Sandwich structures of [email protected]@PANI decorated with TiO2 nanosheets for enhanced electromagnetic wave absorption properties

Sandwich structures of [email protected]@PANI decorated with TiO2 nanosheets for enhanced electromagnetic wave absorption properties

Journal of Alloys and Compounds 662 (2016) 63e68 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://...

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Journal of Alloys and Compounds 662 (2016) 63e68

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Sandwich structures of [email protected]@PANI decorated with TiO2 nanosheets for enhanced electromagnetic wave absorption properties Panbo Liu*, Ying Huang**, Yiwen Yang, Jing Yan, Xiang Zhang Department of Applied Chemistry and The 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

a b s t r a c t

Article history: Received 16 July 2015 Received in revised form 6 November 2015 Accepted 7 December 2015 Available online 10 December 2015

The novel sandwich structures of [email protected]@PANI decorated with TiO2 nanosheets were firstly successfully prepared by simple hydrothermal method and in situ polymerization. VSM results reveal that the nanocomposites have a superparamagnetic behavior. Structure and morphology were characterized by X-ray diffraction, transmission electron microscopy, and field-emission scanning electron microscopy. Electron microscopy images show that TiO2 nanosheets are mostly grown upright on the top of [email protected]@PANI support with a random orientation, and form hierarchical structures. Electromagnetic (EM) wave absorption properties of [email protected]@[email protected] nanosheets containing 50wt% paraffin were investigated in the frequency region of 2e18 GHz. The maximum reflection loss of the nanocomposites is up to 41.8 dB at 14.4 GHz with a thickness of 1.6 mm, and the absorption bandwidth of RL < 10 dB is almost up to 3.5 GHz. Thus, the enhanced EM wave absorption properties of [email protected]@[email protected] nanosheets can be used as promising EM wave absorbers. © 2015 Elsevier B.V. All rights reserved.

Keywords: Graphene Polymers Nanocomposites Electromagnetic wave absorption properties

1. Introduction With the fast development of wireless communication technology, EM interference gradually threats the security of information and physical health [1]. A good way to overcome the problem is exploiting a type of material with high performance EM wave absorption, wide absorption bandwidth, lightweight, and small thickness [2]. Carbon materials itself is very light, and carbon-based EM wave absorption materials have high complex permittivity values. Graphene (GN), a new class of two-dimensional one-atomthick planar sheet carbon material, has attracted much attention for its unique stable structure, high specific surface area and extraordinary electrical properties [3,4]. Nevertheless, the high carrier mobility of GN is an unfavorable factor for the impedance matching mechanism [5]. One of the effective ways to solve the problem is to couple GN with magnetic particles, such as Fe3O4 [6,7], NiFe2O4 [8,9] and CoFe2O4 [10,11]. But the high weight density and large disparity of permittivity and permeability limit its progress in scalable applications [12]. Therefore, kinds of conducting polymers and dielectric materials have been explored to improve the balance

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (P. Liu), [email protected] (Y. Huang). http://dx.doi.org/10.1016/j.jallcom.2015.12.022 0925-8388/© 2015 Elsevier B.V. All rights reserved.

between permittivity and permeability [13e17]. Among many conducting polymers, polyaniline (PANI) has been considered as a promising candidate for electromagnetic wave absorbers due to its high conductivity, low density, easy preparation and good environmental stability [18,19]. Yu et al. fabricated GN/PANI nanorod arrays nanocomposites with a coating layer thickness of 2.5 mm exhibited a maximum absorption of 45.1 dB at 12.9 GHz [20]. Meanwhile, as a dielectric material, TiO2 is very attractive due to its fascinating features such as plentiful polymorphs, strong dielectric characteristics, unique thermal stability, excellent electronic and optical properties. Liu et al. prepared hierarchical [email protected] yolkshell microspheres with a thickness of 2 mm exhibited a maximum absorption of 21.2 dB as well as a bandwidth of 7.8 GHz [21]. Furthermore, besides impedance matching, the EM wave absorption properties are closely related to the structures of EM wave absorber [22]. Some reports demonstrated that excellent EM wave absorption properties can be obtained from hierarchical structures with complicated geometrical morphologies [23,24]. To the best of our knowledge, however, recent research articles mostly focus on two-dimensional graphene-based nanocomposites, and only a few papers study graphene-based hierarchical structures. Herein, the hierarchical structures consisting of [email protected]@[email protected] nanosheets via multi-step route, and the EM wave absorption properties were investigated. The results show that the

