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Reduced graphene oxide-Ni0.5Zn0.5Fe2O4 composite: Synthesis and electromagnetic absorption properties Meng Zong, Ying Huang, Na Zhang
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S0167-577X(15)00113-5 http://dx.doi.org/10.1016/j.matlet.2015.01.100 MLBLUE18392
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Received date: 24 December 2014 Accepted date: 21 January 2015 Cite this article as: Meng Zong, Ying Huang, Na Zhang, Reduced graphene oxide-Ni0.5Zn0.5Fe2O4 composite: Synthesis and electromagnetic absorption properties, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2015.01.100 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Reduced graphene oxide-Ni0.5Zn0.5Fe2O4 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
Abstract The reduced graphene oxide (RGO)-Ni0.5Zn0.5Fe2O4 composite was synthesized by a facile route. The morphology, microstructure and microwave electromagnetic properties of the composite were detected by means of TEM, SEM, XRD, XPS and vector network analyzer. The maximum reflection loss (RL) of the RGO-Ni0.5Zn0.5Fe2O4 reaches -47.8 dB at 10.7 GHz with the thickness of 2.8 mm, and the absorption bandwidth with RL below -10 dB is up to 11.4 GHz (from 6.6 to 18.0 GHz) with the thickness in the range of 2.0-4.0 mm. It is believed that such composite will be applied widely in microwave absorbing area. Key words: Nanocomposites; Magnetic materials; RGO; Microwave absorbing;
1. Introduction Microwave absorbing materials have attracted much attention owing to the expanded electromagnetic interference problems [1-2]. However, most of the traditional microwave absorbing materials fail to satisfy all of the requirements such as strong, wide, lightweight and thin at the same time . Hence, extensive studies have been made to develop lightweight microwave absorption materials with high efficiency and wide absorption frequency . For the past few years, it has been found that the formed heterointerface of composites can enhance microwave absorption properties . Graphene becomes potential nanoscale building blocks for new composite materials due to its special surface properties and layered structure . Recent research shows that inorganic nanoparticles (NPs), such as Fe3O4 , Co3O4 , CoFe2O4  and NiFe2O4  etc., could be attached to graphene to form composite materials, which have application in microwave absorbing field. The microwave absorption performance of the graphene composites is very *
Corresponding author. Tel.: +86 29 88431636 E-mail address: [email protected]
(Y. Huang), [email protected]
sensitive to the morphology and size of ferrites [6-9]. However, to the best of our knowledge, there has been less report on studying the microwave absorption property of RGO combined with Ni0.5Zn0.5Fe2O4. Herein, we design and synthesize composite composed of RGO and Ni0.5Zn0.5Fe2O4 NPs by a facile route. The crystalline structure, morphology and microwave electromagnetic properties of as-prepared composite are investigated. The composite exhibits excellent microwave absorption performances. It is believed that such composite could be used as a kind of candidate microwave absorber.
2. Experimental 2.1 Preparation of GO and Ni0.5Zn0.5Fe2O4 GO was prepared by Hummer's method . Ni0.5Zn0.5Fe2O4 was prepared by hydrothermal route. Ni(NO3)2•6H2O, Zn(NO3)2•6H2O and Fe(NO3)3•9H2O (Ni2+, Zn2+ and Fe3+ with a mole ratio of 0.5:0.5:2) were dispersed in 150 mL of deionized water. 1 M 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 h. The products were washed with deionized water and ethanol, then dried at 60
2.2 Preparation of RGO-Ni0.5Zn0.5Fe2O4 300 mg Ni0.5Zn0.5Fe2O4 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 . The black products were washed several times with deionized water and ethanol, then dried at 60
under vacuum. For comparison purposes, RGO was also prepared in similar procedures
in the absence of Ni0.5Zn0.5Fe2O4. 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 Kα). XPS analysis was characterized by X-ray photoelectron spectrometer (K-Alpha; Thermo Fisher Scientific (SID-Elemental), USA). Electromagnetic (EM) parameters were measured by a vector network analyzer (VNA, HP8720ES) in the range of 2-18 GHz.
3. Results and discussions
Fig.1 TEM images of GO (a), RGO (b) and RGO-Ni0.5Zn0.5Fe2O4 (c,d); HRTEM image (e) and SAED pattern (f) of RGO-Ni0.5Zn0.5Fe2O4.
Fig. 2 SEM images of RGO-Ni0.5Zn0.5Fe2O4 at different magnifications (a,b). Elemental maps of C (c), O (d), Ni (e), Zn (f) and Fe (g) in RGO-Ni0.5Zn0.5Fe2O4, respectively.
