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Reduced graphene oxide-CoFe2O4 composite: Synthesis and electromagnetic absorption properties
Applied Surface Science 345 (2015) 272–278
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Reduced graphene oxide-CoFe2 O4 composite: Synthesis and electromagnetic absorption properties Meng Zong, Ying Huang ∗ , Na 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 710072, PR China
a r t i c l e
i n f o
Article history: Received 29 January 2015 Received in revised form 26 March 2015 Accepted 30 March 2015 Available online 4 April 2015 Keywords: Nanocomposites Magnetic materials RGO Microwave absorbing
a b s t r a c t The reduced graphene oxide (RGO)-CoFe2 O4 composite was synthesized by a facile route. The morphology, microstructure and microwave electromagnetic properties of the composite were detected by means of XRD, TEM, SEM, XPS TGA, VSM and vector network analyzer. The maximum reflection loss (RL ) of the RGO-CoFe2 O4 reaches −44.1 dB at 15.6 GHz with a thickness of 1.6 mm, and the absorption bandwidth with the RL below −10 dB is up to 4.7 GHz (from 13.3 to 18.0 GHz) with a thickness of only 1.5 mm. The result demonstrates that the RGO plays a significant role in the microwave absorption proprieties of the RGO-CoFe2 O4 composite. It is believed that such composite will be applied widely in microwave absorbing area. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Magnetic materials including spinel ferrites are one of the most important materials in advanced technology [1]. In recent years, spinel ferrites (MFe2 O4 , M = Fe, Co, Ni, Zn, etc.) have attracted significant attention due to their interesting magnetic, magnetoresistive, and magneto-optical properties [2]. CoFe2 O4 is a kind of important spinel ferrites. It has remarkable properties, which include a moderate saturation magnetization, excellent chemical stability, and high mechanical hardness [1]. Its magnetic properties can be tuned by the morphology, shape, and size. Microwave absorbing materials have attracted much attention because of the development of modern military and electromagnetic interference problems [3,4]. It has been reported that many materials can be employed to enhance electromagnetic absorption properties of microwave absorbing materials [5]. Rencently, a number of studies on CoFe2 O4 composites have been reported in the literatures [6–10]. Li et al. [6] prepared highly ordered mesoporous CoFe2 O4 via the two-solvent impregnation method using a mesoporous SBA-15 template. The maximum RL could reach −18 dB at about 16 GHz with a thickness of 2 mm and the corresponding absorption bandwidth was 4.5 GHz. Cao and co-workers [7] synthesized actinomorphic tubular ZnO/CoFe2 O4
∗ Corresponding author. Tel.: +86 29 88431636. E-mail addresses: nwpu [email protected] (M. Zong), [email protected] (Y. Huang). http://dx.doi.org/10.1016/j.apsusc.2015.03.203 0169-4332/© 2015 Elsevier B.V. All rights reserved.
nanocomposites via a simple solution method at low temperature and the maximum RL was −28.2 dB at 8.5 GHz. Xi et al. [8] prepared CoFe2 O4 /Co3 Fe7 -Co with the dielectric-core/metallic-shell structure and the maximum RL of the composites was −34.4 dB at 2.4 GHz with a thickness of 4 mm. Li et al. [9] fabricated hollow CoFe2 O4 -Co3 Fe7 microspheres by a two step method and the maximum RL of the composites was −41.6 dB at 9.0 GHz with a thickness of 2 mm. Chen et al. [10] demonstrated a straightforward strategy to fabricate expanded graphite/polyaniline/CoFe2 O4 . The maximum absorption of the composites reached −19.1 dB with a thickness of 0.5 mm. Carbon-based materials, as a class of promising microwave absorbing materials, exhibited several exceptional properties including lightweight, wide absorption frequency [11], high thermal stability [12–14], and high chemical stability [3]. Graphene is a novel carbon nanomaterial consisting of one-atom-thick, hexagonally arranged carbon atoms [15,16]. Owing to its special surface properties and layered structure, graphene becomes potential nanoscale building blocks for new microwave absorbing materials [17]. But its high conductivity may degrade its microwave absorption ability [18]. Herein we design and synthesize composite composed of RGO and CoFe2 O4 NPs by a facile route. The crystalline structure, morphology and microwave electromagnetic properties of asprepared composite were investigated. The composite exhibits excellent microwave absorption performances. It is believed that such composite can find applications in microwave absorbing area.