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nanocomposites exhibit enhanced EM wave absorption in terms of the maximum reflection loss (RL) value and the absorption bandwidth. The maximum reflection loss value can reach 41.8 dB at 14.4 GHz with a thickness of 1.6 mm and the absorption bandwidth of reflection loss below 10 dB is 3.5 GHz. 2. Experimental 2.1. Preparation The quaternary nanocomposites were prepared as illustrated in Fig. 1. Graphene oxide (GO) was synthesized from flake graphite according to the Hummers method [25]. The preparation of [email protected] was carried out by a solvothermal route [7]. In detail, 0.3 g GO was dissolved into 150 mL ethylene glycol followed by the addition of 3.0 g FeCl3$6H2O, 9.0 g CH3COONa and 30 mL ethylene diamine (EDA) to form a homogeneous solution through ultrasonic dispersion. The mixture was then transferred to a 200 mL Teflonlined stainless steel autoclave for solvothermal reaction at 200  C for 8 h. The final product was washed with distilled water and ethanol several times and then dried at 60  C under vacuum. [email protected]@[email protected] nanosheets were synthesized according to the literature method [5,26]. Briefly, the as-prepared [email protected] was dispersed in 300 mL HCl solution (1M), then 0.2 mL aniline was added into the above solution and stirred for 30 min at 0e5  C. Afterward, (NH4)2S2O8 (APS), the oxidation, was dissolved in 40 mL HCl solution (aniline/APS ¼ 1.5) and cooled to 0e5  C. The polymerization was performed by rapid addition of the pre-cooled oxidant solution and stirred for 24 h at 0e5  C. The resulting products were washed by deionized water and ethanol till PH ¼ 7, and then dried at 60  C. Then, the [email protected]@PANI was dispersed in 166 mL isopropanol (IPA) before 0.03 mL diethylenetriamine (DETA) was added. After stirred gently by hand, 1 mL titanium isopropoxide (TIP) was added into the solution. Subsequently, the mixture was transferred into a 250 mL Teflon-lined stainless steel autoclave and heated at 200  C for 24 h. After cooled to temperature, the white products were washed thoroughly with ethanol through magnetic decantation, and dried at 60  C under vacuum. 2.2. Characterization

magnetometer (VSM), X-ray photoelectron spectroscopy (XPS, PHI 5300X), field emission transmission electron microscopy (FETEM: Tecnai F30 G2), field emission scanning electron microscopy (FESEM: Hitachi S-5500). For the measurement of electromagnetic wave absorption properties, the samples were dispersed in paraffin homogeneously with weight ratio of 1:1 (sample/paraffin) at 70  C, and then the mixture was pressed into the toroidal shape whose inner diameter is 3.0 mm and outer diameter is 7.0 mm. The relative complex permittivity (ε0 , ε00 ) and permeability (m0 , m00 ) were carried out by the HP8753D vector network analyzer at the frequency range of 2e18 GHz.

3. Results and discussion The crystal structure was determined by XRD and the results are shown in Fig. 2. For graphene oxide (GO), with a characteristic reflection plane (001) at 2q ¼ 10.5 , compared with the peaks of [email protected], the intensities of its diffraction peak (001) were falling rapidly, which confirms the reduction of GO. For [email protected] in Fig 2b, it can be observed that six diffraction peaks at 2q ¼ 30.12 , 35.53 , 43.18 , 53.73 , 57.26 and 62.73 , corresponding to (220), (311), (400), (422), (511) and (440) planes of Fe3O4, match well with the standard pattern of Fe3O4 (JCPDS Card no.19e0629) [7]. In particular, there was no characteristic peak of GN in [email protected] nanocomposites. The reason was that the excessive amounts of Fe3O4 prevent the reduced graphene sheets stacking with each other to form crystalline structures due to the increasing coverage of Fe3O4 nanoparticles on the graphene sheets [7]. As shown in Fig. 2c and d, the main peaks of [email protected] observed in [email protected]@PANI and [email protected]@[email protected] nanosheets confirm the presence of [email protected] Fe3O4 in the nanocomposites. Furthermore, for [email protected]@PANI, the extra diffraction peaks at 15 , 20 , and 25.5 arised from PANI [27]; for [email protected]@[email protected] nanosheets, there were two extra weak peaks located at 2q ¼ 25 and 48 being attributed to (101) and (200) planes, respectively [28]. The results indicate that the pristine TiO2 nanosheets are poorly crystalline. To further analyze the surface composition of the nanocomposites, XPS spectroscopy measurement is carried out in Fig. 3. In Fig. 3a, the C1s spectrum of GO is deconvoluted into four

