Composites Science and Technology 95 (2014) 107–113
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Superparamagnetic NiFe2O4 particles on poly (3,4-ethylenedioxythiophene)–graphene: Synthesis, characterization and their excellent microwave absorption properties Panbo Liu, Ying Huang ⇑, Xiang Zhang Key Laboratory of Space Applied Physics and Chemistry, Ministry of Education, School of Science, Northwestern Polytechnical University, Xi’an 710129, PR China
a r t i c l e
i n f o
Article history: Received 14 October 2013 Received in revised form 12 February 2014 Accepted 16 February 2014 Available online 4 March 2014 Key Words: A. Polymers Oxides A. Nanocomposites Electrical properties
a b s t r a c t The ternary nanocomposites of poly(3,4-ethylenedioxythiophene), graphene and NiFe2O4 (PEDOT–GN– NiFe2O4) were synthesized via a two-step method, in situ polymerization of poly(3,4-ethylenedioxythiophene) in the ﬁrst step and followed by the crystallization of NiFe2O4 particles in the second step. The ternary nanocomposites were characterized by XRD, TEM, XPS, Raman spectroscopy, TG and VSM. TEM results indicated that NiFe2O4 particles ranged from 10 to 15 nm uniformly dispersed on PEDOT–GN. The investigation of the electromagnetic absorbability reveals that the ternary nanocomposites exhibit excellent microwave absorption properties and wide absorption bandwidths. When the thickness is 2 mm, the maximum reﬂection loss is 45.4 dB at 15.6 GHz and the absorption bandwidths with the reﬂection loss below 10 dB and 20 dB are 4.6 GHz and 1.7 GHz, respectively. Compared to PEDOT– GN, GN–NiFe2O4 and PEDOT–NiFe2O4, the enhanced microwave absorption properties of PEDOT–GN– NiFe2O4 are due to the improved impedance matching and the enhanced interfacial effects. Furthermore, our development strategy provides a feasible method to obtain ternary nanocomposites based on conducting polymers, graphene and magnetic particles. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Microwave absorption materials can absorb electromagnetic (EM) waves effectively and convert EM energy into thermal energy or dissipate the EM waves by interference. Hence, increasing attentions have been given to the microwave absorption materials. Recently, many researchers are focus on the development of microwave absorption materials, such as nickel , cobalt , barium titanates [3,4], carbon nanotubes [5,6] and conducting polymers [7–9]. However, the attenuation mechanism of those materials is mainly based on either dielectric loss or magnetic loss, and the impedance matching characteristics have seldom been studied. Poly(3,4-ethylenedioxythiophene) (PEDOT), one of the most promising conducting polymers with excellent physical and chemical properties, can be used as microwave absorption materials. However, the maximum reﬂection loss is only 24 dB with a thickness of 2 mm . Previous reports demonstrate that the combination of PEDOT with Fe3O4 , barium ferrite  or iron oxides  could enhance its microwave absorption properties.
⇑ Corresponding author. Tel.: +86 29 88431636. E-mail address: [email protected]
(Y. Huang). http://dx.doi.org/10.1016/j.compscitech.2014.02.018 0266-3538/Ó 2014 Elsevier Ltd. All rights reserved.
