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NiFe2O4 nanocomposite
Synthetic Metals 189 (2014) 34–41
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Synthesis, characterization, magnetic and electrical properties of polyaniline/NiFe2 O4 nanocomposite M. Khairy ∗ Chemistry Department, Faculty of Science, Benha University, Benha, Egypt
a r t i c l e
i n f o
Article history: Received 28 October 2013 Received in revised form 27 December 2013 Accepted 28 December 2013 Available online 17 January 2014 Keywords: Conducting polymer Nanocomposites NiFe2 O4 nanoparticle Magnetic properties Electrical conductivity Microwave absorption
a b s t r a c t Nickel ferrite (NiFe2 O4 ) nanoparticles were prepared using sol–gel technique; while Polyaniline (PANI)/NiFe2 O4 nanocomposites were synthesized via chemical oxidative polymerization of aniline monomer (ANI) in the presence of various ferrite amounts (nickel ferrite/ANI = 2.5, 5, 25 and 50 wt%). The morphology, structure, magnetic, electrical and microwave absorption properties of ferrite powders and nanocomposites were characterized by Fourier transform infrared (FTIR), powder X-ray diffraction (XRD), UV–vis absorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), vibrating sample magnetometer (VSM), four-point probe resistivity instrument and vector network analyzer. The crystalline size of nickel ferrite was found in the range of 23–27 nm. The results of TGA, FTIR and UV–vis spectra indicated that nickel ferrite particles improved the thermal stability of composite, and there were interactions between ferrite particles and PANI. The nanocomposites under applied magnetic field exhibited the hysteresis loops of ferromagnetic nature at room temperature. The conductivity of all measured samples decreased with decreasing temperature, exhibiting typical semiconductor behavior. The microwave absorbing properties of the nickel ferrite and composite specimens with the coating thickness of 2 mm were investigated using waveguide method in the frequency range of 1–6 GHz. The results showed a microwave absorption band with frequency shifts to the higher frequency region with the increase in PANI content. The maximum reflection loss reaches −12.5 dB at 2.5 GHz for the nickel ferrite and −20.3 dB at 3.78 GHz for the composite containing 95 wt% aniline. The electronic and magnetic properties of the nanocomposites are tailored by controlling the ferrite content. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Conducting polymers have attracted significant attention in recent decades because of their potential applications in various fields such as electromagnetic interference (EMI) shielding, rechargeable battery, chemical sensor, corrosion devices and microwave absorption [1–5]. Among the known conducting polymers, polyaniline (PANI) has been extensively studied due to its unique electrochemical and physicochemical behavior, good environment stability and relatively easy preparation [6,7]. Magnetite (Fe3 O4 ) and ferrite spinel nanocrystals are regarded as one of the most important inorganic nanomaterials because of their electronic, optical, electrical, magnetic and catalytic properties, all of which are different from their bulk counterparts. Ferrites have the general formula (Me1− Fe )[Me Fe2− ]O4 , where parentheses and square brackets denote cation sites of tetrahedral (A) and octahedral [B] co-ordination respectively and Me is a divalent
∗ Tel.: +20 0223598143. E-mail addresses: [email protected], mousa [email protected] 0379-6779/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2013.12.022
metallic ion. represents the so-called degree of inversion. In the normal spinel structure ( = 0) all the divalent ions are located in the A sites, whereas in the inverse spinel ( = 1) the 8 A sites are filled with trivalent Fe3+ ions and the 16 B sites are equally shared by both divalent and trivalent ions [8,9]. Among spinel ferrites, nickel ferrite (NiFe2 O4 ) has an inverse spinel structure which, in its ideal state, all Ni2+ ions are in B sites and Fe3+ ions are equally distributed between A and B sites. This has been widely studied due to its high electromagnetic performance, excellent chemical stability and mechanical hardness, high coercivity, and moderate saturation magnetization, making it a good contender for the application as soft magnets and low loss materials at high frequencies [10,11]. These properties are dependent on chemical composition and microstructural characteristics where the particle size and shape might be controlled in the fabrication processes. Inorganic nanomaterials composited with PANI could enhance the mechanic property and other properties depending on the additives used. For example, conducting polymer/ferrite composites with an organized structure provide a new functional hybrid between organic and inorganic materials. Coating of PANI on ferrite nanoparticles can enhance compatibility with organic ingredients,
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reduce susceptibility to leaching, and probably avoid aggregation. Recently, many interesting researches have focused on the PANI/ferrite composites to obtain the materials with synergetic or complementary behavior between PANI and ferrite nanoparticles [12–18]. This study is inspired in development of materials with magnetically and electrically controlled attenuation. Thus, magnetic and conducting composites containing polyaniline-coated nickel ferrite nanoparticles with various compositions were synthesized by in situ polymerization of aniline on the surface of ferrite particles. The samples were characterized by various experimental techniques, magnetic and electrical properties were investigated. In this paper, magnetic and conducting composites containing polyaniline-coated NiFe2 O4 were synthesized by chemical oxidative polymerization of aniline monomer (ANI) in the presence of ferrite nanoparticles. The samples were characterized by various experimental techniques, and the properties of composite were investigated. The influence of the PANI content with respect to the electromagnetic properties of PANI/nickel ferrite composites has been investigated. More detailed information on the nature of the coating, an interface between ferrite and conducting polymer, is needed for the understanding of its performance. 2. Experimental 2.1. Materials Aniline (Adwic 99%) was used after double distillation. Other chemicals used were of AR grade. Water used in this investigation was de-ionized water. The nano nickel ferrite powder was synthesized by sol–gel method. Ni(NO3 )2 ·6H2 O and Fe(NO3 )3 ·9H2 O were separately dissolved in de-ionized water. Then, appropriate amounts of the clear solutions were mixed together with citric acid and stirred. The temperature of the solution was then raised from room temperature to about 70 ◦ C, and kept for a few hours. The solution became viscous and gel was formed. The gel was then washed with de-ionized water several times to remove possible residues and then dried at 70 ◦ C for 24 h and calcined at 500 ◦ C for 2 h. Ferrite-PANI composites with different ferrite:aniline monomer ratio wt/wt% (C1: 2.5%, C2: 5%, C3: 25% and C4: 50%) were prepared by the in situ polymerization of aniline in aqueous solution of hydrochloric acid, with different amount of ferrite powder. In a typical procedure, aniline (0.2 M) dissolved in 100 ml of HCl (1 M) was taken in 250 ml round bottom flask and stirred well. Appropriate amounts of nano NiFe2 O4 powder were then added to the above mixture under vigorous stirring to keep NiFe2 O4 powder suspended in the solution. The reaction occurred in ice bath and the pre-cooled solution of ammonium persulfate (0.25 M) was slowly added over a period of 2 h as oxidizing agent. The reaction was allowed to proceed for 6 h. At the end of the reaction, PANI/NiFe2 O4 composite formed was collected by filtration, washed with distilled water and acetone several times until filtrate was colorless. The collected composite was dried at 80 ◦ C until constant weight was attained. Under the same conditions PANI was also synthesized in an absence of NiFe2 O4 . 2.2. Characterization X-ray diffraction spectra of pure PANI, NiFe2 O4 and PANI/NiFe2 O4 composites (C1: 2.5% and C4: 50%) were performed at room temperature in the range from 2 = 2◦ to 80◦ on a Diano (made by Diano Corporation, U.S.A.), using Cu K␣ ˚ The FT-IR spectra of the samples were radiation ( = 1.5406 A). recorded using a KBr pellet on a Brucker-FTIR (Vector 22), made
