CoFe2O4 nanocomposite

Applied Surface Science 357 (2015) 1289–1293

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Preparation and microwave absorption property of graphene/BaFe12 O19 /CoFe2 O4 nanocomposite Haibo Yang ∗ , Ting Ye, Ying Lin, Miao Liu School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi’an 710021, PR China

a r t i c l e

i n f o

Article history: Received 26 July 2015 Received in revised form 6 September 2015 Accepted 16 September 2015 Available online 21 September 2015 Keywords: BaFe12 O19 /CoFe2 O4 Nanocomposites Graphene Microwave absorbing property

a b s t r a c t The graphene/BaFe12 O19 /CoFe2 O4 nanocomposite powders were successfully synthesized by a deoxidation technique. The phase composition, morphology and electromagnetic properties of the nanocomposites were characterized by various instruments. The as-prepared graphene/BaFe12 O19 /CoFe2 O4 nanocomposite exhibits a saturated magnetization of 50.42 emu/g. The electromagnetic parameters of graphene/BaFe12 O19 /CoFe2 O4 nanocomposite were investigated in the 2–18 GHz range. The graphene/BaFe12 O19 /CoFe2 O4 nanocomposite exhibits better microwave absorbing properties than BaFe12 O19 /CoFe2 O4 nanocomposite. The excellent microwave absorbing property is achieved and the minimum reflection loss of the nanocomposite can reach −32.4 dB, which is obviously enhanced compared with that of the BaFe12 O19 /CoFe2 O4 nanocomposite. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Nowadays, there has been a considerable interest in microwave absorbing materials for the serious electromagnetic pollution of microwave irradiation and the development of electronic technology [1–4]. The attenuation mechanism of those materials is mainly based on dielectric loss or magnetic loss. Therefore, ferrite, carbon nanotubes or conducting polymers have been developed to attenuate electromagnetic wave [5–8]. Over the past decade, extensive studies have been made for the development of novel microwave absorption materials with high efficiency and wide absorption frequency [9]. Ferrites, as magnetic materials, have long been subjected to intensive research because of their low cost, environmental stability, high saturation magnetization and magnetic loss, but ferrite absorbents have heavier weight and lower dielectric loss which restrain them from being widely used in the field of microwave absorbing [10–12]. In order to improve the magnetic and electromagnetic absorption properties of magnetic materials, many studies have focused on new systems [13,14], such as CoFe2 O4 /ZnFe2 O4 [15], earth-iron-boron [16] and Fe/Z type ferrite [17], etc. The magnetic properties of hard/soft CoFe2 O4 /ZnFe2 O4 have been studied in literature [15]. As we all know, cobalt ferrite (CoFe2 O4 ) is a kind of typical soft ferrites, which has strong

∗ Corresponding author. E-mail address: [email protected] (H. Yang). http://dx.doi.org/10.1016/j.apsusc.2015.09.147 0169-4332/© 2015 Elsevier B.V. All rights reserved.

anisotropy, high saturation magnetization and moderate coercivity at room temperature [18,19]. Hard magnetic barium hexaferrite BaFe12 O19 is widely known due to its high electrical resistivity, low cost, excellent oxidation and corrosion resistance compared with other hard ferrites [7]. To overcome the shortcomings of ferrites in the previous, dielectric loss fillers such as conducting polymers, carbon nanotubes and graphene are always added into the system which can obtain higher absorption and broaden the frequency band. As a unique two-dimensional carbon material which is composed of sp2 -bonded carbon atoms, reduced graphene oxide (RGO) has received much attention due to its structure and good electric conductivity properties [20–22] which can remedy the defect of ferrite absorbents. In our work, the BaFe12 O19 /CoFe2 O4 nanocomposite powders were successfully synthesized by a sol–gel method, and then the BaFe12 O19 /CoFe2 O4 was deposited on the surface of graphene sheets. The samples can obtain good electrical conductivity and electromagnetic properties, which significantly increase the complex permittivity of the nanocomposite and provide the material with interesting shielding properties. 2. Experimental procedure 2.1. Preparation of graphite oxide (GO) In the first step, 2 g portion of natural flake graphite with 2 g of NaNO3 , and 96 mL of concentrated H2 SO4 were mixed at 0 ◦ C. During the following stages the mixture was continuously stirred

