Broadband and Lightweight Microwave Absorber Constructed by in

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Functional Nanostructured Materials (including low-D carbon)

Broadband and Lightweight Microwave Absorber Constructed by in-situ Growth of Hierarchical CoFeO/rGO Porous Nanocomposites 2

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Yang Liu, Zhuo Chen, Yang Zhang, Rui Feng, Xiao Chen, Chuanxi Xiong, and Lijie Dong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02137 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018

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ACS Applied Materials & Interfaces

Broadband

and

Lightweight

Microwave

Absorber Constructed by in-situ Growth of Hierarchical

CoFe2O4/rGO

Porous

Nanocomposites Yang Liu,a Zhuo Chen,a Yang Zhang,a Rui Feng,a Xiao Chen,a Chuanxi Xiong,a Lijie Dong*, a a

Center for Smart Materials and Devices, State Key Laboratory of Advanced

Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China. ABSTRACT: Broadband and lightweight microwave absorber has attracted soaring research interest because of the increasing demand for electronic reliability and defense security. Lightweight ferrites/graphene porous composites with abundant interfaces are potential high-performance absorbers owing to their balanced attenuation ability and impedance matching. Herein, we synthesized hierarchical CoFe2O4/reduced graphene oxide (CFO/rGO) nanocomposites with a porous structure via an in-situ solvothermal method. The electromagnetic parameters of CFO/rGO nanocomposites can be well adjusted by modulating the weight fraction of rGO. Hierarchical porous structure and proper electromagnetic parameters result in the enhancement of impedance matching and attenuation ability. Benefiting from the controllable composition, hierarchical porous structure, and strong synergetic effect between CFO and rGO sheets, as expected, CFO/rGO nanocomposites exhibit superior microwave absorption

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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performance with an ultra-broad bandwidth reaching 5.8 GHz (8.3-14.1 GHz) with a thin thickness of 2.8 mm. Meanwhile, a strong reflection loss of -57.7 dB at the same thickness is achieved. Considering the outstanding microwave absorption performance, the hierarchical CFO/rGO porous nanocomposites can be employed as highperformance microwave absorber. KEYWORDS: CoFe2O4/rGO, hierarchical structure, multiple interfaces, broadband microwave absorption, impedance matching, lightweight absorber 1. INTRODUCTION Microwave absorption materials (MAMs) with broadband absorption performance are highly demanded in electronic stability, healthcare and military fields because of the inundant electromagnetic interference generated by the explosive usage of broadband and high-power electronic instruments.1-2 Recently, MAMs enjoying the advantages of lightweight and thin thickness have attracted increasing demand for aerospace, automobile and miniature electronics.3-4 It is well acknowledged that microwave contains electric and magnetic components, and the absorption of microwave energy of MAMs is basically decided by the dielectric and/or magnetic loss.5 However, materials with only single loss factor cannot exhibit strong and broadband microwave absorption features because of the impedance mismatching and limited loss mechanism. Fortunately, based on the elaborate structure and abundant interfaces, magnetic/dielectric composites can simultaneously exhibit matched impedance

and

strong

attenuation

ability.6-8

Therefore,

developing

the

magnetic/dielectric composites with adjustable electromagnetic parameters and different loss mechanisms has become an effective way to fabricate high-performance MAMs.


