Synthetic Metals 194 (2014) 52–58
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Enhanced electromagnetic wave absorption performances of Co3 O4 nanocube/reduced graphene oxide composite Xiubing Li a , Siwei Yang a , Jing Sun a , Peng He a , Xipeng Pu b , Guqiao Ding a,∗ a State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China b School of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 24 December 2013 Received in revised form 8 April 2014 Accepted 13 April 2014 Keywords: Co3 O4 Reduced graphene oxide Nanocubes Electromagnetic absorption
a b s t r a c t The Co3 O4 nanocube/reduced graphene oxide (Co3 O4 /RGO) composite paper has been ﬁrstly fabricated via a simple process. Several analytical techniques including X-ray diffraction (XRD), Raman spectroscopy, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) have been employed to characterize the Co3 O4 /RGO composite. The results indicated that the Co3 O4 nanocubes attached to the RGO sheets, and that the average edge length of Co3 O4 nanocubes is about 200 nm. The obtained composite exhibited a maximum reﬂection loss of −32.3 dB at 12.4 GHz with a coating layer thickness of 2.5 mm, and the effective absorption bandwidth with reﬂection loss less than −10 dB is up to 10.5 GHz (from 5.5 to 16.0 GHz) when an appropriate absorber thickness between 2 and 5 mm is chosen. Such high microwave absorption composite can be used as promising candidate for the new type of electromagnetic wave absorptive material. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Electromagnetic (EM) wave absorption materials in the gigahertz (GHz) range have attracted much attention because of their potential applications in electronic devices and telecommunications [1,2]. Recently, extensive efforts have been done to develop high performance EM wave absorption materials with wide absorption frequency range, small thickness, light weight and strong absorption characteristics [3–5]. According to the EM energy conversion principle, the reﬂection and attenuation characteristics of EM absorption materials mainly depend on dielectric loss and magnetic loss [6,7]. Traditional EM wave absorption materials such as magnetic metal oxides [8,9] and magnetic metal powders [10,11] produced large magnetic loss due to their high complex permeability. However, the magnetic absorbers have relatively larger densities and can only be produced with large thickness, which restrains their practical application . Therefore, it is urgent to synthesize lightweight and stable EM wave absorption materials for meeting the actual application. Carbon itself is very light, and carbon based EM wave absorption materials have high complex permittivity values. In addition, carbon materials exhibit several
∗ Corresponding author. Tel.: +86 21 62511070. E-mail addresses: [email protected]
, [email protected]
(G. Ding). http://dx.doi.org/10.1016/j.synthmet.2014.04.012 0379-6779/© 2014 Elsevier B.V. All rights reserved.
exceptional properties, including high thermal stability and light weight [13–16]. As a rising carbon material, graphene is attractive of its extraordinary thermal, electrical and mechanical properties, coming from its unique two-dimensional one-atom-thick planar sheet of sp2 hybridized carbon atoms . Due to their unique structures and superior properties, graphene-based composite materials are utilized for supercapacitors [18,19], lithium ion batteries [20,21], ﬁeld effect transistors  and sensors [23,24]. Recently, chemically reduced graphene oxide (RGO) and their composites are expected to be promising EM wave absorption materials because of their high surface area, good electric conductivity, and excellent mechanical stability [25,26]. Some reports demonstrated that the combination of RGO with magnetic particles would enhance its EM wave absorption property [27–30]. Co3 O4 is one of the most intriguing magnetic p-type semiconductors, and has found use in applications in many ﬁelds, such as heterogeneous catalysts, electrochromic devices, and solid-state sensors [31–33]. Recently, composites of Co3 O4 with graphene were successfully prepared by liquid phase and microwave-assisted methods and greatly improved electrochemical performance and gas sensing properties [34–38]. For example, Lian et al. reported an electro-static spray deposition method to synthesize a Co3 O4 -graphene composite coating was uniformly deposited on current collectors, which enables fast ion transport during the charge and discharge processes and