Graphene, a new kind of carbon based material with low density and high complex permittivity, also can be used as microwave absorption materials. However, graphene is found to be nonmagnetic and the microwave absorption mostly contributes to the dielectric loss, which is not an ideal microwave absorption material . According to EM energy conversion principle, apart from dielectric loss and magnetic loss, a proper matching between the dielectric loss and magnetic loss also determines the reﬂection and attenuation characteristics of EM absorbers. Based on the impedance matching strategy, one of the effective ways to solve the problem is to couple graphene with Fe3O4 particles [15–19] or Co3O4 particles , but these reports are conﬁned to study the microwave absorption property of the binary composites. Recently, the novel ternary composites of EG/PANI/CF  have been synthesized, but the minimum reﬂection loss is only 19.1 dB at the frequency of 13.28 GHz with a thickness of 0.5 mm. Singh et al.  developed a 3D nano-architecture in polyaniline by incorporating the hybrid structure of graphene/Fe3O4 and the EM interference shielding effectiveness is only 26 dB. In our recent study, we have revealed the microwave absorption ability of RGO–PPy–Co3O4, and the maximum reﬂection loss is 33.5 dB with a thickness of 2.5 mm . As for RGO–PEDOT–Co3O4, the maximum reﬂection loss is 46.5 dB with a thickness of 3.1 mm,
P. Liu et al. / Composites Science and Technology 95 (2014) 107–113
but the absorption bandwidths with the reﬂection loss below 10 dB are very narrow (from 6.4 to 8.5 GHz) . Furthermore, the synthesis procedure of these ternary composites requires multiple steps. As one of the most important spinel ferrites, NiFe2O4 particles have higher magnetic loss factors, which can be used as microwave absorption materials [25,26]. Fu et al.  synthesized NiFe2O4 nanorod–graphene composites by a facile one-step hydrothermal process in the presence of an ionic liquid and the result indicates that the reﬂection loss below 10 dB ranges from 13.6 to 18 GHz with a thickness of 2.0 mm. Though the research has reported the microwave absorption properties of graphene with Fe3O4, Co3O4 and NiFe2O4 particles, the focus is mainly on the binary composites and the microwave absorption properties and the absorption bandwidths of the presented ternary composites are not satisfactory. To the best of our knowledge, the microwave absorption properties of the ternary nanocomposites composed of PEDOT, graphene and NiFe2O4 particles have never been reported. Inspired by this thought, in order to synthesize absorbing materials with excellent microwave absorption properties and wide absorption bandwidths, we motivate to prepare PEDOT– GN–NiFe2O4 by a two-step method. It is a great challenge to design and prepare the ternary nanocomposites based on PEDOT, GN and NiFe2O4 particles because the ternary nanocomposites possess the excellent microwave absorption properties and wide absorption bandwidths. In this work, we present a two-step method to synthesize PEDOT–GN–NiFe2O4, and the structure is investigated by XRD, TEM, XPS, TG, Raman spectroscopy and VSM. Conﬁrming that PEDOT with the incorporation of GN and NiFe2O4 particles can effectively improve the microwave absorption properties, it is very valuable to study the relationship of the relative permittivity and permeability. The results indicate that the electromagnetic attenuation mechanism of PEDOT–GN–NiFe2O4 may be ascribed to the improved impedance matching and the enhanced interfacial effects. The maximum reﬂection loss of the ternary nanocomposites is up to 45.4 dB at 15.6 GHz and the absorption bandwidths with the reﬂection loss below 10 dB are 4.6 GHz (from 12.6 to 17.2 GHz) with a coating layer thickness of 2 mm, which are much wider than that of RGO–PEDOT–Co3O4 (only 2.1 GHz).
