Crystalline-Amorphous Permalloy@Iron Oxide Core-Shell

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Functional Nanostructured Materials (including low-D carbon)

Crystalline-Amorphous Permalloy@Iron Oxide Core-Shell Nanoparticles Decorated on Graphene as High-Efficiency, Lightweight and Hydrophobic Microwave Absorbents Yong Sun, Junwei Zhang, Yan Zong, Xia Deng, Hongyang Zhao, Juan Feng, Mi He, Xing Hua Li, Yong Peng, and Xinliang Zheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18875 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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ACS Applied Materials & Interfaces

Crystalline-Amorphous

Permalloy@Iron

Oxide

Core-Shell

Nanoparticles

Decorated on Graphene as High-Efficiency, Lightweight and Hydrophobic Microwave Absorbents

Yong Sun†§, Junwei Zhang‡§, Yan Zong†, Xia Deng‡, Hongyang Zhao‖, Juan Feng†, Mi He†, Xinghua Li†*, Yong Peng‡*, Xinliang Zheng†

†

‡

School of Physics, Northwest University, Xi’an, 710069, China Key Laboratory of Magnetism and Magnetic Materials of the Ministry of

Education, Lanzhou University, Lanzhou, 730000, China ‖

School of science, Xi’an Jiaotong University, Xi’an, Shaanxi 710054, China

§

These authors contributed equally to this work

*Corresponding author: [email protected] (X. Li) & [email protected] (Yong Peng)

Keywords: graphene, magnetic nanostructure, core-shell structure, hydrophobic, microwave absorption.

Abstract The exploration of high-efficiency microwave absorption materials with lightweight and hydrophobic features is highly expected to reduce or eliminate the electromagnetic

pollution.

Graphene-based

nanocomposites

ACS Paragon Plus Environment

are

universally

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

acknowledged as promising candidates for absorbing microwaves due to their remarkable dielectric properties and lightweight characteristic. However, the hydrophilicity of graphene may reduce their stability and restrict the applications in moist environment. Herein, a well-designed heterostructure composed of crystalline permalloy core and amorphous iron oxide shell was uniformly adhered on oleylamine-modified graphene nanosheets by a one-pot thermal decomposition method. Compared with the recognized hydrophilic graphene-based hybrid materials, the permalloy@iron oxide/graphene nanocomposites show excellent hydrophobic and water resistant features with a water contact angle of 136.5°. Besides, the nanocomposites show high-efficiency microwave absorption performance, benefiting from the tunneling effect, polarization, interface interaction, impedance matching condition and synergistic effect between core-shell permalloy@iron oxide nanoparticles and graphene nanosheets. A broad effective absorption bandwidth with reflection loss (RL) value exceeding -10 dB can be obtained from 4.25 to 18 GHz, covering about 86% measured frequency range when the absorber thickness is 2.0-5.0 mm. And the microwave absorption performance of nanocomposites can be tuned by changing the amount of graphene. More importantly, a greatly improved microwave absorption effectiveness of -71.1 dB can be achieved for the nanocomposites in comparison with the bare permalloy@iron oxide nanoparticles (-5.6 dB) and oleylamine-modified GO nanosheets (-3.56 dB). The lightweight and hydrophobic permalloy@iron oxide/graphene nanocomposites with high-efficiency microwave absorption performance are highly promising to improve the environmental


