Facile design of cubic-like cerium oxide nanoparticles decorated reduced graphene oxide with enhanced microwave absorption properties

Facile design of cubic-like cerium oxide nanoparticles decorated reduced graphene oxide with enhanced microwave absorption properties

Journal of Alloys and Compounds xxx (xxxx) xxx Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://ww...

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Journal of Alloys and Compounds xxx (xxxx) xxx

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Facile design of cubic-like cerium oxide nanoparticles decorated reduced graphene oxide with enhanced microwave absorption properties Yue Wu a, Ruiwen Shu a, b, *, Xiameng Shan a, Jiabin Zhang a, Jianjun Shi a, Yin Liu c, Mingdong Zheng a, ** a b c

School of Chemical Engineering, Anhui University of Science and Technology, Huainan, 232001, People’s Republic of China School of Earth and Environment, Anhui University of Science and Technology, Huainan, 232001, People’s Republic of China School of Materials Science and Engineering, Anhui University of Science and Technology, Huainan, 232001, People’s Republic of China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 August 2019 Received in revised form 19 October 2019 Accepted 21 October 2019 Available online xxx

Herein, reduced graphene oxide/cerium oxide (RGO/CeO2) hybrid nanocomposites were synthesized by a facile one-pot hydrothermal strategy. Results of morphology observations revealed that the as-prepared hybrid nanocomposites showed improved dispersion of particles with the increasing of contents of RGO and numerously cubic-like CeO2 nanoparticles were uniformly loaded on the crumpled surfaces or edges of thinly flake-like RGO. Moreover, the effects of contents of RGO and concentrations of oxygen vacancies on the microwave absorption properties of RGO/CeO2 hybrid nanocomposites were carefully investigated. It was found that complexing of RGO notably enhanced the microwave absorption properties of CeO2 nanoparticles, and the microwave absorption properties of as-prepared hybrid nanocomposites could be optimized by facilely modulating the contents of RGO. Remarkably, the obtained hybrid nanocomposites with the content of RGO of 4.1 wt% exhibited the best microwave absorption performance, i.e. the minimum reflection loss reached 49.2 dB at 5.2 GHz with a matching thickness of 4.46 mm, and effective absorption bandwidth achieved 4.1 GHz (13.4e17.5 GHz) with a thin thickness of merely 1.7 mm. Besides, the underlying microwave absorption mechanisms of as-prepared hybrid nanocomposites were proposed. It was believed that our results could shed light on the design and fabrication of graphene-based hybrid composites as high-efficient microwave absorbers. © 2019 Elsevier B.V. All rights reserved.

Keywords: Reduced graphene oxide Cerium oxide Oxygen vacancy Hybrid nanocomposites Microwave absorption

1. Introduction With the increasingly serious problem of electromagnetic pollution derived from the wide usage of electronic equipment, microwave absorbing materials (MAMs) have gained great attentions in the field of functional materials [1e3]. Generally, strong absorption, broad bandwidth, thin thickness and light weight are four key factors need to be considered for designing ideal microwave absorbers [4e6]. As a novel carbon nanomaterial, reduced graphene oxide (RGO) has been considered as a potential candidate for microwave

* Corresponding author. ** Corresponding author. School of Chemical Engineering, Anhui University of Science and Technology, Huainan, 232001, People’s Republic of China. E-mail addresses: [email protected] (R. Shu), [email protected] (M. Zheng).

absorption due to the advantages, such as uniquely laminated structure, large specific surface areas, low density and strong dielectric loss [2,7e10]. However, single RGO used as MAMs suffers from inferior impedance matching and poor microwave attenuation [4,7e10]. Thus, it is very urgent to improve impedance matching and enhance microwave absorption performance of RGO for dealing with the growing problem of electromagnetic pollution. According to the electromagnetic theories, both attenuation loss and impedance matching play key roles for achieving good microwave absorption [1,5]. Recently, numerous investigations demonstrated that the complexation of RGO with other magnetic or dielectric loss materials for fabricating RGO-based hybrid nanocomposites was an effective route strategy to enhance the microwave attenuation capacity of RGO [8,9,11e23]. For instance, Cao et al. used a confined growth method to fabricate the NiFe2O4‒ RGO nanocomposites and the results showed that the minimum reflection loss (RLmin) was 58.0 dB and effective absorption

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bandwidth (EAB, RL  10 dB) was 4.08 GHz with a matching thickness of 2.7 mm [12]. Yin et al. synthesized the RGO-wrapped ZnO hollow spheres composites by a two-step process and observed that the as-prepared composites presented the RLmin of 45.05 dB at 9.7 GHz with a thickness of only 2.2 mm [17]. Wu et al. prepared the MoS2/RGO hybrids and found that the obtained hybrids exhibited the RLmin of 50.9 dB at 11.68 GHz with a thickness of 2.3 mm and EAB of 5.72 GHz with a thickness of merely 1.9 mm [18]. Cerium oxide (CeO2) as a popular rare earth compound, which has been proven to be a potential candidate for microwave absorption owing to its dielectric loss characteristic and charge polarization relaxation inducing by oxygen vacancy defects [24e30]. However, weak microwave absorption performance and high density limit the practical applications of CeO2 in the field of electromagnetic absorption. In view of the merits of CeO2 with notable oxygen vacancies defects, good chemical stability, facile synthesis and low cost [24e30], RGO with large specific surface areas, low density and strong dielectric loss [7e10], it is believed that superior microwave absorption properties could be achieved through fabricating the

