Superparamagnetic [email protected] nanoparticles on graphene-polyaniline: Synthesis and enhanced electromagnetic wave absorption properties

Superparamagnetic [email protected] nanoparticles on graphene-polyaniline: Synthesis and enhanced electromagnetic wave absorption properties

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Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎

Contents lists available at ScienceDirect

Ceramics International journal homepage: www.elsevier.com/locate/ceramint

Superparamagnetic [email protected] nanoparticles on graphenepolyaniline: Synthesis and enhanced electromagnetic wave absorption properties Yan Wang n, Wenzhi Zhang, Chunyan Luo, Xinming Wu, Gang Yan School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an 710021, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 16 April 2016 Received in revised form 4 May 2016 Accepted 6 May 2016

A superparamagnetic quaternary nanocomposite of [email protected]@[email protected] was prepared via a three-step method. The obtained quaternary nanocomposite was characterized by XRD, TEM, XPS, FTIR, VSM and VNA analysis. The electromagnetic parameters indicate that [email protected]@[email protected] exhibits enhanced electromagnetic absorption properties compared to FeCo and [email protected], which can be mainly attributed to the improved impedance matching. The possible absorption mechanism for the quaternary nanocomposite was also discussed in detail. The maximum reflection loss of [email protected] @[email protected] can reach  39.8 dB at 6.4 GHz and the absorption bandwidth with the reflection loss exceeding  10 dB is 3.1 GHz (from 4.6 to 7.7 GHz) with the thickness of 3 mm. Our results demonstrate that the [email protected]@[email protected] nanocomposite can serve as an excellent microwave absorber. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Graphene Polymers Magnetic materials Microwave absorption Nanocomposites

1. Introduction With the rapid development of electronic devices and communication facilities, electromagnetic (EM) radiation interference gradually threats the security of information and living environments of human being [1,2]. One of potential approaches to solve this problem is to use microwave absorption materials to attenuate the undesired EM wave. The attenuation mechanism of microwave absorption materials is mainly based on dielectric loss or magnetic loss. Ideal EM wave absorbing materials are required to have wide absorption frequency range, strong absorption, light weight and thin thickness feature [3]. To achieve the low density of ideal EM absorbers, many researchers are focusing on the development of carbon-based materials, with high complex permittivity and light weight density [4]. Graphene (RGO), a monolayer or few layers two-dimensional planar sheets composed by sp2-bonded carbon atoms, has attracted much attention in the material science field due to its superior structure, high specific surface, extraordinary electrical properties and may be used as a lightweight EM wave absorber [5–7]. Nevertheless, the sole graphene suffers from impedance mismatch owing to high conductivity, resulting in weak absorption and narrow absorption frequency [8]. One of the effective ways is to combine graphene with magnetic particles, such as Ni0.8Zn0.2Ce0.06Fe1.94O4 [9], n

Corresponding author. E-mail address: [email protected] (Y. Wang).

ZnFe2O4 [10], CoFe2O4 [11], FeCo [12], Fe3O4 [13], BaFe12O19/ CoFe2O4 [14] and Fe3O4-Fe [3] by tuning the permittivity of RGO to enhance electromagnetic attenuation. But the high density limits its progress in microwave absorption. Therefore, kinds of conducting polymers and dielectric materials have been explored to improve the balance between permittivity and permeability [4]. Theoretically, because of their synergetic effect, interfacial polarization and dipole polarization, composite materials will exhibit outstanding electromagnetic absorption properties [15]. Among various conducting polymers, polyaniline (PANI) has been regarded as a promising candidate for microwave absorbers due to its high electrical conductivity, easy preparation, low cost and excellent environment stability [16–18]. Wang et al. [19], fabricated RGO foam/PANI nanorods, which exhibited a maximum absorption of  52.5 dB at 13.8 GHz with the absorption thickness of 2 mm. Meanwhile, SnO2 is one of the most important n-type semiconductor materials and has many unique properties such as high optical transparency, electrical conductivity and chemical sensitivity [2]. In recent years, SnO2 has been an attractive material in microwave absorption [20,21]. Zhao et al. [22], prepared Ni-SnO2 composites and the optimal RL reached  42.8 dB at 9.8 GHz for a layer thickness of 3 mm. In this paper, the unique properties of magnetic particles, PANI, RGO and SnO2 prompted us to synthesize the quaternary nanocomposite of [email protected]@[email protected] The results show that the nanocomposite exhibits enhanced EM wave absorption and the maximum reflection loss value can reach  39.8 dB at 6.4 GHz.

http://dx.doi.org/10.1016/j.ceramint.2016.05.038 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Y. Wang, et al., Superparamagnetic [email protected] nanoparticles on graphene-polyaniline: Synthesis and enhanced electromagnetic wave absorption properties, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.038i

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Fig. 1. Schematic illustration of the synthesis procedure of [email protected]@[email protected] composite.