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in Germany. SEM and TEM of the pure PANI and PANI/NiFe2 O4 composites were recorded using JEOL JSM 6400 and TEM (JEOL JEM-2010) microscopes, respectively. TGA thermograms of pure PANI, NiFe2 O4 and PANI/NiFe2 O4 composites were recorded under nitrogen atmosphere and in a temperature range of 25–700 ◦ C and at a heating rate of 10 ◦ C/min using Shimadzu DT-50 thermal analyzer. UV–Vis absorption spectra were recorded on a Shimadzu UV-2501PC spectrophotometer in the range of 300–800 nm. The hysteresis loop, saturation magnetization (Ms ) and coercivity (Hc ) of the materials under study were also measured by means of VSM at a maximum applied field of 5 kG at room temperature. The conductivity was measured on a SDY-4 four-probe resistivity instrument using pressed pellets of samples powder with the thickness of about 1 mm and diameter of 7 mm under 40 MPa. To measure the microwave absorption properties, the studied samples were dispersed into epoxy, the mass ratio of the sample to epoxy is 2:1, to form a paste, and then moved onto an 180 mm × 180 mm aluminum substrate. After curing 10 min in the air, the mixture was pressed using another aluminum substrate and further dried. All the five samples with the coating thickness of 2 mm were cut into rectangular shapes with dimensions as desired for waveguide measurements at different microwave frequency bands. The sample is then inserted in the waveguide so that it fills the entire cross-section in order to prevent any leakage of electromagnetic energy. The permittivity (ε , ε ) and permeability ( , ) of the samples were measured by a vector network analyzer (Agilent Technologies Inc. 8753ES) in the range of 1–6 GHz at room temperature. The reflection loss (RL ) were calculated from complex permittivity (ε = ε − jε ) and complex permeability ( = − j ) at given frequency f and absorber thickness d with following equations [19]:
(Zin − 1) (Z + 1)
RL (dB) = 20 log
(1)
in
where Zin is the normalized input impedance given by Zin =
1/2 ε
2fd
tanh j
c
(ε)1/2
(2)
where c is the velocity of microwave in vacuum. ε , associated with electric field loss, results mostly from electric-dipole polarization at microwave frequencies. , associated with magnetic field loss, results from magnetic-dipole magnetization. Further, the loss tangent of the dielectric/magnetic can be expressed as tan ıε = ε /ε and tan ı = / , respectively, which means that the ratio of the energy loss/unit radian in the dielectric to the energy storage in the dielectric can be determined. 3. Results and discussion 3.1. X-ray diffraction X-ray diffraction patterns of pure PANI, NiFe2 O4 and PANI/NiFe2 O4 composites (C1: 2.5% and C4: 50%) are shown in Fig. 1. Fig. 1(a) shows the characteristic peaks of NiFe2 O4 at 2 = 30.55◦ , 35.94◦ , 43.64◦ , 54.07◦ , 57.63◦ , 63.26◦ , 71.76◦ with the reflection of Fd3m cubic spinel group (PDF 10-0325) [20], which indicates the formation of a single-spinel phase. The broadening nature of the diffraction peaks suggests that the as-prepared sample is very small in dimensions. The crystallite size D is calculated from XRD peak broadening of the (3 1 1) peak using Debye–Scherrer formula: D = 0.9/ˇ cos where D is the average crystallite size, is the wavelength of Cu K␣, ˇ is the full width at half maximum (FWHM) of the diffraction peaks and is the Bragg’s angle. Correction for the line broadening by the instrument was applied using a large particle size silicon standard and the 2 − B2 where B relationship B2 = BM M and BS are the measured S
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Fig. 2. FTIR spectra of NiFe2 O4 (a); PANI (b) and NiFe2 O4 /PANI (C4: 50%) (C).
Fig. 1. XRD pattern of NiFe2 O4 (a); PANI (b); and NiFe2 O4 /PANI (C1: 2.5%) (c); NiFe2 O4 /PANI (C4: 50%) (d).
widths at half-maximum intensity of the lines from the sample and the standard, respectively. The crystalline size is estimated to be in the range of 23–27 nm. XRD pattern of PANI (Fig. 1(b)) shows the main diffraction peaks with amorphous nature at about 2 = 20.3◦ and 25.1◦ , which is ascribed to the periodicity parallel and perpendicular to PANI chains, respectively [21]. The XRD of the composite samples (C1 and C4), are shown in Fig. 1(c) and (d). The XRD exhibit both the characteristic peaks of the NiFe2 O4 ferrite and the broad diffraction peaks of PANI. These results confirmed the formation of PANI/NiFe2 O4 composites. 3.2. FTIR spectra Fig. 2 shows the FTIR spectra of NiFe2 O4 , PAN and PANI/NiFe2 O4/ (C4: 50%) composite samples. Spectrum (2 a) shows bands in the region 400–600 cm−1 only which are attributed to pure NiFe2 O4 in absence of any organic moiety. Polymerization of aniline can be confirmed by the presence of spectral peaks at 1456 and 1564 cm−1 , Fig. 2(b), which are attributed to stretching of the benzenoid and quinoid rings, respectively, for the HCl doped PANI. These results indicate that the pure PANI is highly doped and exists in conducting emeraldine salt form [22]. The peak at 1299 cm−1 corresponding to C N stretching of secondary amine in polymer main chain can be clearly seen. The third spectrum in Fig. 2(c) shows peaks corresponding to both the organic moiety (1641 and 1463 cm−1 quinoid and benzenoid ring of polyaniline, 1246 C N stretching vibrations and 1080 cm−1 for C H bending mode) and NiFe2 O4 nanoparticles (400–600 cm−1 ).These specific peaks in the
functional group region of the FTIR are observed at slightly higher frequency than their actual values because of the interaction with nickel ferrite moiety. The results reveal that there exists an interaction between ferrite particles and PANI chains. This is produced from the – interaction between metal oxide and PANI, which includes (1) the molecular orbital of PANI overlaps the empty d-orbital of metal ions to form the -bond where metallic empty d-orbital is the electron pair acceptor; (2) the ∗ molecular orbital of PANI overlaps the d-orbital of metal ions to form the -bond, in which the metal ions is the electron pair donor. In addition, the hydrogen bonding interaction between the polyaniline chains and the oxygen atoms on the ferrite surface occurs in the composites, which make ferrite particles be embedded into polymer chain of PANI [23]. 3.3. Electron microscopy analysis The morphology and particle size of magnetic polyaniline, nickel ferrite and PANI/nickel ferrite composites is determined by SEM and TEM. Fig. 3(b), shows a fiber structure for PANI. Whereas, the SEM and TEM micrographs (Figs. 3(a) and 4(a)) of nickel ferrite reveal that the particles are approximately spherical in shape with diameter ranging from 25–35 nm. It is also observed that the ferrite nanoparticles exhibit some extent of agglomeration because of magnetic dipole interaction between ferrite particles. The particle sizes of ferrite obtained by TEM technique are slightly larger than the average crystallite size of 25 nm estimated from the XRD peaks using the Scherrer formula. This difference can be attributed to the agglomeration behavior of the ferrite nano-particles and due to the different approach of the two techniques. Figs. 3(c) and (d) and 4(b)–(e) show the SEM and TEM images of the core–shell PANI/nickel ferrite. The images indicate that the ferrite core–shell are coated by polyaniline into approximately spherical coral-like particles, which have diameter around 30–45 nm and connected by polymer. The black core is magnetic ferrite particles, and the light grey shell is PANI in the nanocomposite. The difference in colors is due to the different electron penetrability. It can be also seen from Fig. 4(b)–(e) that the degree of coating on the surface of NiFe2 O4 increases with PANI content. Meanwhile, the agglomeration of the ferrite particles is reduced to the coating of PANI on them, which is good to the dispersion and stabilization of the ferrite particles. From the results of SEM and TEM, it is clear that synthetic method in this study is applicable to the synthesis of PANI/nickel ferrite
M. Khairy / Synthetic Metals 189 (2014) 34–41
Fig. 3. SEM of NiFe2 O4 (a); PANI (b); NiFe2 O4 /PANI (C2: 5%) (c); NiFe2 O4 /PANI (C3: 25%) (d) and NiFe2 O4 /PANI (C4: 50%) (e).