2. Experimental section 2.1. Preparation Graphene oxide (GO) was synthesized by Hummers method . The PEDOT–GN–NiFe2O4 nanocomposites were prepared via a two-step method as follows: Firstly, 3,4-ethylenedioxythiophene monomer (0.2 mL) was dissolved in GO (100 mL, 1 mg/mL) solution and sonicated for 2 h. After cooling to 5 °C, 2 mL concentrated H2SO4 and 0.95 g ammonium persulphate (APS) dissolved in deionized water were added. The mixture was stirred overnight and the resulting precipitates were washed with deionized water. Secondly, 0.3 g Ni(NO3)26H2O and 0.8 g Fe(NO3)39H2O were added in the resulting precipitates under stirring at room temperature for a period time, the dissolved Ni2+ and Fe3+ ions were adsorbed on the surface of GO via electrostatic interaction, then NH3H2O (20 mL) was added to the solution slowly. After that, the mixture was transferred to a Teﬂon-lined autoclave and maintained at 180 °C for 12 h. The composites were washed with deionized water several times and dried at 50 °C for 12 h. In our work, NH3H2O is selected to accomplish two functions. In the ﬁrst place, NH3H2O plays an important role in the formation of NiFe2O4 particles. In the second place, NH3H2O is a reducing agent, promoting the reduction reaction of GO in the solvothermal system. PEDOT–GN was prepared in the same way without the presence of Ni(NO3)2
6H2O and Fe(NO3)39H2O. GN–NiFe2O4 and PEDOT–NiFe2O4 were prepared in the same way without the presence of PEDOT and GN, respectively. 2.2. Characterization Crystal phase analysis was performed by powder X-ray diffraction (XRD) diffraction with Cu Ka radiation (XRD, Philips X-ray diffractometer, PW3040). Morphology observation was conducted with a ﬁeld emission transmission electron microscope (FETEM: Tecnai F30 G2). X-ray photoelectron spectroscopy (XPS, Thermal Scientiﬁc K Alpha) was performed with a Phoibos 100 spectrometer. Raman scattering was performed on a Jobin-Yvon HR800 Raman spectrometer. Thermal gravimetric (TG) analysis was performed on a Q2000 thermogravimetric analyzer at a heating rate of 10 °C min1 in air. The magnetic property of the product was measured by a vibrating sample magnetometer (VSM). For measurement of the microwave properties, the samples were dispersed in parafﬁn homogeneously with a sample-to-parafﬁn weight ratio of 1:1, and then the mixture was pressed into a toroidal shape with an inner diameter of 3.0 mm and an outer diameter of 7.0 mm. The relative complex permittivity (e0 and e00 ) and permeability (l0 and l00 ) were carried out by a HP8753D vector network analyzer at the frequency range of 2–18 GHz. 3. Results and discussion To investigate the formation of NiFe2O4 particles, XRD pattern of GN and PEDOT–GN–NiFe2O4 is presented in Fig. 1. In Fig. 1a, it can be seen that GN shows a very broad diffraction peak at about 24.8°, suggesting that GO has been reduced to GN. For PEDOT– GN–NiFe2O4 in Fig. 1b, eight diffraction peaks at 2h = 18.5°, 30.3°, 35.7°, 37.3°, 43.4°, 53.8°, 57.4° and 63.0° are assigned to reﬂections from the (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) planes of spinel NiFe2O4, which is similar to the standard patterns of NiFe2O4. The relatively weak intensities of these peaks demonstrate NiFe2O4 particles are small and the narrow sharp peaks suggest NiFe2O4 particles are highly crystalline. Furthermore, apart from the characteristic peaks of NiFe2O4, an additional small and weak broad diffraction peak (0 0 2) appearing at about 25° can be indexed to the disorderedly stacked GN sheets . Notably, no obvious diffraction peaks for PEDOT can be observed, which might be due to the relatively low diffraction intensity of PEDOT in the nanocomposites. To investigate the morphology of the samples, TEM is recorded for PEDOT–GN and PEDOT–GN–NiFe2O4 and the results are shown in Fig. 2. As can be seen in Fig. 2a, PEDOT–GN exhibits a paper-like morphology and the surface is covered with many nanowhiskers,
Fig. 1. XRD pattern of GN (a) and PEDOT–GN–NiFe2O4 (b).
P. Liu et al. / Composites Science and Technology 95 (2014) 107–113
Fig. 2. TEM image of PEDOT–GN (a), inset is SAED pattern, TEM images of PEDOT–GN–NiFe2O4 (b–e), inset in (e) is SAED pattern and HRTEM image of PEDOT–GN–NiFe2O4 (f).