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Reduced graphene oxide (rGO), a two-dimensional material, draws considerable attention in MAMs field for the distinct and outstanding properties including low density, abundant defects, high specific surface area and large aspect ratio.9-11 Nevertheless, impedance mismatching caused by improper high electric conductivity, single loss mechanism and severe agglomeration of pristine rGO sheets in matrix limit its application as broadband MAMs.12-13 Extensive studies have been reported that modification on the surface of rGO with nanomaterials is a feasible approach to overcome the matters.14-17 Chen and Cao et al. investigated that rGO-hematite nanocomposites showed improved microwave absorption performance because of the quasi core-shell structure of the nanocomposites.15 Zhang et al. synthesized CoS2 nanocrystals embedded into rGO and investigated microwave absorption performance of CoS2/rGO nanocomposites.18 However, to date, rGO-based composites cannot achieve powerful RL values in wide frequency range due to the limited magnetic loss factor. Therefore, combining rGO sheets with magnetic ferrites undoubtedly becomes an advisable strategy. Cobalt ferrite (CoFe2O4， CFO), exhibits moderate room-temperature saturation magnetization, rather low real part permittivity and excellent chemical stability, making it a promising candidate as MAMs.4, 19 Unfortunately, sole CFO materials suffer from shortcomings including poor attenuation ability, high density, aggregation and large coating thickness, which hamper its practical applications. Thus, researchers have focused on the design of CFO-based nanocomposites and investigation into their microwave absorption performance,20-22 e.g., ZnO/CoFe2O4，23 α-Fe2O3@CoFe2O4,24 polyaniline-CoFe2O4,25 BaFe12O19/CoFe2O4,26 CBC/CoFe2O4 and CoFe2O4-carbonyl iron.21, 27 Furthermore, considering that the broadband and lightweight requirements of the

absorber,

CFO

nanoparticles/graphene

nanocomposites


with

adjustable 3


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composition, microstructure and performance have aroused intensive attention on preparing broadband and lightweight MAMs. Fu et al. fabricated hollow CoFe2O4 sphere/graphene composites and investigated the enhanced microwave absorption performance and possible mechanism.28 Sandwich-like CoFe2O4/graphene composites prepared by one-pot polyol route showed a maximum RL of -36.4 dB at 2.5 mm.29 Similarly, rGO-CoFe2O4 composites provided an RL value of -44.1 dB with the effective bandwidth of 4.7 GHz (13.3-18.0 GHz).30 Although these CFO and graphene composites perform the excellent attenuation ability, the broadband performance of the composites is disappointing. Based on previous research, we may conclude that the hierarchical structure and synergistic effect of rGO-based nanocomposites can adjust the impedance matching condition by balancing the complex permittivity and permeability. Furthermore, the nanoparticles with the plate,24 sphere28 and rod31 structure can provide new channels for the propagation of microwave, leading to more multiple reflection and scattering. The microstructure of the nanoparticles and their distribution on rGO sheets can form abundant interfaces, inducing more interfacial polarization and leading to the significant enhancement of microwave attenuation. Furthermore, the hierarchical structure formed by nanoparticles and rGO sheets can prevent the agglomeration of both nanoparticles and rGO sheets effectively.32 In this study, we prepare the hierarchical CFO/rGO porous nanocomposites via a facile in-situ solvothermal method and investigate their microwave absorption performance. The synthesized uniform-size CFO nanospheres exhibit a porous structure. Porous CFO nanospheres are uniformly embedded in rGO sheets and form the hierarchical structure. Moreover, it is realized that hierarchical porous structure and strong synergetic effect between CFO and rGO sheets can effectively improve


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impedance matching condition and attenuation ability of CFO/rGO nanocomposites, leading to superior microwave absorption performance. Moreover, the possible microwave absorbing mechanism of CFO/rGO nanocomposites is also explored. 2. EXPERIMENTAL SECTION 2.1. Materials. Graphite powder (325 mesh, 99.95%) was provided by Nanjing XFNANO Materials Tech Co. Ltd. Graphene oxide (GO) was synthesized according to a modified Hummer’s method (Supporting Information). FeCl3·6H2O, CoCl2·6H2O and Sodium acetate anhydrous (NaAc) were obtained from Aladdin Co. Ltd. Ethylene glycol (EG), diethylene glycol (DEG) and ethanol were analytical grade and used without further purification. 2.2. Synthesis of CoFe2O4/RGO (CFO/rGO) nanocomposites. CFO/rGO nanocomposites were prepared via a facile in-situ solvothermal method. Typically, 23.4 mg graphene oxide (GO) was dispersed into 20 ml mixed solution of EG and DEG (3:1 in volume) by ultrasonically treated for 2 h to produce a homogeneous brown-yellow solution (solution A). Afterwards, CoCl2·6H2O (1 mmol) and FeCl3·6H2O (2 mmol) were dissolved in solution A by ultrasonically treated for another 1 h. Subsequently, 20 ml NaAc-EG/DEG solution (0.041 g/ml) was added to the above mixture. The brown mixture was magnetically stirred for 1 h before transferred into a 50 ml Teflon-lined stainless steel autoclave for solvothermal reaction at 200 oC for 4 h. When the autoclave was cooled down to room temperature, the as-prepared black CFO/rGO nanocomposites was washed by deionized water and ethanol several times and then freeze-dried, which was remarked as CFO/rGO 10:1. The CFO/rGO nanocomposites were named as CFO/rGO 1:1, CFO/rGO 5:1 and CFO/rGO 20:1, which is consistent with the different rGO contents (234.0 mg, 46.7 mg and 11.7mg, respectively). The


