/ PANI and 8-Fe3O4/PANI, which indicates that the adsorption of PANI to the surface of the ferrite particles takes place through the hydrogen bonding between N–H groups in the PANI and OH group of bonded water molecules in the ferrofluid, resulting in the encapsulation of ferrite particles by PANI. The FT-IR spectrum confirms the bonding of PANI strongly to the protruding surface hydroxyl groups on ferrite nanoparticles. 4.4. UV–vis spectra
Fig. 2. FT-IR spectra of Fe3O4/PANI nanocomposites.
583 cm1 confirming the presence of ferrite particles and the composite nature of the Fe3O4/PANI composite [24,25]. Incase of 6-Fe3O4/PANI, the benzenoid–quinonoid vibrations, CN vibration peaks, and the ferrite peaks are found to shift by 10, 37, 9, 12, and 15 cm1, respectively. The increase in the intensity of the ferrite peaks is observed with the increase in the concentration of the ferrofluid. The shifting as well as broadening of the peaks can be attributed to interaction of 3d orbit of ferrite with N atom in PANI to form coordinate bond [26]. In accordance with
The UV–vis spectra of 2-Fe3O4/PANI nanocomposite (Fig. 3) show absorption maxima at 350 nm as well as 600 nm. The former is assigned to P–P* transitions while the later peak can be correlated to the polaronic transitions. The presence of polaronic transition peak confirms the emeraldine doped state of PANI in Fe3O4/PANI nanocomposite. The exciton transitions of the quinonoid rings appear to be highly suppressed and show flattening in 6-Fe3O4PANI and 8-Fe3O4/PANI revealing the increase in the extent of doping of PANI by the ferrite particles upon encapsulation. 4.5. Magnetization It is well known that when the grain size of ferrite particles decreases to a critical size (an order of 10 nm), the
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coercive force HC decreases to zero and the nanocomposite exhibits a super paramagnetic behavior, which can be usually observed at room temperature (above the blocking
Fig. 3. UV spectra of Fe3O4/PANI nanocomposites.
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temperature) [13,19,22]. The applied magnetic field H dependence of the magnetization M can be described by Langevin equation [24]: M ¼ MS(coth y1/y), where Ms is the saturation magnetization and y ¼ mH/kBT, m is the average magnetic moment of an individual particle in the sample and B is the Boltzmann constant. Fig. 4(a)–(c) shows the plots of the magnetization M versus the applied magnetic field H (between 71000 H (MT) for the PANI/Fe3O4 nanocomposites at 300 K. For 2Fe3O4/PANI and 6-Fe3O4/PANI nanocomposite (Fig. 4(a), (b)) the Ms values were found to be 0.010 and 0.5 emu/g, respectively, while for 8-Fe3O4/PANI nanocomposite (Fig. 4(c)) Ms was found to be around 3.2 emu/g. In this case, remanent magnetization Mr and coercive force Hc were found to be zero indicating a super paramagnetic behavior. Long et al. [24] have reported, Ms of 3.5 emu/g at 20 wt% loading of ferrite in PANI. Interestingly, in our case, similar values of paramagnetism have been obtained
Fig. 4. Magnetization curve of (a) 2-Fe3O4/PANI, (b) 6-Fe3O4/PANI, and (c) 8-Fe3O4/PANI.
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at reasonably lower loading. This variation in the values of magnetization can be attributed to the varying size of ferrite particles synthesized in each case. However, the interactions between the polymer matrix and iron oxide nanoparticles are also found to play a significant role in attaining the magnetic nature of these composites. The specific morphology of PANI as well as the surface chemistry influences the surface interactions of the magnetic nanoparticle strongly. Encapsulation of ferrite nanoparticles by PANI leads to a significant decrease in the magnetic nature of the overall nanocomposite owing to the diamagnetic nature of PANI. Therefore, the measured values of Ms differ than the reported ones mainly due to the difference in the interactions developed at the interphases of Fe3O4/PANI nanocomposite. Fig. 5. SEM micrograph of 6-Fe3O4/PANI nanocomposite.