M. Zong et al. / Applied Surface Science 345 (2015) 272–278
2. Experimental 2.1. Preparation of GO and CoFe2 O4 GO was prepared by Hummer’s method [19]. CoFe2 O4 was prepared by hydrothermal route [20]. Co(NO3 )2 •6H2 O and Fe(NO3 )3 •9H2 O (Co2+ and Fe3+ with a mole ratio of 1:2) were dispersed in 150 mL of deionized water. NaOH aqueous solution was added to the suspension until the pH = 11. The solution was stirred for 10 min, followed by a hydrothermal treatment at 180 ◦ C for 10 h. The products were washed with deionized water and ethanol, then dried at 60 ◦ C under vacuum. 2.2. Preparation of RGO-CoFe2 O4 100 mg GO was dispersed in 300 mL of deionized water. 400 mg CoFe2 O4 was added to the suspension of GO. The solution was stirred for 1 h. Subsequently, freshly prepared NaBH4 aqueous solution was added dropwise with stirring and the mixture was stirred for 2 h at 80 ◦ C. The black products were washed several times with deionized water and ethanol, then dried at 60 ◦ C under vacuum. 2.3. Characterization The morphology and the size of synthesized samples were characterized by scanning electron microscope (SEM, Supra 55, German ZEISS) and transmission electron microscopy (TEM, Tecnai F30 G2, FEI, USA). The crystal structure was determined by X-ray diffraction (XRD, Rigaku, model D/max-2500 system at 40 kV and 100 mA of Cu K␣). XPS analysis was characterized by X-ray photoelectron spectrometer (K-Alpha; Thermo Fisher Scientific (SID-Elemental), USA). The thermal stabilities of the composites were analyzed by using a thermogravimetric analysis (TGA, Model Q50, TA, USA) from room temperature to 800 ◦ C in air atmosphere, with a heating rate of 20 ◦ C/min. The magnetic properties were measured by vibrating sample magnerometer (VSM, Riken Denshi, BHV-525) at room temperature. Electromagnetic (EM) parameters were measured by a vector network analyzer (VNA, HP8720ES) in the range of 2–18 GHz. 3. Results and discussions X-ray diffraction was employed to investigate the phase and structure of the synthesized samples. Fig. 1 shows the XRD patterns of GO, RGO-CoFe2 O4 and CoFe2 O4 . The XRD pattern of GO
Fig. 1. XRD patterns of GO (a), RGO-CoFe2 O4 (b) and CoFe2 O4 (c).
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shows a sharp peak at 2 = 11.1◦ , corresponding to the (001) reflection. The peaks at 2 = 18.47◦ , 30.34◦ , 35.70◦ , 43.36◦ , 53.80◦ , 57.38◦ and 62.92◦ correspond to (111), (220), (311), (400), (422), (511) and (440) planes of CoFe2 O4 , which are indexed to a single spinel structure with the characteristic reflections of the Fd3 m cubic group (JCPDS no. 22-1086) [10]. For RGO-CoFe2 O4 , all the observed peaks have been matched with CoFe2 O4 . It shows that the adding of graphene will not affect the crystalline structure in the reaction. Notably, it can be seen that except the peaks assigned to CoFe2 O4 , not any other diffraction peaks results from GO or graphene can be found, which indicates that the GO is effectively reduced into graphene and the self-restacking of the as-reduced graphene sheets is well prevented. No other peaks can be observed, indicating the high purity of RGO-CoFe2 O4 . In order to investigate the morphology of as-prepared RGOCoFe2 O4 composite, TEM and SEM-EDS images are shown in Figs. 2 and 3. As seen from the low magnification TEM image (Fig. 2a) of RGO-CoFe2 O4 , CoFe2 O4 NPs with a relatively uniform size are anchored on the surface of the RGO nanosheets. As Fig. 1b shows, CoFe2 O4 NPs can deposit on the surface of RGO nanosheets in a dense and orderly with an average diameter in the range of 10–25 nm. These CoFe2 O4 NPs are firmly attached on RGO, even sonication is applied during the preparation of TEM specimens, indicating that an excellent adhesion between RGO and CoFe2 O4 NPs is obtained [21]. To verify the crystalline structure of CoFe2 O4 NPs, HRTEM image of RGO-CoFe2 O4 is presented in Fig. 2c. The lattice fringes with interplanar distances of 0.25 nm (Fig. 2d and Fig. 2f) and 0.48 nm (Fig. 2e) can be assigned to the (311) and (111) planes of the cubic spinel crystal CoFe2 O4 . The selected-area electron diffraction pattern (SAED, Fig. 2g) clearly shows the ring pattern arising from the cubic spinel crystal CoFe2 O4 , further confirming the crystalline nature of CoFe2 O4 . As shown in Fig. 3, SEM images of RGO-CoFe2 O4 are consistent with the above TEM analysis. In Fig. 3d, RGO sheets have a crumpled and rippled structure which is due to deformation upon the exfoliation and restacking process. CoFe2 O4 NPs are anchored on the surface of RGO, as indicated by the arrows. Moreover, energy dispersive spectrometer (EDS) mapping results (elemental distribution of C, O, Fe and Co) further confirm that the CoFe2 O4 NPs were well dispersed onto the surface of the RGO nanosheets. The elemental components of GO and RGO-CoFe2 O4 were further investigated by XPS. Fig. 5a shows a general XPS profile of GO and RGO-CoFe2 O4 . The figure reveals that the composite is composed of C, O, Co and Fe elements, which is consistent with the results of the EDS elemental maps (Fig. 3). Compared GO, the C/O ratio in the RGO-CoFe2 O4 increases remarkably [22]. Such a higher C/O ratio of RGO-CoFe2 O4 implies a good electronic conductivity. The C 1s spectrum of GO (Fig. 4b) and RGO-CoFe2 O4 (Fig. 4c) consist of three main components, arising from C C/C–C in the aromatic rings, C–O of epoxy and alkoxy and C O groups [23]. It is clearly seen that the peak at 286.4 eV has almost disappeared, which confirms a considerable degree of reduction. As shown in Fig. 4d, the Co 2p3/2 signal appeared at 780.1 eV, and the peak at 795.6 eV is ascribed to the Co 2p1/2 level. The peaks of Fe 2p3/2 and Fe 2p1/2 are located at 710.8 eV and 724.6 eV, respectively (Fig. 4e). The presence of CoFe2 O4 can be further confirmed by the O 1s peak at 529.7 eV (Fig. 4f), which corresponds to the oxygen species in the CoFe2 O4 phase [24]. The other O 1s peak at 531.5 eV indicates the presence of oxygen-containing groups which bonded with C atoms in the RGO [25]. Thermogravimetric analysis curves of GO and RGO-CoFe2 O4 in an air atmosphere at a heating rate of 20 ◦ C min−1 are shown in Fig. 5. It can be noted that GO and RGO-CoFe2 O4 show a weight loss below 150 ◦ C, which is due to the evaporation of moisture and solvent residue. TG curve of GO shows two major weight losses. The first weight loss in the range of 175–240 ◦ C is due to the loss
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Fig. 2. TEM images (a,b), HRTEM image (c,d,e,f) and SAED pattern (g) of RGO-CoFe2 O4 .