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pure CFO nanospheres were synthesized by the same procedures and conditions only without GO. 2.3. Characterization. The morphology of CFO/rGO nanocomposites were investigated by a scanning electron microscope (SEM Zeiss Ultra Plus) and a highresolution transmission electron microscopy (HRTEM, JEM2100F). The crystal structure of the samples was measured using X-ray diffraction (XRD) under Cu-Kα radiation on a Bruker D8 Advance X-ray diffractometer. Raman spectra were recorded using a RM 2000 Microscopic Confocal Raman Spectrometer (Renishaw PLC) under Ar ion laser with an excitation wavelength of 514.5 nm. The magnetic properties of the samples were measured by vibrating sample magnetometry (Micro Sense Easy VSM9) at room temperature. The thermal stabilities of the nanocomposites were characterized by a thermogravimetric analyzer (TG, TA Discovery) from ambient temperature to 1000 oC under air atmosphere with a heating rate of 20 oC/min. 2.4. Measurements of electromagnetic parameters. The electromagnetic parameters measurements were performed by a vector network analyzer (VNA, Agilent N5247A) in the frequency range of 2-18 GHz. The CFO/rGO nanocomposites were pressed into toroidal shape with φout 7.00 mm and φin 3.04 mm, in which 50 wt % paraffin was used as the binder. The reflection loss curves of CFO/rGO nanocomposites were obtained using the complex permittivity and complex permeability. Based on the transmission line theory, the microwave absorption performance can be estimated by the RL values of the absorber backed by a metal plate. The RL values can be achieved by the relation:33 RL(dB)  20 log ( Zin  Z 0) ( Zin  Z 0)


(1)

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Z in  Z 0 (  r  r ) 1 / 2 tanh[ j ( 2  fd / c )(  r  r ) 1 / 2 ]

(2)

where Zin stands for the input impedance, Z0 is the characteristic impedance of free space (377 Ω), f is the microwave frequency, d represents the thickness of the absorber, c is the velocity of electromagnetic wave in vacuum, εr (εr = ε’-ε’’) and μr (μr = μ’-μ’’) are the complex permittivity and permeability, respectively. 3. RESULTS AND DISCUSSION Scheme 1. Simplified illustration for the synthetic procedure of CFO/rGO nanocomposites.

The overall synthetic procedure for CFO/rGO nanocomposites is illustrated in Scheme 1. First, the ultrasonic treatment is used to fabricate GO sheets loaded with Fe3+ and Co2+ by electrostatic adsorption. Due to the extensive -COOH and -OH groups of GO sheets, Fe3+ and Co2+ can be adsorbed on GO sheets. The electrostatic adsorption between GO sheets and ion precursors can ensure the dispersity of CFO. Second, under solvothermal process, CFO nanocrystals are generated with the addition of NaAc and high temperature (200 oC), and GO is reduced to rGO with the present of EG. Finally,


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CFO nanocrystals assemble into porous nanospheres on the surface of rGO sheets, forming the CFO/rGO nanocomposites. Moderate temperature, NaAc content and EG/DEG proportion are of great importance for the structure and shape of CFO nanospheres and the interfacial interactions between rGO and CFO.34-35 CFO/rGO nanocomposites show high stability, indicating that CFO nanospheres are bonded to the surface of rGO sheets with a strong interaction.