4.6. Conductivity The concentration of ferrite nanoparticles significantly affects conductivity of the resulting composites. The conductivity values of 2-Fe3O4/PANI was found to be 3 103 S/cm, while that of 6-Fe3O4/PANI and 8-Fe3O4/ PANI nanocomposite was found to be 1.8 103 and 1 103 S/cm, respectively. The higher conductivity of the 2-Fe3O4/PANI nanocomposite can be attributed to the optimum level of doping induced by the interaction between ferrite nanoparticles and PANI backbone. The decrease in the conductivity of 8-Fe3O4/PANI nanocomposites upon increasing the ferrite concentration can be attributed to the insulating behavior of the iron oxide in the core of the nanoparticles, which hinders the charge transfer thereby lowering the conductivity. 4.7. SEM The SEM of 6-Fe3O4/PANI (Fig. 5) shows a homogeneous nanoporous structure of the nanocomposite with the uniform distribution of particles. The phase contrast of the nanocomposite appears to be less pronounced owing to the engulfment of the ferrite particles by PANI. The micrograph of ferrite–PANI nanocomposite exhibits a twophase system where the bright phase corresponds to the existence of ferrite, while dark phase constitutes PANI. The morphology of nanocomposites is found to be porous in nature, where the PANI as well as iron oxide phase form a network like structure and appear to be well interconnected. As the concentration of sol increases, the bright phase becomes predominant with the aggregation of ferrite particles through magnetic as well as intermolecular interaction. 5. Conclusion A simple chemical method was adopted for the synthesis of ‘‘agglomerate free’’ Fe3O4/PANI nanocomposite where the nanoparticles are entrapped in the growing PANI chains. The improvements made in physical properties of
the present nanocomposites are expected to enhance the application potential of the polymer without hampering its magnetic properties. The structural characterizations of the Fe3O4/PANI nanocomposites reveal that the encapsulation of ferrite particles by PANI results in significant amount of doping, which enhances the electrical conducting properties of Fe3O4/PANI nanocomposites, while the inherent magnetic behavior of ferrites is responsible for the magnetic properties. The observed super paramagnetic properties in the 8-Fe3O4/PANI nanocomposites hold potential for application in microwave absorption devices [28–31]. Acknowledgment The authors wish to thank the CSIR, New Delhi, India, for its financial support vide sanction no. 01(1953)/04/ EMR-II. References [1] [2] [3] [4] [5]
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ARTICLE IN PRESS J. Alam et al. / Journal of Magnetism and Magnetic Materials 314 (2007) 93–99 [14] S.R. Vadera, R. Mathur, M. Parihar, N.K. Kumar, Nanostruct. Mater. 8 (1997) 889. [15] S.B. Darling, N.A. Yufa, A.L. Cisse, S.D. Bader, S.J. Sibener, Adv. Mater. 17 (2005) 2446. [16] J. Ugelstad, P. Stenstad, L. Kilaas, W.S. Prestvik, R. Herje, A. Berge, E. Hornes, Blood Purif. 11 (1993) 349. [17] N. Yanase, H. Noguchi, H. Asakura, T. Suzuta, J. Appl. Polym. Sci. 50 (1993) 765. [18] G. Xie, Q. Zhang, Z. Luo, M. Wu, T. Li, J. Appl. Polym. Sci. 87 (2003) 1733. [19] Y. Deng, L. Wang, W. Yang, S. Fu, A. Elassari, J. Magn. Magn. Mater. 257 (2003) 69. [20] O. Yavuz, M.K. Ram, M. Aldissi, P. Poddar, S. Hariharan, J. Mater. Chem. 15 (2005) 810. [21] W. Luzny, E. Banka, Macromolecules 33 (2000) 425. [22] Y. Xia, J.M. Wiesinger, A.G. MacDiarmid, Chem. Mater. 7 (1995) 443.