of oxide groups. The second weight loss from 500 to 600 ◦ C has been attributed to the degradation of the oxidation of carbon. The residual weight is about 2.2%. For RGO-CoFe2 O4 , a gradual weight loss occurs between 150 and 450 ◦ C, which can be assigned to the removal of the labile oxygen-containing functional groups on the surfaces of RGO. The major loss from 450 to 525 ◦ C is due to the decomposition of RGO. In addition, the residual weight for RGOCoFe2 O4 is about 82.7% at 700 ◦ C. The skeleton of the graphene in the RGO-CoFe2 O4 composite decomposes at a lower temperature compared to that of the GO, which might be caused by the reaction of skeleton carbon atoms and the oxide NPs [26].
Fig. 6 shows the magnetization curves of as-prepared CoFe2 O4 NPs and RGO-CoFe2 O4 composite measured at room temperature by VSM. The saturation magnetization (Ms ), coercivity (Hc ), and remnant magnetization (Mr ) are 63.7 emu/g, 792 Oe, and 25.6 emu/g for the CoFe2 O4 NPs, and 53.6 emu/g, 768 Oe, and 25.3 emu/g for the RGO-CoFe2 O4 composite, respectively. The decrease of the Ms can be attributed to the presence of nonmagnetic graphene. Fig. 7 shows the complex permittivity (εr = ε − jε ), the complex permeability (r = − j ), the loss tangent and the RL of RGO-CoFe2 O4 and CoFe2 O4 . The real permittivity (ε ) and real
Fig. 3. SEM image of RGO-CoFe2 O4 (a) and corresponding EDS elemental maps of C (e), O (f), Fe (g) and Co (h); SEM images of RGO-CoFe2 O4 at different magnifications (b, c and d).
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Fig. 4. XPS spectra: wide scan of GO and RGO-CoFe2 O4 (a); C 1s spectrum of GO (b); C 1s spectrum (c), Co 2p spectrum (d), Fe 2p spectrum (e) and O 1s spectrum (f) of RGO-CoFe2 O4 .
permeability ( ) symbolize the storage ability of electric and magnetic energy, while the imaginary permittivity (ε ) and imaginary permeability ( ) are related to the dissipation of electric and magnetic energy [27]. For RGO-CoFe2 O4 , both ε and ε values decrease gradually with several small fluctuations in the frequency range of 2–18 GHz. The values of ε and ε are in the range of 8.7–15.8 and 0.2–10.7, respectively, which are relatively higher than those of CoFe2 O4 NPs. The values of and are in the range of 0.7–1.2 and −0.1–1.2, respectively. For CoFe2 O4 , the and maintain around 1 and 0 in the whole frequency range.
We have also calculated both the dielectric loss tangent (tan ıE = ε /ε ) and the magnetic loss tangent (tan ıM = / ) of RGO-CoFe2 O4 and CoFe2 O4 based on the permeability and permittivity of samples measured as above, shown in Fig. 7b and Fig. 7e. For RGO-CoFe2 O4 , the values of tan ıE are larger than 0.3 at almost 2–18 GHz. These results suggest that RGO-CoFe2 O4 has distinct dielectric loss properties. Compared with tan ıM , the values of tan ıE are much higher in frequencies ranging from 2 to 18 GHz. Consequently, the enhanced microwave absorption of the composite results mainly from dielectric loss rather than magnetic loss.
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Fig. 5. TG curves of GO and RGO-CoFe2 O4 .
