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Preparation of pod-like 3D Ni0.33Co0.67Fe2O4@rGO composites and their microwave absorbing properties
Author’s Accepted Manuscript Preparation of pod-like 3D Ni0.33Co0.67Fe2O4@rGO composites and their microwave absorbing properties Meimei Gao, Yun Zhao, Shanshan Wang, Yingchun Xu, Caihong Feng, Daxin Shi, Qingze Jiao www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(18)33621-6 https://doi.org/10.1016/j.ceramint.2018.12.226 CERI20452
To appear in: Ceramics International Received date: 13 November 2018 Revised date: 21 December 2018 Accepted date: 31 December 2018 Cite this article as: Meimei Gao, Yun Zhao, Shanshan Wang, Yingchun Xu, Caihong Feng, Daxin Shi and Qingze Jiao, Preparation of pod-like 3D Ni0.33Co0.67Fe2O4@rGO composites and their microwave absorbing properties, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.12.226 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.
Preparation of pod-like 3D Ni0.33Co0.67Fe2O4@rGO composites and their microwave absorbing properties Meimei Gaoa, Yun Zhaoa*, Shanshan Wanga, Yingchun Xua, Caihong Fenga, Daxin Shia and Qingze Jiaoa,b*. a
School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China
b
School of Materials and the Environment, Beijing Institute of Technology, Zhuhai, Zhuhai 519085, P. R. China *Corresponding author. Email: [email protected]; [email protected]
Abstract The ferrite/reduced graphene oxide (rGO) composites have attracted increasing attention due to the combination of the dielectric loss of rGO and the magnetic loss of ferrites. In this paper, pod-like 3D Ni0.33Co0.67Fe2O4@rGO composites were prepared using a solvothermal reaction followed by cold quenching. The structures and morphologies of as obtained composites were characterized using X-ray diffractometer, Raman microscope, photoelectron spectroscopy, scanning electron microscope
and
transmission
electron
microscope.
The
Ni0.33Co0.67Fe2O4
microspheres with a diameter of 100-150 nm were wrapped in rGO rolls due to the shrinkage of rGO in liquid nitrogen. The rGO sheets with ferrite microspheres wrapped in form the pod-like 3D network morphology. The minimum reflection loss 1
of as-prepared composites reaches -47.5 dB and the absorption bandwidth (RL<-10 dB) is 5.02 GHz. The composites show much better absorbing performances than pure Ni0.33Co0.67Fe2O4 microspheres and Ni0.33Co0.67Fe2O4-rGO mixture formed by mechanically blending of cold quenched pure rGO and ferrite microspheres. Keyword: D. Ferrites; E. Functional applications; Reduced graphene oxide; Electromagnetic wave absorption mechanism
1. Introduction In recent years, accompanying the rapid development of information technology and extensive use of electronic devices, the issue of electromagnetic interference and pollution has become more and more serious [1, 2]. It is paramount to design and develop microwave absorbing materials with thin thickness, wide bandwidth, lightweight, and strong absorbing ability. Graphene, a monolayer of carbon atoms patterned in close-packed hexagonal lattices, has been considered to be a revolutionary material of the future due to its high electrical and thermal conductivity, large specific surface aera and excellent mechanical strength [3-6]. In the past few years, considerable efforts have been expended in the fabrication of graphene-based composites which integrate the excellent performance of graphene with other functional nanomaterials thereby broaden the application field of graphene materials [7]. The application of graphene-based composites is an excellent choice for electromagnetic wave 2
absorption field. In recent years, magnetic materials have been incorporated into graphene to improve electromagnetic wave absorption in view of the principle of impedance matching. Tsung-Yung Wu et al. [8] synthesized Fe3O4 nanoparticle/graphene hybrids with the minimum reflection loss of -22 dB at thicknesses of 4 and 5 mm through a single-step microwave-assisted solvothermal method. Shu R et al. [9] fabricated reduced graphene oxide/zinc ferrite (rGO/ZnFe2O4) hybrid nanocomposites by a facile solvothermal route and they exhibited excellent microwave absorption performance with the minimum reflection loss of -41.1 dB for a thickness of 2.5 mm and effective absorption bandwidth (less than -10 dB) of 3.2 GHz for a thickness of 2.0 mm. Liu Z et al. [10] successfully synthesized CoFe2O4/rGO nanocomposites via Cu (I)-catalyzed click reaction and the minimum reflection loss of CoFe2O4/rGO nanocomposites could reach -40 dB at 6.8 GHz with a matching thickness of 4.0 mm, and also it had a bandwidth below -10 dB ranging from 5.8 to 8.5 GHz. In fact, not only does the composition of the materials affect performances, but the morphology and structure of the composites has a significant impact on their performances. Min Fu et al. [11] reported that the absorbing performance of NiFe2O4 nanorod/graphene composites prepared through a one-step hydrothermal process was better than that of the NiFe2O4 nanoparticle-graphene composites. Especially, three-dimensional (3D) graphene architectures have shown excellent absorbing properites due to high specific surface area and excellent mechanical properties [12]. Li C et al. [13] synthesized porous Fe3O4/C microspheres with 3D interconnected porous structure via a facile 3
hydrothermal process, and a minimum reflection loss value of -31.75 dB was achieved at an absorber thickness of 3.0 mm. Herein, we synthesized pod-like 3D Ni0.33Co0.67Fe2O4@rGO composites by combination of a solvothermal reaction and cold quenching. By cold quenching ferrite microspheres were well coated with rGO sheets to form a 3D network structure. Structural, morphological and microwave absorbing properties of as-obtained composites were investigated. 2. Experimental 2.1. Materials Graphene oxide (GO) was synthesized by the Hummers method [14]. The raw materials used for the synthesis of Ni0.33Co0.67Fe2O4 were FeCl3·6H2O, NiCl2·6H2O, CoCl2·6H2O, urea and PVP (polyvinyl pyrrolidone). All chemicals and reagents for experiments and analysis were purchased from Beijing Chemicals and of analytical grade and used without further purification. 2.2. Preparation of Ni0.33Co0.67Fe2O4 microspheres The metal chloride salts were dissolved in ethylene glycol at a molar ratio of 4:1:1 for Fe: Ni: Co. After magnetic stirring for 1 h, appropriate amounts of urea and PVP were added, and stirring was continued for 1 h to form homogeneous suspension. The above suspension was sealed in the teflon lined stainlessteel autoclave and maintained at 180 °C for 20 h. Later the autoclave was naturally cooled to room temperature. The black precipitate was collected, washed with ethanol and deionized water respectively and then dried in a vacuum oven at 60 °C for 12 h. 4
2.3. Preparation of Ni0.33Co0.67Fe2O4@rGO composites 50 mg Ni0.33Co0.67Fe2O4 microspheres were added into a 20 mL GO suspension (0.5 mg/mL) to form a suspension. After 30 min of ultrasonication, the suspension was
dripped
into
liquid
nitrogen
with
vigorous
stirring.