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Fig. 4. TEM of NiFe2 O4 (a); NiFe2 O4 /PANI (C1: 2.5%) (b); NiFe2 O4 /PANI (C2: 5%) (c); NiFe2 O4 /PANI (C3: 25%) (d) and NiFe2 O4 /PANI (C4: 50%) (e).
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M. Khairy / Synthetic Metals 189 (2014) 34–41
Fig. 6. UV–visible spectra of PANI, (—); NiFe2 O4 /PANI (C1: 2.5%), (. . ...); (C2: 5%), (- - - - - -); (C3: 25%), (- -) and (C4: 50%), (-.-.).
Fig. 5. TGA of PANI (a); NiFe2 O4 /PANI (C3: 25%) (b); (C4: 50%) (c) and NiFe2 O4 (d).
composites. The results obtained are quite similar to other systems reported elsewhere [13,15,24]. 3.4. Thermal analysis TG analysis of NiFe2 O4 , PANI and PANI/NiFe2 O4 composites (C3: 25% and C5: 50%) is shown in Fig. 5. NiFe2 O4 shows thermal stability, whereas PANA and the composites decompose over the temperature range investigated. The thermogram of PANI (Fig. 5(a)) shows a three-step weight loss. The initial weight loss at lower temperature (less than 70 ◦ C) is due to the loss of water retained in PANI matrix after drying. The second weight loss ranging from 115 to 300 ◦ C is attributed to the loss of acid dopant anions compensated the positive charge of PANI Chains [25,26]. The third weight loss starting at about 300 ◦ C can be ascribed to the skeletal PANI chain decomposition after the elimination of dopant anions. It is seen from Fig. 5(a)–(c) that the 40% weight loss occurred at 415 ◦ C for pure PANI and 460 ◦ C for (C3: 25%) as well as 490 ◦ C for (C4: 50%) PANI/NiFe2 O4 composites. This reveals that the thermal stability of the composites is higher than that of pure PANI. This may be caused by the interaction between ferrite particles and PANI chains. This finding agrees well with the results obtained by Wang et al. [27]. 3.5. UV–visible spectra UV–vis absorption spectra of PANI and PANI/NiFe2 O4 composites are shown in Fig. 6. From which it is seen that PANI has two characteristic absorption bands at 330 and 610 nm. The absorption band at 330 nm is attributed to –* transition of the benzenoid ring [28,29], while the peak at 610 nm corresponds to the benzenoid-to-quinoid excitonic transition [30,31]. For PANI/NiFe2 O4 composite (C4: 50%), the absorption peaks are found at around 360 and 630 nm, which have a red shift of 30 and 20 nm, respectively, as compared with that of PANI. These results supported our suggestion of the presence of some sorts of interactions between the ferrite and PANI chains [32]. The results obtained show also a decrease in the intensities of the two absorption peaks
Fig. 7. Magnetic hysteresis loop of NiFe2 O4 (a); NiFe2 O4 /PANI (C4: 50%) (b); (C3: 25%) (c); (C2: 5%) (d) and (C1: 2.5%) (e)
with increasing the ferrite content due to their interactions with the polyaniline. 3.6. Magnetic properties Fig. 7 shows the magnetization, M, versus the applied magnetic field, H, for NiFe2 O4 nanoparticles, pure NiFe2 O4 and PANI/NiFe2 O4 nanocomposites of various compositions (C1: 2.5%, C2: 5%, C3 25% and C4: 50%) at room temperature. The magnetization of NiFe2 O4 and its composites exhibits a clear hysteretic behavior. The magnetic parameters such as saturation magnetization (Ms ), remnant magnetization (Mr ) and coercivity (Hc ) that determined by the hysteresis loops measurement are given in Table 1. The Ms -values of composites are less than that obtained for pure nickel ferrite, and decreases with decreasing the ferrite amount in the composite. This behavior is due to the non-magnetic coating layer, can be envisaged as a magnetic dead layer on the surface, thus affecting the magnitude of magnetization due to quenching of the surface moment Table 1 Saturation magnetization (Ms ), remnant magnetization (Mr ) and coercivity (Hc ) of the studied samples. Sample
Ms (emu/g)
Mr (emu/g)
Mr /Ms
Hc (Oe)
NiFe2 O4 NiFe2 O4 /PANI (C4: 50%) NiFe2 O4 /PANI (C3: 25%) NiFe2 O4 /PANI (C2: 5%) NiFe2 O4 /PANI (C1: 2.5%)
24 13.36 6.24 1.25 0.13
13 8.59 3.95 0.76 0.094
0.542 0.64 0.63 0.51 0.72
330 307 303 298 294
M. Khairy / Synthetic Metals 189 (2014) 34–41
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Table 2 Electrical conductivity data of the studied samples. Sample
* × 103 ( −1 cm−1 )
E (eV)
Tt # (K)
Pure PANI NiFe2 O4 /PANI (C1: 2.5%) NiFe2 O4 /PANI (C2: 5%) NiFe2 O4 /PANI (C4: 50%) NiFe2 O4
23.2 7.1 4.3 2.0 3.6 × 10−4
0.044 0.056 0.077 0.092 0.32
390 405 415 425
* : conductivity at 300 K; Tt # : transition temperature on ln vs. 1/T curve, conductivity of NiFe2 O4 at 470 K.
Fig. 8. Effect of temperature on the conductivity of: PANI (a); NiFe2 O4 /PANI (C1:2.5%) (b); (C2: 5%) (c); (C4: 50%) (d); and Pure NiFe2 O4 (e).