reduced graphene oxide/cerium oxide (RGO/CeO2) hybrid nanocomposites. To the best of our knowledge, the investigations of contents of RGO on oxygen vacancies defects, electromagnetic parameters and microwave absorption properties of RGO/CeO2 hybrid nanocomposites were seldom reported in the literatures. Herein, we fabricated RGO/CeO2 hybrid nanocomposites through a facile one-pot hydrothermal strategy by using graphene oxide (GO) as the template. Various characterization techniques were adopted to explore the relationships among structure, compositions and microwave absorption properties of RGO/CeO2 hybrid nanocomposites. Moreover, the effects of contents of RGO on the micromorphology, oxygen vacancies concentrations, electromagnetic parameters and microwave absorption properties of asprepared hybrid nanocomposites were systematically investigated. Results revealed that the as-prepared hybrid nanocomposites showed excellent microwave absorption performance with strong absorption, broad bandwidth and thin thickness, which could be used as potential candidates for electromagnetic absorption. Besides, the possible microwave absorption mechanisms of obtained hybrid nanocomposites were clarified.

Fig. 1. Schematic illustration of the synthesis process of RGO/CeO2 hybrid nanocomposites.

Fig. 2. (a) XRD patterns and (b) Raman spectra for the samples of S1, S2 and S3; (c) TGA curves for the samples of S2 and S3.

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2. Experimental The detailed synthesis procedures and characterization of RGO/ CeO2 hybrid nanocomposites could be found in the electronic supplementary materials. For simplicity, pure CeO2 was marked as S1 and the as-prepared RGO/CeO2 hybrid nanocomposites with different additive amounts of graphite oxide were labeled as S2 (30 mg) and S3 (60 mg), respectively. 3. Results and discussion 3.1. Formation process of RGO/CeO2 hybrid nanocomposites Fig. 1 describes the schematic synthesis procedures of RGO/CeO2 hybrid nanocomposites. Firstly, positively charged Ce3þ could be attached to the negatively charged ‒COO- on the surfaces or edges of GO by electrostatic attraction interactions under alkaline conditions in the aqueous GO dispersions [28,30]. Secondly, CeO2 particles were in situ deposited on the surfaces or edges of RGO after hydrothermal reactions and thus RGO/CeO2 hybrid nanocomposites were formed. 3.2. Structural analysis As depicted in Fig. 2(a), the diffraction peaks from X-ray diffraction (XRD) patterns of the samples of S1, S2 and S3 appearing at 2q ¼ 28.7, 33.2, 47.5, 56.5, 59.1, 69.3 and 76.7 are in good accordance with the (111), (200), (220), (311), (222), (400) and (331) crystal planes of cubic fluorite CeO2 (JCPDS No. 34e0394) [25,30]. However, it is difficult to distinguish the diffraction peaks of RGO in the patterns of S2 and S3, which could be explained by the relatively low diffraction strength of RGO compared with CeO2 in the binary nanocomposites.

3

Degree of graphitization of obtained samples could be evaluated by Raman spectroscopy. From Fig. 2(b), both S2 and S3 exhibit two obvious scattering peaks located at around 1588 cm1 (G band) and 1349 cm1 (D band), respectively. The D band signifies the sp3 defects or disorder, while G band indicates the sp2 hybridization [30e32]. Generally, ID/IG is often used to reflect the degree of disorder [30e32]. The S3 shows slightly larger ID/IG than S2, which suggests that the degree of defects or disorder become higher with the increasing of additive amounts of GO. Furthermore, it is obvious that the Raman scattering peaks at around 461 cm1 for the samples of S1, S2 and S3 could be assigned to the characteristic peak of CeO2 [25,30]. Thermal gravimetric analysis (TGA) measurements were conducted to determine the contents of RGO in the samples of S2 and S3. As shown in Fig. 2(c), the thermal decomposition behaviors could be divided into two stages. Firstly, a small weight loss (~3.9 wt%) below 250  C mainly originates from the loss of adsorbed water on the surfaces of samples [18]. Secondly, an obvious weight loss from 250  C to 600  C, which could be ascribed to the degradation of RGO in air atmosphere. Besides, the residual products are deduced to the constituent of CeO2. Thus, the contents of RGO in S2 and S3 are estimated as 4.1 wt% and 5.2 wt%, respectively. X-ray photoelectron spectroscopy (XPS) analysis was used to reveal the surface chemical compositions and valence states of the sample of S2. Fig. 3(a) shows the typical spectrum of wide scan, which confirms that the S2 contains the elements of Ce, O and C. As shown in Fig. 3(b), the peaks of C 1s at 289.0, 285.8, 285.0 and 284.4 eV can be assigned to OeC]O, C]O, CeO and CeC/C]C bonds, respectively [27,30]. From Fig. 3(c), the O 1s spectra can be fitted into three peaks of CeeO (529.7 eV), OeH (531.4 eV) and O2 (532.1 eV) [30]. Fig. 3(d) shows the typical Ce 3d spectra, which can be fitted into eight peaks. The peaks of U, U00, U000, V, V00 and V000 originate from Ce4þ, while the peaks of U0 and V0 derive from Ce3þ

Fig. 3. XPS spectra: (a) wide scan, (b) C 1s, (c) O 1s and (d) Ce3þ spectra for the sample of S2.