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[23] A.B. Diaz, N.D.S. Mohallem, R.D. Sinisterra, J. Braz. Chem. Soc. 14 (6) (2003) 936. [24] Y. Long, Z. Chena, J.L. Duvailb, Z. Zhang, M. Wan, Physica B 370 (2005) 121. [25] P. Li, B. Yu, X. Wei, J. Appl. Polym. Sci. 93,362 (2004) 894. [26] S. Wei, Y. Zhu, Y. Zhang, J. Xu, React. Funct. Polym. 66 (11) (2006) 1272. [27] A. Bocanegra, N.D.S. Mohallem, R.D. Sinisterra, Mater. Res. Soc. Symp. Proc. 711 (2002) 30.1. [28] D.K. Kim, M.S. Toprak, M. Mikhaylova, Y.S. Lo, S.J. Savage, H.B. Lee, T. Tsakalakos, M. Muhammed, Solid State Phenom. 99–100 (2004) 165. [29] O.Yu. Posudievskii, V.D. Pokhodenko, Theor. Exp. Chem. 32 (1996) 213. [30] P. Poddar, J.L. Wilson, H. Srikanth, S.A. Morrison, E.E. Carpenter, Nanotechnology 15 (2004) S570. [31] Y.W. Yun, S.W. Kim, G.Y. Kim, Y.B. Kim, Y.C. Yun, K.S. Lee, J. Electroceram. 17 (2006) 467.
Journal of Magnetism and Magnetic Materials 314 (2007) 93–99 www.elsevier.com/locate/jmmm
Effect of ferrofluid concentration on electrical and magnetic properties of the Fe3O4/PANI nanocomposites Javed Alam, Ufana Riaz, Sharif Ahmad Materials Research Laboratory, Department of Chemistry, Jamia Millia Islamia, New Delhi 110025, India Received 4 January 2007; received in revised form 5 February 2007 Available online 7 March 2007
Abstract Magnetic nanocomposites can be controlled and tailored to provide the desired mechanical, physical, chemical, and biomedical properties depending on the final applications. The coating of ferrite nanoparticles with polymers affords the possibility of minimizing agglomeration in large-scale commercial synthesis of nanocomposite materials. The process of coating not only provides effective encapsulation of individual nanoparticles, but also controls the growth in size, thus, yielding a better overall size distribution. In this paper, in-situ polymerization of aniline was carried out in different concentration of the ferrofluid with the aim to obtain agglomerate free nanocomposites. The role of the ferrite concentration was investigated by the spectral, morphological, conductivity, and magnetic properties of Fe3O4/polyaniline (PANI) nanocomposites. XRD revealed the presence of spinel phase of Fe3O4 and the particle size was calculated to be 14.3 nm. Spectral analysis confirmed the formation of PANI encapsulated Fe3O4 nanocomposite. Conductivity of the nanocomposites was found to be in the range of 0.001–0.003 S/cm. Higher saturation magnetization of 3.2 emu/g was observed at 300 K, revealing a super paramagnetic behavior of this nanocomposite. r 2007 Elsevier B.V. All rights reserved. Keywords: Polyaniline; Nanocomposite; Super paramagnetism; Conductivity
1. Introduction Magnetic polymer nanocomposites represent a class of functional materials, where magnetic nanoparticles are embedded in polymer matrices. These nanocomposites hold immense potential for application in cell separation, enzyme immunoassay, drug targeting, electromagnetic device applications, and electromagnetic interference suppression [1–4]. Multi-component conducting polymer systems with nanoparticles of metal oxides can be tailored to obtain desired mechanical properties besides novel electrical, magnetic, and optical properties. A number of articles have been published on the magnetic and conducting polymeric nanocomposites of polyaniline (PANI) as well as polypyrrole composites containing nanoparticles such as TiO2 [5], ZrO2 [6], g-Fe2O3 [7], Fe3O4 [8], and SnO2 [9]. The properties of these systems are sensitive to the particle size, inter particle interaction, and temperature Corresponding author. Tel.: +91 26982815.