The morphology and structure of the samples were investigated by TEM and SEM, which are shown in Fig. 1 and Fig. 2. The RGO (Fig. 1b) sheets are transparent as thin films. Comparing with GO (Fig. 1a), RGO have a crumpled and rippled structure which is due to deformation upon the exfoliation and restacking process . As seen from the low magnification TEM images (Fig. 1c,d) and SEM images (Fig. 2a,b) of RGO-Ni0.5Zn0.5Fe2O4, the RGO nanosheets are loaded by Ni0.5Zn0.5Fe2O4 NPs with diameters about 10-20 nm. The HRTEM image shown in Fig. 1e also reveals the crystalline structure of the Ni0.5Zn0.5Fe2O4 NPs, and the lattice fringes with interplanar distances of 0.25 nm can be assigned to the (311) planes of the
cubic spinel crystal Ni0.5Zn0.5Fe2O4. The selected-area electron diffraction pattern (SAED, Fig. 1f) clearly shows the ring pattern arising from the cubic spinel crystal Ni0.5Zn0.5Fe2O4, further confirming the crystalline nature of Ni0.5Zn0.5Fe2O4. Importantly, the energy dispersive Spectrometer (EDS) mapping analyses of the RGO-Ni0.5Zn0.5Fe2O4 in Fig. 2c-g reveal that the Ni, Zn and Fe mapping image (Fig. 2e-g) has a distribution similar to that of C (Fig. 2c) elements, which directly confirms that the Ni0.5Zn0.5Fe2O4 NPs are well dispersed onto the surface of the RGO nanosheets.
Fig. 3 XRD patterns of GO, RGO, Ni0.5Zn0.5Fe2O4 and RGO-Ni0.5Zn0.5Fe2O4 (a); XPS spectra: wide scan (b), C 1s spectrum (c), Fe 2p spectrum (d) of RGO-Ni0.5Zn0.5Fe2O4.
Fig. 3a shows the XRD patterns of GO, RGO, Ni0.5Zn0.5Fe2O4 and RGO-Ni0.5Zn0.5Fe2O4. The diffraction peak of RGO at 25.6° can be ascribed to the graphite-like structure (002). The absence of the sharp peaks at 11.1° suggests that oxygen groups have been removed and GO is effectively reduced to RGO . For RGO-Ni0.5Zn0.5Fe2O4, seven peaks at 18.3°, 30.2°, 35.6°, 43.2°, 53.2°, 57.1° and 62.6° are observed, which are very similar to that of the pure Ni0.5Zn0.5Fe2O4, could be indexed as the characteristic (111), (220), (311), (400), (422), (511) and (440) reflections of the cubic spinel crystal structure of Ni0.5Zn0.5Fe2O4 (JCPDS no. 52-0278). The crystallite size of Ni0.5Zn0.5Fe2O4 was calculated to be 15.0 ± 0.4 nm based on the width of the XRD peak at 2θ = 35.6° by using the Scherrer’s equation . The chemical state of elements in GO and RGO-Ni0.5Zn0.5Fe2O4 were further investigated by XPS. The wide scan XPS spectrum (Fig. 3b) of RGO-Ni0.5Zn0.5Fe2O4 shows photoelectron lines at a binding energy of about 285, 532, 711, 856 and 1022 eV attributed to C 1s, O 1s, Fe 2p, Ni 2p and Zn 2p, respectively. The peaks of Ni 2p3/2 and Ni 2p1/2 are located at 856.6 eV and 874.7 eV. The Zn 2p3/2 signal appears at 1022.6 eV, and the peak at 1045.7 eV is ascribed to the Zn 2p1/2 level. The C 1s spectrum of RGO-Ni0.5Zn0.5Fe2O4 (Fig. 3c) consists of four main components, arising from C=C/C-C in the aromatic rings, C-O of epoxy and alkoxy, C=O and O-C=O groups . Comparing with GO, the C/O ratio in the RGO-Ni0.5Zn0.5Fe2O4 increases remarkably, suggesting a remarkable reduction of GO. Such a higher C/O ratio of RGO-Ni0.5Zn0.5Fe2O4 implies a good electronic conductivity, which is favorable for enhancing microwave absorbing .
Fig. 4 Frequency dependence of the complex permittivity and the complex permeability, the loss tangent and the reflection loss of Ni0.5Zn0.5Fe2O4 (a,b,c) and RGO-Ni0.5Zn0.5Fe2O4 (d,e,f).