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using a magnet stirrer. 12 g of KMnO4 was gradually added to the above mixture for 1.5 h. After that, the solution was transferred to a 35 ◦ C water bath and stirred for about 2 h, forming a thick paste. Then 80 mL of distilled water was slowly dropped into the above paste and the resulting solution was stirred over a period of around 30 min while the temperature was raised to 90 ◦ C. Finally, 200 mL of water containing 10 mL of H2 O2 was added for 10 min until the color of the solution turned from dark brown to yellow. The graphite oxide deposit was collected from the graphite oxide suspension by high-speed centrifugation, and repeatedly washed with distilled water until the pH = 7 and then was dried at 60 ◦ C. 2.2. Preparation of BaFe12 O19 /CoFe2 O4 nanocomposite powders BaFe12 O19 /CoFe2 O4 nanocomposite powders with a mass ratio of 7:3 were synthesized by a one-step sol–gel method. The aqueous solution was prepared by dissolving Ba(NO3 )2 , Fe(NO3 )3 ·9H2 O, Co(NO3 )2 ·6H2 O, and C6 H8 O7 ·H2 O into distilled water and magnetically stirred at 80 ◦ C. Ammonia was added to the above solution to adjust the pH value to 7. After that the mixture solution was dried at 200 ◦ C for 2 h, the black floppy carbonaceous precursor powders were obtained. Finally, the precursor powders were calcined at 1100 ◦ C for 4 h to obtain the BaFe12 O19 /CoFe2 O4 nanocomposite powders.

Fig. 1. XRD patterns of (a) BaFe12 O19 /CoFe2 O4 graphene/BaFe12 O19 /CoFe2 O4 nanocomposite.

nanocomposite

and

(b)

graphene will not affect the crystalline structure in the reaction. So, it can be concluded that graphene/BaFe12 O19 /CoFe2 O4 nanocomposite has been obtained during the reaction process.

2.3. Preparation of graphene/BaFe12 O19 /CoFe2 O4 nanocomposite

3.2. Structural analysis

Graphene/BaFe12 O19 /CoFe2 O4 nanocomposite powders were prepared with the mass ratio of 1:5. In a typical step, a certain amount of graphite oxide (GO) were dispersed in glycol by ultrasonication for 4 h. And then BaFe12 O19 /CoFe2 O4 was added and continued dispersing for 1 h. After that, the mixture was reacted at 200 ◦ C for 24 h by hydrothermal method. The resulting precipitate was filtrated, washed with distilled water and ethanol repeatedly and dried under vacuum at 60 ◦ C for about 24 h.

Raman spectroscopy is also one of the most sensitive and informative techniques to characterize disorder in sp2 carbon materials. Fig. 2 presents the Raman spectrum of GO and graphene/BaFe12 O19 /CoFe2 O4 nanocomposite. There are two prominent peaks at about ∼1352 cm−1 and ∼1590 cm−1 , which correspond to the G and D peaks of graphene, respectively. The G band and D band arising from E2g phonon of carbon sp2 atoms and the A1g phonon of carbon sp3 atoms from defects and boundaries of lattice, respectively [23]. The intensity ratio of D to G band (ID /IG ) of graphene is 1.06 which is higher than that of well-crystallized graphite oxide (0.90), this can be due to the decrease in-plane sp2 domains size and a partially ordered crystal structure of the graphene.

2.4. Characterization The phase composition of the samples was detected by an X-ray diffractometer (XRD) with Cu K␣ radiation (Rigaku D/MAX-2400, Japan). The Transmission electron microscopy (TEM, American FEI Tecnai G2 F20 S-TWIN) is employed to analyze the morphology of the nanocomposite powders. The Raman spectra of the nanocomposite samples were obtained using an in Via LaserRaman spectrometer (Renishaw Co, England) with a 514 nm radiation. The magnetic hysteresis loops of the nanocomposite powders were measured by a vibrating sample magnetometer 113 (VSM) (Lake Shore 7410, USA). The electromagnetic parameters of them were analyzed by using a HP8720ES vector network analyzer and the samples were pressed to be toroidal samples with the height about 3 mm according to the mass ratio 1:1 of paraffin and nanocomposite powders.