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Figure 1. SEM images of (a) CFO/rGO 20:1, (b) CO/rGO 10:1, (c) CFO/rGO 5:1 and (d) CFO/rGO 1:1 nanocomposites. Representative (e) TEM and (f) HRTEM images of CFO/rGO 10:1 nanocomposites. Figure 1 shows the SEM and TEM images of CFO/rGO nanocomposites with the different weight fraction of rGO. For CFO/rGO 20:1 nanocomposites (Figure 1a), rGO sheets are difficult to be observed. The CFO nanoparticles are agglomerated without uniform shape and dimension. As shown in Figure 1b-d, CFO nanoparticles have typical spherical morphology with the average diameters of ~200 nm, which are uniformly embedded in the flexible rGO sheets, forming a hierarchical 3D network of CFO/rGO nanocomposites. It is clearly observed that CFO nanospheres are distributed on both sides of rGO sheets without agglomeration as the increasing rGO weight fraction. This is probably ascribed to the fact that a large amount of rGO sheets can provide sufficient active sites for CFO precursors, leading to the uniform dimension and distribution of CFO nanospheres. High-magnification SEM images of CFO/rGO nanocomposites reveal that the as-prepared CFO nanospheres exhibit nanoscale rough surface. As presented in Figure 1e, CFO nanospheres wrapped by rGO sheets form the hierarchical structure. The porous structure of CFO nanospheres and low-density rGO sheets indicate the lightweight feature of CFO/rGO nanocomposites. From Figure 1f,


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the inner structure of CFO nanospheres obviously indicates the porous structure of CFO/rGO nanocomposites assembled by the smaller nanocrystals of ~12 nm. The porous structure can introduce more propagation paths, multiple reflection and scattering of microwave. The nanoscale rough surface and porous structure of CFO nanospheres along with the hierarchical structure of CFO/rGO nanocomposites can generate abundant interfacial polarization, which will efficiently enhance the loss of microwave.36

Figure 2. (a) XRD spectra of CFO and CFO/rGO nanocomposites and (b) Raman spectra of GO and CFO/rGO nanocomposites. The structure features of CFO and CFO/rGO nanocomposites are measured using XRD analysis. As shown in Figure 2a, all CFO and CFO/rGO samples exhibit six peaks at 30.0o, 35.4o, 43.0o, 53.4o, 56.9o and 62.5o， corresponding to (220), (311), (400), (422), (511) and (440) of the spinel structured CoFe2O4 (JCPDS card No.22-1086), respectively. Meanwhile, on diffraction peaks at about 11.0o (graphene oxide) or 26.5o (graphite) of CFO/rGO nanocomposites are detected, suggesting the successful reduction of GO without re-stacking during the solvothermal process. No additional


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peaks are observed, confirming that the high purity CFO/rGO nanocomposites are successfully fabricated during the in-situ solvothermal method. The Raman spectra are measured to further explore the structural information of carbon atoms (Figure 2b). CFO/rGO nanocomposites display a peak at around 1330 cm-1 named D band, which represents the vibration of sp3 defects; and the peak at around 1595 cm-1 stands for the vibration of sp2 hybridization (G band). The D band and G band of CFO/rGO nanocomposites present a red-shift owing to the recovery of the hexagonal carbon-network, comparing with 1359 cm-1 and 1604 cm-1 of GO.33 Moreover, the ID/IG values of CFO/rGO 20:1, CFO/rGO 10:1, CFO/rGO 5:1 and CFO/rGO 1:1 nanocomposites relatively increase to 1.15, 1.39, 1.18 and 1.05 from 0.74 of GO. The higher ID/IG values indicate that more defects and disorders are introduced into CFO/rGO nanocomposites during the solvothermal process.12, 37 This is probably caused by the reduction of oxygen-containing groups and porous CFO nanospheres embedded in rGO sheets. The higher ID/IG values indicate that the hierarchical CFO/rGO porous nanocomposites can exhibit higher dielectric loss, which is an important factor of superior MAMs.29