2. Experimental 2.1. Synthesis of FeCo In a typical procedure, 1.2 g CoCl2  6H2O and 2.8 g FeSO4  7H2O were dissolved in 50 mL deionized water and magnetic stirred for 10 min Then 2.0 g NaOH was dissolved in 30 mL deionized water and added drop wise into 50 mL metal solution. Subsequently, 40 mL hydrazine hydrate (N2H4  H2O) was added to the solution as a reducing agent. The mixture was stirred again for 30 min and transferred in a Teflon cup in a stainless steel-lined autoclave at 110 °C for 2 h. The products were cooled and washed, then dried at 60 °C under vacuum oven. 2.2. Synthesis of [email protected] Briefly, 0.5 g as-prepared FeCo particles were dispersed in a mixture of 1,2- propanediol (75 mL), distilled water (75 mL), and ammonia solution (10 mL). Then, 2.25 g SnCl2  2H2O was added and stirred for 30 min Subsequently, 5 mL H2O2 was introduced and transferred into a stainless steel-lined autoclave, and kept at 150 °C for 12 h. The cooled products were washed with distilled water and absolute ethanol and dried at 60 °C under vacuum oven. 2.3. Synthesis of [email protected]@[email protected] Firstly, 0.16 mL of aniline monomer was added into 200 mL GO solution (1 mg/mL) and stirred for 30 min 0.4 g (NH4)2S2O8 dissolved in 40 mL distilled water was added drop by drop to the mixture with constant stirring. The polymerization was carried out at 0 °C for 12 h and the products of GO-PANI composites were washed with distilled water and absolute ethanol. Secondly, certain amount of [email protected] nanoparticles was dispersed in GOPANI solution under stirring and diluted with water to 150 mL. Then transferred into a 200 mL Teflon-lined stainless steel autoclave and maintained in an oven at 180 °C for 12 h. After cooling down to room temperature, the final products were washed with deionized water and ethanol several times, then dried at 60 °C under vacuum oven. 2.4. Characterization The crystal structure and morphology were analyzed by X-ray diffraction (XRD) patterns (German Bruker D8 with Cu-Kα radiation), transmission electron microscopy (TEM, American FEI F30 G2). XPS analysis was characterized by X-ray photoelectron spectrometer (ESCALAB 250, Thermofisher Co). Fourier transform infrared spectroscopy (FTIR) spectra of the samples in KBr pallets was obtained using Model NIcolETiS10 fourier transform

spectrometer (Thermo SCIENTIFIC Co. USA). The magnetic properties were measured using a vibrating sample magnetometer (VSM, Lake Shore7307) with a maximum applied field of 13500 Oe. The relative complex permittivity (ε′ and ε″) and permeability (μ′ and μ″) were carried out by a HP8720ES vector network analyzer, and the measured samples were prepared by uniformly mixing 30 wt% of the sample in a paraffin matrix and pressed into toroidal shaped samples (OD 7 mm, ID 3.04 mm and height about 3 mm).