which is attributed to the PEDOT coating onto GN, as indicated by the arrows. Because PEDOT is an amorphous material, the SAED pattern (inset in Fig. 2a) indicates that the lack obvious crystalline character of PEDOT–GN is due to the perfect coverage of PEDOT. As shown in Fig. 2b–d, it can be observed that a large quantity of NiFe2O4 particles with a relatively uniform size is distributed on the surface of PEDOT–GN, and both the outline of PEDOT–GN and NiFe2O4 particles can be clearly observed. Besides, these NiFe2O4 particles are ﬁrmly attached onto PEDOT–GN, even sonication was used during the preparation of TEM specimens, indicating an excellent adhesion between PEDOT–GN and NiFe2O4 particles is formed. Fig. 2e further demonstrates that NiFe2O4 particles with the average size in the range of 10–15 nm are covered on the surface of PEDOT–GN. The SAED pattern (inset in Fig. 2e) demonstrates the crystalline feature of NiFe2O4 particles. In order to verify the crystalline structure of NiFe2O4 particles, HRTEM image of PEDOT–GN–NiFe2O4 is presented in Fig. 3f. We can observed that some clear lattice fringes are observed, and the crystal lattice fringes with a spacing of 0.25 nm (inset in Fig. 2f) can be assigned to the (3 1 1) plane of NiFe2O4 particles, which is in accordance with the XRD results. These above analyses conﬁrm the successful formation of the ternary nanocomposites. The elemental components of PEDOT–GN–NiFe2O4 are further investigated by XPS in Fig. 3. In Fig. 3a, the C1s spectrum of GO can be deconvoluted into four different peaks. The peaks at 284.6, 286.4, 287.8, and 289.3 eV have been assigned to the CAC/ [email protected]
in the aromatic rings, CAO of epoxy and/or alkoxy, [email protected]
and [email protected]
groups, respectively . For PEDOT–GN–NiFe2O4 in Fig. 3b, the intensities of C 1s peaks of the carbon binding to oxygen are very weak, suggesting most of the epoxide and hydroxyl functional groups are successfully removed. Meanwhile, a new peak centered at 285.3 eV can be assigned to the CAS group [31,32]. In Fig. 3c, the wide scan XPS spectrum indicates that the nanocomposites are completely composed of S, C, O, Fe and Ni elements. No other elemental signals are detected in the general XPS spectrum. The S 2p XPS spectra (Fig. 3d) shows the presence of sulfur spin-split doublet at around 164.0 eV (S 2p3/2) and 165.1 eV
(S 2p1/2) with an energy splitting of 1.1 eV. The higher binding energy doublet at around 168.2 and 169.5 eV would be ascribed to sulfur spin-split coupling from SO2 4 due to the incorporation of counterion SO2 4 into PEDOT, suggesting the formation of doped PEDOT . The Ni 2p XPS spectra of the nanocomposites in Fig. 3e exhibit two peaks at 852.5 and 870.2 eV, which are assigned to the binding energy of Ni 2p3/2 and Ni 2p1/2, respectively. As shown in Fig. 3f, the core level binding energy at 711.6 and 725.1 eV are ascribed to the characteristic doublets of Fe 2p3/2 and Fe 2p1/2, respectively. Raman spectra of GN, NiFe2O4, PEDOT and PEDOT–GN–NiFe2O4 are presented in Fig. 4. It can be clearly observed that GN in Fig. 4a shows two Raman peaks centered at 1347 cm1 (D band) and 1581 cm1 (G band). The D band is assigned to the vibrations of sp3 carbon atoms of disordered graphite and the G band is mainly assigned to the in-plane vibration of sp2 carbon atoms in a 2D hexagonal lattice, respectively [34,35]. For NiFe2O4 in Fig. 4b, the peaks appeared in the range of 400–800 cm1 is in good agreement with published work on NiFe2O4 crystalline . For PEDOT in Fig. 4c, the band at 989 cm1 is assigned to oxyethylene ring deformation, and the bands at 1430 and 1508 cm1 are attributed to [email protected]