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[25,30]. Thus, both Ce4þ and Ce3þ coexist in the hybrid nanocomposites, which suggests the existence of oxygen vacancies originated from valence state transformation from Ce4þ to Ce3þ [25,30]. Furthermore, we have estimated the concentration of Ce3þ in CeO2 by calculating the integral areas of fitted eight peaks in Ce 3d spectrum. It should be noted that the concentration of Ce3þ in S2 (16.6 wt%) is slightly higher than that of S3 (14.4 wt%). Besides, the concentrations of Ce3þ in both S2 and S3 are much higher than the value of 9.4 wt% for pure CeO2. These findings reveal that the oxygen vacancies increase by hybridizing CeO2 nanoparticles with RGO and the concentrations of oxygen vacancies could be regulated by modulating the contents of RGO in the hybrid nanocomposites. Therefore, the electric conductivity enhances with the increasing of oxygen vacancy defects. The enhanced electric conductivity is beneficial to enhancing the conduction loss and charge polarization relaxation of RGO/CeO2 hybrid nanocomposites [28,30]. 3.3. Morphological analysis Scanning electron microscopy (SEM) was used to observe the

micromorphology of the samples of S1, S2 and S3, as shown in Fig. 4. From Fig. 4(a)e(c), it can be seen that numerous CeO2 particles are densely packed together with obvious aggregation in the sample of S1. Notably, RGO with a rippled and crumpled morphology is clearly observed in the samples of S2 and S3 (marked by white arrows in Fig. 4(f) and (g)). Furthermore, the dispersion of particles becomes better after hybridization CeO2 with RGO and small cubic-like CeO2 particles were uniformly distributed on the surfaces or edges of RGO in the S2. Therefore, it is believed that abundantly heterogeneous interfaces between CeO2 and RGO could be formed, which notably enhance the interfacial polarization and dielectric loss of S2 and S3 under the alternating electromagnetic fields [32]. The energy dispersive spectrum (EDS) pattern of S2 clearly reveals the existence of C, O, and Ce elements (Fig. S1), which is in good accordance with the results of XPS analysis. The micromorphology and structure of the sample of S2 were further characterized by transmission electron microscopy (TEM), as displayed in Fig. 5. From Fig. 5(a)e(c), it is clear that cubic-like CeO2 nanoparticles with slight aggregation are loaded on the

Fig. 4. SEM images with different magnifications for the samples: (a)e(c) of S1, (d)e(f) of S2 and (g)e(i) of S3.

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Fig. 5. (a)e(c) TEM images with different magnification, (d) HRTEM image and (e) histogram of particle size distribution of CeO2 nanoparticles for the sample of S2.

crumpled surfaces of thinly flake-like RGO. Furthermore, it should be noted that almost no CeO2 nanoparticles drop off from the surfaces of RGO under powerful ultrasound treatment [31]. Thus, we believe that the CeO2 nanoparticles are tightly anchored on the

surfaces of RGO. As shown in the high-resolution transmission electron microscopy (HRTEM) image of Fig. 5(d), the inter-plane distance of 0.311 nm corresponds to the (311) crystal plane of CeO2. Fig. 5(e) demonstrates that the CeO2 nanoparticles exhibit a

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Fig. 6. Frequency dependence of reflection loss with different thicknesses and 3D plots of reflection loss for the samples: (a) and (a’) S1, (b) and (b’) S2, (c) and (c’) S3.

small statistical average size of 12.45 nm. 3.4. Microwave absorption properties The microwave absorption properties of absorbers can be evaluated by the reflection loss (RL), which are calculated by the following equations according to the transmission line theory [1e5]:

  Z  Z 0   RLðdBÞ ¼ 20lg in Zin þ Z0  Zin ¼ Z0

rffiffiffiffiffi    mr 2pfd pffiffiffiffiffiffiffiffiffi tanh j mr εr c εr

(1)

(2)

Herein Zin is the input impedance of absorber, Z0 is the impedance of free space, εr is the relative complex permittivity, mr is the relative complex permeability, d is the thickness of the absorber, c is the velocity of light in free space, and f is the frequency. As described in Fig. 6(a) and (a’), the sample of S1 shows the RLmin of-6.6 dB with a matching thickness of 5.0 mm, which

manifests that the pure CeO2 has weak microwave absorption performance. From Fig. 6(b) and (b’), the sample of S2 exhibits the RLmin of 49.2 dB at 5.2 GHz with a thickness of 4.46 mm and EAB of 4.1 GHz (13.4e17.5 GHz) with a thin thickness of merely 1.7 mm. As shown in Fig. 6(c) and (c’), the sample of S3 presents the RLmin of 13.1 dB at 12.1 GHz with a thickness of 1.5 mm. Thus, the microwave absorption properties including RLmin and EAB of the samples firstly enhance and then decrease as the contents of RGO increase. Significantly, the sample of S2 shows the optimal microwave absorption performance among all the samples. Furthermore, the three-dimensional (3D) plots of reflection loss demonstrate that the RLmin corresponding to the maximum microwave absorption could locate at various frequencies by modulating the thicknesses of absorbers [5,33]. Generally, the electromagnetic parameters (ε0 , ε’’, m0 , m’’) are vitally important to determine the microwave absorption properties of absorbers [5,32,33]. The real permittivity (ε0 ) and real permeability (m0 ) represent the storage ability of electric and magnetic field energies, whereas the imaginary permittivity (ε’’) and imaginary permeability (m’’) indicate the dissipation capacity of electric and magnetic field energies, respectively [5,32,33]. Thus,

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Fig. 7. Frequency dependence of (a) ε0 , (b) ε’’, (c) m0 , (d) m’’, (e) tande and (f) tandm for the samples of S1, S2 and S3. According to Debye theory, dielectric loss consists of both conductive loss and polarization loss [47]. According to Debye theory, the ε0 and ε’’ follow the equation [32,33,47].