E-mail address: [email protected] (S. Ahmad). 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.02.195
[10]. Till date, the most commonly studied magnetic nanomaterials are the oxides used in the form of ferrites and substituted ferrites in stable colloidal suspensions known as ferrofluids. Ferrofluids are the colloidal dispersion of nanosize 2–10 nm magnetic particles in some suitable carrier medium. Ferro-fluid-polymer composites materials are formed by the dispersion of ferro-ferrimagnetic materials with other non-magnetic particles in a host matrix. Although, it is an established fact that nanoparticles exhibit novel magnetic properties, the ideal response in general is associated with isolated particles. A critical obstacle in assembling and maintaining a nanoscale magnetic material is usually its tendency to agglomerate, which is a deterrent to its applications [10]. In practical applications, one invariably has to consider a collection or aggregate of particles forming nanopowders. Further, several applications require consolidation and sintering of nanophase materials into solid blocks or thin films. These processing routes often lead to unavoidable formation of agglomerations and larger grains that effectively prevent the materials from attaining
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their full potential in terms of the desirable magnetic response associated with nanoparticles [11]. Polymer encapsulated magnetic nanoparticles are of great technological interest as the coating provides a matrix for binding of the particles and also prevents grain growth and agglomeration. The process of coating not only provides effective encapsulation of individual nanoparticles, but also controls the growth in size thus, yielding uniform overall size distribution. Encapsulation of magnetic particles with preformed natural or synthetic polymers is the simple and classical method to prepare magnetic polymeric particles [12–15]. Ugelstad et al. [16] developed an effective method through direct precipitation of iron salt inside the porous polystyrene particles. The development of emulsion polymerization, such as the conventional emulsion polymerization, soap-free emulsion polymerization, miniemulsion polymerization, and microemulsion polymerization, was led to new synthesis methods [17,18]. However, the control over the dispersion of the nanoparticle phase embedded in a polymer matrix is still in its embryonic stage at the research level and requires extensive investigation. To achieve excellent dispersion, competition between polymer–polymer and polymer–particle interactions have to be balanced to avoid clustering of particles in polymer nanocomposites. Hence, to exploit the full potential of the technological applications of these materials, it is important to address the above problems. In the present work, the effect of ferrofluid concentration on the spectral, morphological, conductivity, and magnetic properties of the Fe3O4/PANI nanocomposite has been investigated. Our aim was to obtain the ferrite/polyaniline nanocomposites with less agglomeration along with promising magnetic properties. The Fe3O4/PANI nanocomposites were prepared by in-situ polymerization of aniline in ferric hydroxide sol of 2%, 6% and 8% concentrations, respectively. The resulting nanocomposites were analyzed by FT-IR, UV–vis, XRD, conductivity, and SEM as well as VSM studies.
purification. Aniline (Merck, India) was double distilled and was stored in refrigerator prior to use. All chemicals were of AR grade. 2.1. Preparation of ferrofluid The preparation of ferrofluid was carried out at room temperature (25 1C) by the addition of NH4OH solution (5 N) to aqueous solution of different concentrations of ferric chloride (2, 6 and 8 wt%) with slow stirring. The pH of the solution was maintained between 4 and 5. Ammonium hydroxide solution was added until a red cherry color appeared indicating formation of ferric hydroxide sol of pH ¼ 6, with inherent viscosity Z ¼ 0.466, and specific gravity 1.0642. The 2, 6 and 8 wt% ferrofluids obtained were then used as oxidants for the polymerization of PANI. 2.2. Preparation of polyaniline–ferrite nanocomposite A fixed volume of ferrofluid was added drop wise to a 250 ml round bottom flask containing HCl (8 ml, 1 N) and double distilled aniline (5 ml) with slow and continuous stirring maintained at a constant temperature and pH, as shown in Table 1. After an induction period of 30 min, the color of the sol changed from red to green, which confirmed the polymerization of aniline in conducting form. Polymerization was further continued for 12 h at room temperature. The green precipitate of Fe3O4/PANI composite obtained was filtered, washed several times with distilled water, methanol, and then dried in vacuum for 72 h at 60 1C. The nanocomposites were designated as 2Fe3O4/PANI, 6-Fe3O4/PANI and 8-Fe3O4/PANI, according to the concentration of the ferrofluid used in the preparation. 3. Characterization 3.1. Spectral analysis
2. Experimental Ammonium hydroxide solution (25%) and ferric chloride (MW 162.21) (Merck, India) were used without further
FT-IR spectra of the composites were taken in dried KBr powder on Perkin-Elmer spectrometer model 1750. UV–vis spectra were taken on Perkin-Elmer lambda EZ-221.