Pod-like
Ni0.33Co0.67Fe2O4@GO composites was prepared due to the shrinkage of GO sheets in liquid nitrogen. They were then freeze-dried for 24 h. Finally, pod-like Ni0.33Co0.67Fe2O4@rGO composites were obtained by thermal-reduction at 500 ° C in an argon atmosphere for 30 min. RGO was prepared using the same process without the introduction of Ni0.33Co0.67Fe2O4 microspheres. 2.4. Characterization The morphology of the samples was observed and analyzed using a Hitachi JSM-7500F Field Scanning Electron Microscope. TEM observations were performed with a Hitachi-800 transmission electron microscope and a JEOL high-resolution transmission electron microscope (JEM-2100) using an operating voltage of 120 kV. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku multi-function X-ray diffractometer (Ultima IV) at 40 KV and 150 mA with the Cu-Kα radiation. XRD data were collected between 5 and 80 º at a scan rate of 20 º/min. Raman spectroscopy was carried out on a Raman microscope (LabRam HR Evolution). 5
The chemical states of samples were obtained by a Thermo Scientific X-ray photoelectron spectroscopy (XPS, ESCALAB 250 XI) The magnetic properties were measured using a Vibrating sample magnetometer (VSM, Squid-VSM). Thermogravimetric analysis (TGA) was performed using a thermogravimetric analyzer (Q5000) from 50 ºC to 700 ºC in air at a heating rate of 10 ºC min-1. The complex permittivity and complex permeability of the composites were measured using an Agilent PNA-X Microwave Network Analyzer (PNA-N5244A) in the 1–18 GHz frequency range. For measurement, the composites were mixed with paraffin at a ratio of 20 wt% and compressed to a block with an outer diameter of 7 mm, an inner diameter of 3 mm, and a thickness of 2 mm. 3. Results and discussion The XRD patterns of the Ni0.33Co0.67Fe2O4, Ni0.33Co0.67Fe2O4@rGO composites, cold quenched pure rGO and GO are shown in Fig.1. The diffraction peaks of Ni0.33Co0.67Fe2O4 can match well with the crystal planes of (220), (311), (222), (400), (422), (511) and (440), respectively, and the relative intensities and purity phases indicate the high purity of the Ni0.33Co0.67Fe2O4 with very good crystallinity [10, 11]. The peak intensity of Ni0.33Co0.67Fe2O4@rGO composites has a quite decrease due to the introduction of rGO. Moreover, there is a weak peak at about 2θ=25° indicating rGO in the spectrum of Ni0.33Co0.67Fe2O4@rGO composites, and the characteristic peak of GO disappears through thermal reduction. Fig.2a is a SEM image of as-synthesized Ni0.33Co0.67Fe2O4 microspheres. It can be 6
clearly observed that the size distribution of Ni0.33Co0.67Fe2O4 is in the range of 100-150 nm with the shape of sphere. Since the particle size is small, agglomeration occurs. As shown in Fig.2b, the sheet structure of rGO can be clearly seen and the graphene sheets have a tendency to shrink due to the cold quench in liquid nitrogen. In case of Ni0.33Co0.67Fe2O4@rGO composites (Fig.2c and d), the rGO sheets with ferrite microspheres wrapped in form the pod-like 3D network morphology. In order to have a further observation of morphologies and microstructures of the Ni0.33Co0.67Fe2O4@rGO composites, TEM measurements were carried out shown in Fig.3. It can be further observed from Fig.3a that the Ni0.33Co0.67Fe2O4 microspheres are assembled by nanoparticles of about 100-150 nm. Fig.3b shows a partly rolled up rGO sheet with many folds. Fig.2d further demonstrates that Ni0.33Co0.67Fe2O4 microspheres are wrapped in rGO sheets. As shown in Fig.3c, compared to GO, the lattice spacing of rGO for Ni0.33Co0.67Fe2O4@rGO composites reduces to 0.29 nm, indicating the elimination of oxygen-containing functional groups after thermal reduction. Moreover, the lattice spacing of 0.48 nm is assigned to the (111) plane of Ni0.33Co0.67Fe2O4. The Raman spectra of GO and Ni0.33Co0.67Fe2O4@rGO composites are illustrated in Fig.4. There are two characteristic peaks on the spectrum of GO, at 1344 cm-1 and 1587 cm-1, corresponding to the D and G band, respectively. The D band generally arises from the edges, disordered carbon and other defects, while the G band is assigned to the ordered sp2–bonded carbon atoms [15]. The Raman spectrum of the Ni0.33Co0.67Fe2O4@rGO composites displays D and G band at 1334 cm-1 and 1595 7
cm-1. In addition, some weak peaks appear in the range of 200-1000 cm-1 and they are attributed to Ni0.33Co0.67Fe2O4 microspheres. The ratio of the intensity of D and G peaks, ID/IG, is used to measure the disorder degree of the sp2 domains. The ID/IG value of Ni0.33Co0.67Fe2O4@rGO nanocomposites is 1.16, compared to that of GO, the increase of ID/IG is due to the decrease in the sp2 domain size and more defects after thermal reduction [16,17]. XPS was measured to investigate the surface chemical composition of the Ni0.33Co0.67Fe2O4@rGO composites. Fig.5a shows that the Ni0.33Co0.67Fe2O4@rGO consist of C, O, Fe, Co and Ni elements, and the atomic ratio is 78.16, 17.27, 2.93, 1.09 and 0.56 %, respectively. As depicted in Fig.5b, there are four peaks appearing in the C1s spectrum of Ni0.33Co0.67Fe2O4@rGO, corresponding to C–C/C=C (284.6 eV), C–O (286.4 eV), C=O (288.3 eV) and O–C=O (289.1 eV) groups [18]. Compared with that of GO shown in Fig.5c, the oxygen content of Ni0.33Co0.67Fe2O4@rGO composites decrease rapidly, especially for C–O, denoting a prominent reduction of GO after the hydrothermal reaction. In Fig.5d, the binding energy peaks at 711.2 and 724.7 eV are attributed to Fe2p 3/2 and Fe2p 1/2, respectively, and the separation between the peaks Fe2p 3/2 and Fe2p 1/2 is owing to the spin orbital spitting of energy [19]. In Fig. 5e, the Co2p 3/2 signal appears at 780.8 eV, and the peak at 796.6 eV is ascribed to the Co2p 1/2 level, and there is a shake-up satellite peak located at 786.8 eV. In the spectrum of Ni2p showed in Fig. 5f, the peaks of Ni2p 3/2 and Ni2p 1/2 are located at 855.4 and 873.64 eV, respectively. 8
Fig.6
displays
the
magnetization hysteresis
loops
of
Ni0.33Co0.67Fe2O4
microspheres and Ni0.33Co0.67Fe2O4@rGO composites at room temperature measured by a vibrating sample magnetometer. It is clearly seen that both two samples possess ferromagnetic behavior. The saturation magnetization (Ms) of Ni0.33Co0.67Fe2O4@rGO composites is 49.48 emu/g, and it is lower than 70.78 emu/g of Ni0.33Co0.67Fe2O4 microspheres. The decreased Ms results from the presence of non-magnetic rGO. Simultaneously, Hc of the Ni0.33Co0.67Fe2O4@rGO composites is 424.74 Oe higher than that of Ni0.33Co0.67Fe2O4 (227.60 Oe), which results in a high frequency resonance and futher larger magnetic loss [20]. To explore the influence of pod-like morphology on absorbing properties, cold quenched pure rGO are mixed with Ni0.33Co0.67Fe2O4 microspheres by mechanically blending according to the same ratio of the above composites as a comparative sample, named as Ni0.33Co0.67Fe2O4-rGO. TGA was performed in air to estimate the content of rGO for the Ni0.33Co0.67Fe2O4@rGO composites. As shown in Fig.7, there is a major mass loss between 350 and 450 ºC resulting from the combustion of rGO. Therefore the rGO content for the Ni0.33Co0.67Fe2O4@rGO composites is estimated to be 16.7 wt%. To have a further understanding of the EM wave absorption mechanism of samples, the complex permittivity, the complex permeability, dielectric loss tangent and magnetic loss tangent are depicted in Fig.8. As shown in Fig.8a, the real parts (𝜀′) of the complex permittivity of Ni0.33Co0.67Fe2O4@rGO composites and Ni0.33Co0.67Fe2O4-rGO mixture decrease with increasing the frequency, while that of 9
Ni0.33Co0.67Fe2O4 maintains around 3 in the whole frequency range. It is clearly seen that the value of Ni0.33Co0.67Fe2O4@rGO composites are relatively higher than the other two samples. The imaginary parts (ε′′) of the samples shown in Fig.8b exhibit the same trends with the real part(ε′). As shown in Fig.8c and d, the real part (μ′) and imaginary part (μ′′) of the complex permeability of Ni0.33Co0.67Fe2O4, Ni0.33Co0.67Fe2O4@rGO composites and Ni0.33Co0.67Fe2O4-rGO mixture show the similar trend. The only difference is that the fluctuation range of Ni0.33Co0.67Fe2O4@rGO composites is smaller than the other two simples. As depicted in Fig.8e, the dielectric loss tangent of the three samples are calculated. The values of tan δε for Ni0.33Co0.67Fe2O4@rGO composites and Ni0.33Co0.67Fe2O4-rGO mixture are in the range of 0.51–0.39 and 0.29–0.23, respectively, higher than that of pure Ni0.33Co0.67Fe2O4 microspheres. It indicates that the
introduction
of
rGO
causes
high
dielectric
loss.