The microwave absorbing properties of the materials can be defined by the RL . Based on the measured data of the electromagnetic parameters, the microwave absorbing properties of the obtained samples can be calculated by the following equations [1,2]:
Zin − 1 Z +1
RL (dB) = 20 log
Zin =
(1)
in
2fd √
r tanh j εr
c
εr r
(2)
where Zin is the input impedance of the absorber, r and εr are respectively the relative complex permeability and permittivity, f is the frequency of microwaves, d is the thickness of the absorber, and c is the velocity of electromagnetic waves in free space, respectively. The calculated results are shown in Fig. 7c and Fig. 7f. It can be seen that the RL values of CoFe2 O4 (Fig. 7f) are less than −4 dB with a thickness of 2–5 mm over 2–18 GHz. Compared with CoFe2 O4 , the microwave absorption properties of RGO-CoFe2 O4 are significantly enhanced and the results are presented in Fig. 7c. It is noted that there is one sharp and strong wave absorption peak at 15.6 GHz, the maximum RL is up to −44.1 dB and the bandwidths corresponding to the RL values below −10 dB are 4.4 GHz
(from 12.6 to 17.0 GHz) with a coating layer thickness of 1.6 mm. In addition, the bandwidth corresponding to RL at −10 dB can reach 4.7 GHz (from 13.3 to 18.0 GHz) with the thickness of only 1.5 mm. The microwave absorption properties of some recently reported CoFe2 O4 composites are summarized in Table 1. The above analysis and Table 1 show that the RGO-CoFe2 O4 obtained in this work are attractive candidates for the new type of microwave absorptive materials. The enhanced absorption properties of RGO-CoFe2 O4 are mainly attributed to two key factors: impedance matching and electromagnetic wave attenuation. On one hand, the introduction of CoFe2 O4 NPs has lowered the εr values of graphene, and improved the equality of εr and r , which helps to improve the level of impedance matching. As we all know, impedance matching is a key factor to enhanced absorption properties of microwave absorbing materials [28,29]. On the other hand, high loss tangent is a necessary factor to enhance microwave absorption. Firstly, the interfaces are advantageous for electromagnetic attenuation due to the existing interfacial polarization [30]. Secondly, the existence of residual defects and groups in RGO do not significantly alter the graphene lamellar structure, but they can act as polarized centers, which are in favor of the electromagnetic energy absorption [26]. Thirdly, dipole polarizations are presented in CoFe2 O4 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 [31]. Finally, the huge aspect ratio, layered structure and high conductivity of RGO sheets can enhance the dielectric loss. On the basis of the analyses above, we can draw a conclusion that the RGO plays a significant role in the microwave absorption proprieties of the RGO-CoFe2 O4 composite, which is in accordance with the following literatures [32,33]. It is well known that the magnetic loss is related to the eddy current effect and the natural resonance of the composite. For the ferromagnetic absorber, the microwave absorption properties are usually subject to degradation caused by the eddy current effect in the high-frequency region. The eddy current loss can be expressed by: ≈
20 ( )2 · d2 f 3
(3)
where and 0 are the electric conductivity and the permeability in vacuum, respectively. Thus, C0 can be described by: C0 = ( )
−2 −1
f
=
20 d2 3
(4)
If the magnetic loss results from eddy current loss, the values of C0 are constant when the frequency is varied [34]. Fig. 8 shows the C0 –f curves of the pure CoFe2 O4 NPs and the RGO-CoFe2 O4 composite. For pure CoFe2 O4 NPs, the values of C0 drastically decrease at the frequency range from 2.0 to 6.0 GHz. However, the values of C0 are almost constant in the high-frequency region (6.0–18.0 GHz). It implies that pure CoFe2 O4 NPs have significant eddy current effect in the high-frequency region. Besides, the magnetic loss in RGO-CoFe2 O4 composite is mainly caused by the natural resonance in the frequency range 2.0–6.0 GHz and 12.4–15.8 GHz. For RGO-CoFe2 O4 composite, the natural resonance can be attributed to the small size effect and the introduction of RGO. According to the natural-resonance equation [35],
Fig. 6. Magnetization curves of CoFe2 O4 and RGO-CoFe2 O4 composite at room temperature.
2fr = rHa
(5)
Ha =
(6)
4 K1
30 Ms
where r is the gyromagnetic ratio, Ha is the anisotropy energy, and |K1 | is the anisotropy coefficient. According to the Eq. (6), the anisotropy energy increased with decreasing of saturation
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Table 1 The microwave absorption properties of some recently reported CoFe2 O4 composites. Samples
Maximal RL (dB)
dm (mm)
fm (GHz)
RL < −10 dB (GHz)
References
CoFe2 O4 NPs Porous rugby-shaped CoFe2 O4 CoFe2 O4 hollow sphere/graphene Mesoporous CoFe2 O4 CoFe2 O4 /Co3 Fe7 -Co core/shell Hollow CoFe2 O4 -Co3 Fe7 RGO-CoFe2 O4 composite
−5 −34.1 −18.5 −18 −34.4 −41.6 −44.1
2.5 2.5 2.0 2.0 4.0 2.0 1.6
12.4 13.4 12.9 16.0 2.4 9.0 15.6
/ 12.3–14.9 11.3–15.0 13.5–18.0 ∼2.0–3.0 7.4–10.4 12.6–17.0
[1] [1] [2] [6] [8] [9] Present Work
Fig. 7. Frequency dependence of the complex permittivity and the complex permeability, the loss tangent and the RL of RGO-CoFe2 O4 (a,b,c) and CoFe2 O4 (d,e,f).
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Foundation of China (2014-HT-XGD), the Spaceflight Innovation Foundation of China (2014KC11023). References
Fig. 8. C0 –f curves of RGO-CoFe2 O4 composite and CoFe2 O4 NPs, inset is the expanded of the curves.