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accommodates the large volume expansion of Co3 O4 when Li+ inserts or extracts into its lattice . Li et al. adopted a hydrothermal method to prepare Co3 O4 intercalated RGO based thick ﬁlm semiconductor sensors. The sensor showed an enhanced response to NO2 and excellent response and recovery to methanol . Qiu and coworkers reported a hydrothermal method to synthesize the composite of Co3 O4 /RGO, and the composite exhibit excellent electrochemical performance for supercapacitors . More recently, Huang et al. fabricated polyaniline/RGO/Co3 O4 nanocomposite by a three-step method and RGO/Co3 O4 nanocomposite by hydrothermal method [39,40]. The results show that small Co3 O4 nanoparticles with the sizes in the range of 5–20 nm are anchored on the surface of polyaniline/RGO. The simulation results show that the polyaniline/RGO/Co3 O4 nanocomposite exhibits high values of reﬂection loss (<−10 dB) over a wide frequency range of 3.4–11.8 and 12.9–18 GHz and the maximum loss is −32.6 dB at 6.3 GHz with a thickness of 3 mm. For the RGO/Co3 O4 composite, the microwave adsorption properties show that the maximum reﬂection loss of RGO/Co3 O4 is up to −42.7 dB at 13.8 GHz and the absorption bandwidth with the reﬂection loss below −10 dB is 4.6 GHz with a thickness of 3.3 mm. Shi et al.  synthesized Fe3 O4 /RGO composite through a facile method, exhibiting a variety of good electrochemical characteristics. However, the EM wave absorption properties of Co3 O4 /RGO composite have not been adequately studied [39–41]. Herein, we tried to supported Co3 O4 nanocubes on the RGO sheet surface, which were expected to exhibit lightweight, wide-frequency and strong microwave absorption performance. In this work, we fabricated a free-standing Co3 O4 /RGO composite paper by simple ﬁltration combined with a hydrothermal reduction process. Several analytical techniques, including XRD, XPS, TGA, SEM and TEM, have been employed to characterize the resulting composite. Moreover, the Co3 O4 /RGO composite exhibits excellent EM absorption properties. The maximum reﬂection loss (RL) of the as-prepared composite is −32.3 dB at 12.4 GHz with a thickness of 2.5 mm, and the absorption bandwidth with the reﬂection loss below −10 dB is up to 10.5 GHz (from 5.5 to 16.0 GHz) with a thickness in the range of 2–5 mm, which demonstrate an enhanced EM absorption property and wide absorption bandwidth. 2. Materials and methods 2.1. Materials Co(CH3 COO)2 ·4H2 O was purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All chemicals were of analytical grade and used as received without further puriﬁcation. Ultrapure water (18 M cm) was used for all experiments. 2.2. Preparation of Co3 O4 /RGO composite paper Graphene oxide (GO) was synthesized by acid oxidation of natural graphite ﬂakes according to a modiﬁed Hummers’s method . GO solution was achieved by ultrasonication in anhydrous ethanol for 2 h and the concentration of the ﬁnal GO ethanol suspension was about 0.3 mg/ml. The Co3 O4 /RGO composite was synthesized via a simple process. In a typical synthesis, ﬁrstly, 0.5 ml of 0.2 mol/L Co(CH3 COO)2 aqueous solution was added to 20 ml of GO ethanol suspension. The reaction was stirred for 15 h at 80 ◦ C. Secondly, the reaction mixture was transferred to a 50 ml autoclave for hydrothermal reaction at 180 ◦ C for 3 h. Free-standing Co3 O4 /RGO composite paper was prepared by ﬁltrating the above mixture through a 0.2 m polytetraﬂuoroethylene (PTFE) membrane ﬁlter, followed by washing, air drying, and peeling off from