Compared
with
Ni0.33Co0.67Fe2O4-rGO mixture, the Ni0.33Co0.67Fe2O4@rGO composites has higher dielectric loss, which is probably due to the unique pod-like structure of the composite and the interfacial polarization effects [21]. For the magnetic loss tangent shown in Fig.8f, the value of Ni0.33Co0.67Fe2O4 microsheres is greater than those of other two samples. In view of microwave absorption principle, except dielectric loss and magnetic loss, there is another aspect, impedance matching characteristics, that affects microwave absorption. To have a further understanding of the EM wave absorption properties, the 10
reflection loss (RL) are deduced in accordance with transmission-line theory as the following equations: 𝑍 −𝑍
RL(𝑑𝐵) = 20 log |𝑍𝑖𝑛+𝑍0 | 𝑖𝑛
𝜇
𝑍𝑖𝑛 = 𝑍0 √ 𝜀 𝑟 tan ℎ( 𝑗 𝑟
2𝜋𝑓𝑑 𝑐
0
√𝜇𝑟 𝜀𝑟 )
(1) (2)
Where 𝑍𝑖𝑛 is the input impedance of the absorber, 𝑍0 is the impedance of free space,
𝜇𝑟 and 𝜀𝑟 are the relative complex permittivity and permeability of the
absorber, f is the frequency of the electromagnetic wave, c is the velocity of electromagnetic wave in free space, and d is the thickness of the absorber [22]. Fig.9a, b, c and d-a show the deduced RL values of three above-mentioned samples. As shown in Fig.9a, there is almost no RL for Ni0.33Co0.67Fe2O4 microspheres, while the minimum RL values of Ni0.33Co0.67Fe2O4-rGO mixture shown in Fig.9c are around -10 dB, and the minimum RL reaches -11 dB at 8.2 GHz with the thickness of 4.5 mm. The reason leading to this result is that the addition of graphene brings about dielectric loss, which further enhances EM wave absorption. Moreover, as-prepared Ni0.33Co0.67Fe2O4@rGO composites display much higher RL depicted in Fig.9b. The minimum RL can reach -47.5 dB at 9.1 GHz with the thickness of 3 mm and the absorption bandwidth(RL<-10 dB) is 5.02 GHz from 12.39 to 17.41 GHz with the thickness of 2.0 mm.
In addition, as the absorber thickness is increased, the
minimum absorption peaks shift toward the lower frequency, which is in accordance with literatures [11,23]. Recently, more and more researches explain the absorption properties using the quarter-wave matching theory, in which the 𝑡𝑚 and 𝑓𝑚 obey the following equation 11
[24]: 𝑡𝑚 =
𝑛𝜆 𝑛𝑐 (𝑛 = 1,3,5, … ) = 4 4𝑓𝑚 √|𝜀𝑟 𝜇𝑟 |
(3)
Herein 𝜆 represents the wavelength of EM wave, c represents veleocity of light in vacuum, |𝜇𝑟 | and |𝜀𝑟 | represent the modulus of 𝜇𝑟 and 𝜀𝑟 , respectively. According to the quarter-wave matching theory, if 𝑡𝑚 and 𝑓𝑚 meet the above equation, the phase difference between the incident and reflected microwaves in the absorber is 180 ℃, arousing an extinction of the two waves at the air-abssorber interface [21,25 ]. As shown in Fig.9d-b, the curve is the simulation of the relationship between 𝑡𝑚 and 𝑓𝑚 in view of the quarter-wave matching theory. It can be seen that 𝑒𝑥𝑝 the red arrows refer to experimental matching thickness(𝑡𝑚 ) are almost located in
the quarter-wave curve, indicating that 𝑡𝑚 and 𝑓𝑚 obey the quarter-wave model. Furthermore, the impedance matching characteristic is another indispensable aspect for the value of RL. Only when Z(𝑍 = |𝑍𝑖𝑛 ⁄𝑍0 |)
is equal to or similar to 1, the EM
waves entering the inside of the material can be well absorbed. In Fig.9(d-c), when the frequency is 9.1 GHz with the thickness of 3 mm, the value of Z is about 1 and it can be clearly seen from Fig.9(d-a) that it is under this condition that the RLmin was obtained. The enhanced microwave absorption properties of Ni0.33Co0.67Fe2O4@rGO composites can be attributed to their unique pod-like 3D network structure. First, unlike the mixture of Ni0.33Co0.67Fe2O4 microspheres and rGO, Ni0.33Co0.67Fe2O4 microspheres are wrapped in the rGO sheets for the Ni0.33Co0.67Fe2O4@rGO composites.
However,
in
case
of
the 12
Ni0.33Co0.67Fe2O4@rGO
composites,
Ni0.33Co0.67Fe2O4 microspheres are wrapped in the rGO rolls. Once the electromagnetic wave enters into the rGO roll, the presence of internal ferrites will increase multiple reflections and absorption of electromagnetic waves. In addition, GO cannot be thoroughly reduced during the thermal reduction process, thus some defects and oxygen groups existing in rGO for the composites cause polarization relaxation [22]. The synergistic loss mechanism of rGO and ferrites enhances the microwave absorption. Besides, compared to Ni0.33Co0.67Fe2O4-rGO mixture, rGO in Ni0.33Co0.67Fe2O4@rGO composites not only acts as a dielectric matrix, but also prevents Ni0.33Co0.67Fe2O4 microspheres from agglomeration. Thirdly, there are many interfaces between the uniformly wrapped Ni0.33Co0.67Fe2O4 microspheres and rGO, which causes interface polarization [26]. This is also what Ni0.33Co0.67Fe2O4-rGO mixture does not possess. At last, the as-formed 3D network structure provided a large specific area, and caused multiple reflections and absorption of EM waves. 4. Conclusions In summary, the pod-like Ni0.33Co0.67Fe2O4@rGO composites were prepared successfully by combination of a solvothermal reaction and cold quenching. The 100-150 nm of Ni0.33Co0.67Fe2O4 microspheres are wrapped in rGO sheet roll to further form a 3D network structure. The minimum reflection loss of as-prepared composites reaches -47.5 dB at 9.1 GHz with the thickness of 3 mm and the absorption bandwidth (RL<-10 dB) is 5.02 GHz from 12.39 to 17.41 GHz with the thickness of 2.0 mm. Compared to the absorbing performance of Ni0.33Co0.67Fe2O4 microspheres and Ni0.33Co0.67Fe2O4-rGO mixture, Ni0.33Co0.67Fe2O4@rGO composite 13
has wider frequency bandwidth and stronger absorption capacity, performing great promise in the application of absorbing materials.