magnetization. The Ms value of RGO-CoFe2 O4 composite is much lower than that of the pure CoFe2 O4 NPs (Fig. 6). Hence, the anisotropy energy of RGO-CoFe2 O4 composite is higher than that of the pure CoFe2 O4 NPs. Besides, the anisotropy energy of small size materials, especially on the nanoscale, would be remarkably increased due to the surface anisotropic field by the small size effect [36]. The higher anisotropy energy is helpful to the improvement of microwave absorption properties [31]. 4. Conclusion In summary, the RGO-CoFe2 O4 was synthesized by a facile route. The maximum RL of the RGO- CoFe2 O4 is −44.1 dB at 15.6 GHz and the bandwidths corresponding to the RL values below −10 dB are 4.4 GHz (from 12.6 to 17.0 GHz) with a coating layer thickness of 1.6 mm. The absorption bandwidth with the RL below −10 dB is up to 12.3 GHz (from 5.7 to 18.0 GHz) with a thickness in the range of 1.5–3.5 mm. The microwave absorption mechanism of RGO-CoFe2 O4 is mainly dependent on the dielectric loss. The result demonstrates that the RGO plays a significant role in the microwave absorption proprieties of the RGO-CoFe2 O4 composite. It is believed that such composite could be used as a kind of candidate microwave absorber. Acknowledgements This work was supported by the Doctorate Foundation of Northwestern Polytechnical University (CX201430), the Spaceflight
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Reduced graphene oxide-CoFe2 O4 composite: Synthesis and electromagnetic absorption properties Meng Zong, Ying Huang ∗ , Na 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 710072, PR China
a r t i c l e
i n f o
Article history: Received 29 January 2015 Received in revised form 26 March 2015 Accepted 30 March 2015 Available online 4 April 2015 Keywords: Nanocomposites Magnetic materials RGO Microwave absorbing
a b s t r a c t The reduced graphene oxide (RGO)-CoFe2 O4 composite was synthesized by a facile route. The morphology, microstructure and microwave electromagnetic properties of the composite were detected by means of XRD, TEM, SEM, XPS TGA, VSM and vector network analyzer. The maximum reflection loss (RL ) of the RGO-CoFe2 O4 reaches −44.1 dB at 15.6 GHz with a thickness of 1.6 mm, and the absorption bandwidth with the RL below −10 dB is up to 4.7 GHz (from 13.3 to 18.0 GHz) with a thickness of only 1.5 mm. The result demonstrates that the RGO plays a significant role in the microwave absorption proprieties of the RGO-CoFe2 O4 composite. It is believed that such composite will be applied widely in microwave absorbing area. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Magnetic materials including spinel ferrites are one of the most important materials in advanced technology [1]. In recent years, spinel ferrites (MFe2 O4 , M = Fe, Co, Ni, Zn, etc.) have attracted significant attention due to their interesting magnetic, magnetoresistive, and magneto-optical properties [2]. CoFe2 O4 is a kind of important spinel ferrites. It has remarkable properties, which include a moderate saturation magnetization, excellent chemical stability, and high mechanical hardness [1]. Its magnetic properties can be tuned by the morphology, shape, and size. Microwave absorbing materials have attracted much attention because of the development of modern military and electromagnetic interference problems [3,4]. It has been reported that many materials can be employed to enhance electromagnetic absorption properties of microwave absorbing materials [5]. Rencently, a number of studies on CoFe2 O4 composites have been reported in the literatures [6–10]. Li et al. [6] prepared highly ordered mesoporous CoFe2 O4 via the two-solvent impregnation method using a mesoporous SBA-15 template. The maximum RL could reach −18 dB at about 16 GHz with a thickness of 2 mm and the corresponding absorption bandwidth was 4.5 GHz. Cao and co-workers [7] synthesized actinomorphic tubular ZnO/CoFe2 O4
∗ Corresponding author. Tel.: +86 29 88431636. E-mail addresses: nwpu [email protected] (M. Zong), [email protected] (Y. Huang). http://dx.doi.org/10.1016/j.apsusc.2015.03.203 0169-4332/© 2015 Elsevier B.V. All rights reserved.
nanocomposites via a simple solution method at low temperature and the maximum RL was −28.2 dB at 8.5 GHz. Xi et al. [8] prepared CoFe2 O4 /Co3 Fe7 -Co with the dielectric-core/metallic-shell structure and the maximum RL of the composites was −34.4 dB at 2.4 GHz with a thickness of 4 mm. Li et al. [9] fabricated hollow CoFe2 O4 -Co3 Fe7 microspheres by a two step method and the maximum RL of the composites was −41.6 dB at 9.0 GHz with a thickness of 2 mm. Chen et al. [10] demonstrated a straightforward strategy to fabricate expanded graphite/polyaniline/CoFe2 O4 . The maximum absorption of the composites reached −19.1 dB with a thickness of 0.5 mm. Carbon-based materials, as a class of promising microwave absorbing materials, exhibited several exceptional properties including lightweight, wide absorption frequency [11], high thermal stability [12–14], and high chemical stability [3]. Graphene is a novel carbon nanomaterial consisting of one-atom-thick, hexagonally arranged carbon atoms [15,16]. Owing to its special surface properties and layered structure, graphene becomes potential nanoscale building blocks for new microwave absorbing materials [17]. But its high conductivity may degrade its microwave absorption ability [18]. Herein we design and synthesize composite composed of RGO and CoFe2 O4 NPs by a facile route. The crystalline structure, morphology and microwave electromagnetic properties of asprepared composite were investigated. The composite exhibits excellent microwave absorption performances. It is believed that such composite can find applications in microwave absorbing area.
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2. Experimental 2.1. Preparation of GO and CoFe2 O4 GO was prepared by Hummer’s method [19]. CoFe2 O4 was prepared by hydrothermal route [20]. Co(NO3 )2 •6H2 O and Fe(NO3 )3 •9H2 O (Co2+ and Fe3+ with a mole ratio of 1:2) were dispersed in 150 mL of deionized water. NaOH aqueous solution was added to the suspension until the pH = 11. The solution was stirred for 10 min, followed by a hydrothermal treatment at 180 ◦ C for 10 h. The products were washed with deionized water and ethanol, then dried at 60 ◦ C under vacuum. 2.2. Preparation of RGO-CoFe2 O4 100 mg GO was dispersed in 300 mL of deionized water. 400 mg CoFe2 O4 was added to the suspension of GO. The solution was stirred for 1 h. Subsequently, freshly prepared NaBH4 aqueous solution was added dropwise with stirring and the mixture was stirred for 2 h at 80 ◦ C. The black products were washed several times with deionized water and ethanol, then dried at 60 ◦ C under vacuum. 2.3. Characterization The morphology and the size of synthesized samples were characterized by scanning electron microscope (SEM, Supra 55, German ZEISS) and transmission electron microscopy (TEM, Tecnai F30 G2, FEI, USA). The crystal structure was determined by X-ray diffraction (XRD, Rigaku, model D/max-2500 system at 40 kV and 100 mA of Cu K␣). XPS analysis was characterized by X-ray photoelectron spectrometer (K-Alpha; Thermo Fisher Scientific (SID-Elemental), USA). The thermal stabilities of the composites were analyzed by using a thermogravimetric analysis (TGA, Model Q50, TA, USA) from room temperature to 800 ◦ C in air atmosphere, with a heating rate of 20 ◦ C/min. The magnetic properties were measured by vibrating sample magnerometer (VSM, Riken Denshi, BHV-525) at room temperature. Electromagnetic (EM) parameters were measured by a vector network analyzer (VNA, HP8720ES) in the range of 2–18 GHz. 3. Results and discussions X-ray diffraction was employed to investigate the phase and structure of the synthesized samples. Fig. 1 shows the XRD patterns of GO, RGO-CoFe2 O4 and CoFe2 O4 . The XRD pattern of GO
Fig. 1. XRD patterns of GO (a), RGO-CoFe2 O4 (b) and CoFe2 O4 (c).