the ﬁlter. RGO was obtained from GO via a similar process without the presence of Co(CH3 COO)2 . 2.3. Characterizations FT-IR spectra were recorded using FT-IR NICOLET-4700 in the range 500–4000 cm−1 . X-ray diffraction (XRD) powder patterns were taken on a Rigaku X-ray diffractometer with Cu-K␣ irradiation. Raman spectra were recorded on a Thermo Fisher DXR using an Ar+ laser (wavelength 532 nm, 2 mW) with 1 m laser spot equipped with an optical microscopy. Thermogravimetric analysis (TGA) was conducted by a TA Q5000IR with a heating rate of 5 ◦ C min−1 from room temperature to 800 ◦ C under ﬂowing air. The X-ray photoelectron spectrum (XPS) was conducted on an Xray photoelectron spectrometer using an Mg-K␣ radiation exciting source (AXIS ULTRA DLD, Kratos). The atomic force microscope (AFM, SPM-9600) was employed to evaluate the morphology of GO sheet, with a special emphasis on estimating its thickness. The structure and morphology of the sample were investigated by scanning electron microscope (SEM) (FEI Sirion-F250) and transmission electron microscope (TEM) (FEI Tecnai G2 20 STWIN). The composites used for EM absorption measurements were prepared by mixing 10, 20 and 30 wt% of the Co3 O4 /RGO composite sample with a parafﬁn matrix. The mixtures were then pressed into toroidal-shaped samples (˚out = 7.00 mm and ˚in = 3.04 mm). The complex permittivity and permeability values were measured in the 1–18 GHz range with an Agilent 8722ES vector network analyzer by using the transmission/reﬂection coaxial wire method. 3. Results and discussions Fig. 1a shows the typical XRD patterns of pristine natural graphite, GO, RGO and Co3 O4 /RGO sample. The natural graphite displays a characteristic peak at 2 = 26.6◦ indicating an interlayer spacing of 0.34 nm with an index of (0 0 2). After oxidation, the characteristic graphite peak disappeared and was replaced by a well-deﬁned peak at 2 = 12.1◦ corresponds to the (0 0 1) reﬂection of GO, and the interlayer spacing (0.73 nm) is much larger than that of natural graphite. The increased d-spacing of GO is ascribed to the presence of copious oxygen-containing functional groups. For RGO, there is a characteristic broad peak of graphene at 23.9◦ , representing an interplanar spacing of 0.37 nm with an index of (0 0 2), which is slightly higher than that of natural graphite. Whereas, for Co3 O4 /RGO composite, the diffraction peak for C (0 0 2) is relatively low indicating that signiﬁcant orderedly staked graphene sheets are absent due to the introduction of Co3 O4 nanocubes on both sides of graphene sheets and the presence of a small amount of oxygen-containing functional groups. All the other characteristic (1 1 1), (2 2 0), (3 1 1), (4 0 0), (5 1 1) and (4 4 0) peaks are well observed, reﬂecting the presence of Co3 O4 phase (JCPDS card no. 43-1467) in the composite . Fig. 1b shows the Raman spectrum of GO and Co3 O4 /RGO composite. In the Raman of GO, there are two broad peaks centered at 1346 and 1598 cm−1 , assigning to sp3 (D band) and sp2 (G band) hybridization carbon atoms, respectively. The Raman spectrum of the composite also contains both peaks at about 1350 and 1594 cm−1 , which are attributed the D and G bands of RGO. The intensity ratio of the D band to the G band (ID /IG ) of RGO (0.83) is low than that of GO (0.96), which conﬁrms the reduction of GO . The signals located at approximately 189, 470, 516, and 676 cm−1 correspond to the Eg , F2g 1 , F2g 2 and A1g vibration modes of the Co3 O4 , respectively . These XRD and Raman results demonstrate the existence of both RGO and well-crystallized Co3 O4 . TGA was carried out in an air ﬂow of 50 mL/min to quantify the amount of graphene and Co3 O4 in the Co3 O4 /RGO composite. As shown in Fig. 1c, TGA-DTA curves of Co3 O4 /RGO clarify the weight ratio of
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Fig. 1. XRD patterns of natural graphite, GO, RGO and Co3 O4 /RGO composite (a), Raman spectra of GO and Co3 O4 /RGO composite (b), and (c) TGA-DTA curves of the Co3 O4 /RGO composite.