Acknowledgments The authors of this paper would like to thank the National Natural Science Foundation of China (No. 21376029) and the Analysis & Testing Center, Beijing Institute of Technology for sponsoring this research.
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17
Fig.1 XRD patterns of (a) Ni0.33Co0.67Fe2O4, (b) Ni0.33Co0.67Fe2O4@rGO composites, (c) cold quenched pure rGO and (d) GO Fig.2 SEM images of (a) Ni0.33Co0.67Fe2O4, (b) cold quenched pure rGO, and (c, d) Ni0.33Co0.67Fe2O4@rGO composites Fig.3 TEM images of (a) Ni0.33Co0.67Fe2O4, (b) cold quenched pure rGO, (c, d) as-prepared Ni0.33Co0.67Fe2O4@rGO composites Fig.4 Raman spectra of GO and Ni0.33Co0.67Fe2O4@rGO composites Fig.5 XPS spectra of the (a) survey scan, (b) C1s of Ni0.33Co0.67Fe2O4@rGO composites, (c) C1s of GO, (d) Fe2p region, (e) Co2p region, (f) Ni2p region of the Ni0.33Co0.67Fe2O4@rGO composites Fig.6 Magnetic hysteresis loops for (a) Ni0.33Co0.67Fe2O4 microspheres, (b) as-prepared Ni0.33Co0.67Fe2O4@rGO composites Fig.7 TG curve of Ni0.33Co0.67Fe2O4@rGO composites Fig.8 (a) Real part (𝜀′)and (b) imaginary part(𝜀′′) of the complex permittivity, (c) real part (μ′) and (d) imaginary part (μ′′)of the complex permeability, (e) dielectric loss (tan 𝛿𝜀 ), and (f) magnetic loss (tan 𝛿𝜇 ) of Ni0.33Co0.67Fe2O4 microspheres, Ni0.33Co0.67Fe2O4@rGO composites and Ni0.33Co0.67Fe2O4-rGO mixture Fig.9 The calculated reflection loss of (a) Ni0.33Co0.67Fe2O4 microspheres, (b and d-a) Ni0.33Co0.67Fe2O4@rGO composites and (c) Ni0.33Co0.67Fe2O4-rGO mixture with different thicknesses, (d-b) simulations of the absorber thickness (tm) versus peak frequency (fm) under n = 1; (d-c) the impedance matching characteristics (Z = |𝑍𝑖𝑛 ⁄𝑍0 |) for Ni0.33Co0.67Fe2O4@rGO composites 18
(311)
(440)
Intensity(a.u.)
(220) (111)
(511)
(400)
(422)
(222)
a b c d
10
20
30
40
50
2(degree)
19
60
70
80
Ni0.33Co0.67Fe2O4/RGO
D
G
Intensity(a.u.)
ID/IG=1.16
D
G
GO
500
ID/IG=0.94
1000
1500
Raman Shift(cm-1)
20
2000
2500
a
250000
C1s
C-C
b
40000
150000
Intensity(a.u.)
Intensity(a.u.)
200000
Ni2p
O1s
Co2p Fe2p 100000
30000
20000
C-O C=O C(O)O
10000
50000
0 0
200
400
600
800
0 280
1000
285
Binding energy(eV)
290
295
Binding Energy(eV)
8000
c
7000
C-O
11000
C-C
Fe2p3/2 Fe2p1/2
10000
5000
Intensity(a.u.)
Intensity(a.u.)
6000
d
4000 3000
C=O
2000
C(O)O
9000
8000
7000
1000 6000
0 280
285
290
295
710
Binding Energy(eV) 11800 11600 11400
730
740
14400
e
Co2p3/2
14200
Co2p1/2
f
Ni2p3/2 Ni2p1/2
14000
Intensity(a.u.)
11200
Intensity(a.u.)
720
Binding Energy(eV)
11000 10800 10600 10400
13800 13600 13400
10200
13200
10000
13000
9800
12800 780
790
800
810
Binding Energy(eV)
845
850
855
860
865
Bingding Energy(eV)
21
870
875
80
a
Magnetization(emu/g)
60
b
40 20 0 -20 -40 -60 -80 -30000 -20000 -10000
0
10000 20000 30000
Magnetic field(Oe)
100
Weight Loss(%)
95 90 85 80 75 70 100
200
300
400
Temperature(
22
500 ℃
)
600
700
14
a
Ni0.33Co0.67Fe2O4
7
Ni0.33Co0.67Fe2O4@rGO
6
b
Ni0.33Co0.67Fe2O4 Ni0.33Co0.67Fe2O4@rGO
Ni0.33Co0.67Fe2O4-rGO
12
Ni0.33Co0.67Fe2O4-rGO 5
10
''
'
4 8
3 2
6
1
4
0 2 2
4
6
8
10
12
14
16
18
2
4
Frequency(GHz) 1.10
c
8
10
12
14
16
18
Frequency(GHz)
Ni0.33Co0.67Fe2O4
0.08
Ni0.33Co0.67Fe2O4@rGO
1.08
6
Ni0.33Co0.67Fe2O4-rGO
d
Ni0.33Co0.67Fe2O4 Ni0.33Co0.67Fe2O4@rGO Ni0.33Co0.67Fe2O4-rGO
0.06
1.06
0.04
'
''
1.04 1.02
0.02
1.00
0.00 0.98 0.96
-0.02 2
4
6
8
10
12
14
16
18
2
4
Frequency(GHz)
6
8
10
12
14
16
18
Frequency(GHz)
0.6
0.5
Ni0.33Co0.67Fe2O4
e
0.08
Ni0.33Co0.67Fe2O4@rGO Ni0.33Co0.67Fe2O4-rGO
tan
tan
0.2
Ni0.33Co0.67Fe2O4@rGO Ni0.33Co0.67Fe2O4-rGO
0.06
0.4
0.3
Ni0.33Co0.67Fe2O4
f
0.04
0.02
0.1 0.00
a 0.0
-0.02
2
4
6
8
10
12
14
16
18
2
Frequency(GHz)
4
6
8
10
12
Frequency(GHz)
23
14
16
18
0
(d) 0 1.0mm 1.5mm 2.0mm 2.5mm 3.0mm 3.5mm 4.0mm 4.5mm 5.0mm 5.5mm
-2
2
(b)
4
6
8
10
12
14
16
18
Rrequency/GHz
0
Reflection Loss(dB)
Reflection loss/dB
(a)
-10 -20
-40 -50 8
b 6
-20 -30
1.0mm 1.5mm 2.0mm 2.5mm 3.0mm 3.5mm 4.0mm 4.5mm 5.0mm 5.5mm
-40 -50 2
4
6
8
10
12
14
16
tm(mm)
Reflection loss/dB
1.0 mm 1.0 mm 1.5 mm 2.0 mm 2.5 mm 3.0 mm 3.5 mm 4.0 mm 4.5 mm 5.0 mm 5.5 mm
-30
-10
tm 4
2
18
Frequency/GHz
(c)
a
c
0
1
lZin/Z0l
Reflection loss/dB
-2 -4 -6 1.0 mm 1.5 mm 2.0 mm 2.5 mm 3.0 mm 3.5 mm 4.0 mm 4.5 mm 5.0 mm 5.5 mm
-8 -10 -12 2
4
0 2 6
8
10
12
14
16
4
6
8
10
12
Frequency(GHz)
18
Frequency/GHz
24
14
16
18
PII: DOI: Reference:
S0272-8842(18)33621-6 https://doi.org/10.1016/j.ceramint.2018.12.226 CERI20452
To appear in: Ceramics International Received date: 13 November 2018 Revised date: 21 December 2018 Accepted date: 31 December 2018 Cite this article as: Meimei Gao, Yun Zhao, Shanshan Wang, Yingchun Xu, Caihong Feng, Daxin Shi and Qingze Jiao, Preparation of pod-like 3D Ni0.33Co0.67Fe2O4@rGO composites and their microwave absorbing properties, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.12.226 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.