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shows a sharp peak at 2 = 11.1◦ , corresponding to the (001) reflection. The peaks at 2 = 18.47◦ , 30.34◦ , 35.70◦ , 43.36◦ , 53.80◦ , 57.38◦ and 62.92◦ correspond to (111), (220), (311), (400), (422), (511) and (440) planes of CoFe2 O4 , which are indexed to a single spinel structure with the characteristic reflections of the Fd3 m cubic group (JCPDS no. 22-1086) [10]. For RGO-CoFe2 O4 , all the observed peaks have been matched with CoFe2 O4 . It shows that the adding of graphene will not affect the crystalline structure in the reaction. Notably, it can be seen that except the peaks assigned to CoFe2 O4 , not any other diffraction peaks results from GO or graphene can be found, which indicates that the GO is effectively reduced into graphene and the self-restacking of the as-reduced graphene sheets is well prevented. No other peaks can be observed, indicating the high purity of RGO-CoFe2 O4 . In order to investigate the morphology of as-prepared RGOCoFe2 O4 composite, TEM and SEM-EDS images are shown in Figs. 2 and 3. As seen from the low magnification TEM image (Fig. 2a) of RGO-CoFe2 O4 , CoFe2 O4 NPs with a relatively uniform size are anchored on the surface of the RGO nanosheets. As Fig. 1b shows, CoFe2 O4 NPs can deposit on the surface of RGO nanosheets in a dense and orderly with an average diameter in the range of 10–25 nm. These CoFe2 O4 NPs are firmly attached on RGO, even sonication is applied during the preparation of TEM specimens, indicating that an excellent adhesion between RGO and CoFe2 O4 NPs is obtained [21]. To verify the crystalline structure of CoFe2 O4 NPs, HRTEM image of RGO-CoFe2 O4 is presented in Fig. 2c. The lattice fringes with interplanar distances of 0.25 nm (Fig. 2d and Fig. 2f) and 0.48 nm (Fig. 2e) can be assigned to the (311) and (111) planes of the cubic spinel crystal CoFe2 O4 . The selected-area electron diffraction pattern (SAED, Fig. 2g) clearly shows the ring pattern arising from the cubic spinel crystal CoFe2 O4 , further confirming the crystalline nature of CoFe2 O4 . As shown in Fig. 3, SEM images of RGO-CoFe2 O4 are consistent with the above TEM analysis. In Fig. 3d, RGO sheets have a crumpled and rippled structure which is due to deformation upon the exfoliation and restacking process. CoFe2 O4 NPs are anchored on the surface of RGO, as indicated by the arrows. Moreover, energy dispersive spectrometer (EDS) mapping results (elemental distribution of C, O, Fe and Co) further confirm that the CoFe2 O4 NPs were well dispersed onto the surface of the RGO nanosheets. The elemental components of GO and RGO-CoFe2 O4 were further investigated by XPS. Fig. 5a shows a general XPS profile of GO and RGO-CoFe2 O4 . The figure reveals that the composite is composed of C, O, Co and Fe elements, which is consistent with the results of the EDS elemental maps (Fig. 3). Compared GO, the C/O ratio in the RGO-CoFe2 O4 increases remarkably [22]. Such a higher C/O ratio of RGO-CoFe2 O4 implies a good electronic conductivity. The C 1s spectrum of GO (Fig. 4b) and RGO-CoFe2 O4 (Fig. 4c) consist of three main components, arising from C C/C–C in the aromatic rings, C–O of epoxy and alkoxy and C O groups [23]. It is clearly seen that the peak at 286.4 eV has almost disappeared, which confirms a considerable degree of reduction. As shown in Fig. 4d, the Co 2p3/2 signal appeared at 780.1 eV, and the peak at 795.6 eV is ascribed to the Co 2p1/2 level. The peaks of Fe 2p3/2 and Fe 2p1/2 are located at 710.8 eV and 724.6 eV, respectively (Fig. 4e). The presence of CoFe2 O4 can be further confirmed by the O 1s peak at 529.7 eV (Fig. 4f), which corresponds to the oxygen species in the CoFe2 O4 phase [24]. The other O 1s peak at 531.5 eV indicates the presence of oxygen-containing groups which bonded with C atoms in the RGO [25]. Thermogravimetric analysis curves of GO and RGO-CoFe2 O4 in an air atmosphere at a heating rate of 20 ◦ C min−1 are shown in Fig. 5. It can be noted that GO and RGO-CoFe2 O4 show a weight loss below 150 ◦ C, which is due to the evaporation of moisture and solvent residue. TG curve of GO shows two major weight losses. The first weight loss in the range of 175–240 ◦ C is due to the loss
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Fig. 2. TEM images (a,b), HRTEM image (c,d,e,f) and SAED pattern (g) of RGO-CoFe2 O4 .