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Figure 3. Room temperature hysteresis loops of CFO nanoparticles and CFO/rGO nanocomposites (Inset is the local enlargement). Figure 3 shows the hysteresis loops of CFO nanoparticles and CFO/rGO nanocomposites at room temperature (300K). The high saturation magnetization (Ms=65.8 emu/g), small remanent magnetization (Mr = 2.4 emu/g) and coercivity (Hc = 35.3 Oe) indicate a quasi superparamagnetic characteristic of CFO nanospheres. The Ms values of CFO/rGO 20:1, CFO/rGO 10:1, CFO/rGO 5:1 and CFO/rGO 1:1 nanocomposites are 59.4, 53.5, 46.7 and 34.2 emu/g, respectively. Ms values of CFO/rGO nanocomposites is smaller than CFO nanoparticles due to the different weight fraction of nonmagnetic rGO in nanocomposites, which corresponds to the results of TG (Figure S2). Nevertheless, Mr and Hc values of CFO/rGO nanocomposites increase slightly as shown in the inset image. Among CFO/rGO 20:1, CFO/rGO 10:1, CFO/rGO 5:1 and CFO/rGO 1:1 nanocomposites, the Mr and Hc value of CFO/rGO 10:1 nanocomposites reach the maximum value, which is 12.9 emu/g and 347.5 Oe, respectively. Higher coercivity can enhance absorption performance at gigahertz due to the increased natural resonance frequency of ferrite.38


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Figure 4. Frequency dependence of (a, b) complex permittivity and (c, d) complex permeability of CFO/rGO nanocomposites. Microwave absorption performance is highly associated with the complex permittivity (εr = ε’-ε’’) and complex permeability (μr = μ’-μ’’) of the absorber. The real part (ε’ and μ’) and the imaginary part (ε’’ and μ’’) represents the storage ability and loss ability of electric and magnetic energy, respectively.39-41 As shown in Figure 4a and b, ε’ and ε’’ values of CFO/rGO nanocomposites increase greatly with the introduction of rGO sheets, which are much higher than the pure CFO nanoparticles (Figure S4). ε’ values of CFO/rGO 20:1, CFO/rGO 10:1, CFO/rGO 5:1 and CFO/rGO 1:1 nanocomposites range from 7.3 to 4.3, 9.4~5.1, 18.1~6.6, and 37.6 ~ 8.9 in 2-18 GHz. It is obvious that ε’’ values (Figure 4b) show a similar trend with the ε’ values. Moreover, ε’ and ε’’ values of CFO/rGO nanocomposites present an increasing trend with the incremental rGO weight fraction in CFO/rGO nanocomposites. The results indicate that combining with rGO sheets can greatly increase the permittivity of CFO/rGO nanocomposites, which caused by the high conductivity of rGO and the synergistic effect of CFO/rGO nanocomposites. In addition, the polarization relaxation caused by the defects of rGO can enhance the permittivity in the microwave range. Considering that the important role of Debye relaxation in the microwave absorbing mechanism, the


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Cole-Cole semicircle of CFO/rGO 10:1 is shown in Figure S3. Based on the result of many semicircles, there may be multiple Debye relaxation processes in CFO/rGO 10:1 nanocomposites.42 The μ’ values of CFO/rGO 20:1, CFO/rGO 10:1, CFO/rGO 5:1 and CFO/rGO 1:1 nanocomposites are in the range of 1.02-1.25, 1.21-1.51, 1.17-1.37 and 1.11-1.31 respectively (Figure 4c). Meanwhile, the μ’’ values of CFO/rGO 20:1, CFO/rGO 10:1, CFO/rGO 5:1 and CFO/rGO 1:1 nanocomposites, as presented in Figure 4d, are in the range of 0.11- (-0.12), 0.11- (-0.22), 0.02- (-0.17) and 0.02- (-0.22) respectively. The complex permeability of CFO/rGO nanocomposites increase slightly compared with CFO nanoparticles. This is because that the permeability of ferrite-based materials is difficult to modulate. However, the slight increase of μ’ and μ’’ indicates the improvement of the impedance matching condition, inducing the enhanced microwave absorption performance.14, 43

Figure 5. Attenuation constant of CFO/rGO nanocomposites.