stretching, suggesting the formation of PEDOT . Notably, for PEDOT–GN–NiFe2O4 in Fig. 4d, the bands at about 1511 and 1434 cm1 can be due to the p–p interaction of PEDOT with GN . In addition, the peaks centered at 572 and 704 cm1 can be attributed to the A1g vibration mode of NiFe2O4, and the peaks at 493 and 667 cm1 can be attributed to the T2g and Eg vibration modes of NiFe2O4, respectively . These results demonstrate the existence of PEDOT, GN and NiFe2O4 in the nanocomposites. Fig. 5 shows the thermal stability of GN, PEDOT, PEDOT–GN, PEDOT–NiFe2O4 and PEDOT–GN–NiFe2O4 in an air atmosphere, respectively. It can be noted that all the samples show a little weight loss below 200 °C, which is due to the evaporation of moisture and solvent residue in the samples. In Fig. 5a, it is observed that GN shows a slight weight loss from 200 to 500 °C and an obvious rapid weight loss range from 500 to 600 °C, which can be attributed to the decomposition of oxide groups and the oxidation of carbon, respectively, and the residual weight of GN is about 3.6%
P. Liu et al. / Composites Science and Technology 95 (2014) 107–113
Fig. 3. C 1s XPS spectra of GO (a) and PEDOT–GN–NiFe2O4 (b), survey scan (c), S 2p (d), Ni 2p (e) and Fe 2p (f) of PEDOT–GN–NiFe2O4.
Fig. 4. Raman spectra of GN (a), NiFe2O4 (b), PEDOT (c) and PEDOT–GN–NiFe2O4 (d).
at 600 °C. For PEDOT in Fig. 5b, the signiﬁcant mass loss occurs at 300–560 °C, which is accounted for the decomposition of PEDOT, and the residual weight is about 1.8% 560 °C. For PEDOT–GN (Fig. 5c), a major weight loss in the range of 300–500 °C is mainly supposed to the decomposition of PEDOT, and an obvious weight loss from 500 to 600 °C is most probably due to the decomposition of GN and PEDOT. In Fig. 5d, the residual weight of GN–NiFe2O4 is about 85.1% at 800 °C. As for PEDOT–GN–NiFe2O4 (Fig. 5e), the weight loss is about 31.4% at 800 °C. According to the mass loss, we can draw a conclusion that the weight percentage of NiFe2O4 particles in the ternary nanocomposites is about 68.6%. The magnetic property of PEDOT–GN–NiFe2O4 is measured with VSM at room temperature and the measurement result is presented in Fig. 6. The signiﬁcant hysteresis loop of the ternary nanocomposites shows no remanence or coercivity, suggesting a superparamagnetic character. The saturation magnetization (Ms) value of PEDOT–GN–NiFe2O4 is 31.7 emu g1. The much lower Ms of the ternary nanocomposites compared to NiFe2O4 particles
Fig. 5. TG curves of GN (a), PEDOT (b), PEDOT–GN (c), PEDOT–NiFe2O4 (d) and PEDOT–GN–NiFe2O4 (e).
(50.0 emu g1) can be attributed to the presence of nonmagnetic PEDOT and GN in the nanocomposites . To reveal the microwave absorption properties of PEDOT– GN–NiFe2O4 are greatly enhanced, the reﬂection losses (RL) are calculated according to transmission line theory as follows :
Z in 1 RL ðdBÞ ¼ 20 log Z in þ 1 Z in ¼
qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ pﬃﬃﬃﬃﬃﬃﬃﬃﬃ lr =er tan h jð2pfd=cÞ er lr
where Zin is the input impedance of the absorber, c is the velocity of electromagnetic waves in free space, f is the frequency and d is the layer thickness. In order to indicate that the microwave absorption properties of PEDOT–GN–NiFe2O4 are greatly enhanced compared to the binary composites, the RL curves of PEDOT–GN, PEDOT– NiFe2O4, GN–NiFe2O4 and PEDOT–GN–NiFe2O4 with different
P. Liu et al. / Composites Science and Technology 95 (2014) 107–113
Fig. 6. Magnetization hysteresis loops of PEDOT–GN–NiFe2O4 measured at room temperature.