3. Results and discussion The formation mechanism of [email protected]@[email protected] is schematically depicted in in Fig. 1. Firstly, FeCo microspheres were prepared via a hydrothermal treatment at 110 °C. Secondly, [email protected] particles were obtained by oxidation of SnCl2  2H2O with H2O2. Meanwhile, [email protected] composite was synthesized via a situ polymerization at 0 °C by polymerization of aniline with (NH4)2S2O8. Finally, [email protected]@[email protected] hierarchical structures were fabricated by introducing [email protected] on [email protected] surfaces by hydrothermal process, which plays an important role in the formation of graphene. The crystal structure and phase purity of GO, RGO, FeCo, [email protected] and [email protected]@[email protected] were analyzed by XRD and the results were shown in Fig. 2. As shown in Fig. 2(a), for GO, a characteristic peak (0 0 1) appeared at 2θ ¼ 10.76° indicates that a highly oxidized GO was synthesized. For RGO, the peak at 24.4° can be ascribed to the (0 0 2) plane of RGO, and the absence of the peaks at 10.76° suggests that oxygen groups have been removed and GO is reduced to RGO. In Fig. 2(b), the diffraction peaks of FeCo at 44.56°, 65.26°, 82.32° are attributed to the (1 1 0), (2 0 0), (2 1 1) planes of cubic structure of FeCo and no detected peaks of impurities were found [23]. As for the [email protected], the XRD pattern shows new characteristic diffraction peaks, which can be assigned to SnO2. The diffraction peaks of SnO2 at 26.55°, 34.03°, 51.85° are attributed to the (1 1 0), (1 0 1), (2 1 1) planes of tetragonal rutile SnO2 (JCPDs 41-1445) [22]. It suggests that the composites were composed of FeCo and SnO2. The major diffraction peak positions of [email protected]@[email protected] composite are similar to that of [email protected], but the intensities of the FeCo peak are relatively low. In addition, no other diffraction peaks arising from graphene and PANI can be found, which is due to the weak intensity of PANI compared with [email protected] and GO has been reduced. The elemental components of the [email protected]@[email protected] composite were further confirmed by XPS measurements and the results were shown in Fig. 3. Fig. 3a shows that the quaternary composite is composed of C, Sn, O, Fe and Co elements, which is

Please cite this article as: Y. Wang, et al., Superparamagnetic [email protected] nanoparticles on graphene-polyaniline: Synthesis and enhanced electromagnetic wave absorption properties, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.038i

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Fig. 2. XRD patterns of GO, RGO (a), FeCo, [email protected] and [email protected]@[email protected] (b).

consistent with the results of experimental value. No other elemental signals are detected in the general XPS spectrum. In Fig. 3b, the C1s spectrum can be divided into four different peaks. The peaks at 284.6, 286.2, 287.4 and 289.1 eV have been assigned to the C–C/C ¼C in the aromatic rings, C–O of epoxy and alkoxy, C ¼O and O–C ¼O groups, respectively [8]. The core level binding energy at 717.8, 722.1 and 725.1 eV is the characteristic peaks of Fe 2p3/2 and Fe 2p1/2, respectively, as shown in Fig. 3c. The peaks centered 486.2 and 494.8 eV in Fig. 3d are attributed to Sn 3d5/2 and Sn 3d3/2, which accord with the data for Sn 3d in SnO2 [21]. The Co 2p3/2 (Fig. 3e) signal appears at 782.5 eV, and the peak at 790.2 eV is ascribed to the Co 2p1/2 level [24]. Fig. 3f exhibits two peaks at 530.2 eV and 531.7 eV, respectively, which are assigned to the bonding energy of O 1 s The existence of these peaks illustrates that the [email protected]@[email protected] composite is formed. Fig. 4 shows the FTIR spectra of [email protected]@[email protected] composite. The characteristic bands at 1585 cm  1 and 1475 cm  1 are attributed to the C ¼ N and C ¼ C stretching of the quinonoid and benzenoid units, respectively, the peaks at 1360 cm  1 and 1140 cm  1 are ascribed to C-N stretching [19]. The peak located at 1014 cm  1 is due to the C–O stretching vibration of the graphene [25]. Additionally, the strong and wide absorption peak at 612 cm  1 is ascribed to the stretching vibrations of metal ions and the Sn–O–Sn antisymmetric vibrations [22], which confirms the formation of [email protected]@[email protected] In order to characterize the morphology of as-prepared composites, TEM measurement was carried out and the results were presented in Fig. 5. In Fig. 5a, it can be clearly seen that the FeCo