Table 1 Characteristics of Fe3O4/PANI nanocomposites Nanocomposite
Sol concentration (M)
pH
Temperature (1C)
Yield (%)
Reaction time
Total time (days)
Conductivity (S/cm)
2-Fe3O4/PANI 2-Fe3O4/PANI 2-Fe3O4/PANI 6-Fe3O4/PANI 6-Fe3O4/PANI 6-Fe3O4/PANI 8-Fe3O4/PANI 8-Fe3O4/PANI 8-Fe3O4/PANI
0.49 0.612 0.612 0.073 0.073 0.073 0.307 0.615 0.615
1 3 5 1 3 5 1 3 5
6 10 15 6 10 15 6 10 15
13 14 16 12 21 23 18 21 25
3 6 12 3 6 12 3 6 12
7 7 7 3 3 3 3 3 3
— — 0.003 — — 0.0018 — — 0.001
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3.2. Morphological analysis
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X-ray diffractograms were recorded on X-ray diffractometer model Philips PW3710 using copper Ka radiations. Scanning electron micrographs were taken on JEOL JSM840 scanning electron microscope under thin gold film.
nanocomposite was obtained (Table 1) owing to the formation of excess of ferrite nanoparticles. Maximum yield was obtained when polymerization was carried out at room temperature and pH ¼ 5, while the percent yield was found to vary slightly with the increase in the polymerization time after 12 h.
3.3. Conductivity studies
4.2. X-ray diffraction analysis
Conductivity was measured by standard two-probe method using Keithley multimeter models 2001 at 30 1C.
The X-ray diffractogram of Fe3O4 (Fig. 1) shows the characteristic diffraction peaks of Fe3O4 (2 2 0), Fe3O4 (3 1 1), Fe3O4 (4 0 0), Fe3O4 (4 2 2), Fe3O4 (5 1 1) and Fe3O4 (4 4 0). The size D of the Fe2O3 particles was calculated by Scherrer’s equation [24]
3.4. Magnetic measurements The magnetic measurements were performed using Vibrating Sample Magnetometer Model EG & G Princeton at 300 K by applying a field of 1000 A/m and at a frequency of 16 Hz. 4. Results and discussion 4.1. Effect of ferrofluid concentration on the properties of the nanocomposite The Fe3O4/PANI composite was obtained and a possible mechanism of its formation is predicted in Scheme 1. The ferrite particles in the stable ferrofluid dispersion interact with the conducting polymer via hydrogen bonding, which gets adsorbed to the surface of the ferrite resulting in encapsulation of the ferrite nanoparticles by PANI. The nanocomposites were found to move in the direction of applied magnetic field, which indicates their magnetic behavior. This property can be utilized in the magnetic separation of materials from suspension. To reduce the formation of agglomerates of ferrite nanoparticles during the preparation of polyaniline–ferrite nanocomposites, we adopted the in-situ polymerization technique for the synthesis of Fe3O4/PANI nanocomposites. The color of the solution was found to change from red to green upon polymerization of aniline in the presence of ferric hydroxide sol, which confirmed the polymerization of aniline in the former. It was also observed that with the increase in the ferrite concentration, higher yield of the PANI-Fe3O4
Scheme 1. Mechanism of formation of Fe3O4/PANI nanocomposite.