Fig. 4 shows the complex permittivity (εr=ε'-jε''), the complex permeability (µr=µ'-jµ''), the loss tangent and the reflection loss of Ni0.5Zn0.5Fe2O4 and RGO-Ni0.5Zn0.5Fe2O4. For RGO-Ni0.5Zn0.5Fe2O4, the values of ε' and ε'' are in the range of 5.9-9.5 and 1.7-10.6, respectively, which are relatively higher than those of Ni0.5Zn0.5Fe2O4 NPs. While the µ' and µ'' maintain around 1 and 0 in the whole frequency range, respectively, similar to those of Ni0.5Zn0.5Fe2O4. For RGO-Ni0.5Zn0.5Fe2O4, the values of dielectric loss tangent (tan δE) are larger than 0.3 at almost 2-18 GHz. Meanwhile, the magnetic loss tangent (tan δM) are largely lower than tan δE at almost 2-18 GHz. These results suggest RGO-Ni0.5Zn0.5Fe2O4 has distinct dielectric loss properties, and the microwave absorption mechanism is mainly dependent on the dielectric loss. In terms of the electromagnetic theory, the dielectric loss of RGO-Ni0.5Zn0.5Fe2O4 may be attributed to the unique layered nanostructures, natural resonance, Debye dipolar relaxation and electron polarization etc. . According to the transmit-line theory, the reflection loss (RL) can be calculated by the following equations:
RL ( dB) = 20 log ( Z in − 1) /( Z in + 1)
Z in = µ r / ε r tanh[ j (2πfd / c) ε r µ r ]
where Zin is the input impedance of the absorber, µr and εr are respectively the relative complex permeability and permittivity, ƒ is the frequency of microwaves, d is the thickness of the absorber, and c is the velocity of electromagnetic waves in free space. The calculated results are shown in Fig. 4f, which indicate that the maximum RL reaches -47.8 dB at 10.7 GHz and the bandwidth corresponding to RL at -10 dB can reach 3.2 GHz (from 9.3 to 12.5 GHz) for a layer of 2.8 mm thickness. The bandwidth
corresponding to RL at -10 dB can reach 4.8 GHz (from 13.2 to 18.0 GHz) with the thickness of 2.0 mm. Additionally, the microwave absorption properties are better than Ni0.5Zn0.5Fe2O4 and other graphene composites [3,5-7,9]. The excellent microwave absorbing performance of the RGO-Ni0.5Zn0.5Fe2O4 composite are mainly attributed to two key factors: impedance matching and electromagnetic wave attenuation . In one hand, the existence of Ni0.5Zn0.5Fe2O4 NPs has lowered the εr of the RGO, and improved the equality of the εr and µr, which helps to improve the level of impedance matching. In the other hand, the RGO-Ni0.5Zn0.5Fe2O4 composite have strong electromagnetic wave attenuation, which determined by their dielectric loss and magnetic loss.
4. Conclusion In summary, the RGO-Ni0.5Zn0.5Fe2O4 was synthesized by a facile route. The maximum RL of RGO-Ni0.5Zn0.5Fe2O4 is -47.8 dB at 10.7 GHz with the thickness of 2.8 mm, and the absorption bandwidth with the RL below -10 dB is up to 4.8 GHz (from 13.2 to 18.0 GHz) with a thickness of 2.0 mm. The microwave absorption mechanism of RGO-Ni0.5Zn0.5Fe2O4 is mainly dependent on the dielectric loss. The results demonstrate that RGO plays a significant role in the microwave absorption proprieties of the RGO-Ni0.5Zn0.5Fe2O4 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 Foundation of China (2014-HT-XGD), the Spaceflight Innovation Foundation of China (No. 2011XT110002C110002).
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Figure captions Fig.1 TEM images of GO (a), RGO (b) and RGO-Ni0.5Zn0.5Fe2O4 (c,d); HRTEM image (e) and SAED pattern (f) of RGO-Ni0.5Zn0.5Fe2O4. Fig. 2 SEM images of RGO-Ni0.5Zn0.5Fe2O4 at different magnifications (a,b). Elemental maps of C (c), O (d), Ni (e), Zn (f) and Fe (g) in RGO-Ni0.5Zn0.5Fe2O4, respectively. Fig. 3 XRD patterns of GO, RGO, Ni0.5Zn0.5Fe2O4 and RGO-Ni0.5Zn0.5Fe2O4 (a); XPS spectra: wide scan (b), C 1s spectrum (c), Fe 2p spectrum (d) of RGO-Ni0.5Zn0.5Fe2O4. Fig. 4 Frequency dependence of the complex permittivity and the complex permeability, the loss tangent and the reflection loss of Ni0.5Zn0.5Fe2O4 (a,b,c) and RGO-Ni0.5Zn0.5Fe2O4 (d,e,f).