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nanoparticles with a size of about 200 nm mainly present spherical and distribute uniformly. It can be seen from Fig. 5b that the FeCo microspheres were coated by a layer of SnO2 to form a core-shell structure. Moreover, the thickness of SnO2 shell is about 20–30 nm. With a higher magnification in Fig. 5c, the HRTEM image of [email protected] clearly shows sets of the lattice fringes, indicating the crystal nature of the nanoparticles. The crystal lattice fringes with a spacing of 0.26 nm and 0.33 nm can be assigned to the (1 0 1) plane and (1 1 0) plane of tetragonal rutile SnO2, respectively. The lattice fringe of FeCo is about 0.21 nm, corresponding to the (1 1 0) plane of FeCo, which is in good accordance with the XRD results. Furthermore, the coreshell structure of [email protected] does not destroy the structure of FeCo. As shown in Fig. 5d, we can see that [email protected] nanoparticles distribute on the surface of [email protected] sheets with a crumpled and rippled structure. The magnetic properties of FeCo, [email protected] and [email protected] @[email protected] composite were characterized by VSM measurement at room temperature, as shown in Fig. 6. In this study, FeCo possesses a saturation magnetization (MS) of 172.51 emu/g, remnant magnetization (Mr) of 18.28 emu/g and coercivity (HC) of 208.73 Oe at room temperature, which indicate its typical ferromagnetic properties. The FeCo particles have a strong magnetic interaction by dipolar interactions and magnetic moments of it would turn to the applied magnetic field direction under a strong magnetic interaction [26]. The MS value of [email protected] (36.19 emu/ g) and [email protected]@[email protected] (14.18 emu/g) composite is smaller than that of FeCo nanocrystal, which is attributed to the existence of non-magnetic RGO and PANI in the nanocomposites, and the decreasing content of magnetic particles leads to the decline in saturation magnetization. When a magnet is placed beside a bottle, the nanocomposites dispersed in ethanol quickly accumulate near the magnetic field within a few minutes, leaving the solution transparent (Fig. 6). The relative permittivity real part (ε′), permittivity imaginary part (ε″), permeability real part (m′) and permeability imaginary part (m″) of FeCo, [email protected] and [email protected]@graphene @PANI are shown in Fig. 7. As shown in Fig. 7(a) and (b), it can be found that the real part (ε′) and imaginary part (ε″) of complex permittivity for FeCo are in the range of 2.51–3.76 and 0.06–0.85, respectively. However, ε′ and ε″ values of [email protected] are in the range of 2.27–2.78 and 0.01- 1.34, respectively, which is smaller than those of FeCo. Meanwhile, it is observed that the ε′ and ε″ values of [email protected]@[email protected] are higher than FeCo and [email protected], which is due to the high electrical conductivity of PANI directly covered on the surface of RGO, suggesting that the microwave attenuation mechanism of [email protected]@[email protected] is mainly ascribed to dielectric loss. From Fig. 7c, we can observe that the m′ values of all samples exhibit complex variation in the frequency range of 2–18 GHz. The m″ values (Fig. 7d) of [email protected]@ [email protected] are smaller than those of FeCo and [email protected], which indicates a lower magnetic loss. Generally, apart from dielectric loss and magnetic loss, another important factor relating to excellent microwave absorption is dependent on the impedance match, where the characteristic impedance of the absorbing materials should be close to that of the free space (377 Ω sq  1) to achieve zero reflection on the surface of the materials [27]. Sometimes the high dielectric loss and magnetic loss are harmful to the impedance match and result in strong reflection and weak absorption [28]. The high dielectric and magnetic loss only suggest electromagnetic wave can be absorbed by materials, and the impedance matching indicates that electromagnetic wave can maximum enter material interior without being reflected by material surface. In order to investigate EM absorption properties of FeCo, [email protected] and [email protected]@[email protected] composite, the

Please cite this article as: Y. Wang, et al., Superparamagnetic [email protected] nanoparticles on graphene-polyaniline: Synthesis and enhanced electromagnetic wave absorption properties, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.038i

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Fig. 3. The general XPS (a), C 1 s (b), Fe 2p (c), Sn 3d (d), Co 2p (e) and O 1 s (f) of [email protected]@[email protected]

reflection loss (RL) values of the samples are calculated according to the transmission line theory, which is summarized as follows [29]:

Zin − 1 Zin + 1

(1)

⎞ ⎛ 2πfd μr tanh ⎜ j μ rϵr ⎟ ⎠ ⎝ c ϵr

(2)

R (dB) = 20 lg

Zin =

where Zin is the input impedance of the absorber, f is the frequency, d is the coating thickness of the absorb layer, c is the

velocity of electromagnetic wave in free space, and εr and mr are the complex relative permittivity and permeability of the absorber medium, respectively. Fig. 8 shows the RL data for the FeCo, [email protected] and [email protected]@[email protected] In Fig. 8a, it can be observed that the maximum RL of FeCo is only 1.9 dB at 14.2 GHz when its thickness is 1.5 mm, and the maximum RL values obviously enhance as the thickness of the absorber increase from 1.5 to 4 mm. In Fig. 8b, the maximum RL of [email protected] is  26.7 dB at 6.8 GHz with a thickness of 4 mm, and the absorption bandwidth exceeding  10 dB is very narrow (6.7–7 GHz). These results demonstrate that FeCo and [email protected] have a very weak ability to absorb electromagnetic wave. The RL curves of [email protected]

Please cite this article as: Y. Wang, et al., Superparamagnetic [email protected] nanoparticles on graphene-polyaniline: Synthesis and enhanced electromagnetic wave absorption properties, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.038i

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Fig. 6. Magnetization hysteresis loops of FeCo (a), [email protected] (b) and [email protected] @graphene @PANI (c).