ε0 

ε  ε 2 εs  ε∞ 2 00 s ∞ þ ðε Þ2 ¼ 2 2

the frequency dependence of electromagnetic parameters for the samples of S1, S2 and S3 was carefully investigated, as depicted in Fig. 7(a)e(d). From Fig. 7(a), the ε0 of hybrid nanocomposites shows a decline trend with the increasing of frequency and reveals a frequency dispersion effect, which is beneficial to the microwave absorption [34]. Specifically, the ε0 decreases from 23.3 to 13.8, 12.1 to 8.7 for S3 and S2, respectively. While the ε0 of pure CeO2 almost keeps constant around 4.6, which is much lower than that of S2 and S3. This result indicates that the ε0 enhances as the contents of RGO

(3)

increase. As shown in Fig. 7(b), the ε’’ shows a similar increase trend as ε0 with the increasing of contents of RGO. On the basis of free electron theory, it can be deduced that the ε’’ enhances with the increasing of electric conductivity [5,30,34]. We have measured the electrical conductivity of RGO/CeO2 hybrid nanocomposites (S2 and S3) by a four-point probe method. The values of electrical conductivity for S2 and S3 are 76.9 and 303.0 S/m, respectively. However, the pure CeO2 is an electrical insulator [25,35]. Thus, the electrical conductivity of hybrid nanocomposites enhances with

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Fig. 8. Cole‒Cole semicircle (ε’’‒ε0 ) curves of the samples: (a) S1, (b) S2 and (c) S3; (d) Frequency dependence of C0 for the samples of S1, S2 and S3.

Fig. 9. Frequency dependence of (a) attenuation constant (a) and (b) normalized impedance matching (Z) with a thickness of 4.46 mm for the samples of S1, S2 and S3.

the increasing of contents of RGO, leading to the enhancement of ε’’. Numerous investigations demonstrated that ε0 kept pace with the variation in electric conductivity and there existed identical change rules about ε0 and ε’’ [34,36,37]. Therefore, the S3 exhibits the biggest ε0 and ε’’, which manifests the enhanced storage capability of electric energy and dielectric loss [34,38]. From Fig. 7(c), the m0 of S1, S2 and S3 presents some fluctuations with the increasing of frequency and the values of m0 are in the range of 0.95e1.35. As described in Fig. 7(d), the m’’ of S1, S2 and S3 presents a slightly decreasing trend as the frequency increases. Furthermore, the m0 and m’’ of S1 show obvious peak values in the low frequency region (4e5 GHz). Besides, it should be noted that the samples of S2 and S3 exhibit negative m’’ in the high frequency region. Recently, the negative permeability phenomena have been observed in many low resistivity absorbent systems [39e43]. Meanwhile, more investigations pointed out that the negative m’’ means the radiation of magnetic energy induced by eddy current loss in low resistivity composites [44e46]. Attenuation loss includes dielectric loss and magnetic loss,

which plays a vital role in microwave attenuation [5,32]. Thus, the frequency-dependent dielectric loss tangent (tande ¼ ε’’/ε0 ) and magnetic loss tangent (tandm ¼ m’’/m0 ) for the samples of S1, S2 and S3 were investigated. From Fig. 7(e), the values of tande for S1 (pure CeO2) are in the range of 0e0.1, indicating the weak dielectric loss capacity. However, the S2 and S3 exhibit obviously enhanced tande than S1 in the whole frequency range, suggesting the improved dielectric loss capacity. Notably, the S3 displays the strongest dielectric loss capacity among the three samples. The obviously enhanced dielectric loss could be attributed to the notably increased ε0 and ε’’ of S3. As depicted in Fig. 7(f), the tandm of S1, S2 and S3 displays a similar tendency as m’’ with the increasing of frequency. Furthermore, it can be found that the tandm decreases with the increasing of contents of RGO. Besides, combined Fig. 7(e) with Fig. 7(f), it is clear that both dielectric loss and magnetic loss play important roles for the microwave attenuation. Herein εs, ε∞, ε0 and ε’’ are the static permittivity, relative dielectric permittivity at high-frequency limit, real part and imaginary part of permittivity, respectively [32,33,47]. Based on

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equation (3), the curve of (ε00 ~ε0 ) should be a single semicircle, which is known as Cole-Cole semicircle [32,33,47]. Each semicircle represents a Debye relaxation process [32,33,47]. Fig. 8(a)‒(c) shows the Cole‒Cole plots for the samples of S1, S2 and S3. Fig. 8(a) shows the S1 with none of Cole-Cole semicircle, which suggests no Debye dipolar relaxation process [32,33,48]. However, from Fig. 8(b) and (c), the S2 and S3 present at least two semicircles, which indicate the existing of dual Debye dipolar relaxation processes. Furthermore, it can be observed that the semicircles are distorted in some degree, which suggests that the Debye relaxation is not the only mechanism for dielectric loss and other mechanisms such as conductive loss and interfacial polarization could be responsible for microwave absorption [32,33,48]. Notably, the straight line in the high ε0 region for the samples of S2 and S3 could be ascribed to the conduction loss [49]. Generally, eddy current loss and natural resonance are the main reasons for magnetic loss in the microwave frequency range [32,47,50]. The eddy current loss can be evaluated by the following equations [32,47,50]:

m’’ z2pm0 ðm’ Þ2 sd2 f

.