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Generally, attenuation constant (α) can estimate the microwave attenuation ability of the absorber. The attenuation constant can be defined using the following formula:44



2f  (  ' '  ' '  '  ' )  (  ' '  ' '  '  ' ) 2  ( '  ' ' ' '  ' ) 2 c

(3)

The attenuation constant versus frequency of CFO/rGO nanocomposites is exhibited in Figure 5. Obviously, CFO/rGO nanocomposites perform larger α values than CFO nanoparticles (Figure S5). Moreover, CFO/rGO 1:1 and CFO/rGO 5:1 nanocomposites exhibit larger α values than CFO/rGO 10:1 and CFO/rGO 20:1 nanocomposites. Furthermore, the α values increase as the increasing weight fraction of rGO in CFO/rGO nanocomposites, which results from the cooperation of dielectric and magnetic loss mechanisms. The microwave attenuation ability of CFO/rGO nanocomposites may be mainly derived from the high dielectric loss of rGO sheets. Considering that the loss factor and attenuation constant, there may be a speculation that the microwave absorption performance of CFO/rGO 1:1 and CFO/rGO 5:1 nanocomposites will be superior to CFO/rGO 10:1 and CFO/rGO 20:1 nanocomposites. Unfortunately, the speculation is inconsistent with the results of the calculated RL values shown in Figure 6.


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Figure 6. RL values at different thickness versus frequency and microwave reflection maps of (a, b) CFO/rGO 20:1, (c, d) CFO/rGO 10:1, (e, f) CFO/rGO 5:1 and (g, h) CFO/rGO 1:1.


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Figure 6 exhibits the RL values and microwave reflection maps of CFO/rGO nanocomposites in the frequency range of 2-18 GHz. CFO/rGO 20:1 nanocomposites (Figure 6a and 6b) reach a maximum RL value of -30.8 dB with effective absorption bandwidth of 6.2 GHz (11.8-18.0 GHz) at the thickness of 2.4 mm. As observed in Figure 6c and 6d, CFO/rGO 10:1 nanocomposites exhibit superior absorption performance, obtaining a strong intensity of -57.7 dB and an ultra-broad effective absorption bandwidth of 5.8 GHz (8.3-14.1 GHz) at a relatively thin thickness of 2.8 mm. From Figure 6e-f and g-h, CFO/rGO 5:1 and CFO/rGO 1:1 nanocomposites display a rather poor microwave absorption ability, especially that the RL values of CFO/rGO 1:1 nanocomposites cannot exceed -10 dB in the whole frequency range. The results demonstrate that CFO/rGO 10:1 nanocomposites show strong and broadband microwave absorption performance almost covering the X bands (8-12 GHz). Besides, the absorbing range shifts as changing the thickness of the absorber. Based on the quarter-wavelength matching theory, the response frequency takes place at the lower frequency along with increasing the sample thickness.4, 14, 45

Figure 7. (a) the calculated modulus of normalized characteristic impedance and (b) RL values of CFO/rGO nanocomposites at 2.8 mm.


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For further analyzing the microwave absorption performance of CFO/rGO nanocomposites, the calculated modulus of normalized characteristic impedance of CFO/rGO nanocomposites Z=|Zin/Z0| (at 2.8 mm) are shown in Figure 7a. It is widely recognized that the impedance matching condition acts a pivotal role in controlling the microwave absorption performance of the absorber46-47. The Z values of CFO/rGO 20:1 and CFO/rGO 10:1 nanocomposites are close to 1.0 (Z=1.0 means the perfect impedance matching performance), indicating the good impedance matching performance. Especially, for CFO/rGO 10:1 nanocomposites, the appropriate composition, hierarchical structure and synergistic effect of conductive rGO sheets and magnetic CFO porous nanospheres contribute to its excellent impedance matching performance. With the excellent impedance matching performance of CFO/rGO 10:1 nanocomposites, more microwave can enter the absorber, and then CFO/rGO 10:1 nanocomposites can absorb the microwave energy due to its excellent attenuation ability (Figure 5). On the contrary, the Z values of CFO/rGO 1:1 and CFO/rGO 5:1 nanocomposites are much lower than 1.0, showing the poor impedance matching performance. Therefore, large amount reflection of microwave occurs at the surface of the absorber, resulting in weak microwave absorption performance of CFO/rGO 5:1 and CFO/rGO 1:1 nanocomposites. The favorable matching of complex permittivity and complex permeability of CFO/rGO 10:1 nanocomposites (Figure 4) causes the better impedance matching performance, contributing to the superior microwave absorption

performance.