[33,34]. Generally, the magnetic behavior and the magnetic values (Ms ) of the nanocomposites can be also explained according to the equation Ms = ϕms , where ϕ is the volume fraction of the magnetic particles and ms is the saturation moment of a single particle [35–37]. PANI/NiFe2 O4 nanocomposites have less magnetization than that observed for the pure nickel ferrite nanoparticles, which means that the total magnetic behavior of the nanocomposite could be tuned and tailored depending on these two parameters. Inspection of Table 1 shows that the coercivity Hc of the PANI/ferrite composites increases with the increase in the nickel ferrite content. This can be attributed to the microstructure nature of PANI/NiFe2 O4 I nanocomposites. It is well known that nano-sized ferrites have an irregular structure, geometric and crystallographic nature, such as pores, cracks, surface roughness and impurities. In the polymerization process, depositing PANI on the ferrite surface and crystallite boundary, has a healing effect to cover the ferrite surface defects, such as pores and cracks, leading to a decrease in magnetic surface anisotropy of ferrite particles, consequently, PANI/NiFe2 O4 nanocomposite presents a lower value of coercivity than that of NiFe2 O4 [35–37]. 3.7. Electrical conductivity It is well known that electrical conduction in semiconductor materials is a thermally activation process and is due to the ordered motion of weakly bound charged particles under the influence of an electric field. It depends on the nature of the charge carriers that dominate the conduction process. Therefore, due to the presence of some residual waters in the investigated samples (as shown from FT-IR and thermal analyses results), that may influence their conductivity values; the samples were heated at 70 ◦ C for 3 h before performing the conductivity measurements. The temperature dependence of dc-conductivity ( dc ) for NiFe2 O4 , PANI and PANI/NiFe2 O4 composites in a temperature range between 30 and 250 ◦ C is shown in Fig. 8, the insert figure is plotted to show clearly the difference in conductivity values between PANI and its composites. It can be seen that the conductivity of pure PANI is higher than the NiFe2 O4 and composite samples. There are many influencing factors on conductivity, such as dopant, doping level, crystallinity, the length of the conjugate chain, the interactions
between conjugated chains and molecular weight, etc. It is known that PANI is a kind of conducting polymer while the ferrite particles are insulators, so the conductivity of the composites could decrease with the increase of NiFe2 O4 content. There are several reasons for the decrease in conductivity; one reason is that the introduction of the ferrite particles would affect the polymerization of aniline, and further result in destroying of the continuity and regularity of the conjugated chain and blockage of conducting path. Besides, interactions between the ferrite particles and PANI chains, and possible bonding effect between the metal cations and PANI would reduce the electronic density of the polymer chain, and increases the charge carrier scattering and further result in the decrease of the conductivity. Fig. 8 shows that the dc-conductivity of all measured samples decreased with decreasing temperature, exhibiting typical semiconductor behavior and obeys Arrhenius equation (Eq. (3)) up to a transition temperature Tt of 390 ◦ C for PANI and 405, 415 and 425 K for the composites containing 2.5, 5 and 50 wt% NiFe2 O4 , respectively. = 0 exp
E a
kt
(3)
where 0 is preexponential factor and Ea is the activation energy for conduction process. The conductivity data are summarized in Table 2. Inspection of Fig. 8 shows that at temperatures higher than Tt , dc decreases gradually. A similar trend has been reported for similar systems [38,39]. The decrease in conductivity after the well noticeable transition temperature (Tt ) is attributed to starting the thermal dissociation of the samples, as confirmed by thermal analyses. The above results mean that desired level of thermal stability and electrical conductivity can be attained by taking suitable composition of PANI/NiF2 O4 nanocomposite for specific application such as in the preparation of high temperature conducting polymers. 3.8. Microwave absorption properties The frequency dependence of dissipation factors represented by the dielectric loss (tan ı ) and magnetic loss (tan ı ) are shown in Fig. 9. The change of these losses with frequency may explain the microwave absorption in the samples. The values of tan ı for epoxy–PANI/nickel ferrite composites are larger than that of epoxy–nickel ferrite, and the value of tan ı for epoxy–nickel ferrite is larger than that of epoxy–PANI/nickel ferrite composites in the frequency range of 1–6 GHz. These results demonstrate that the magnetic loss (tan ı ) is the main microwave absorbing mechanism for epoxy–nickel ferrite sample, and the microwave absorption enhancement of epoxy–PANI/nickel ferrite composites is attributed to dielectric loss (tan ı ) rather than magnetic loss. In order to reveal the microwave absorbing properties of the studied samples, reflection losses (RL ) are calculated by Eqs. (1) and (2) using various measured values of ε , ε , , and results
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4. Conclusions
Fig. 9. Frequency dependence of dielectric and magnetic losses of nickel ferrite and its NiFe2 O4 /PANI composites: (a) tan ı for C2: 5%, (b) tan ı for C3: 25%, (c) tan ı for ferrite, (d) tan ı for C2: 50%, (e) tan ı for ferrite, (f) tan ı for C2: 50%, (g) tan ı for C3: 25% and (h) tan ı : for C2: 5%.
are shown in Fig. 10. One microwave absorption band is observed in the specimens of nickel ferrite and its composites. The minimum reflection point shifts toward higher frequency with the increase in PANI content, and the intensity increases. The maximum reflection loss reaches −12.5 dB at 2.5 GHz for the nickel ferrite, −14.2 dB at 3.0 GHz, −17 dB at 3.55 GHz and −20.3 dB at 3.78 GHz for C4, C3 and C2 composites, respectively. From the above analysis we can infer that the enhancement observed in microwave absorption of composite samples is due to the change occurring in dielectric loss due to the addition of PANI on Nickel ferrite surface. This behavior can be explained on the basis that the inorganic insulating NiFe2 O4 in the PANI/NiFe2 O4 composite destroys the conductive network of PANI to a certain extent. As such, the input impedance of the composite increased and the electromagnetic wave could then be transmitted to the composite interiors adequately, a necessary condition for the loss of incident electromagnetic waves [40]. TEM results prove that NiFe2 O4 was decorated with PANI, so that the electrons and electron charges on the molecular chain aggregated on the surface of the NiFe2 O4 particles. Under the alternating electromagnetic field, interfacial and space–charge polarization were easily formed on the interface of the NiFe2 O4 and PANI. The reflection loss may be attributed to the resistance loss of local conductive particles, the dielectric loss of NiFe2 O4 , and polarization relaxation losses of the interface. Under these various loss mechanisms, the microwave absorbing properties of the PANI/NiFe2 O4 composites were better than that of pure NiFe2 O4 .
Fig. 10. Reflection loss dependence on the frequency for: (a) PANI, (b) NiFe2 O4 , (c) NiFe2 O4 /PANI (C4: 50%), (d) NiFe2 O4 /PANI (C3: 25%) and (e) NiFe2 O4 /PANI (C2; 5%)
Conductive and magnetic NiFe2 O4 /PANI nanocomposites have been prepared by in situ polymerization of aniline in the presence of different amounts of NiFe2 O4 nanoparticles by ammonium persulfate oxidant in HCl medium. NiFe2 O4 nanoparticles were prepared via sol–gel method. Characterizations of the samples were carried out using XRD, TGA, FT-IR and SEM techniques. XRD confirm spinel structure for NiFe2 O4 sample. The thermal stability increases with increasing the ferrite content in PANI matrix. The results of thermal and spectro-analysis indicate that there is an interaction between PANI chains and ferrite nanoparticles. The PANI/ferrite nanocomposites under applied magnetic field exhibit the hysteresis loop of the ferromagnetic nature at room temperature. The values of Ms and Hc increase with increasing the ferrite content in the composite. The conductivities of PANI/NiFe2 O4 composites are lower than that of pure PANI, and are reduced with the increase in the nickel ferrite content. The results obtained indicate that the total magneto-electrical behavior of the nanocomposites could be tuned and tailored depending on the volume fraction of the magnetic ferrite particles (ϕ), and on the contribution of the non-magnetic PANI coating layer. Our results indicate that the composites exhibit good microwave absorption performances. The PANI/nickel ferrite composites show absorption bands better than that of nickel ferrite at frequency range of 1–6 GHz. The absorption band shifts to the higher frequency region with the increase in the content of PANI in composites and the effectual absorption band for all samples (below −12 dB) reaches 2.5, 3, 3.55 and 3.8 GHz, respectively. The microwave absorption of nickel ferrite is mainly due to magnetic loss, while those in composites are due to dielectric loss. The loss mechanism may be attributed to the resistance loss of the local conductive particles, the dielectric loss of nickel ferrite, polarization relaxation losses of the interface and the changes of boundary condition of the microwave field at the interface between the ferrite particles and polymer matrix. The results showed that the PANI/NiFe2 O4 nanocomposites can operate as suitable microwave absorbers.