3

C0 ¼ m’’ ðm’ Þ2 f 1

(4) (5)

Herein C0 is eddy current coefficient, s is the electric conductivity, m0 is the vacuum permeability and d is the thickness of absorber. If magnetic loss exclusively results from eddy current effect, the values of C0 should keep constant as the frequency increasing [32,47,50]. However, the curves of C0 ~ f for all the samples in Fig. 8(d) display notable fluctuations with the increasing of frequency, thus excluding the eddy current loss [47]. For all the samples, an obvious change of C0 with the increasing of frequency can be observed in the frequency range of 2e6 GHz, which suggests that the natural resonance should be responsible for magnetic loss under this situation [47,50]. In general, an ideal microwave absorber should satisfy the two requirements of impedance matching and maximum attenuation. Normalized impedance matching (Z) is often described as follows [5,32,50e52]:

  Z  Z ¼  in  ¼ Z0

rffiffiffiffiffi     mr 2pfd pffiffiffiffiffiffiffiffiffi   tanh j m ε r r  ε  c r

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moderate electromagnetic attenuation capacity could effectively transform the electromagnetic energies into thermal energies [5,32,50]. Therefore, the S2 demonstrates the best microwave absorption performance among the three samples. The relationship between absorption peak frequency (fm) and matching thickness (tm) can be well clarified by quarterwavelength (l/4) matching theory, which is usually described as follows [5,33,35]:

tm ¼

nl nc pffiffiffiffiffiffiffiffiffiffiffiffi ðn ¼ 1; 2; 5:::Þ ¼ 4 4fm jεr mr j

(8)

If tm and fm meet the above equation, a phase cancellation effect could effectively attenuate the incident microwaves [5,33,35]. As depicted in Fig. 10(a), it can be observed that the RL peaks of S2 shift to lower frequency with the increasing of tm. Fig. 10(b) shows the simulations of tm versus fm under the l/4 model. The pentagram signifies the experimental tm (denoted as texp m ). Significantly, all the texp m are exactly located at the l/4 curve. This finding indicates that the l/4 matching theory essentially determines the relationship between tm and fm. Therefore, it is meaningful to design the thickness of absorbers according to the quarterwavelength matching theory. Besides, the strongest RL peak (49.2 dB at 5.2 GHz and 4.46 mm) corresponds well with the optimal impedance matching of Z equal to 1 (Fig. 10(c)). Fig. 11 describes the possible microwave absorption mechanisms of RGO/CeO2 hybrid nanocomposites. Firstly, the residual groups (‒COOH and eOH) and structure defects on the surfaces or edges of RGO could induce the dipole polarization and defect polarization under the alternating electromagnetic fields, respectively [31,35]. Meanwhile, the CeO2 nanoparticles can also act as the polarized centers, which further enhance the dipole polarization relaxation [25,30]. Secondly, numerously heterogeneous interfaces among paraffin matrix, RGO and CeO2 could be considered as the capacitor-like structure [30,32,55]. According to the model proposed by Cao et al. [55], the capacitor-like structure at the interfaces could attenuate the power of incident microwaves by aligning the polar bonds or charges under the alternating electromagnetic fields [55]. Thirdly, according to the Cao’s Electron-Hopping model [56,57], the electrons can absorb electromagnetic energies to

(6)

Electromagnetic attenuation capacity is often reflected by the attenuation constant (a), which can be expressed as follows [5,32,50,53,54]:

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2pf  ðm’’ε’’  m’ε’Þ þ ðm’’ε’’  m’ε’Þ2 þ ðε’m’’ þ ε’’m’Þ2 a¼ c (7) Fig. 9 shows the frequency-dependent a and Z for the samples of S1, S2 and S3. From Fig. 9(a), it is clear that the S3 displays the biggest value of a (365.2), suggesting the strongest electromagnetic attenuation capacity. The enhanced attenuation capacity mainly originates from the enhancement of dielectric loss. However, the S3 does not display the best microwave absorption performance, as shown in Fig. 6(c) and (c’). Therefore, the impedance matching characteristic should be further considered. As described in Fig. 9(b), the values of Z for pure CeO2 are far from 1, which suggests inferior impedance matching. Moreover, the values of Z for S2 are much closer to the line of optimal impedance matching (Z ¼ 1) than S1 and S3, which indicate the improved impedance matching. As the optimal impedance matching is achieved, most of the incident microwaves can enter into the specimen [5,32,50]; meanwhile, a

Fig. 10. (a) Frequency-dependent reflection loss, (b) simulations of tm versus fm under the l/4 model and (c) normalized impedance matching (Z) as a function of frequency for the sample of S2 with different thicknesses.

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Fig. 11. Schematic illustration of the possible microwave absorption mechanisms of RGO/CeO2 hybrid nanocomposites.

migrate on the surfaces of RGO, which enhance the conduction loss capacity and further covert electromagnetic energies into thermal energies by colliding with the lattice [58]. Lastly, numerous oxygen vacancies defects in the hybrid nanocomposites evidenced from the XPS analysis could induce the conduction loss and charge polarization relaxation [25,30].