The obtained products were characterized by X-ray diffraction (XRD, D/max 2550V, Cu Ka radiation), vibrating sample

Fig. 1. Schematic illustration of the fabrication of the [email protected]@[email protected] nanosheets.

Fig. 2. XRD patterns of GO (a), [email protected] (b), [email protected]@PANI (c) and [email protected]@[email protected] nanosheets (d).

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Fig. 3. C 1s spectrum of GO (a), C 1s spectrum (b), survey scan (c), N 1s spectrum (d), Fe 2p spectrum (e), and Ti 2p spectrum (f) of [email protected]@[email protected] nanosheets.

different peaks. The peaks centered at 284.3, 286.4, 287.8, and 289.3 eV can be assigned to the CeC/C]C in the aromatic rings, CeO of epoxy and/or alkoxy, C]O and OeC]O groups, respectively [29]. Compared with GO, the C1s spectrum of [email protected]@[email protected] nanosheets as shown in Fig. 3b, the intensities of the carbon binding to oxygen of decreases rapidly, indicating a remarkable reduction of GO further. Meanwhile, the new peak centered at 285.3 eV can be assigned to the CeN group. The wide scan of XPS spectrum (Fig. 3c) shows the nanocomposites consists of C, N, O, Fe and Ti elements. In Fig. 3d, N 1s spectra of the nanocomposites can be deconvoluted into three peaks. The binding energy at 398.2 ev is attributed to the quinoid imine (-N ¼ ), the peak at 399.0 ev is related to the benzenoid amine (eNHe) and the peak at 400.4 eV is assigned to the cationic nitrogen atoms (-Nþ-), respectively. The observed three peaks clearly indicate the existence of PANI in the nanocomposites [30]. Fe 2p spectra is presented in Fig. 3e, in which the peaks located at 710.7 and 724.3 eV are attributed to the binding energy of Fe 2p3/2 and Fe 2p1/2, respectively. In Fig. 3f, the binding energy of Ti 2p3/2 (457.98 eV) and Ti 2p1/2 (464.08 eV) are assigned to Ti ions in TiO2. These spectra indicate the successful formation of [email protected]@[email protected] nanosheets. The morphology of [email protected]@[email protected] nanosheets is characterized by TEM and SEM in Figs. 4 and 5. Fig. 4a displays the TEM image of GN, in which can be found that the bare GN sheets appear flat and transparent, except for some wrinkles at the edges. In Fig. 4b, the Fe3O4 nanoparticles are well distributed on GN sheets with the size of about 5e10 nm, and the lattice fringe spacing (0.258 nm) is pointed out in HRTEM image (the inset in Fig. 4b). Fig. 4c shows that when introduced into PANI via in situ polymerization, the shapes of Fe3O4 nanoparticles become blurred. What's more, the introduction of PANI can protect Fe3O4 particles from oxidation to some degree. From Fig. 4d and e, we can found that the dark line demarcates TiO2 nanosheets approximately oriented perpendicular to [email protected]@PANI and the wrinkles indicating the existence of TiO2 nanosheets. In Fig. 5a, TiO2 nanosheets are mostly grown upright with a random orientation on top of the [email protected]@PANI support, which indicate that the SEM image of the

nanocomposites are consistent with above TEM analysis. Moreover, the corresponding energy-dispersive X-ray (EDX) image (Fig. 5b) confirms the presence of C, N, O, Fe, Ti elements in the nanocomposites, which is consistent with the analysis of XPS. The field-dependent magnetization of the nanocomposites was measured by VSM at room temperature. As shown in Fig. 6, both [email protected] and [email protected]@[email protected] nanosheets exhibit superparamagnetic behavior at room temperature with no coercivity and remanence. The value of Ms (saturation magnetization) decreases from 57.56 emu$g1 for [email protected] to 18.466 emu$g1 for [email protected]@[email protected] nanosheets. This decrease in magnetism is mainly attributed to the decrease in the weight ratio of Fe3O4 in the nanocomposites. In order to further study the EM wave absorption properties of the nanocomposites, the reflection loss (RL) is calculated according to transmission line theory:

  Z  1   RL ðdBÞ ¼ 20 log in Zin þ 1 Zin ¼

pffiffiffiffiffiffiffiffiffiffiffi  pffiffiffiffiffiffiffiffiffi mr =εr tanh jð2pfd=cÞ εr mr

(1)

(2)

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. According to the above equations, the calculated RL values of [email protected], [email protected]@PANI and [email protected]@[email protected] nanosheets with different thicknesses are shown in Fig. 7. In Fig. 7a, it can be observed that the maximum RL of [email protected] is only 21.2 dB at the frequency of 16.3 GHz with a layer thickness of 1.6 mm in the range of 1e4 mm. As for [email protected]@PANI in Fig. 7b, the maximum RL increases to 29.1 dB at 15.3 GHz with a layer thickness of 2 mm. As is shown in Fig. 7c, it can be found that [email protected]@[email protected] nanosheets exhibit significantly enhanced EM wave absorption compared with [email protected] and [email protected]@PANI, the maximum RL value of [email protected]@[email protected] nanosheets is 41.8 dB, and the absorption bandwidth of RL below 10 dB is

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Fig. 4. TEM images of GN (a), [email protected] (b), [email protected]@PANI (c) and [email protected]@[email protected] nanosheets (d,e).

Fig. 5. SEM image (a) and EDS spectra (b) of [email protected]@[email protected] nanosheets.

Fig. 6. Room-temperature magnetization curves of [email protected] (a) and [email protected]@[email protected] nanosheets (b).

almost up to 7.5 GHz (7.0e8.5 GHz, 10.2e16.2 GHz) with a thickness of 1.6e3 mm, which is attributed to the better impedance matching between the dielectric loss and magnetic loss, and the maximum RL values obviously shift to the lower frequency ranges as the thickness of the absorber increase from 1 to 4 mm. Therefore, [email protected]@[email protected] nanosheets can be designed to the very promising EM wave absorption materials. To understand the possible EM wave absorption mechanism, the real part (ε0 ) and imaginary part (ε00 ) of the relative complex permittivity (εr ¼ ε0-jε00 ), the real part (m0 ) and imaginary part (m00 ) of the relative complex permeability (mr ¼ m0 -jm00 ), dielectric loss tangent (tandε ¼ ε00 /ε0 ) and magnetic loss tangent (tandm ¼ m00 /m0 ) of the composites are shown in Fig. 8. As shown in Fig. 8a, the ε0 values of [email protected], [email protected]@PANI and [email protected]@[email protected] nanosheets decrease gradually from 16.8 to 6.1, 12.1 to 6.2 and 17.9 to 10.5 respectively, with several fluctuations in the 2e18 GHz ranges. Moreover, it is observed that the ε0 values of [email protected]@[email protected] nanosheets are higher than that of [email protected] and [email protected]@PANI. In Fig. 8b, the ε00 values of [email protected] are higher than [email protected]@PANI and [email protected]@[email protected] nanosheets, implying that the strong dielectric loss is responsible for EM wave absorption properties of [email protected] In Fig. 8c, we can see that the m0 values of

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Fig. 7. The reflection loss curves of [email protected] (a), [email protected]@PANI (b) and [email protected]@[email protected] nanosheets (c).

Fig. 8. The ε0 (a), ε00 (b), m0 (c), m00 (d), tandε (e) and tandm (f) of [email protected] (a), [email protected]@PANI (b) and [email protected]@[email protected] nanosheets (c).