Preparation of pod-like 3D Ni0.33Co0.67Fe2O4@rGO composites and their microwave absorbing properties Meimei Gaoa, Yun Zhaoa*, Shanshan Wanga, Yingchun Xua, Caihong Fenga, Daxin Shia and Qingze Jiaoa,b*. a
School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China
b
School of Materials and the Environment, Beijing Institute of Technology, Zhuhai, Zhuhai 519085, P. R. China *Corresponding author. Email: [email protected]; [email protected]
Abstract The ferrite/reduced graphene oxide (rGO) composites have attracted increasing attention due to the combination of the dielectric loss of rGO and the magnetic loss of ferrites. In this paper, pod-like 3D Ni0.33Co0.67Fe2O4@rGO composites were prepared using a solvothermal reaction followed by cold quenching. The structures and morphologies of as obtained composites were characterized using X-ray diffractometer, Raman microscope, photoelectron spectroscopy, scanning electron microscope
and
transmission
electron
microscope.
The
Ni0.33Co0.67Fe2O4
microspheres with a diameter of 100-150 nm were wrapped in rGO rolls due to the shrinkage of rGO in liquid nitrogen. The rGO sheets with ferrite microspheres wrapped in form the pod-like 3D network morphology. The minimum reflection loss 1
of as-prepared composites reaches -47.5 dB and the absorption bandwidth (RL<-10 dB) is 5.02 GHz. The composites show much better absorbing performances than pure Ni0.33Co0.67Fe2O4 microspheres and Ni0.33Co0.67Fe2O4-rGO mixture formed by mechanically blending of cold quenched pure rGO and ferrite microspheres. Keyword: D. Ferrites; E. Functional applications; Reduced graphene oxide; Electromagnetic wave absorption mechanism
1. Introduction In recent years, accompanying the rapid development of information technology and extensive use of electronic devices, the issue of electromagnetic interference and pollution has become more and more serious [1, 2]. It is paramount to design and develop microwave absorbing materials with thin thickness, wide bandwidth, lightweight, and strong absorbing ability. Graphene, a monolayer of carbon atoms patterned in close-packed hexagonal lattices, has been considered to be a revolutionary material of the future due to its high electrical and thermal conductivity, large specific surface aera and excellent mechanical strength [3-6]. In the past few years, considerable efforts have been expended in the fabrication of graphene-based composites which integrate the excellent performance of graphene with other functional nanomaterials thereby broaden the application field of graphene materials [7]. The application of graphene-based composites is an excellent choice for electromagnetic wave 2
absorption field. In recent years, magnetic materials have been incorporated into graphene to improve electromagnetic wave absorption in view of the principle of impedance matching. Tsung-Yung Wu et al. [8] synthesized Fe3O4 nanoparticle/graphene hybrids with the minimum reflection loss of -22 dB at thicknesses of 4 and 5 mm through a single-step microwave-assisted solvothermal method. Shu R et al. [9] fabricated reduced graphene oxide/zinc ferrite (rGO/ZnFe2O4) hybrid nanocomposites by a facile solvothermal route and they exhibited excellent microwave absorption performance with the minimum reflection loss of -41.1 dB for a thickness of 2.5 mm and effective absorption bandwidth (less than -10 dB) of 3.2 GHz for a thickness of 2.0 mm. Liu Z et al. [10] successfully synthesized CoFe2O4/rGO nanocomposites via Cu (I)-catalyzed click reaction and the minimum reflection loss of CoFe2O4/rGO nanocomposites could reach -40 dB at 6.8 GHz with a matching thickness of 4.0 mm, and also it had a bandwidth below -10 dB ranging from 5.8 to 8.5 GHz. In fact, not only does the composition of the materials affect performances, but the morphology and structure of the composites has a significant impact on their performances. Min Fu et al. [11] reported that the absorbing performance of NiFe2O4 nanorod/graphene composites prepared through a one-step hydrothermal process was better than that of the NiFe2O4 nanoparticle-graphene composites. Especially, three-dimensional (3D) graphene architectures have shown excellent absorbing properites due to high specific surface area and excellent mechanical properties [12]. Li C et al. [13] synthesized porous Fe3O4/C microspheres with 3D interconnected porous structure via a facile 3
hydrothermal process, and a minimum reflection loss value of -31.75 dB was achieved at an absorber thickness of 3.0 mm. Herein, we synthesized pod-like 3D Ni0.33Co0.67Fe2O4@rGO composites by combination of a solvothermal reaction and cold quenching. By cold quenching ferrite microspheres were well coated with rGO sheets to form a 3D network structure. Structural, morphological and microwave absorbing properties of as-obtained composites were investigated. 2. Experimental 2.1. Materials Graphene oxide (GO) was synthesized by the Hummers method [14]. The raw materials used for the synthesis of Ni0.33Co0.67Fe2O4 were FeCl3·6H2O, NiCl2·6H2O, CoCl2·6H2O, urea and PVP (polyvinyl pyrrolidone). All chemicals and reagents for experiments and analysis were purchased from Beijing Chemicals and of analytical grade and used without further purification. 2.2. Preparation of Ni0.33Co0.67Fe2O4 microspheres The metal chloride salts were dissolved in ethylene glycol at a molar ratio of 4:1:1 for Fe: Ni: Co. After magnetic stirring for 1 h, appropriate amounts of urea and PVP were added, and stirring was continued for 1 h to form homogeneous suspension. The above suspension was sealed in the teflon lined stainlessteel autoclave and maintained at 180 °C for 20 h. Later the autoclave was naturally cooled to room temperature. The black precipitate was collected, washed with ethanol and deionized water respectively and then dried in a vacuum oven at 60 °C for 12 h. 4
2.3. Preparation of Ni0.33Co0.67Fe2O4@rGO composites 50 mg Ni0.33Co0.67Fe2O4 microspheres were added into a 20 mL GO suspension (0.5 mg/mL) to form a suspension. After 30 min of ultrasonication, the suspension was
dripped
into
liquid
nitrogen
with
vigorous
stirring.