Co3 O4 nanocubes in the composite. The mass loss between 15 and 100 ◦ C and 222–340 ◦ C can be attributed to the loss of adsorbed water and the pyrolysis of graphene, respectively. According to the changes in weight, the percentage of RGO in the composite was found to be about 25%, and the residual weight corresponds to the loading of Co3 O4 nanocubes depositing onto graphene sheets of the Co3 O4 /RGO composite is estimated to be 68%. Fig. 2 shows FT-IR spectra of GO and Co3 O4 /RGO composite. For GO, the broad peak at 3407 cm−1 can be ascribed to the deformation vibration of hydroxyl groups and the stretching vibration of adsorbed water molecules. The C O H deformation of carboxyl groups was observed at 1733 cm−1 . The skeletal vibrations of C C bonds were observed around 1635 cm−1 . The peaks at 1406, 1223 and 1069 cm−1 correspond to C O H deformation,
Fig. 2. FT-IR spectra of GO and Co3 O4 /RGO composite.
C OH stretching (epoxyl groups) and C O stretching vibrations (alkoxy groups), respectively. In comparison with GO, the characteristic bonds of oxygen-based functionalities on the spectra of Co3 O4 /RGO composite have disappeared, and the intensities of all peaks are weaker. This result proves that the GO has been chemically reduced into RGO after the preparation of Co3 O4 /RGO composite. The chemical states of elements in GO and Co3 O4 /RGO composite were further provided by XPS measurements in the region of 0–1250 eV, are presented in Fig. 3. The survey spectra of GO and asprepared Co3 O4 /RGO composite are shown in Fig. 3a, and it is clear that C, O, and Co elements coexist in the as-prepared composite. The C 1s spectrum of GO can be deconvoluted into four components that correspond to carbon atoms in different functional groups: the aromatic carbon (284.6 eV), the carbon in C O bonds of epoxy and alkoxy (285.3 eV), the carbonyl carbon (287.2 eV) and the carboxylate carbon (288.7 eV). Although the C 1s spectrum of the Co3 O4 /RGO composite exhibit the same oxygen-containing functionalites, the peak intensities of all C 1s peaks of the carbon binding to oxygen are much smaller than those in GO, conﬁrming that most of the oxygen-containing functional groups are removed (Fig. 3b.) [47,48]. Compared to that of GO, the XPS spectrum of Co3 O4 /RGO composite exhibits a low O 1s peak and an additional Co 2p peaks. The Co 2p XPS spectrum of the composite exhibits two major peaks with binding energies at 780.5 and 796.3 eV, corresponding to the Co 2p3/2 and Co 2p1/2 spin-orbit peaks of Co3 O4 , respectively, and two shake-up satellite peaks located at approximately 6 eV above the main peaks, which is characteristic of a Co3 O4 phase and in good agreement with the reported data (Fig. 3c.) [49,50]. The deconvoluted O 1s spectrum in Fig. 3d displays two peaks centered at 530.0 and 531.7 eV, respectively, which can be assigned to the lattice oxygen in the Co O phase and the oxygen of the hydroxide ions .
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Fig. 3. XPS survey spectra of GO and Co3 O4 /RGO composite (a), C 1s XPS spectra of GO and Co3 O4 /RGO composite (b), Co 2p XPS spectrum of Co3 O4 /RGO composite (c), and (d) O 1s XPS spectrum of Co3 O4 /RGO composite.