Appendix A. Supplementary data

4. Conclusions

[1] J. Luo, K. Zhang, M. Cheng, M. Gu, X. Sun, MoS2 spheres decorated on hollow porous ZnO microspheres with strong wideband microwave absorption, Chem. Eng. J. 380 (2020) 122625. [2] Y. Wang, X. Gao, X. Wu, W. Zhang, C. Luo, P. Liu, Facile design of 3D hierarchical NiFe2O4/N-GN/ZnO composite as a high performance electromagnetic wave absorber, Chem. Eng. J. 375 (2019) 121942. [3] P. Liu, Y. Zhang, J. Yan, Y. Huang, L. Xia, Z. Guang, Facile synthesis of ultralightweight N-doped graphene foams with open reticular structures for high-efficiency electromagnetic wave absorption performance, Chem. Eng. J. 368 (2019) 285e298. [4] J. Yu, J. Yu, T. Ying, C. Cui, Y. Sun, X. Liu, The design and the preparation of mesoporous Ag3PO4 nanorod/SrFe12O19 hexagonal nanoflake heterostructure for excellent microwave absorption, J. Alloy. Comp. 775 (2019) 225e232. [5] R. Shu, W. Li, Y. Wu, J. Zhang, G. Zhang, Nitrogen-doped Co-C/MWCNTs nanocomposites derived from bimetallic metal-organic frameworks for electromagnetic wave absorption in the X-band, Chem. Eng. J. 362 (2019) 513e524. [6] Y. Wang, X. Gao, C. Lin, L. Shi, X. Li, G. Wu, Metal organic frameworks-derived Fe-Co nanoporous carbon/graphene composite as a high-performance electromagnetic wave absorber, J. Alloy. Comp. 785 (2019) 765e773. [7] C. Wang, X. Han, P. Xu, X. Zhang, Y. Du, S. Hu, J. Wang, X. Wang, The electromagnetic property of chemically reduced graphene oxide and its application as microwave absorbing material, Appl. Phys. Lett. 98 (2011), 072906. [8] K. Zhang, J. Luo, N. Yu, M. Gu, X. Sun, Synthesis and excellent electromagnetic absorption properties of reduced graphene oxide/PANI/BaNd0.2Sm0.2Fe11.6O19 nanocomposites, J. Alloy. Comp. 779 (2019) 270e279. [9] J. Luo, L. Yue, H. Ji, K. Zhang, N. Yu, Investigation on the optimization, design and microwave absorption properties of BaTb0.2Eu0.2Fe11.6O19/PANI decorated on reduced graphene oxide nanocomposites, J. Mater. Sci. 54 (2019) 6332e6346. [10] M. Cao, C. Han, X. Wang, M. Zhang, Y. Zhang, J. Shu, H. Yang, X. Fang, J. Yuan, Graphene nanohybrids: excellent electromagnetic properties for the absorbing and shielding of electromagnetic waves, J. Mater. Chem. C 6 (2018) 4586e4602. [11] J. He, X. Wang, Y. Zhang, M. Cao, Small magnetic nanoparticles decorating reduced graphene oxides to tune the electromagnetic attenuation capacity, J. Mater. Chem. C 4 (2016) 7130e7140. [12] Y. Zhang, X. Wang, M. Cao, Confinedly implanted NiFe2O4-rGO: Cluster tailoring and highly tunable electromagnetic properties for selectivefrequency microwave absorption, Nano Res 11 (2018) 1426e1436. [13] Y. Liu, Z. Chen, Y. Zhang, R. Feng, X. Chen, C. Xiong, L. Dong, Broadband and lightweight microwave absorber Constructed by in situ growth of hierarchical CoFe2O4/reduced graphene oxide porous nanocomposites, ACS Appl. Mater. Interfaces 10 (2018) 13860e13868. [14] F. Yan, D. Guo, S. Zhang, C. Li, C. Zhu, X. Zhang, Y. Chen, Ultra-small NiFe2O4 hollow particle/graphene hybrid: fabrication and electromagnetic wave absorption property, Nanoscale 10 (2018) 2697e2703. [15] R. Shu, G. Zhang, J. Zhang, X. Wang, M. Wang, Y. Gan, J. Shi, J. He, Synthesis and high-performance microwave absorption of reduced graphene oxide/zinc

In summary, RGO/CeO2 hybrid nanocomposites were successfully prepared by a facile one-pot hydrothermal strategy. Results revealed that the dispersion of particles became better after hybridization CeO2 with RGO and numerously cubic-like CeO2 nanoparticles with a statistical average size of 12.45 nm were uniformly loaded on the surfaces or edges of RGO. Moreover, the contents of RGO had remarkable influence on the micromorphology, oxygen vacancies concentrations, electromagnetic parameters and microwave absorption properties of RGO/CeO2 hybrid nanocomposites. Significantly, the as-prepared hybrid nanocomposites with the content of RGO of 4.1 wt% displayed the optimal RL of 49.2 dB and EAB of 4.1 GHz for a thin thickness of only 1.7 mm. Besides, the possible microwave absorption mechanisms of obtained hybrid nanocomposites were proposed, which could be ascribed to the synergistic effects of dipole polarization, defect polarization and interfacial polarization, enhanced conduction loss derived from electron migration, charge polarization relaxation originated from oxygen vacancies defects. Therefore, the obtained hybrid nanocomposites could be used as high-efficiency microwave absorbers in the field of electromagnetic absorption. Declaration of competing interest We declare no competing financial interest. Acknowledgments This work was financially supported by the Foundation of Provincial Natural Science Research Project of Anhui Colleges (Grant No. KJ2019A0119), China Postdoctoral Science Foundation (Grant No. 2019M652160), National Natural Science Foundation of China (Grant No. 51507003), Key Project of Science and Technology of Huainan (Grant No. 2018A362), Lift Engineering of Young Talents and Doctor’s Start-up Research Foundation of Anhui University of Science and Technology (Grant No. ZY537).

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.152766. References

Please cite this article as: Y. Wu et al., Facile design of cubic-like cerium oxide nanoparticles decorated reduced graphene oxide with enhanced microwave absorption properties, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152766