Pod-like
Ni0.33Co0.67Fe2O4@GO composites was prepared due to the shrinkage of GO sheets in liquid nitrogen. They were then freeze-dried for 24 h. Finally, pod-like Ni0.33Co0.67Fe2O4@rGO composites were obtained by thermal-reduction at 500 ° C in an argon atmosphere for 30 min. RGO was prepared using the same process without the introduction of Ni0.33Co0.67Fe2O4 microspheres. 2.4. Characterization The morphology of the samples was observed and analyzed using a Hitachi JSM-7500F Field Scanning Electron Microscope. TEM observations were performed with a Hitachi-800 transmission electron microscope and a JEOL high-resolution transmission electron microscope (JEM-2100) using an operating voltage of 120 kV. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku multi-function X-ray diffractometer (Ultima IV) at 40 KV and 150 mA with the Cu-Kα radiation. XRD data were collected between 5 and 80 º at a scan rate of 20 º/min. Raman spectroscopy was carried out on a Raman microscope (LabRam HR Evolution). 5
The chemical states of samples were obtained by a Thermo Scientific X-ray photoelectron spectroscopy (XPS, ESCALAB 250 XI) The magnetic properties were measured using a Vibrating sample magnetometer (VSM, Squid-VSM). Thermogravimetric analysis (TGA) was performed using a thermogravimetric analyzer (Q5000) from 50 ºC to 700 ºC in air at a heating rate of 10 ºC min-1. The complex permittivity and complex permeability of the composites were measured using an Agilent PNA-X Microwave Network Analyzer (PNA-N5244A) in the 1–18 GHz frequency range. For measurement, the composites were mixed with paraffin at a ratio of 20 wt% and compressed to a block with an outer diameter of 7 mm, an inner diameter of 3 mm, and a thickness of 2 mm. 3. Results and discussion The XRD patterns of the Ni0.33Co0.67Fe2O4, Ni0.33Co0.67Fe2O4@rGO composites, cold quenched pure rGO and GO are shown in Fig.1. The diffraction peaks of Ni0.33Co0.67Fe2O4 can match well with the crystal planes of (220), (311), (222), (400), (422), (511) and (440), respectively, and the relative intensities and purity phases indicate the high purity of the Ni0.33Co0.67Fe2O4 with very good crystallinity [10, 11]. The peak intensity of Ni0.33Co0.67Fe2O4@rGO composites has a quite decrease due to the introduction of rGO. Moreover, there is a weak peak at about 2θ=25° indicating rGO in the spectrum of Ni0.33Co0.67Fe2O4@rGO composites, and the characteristic peak of GO disappears through thermal reduction. Fig.2a is a SEM image of as-synthesized Ni0.33Co0.67Fe2O4 microspheres. It can be 6
clearly observed that the size distribution of Ni0.33Co0.67Fe2O4 is in the range of 100-150 nm with the shape of sphere. Since the particle size is small, agglomeration occurs. As shown in Fig.2b, the sheet structure of rGO can be clearly seen and the graphene sheets have a tendency to shrink due to the cold quench in liquid nitrogen. In case of Ni0.33Co0.67Fe2O4@rGO composites (Fig.2c and d), the rGO sheets with ferrite microspheres wrapped in form the pod-like 3D network morphology. In order to have a further observation of morphologies and microstructures of the Ni0.33Co0.67Fe2O4@rGO composites, TEM measurements were carried out shown in Fig.3. It can be further observed from Fig.3a that the Ni0.33Co0.67Fe2O4 microspheres are assembled by nanoparticles of about 100-150 nm. Fig.3b shows a partly rolled up rGO sheet with many folds. Fig.2d further demonstrates that Ni0.33Co0.67Fe2O4 microspheres are wrapped in rGO sheets. As shown in Fig.3c, compared to GO, the lattice spacing of rGO for Ni0.33Co0.67Fe2O4@rGO composites reduces to 0.29 nm, indicating the elimination of oxygen-containing functional groups after thermal reduction. Moreover, the lattice spacing of 0.48 nm is assigned to the (111) plane of Ni0.33Co0.67Fe2O4. The Raman spectra of GO and Ni0.33Co0.67Fe2O4@rGO composites are illustrated in Fig.4. There are two characteristic peaks on the spectrum of GO, at 1344 cm-1 and 1587 cm-1, corresponding to the D and G band, respectively. The D band generally arises from the edges, disordered carbon and other defects, while the G band is assigned to the ordered sp2–bonded carbon atoms [15]. The Raman spectrum of the Ni0.33Co0.67Fe2O4@rGO composites displays D and G band at 1334 cm-1 and 1595 7
cm-1. In addition, some weak peaks appear in the range of 200-1000 cm-1 and they are attributed to Ni0.33Co0.67Fe2O4 microspheres. The ratio of the intensity of D and G peaks, ID/IG, is used to measure the disorder degree of the sp2 domains. The ID/IG value of Ni0.33Co0.67Fe2O4@rGO nanocomposites is 1.16, compared to that of GO, the increase of ID/IG is due to the decrease in the sp2 domain size and more defects after thermal reduction [16,17]. XPS was measured to investigate the surface chemical composition of the Ni0.33Co0.67Fe2O4@rGO composites. Fig.5a shows that the Ni0.33Co0.67Fe2O4@rGO consist of C, O, Fe, Co and Ni elements, and the atomic ratio is 78.16, 17.27, 2.93, 1.09 and 0.56 %, respectively. As depicted in Fig.5b, there are four peaks appearing in the C1s spectrum of Ni0.33Co0.67Fe2O4@rGO, corresponding to C–C/C=C (284.6 eV), C–O (286.4 eV), C=O (288.3 eV) and O–C=O (289.1 eV) groups [18]. Compared with that of GO shown in Fig.5c, the oxygen content of Ni0.33Co0.67Fe2O4@rGO composites decrease rapidly, especially for C–O, denoting a prominent reduction of GO after the hydrothermal reaction. In Fig.5d, the binding energy peaks at 711.2 and 724.7 eV are attributed to Fe2p 3/2 and Fe2p 1/2, respectively, and the separation between the peaks Fe2p 3/2 and Fe2p 1/2 is owing to the spin orbital spitting of energy [19]. In Fig. 5e, the Co2p 3/2 signal appears at 780.8 eV, and the peak at 796.6 eV is ascribed to the Co2p 1/2 level, and there is a shake-up satellite peak located at 786.8 eV. In the spectrum of Ni2p showed in Fig. 5f, the peaks of Ni2p 3/2 and Ni2p 1/2 are located at 855.4 and 873.64 eV, respectively. 8
Fig.6
displays
the
magnetization hysteresis
loops
of
Ni0.33Co0.67Fe2O4
microspheres and Ni0.33Co0.67Fe2O4@rGO composites at room temperature measured by a vibrating sample magnetometer. It is clearly seen that both two samples possess ferromagnetic behavior. The saturation magnetization (Ms) of Ni0.33Co0.67Fe2O4@rGO composites is 49.48 emu/g, and it is lower than 70.78 emu/g of Ni0.33Co0.67Fe2O4 microspheres. The decreased Ms results from the presence of non-magnetic rGO. Simultaneously, Hc of the Ni0.33Co0.67Fe2O4@rGO composites is 424.74 Oe higher than that of Ni0.33Co0.67Fe2O4 (227.60 Oe), which results in a high frequency resonance and futher larger magnetic loss [20]. To explore the influence of pod-like morphology on absorbing properties, cold quenched pure rGO are mixed with Ni0.33Co0.67Fe2O4 microspheres by mechanically blending according to the same ratio of the above composites as a comparative sample, named as Ni0.33Co0.67Fe2O4-rGO. TGA was performed in air to estimate the content of rGO for the Ni0.33Co0.67Fe2O4@rGO composites. As shown in Fig.7, there is a major mass loss between 350 and 450 ºC resulting from the combustion of rGO. Therefore the rGO content for the Ni0.33Co0.67Fe2O4@rGO composites is estimated to be 16.7 wt%. To have a further understanding of the EM wave absorption mechanism of samples, the complex permittivity, the complex permeability, dielectric loss tangent and magnetic loss tangent are depicted in Fig.8. As shown in Fig.8a, the real parts (𝜀′) of the complex permittivity of Ni0.33Co0.67Fe2O4@rGO composites and Ni0.33Co0.67Fe2O4-rGO mixture decrease with increasing the frequency, while that of 9
Ni0.33Co0.67Fe2O4 maintains around 3 in the whole frequency range. It is clearly seen that the value of Ni0.33Co0.67Fe2O4@rGO composites are relatively higher than the other two samples. The imaginary parts (ε′′) of the samples shown in Fig.8b exhibit the same trends with the real part(ε′). As shown in Fig.8c and d, the real part (μ′) and imaginary part (μ′′) of the complex permeability of Ni0.33Co0.67Fe2O4, Ni0.33Co0.67Fe2O4@rGO composites and Ni0.33Co0.67Fe2O4-rGO mixture show the similar trend. The only difference is that the fluctuation range of Ni0.33Co0.67Fe2O4@rGO composites is smaller than the other two simples. As depicted in Fig.8e, the dielectric loss tangent of the three samples are calculated. The values of tan δε for Ni0.33Co0.67Fe2O4@rGO composites and Ni0.33Co0.67Fe2O4-rGO mixture are in the range of 0.51–0.39 and 0.29–0.23, respectively, higher than that of pure Ni0.33Co0.67Fe2O4 microspheres. It indicates that the
introduction
of
rGO
causes
high
dielectric
loss.