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Synthesis, characterization, magnetic and electrical properties of polyaniline/NiFe2 O4 nanocomposite M. Khairy ∗ Chemistry Department, Faculty of Science, Benha University, Benha, Egypt
a r t i c l e
i n f o
Article history: Received 28 October 2013 Received in revised form 27 December 2013 Accepted 28 December 2013 Available online 17 January 2014 Keywords: Conducting polymer Nanocomposites NiFe2 O4 nanoparticle Magnetic properties Electrical conductivity Microwave absorption
a b s t r a c t Nickel ferrite (NiFe2 O4 ) nanoparticles were prepared using sol–gel technique; while Polyaniline (PANI)/NiFe2 O4 nanocomposites were synthesized via chemical oxidative polymerization of aniline monomer (ANI) in the presence of various ferrite amounts (nickel ferrite/ANI = 2.5, 5, 25 and 50 wt%). The morphology, structure, magnetic, electrical and microwave absorption properties of ferrite powders and nanocomposites were characterized by Fourier transform infrared (FTIR), powder X-ray diffraction (XRD), UV–vis absorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), vibrating sample magnetometer (VSM), four-point probe resistivity instrument and vector network analyzer. The crystalline size of nickel ferrite was found in the range of 23–27 nm. The results of TGA, FTIR and UV–vis spectra indicated that nickel ferrite particles improved the thermal stability of composite, and there were interactions between ferrite particles and PANI. The nanocomposites under applied magnetic field exhibited the hysteresis loops of ferromagnetic nature at room temperature. The conductivity of all measured samples decreased with decreasing temperature, exhibiting typical semiconductor behavior. The microwave absorbing properties of the nickel ferrite and composite specimens with the coating thickness of 2 mm were investigated using waveguide method in the frequency range of 1–6 GHz. The results showed a microwave absorption band with frequency shifts to the higher frequency region with the increase in PANI content. The maximum reflection loss reaches −12.5 dB at 2.5 GHz for the nickel ferrite and −20.3 dB at 3.78 GHz for the composite containing 95 wt% aniline. The electronic and magnetic properties of the nanocomposites are tailored by controlling the ferrite content. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Conducting polymers have attracted significant attention in recent decades because of their potential applications in various fields such as electromagnetic interference (EMI) shielding, rechargeable battery, chemical sensor, corrosion devices and microwave absorption [1–5]. Among the known conducting polymers, polyaniline (PANI) has been extensively studied due to its unique electrochemical and physicochemical behavior, good environment stability and relatively easy preparation [6,7]. Magnetite (Fe3 O4 ) and ferrite spinel nanocrystals are regarded as one of the most important inorganic nanomaterials because of their electronic, optical, electrical, magnetic and catalytic properties, all of which are different from their bulk counterparts. Ferrites have the general formula (Me1− Fe )[Me Fe2− ]O4 , where parentheses and square brackets denote cation sites of tetrahedral (A) and octahedral [B] co-ordination respectively and Me is a divalent
∗ Tel.: +20 0223598143. E-mail addresses: [email protected], mousa [email protected] 0379-6779/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2013.12.022
metallic ion. represents the so-called degree of inversion. In the normal spinel structure ( = 0) all the divalent ions are located in the A sites, whereas in the inverse spinel ( = 1) the 8 A sites are filled with trivalent Fe3+ ions and the 16 B sites are equally shared by both divalent and trivalent ions [8,9]. Among spinel ferrites, nickel ferrite (NiFe2 O4 ) has an inverse spinel structure which, in its ideal state, all Ni2+ ions are in B sites and Fe3+ ions are equally distributed between A and B sites. This has been widely studied due to its high electromagnetic performance, excellent chemical stability and mechanical hardness, high coercivity, and moderate saturation magnetization, making it a good contender for the application as soft magnets and low loss materials at high frequencies [10,11]. These properties are dependent on chemical composition and microstructural characteristics where the particle size and shape might be controlled in the fabrication processes. Inorganic nanomaterials composited with PANI could enhance the mechanic property and other properties depending on the additives used. For example, conducting polymer/ferrite composites with an organized structure provide a new functional hybrid between organic and inorganic materials. Coating of PANI on ferrite nanoparticles can enhance compatibility with organic ingredients,
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reduce susceptibility to leaching, and probably avoid aggregation. Recently, many interesting researches have focused on the PANI/ferrite composites to obtain the materials with synergetic or complementary behavior between PANI and ferrite nanoparticles [12–18]. This study is inspired in development of materials with magnetically and electrically controlled attenuation. Thus, magnetic and conducting composites containing polyaniline-coated nickel ferrite nanoparticles with various compositions were synthesized by in situ polymerization of aniline on the surface of ferrite particles. The samples were characterized by various experimental techniques, magnetic and electrical properties were investigated. In this paper, magnetic and conducting composites containing polyaniline-coated NiFe2 O4 were synthesized by chemical oxidative polymerization of aniline monomer (ANI) in the presence of ferrite nanoparticles. The samples were characterized by various experimental techniques, and the properties of composite were investigated. The influence of the PANI content with respect to the electromagnetic properties of PANI/nickel ferrite composites has been investigated. More detailed information on the nature of the coating, an interface between ferrite and conducting polymer, is needed for the understanding of its performance. 2. Experimental 2.1. Materials Aniline (Adwic 99%) was used after double distillation. Other chemicals used were of AR grade. Water used in this investigation was de-ionized water. The nano nickel ferrite powder was synthesized by sol–gel method. Ni(NO3 )2 ·6H2 O and Fe(NO3 )3 ·9H2 O were separately dissolved in de-ionized water. Then, appropriate amounts of the clear solutions were mixed together with citric acid and stirred. The temperature of the solution was then raised from room temperature to about 70 ◦ C, and kept for a few hours. The solution became viscous and gel was formed. The gel was then washed with de-ionized water several times to remove possible residues and then dried at 70 ◦ C for 24 h and calcined at 500 ◦ C for 2 h. Ferrite-PANI composites with different ferrite:aniline monomer ratio wt/wt% (C1: 2.5%, C2: 5%, C3: 25% and C4: 50%) were prepared by the in situ polymerization of aniline in aqueous solution of hydrochloric acid, with different amount of ferrite powder. In a typical procedure, aniline (0.2 M) dissolved in 100 ml of HCl (1 M) was taken in 250 ml round bottom flask and stirred well. Appropriate amounts of nano NiFe2 O4 powder were then added to the above mixture under vigorous stirring to keep NiFe2 O4 powder suspended in the solution. The reaction occurred in ice bath and the pre-cooled solution of ammonium persulfate (0.25 M) was slowly added over a period of 2 h as oxidizing agent. The reaction was allowed to proceed for 6 h. At the end of the reaction, PANI/NiFe2 O4 composite formed was collected by filtration, washed with distilled water and acetone several times until filtrate was colorless. The collected composite was dried at 80 ◦ C until constant weight was attained. Under the same conditions PANI was also synthesized in an absence of NiFe2 O4 . 2.2. Characterization X-ray diffraction spectra of pure PANI, NiFe2 O4 and PANI/NiFe2 O4 composites (C1: 2.5% and C4: 50%) were performed at room temperature in the range from 2 = 2◦ to 80◦ on a Diano (made by Diano Corporation, U.S.A.), using Cu K␣ ˚ The FT-IR spectra of the samples were radiation ( = 1.5406 A). recorded using a KBr pellet on a Brucker-FTIR (Vector 22), made