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adaptability of electric devices, especially in the wet environment. 1. Introduction Electromagnetic-radiation pollution is becoming more and more serious with the expanded usage of electronic devices, since the electromagnetic radiation not only damages the human health, but also disturbs the normal work of electronic equipments.1-3 Therefore, the control or mitigation of electromagnetic-radiation pollution is urgently desired. Microwave absorption materials, which can consume the incident microwaves and weaken the reflected microwaves, have been drawn much attention in civilian and military applications. Nowadays, several novel composites with special structure have been designed for microwave absorption applications.4-10 Ideal microwave absorption materials should fulfil four features, including high-efficient absorption performance, broad effective absorption bandwidth, light weight and thin thickness, which can be determined by the complex permeability and permittivity. Nowadays, traditional magnetic nanostructures, such as ferrites11-15 and metals/alloys16-19, have been commonly used as high-efficient microwave absorption materials owing to their large saturation magnetization, high permeability and excellent magnetic loss. The features of low cost, abundant and chemical stability make ferrites good candidates for practical applications. However, due to the low Snoek’s limit and small resonance frequency, the complex permeability of ferrites quickly reduces by increasing the frequency in gigahertz range, which makes the usage of ferrites only in relative low frequency range (< 3 GHz).20 In comparison with ferrites, the magnetic metals/alloys have high permeability and broad working



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frequency in GHz frequency range because of the higher Snoek’s limit, which can overcome the problem.21 But the attributes of easy-oxidation and poor corrosion resistance are undesirable for the practical applications. Moreover, the sole magnetic materials have common inherent drawbacks: the large density and poor flexibility restrict their practical applications; the lack of dielectric loss may cause impendence mismatch, resulting in high reflection and poor absorption performance.22,

23

To

achieve superior microwave absorption performance, constructing magnetic/dielectric nanocomposites by integrating magnetic materials with dielectric materials has been proved to be novel microwave absorbents.6, 24-29 Graphene as the thinnest and lightest carbon material has been considered to be an excellent dielectric material for microwave absorption due to its lightweight, large surface area, excellent electrical conductivity, remarkable thermal conductivity, high electron mobility, strong mechanical stiffness and flexibility.30-35 Although the sole graphene has achieved some advances in microwave absorption, it is still a big challenge to get super microwave absorption performance at small thickness. An intrinsic reason is that the unilateral graphene cannot meet impendence matching condition. Nowadays, anchoring magnetic nanostructures on graphene nanosheets has been widely reported as superior microwave absorbents with lightweight and high-efficient performance.36-38 However, most of these magnetic graphene-based nanocomposites were fabricated in aqueous phase or using hydrophilic graphene oxide as raw material, which makes the nanocomposites show hydrophilic characteristic. These situations may affect their reliability and stability when the


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graphene-based nanocomposites were used in moist environments. Besides, if the magnetic component in the nanocomposites is metals or alloys, they are easily oxidized in the atmosphere environment. Therefore, the exploration of novel magnetic graphene-based microwave absorbents with hydrophobic and antioxidative features is urgently demanded.25, 31 Herein, core-shell permalloy@iron oxide nanosparticles were anchored on hydrophobic graphene nanosheets by a facile one-pot thermal decomposition method. Compared to the conventional hydrophilic graphene-based nanocomposites, our graphene-based materials show hydrophobic and antioxidative features in wet environment. In addition, a greatly improved microwave absorption performance was acquired. These hydrophobic magnetic graphene-based nanocomposites are innovative environment-friendly microwave absorption materials, which can protect the practical applications of electric nanodevices in wet environment. 2. Experimental section 2.1 Synthesis of permalloy@iron oxide/graphene nanocomposites Graphene oxide (GO) was synthesized from natural graphite flakes via a modified Hummers method.39 In a typical process, a certain amount of GO was added into 20 ml of oleylamine, and treated by ultrasound for 3 h. Then 67 mg of Iron (III) acetylacetonate (Fe(acac)3) and 33 mg of Nickel (II) acetylacetonate (Ni(acac)2) were dissolved into the above GO-containing solution and ultra-sonicated for another 0.5 h. Under magnetically stirring, the mixture solution was dehydrated at 120 oC for 1 h, then heated to 200 ˚C with a heating rate of 8 oC/min and maintained for another 1 h.