Compared
with
Ni0.33Co0.67Fe2O4-rGO mixture, the Ni0.33Co0.67Fe2O4@rGO composites has higher dielectric loss, which is probably due to the unique pod-like structure of the composite and the interfacial polarization effects [21]. For the magnetic loss tangent shown in Fig.8f, the value of Ni0.33Co0.67Fe2O4 microsheres is greater than those of other two samples. In view of microwave absorption principle, except dielectric loss and magnetic loss, there is another aspect, impedance matching characteristics, that affects microwave absorption. To have a further understanding of the EM wave absorption properties, the 10
reflection loss (RL) are deduced in accordance with transmission-line theory as the following equations: 𝑍 −𝑍
RL(𝑑𝐵) = 20 log |𝑍𝑖𝑛+𝑍0 | 𝑖𝑛
𝜇
𝑍𝑖𝑛 = 𝑍0 √ 𝜀 𝑟 tan ℎ( 𝑗 𝑟
2𝜋𝑓𝑑 𝑐
0
√𝜇𝑟 𝜀𝑟 )
(1) (2)
Where 𝑍𝑖𝑛 is the input impedance of the absorber, 𝑍0 is the impedance of free space,
𝜇𝑟 and 𝜀𝑟 are the relative complex permittivity and permeability of the
absorber, f is the frequency of the electromagnetic wave, c is the velocity of electromagnetic wave in free space, and d is the thickness of the absorber [22]. Fig.9a, b, c and d-a show the deduced RL values of three above-mentioned samples. As shown in Fig.9a, there is almost no RL for Ni0.33Co0.67Fe2O4 microspheres, while the minimum RL values of Ni0.33Co0.67Fe2O4-rGO mixture shown in Fig.9c are around -10 dB, and the minimum RL reaches -11 dB at 8.2 GHz with the thickness of 4.5 mm. The reason leading to this result is that the addition of graphene brings about dielectric loss, which further enhances EM wave absorption. Moreover, as-prepared Ni0.33Co0.67Fe2O4@rGO composites display much higher RL depicted in Fig.9b. The minimum RL can reach -47.5 dB at 9.1 GHz with the thickness of 3 mm and the absorption bandwidth(RL<-10 dB) is 5.02 GHz from 12.39 to 17.41 GHz with the thickness of 2.0 mm.
In addition, as the absorber thickness is increased, the
minimum absorption peaks shift toward the lower frequency, which is in accordance with literatures [11,23]. Recently, more and more researches explain the absorption properties using the quarter-wave matching theory, in which the 𝑡𝑚 and 𝑓𝑚 obey the following equation 11
[24]: 𝑡𝑚 =
𝑛𝜆 𝑛𝑐 (𝑛 = 1,3,5, … ) = 4 4𝑓𝑚 √|𝜀𝑟 𝜇𝑟 |
(3)
Herein 𝜆 represents the wavelength of EM wave, c represents veleocity of light in vacuum, |𝜇𝑟 | and |𝜀𝑟 | represent the modulus of 𝜇𝑟 and 𝜀𝑟 , respectively. According to the quarter-wave matching theory, if 𝑡𝑚 and 𝑓𝑚 meet the above equation, the phase difference between the incident and reflected microwaves in the absorber is 180 ℃, arousing an extinction of the two waves at the air-abssorber interface [21,25 ]. As shown in Fig.9d-b, the curve is the simulation of the relationship between 𝑡𝑚 and 𝑓𝑚 in view of the quarter-wave matching theory. It can be seen that 𝑒𝑥𝑝 the red arrows refer to experimental matching thickness(𝑡𝑚 ) are almost located in
the quarter-wave curve, indicating that 𝑡𝑚 and 𝑓𝑚 obey the quarter-wave model. Furthermore, the impedance matching characteristic is another indispensable aspect for the value of RL. Only when Z(𝑍 = |𝑍𝑖𝑛 ⁄𝑍0 |)
is equal to or similar to 1, the EM
waves entering the inside of the material can be well absorbed. In Fig.9(d-c), when the frequency is 9.1 GHz with the thickness of 3 mm, the value of Z is about 1 and it can be clearly seen from Fig.9(d-a) that it is under this condition that the RLmin was obtained. The enhanced microwave absorption properties of Ni0.33Co0.67Fe2O4@rGO composites can be attributed to their unique pod-like 3D network structure. First, unlike the mixture of Ni0.33Co0.67Fe2O4 microspheres and rGO, Ni0.33Co0.67Fe2O4 microspheres are wrapped in the rGO sheets for the Ni0.33Co0.67Fe2O4@rGO composites.
However,
in
case
of
the 12
Ni0.33Co0.67Fe2O4@rGO
composites,
Ni0.33Co0.67Fe2O4 microspheres are wrapped in the rGO rolls. Once the electromagnetic wave enters into the rGO roll, the presence of internal ferrites will increase multiple reflections and absorption of electromagnetic waves. In addition, GO cannot be thoroughly reduced during the thermal reduction process, thus some defects and oxygen groups existing in rGO for the composites cause polarization relaxation [22]. The synergistic loss mechanism of rGO and ferrites enhances the microwave absorption. Besides, compared to Ni0.33Co0.67Fe2O4-rGO mixture, rGO in Ni0.33Co0.67Fe2O4@rGO composites not only acts as a dielectric matrix, but also prevents Ni0.33Co0.67Fe2O4 microspheres from agglomeration. Thirdly, there are many interfaces between the uniformly wrapped Ni0.33Co0.67Fe2O4 microspheres and rGO, which causes interface polarization [26]. This is also what Ni0.33Co0.67Fe2O4-rGO mixture does not possess. At last, the as-formed 3D network structure provided a large specific area, and caused multiple reflections and absorption of EM waves. 4. Conclusions In summary, the pod-like Ni0.33Co0.67Fe2O4@rGO composites were prepared successfully by combination of a solvothermal reaction and cold quenching. The 100-150 nm of Ni0.33Co0.67Fe2O4 microspheres are wrapped in rGO sheet roll to further form a 3D network structure. The minimum reflection loss of as-prepared composites reaches -47.5 dB at 9.1 GHz with the thickness of 3 mm and the absorption bandwidth (RL<-10 dB) is 5.02 GHz from 12.39 to 17.41 GHz with the thickness of 2.0 mm. Compared to the absorbing performance of Ni0.33Co0.67Fe2O4 microspheres and Ni0.33Co0.67Fe2O4-rGO mixture, Ni0.33Co0.67Fe2O4@rGO composite 13
has wider frequency bandwidth and stronger absorption capacity, performing great promise in the application of absorbing materials.