35
in Germany. SEM and TEM of the pure PANI and PANI/NiFe2 O4 composites were recorded using JEOL JSM 6400 and TEM (JEOL JEM-2010) microscopes, respectively. TGA thermograms of pure PANI, NiFe2 O4 and PANI/NiFe2 O4 composites were recorded under nitrogen atmosphere and in a temperature range of 25–700 ◦ C and at a heating rate of 10 ◦ C/min using Shimadzu DT-50 thermal analyzer. UV–Vis absorption spectra were recorded on a Shimadzu UV-2501PC spectrophotometer in the range of 300–800 nm. The hysteresis loop, saturation magnetization (Ms ) and coercivity (Hc ) of the materials under study were also measured by means of VSM at a maximum applied field of 5 kG at room temperature. The conductivity was measured on a SDY-4 four-probe resistivity instrument using pressed pellets of samples powder with the thickness of about 1 mm and diameter of 7 mm under 40 MPa. To measure the microwave absorption properties, the studied samples were dispersed into epoxy, the mass ratio of the sample to epoxy is 2:1, to form a paste, and then moved onto an 180 mm × 180 mm aluminum substrate. After curing 10 min in the air, the mixture was pressed using another aluminum substrate and further dried. All the five samples with the coating thickness of 2 mm were cut into rectangular shapes with dimensions as desired for waveguide measurements at different microwave frequency bands. The sample is then inserted in the waveguide so that it fills the entire cross-section in order to prevent any leakage of electromagnetic energy. The permittivity (ε , ε ) and permeability ( , ) of the samples were measured by a vector network analyzer (Agilent Technologies Inc. 8753ES) in the range of 1–6 GHz at room temperature. The reflection loss (RL ) were calculated from complex permittivity (ε = ε − jε ) and complex permeability ( = − j ) at given frequency f and absorber thickness d with following equations [19]:
(Zin − 1) (Z + 1)
RL (dB) = 20 log
(1)
in
where Zin is the normalized input impedance given by Zin =
1/2 ε
2fd
tanh j
c
(ε)1/2
(2)
where c is the velocity of microwave in vacuum. ε , associated with electric field loss, results mostly from electric-dipole polarization at microwave frequencies. , associated with magnetic field loss, results from magnetic-dipole magnetization. Further, the loss tangent of the dielectric/magnetic can be expressed as tan ıε = ε /ε and tan ı = / , respectively, which means that the ratio of the energy loss/unit radian in the dielectric to the energy storage in the dielectric can be determined. 3. Results and discussion 3.1. X-ray diffraction X-ray diffraction patterns of pure PANI, NiFe2 O4 and PANI/NiFe2 O4 composites (C1: 2.5% and C4: 50%) are shown in Fig. 1. Fig. 1(a) shows the characteristic peaks of NiFe2 O4 at 2 = 30.55◦ , 35.94◦ , 43.64◦ , 54.07◦ , 57.63◦ , 63.26◦ , 71.76◦ with the reflection of Fd3m cubic spinel group (PDF 10-0325) [20], which indicates the formation of a single-spinel phase. The broadening nature of the diffraction peaks suggests that the as-prepared sample is very small in dimensions. The crystallite size D is calculated from XRD peak broadening of the (3 1 1) peak using Debye–Scherrer formula: D = 0.9/ˇ cos where D is the average crystallite size, is the wavelength of Cu K␣, ˇ is the full width at half maximum (FWHM) of the diffraction peaks and is the Bragg’s angle. Correction for the line broadening by the instrument was applied using a large particle size silicon standard and the 2 − B2 where B relationship B2 = BM M and BS are the measured S
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Fig. 2. FTIR spectra of NiFe2 O4 (a); PANI (b) and NiFe2 O4 /PANI (C4: 50%) (C).
Fig. 1. XRD pattern of NiFe2 O4 (a); PANI (b); and NiFe2 O4 /PANI (C1: 2.5%) (c); NiFe2 O4 /PANI (C4: 50%) (d).
widths at half-maximum intensity of the lines from the sample and the standard, respectively. The crystalline size is estimated to be in the range of 23–27 nm. XRD pattern of PANI (Fig. 1(b)) shows the main diffraction peaks with amorphous nature at about 2 = 20.3◦ and 25.1◦ , which is ascribed to the periodicity parallel and perpendicular to PANI chains, respectively [21]. The XRD of the composite samples (C1 and C4), are shown in Fig. 1(c) and (d). The XRD exhibit both the characteristic peaks of the NiFe2 O4 ferrite and the broad diffraction peaks of PANI. These results confirmed the formation of PANI/NiFe2 O4 composites. 3.2. FTIR spectra Fig. 2 shows the FTIR spectra of NiFe2 O4 , PAN and PANI/NiFe2 O4/ (C4: 50%) composite samples. Spectrum (2 a) shows bands in the region 400–600 cm−1 only which are attributed to pure NiFe2 O4 in absence of any organic moiety. Polymerization of aniline can be confirmed by the presence of spectral peaks at 1456 and 1564 cm−1 , Fig. 2(b), which are attributed to stretching of the benzenoid and quinoid rings, respectively, for the HCl doped PANI. These results indicate that the pure PANI is highly doped and exists in conducting emeraldine salt form [22]. The peak at 1299 cm−1 corresponding to C N stretching of secondary amine in polymer main chain can be clearly seen. The third spectrum in Fig. 2(c) shows peaks corresponding to both the organic moiety (1641 and 1463 cm−1 quinoid and benzenoid ring of polyaniline, 1246 C N stretching vibrations and 1080 cm−1 for C H bending mode) and NiFe2 O4 nanoparticles (400–600 cm−1 ).These specific peaks in the
functional group region of the FTIR are observed at slightly higher frequency than their actual values because of the interaction with nickel ferrite moiety. The results reveal that there exists an interaction between ferrite particles and PANI chains. This is produced from the – interaction between metal oxide and PANI, which includes (1) the molecular orbital of PANI overlaps the empty d-orbital of metal ions to form the -bond where metallic empty d-orbital is the electron pair acceptor; (2) the ∗ molecular orbital of PANI overlaps the d-orbital of metal ions to form the -bond, in which the metal ions is the electron pair donor. In addition, the hydrogen bonding interaction between the polyaniline chains and the oxygen atoms on the ferrite surface occurs in the composites, which make ferrite particles be embedded into polymer chain of PANI [23]. 3.3. Electron microscopy analysis The morphology and particle size of magnetic polyaniline, nickel ferrite and PANI/nickel ferrite composites is determined by SEM and TEM. Fig. 3(b), shows a fiber structure for PANI. Whereas, the SEM and TEM micrographs (Figs. 3(a) and 4(a)) of nickel ferrite reveal that the particles are approximately spherical in shape with diameter ranging from 25–35 nm. It is also observed that the ferrite nanoparticles exhibit some extent of agglomeration because of magnetic dipole interaction between ferrite particles. The particle sizes of ferrite obtained by TEM technique are slightly larger than the average crystallite size of 25 nm estimated from the XRD peaks using the Scherrer formula. This difference can be attributed to the agglomeration behavior of the ferrite nano-particles and due to the different approach of the two techniques. Figs. 3(c) and (d) and 4(b)–(e) show the SEM and TEM images of the core–shell PANI/nickel ferrite. The images indicate that the ferrite core–shell are coated by polyaniline into approximately spherical coral-like particles, which have diameter around 30–45 nm and connected by polymer. The black core is magnetic ferrite particles, and the light grey shell is PANI in the nanocomposite. The difference in colors is due to the different electron penetrability. It can be also seen from Fig. 4(b)–(e) that the degree of coating on the surface of NiFe2 O4 increases with PANI content. Meanwhile, the agglomeration of the ferrite particles is reduced to the coating of PANI on them, which is good to the dispersion and stabilization of the ferrite particles. From the results of SEM and TEM, it is clear that synthetic method in this study is applicable to the synthesis of PANI/nickel ferrite
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Fig. 3. SEM of NiFe2 O4 (a); PANI (b); NiFe2 O4 /PANI (C2: 5%) (c); NiFe2 O4 /PANI (C3: 25%) (d) and NiFe2 O4 /PANI (C4: 50%) (e).