D¼
0:89l , b cos y
where l is the X-ray wavelength, b is the full-width at halfmaxima (FWHM) of the X-ray diffraction peak, and y is the Bragg angle. According to the b values of Fe3O4 (3 1 1), the particle size was calculated to be about 14.3 nm, which confirms the nanosize of ferrite. The Fe3O4/PANI nanocomposites however, reveal broadening of the ferrite peaks. It is interesting to note that the amorphous nature of the nanocomposite increases with the increase in the concentration of the ferrite sol. This can be attributed to the presence of higher amount of PANI in the composite. The presence of diffused broad peaks indicates lower crystalline order owing to the formation of larger fraction of PANI [20]. The preponderance of amorphous peaks of PANI indicates that the crystalline behavior of ferrite is suppressed as a result of its encapsulation by PANI, as PANI displays a diffuse broad peak ranging from 201 to 351.The results obtained are consistent with the ones obtained by other research groups [21,22]. This observation is further corroborated by the spectral as well as SEM studies discussed in the proceeding sections. 4.3. Spectral analysis The FT-IR spectra of pristine ferrite (Fig. 2) exhibit a small peak at 3000 cm1, which can be associated with the O–H stretching vibrations in physically adsorbed hydrogen-bonded water molecule. The small peaks in the region 1500–1400 cm1 can also be correlated to the different modes of bonded water molecules existing in the ferrofluid while the prominent peak at 510 cm1, is attributed to the stretching and torsional vibration modes of the Fe–O bonds in tetrahedral and octahedral sites, respectively [23]. The spectra of 2-Fe3O4/PANI (Fig. 2) reveals the N–H stretching vibration peak corresponding to PANI at 3394 cm1 while the peaks corresponding to benzenoid (NH–B–NH) and quinonoid (NH–Q–NH) rings are observed at 1590, 1498, and 1305 cm1, respectively. The CN vibration peak is observed at 1166 cm1 while the characteristic peak of ferrite is observed at around
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Fig. 1. X-ray diffractograms of Fe3O4/PANI nanocomposites.
previous works, the suppression of –OH vibrational modes in the 3000 cm1 region has been related to evidence of host–guest interaction as a consequence of complete water release upon inclusion of ferrite in the polymer [27]. In our case, the increase in the broadness and intensity in O–H stretching vibration peak is observed incase of 6-Fe3O4/ PANI and 8-Fe3O4/PANI, which indicates that the adsorption of PANI to the surface of the ferrite particles takes place through the hydrogen bonding between N–H groups in the PANI and OH group of bonded water molecules in the ferrofluid, resulting in the encapsulation of ferrite particles by PANI. The FT-IR spectrum confirms the bonding of PANI strongly to the protruding surface hydroxyl groups on ferrite nanoparticles. 4.4. UV–vis spectra
Fig. 2. FT-IR spectra of Fe3O4/PANI nanocomposites.
583 cm1 confirming the presence of ferrite particles and the composite nature of the Fe3O4/PANI composite [24,25]. Incase of 6-Fe3O4/PANI, the benzenoid–quinonoid vibrations, CN vibration peaks, and the ferrite peaks are found to shift by 10, 37, 9, 12, and 15 cm1, respectively. The increase in the intensity of the ferrite peaks is observed with the increase in the concentration of the ferrofluid. The shifting as well as broadening of the peaks can be attributed to interaction of 3d orbit of ferrite with N atom in PANI to form coordinate bond [26]. In accordance with
The UV–vis spectra of 2-Fe3O4/PANI nanocomposite (Fig. 3) show absorption maxima at 350 nm as well as 600 nm. The former is assigned to P–P* transitions while the later peak can be correlated to the polaronic transitions. The presence of polaronic transition peak confirms the emeraldine doped state of PANI in Fe3O4/PANI nanocomposite. The exciton transitions of the quinonoid rings appear to be highly suppressed and show flattening in 6-Fe3O4PANI and 8-Fe3O4/PANI revealing the increase in the extent of doping of PANI by the ferrite particles upon encapsulation. 4.5. Magnetization It is well known that when the grain size of ferrite particles decreases to a critical size (an order of 10 nm), the
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coercive force HC decreases to zero and the nanocomposite exhibits a super paramagnetic behavior, which can be usually observed at room temperature (above the blocking
Fig. 3. UV spectra of Fe3O4/PANI nanocomposites.