3.3. Morphology Fig. 3(a–c) shows the TEM micrographs of graphene/ BaFe12 O19 /CoFe2 O4 nanocomposite powders. It can be seen

3. Results and discussion 3.1. XRD analysis The XRD patterns of BaFe12 O19 /CoFe2 O4 and graphene/ BaFe12 O19 /CoFe2 O4 nanocomposite are shown in Fig. 1. One can notice that the characteristic peaks for the two phases of BaFe12 O19 and CoFe2 O4 can be found in all the samples and there is no any other impurity phase can be detected. The XRD patterns are consistent with BaFe12 O19 phase (JCPDS 27-1029) and CoFe2 O4 phase (JCPDS 22-1086). The diffraction peaks can be perfectly indexed to the hexagonal magnetoplumbite structure of BaFe12 O19 and the cubic spinel structure of CoFe2 O4 . It can be found that the adding of

Fig. 2. Raman spectra of (a) GO and (b) graphene/BaFe12 O19 /CoFe2 O4 nanocomposite.

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Fig. 3. (a–c) TEM photographs and (d) HRTEM image of the graphene/BaFe12 O19 /CoFe2 O4 nanocomposite.

that the BaFe12 O19 /CoFe2 O4 grains are deposited on the surface of graphene sheets and between the layers of graphene sheets with high density and high uniformity. The detailed morphologies of graphene/BaFe12 O19 /CoFe2 O4 graphene have been further examined by HR-TEM. Fig. 3(d) shows the sets of lattice fringes of BaFe12 O19 ferrite (1 1 4) and CoFe2 O4 ferrite (3 1 1). This confirms that the BaFe12 O19 phase and CoFe2 O4 phase can coexist in the nanocomposite powders, which corresponding to the above XRD results.

3.4. Magnetic property Fig. 4 shows the magnetic hysteresis (M-H) loops of the BaFe12 O19 /CoFe2 O4 and graphene/BaFe12 O19 /CoFe2 O4 nanocomposite powders. The saturation magnetization (Ms ) of BaFe12 O19 /CoFe2 O4 and graphene/BaFe12 O19 /CoFe2 O4 is 54.6 and 38.6 emu/g, respectively. It is well known that the ferrite is shown to be a ferromagnetic phase while graphene is nonmagnetic. According to the equation Ms = ϕms [24], Ms is related to the volume fraction of the magnetic phases (ϕ) and the saturation moment (ms ) of the magnetic phase. The decrease of the saturation magnetization can be attributed to the existence of nonmagnetic graphene. Besides, the nonmagnetic graphene can isolate the magnetic particles, which results in the transformation of colinear ferromagnetic order of ferrite into non-colinear arrangement and disrupts ferrimagnetic order [25]. Therefore, the saturation magnetization of the graphene/BaFe12 O19 /CoFe2 O4 nanocomposite is lower than that of the BaFe12 O19 /CoFe2 O4 nanocomposite.

Fig. 4. Magnetic hysteresis (M-H) loops of the samples: (a) BaFe12 O19 /CoFe2 O4 nanocomposite, (b) graphene/BaFe12 O19 /CoFe2 O4 nanocomposite.

3.5. Electromagnetic property analyses Fig. 5 shows frequency dependence of the complex permittivity, complex permeability and loss tangent of dielectric/magnetic of the samples between 2 GHz and 18 GHz. It can be seen from Fig. 5(A) and (B) that the ε and ε of graphene/BaFe12 O19 /CoFe2 O4 nanocomposite increase while the  and  decrease after the introduction of graphene. Compared with BaFe12 O19 /CoFe2 O4 , the increase in ε and ε of the nanocomposites can be attributed to the improved electrical conductivity. Moreover, the addition of

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Fig. 5. (A–F) Frequency dependence of real and imaginary parts of the complex permittivity and the complex permeability of (a) BaFe12 O19 /CoFe2 O4 nanocomposite and (b) graphene/BaFe12 O19 /CoFe2 O4 nanocomposite.

graphene enlarges the interface of graphene/BaFe12 O19 /CoFe2 O4 nanocomposite which makes the dipole and interfacial polarizability of the nanocomposite enhanced in the electromagnetic field [26] and the larger dielectric loss has been obtained. On the contrary, the magnetic loss is smaller. Compared with tanı␮ , the values of tanıε are much higher in frequencies ranging from 2 to 18 GHz. Consequently, the enhanced microwave absorption of the nanocomposite results mainly from dielectric loss rather than magnetic loss. To further evaluate the microwave absorbing performance, the reflection loss [RL(dB)] of the specimens was calculated according to the following equations [27].