of oxide groups. The second weight loss from 500 to 600 ◦ C has been attributed to the degradation of the oxidation of carbon. The residual weight is about 2.2%. For RGO-CoFe2 O4 , a gradual weight loss occurs between 150 and 450 ◦ C, which can be assigned to the removal of the labile oxygen-containing functional groups on the surfaces of RGO. The major loss from 450 to 525 ◦ C is due to the decomposition of RGO. In addition, the residual weight for RGOCoFe2 O4 is about 82.7% at 700 ◦ C. The skeleton of the graphene in the RGO-CoFe2 O4 composite decomposes at a lower temperature compared to that of the GO, which might be caused by the reaction of skeleton carbon atoms and the oxide NPs [26].
Fig. 6 shows the magnetization curves of as-prepared CoFe2 O4 NPs and RGO-CoFe2 O4 composite measured at room temperature by VSM. The saturation magnetization (Ms ), coercivity (Hc ), and remnant magnetization (Mr ) are 63.7 emu/g, 792 Oe, and 25.6 emu/g for the CoFe2 O4 NPs, and 53.6 emu/g, 768 Oe, and 25.3 emu/g for the RGO-CoFe2 O4 composite, respectively. The decrease of the Ms can be attributed to the presence of nonmagnetic graphene. Fig. 7 shows the complex permittivity (εr = ε − jε ), the complex permeability (r = − j ), the loss tangent and the RL of RGO-CoFe2 O4 and CoFe2 O4 . The real permittivity (ε ) and real
Fig. 3. SEM image of RGO-CoFe2 O4 (a) and corresponding EDS elemental maps of C (e), O (f), Fe (g) and Co (h); SEM images of RGO-CoFe2 O4 at different magnifications (b, c and d).
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Fig. 4. XPS spectra: wide scan of GO and RGO-CoFe2 O4 (a); C 1s spectrum of GO (b); C 1s spectrum (c), Co 2p spectrum (d), Fe 2p spectrum (e) and O 1s spectrum (f) of RGO-CoFe2 O4 .
permeability ( ) symbolize the storage ability of electric and magnetic energy, while the imaginary permittivity (ε ) and imaginary permeability ( ) are related to the dissipation of electric and magnetic energy [27]. For RGO-CoFe2 O4 , both ε and ε values decrease gradually with several small fluctuations in the frequency range of 2–18 GHz. The values of ε and ε are in the range of 8.7–15.8 and 0.2–10.7, respectively, which are relatively higher than those of CoFe2 O4 NPs. The values of and are in the range of 0.7–1.2 and −0.1–1.2, respectively. For CoFe2 O4 , the and maintain around 1 and 0 in the whole frequency range.
We have also calculated both the dielectric loss tangent (tan ıE = ε /ε ) and the magnetic loss tangent (tan ıM = / ) of RGO-CoFe2 O4 and CoFe2 O4 based on the permeability and permittivity of samples measured as above, shown in Fig. 7b and Fig. 7e. For RGO-CoFe2 O4 , the values of tan ıE are larger than 0.3 at almost 2–18 GHz. These results suggest that RGO-CoFe2 O4 has distinct dielectric loss properties. Compared with tan ıM , the values of tan ıE are much higher in frequencies ranging from 2 to 18 GHz. Consequently, the enhanced microwave absorption of the composite results mainly from dielectric loss rather than magnetic loss.
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Fig. 5. TG curves of GO and RGO-CoFe2 O4 .
The microwave absorbing properties of the materials can be defined by the RL . Based on the measured data of the electromagnetic parameters, the microwave absorbing properties of the obtained samples can be calculated by the following equations [1,2]:
Zin − 1 Z +1
RL (dB) = 20 log
Zin =
(1)
in
2fd √
r tanh j εr
c
εr r
(2)
where Zin is the input impedance of the absorber, r and εr are respectively the relative complex permeability and permittivity, f is the frequency of microwaves, d is the thickness of the absorber, and c is the velocity of electromagnetic waves in free space, respectively. The calculated results are shown in Fig. 7c and Fig. 7f. It can be seen that the RL values of CoFe2 O4 (Fig. 7f) are less than −4 dB with a thickness of 2–5 mm over 2–18 GHz. Compared with CoFe2 O4 , the microwave absorption properties of RGO-CoFe2 O4 are significantly enhanced and the results are presented in Fig. 7c. It is noted that there is one sharp and strong wave absorption peak at 15.6 GHz, the maximum RL is up to −44.1 dB and the bandwidths corresponding to the RL values below −10 dB are 4.4 GHz
(from 12.6 to 17.0 GHz) with a coating layer thickness of 1.6 mm. In addition, the bandwidth corresponding to RL at −10 dB can reach 4.7 GHz (from 13.3 to 18.0 GHz) with the thickness of only 1.5 mm. The microwave absorption properties of some recently reported CoFe2 O4 composites are summarized in Table 1. The above analysis and Table 1 show that the RGO-CoFe2 O4 obtained in this work are attractive candidates for the new type of microwave absorptive materials. The enhanced absorption properties of RGO-CoFe2 O4 are mainly attributed to two key factors: impedance matching and electromagnetic wave attenuation. On one hand, the introduction of CoFe2 O4 NPs has lowered the εr values of graphene, and improved the equality of εr and r , which helps to improve the level of impedance matching. As we all know, impedance matching is a key