Fig. 4. FTIR spectra of [email protected]@[email protected]

@[email protected] with different thicknesses are presented in Fig. 8c. It can be found that the best match thickness of [email protected] @[email protected] is 3 mm between 2 GHz and 18 GHz. The maximum RL of [email protected]@[email protected] is up to  39.8 dB at 6.4 GHz and the absorption bandwidth exceeding  10 dB is 3.1 GHz (from 4.6 to 7.7 GHz) with a layer thickness of 3 mm. All of the maximum RL values are less than  20 dB when the thickness is in the range of 1.5–4 mm. Moreover, the matching thickness of

the composites has an important influence on the microwave attenuation. The attenuation peaks shift to low frequency with the increasing of thickness. It can be explained as follows [30]:

d = nλm/4

λm =

f

(n = 1, 3, 5, 7, 9… …)

c ε μ

(3)

(4)

where ε and m are the complex relative permittivity and permeability of the absorber, respectively.

Fig. 5. TEM images of FeCo (a), [email protected] (b), [email protected]@[email protected] (d) and HRTEM image of [email protected] (c).

Please cite this article as: Y. Wang, et al., Superparamagnetic [email protected] nanoparticles on graphene-polyaniline: Synthesis and enhanced electromagnetic wave absorption properties, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.038i

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Y. Wang et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎

Fig. 7. The complex permittivity (a-b), the complex permeability ((c-d)) of the samples.

Fig. 8. The reflection losses of FeCo (a), [email protected] (b) and [email protected]@[email protected] (c).

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From the formula ((3) and (4)), we can conclude that the attenuation peaks shift to low frequency with increasing thickness. Compared with FeCo and [email protected], [email protected]@[email protected] composite exhibits more excellent electromagnetic absorption performance. The excellent electromagnetic absorption performance of [email protected]@[email protected] composite can be explained as follows: Firstly, high specific surface area, residual defects and groups of RGO can served as polarized centers, which increase polarization relaxation process and multiple reflections [31–33]. The migrating and hopping electron may enhance eddy current between graphene and PANI, which the electric energy coverts into heat energy. Secondly, SnO2, graphene and PANI are dielectric loss absorbers, and FeCo particles are magnetic loss absorbers; an improved impedance matching between dielectric loss and magnetic loss plays an important role in enhancing microwave absorption properties. Thirdly, the multi-interfaces between FeCo, SnO2, PANI and graphene can act as polarization centers. Meanwhile, the existence of these interfaces generates the dipole and interfacial polarization of the composites due to the synergistic effect of four kinds of materials, which are beneficial to electromagnetic absorption [34]. Lastly, FeCo particles are magnetic loss absorbents. There are dipoles present in FeCo when the sizes of FeCo are nanoscale, which can cause an increase of dipoles. Taken the above analyses, we can conclude that the [email protected]@[email protected] composite can be used as a candidate of microwave absorbers.

4. Conclusion

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

[18] [19]

In summary, the quaternary nanocomposite of [email protected] @[email protected] had been prepared via a three-step method. Compared with FeCo and [email protected], [email protected]@ [email protected] exhibits excellent microwave absorption properties in terms of both the maximum reflection loss and the absorption bandwidth. The maximum RL of it is up to  39.8 dB at 6.4 GHz with a coating layer thickness of 3 mm and the absorption bandwidth with the RL exceeding  10 dB is 3.1 GHz. Thus, it is believed that such quaternary nanocomposite with strong absorption, broad bandwidth, and lightweight can be used as an attractive candidate for applications as a microwave absorber.

[20] [21]

[22]

[23] [24]

[25]

Acknowledgements The authors acknowledge the financial support from the National Natural Science Youth Foundation of China (Grant nos. 51303147 and 21506167) and the National College Students’ Innovative Training Plan (Grant no. 201510702015).

[26]

[27]

[28]

[29]

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Please cite this article as: Y. Wang, et al., Superparamagnetic [email protected] nanoparticles on graphene-polyaniline: Synthesis and enhanced electromagnetic wave absorption properties, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.038i