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Fig. 4. TEM of NiFe2 O4 (a); NiFe2 O4 /PANI (C1: 2.5%) (b); NiFe2 O4 /PANI (C2: 5%) (c); NiFe2 O4 /PANI (C3: 25%) (d) and NiFe2 O4 /PANI (C4: 50%) (e).
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M. Khairy / Synthetic Metals 189 (2014) 34–41
Fig. 6. UV–visible spectra of PANI, (—); NiFe2 O4 /PANI (C1: 2.5%), (. . ...); (C2: 5%), (- - - - - -); (C3: 25%), (- -) and (C4: 50%), (-.-.).
Fig. 5. TGA of PANI (a); NiFe2 O4 /PANI (C3: 25%) (b); (C4: 50%) (c) and NiFe2 O4 (d).
composites. The results obtained are quite similar to other systems reported elsewhere [13,15,24]. 3.4. Thermal analysis TG analysis of NiFe2 O4 , PANI and PANI/NiFe2 O4 composites (C3: 25% and C5: 50%) is shown in Fig. 5. NiFe2 O4 shows thermal stability, whereas PANA and the composites decompose over the temperature range investigated. The thermogram of PANI (Fig. 5(a)) shows a three-step weight loss. The initial weight loss at lower temperature (less than 70 ◦ C) is due to the loss of water retained in PANI matrix after drying. The second weight loss ranging from 115 to 300 ◦ C is attributed to the loss of acid dopant anions compensated the positive charge of PANI Chains [25,26]. The third weight loss starting at about 300 ◦ C can be ascribed to the skeletal PANI chain decomposition after the elimination of dopant anions. It is seen from Fig. 5(a)–(c) that the 40% weight loss occurred at 415 ◦ C for pure PANI and 460 ◦ C for (C3: 25%) as well as 490 ◦ C for (C4: 50%) PANI/NiFe2 O4 composites. This reveals that the thermal stability of the composites is higher than that of pure PANI. This may be caused by the interaction between ferrite particles and PANI chains. This finding agrees well with the results obtained by Wang et al. [27]. 3.5. UV–visible spectra UV–vis absorption spectra of PANI and PANI/NiFe2 O4 composites are shown in Fig. 6. From which it is seen that PANI has two characteristic absorption bands at 330 and 610 nm. The absorption band at 330 nm is attributed to –* transition of the benzenoid ring [28,29], while the peak at 610 nm corresponds to the benzenoid-to-quinoid excitonic transition [30,31]. For PANI/NiFe2 O4 composite (C4: 50%), the absorption peaks are found at around 360 and 630 nm, which have a red shift of 30 and 20 nm, respectively, as compared with that of PANI. These results supported our suggestion of the presence of some sorts of interactions between the ferrite and PANI chains [32]. The results obtained show also a decrease in the intensities of the two absorption peaks
Fig. 7. Magnetic hysteresis loop of NiFe2 O4 (a); NiFe2 O4 /PANI (C4: 50%) (b); (C3: 25%) (c); (C2: 5%) (d) and (C1: 2.5%) (e)
with increasing the ferrite content due to their interactions with the polyaniline. 3.6. Magnetic properties Fig. 7 shows the magnetization, M, versus the applied magnetic field, H, for NiFe2 O4 nanoparticles, pure NiFe2 O4 and PANI/NiFe2 O4 nanocomposites of various compositions (C1: 2.5%, C2: 5%, C3 25% and C4: 50%) at room temperature. The magnetization of NiFe2 O4 and its composites exhibits a clear hysteretic behavior. The magnetic parameters such as saturation magnetization (Ms ), remnant magnetization (Mr ) and coercivity (Hc ) that determined by the hysteresis loops measurement are given in Table 1. The Ms -values of composites are less than that obtained for pure nickel ferrite, and decreases with decreasing the ferrite amount in the composite. This behavior is due to the non-magnetic coating layer, can be envisaged as a magnetic dead layer on the surface, thus affecting the magnitude of magnetization due to quenching of the surface moment Table 1 Saturation magnetization (Ms ), remnant magnetization (Mr ) and coercivity (Hc ) of the studied samples. Sample
Ms (emu/g)
Mr (emu/g)
Mr /Ms
Hc (Oe)
NiFe2 O4 NiFe2 O4 /PANI (C4: 50%) NiFe2 O4 /PANI (C3: 25%) NiFe2 O4 /PANI (C2: 5%) NiFe2 O4 /PANI (C1: 2.5%)
24 13.36 6.24 1.25 0.13
13 8.59 3.95 0.76 0.094
0.542 0.64 0.63 0.51 0.72
330 307 303 298 294
M. Khairy / Synthetic Metals 189 (2014) 34–41
39
Table 2 Electrical conductivity data of the studied samples. Sample
* × 103 ( −1 cm−1 )
E (eV)
Tt # (K)
Pure PANI NiFe2 O4 /PANI (C1: 2.5%) NiFe2 O4 /PANI (C2: 5%) NiFe2 O4 /PANI (C4: 50%) NiFe2 O4
23.2 7.1 4.3 2.0 3.6 × 10−4
0.044 0.056 0.077 0.092 0.32
390 405 415 425
* : conductivity at 300 K; Tt # : transition temperature on ln vs. 1/T curve, conductivity of NiFe2 O4 at 470 K.
Fig. 8. Effect of temperature on the conductivity of: PANI (a); NiFe2 O4 /PANI (C1:2.5%) (b); (C2: 5%) (c); (C4: 50%) (d); and Pure NiFe2 O4 (e).