The binding energy component observed at 533.2 eV is attributed to the residual oxygen-containing groups in RGO . The morphologies of the GO and Co3 O4 /RGO composite were examined by AFM, TEM and SEM. Fig. 4a and c present the AFM and TEM images of GO. The height proﬁle image clearly shows 0.83 nm thickness of pure GO sheet (Fig. 4b), suggesting that singlelayered GO is obtained . Fig. 4d and e shows the top-view SEM images of Co3 O4 /RGO composite paper. It is clearly observed that a large number of Co3 O4 nanocubes covered on the surface of the curved planar graphene sheets, forming Co3 O4 /RGO composite. The inset in Fig. 4d shows Co3 O4 /RGO paper with diameter of approximately 40 mm. As observed from a representative TEM image of the Co3 O4 /RGO composite shown in Fig. 5a, the planar graphene sheets are covered with Co3 O4 nanocubes. The average edge length of Co3 O4 nanocubes is about 200 nm and randomly located on the RGO sheet surface. A higher resolution TEM image of a single Co3 O4 nanocube is shown in Fig. 5b, revealing that the nanocube is not a singe crystal, but consists of a cluster of tiny Co3 O4 particles with a diameter about 10 nm. The SAED pattern from the area focusing on the as-prepared Co3 O4 nanocubes is illutrated in the inset of Fig. 5b, indicating the presence of bright diffraction spots along with diffraction rings. The presence of such rings further conﬁrms that each Co3 O4 nanocube is a polycrystalline. Fig. 5c shows the atomic resolution HR-TEM image of the Co3 O4 /RGO composite, focusing on the Co3 O4 nanocubes. It is clearly shown that there is an obvious interlayer distance of 0.24 nm, conﬁrming the presence of the (1 1 1) plane of the Co3 O4 fcc crystals. To understand the possible EM wave absorption mechanisms, the real (ε ) and imaginary (ε ) parts of the complex permittivity, and the real ( ) and imaginary ( ) parts of the complex
permeability have been measured for three samples composed of 10, 20, and 30 wt% Co3 O4 /RGO composite with wax in the frequency range of 1–18 GHz. It can be found that the ε of 10, 20, and 30 wt% Co3 O4 /RGO decreased gradually with increasing frequency from 3.65 to 3.50, 6.81 to 5.10, and 12.87 to 7.29, respectively, with several small ﬂuctuations over 1–18 GHz, demonstrating a frequency-dependent dielectric response (Fig. 6a). As shown in Fig. 6b, for 10, 20, and 30 wt% Co3 O4 /RGO, the values of ε are in the range of 0.27–0.47, 1.15–1.41, and 3.06–2.32, respectively. Different to the real part, for 10 and 20 wt% Co3 O4 /RGO, the ε value negligibly increases with increasing frequency in the 1–18 GHz range, and the ε value of the 30 wt% Co3 O4 /RGO ﬂuctuates between 2.5 and 4.4. The sample with higher Co3 O4 /RGO ratios shows higher values of ε and ε over the frequency range 1–18 GHz. This can be explained by the fact that the RGO sheets are a kind of conductor material, which may lead to the increasing conductivity of the Co3 O4 /RGO composite. According to the free electron theory, high conductivity would result in high permittivity [15,27]. In terms of EM theory, Co3 O4 nanocubes with a large size may be helpful in absorbing EM waves due to multidomain walls in the Co3 O4 bulk. In addition, the anisotropic energy of Co3 O4 is largely related to its morphology, and thereby, the large size Co3 O4 nanocubes are likely to have an important effect on the magnetic loss for EM waves . Such improved EM absorption characteristics of the Co3 O4 /RGO composite are mainly attributed to the shape anisotropy of Co3 O4 nanocubes and the conductivity of RGO. As presented in Fig. 6c and d, for 10, 20, and 30 wt% Co3 O4 /RGO composites, the values of are in the range of 1.11–1.05, 1.10–1.06, and 1.07–1.11, respectively, in the frequency range of 1–18 GHz and the values of for 10, 20, and 30 wt% Co3 O4 /RGO
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Fig. 4. Morphologies of GO nanosheets and Co3 O4 /RGO composite: AFM (a), the corresponding height proﬁle of the GO used (b), TEM (c) image of GO, and (d and e) SEM images of Co3 O4 /RGO composite, the inset in d is a digital photograph of Co3 O4 /RGO composite paper with diameter of approximately 40 mm.
Fig. 5. TEM images of Co3 O4 /RGO composite with low and high magniﬁcations (a and b). HR-TEM image focusing on cobalt oxide nanocube in Co3 O4 /RGO composite, showing the lattice ﬁngers (c). The inset of (b) shows SAED pattern of Co3 O4 /RGO composite focusing on cobalt oxide nanocube.