thickness are shown in Fig. 7. It can be seen that the RL values of PEDOT–GN (Fig. 7a) are less than 14 dB with a thickness of 2– 4 mm over 2–18 GHz, and the bandwidths of the RL values below 10 dB (90% of EM wave absorption) range from 8.5 to 10.5 GHz with a thickness of 2 mm. For PEDOT–NiFe2O4 in Fig. 7b, the maximum RL is only 9.8 dB at 9.3 GHz with a thickness of 4 mm. As shown in Fig. 7c, the maximum RL of GN–NiFe2O4 is 22.6 dB at 11.0 GHz and the absorption bandwidth with the RL below 10 dB is 4.1 GHz (from 9.3 to 13.4 GHz) with a thickness of 3 mm. Compared with the binary composites, the microwave absorption properties of PEDOT–GN–NiFe2O4 are signiﬁcantly enhanced and the result is presented in Fig. 7d. It is noted that there is one sharp and strong wave absorption peak at 15.6 GHz, the maximum RL is up to 45.4 dB and the bandwidths corresponding to the RL values below 10 dB and 20 dB are 4.6 GHz (from 12.6 to 17.2 GHz) and 1.7 GHz (from 14.5 to 16.2 GHz) with a coating layer thickness of 2 mm, which are better than binary composites such as
Fe3O4–PEDOT , RGO–Fe3O4 [15–19], RGO–Co3O4 , NiFe2O4 nanorod–graphene  and ternary composites of EG/PANI/CF , graphene/Fe3O4/PANI  and RGO–PPy–Co3O4 . When the coating layer thickness goes up to 2.5 mm, the bandwidths of the RL values below 10 dB are more than 7 GHz in the range of 9.4–16.4 GHz and the maximum RL is 30.6 dB at 11.3 GHz, which are much wider than RGO–PEDOT–Co3O4 . All of the above analyses demonstrate that the PEDOT–GN–NiFe2O4 nanocomposites obtained in this work are attractive candidates for the new type of microwave absorption materials. To investigate the possible microwave absorption mechanisms, the complex permittivity real part (e0 ), permittivity imaginary part (e00 ), permeability real part (l0 ) and permeability imaginary part (l00 ), dielectric loss tangent (tan de) and magnetic loss tangent (tan dl) of PEDOT–GN, PEDOT–NiFe2O4, GN–NiFe2O4 and PEDOT– GN–NiFe2O4 are presented in Fig. 8. In Fig. 8a, it can be observed that e0 values of PEDOT–GN decrease gradually from 22.9 to 15.3 with several ﬂuctuations in the frequency range of 2–18 GHz, while e0 values of PEDOT–NiFe2O4, GN–NiFe2O4 and PEDOT–GN– NiFe2O4 decrease from 5.5 to 5.1, 8.5–4.4 and 10.1–5.4, over 2– 18 GHz frequency range, respectively. In Fig. 8b, the e00 values of PEDOT–GN, PEDOT–NiFe2O4, GN–NiFe2O4 and PEDOT–GN–NiFe2O4 vary from 16.7 to 7.7, 1.9–0.5, 3.5–1.9 and 5.1–1.9, respectively, over 2–18 GHz frequency range. Moreover, it is observed that both e0 and e00 values of PEDOT–GN are higher than PEDOT–NiFe2O4, GN– NiFe2O4 and PEDOT–GN–NiFe2O4 in 2–18 GHz, which is due to the high electrical conductivity of PEDOT directly covered on the surface of GN, suggesting the EM attenuation mechanism of PEDOT– GN is mainly ascribed to dielectric loss. Because NiFe2O4 particles have higher magnetic loss factors, the addition of PEDOT and graphene in the ternary nanocomposites is in order to tune the relative permittivity. In Fig. 8c, l0 values of all samples exhibit complex variation in the 12–18 GHz range. In Fig. 8d, l00 values of all samples decrease gradually with several ﬂuctuations in the
Fig. 7. Reﬂection loss curves of PEDOT–GN (a), PEDOT–NiFe2O4 (b), GN–NiFe2O4 (c) and PEDOT–GN–NiFe2O4 (d).