[33,34]. Generally, the magnetic behavior and the magnetic values (Ms ) of the nanocomposites can be also explained according to the equation Ms = ϕms , where ϕ is the volume fraction of the magnetic particles and ms is the saturation moment of a single particle [35–37]. PANI/NiFe2 O4 nanocomposites have less magnetization than that observed for the pure nickel ferrite nanoparticles, which means that the total magnetic behavior of the nanocomposite could be tuned and tailored depending on these two parameters. Inspection of Table 1 shows that the coercivity Hc of the PANI/ferrite composites increases with the increase in the nickel ferrite content. This can be attributed to the microstructure nature of PANI/NiFe2 O4 I nanocomposites. It is well known that nano-sized ferrites have an irregular structure, geometric and crystallographic nature, such as pores, cracks, surface roughness and impurities. In the polymerization process, depositing PANI on the ferrite surface and crystallite boundary, has a healing effect to cover the ferrite surface defects, such as pores and cracks, leading to a decrease in magnetic surface anisotropy of ferrite particles, consequently, PANI/NiFe2 O4 nanocomposite presents a lower value of coercivity than that of NiFe2 O4 [35–37]. 3.7. Electrical conductivity It is well known that electrical conduction in semiconductor materials is a thermally activation process and is due to the ordered motion of weakly bound charged particles under the influence of an electric field. It depends on the nature of the charge carriers that dominate the conduction process. Therefore, due to the presence of some residual waters in the investigated samples (as shown from FT-IR and thermal analyses results), that may influence their conductivity values; the samples were heated at 70 ◦ C for 3 h before performing the conductivity measurements. The temperature dependence of dc-conductivity ( dc ) for NiFe2 O4 , PANI and PANI/NiFe2 O4 composites in a temperature range between 30 and 250 ◦ C is shown in Fig. 8, the insert figure is plotted to show clearly the difference in conductivity values between PANI and its composites. It can be seen that the conductivity of pure PANI is higher than the NiFe2 O4 and composite samples. There are many influencing factors on conductivity, such as dopant, doping level, crystallinity, the length of the conjugate chain, the interactions
between conjugated chains and molecular weight, etc. It is known that PANI is a kind of conducting polymer while the ferrite particles are insulators, so the conductivity of the composites could decrease with the increase of NiFe2 O4 content. There are several reasons for the decrease in conductivity; one reason is that the introduction of the ferrite particles would affect the polymerization of aniline, and further result in destroying of the continuity and regularity of the conjugated chain and blockage of conducting path. Besides, interactions between the ferrite particles and PANI chains, and possible bonding effect between the metal cations and PANI would reduce the electronic density of the polymer chain, and increases the charge carrier scattering and further result in the decrease of the conductivity. Fig. 8 shows that the dc-conductivity of all measured samples decreased with decreasing temperature, exhibiting typical semiconductor behavior and obeys Arrhenius equation (Eq. (3)) up to a transition temperature Tt of 390 ◦ C for PANI and 405, 415 and 425 K for the composites containing 2.5, 5 and 50 wt% NiFe2 O4 , respectively. = 0 exp
E a
kt
(3)
where 0 is preexponential factor and Ea is the activation energy for conduction process. The conductivity data are summarized in Table 2. Inspection of Fig. 8 shows that at temperatures higher than Tt , dc decreases gradually. A similar trend has been reported for similar systems [38,39]. The decrease in conductivity after the well noticeable transition temperature (Tt ) is attributed to starting the thermal dissociation of the samples, as confirmed by thermal analyses. The above results mean that desired level of thermal stability and electrical conductivity can be attained by taking suitable composition of PANI/NiF2 O4 nanocomposite for specific application such as in the preparation of high temperature conducting polymers. 3.8. Microwave absorption properties The frequency dependence of dissipation factors represented by the dielectric loss (tan ı ) and magnetic loss (tan ı ) are shown in Fig. 9. The change of these losses with frequency may explain the microwave absorption in the samples. The values of tan ı for epoxy–PANI/nickel ferrite composites are larger than that of epoxy–nickel ferrite, and the value of tan ı for epoxy–nickel ferrite is larger than that of epoxy–PANI/nickel ferrite composites in the frequency range of 1–6 GHz. These results demonstrate that the magnetic loss (tan ı ) is the main microwave absorbing mechanism for epoxy–nickel ferrite sample, and the microwave absorption enhancement of epoxy–PANI/nickel ferrite composites is attributed to dielectric loss (tan ı ) rather than magnetic loss. In order to reveal the microwave absorbing properties of the studied samples, reflection losses (RL ) are calculated by Eqs. (1) and (2) using various measured values of ε , ε , , and results
40
M. Khairy / Synthetic Metals 189 (2014) 34–41
4. Conclusions
Fig. 9. Frequency dependence of dielectric and magnetic losses of nickel ferrite and its NiFe2 O4 /PANI composites: (a) tan ı for C2: 5%, (b) tan ı for C3: 25%, (c) tan ı for ferrite, (d) tan ı for C2: 50%, (e) tan ı for ferrite, (f) tan ı for C2: 50%, (g) tan ı for C3: 25% and (h) tan ı : for C2: 5%.
are shown in Fig. 10. One microwave absorption band is observed in the specimens of nickel ferrite and its composites. The minimum reflection point shifts toward higher frequency with the increase in PANI content, and the intensity increases. The maximum reflection loss reaches −12.5 dB at 2.5 GHz for the nickel ferrite, −14.2 dB at 3.0 GHz, −17 dB at 3.55 GHz and −20.3 dB at 3.78 GHz for C4, C3 and C2 composites, respectively. From the above analysis we can infer that the enhancement observed in microwave absorption of composite samples is due to the change occurring in dielectric loss due to the addition of PANI on Nickel ferrite surface. This behavior can be explained on the basis that the inorganic insulating NiFe2 O4 in the PANI/NiFe2 O4 composite destroys the conductive network of PANI to a certain extent. As such, the input impedance of the composite increased and the electromagnetic wave could then be transmitted to the composite interiors adequately, a necessary condition for the loss of incident electromagnetic waves [40]. TEM results prove that NiFe2 O4 was decorated with PANI, so that the electrons and electron charges on the molecular chain aggregated on the surface of the NiFe2 O4 particles. Under the alternating electromagnetic field, interfacial and space–charge polarization were easily formed on the interface of the NiFe2 O4 and PANI. The reflection loss may be attributed to the resistance loss of local conductive particles, the dielectric loss of NiFe2 O4 , and polarization relaxation losses of the interface. Under these various loss mechanisms, the microwave absorbing properties of the PANI/NiFe2 O4 composites were better than that of pure NiFe2 O4 .
Fig. 10. Reflection loss dependence on the frequency for: (a) PANI, (b) NiFe2 O4 , (c) NiFe2 O4 /PANI (C4: 50%), (d) NiFe2 O4 /PANI (C3: 25%) and (e) NiFe2 O4 /PANI (C2; 5%)
Conductive and magnetic NiFe2 O4 /PANI nanocomposites have been prepared by in situ polymerization of aniline in the presence of different amounts of NiFe2 O4 nanoparticles by ammonium persulfate oxidant in HCl medium. NiFe2 O4 nanoparticles were prepared via sol–gel method. Characterizations of the samples were carried out using XRD, TGA, FT-IR and SEM techniques. XRD confirm spinel structure for NiFe2 O4 sample. The thermal stability increases with increasing the ferrite content in PANI matrix. The results of thermal and spectro-analysis indicate that there is an interaction between PANI chains and ferrite nanoparticles. The PANI/ferrite nanocomposites under applied magnetic field exhibit the hysteresis loop of the ferromagnetic nature at room temperature. The values of Ms and Hc increase with increasing the ferrite content in the composite. The conductivities of PANI/NiFe2 O4 composites are lower than that of pure PANI, and are reduced with the increase in the nickel ferrite content. The results obtained indicate that the total magneto-electrical behavior of the nanocomposites could be tuned and tailored depending on the volume fraction of the magnetic ferrite particles (ϕ), and on the contribution of the non-magnetic PANI coating layer. Our results indicate that the composites exhibit good microwave absorption performances. The PANI/nickel ferrite composites show absorption bands better than that of nickel ferrite at frequency range of 1–6 GHz. The absorption band shifts to the higher frequency region with the increase in the content of PANI in composites and the effectual absorption band for all samples (below −12 dB) reaches 2.5, 3, 3.55 and 3.8 GHz, respectively. The microwave absorption of nickel ferrite is mainly due to magnetic loss, while those in composites are due to dielectric loss. The loss mechanism may be attributed to the resistance loss of the local conductive particles, the dielectric loss of nickel ferrite, polarization relaxation losses of the interface and the changes of boundary condition of the microwave field at the interface between the ferrite particles and polymer matrix. The results showed that the PANI/NiFe2 O4 nanocomposites can operate as suitable microwave absorbers.
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