Y. Wu et al. / Journal of Alloys and Compounds xxx (xxxx) xxx ferrite hybrid nanocomposite, Mater. Lett. 215 (2018) 229e232. [16] Y. Lin, J. Dong, H. Zong, B. Wen, H. Yang, Synthesis, characterization, and electromagnetic wave absorption properties of composites of reduced graphene oxide with porous LiFe5O8 microspheres, ACS Sustain. Chem. Eng. 6 (2018) 10011e10020. [17] M. Han, X. Yin, L. Kong, M. Li, W. Duan, L. Zhang, L. Chen, Graphene-wrapped ZnO hollow spheres with enhanced electromagnetic wave absorption properties, J. Mater. Chem. 2 (2014) 16403e16409. [18] Y. Wang, D. Chen, X. Yin, P. Xu, F. Wu, M. He, Hybrid of MoS2 and reduced graphene oxide: a lightweight and broadband electromagnetic wave absorber, ACS Appl. Mater. Interfaces 7 (2015) 26226e26234. [19] X. Wang, J. Yu, H. Dong, M. Yu, B. Zhang, W. Wang, L. Dong, Synthesis of nanostructured MnO2, SnO2, and Co3O4: graphene composites with enhanced microwave absorption properties, Appl. Phys. A 119 (2015) 1483e1490. [20] W. Feng, Y. Wang, J. Chen, L. Wang, L. Guo, J. Ouyang, D. Jia, Y. Zhou, Reduced graphene oxide decorated with in-situ growing ZnO nanocrystals: facile synthesis and enhanced microwave absorption properties, Carbon 108 (2016) 52e60. [21] Y. Zuo, J. Luo, M. Cheng, K. Zhang, R. Dong, Synthesis, characterization and enhanced electromagnetic properties of BaTiO3/NiFe2O4-decorated reduced graphene oxide nanosheets, J. Alloy. Comp. 744 (2018) 310e320. [22] J. Zhang, R. Shu, Y. Ma, X. Tang, G. Zhang, Iron ions doping enhanced electromagnetic wave absorption properties of tin dioxide/reduced graphene oxide nanocomposites, J. Alloy. Comp. 777 (2019) 1115e1123. [23] Y. Zhang, S. Gao, H. Xing, Reduced graphene oxide wrapped cube-like ZnSnO3: as a high-performance microwave absorber, J. Alloy. Comp. 777 (2019) 544e553. [24] H. Wu, L. Wang, Y. Wang, S. Guo, Microwave absorption properties of CeO2 and Zn-modified CeO2 microstructures, Appl. Surf. Sci. 258 (2012) 10047e10052. [25] Q. Yin, H. Xing, R. Shu, X. Ji, D. Tan, Y. Gan, Enhanced microwave absorption properties of CeO2 nanoparticles supported on reduced graphene oxide, Nano 11 (2016) 1650058. [26] H. Xing, Q. Yin, Z. Liu, L. Wang, Excellent microwave absorption behaviors of polyaniline composites containing CeO2 nanorods in the X-band, Nano 12 (2017) 1750047. [27] Z. Shen, H. Xing, H. Wang, H. Jia, Y. Liu, A. Chen, P. Yang, Synthesis and enhanced electromagnetic absorption properties of Co-doped CeO2/RGO nanocomposites, J. Alloy. Comp. 753 (2018) 28e34. [28] Z. Wang, P. Zhao, D. He, Y. Cheng, L. Liao, S. Li, Y. Luo, Z. Peng, P. Li, Cerium oxide immobilized reduced graphene oxide hybrids with excellent microwave absorbing performance, Phys. Chem. Chem. Phys. 20 (2018) 14155e14165. [29] Q. Li, J. Liu, Y. Zhao, X. Zhao, W. You, X. Li, R. Che, “Matryoshka doll”-like CeO2 microspheres with hierarchical structure to achieve significantly enhanced microwave absorption performance, ACS Appl. Mater. Interfaces 10 (2018) 27540e27547. [30] Y. Wu, R. Shu, J. Zhang, R. Sun, Y. Chen, J. Yuan, Oxygen vacancy defects enhanced electromagnetic wave absorption properties of 3D net-like multiwalled carbon nanotubes/cerium oxide nanocomposites, J. Alloy. Comp. 785 (2019) 616e626. [31] R. Shu, G. Zhang, J. Zhang, X. Wang, M. Wang, Y. Gan, J. Shi, J. He, Fabrication of reduced graphene oxide/multi-walled carbon nanotubes/zinc ferrite hybrid composites as high-performance microwave absorbers, J. Alloy. Comp. 736 (2018) 1e11. [32] R. Shu, Y. Wu, Z. Li, J. Zhang, Z. Wan, Y. Liu, M. Zheng, Facile synthesis of cobalt-zinc ferrite microspheres decorated nitrogen-doped multi-walled carbon nanotubes hybrid composites with excellent microwave absorption in the X-band, Compos. Sci. Technol. 184 (2019) 107839. [33] R. Shu, G. Zhang, X. Wang, X. Gao, M. Wang, Y. Gan, J. Shi, J. He, Fabrication of 3D net-like MWCNTs/ZnFe2O4 hybrid composites as high-performance electromagnetic wave absorbers, Chem. Eng. J. 337 (2018) 242e255. [34] B. Quan, X. Liang, G. Xu, Y. Cheng, Y. Zhang, W. Liu, G. Ji, Y. Du, A permittivity regulating strategy to achieve high-performance electromagnetic wave absorbers with compatibility of impedance matching and energy conservation, New J. Chem. 41 (2017) 1259e1266. [35] R. Suresh, V. Ponnuswamy, R. Mariappan, Effect of annealing temperature on the microstructural, optical and electrical properties of CeO2 nanoparticles by chemical precipitation method, Appl. Surf. Sci. 273 (2013) 457e464. [36] B. Zhao, B. Fan, G. Shao, W. Zhao, R. Zhang, Facile synthesis of novel heterostructure based on SnO2 nanorods grown on submicron Ni walnut with tunable electromagnetic wave absorption capabilities, ACS Appl. Mater. Interfaces 7 (2015) 18815e18823. [37] B. Zhao, G. Shao, B. Fan, W. Zhao, R. Zhang, Facile synthesis and enhanced

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

[51]

[52]

[53]

[54]

[55]

[56]

[57]

[58]