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temperature) [13,19,22]. The applied magnetic field H dependence of the magnetization M can be described by Langevin equation [24]: M ¼ MS(coth y1/y), where Ms is the saturation magnetization and y ¼ mH/kBT, m is the average magnetic moment of an individual particle in the sample and B is the Boltzmann constant. Fig. 4(a)–(c) shows the plots of the magnetization M versus the applied magnetic field H (between 71000 H (MT) for the PANI/Fe3O4 nanocomposites at 300 K. For 2Fe3O4/PANI and 6-Fe3O4/PANI nanocomposite (Fig. 4(a), (b)) the Ms values were found to be 0.010 and 0.5 emu/g, respectively, while for 8-Fe3O4/PANI nanocomposite (Fig. 4(c)) Ms was found to be around 3.2 emu/g. In this case, remanent magnetization Mr and coercive force Hc were found to be zero indicating a super paramagnetic behavior. Long et al. [24] have reported, Ms of 3.5 emu/g at 20 wt% loading of ferrite in PANI. Interestingly, in our case, similar values of paramagnetism have been obtained
Fig. 4. Magnetization curve of (a) 2-Fe3O4/PANI, (b) 6-Fe3O4/PANI, and (c) 8-Fe3O4/PANI.
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at reasonably lower loading. This variation in the values of magnetization can be attributed to the varying size of ferrite particles synthesized in each case. However, the interactions between the polymer matrix and iron oxide nanoparticles are also found to play a significant role in attaining the magnetic nature of these composites. The specific morphology of PANI as well as the surface chemistry influences the surface interactions of the magnetic nanoparticle strongly. Encapsulation of ferrite nanoparticles by PANI leads to a significant decrease in the magnetic nature of the overall nanocomposite owing to the diamagnetic nature of PANI. Therefore, the measured values of Ms differ than the reported ones mainly due to the difference in the interactions developed at the interphases of Fe3O4/PANI nanocomposite. Fig. 5. SEM micrograph of 6-Fe3O4/PANI nanocomposite.
4.6. Conductivity The concentration of ferrite nanoparticles significantly affects conductivity of the resulting composites. The conductivity values of 2-Fe3O4/PANI was found to be 3 103 S/cm, while that of 6-Fe3O4/PANI and 8-Fe3O4/ PANI nanocomposite was found to be 1.8 103 and 1 103 S/cm, respectively. The higher conductivity of the 2-Fe3O4/PANI nanocomposite can be attributed to the optimum level of doping induced by the interaction between ferrite nanoparticles and PANI backbone. The decrease in the conductivity of 8-Fe3O4/PANI nanocomposites upon increasing the ferrite concentration can be attributed to the insulating behavior of the iron oxide in the core of the nanoparticles, which hinders the charge transfer thereby lowering the conductivity. 4.7. SEM The SEM of 6-Fe3O4/PANI (Fig. 5) shows a homogeneous nanoporous structure of the nanocomposite with the uniform distribution of particles. The phase contrast of the nanocomposite appears to be less pronounced owing to the engulfment of the ferrite particles by PANI. The micrograph of ferrite–PANI nanocomposite exhibits a twophase system where the bright phase corresponds to the existence of ferrite, while dark phase constitutes PANI. The morphology of nanocomposites is found to be porous in nature, where the PANI as well as iron oxide phase form a network like structure and appear to be well interconnected. As the concentration of sol increases, the bright phase becomes predominant with the aggregation of ferrite particles through magnetic as well as intermolecular interaction. 5. Conclusion A simple chemical method was adopted for the synthesis of ‘‘agglomerate free’’ Fe3O4/PANI nanocomposite where the nanoparticles are entrapped in the growing PANI chains. The improvements made in physical properties of
the present nanocomposites are expected to enhance the application potential of the polymer without hampering its magnetic properties. The structural characterizations of the Fe3O4/PANI nanocomposites reveal that the encapsulation of ferrite particles by PANI results in significant amount of doping, which enhances the electrical conducting properties of Fe3O4/PANI nanocomposites, while the inherent magnetic behavior of ferrites is responsible for the magnetic properties. The observed super paramagnetic properties in the 8-Fe3O4/PANI nanocomposites hold potential for application in microwave absorption devices [28–31]. Acknowledgment The authors wish to thank the CSIR, New Delhi, India, for its financial support vide sanction no. 01(1953)/04/ EMR-II. References [1] [2] [3] [4] [5]
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