all samples exhibit complex variation over 2e18 GHz frequency range. From Fig. 8d, it is observed that the m00 values of [email protected]@[email protected] nanosheets are slightly larger than [email protected] and [email protected]@PANI, the slightly larger m00 values indicate a higher magnetic loss. From Fig. 8e and f, we can see that the tandε values of [email protected] and [email protected]@PANI are higher than [email protected]@[email protected] nanosheets, while the tandm values of [email protected]@[email protected] nanosheets are higher than [email protected] and [email protected]@PANI in the frequency range of 2e18 GHz, indicating an improved impedance matching between dielectric loss and magnetic loss. The enhanced absorption properties of [email protected]@[email protected] nanosheets can be explained by the following facts: (1) the improved impedance matching of the nanocomposites. As is well known, the EM absorption strongly depends on the complementarities between the dielectric loss and the magnetic loss. For the [email protected]@[email protected] nanosheets nanocomposites, the Fe3O4 nanoparticles are used as magnetic loss absorbers and the PANI, TiO2 nanosheets is used as the dielectric loss absorbers. The GN sheets serve as an ideal substrate for the deposition of the nanosheets [31,32]. (2) The extra interfacial polarization induced by

hierarchical structures. The multi-interfaces and triple junctions ([email protected], [email protected], and [email protected]) result in the interfacial polarization which is advantageous for attenuation of EM waves [33]. (3) TiO2 nanosheets and the void space existing between Fe3O4 and TiO2 nanosheets result in relatively large specific surfaces areas and high porosities, providing more active sites for reflection and scattering of EM waves interrupt the spread of electromagnetic waves effectively. In addition, the multi-interfaces and triple junctions ([email protected], [email protected], and [email protected]) result in the interfacial polarization, which is advantageous for attenuation of EM waves [34]. 4. Conclusion In summary, the superparamagnetic [email protected]@[email protected] nanosheets with hierarchical structures can be successfully synthesized by the present method. All the Fe3O4 nanoparticles and TiO2 nanosheets are homogeneously coated on the GN sheets and PANI respectively, without significant numbers of vacancies or apparent aggregation. When evaluated as EM wave absorbers, the

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nanocomposites exhibit enhanced EM wave absorption properties in terms of the maximum reflection loss value and the absorption bandwidth. The maximum reflection loss of the nanocomposites is 41.8 dB at 14.4 GHz with a thickness of only 1.6 mm, and the absorption bandwidth of RL < 10 dB is almost up to 3.5 GHz. It indicates that most of the EM wave energy could be attenuated by the nanocomposites. Thus, the [email protected]@[email protected] nanosheets nanocomposites are very promising for application in lightweight, strong absorption, as well as broad absorption frequency bandwidth EM absorbers. Acknowledgments This work was supported by the Space flight Foundation of China (No. 2011XW11000C110001) and the Space flight Innovation Foundation of China (No. 2011XT110002C110002). References [1] P. Saini, V. Choudhary, N. Vijayan, R.K. Kotnala, J. Phys. Chem. C 116 (2012) 13403e13412. [2] X. Bai, Y. Zhai, H.Y. Zhang, J. Phys. Chem. C 115 (2011) 11673e11677. [3] C. Wang, X.J. Han, P. Xu, X.L. Zhang, Y.C. Du, S.R. Hu, J.Y. Wang, X.H. Wang, Appl. Phys. Lett. 98 (2011) 072906. [4] X.B. Li, S.W. Yang, J. Sun, P. He, X.P. Pu, G.Q. Ding, Synth. Met. 194 (2014) 52e58. [5] L. Wang, Y. Huang, C. Li, J.J. Chen, X. Sun, Synth. Met. 198 (2014) 300e307. [6] T.S. Wang, Z.H. Liu, M.M. Lu, B. Wen, Q.Y. Ouyang, Y.J. Chen, J. Appl. Phys. 113 (2013) 024314. [7] X. Sun, J.P. He, G.X. Li, J. Tang, T. Wang, Y.X. Guo, H.R. Xue, J. Mater. Chem. C 1 (2013) 765e777. [8] M. Fu, Q.Z. Jiao, Y.J. Zhao, Mater. Chem. A 1 (2013) 5577e5586. [9] Z. Wang, X. Zhang, Y. Li, J. Mater. Chem. A 1 (2013) 6393e6399. [10] P. Xiong, Q. Chen, M.Y. He, X.Q. Sun, X. Wang, J. Mater. Chem. 22 (2012) 17485e17493. [11] M. Zong, Y. Huang, H.W. Wu, Y. Zhao, Q.F. Wang, X. Sun, Mater. Lett. 114 (2014) 52e55. [12] Z. Wang, L. Wu, J. Zhou, B. Shen, Z. Jiang, RSC Adv. 3 (2013) 3309e3315. [13] X. Bai, Y. Zhai, Y. Zhang, J. Phys. Chem. C 115 (2011) 11673e11677.

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