11

microwave absorption properties of novel hierarchical heterostructures based on a Ni microsphereeCuO nano-rice coreeshell composite, Phys. Chem. Chem. Phys. 17 (2015) 6044e6052. M. Cao, X. Wang, M. Zhang, J. Shu, W. Cao, H. Yang, X. Fang, J. Yuan, Electromagnetic response and energy conversion for functions and devices in low-dimensional materials, Adv. Funct. Mater. (2019) 1807398. Y. Du, W. Liu, R. Qiang, Y. Wang, X. Han, J. Ma, P. Xu, Shell thicknessdependent microwave absorption of coreeshell [email protected] composites, ACS Appl. Mater. Interfaces 6 (2014) 12997e13006. L. Wang, X. Jia, Y. Li, F. Yang, L. Zhang, L. Liu, X. Ren, H. Yang, Synthesis and microwave absorption property of flexible magnetic film based on graphene oxide/carbon nanotubes and Fe3O4 nanoparticles, J. Mater. Chem. 2 (2014) 14940e14946. D. Chen, G. Wang, S. He, J. Liu, L. Guo, M. Cao, Controllable fabrication of mono-dispersed RGOehematite nanocomposites and their enhanced wave absorption properties, J. Mater. Chem. 1 (2013) 5996e6003. Y. Chen, P. Gao, R. Wang, C. Zhu, L. Wang, M. Cao, H. Jin, Porous Fe3O4/SnO2 core/shell nanorods: synthesis and electromagnetic properties, J. Phys. Chem. C 113 (2009) 10061e10064. X. Shi, M. Cao, J. Yuan, X. Fang, Dual nonlinear dielectric resonance and nesting microwave absorption peaks of hollow Cobalt nanochains composites with negative permeability, Appl. Phys. Lett. 95 (2009) 163108e163111. S. Guo, L. Wang, H. Wu, Facile synthesis and enhanced electromagnetic wave absorption of thorny-like FeeNi alloy/ordered mesoporous carbon composite, Adv. Powder Technol. 26 (2015) 1250e1255. J. Zhou, J. He, T. Wang, G. Li, Y. Guo, J. Zhao, Y. Ma, Design of mesostructured gFe2O3/carbon nanocomposites for electromagnetic wave absorption applications, J. Alloy. Comp. 509 (2011) 8211e8214. X. Zhang, P. Guan, X. Dong, Transform between the permeability and permittivity in the close-packed Ni nanoparticles, Appl. Phys. Lett. 97 (2010), 033107. Q. Liu, X. Liu, H. Feng, H. Shui, R. Yu, Metal organic framework-derived Fe/ carbon porous composite with low Fe content for lightweight and highly efficient electromagnetic wave absorber, Chem. Eng. J. 314 (2017) 320e327. R. Shu, W. Li, X. Zhou, D. Tian, G. Zhang, Y. Gan, J. Shi, J. He, Facile preparation and microwave absorption properties of RGO/MWCNTs/ZnFe2O4 hybrid nanocomposites, J. Alloy. Comp. 743 (2018) 163e174. X. Cui, W. Liu, W. Gu, X. Liang, G. Ji, Two-dimensional MoS2 modified using CoFe2O4 nanoparticles with enhanced microwave response in the X and Ku band, Inorg. Chem. Front. 6 (2019) 590e597. R. Shu, W. Li, Y. Wu, J. Zhang, G. Zhang, M. Zheng, Fabrication of nitrogendoped cobalt oxide/cobalt/carbon nanocomposites derived from heterobimetallic zeolitic imidazolate frameworks with superior microwave absorption properties, Compos. B Eng. 178 (2019) 107518. W. Liu, Q. Shao, G. Ji, X. Liang, Y. Cheng, B. Quan, Y. Du, Metaleorganicframeworks derived porous carbon-wrapped Ni composites with optimized impedance matching as excellent lightweight electromagnetic wave absorber, Chem. Eng. J. 313 (2017) 734e744. D. Li, B. Zhang, W. Liu, X. Liang, G. Ji, Tailoring the input impedance of FeCo/C composites with efficient broadband absorption, Dalton Trans. 46 (2017) 14926e14933. L. Wang, X. Bai, B. Wen, Z. Du, Y. Lin, Honeycomb-like Co/C composites derived from hierarchically nanoporous ZIF-67 as a lightweight and highly efficient microwave absorber, Compos. B Eng. 166 (2019) 464e471. X. Zhang, J. Wang, X. Su, S. Huo, Facile synthesis of reduced graphene oxidewrapped CNFs with controllable chemical reduction degree for enhanced microwave absorption performance, J. Colloid Interface Sci. 553 (2019) 402e408. M. Cao, J. Yang, W. Song, D. Zhang, B. Wen, H. Jin, Z. Hou, J. Yuan, Ferroferric oxide/multiwalled carbon nanotube vs polyaniline/ferroferric oxide/multiwalled carbon nanotube multiheterostructures for highly effective microwave absorption, ACS Appl. Mater. Interfaces 4 (2012) 6949e6956. W. Song, M. Cao, Z. Hou, X. Fang, X. Shi, J. Yuan, High dielectric loss and its monotonic dependence of conducting-dominated multiwalled carbon nanotubes/silica nanocomposite on temperature ranging from 373 to 873 K in Xband, Appl. Phys. Lett. 94 (2009) 233110. M. Cao, W. Song, Z. Hou, B. Wen, J. Yuan, The effects of temperature and frequency on the dielectric properties, electromagnetic interference shielding and microwave-absorption of short carbon fiber/silica composites, Carbon 48 (2010) 788e796. M. Cao, X. Wang, W. Cao, X. Fang, B. Wen, J. Yuan, Thermally driven transport and relaxation switching self-powered electromagnetic energy conversion, Small 14 (2018) 1800987.

Please cite this article as: Y. Wu et al., Facile design of cubic-like cerium oxide nanoparticles decorated reduced graphene oxide with enhanced microwave absorption properties, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152766