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Fig. 6. Frequency of dependence of relative complex permittivity: real part (a) and imaginary part (b), and relative complex permeability: real part (c) and imaginary part (d), of various amounts of Co3 O4 /RGO composites mixed with wax, the loss tangent (e–g) with different loadings and the reﬂection loss (h) of the Co3 O4 /RGO-wax composites with a loading of 20 wt% at different thickness from 2 to 5 mm.
composites are in the range of 0.07–(−0.01), 0.09–(−0.01), and 0.04–0.08, respectively. In additional, it should be noted that the value of is negative in part of the frequency region, which is similar to the result for pristine RGO . This is suggested that the EM wave absorption mechanism of the Co3 O4 /RGO composite
consists of both dielectric loss and magnetic loss . From Fig. 6e, f and g, for 10, 20, and 30 wt% Co3 O4 /RGO, we can see that the values of the dielectric loss (tan ıE ) are higher than magnetic loss (tan ıM ), suggesting that the reﬂection of the composite is mainly dependent on the dielectric loss .
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In order to study the EM wave absorption performance of Co3 O4 /RGO composite, based on a generalized transmission line theory, the RL values of Co3 O4 /RGO backed by a metal plate under the normal incidence of an EM ﬁeld were calculated using the relative complex permeability and permittivity data for a given frequency and absorber thickness using the following equations:
Zin = Z0
r fd tanh j 2 εr c
Zin − Z0 Z +Z
RL = 20 log
where Zin is the input impedance of the absorber, Z0 is the intrinsic impedance of free space, f is the frequency, d is the layer thickness, εr and r are the complex permittivity and permeability of the composite absorber, respectively, and c is the velocity of electromagnetic waves in free space. Fig. 6h shows the calculated theoretical RL of the Co3 O4 /RGO composites with different thickness (2–5 mm) in the range of 1–18 GHz with a loading 20 wt%. It can be clearly seen that the thickness of the absorber has a great inﬂuence on the microwave absorbing properties, and the maximum RL gradually appeared at different frequency and shifted toward lower frequency with increasing thickness. At a thickness of 2.5 mm, the minimum RL is −31.7 dB at 12.4 GHz, and the bandwidth of RL values less than −10 dB can reach up to 10.5 GHz (from 5.5 to 16.0 GHz) when an appropriate absorber thickness between 2 and 5 mm is chosen. It demonstrated that the as-prepared Co3 O4 /RGO composite shows enhanced EM wave absorption performances and wider absorption bandwidth, which can be used as an effective candidate for the new EM wave absorption material. 4. Conclusions In summary, Co3 O4 /RGO composite paper with obviously enhanced microwave absorption properties has been ﬁrstly synthesized via a simple process. The 200 nm nanocubes, consisting 10 nm Co3 O4 nanoparticles, attached to RGO sheets to form Co3 O4 /RGO composite. The composite exhibits remarkably improved electromagnetic performance, not only a larger reﬂection loss (−31.7 dB at 12.4 GHz), but also a wider absorption bandwidth (less than −10 dB from 5.5 to 16.0 GHz) have been achieved in the frequency range of 1–18 GHz. Additionally, the microwave absorption properties can be tuned easily by varying the loading mass percentage and the layer thickness of the samples. It provides a facile method to fabricate a potential kind of excellent microwave absorbing material with light weight, strong absorption and wide absorption bandwidth. Our results suggest that the composite with controllable composition and structure can be effective for microwave absorption enhancement, and may be extended to other applications, such as lithium ion batteries and supercapacitors. Acknowledgments This work was supported by projects from China Postdoctoral Science Foundation (2013M531233), the National Science and Technology Major Project (Grant No. 2011ZX02707), the National Natural Science Foundation of China (Grant Nos. 11104303, 11274333, 11204339, 61136005 and 51002069), Chinese Academy of Sciences (Grant Nos. KGZD-EW-303, XDA02040000 and XDB04010500). References  H.B. Zhang, Q. Yan, W.G. Zheng, Z.X. He, Z.Z. Yu, ACS Appl. Mater. Interfaces 3 (2011) 918–924.
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