   Zin − 1   Z +1

RL(dB) = 20lg 

 Zin =

(1)

in



ur 2fd √ tan h j ur εr εr c

 (2)

where f is the microwave frequency, d is the thickness of the absorb layer, c is the velocity of electromagnetic wave in vacuum, and εr and r are the complex relative permittivity and permeability, respectively. The calculated reflection loss curves of the samples are shown in Fig. 6. It is clearly seen that there is nearly no broad peak with a bandwidth lower than −10 dB for BaFe12 O19 /CoFe2 O4 . However, the addition of graphene can significantly enhance the microwave absorbing abilities, an effective absorption band under −10 dB is obtained at 4.5–7.5 GHz and the minimum reflection loss is −32.4 dB. The possible reason for this is that the addition of graphene can well balance the magnetic loss and the dielectric loss, which can improve the microwave absorption performance of the graphene/BaFe12 O19 /CoFe2 O4 nanocomposite. Based on microwave absorbing principle, apart from dielectric loss and magnetic loss, another important effect on microwave absorption is the complementarities between permittivity and permeability [28]. It is believed that proper permittivity and permeability are advantageous to enhance the microwave absorbing properties. In addition, the presence of graphene reduces the

Fig. 6. Reflection loss curves of the samples: (a) BaFe12 O19 /CoFe2 O4 nanocomposite; (b) graphene/BaFe12 O19 /CoFe2 O4 nanocomposite.

magnetic-coupling effect between grains, while increases effective surface anisotropy of the grains [29]. Above these reasons make the as-prepared graphene/BaFe12 O19 /CoFe2 O4 nanocomposite have excellent microwave absorption properties. The synthesized graphene/BaFe12 O19 /CoFe2 O4 nanocomposite can be used as a kind of good microwave absorbing materials. For better comparison, the minimum reflection loss and the absorption bandwidth of some microwave absorbing composites are listed in Table 1. To our knowledge, the minimum reflection loss of the graphene/BaFe12 O19 /CoFe2 O4 nanocomposite (RL = −32.4 dB) and the absorption bandwidth (4 dB) for this work are better than those of the reported microwave absorbing composites, which implies that the as-prepared graphene/BaFe12 O19 /CoFe2 O4 nanocomposite can be potentially used in microwave absorbing field. The microwave absorption performance can be mainly related to the specimen thickness effect [32]. The calculated microwave reflection loss in 2–18 GHz for the graphene/BaFe12 O19 /CoFe2 O4

H. Yang et al. / Applied Surface Science 357 (2015) 1289–1293

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Table 1 The minimum reflection loss and absorption bandwidth of some microwave absorbing composites. Composite

[RL(dB)]min

Absorption bandwidth (GHz)

Ref.

(Z-BCF/SiO2 )@PPy PANI(PPY)-BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 BaFe12 O19 –Ni0.8 Zn0.2 Fe2 O4 /graphene Graphene/BaFe12 O19 /CoFe2 O4

−19.65 −19.7 dB (−21.5 dB) −19.63 −32.4

2 3 (3.5) 1.4 3

[30] [24] [31] This work

2013JQ6004) and the Special Foundation of the Ministry of Shaanxi Province (Grant No. 2013JK0937). References

Fig. 7. Reflection loss curves in 2–18 GHz for graphene/BaFe12 O19 /CoFe2 O4 nanocomposite with different specimen thicknesses.

nanocomposite at different specimen thicknesses is shown in Fig. 7. It is found that the peak value shifts to a lower frequency with specimen thickness increase. The reflection loss values exceeding −20 dB can be obtained by tuning the specimen thickness between 3 and 5 mm. This affirms that the thickness of the sample also plays a major role in the microwave absorption performance of the sample. 4. Conclusions In this article, the graphene/BaFe12 O19 /CoFe2 O4 nanocomposite powders with excellent microwave absorbing properties have been successfully synthesized by a deoxidation technique. Compared with the BaFe12 O19 /CoFe2 O4 nanocomposite, the microwave absorption properties of the graphene/BaFe12 O19 /CoFe2 O4 nanocomposite have been improved and the minimum reflection loss of the nanocomposite can reach to −32.4 dB. Therefore, the prepared graphene/BaFe12 O19 /CoFe2 O4 nanocomposite has potential applications in microwave absorbing field. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 51572159), the Science and Technology Foundation of Shaanxi Province (Grant Nos. 2013KJXX79,

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