P. Liu et al. / Composites Science and Technology 95 (2014) 107–113
Fig. 8. The e0 (a), e00 (b), l0 (c), l00 (d), tan de (e) and tan dl (f) of PEDOT–GN, PEDOT–NiFe2O4, GN–NiFe2O4 and PEDOT–GN–NiFe2O4.
frequency range of 2–18 GHz. Moreover, the negative imaginary part of permeability indicates that magnetic energy is radiated out from these samples which are due to the motion of charges . From Fig. 8e, we can see that tan de values of PEDOT–GN are higher than the other samples in 2–13.8 GHz and tan de values of PEDOT–NiFe2O4 have the lowest values in 4.0–18 GHz. In Fig. 8f, it can be observed that tan dl values of PEDOT–NiFe2O4 are higher than PEDOT–GN, GN–NiFe2O4 and PEDOT–GN–NiFe2O4 in the frequency range of 2–18 GHz while tan dl values of PEDOT–GN are lower than the other samples in 2–13.3 GHz. Generally, apart from dielectric loss and magnetic loss, another important concept relating to excellent microwave absorption is strongly dependent on the efﬁcient complementarities between dielectric loss and magnetic loss , the single higher and lower dielectric loss or magnetic loss is harmful to the impedance match and results in strong reﬂection and weak absorption. Whereas, the ternary nanocomposites could have improved impedance matching between dielectric loss and magnetic loss, which suggests they have excellent electromagnetic wave absorption properties. Firstly, the multiinterfaces between PEDOT, GN and NiFe2O4 particles are helpful to the improvement of EM absorption properties due to the existing interfacial polarization . Secondly, dipole polarizations are presented in NiFe2O4 particles, especially when the size is in nanoscale, the small particles size in our case will increase the dipole polarizations, which can be contributed to the dielectric loss. Thirdly, the magnetic loss is attributed to the outstanding magnetic of NiFe2O4 properties, the complementarities between the dielectric loss and magnetic loss also play an important role in increasing microwave absorption properties. Furthermore, a solid-state charge-transfer complex between GN and PEDOT also facilitates to strong polarization, which provides higher dipole interaction and leads to high microwave absorption properties . On the basis of the analyses above, we can draw a conclusion
that the electromagnetic attenuation mechanism of PEDOT–GN– NiFe2O4 may be ascribed to the enhanced interfacial effects and the improved impedance matching. 4. Conclusions In summary, the PEDOT–GN–NiFe2O4 nanocomposites with excellent microwave absorption properties and wide absorption bandwidths had been successfully synthesized by a facile two-step method. The results indicated that NiFe2O4 particles with a relatively uniform size are ﬁrmly attached onto PEDOT–GN. Compared with PEDOT–GN, GN–NiFe2O4 and PEDOT–NiFe2O4, the ternary nanocomposites exhibit excellent microwave absorption properties and wide absorption bandwidths. The maximum RL of PEDOT–GN–NiFe2O4 is up to 45.4 dB at 15.6 GHz and the bandwidths of RL values below 10 dB and 20 dB are 4.6 GHz and 1.7 GHz, respectively, with a thickness of 2 mm. The ﬁndings provide a feasible method to fabricate ternary nanocomposites with strong absorption and wide absorption bandwidths. Acknowledgements This work was supported by the Spaceﬂight Foundation of the People’s Republic of China (NBXW0001), the Spaceﬂight Innovation Foundation of China (NBXT0002) and the Doctorate Foundation of Northwestern Polytechnical University (CX201328). References  Tong GX, Hu Q, Wu WH, Li W, Qian HS, Liang Y. Submicrometer-sized NiO octahedra: facile one-pot solid synthesis, formation mechanism, and chemical conversion into Ni octahedra with excellent microwave-absorbing properties. J Mater Chem 2012;22:17494–504.
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