factor to enhanced absorption properties of microwave absorbing materials [28,29]. On the other hand, high loss tangent is a necessary factor to enhance microwave absorption. Firstly, the interfaces are advantageous for electromagnetic attenuation due to the existing interfacial polarization [30]. Secondly, the existence of residual defects and groups in RGO do not significantly alter the graphene lamellar structure, but they can act as polarized centers, which are in favor of the electromagnetic energy absorption [26]. Thirdly, dipole polarizations are presented in CoFe2 O4 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 [31]. Finally, the huge aspect ratio, layered structure and high conductivity of RGO sheets can enhance the dielectric loss. On the basis of the analyses above, we can draw a conclusion that the RGO plays a significant role in the microwave absorption proprieties of the RGO-CoFe2 O4 composite, which is in accordance with the following literatures [32,33]. It is well known that the magnetic loss is related to the eddy current effect and the natural resonance of the composite. For the ferromagnetic absorber, the microwave absorption properties are usually subject to degradation caused by the eddy current effect in the high-frequency region. The eddy current loss can be expressed by: ≈
20 ( )2 · d2 f 3
(3)
where and 0 are the electric conductivity and the permeability in vacuum, respectively. Thus, C0 can be described by: C0 = ( )
−2 −1
f
=
20 d2 3
(4)
If the magnetic loss results from eddy current loss, the values of C0 are constant when the frequency is varied [34]. Fig. 8 shows the C0 –f curves of the pure CoFe2 O4 NPs and the RGO-CoFe2 O4 composite. For pure CoFe2 O4 NPs, the values of C0 drastically decrease at the frequency range from 2.0 to 6.0 GHz. However, the values of C0 are almost constant in the high-frequency region (6.0–18.0 GHz). It implies that pure CoFe2 O4 NPs have significant eddy current effect in the high-frequency region. Besides, the magnetic loss in RGO-CoFe2 O4 composite is mainly caused by the natural resonance in the frequency range 2.0–6.0 GHz and 12.4–15.8 GHz. For RGO-CoFe2 O4 composite, the natural resonance can be attributed to the small size effect and the introduction of RGO. According to the natural-resonance equation [35],
Fig. 6. Magnetization curves of CoFe2 O4 and RGO-CoFe2 O4 composite at room temperature.
2fr = rHa
(5)
Ha =
(6)
4 K1
30 Ms
where r is the gyromagnetic ratio, Ha is the anisotropy energy, and |K1 | is the anisotropy coefficient. According to the Eq. (6), the anisotropy energy increased with decreasing of saturation
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Table 1 The microwave absorption properties of some recently reported CoFe2 O4 composites. Samples
Maximal RL (dB)
dm (mm)
fm (GHz)
RL < −10 dB (GHz)
References
CoFe2 O4 NPs Porous rugby-shaped CoFe2 O4 CoFe2 O4 hollow sphere/graphene Mesoporous CoFe2 O4 CoFe2 O4 /Co3 Fe7 -Co core/shell Hollow CoFe2 O4 -Co3 Fe7 RGO-CoFe2 O4 composite
−5 −34.1 −18.5 −18 −34.4 −41.6 −44.1
2.5 2.5 2.0 2.0 4.0 2.0 1.6
12.4 13.4 12.9 16.0 2.4 9.0 15.6
/ 12.3–14.9 11.3–15.0 13.5–18.0 ∼2.0–3.0 7.4–10.4 12.6–17.0
[1] [1] [2] [6] [8] [9] Present Work
Fig. 7. Frequency dependence of the complex permittivity and the complex permeability, the loss tangent and the RL of RGO-CoFe2 O4 (a,b,c) and CoFe2 O4 (d,e,f).
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Foundation of China (2014-HT-XGD), the Spaceflight Innovation Foundation of China (2014KC11023). References
Fig. 8. C0 –f curves of RGO-CoFe2 O4 composite and CoFe2 O4 NPs, inset is the expanded of the curves.
magnetization. The Ms value of RGO-CoFe2 O4 composite is much lower than that of the pure CoFe2 O4 NPs (Fig. 6). Hence, the anisotropy energy of RGO-CoFe2 O4 composite is higher than that of the pure CoFe2 O4 NPs. Besides, the anisotropy energy of small size materials, especially on the nanoscale, would be remarkably increased due to the surface anisotropic field by the small size effect [36]. The higher anisotropy energy is helpful to the improvement of microwave absorption properties [31]. 4. Conclusion In summary, the RGO-CoFe2 O4 was synthesized by a facile route. The maximum RL of the RGO- CoFe2 O4 is −44.1 dB at 15.6 GHz and the bandwidths corresponding to the RL values below −10 dB are 4.4 GHz (from 12.6 to 17.0 GHz) with a coating layer thickness of 1.6 mm. The absorption bandwidth with the RL below −10 dB is up to 12.3 GHz (from 5.7 to 18.0 GHz) with a thickness in the range of 1.5–3.5 mm. The microwave absorption mechanism of RGO-CoFe2 O4 is mainly dependent on the dielectric loss. The result demonstrates that the RGO plays a significant role in the microwave absorption proprieties of the RGO-CoFe2 O4 composite. It is believed that such composite could be used as a kind of candidate microwave absorber. Acknowledgements This work was supported by the Doctorate Foundation of Northwestern Polytechnical University (CX201430), the Spaceflight
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