Acknowledgments The authors of this paper would like to thank the National Natural Science Foundation of China (No. 21376029) and the Analysis & Testing Center, Beijing Institute of Technology for sponsoring this research.
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17
Fig.1 XRD patterns of (a) Ni0.33Co0.67Fe2O4, (b) Ni0.33Co0.67Fe2O4@rGO composites, (c) cold quenched pure rGO and (d) GO Fig.2 SEM images of (a) Ni0.33Co0.67Fe2O4, (b) cold quenched pure rGO, and (c, d) Ni0.33Co0.67Fe2O4@rGO composites Fig.3 TEM images of (a) Ni0.33Co0.67Fe2O4, (b) cold quenched pure rGO, (c, d) as-prepared Ni0.33Co0.67Fe2O4@rGO composites Fig.4 Raman spectra of GO and Ni0.33Co0.67Fe2O4@rGO composites Fig.5 XPS spectra of the (a) survey scan, (b) C1s of Ni0.33Co0.67Fe2O4@rGO composites, (c) C1s of GO, (d) Fe2p region, (e) Co2p region, (f) Ni2p region of the Ni0.33Co0.67Fe2O4@rGO composites Fig.6 Magnetic hysteresis loops for (a) Ni0.33Co0.67Fe2O4 microspheres, (b) as-prepared Ni0.33Co0.67Fe2O4@rGO composites Fig.7 TG curve of Ni0.33Co0.67Fe2O4@rGO composites Fig.8 (a) Real part (𝜀′)and (b) imaginary part(𝜀′′) of the complex permittivity, (c) real part (μ′) and (d) imaginary part (μ′′)of the complex permeability, (e) dielectric loss (tan 𝛿𝜀 ), and (f) magnetic loss (tan 𝛿𝜇 ) of Ni0.33Co0.67Fe2O4 microspheres, Ni0.33Co0.67Fe2O4@rGO composites and Ni0.33Co0.67Fe2O4-rGO mixture Fig.9 The calculated reflection loss of (a) Ni0.33Co0.67Fe2O4 microspheres, (b and d-a) Ni0.33Co0.67Fe2O4@rGO composites and (c) Ni0.33Co0.67Fe2O4-rGO mixture with different thicknesses, (d-b) simulations of the absorber thickness (tm) versus peak frequency (fm) under n = 1; (d-c) the impedance matching characteristics (Z = |𝑍𝑖𝑛 ⁄𝑍0 |) for Ni0.33Co0.67Fe2O4@rGO composites 18
(311)
(440)
Intensity(a.u.)
(220) (111)
(511)
(400)
(422)
(222)
a b c d
10
20
30
40
50
2(degree)
19
60
70
80
Ni0.33Co0.67Fe2O4/RGO
D
G
Intensity(a.u.)
ID/IG=1.16
D
G
GO
500
ID/IG=0.94
1000
1500
Raman Shift(cm-1)
20
2000
2500
a
250000
C1s
C-C
b
40000
150000
Intensity(a.u.)
Intensity(a.u.)
200000
Ni2p
O1s
Co2p Fe2p 100000
30000
20000
C-O C=O C(O)O
10000
50000
0 0
200
400
600
800
0 280
1000
285
Binding energy(eV)
290
295
Binding Energy(eV)
8000
c
7000
C-O
11000
C-C
Fe2p3/2 Fe2p1/2
10000
5000
Intensity(a.u.)
Intensity(a.u.)
6000
d
4000 3000
C=O
2000
C(O)O
9000
8000
7000
1000 6000
0 280
285
290
295
710
Binding Energy(eV) 11800 11600 11400
730
740
14400
e
Co2p3/2
14200
Co2p1/2
f
Ni2p3/2 Ni2p1/2
14000
Intensity(a.u.)
11200
Intensity(a.u.)
720
Binding Energy(eV)
11000 10800 10600 10400
13800 13600 13400
10200
13200
10000
13000
9800
12800 780
790
800
810
Binding Energy(eV)
845
850
855
860
865
Bingding Energy(eV)
21
870
875
80
a
Magnetization(emu/g)
60
b
40 20 0 -20 -40 -60 -80 -30000 -20000 -10000
0
10000 20000 30000
Magnetic field(Oe)
100
Weight Loss(%)
95 90 85 80 75 70 100
200
300
400
Temperature(
22
500 ℃
)
600
700
14
a
Ni0.33Co0.67Fe2O4
7
Ni0.33Co0.67Fe2O4@rGO
6
b
Ni0.33Co0.67Fe2O4 Ni0.33Co0.67Fe2O4@rGO
Ni0.33Co0.67Fe2O4-rGO
12
Ni0.33Co0.67Fe2O4-rGO 5
10
''
'
4 8
3 2
6
1
4
0 2 2
4
6
8
10
12
14
16
18
2
4
Frequency(GHz) 1.10
c
8
10
12
14
16
18
Frequency(GHz)
Ni0.33Co0.67Fe2O4
0.08
Ni0.33Co0.67Fe2O4@rGO
1.08
6
Ni0.33Co0.67Fe2O4-rGO
d
Ni0.33Co0.67Fe2O4 Ni0.33Co0.67Fe2O4@rGO Ni0.33Co0.67Fe2O4-rGO
0.06
1.06
0.04
'
''
1.04 1.02
0.02
1.00
0.00 0.98 0.96
-0.02 2
4
6
8
10
12
14
16
18
2
4
Frequency(GHz)
6
8
10
12
14
16
18
Frequency(GHz)
0.6
0.5
Ni0.33Co0.67Fe2O4
e
0.08
Ni0.33Co0.67Fe2O4@rGO Ni0.33Co0.67Fe2O4-rGO
tan
tan
0.2
Ni0.33Co0.67Fe2O4@rGO Ni0.33Co0.67Fe2O4-rGO
0.06
0.4
0.3
Ni0.33Co0.67Fe2O4
f
0.04
0.02
0.1 0.00
a 0.0
-0.02
2
4
6
8
10
12
14
16
18
2
Frequency(GHz)
4
6
8
10
12
Frequency(GHz)
23
14
16
18
0
(d) 0 1.0mm 1.5mm 2.0mm 2.5mm 3.0mm 3.5mm 4.0mm 4.5mm 5.0mm 5.5mm
-2
2
(b)
4
6
8
10
12
14
16
18
Rrequency/GHz
0
Reflection Loss(dB)
Reflection loss/dB
(a)
-10 -20
-40 -50 8
b 6
-20 -30
1.0mm 1.5mm 2.0mm 2.5mm 3.0mm 3.5mm 4.0mm 4.5mm 5.0mm 5.5mm
-40 -50 2
4
6
8
10
12
14
16
tm(mm)
Reflection loss/dB
1.0 mm 1.0 mm 1.5 mm 2.0 mm 2.5 mm 3.0 mm 3.5 mm 4.0 mm 4.5 mm 5.0 mm 5.5 mm
-30
-10
tm 4
2
18
Frequency/GHz
(c)
a
c
0
1
lZin/Z0l
Reflection loss/dB
-2 -4 -6 1.0 mm 1.5 mm 2.0 mm 2.5 mm 3.0 mm 3.5 mm 4.0 mm 4.5 mm 5.0 mm 5.5 mm
-8 -10 -12 2
4
0 2 6
8
10
12
14
16
4
6
8
10
12
Frequency(GHz)
18
Frequency/GHz
24
14
16
18