Modulating

the

electromagnetic

parameters

of

nanocomposites by the design of composition and structure can be a feasible approach to facilitate the impedance matching performance. Figure 7b presents the RL values of CFO/rGO nanocomposites at the thickness of 2.8 mm. Obviously, CFO/rGO 10:1 nanocomposites exhibit superior microwave absorption performance. Consequently,


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enhanced impedance matching performance as well sufficient microwave attenuation ability simultaneously conduce to CFO/rGO 10:1 nanocomposites applied as highperformance microwave absorber. Scheme 2. Schematic illustration of the microwave propagation and attenuation process in the hierarchical CFO/rGO porous structure.

To explore the possible microwave absorbing mechanism, Scheme 2 demonstrates the possible propagation and attenuation process of the incident microwave in the hierarchical CFO/rGO porous nanocomposites. The impedance matching condition and several loss mechanisms comprehensively determine the absorbing performance. First, the enhanced impedance matching performance of hierarchical CFO/rGO porous nanocomposites allows more microwave to enter the absorber, leading to the outstanding microwave absorption performance. The more proper Z values of the nanocomposites achieve from the appropriate composition, hierarchical structure and synergistic effect of conductive rGO sheets and magnetic CFO porous nanospheres. Second, a 3D network constructed by rGO generates the microcurrent and causes dielectric loss, improving the microwave absorption performance effectively. Third, the


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abundant micropores of CFO nanospheres provide more channels for propagation and reflection of the microwave, enhancing the attenuation of the microwave. Moreover, the interfacial polarization resulting from the multiple interfaces between CFO and rGO, rGO and rGO converts microwave energy into thermal energy, which significantly improves the microwave absorbing performance. Therefore, the hierarchical CFO/rGO porous nanocomposites can be applied as a high-performance broadband and lightweight microwave absorber. 4. CONCLUSIONS In this work, we fabricate the hierarchical CFO/rGO porous nanocomposites applied as broadband and lightweight microwave absorber using an in-situ solvothermal progress. The scaling-up of the synthesis can be completed due to the advantages of the solvothermal method. The CFO porous nanospheres with the average dimension of 200 nm are uniformly embedded in rGO sheets without large aggregation, which forms the hierarchical porous structure. The Raman results reveal that rGO sheets with more defects and disorders are obtained during the solvothermal process, enhancing the dielectric loss of CFO/rGO nanocomposites. The electromagnetic parameters of CFO/rGO nanocomposites can be well modulated by changing the rGO weight fraction of the nanocomposites. CFO/rGO nanocomposites show substantially comprehensive strong and broadband microwave absorption performance. CFO/rGO 10:1 nanocomposites can obtain a maximum RL value of -57.7 dB at a thin thickness of 2.8 mm along with a broad effective absorption bandwidth reaching 5.8 GHz (8.314.1 GHz). The comprehensive features of proper composition, hierarchical porous structure, and abundant interfaces attribute to the balanced impedance matching performance and microwave attenuation of CFO/rGO 10:1 nanocomposites, leading to the superior microwave absorption. We believe that the as-synthesized hierarchical


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CFO/rGO nanocomposites with a porous structure can be applied as a broadband and lightweight microwave absorber. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Synthesis of graphene oxide (GO), SEM images of CFO nanoparticles; TG analysis of CFO/rGO nanocomposites; Cole-Cole semicircle of CFO/rGO 10:1 nanocomposites; electromagnetic parameters, attenuation constant, Z and RL values of CFO nanoparticles. AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Tel: (86) 27-87651775. Fax: (86) 27-87651779. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is financially supported by the National Nature Science Foundation of China (No. 51773163 and No.51706166), the Joint Funds of the Equipment Pre-Research of


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Broadband and Lightweight Microwave Absorber Constructed by in

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