Finally, the solution was heated to 320 oC with a heating rate of 5 oC/min and reacted at this temperature for 2 h. Ar was used during all the processes. When the reaction was finished, 20 ml of isopropyl alcohol was added into the black solution. The products were obtained by centrifuging (10000 rpm, 3 min), and washed by hexane/isopropyl alcohol (1:1 vol) for several times. Under the same conditions, by only changing the amount of GO into 0, 20, 30 and 40 mg, four samples were obtained, which are named by S1, S2, S3 and S4, respectively. 2.2 Electromagnetic measurements The microwave parameters of samples were analyzed using a network analyzer (Agilent Technologies E8363B) in the frequency range of 2-18 GHz by the coaxial wire method. The samples for microwave absorption measurement were prepared by mixing 50 wt% of products with 50 wt% of paraffin. On basic of the standard Nicholson-Ross and Weir theoretical calculations,40 the complex permeability and permittivity were calculated by the experimental scattering parameters. Results and discussions Hydrophobic graphene decorated by core-shell permalloy@iron oxide was fabricated by a straightforward thermal decomposition route using high-boiling solvent and organometallic salt, which was schematically illustrated in Figure 1. Under powerful ultrasonic treatment, the amino in oleylamine was chemically linked with the oxygen-based groups of GO and formed oleylamine-modified GO, during which the hydrophilic GO was converted into hydrophobic feature, as shown in Figure 1a. When the organometallic salts of Fe(acac)3 and Ni(acac)2 were dissolved in the


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GO-containing solvent, part of the organometallic salts was physically absorbed on the hydrophobic oleylamine-modified GO due to the interaction of their hydrophobic groups. The core-shell permalloy@iron oxide nanoparticles were formed and supported on the oleylamine-modified GO through a thermal decomposition method. By controlling the heating rate and aging temperature, the organometallic salts first nucleated on the surface of oleylamine-modified GO, and then the nuclei were grown bigger, forming nanoparticles on the surface of oleylamine-modified GO. During the reaction, part of oleylamine may decompose and generate a tiny amount of H2 and CO at high temperature, which can partly reduce the organometallic salts into permalloy and GO into reduced graphene oxide (rGO). Some Fe(acac)3 can decompose into iron oxide and cover on the surface of permalloy, which can prevent the alloys from oxidization. The as-synthesized permalloy@iron oxide/graphene nanocomposites show a distinguished hydrophobic property and magnetic property. During the reaction, oleylamine plays several roles in the reaction, including solvent, capping agent and reducing agent. Figure S1 and Figure 2a show the TEM images of permalloy@iron oxide nanoparticles and permalloy@iron oxide/graphene nanocomposites measured at different magnification. Obviously, the nanoparticles reveal homogeneous size and core-shell structures, which were uniformly decorated on the surface of graphene nanosheets without agglomeration. HAADF-STEM images (Figure 2c, d) further demonstrate that the nanoparticles are comprised by a bright core coated by a grey shell. The statistical histogram of size distribution of 200 nanoparticles (Figure S2)



shows that the average sizes of shell and core are 1.56 nm and 7.81 nm, respectively. HRTEM image of a single core-shell nanoparticle (Figure 2b) reveal that the core is crystalline and the shell is amorphous. The core shows clear atomic lattice fringes with interplanar spacing distance of 0.208 nm, which is related to the (111) plane of permalloy with face center cubic (FCC) structure. The XRD patterns of GO and permalloy@iron oxide/graphene nanocomposites are shown in Figure S3. In the nanocomposites, GO has been partially reduced, and permalloy and iron oxide are of co-existence. Electron energy loss spectroscopy (EELS) elemental mapping of a single core-shell nanoparticle is displayed in Figure 2e-h. The results show that the core is comprised of Fe and Ni elements, while the shell contains Fe and O elements. Moreover, the EELS spectrum of shell (Figure 2i, area 1 in Figure 2d) shows three peaks of C K, O K and Fe L2,3-edge.41-43 By contrast, the EELS spectrum of core (Figure 2j, area 2 in Figure 2d) has a additional peak of Ni L2,3-edge.43 Combined all the above results, it is clearly seen that the core-shell nanoparicles are composed of crystalline permalloy core and amorphous iron oxide shell, which were anchored on the surface of graphene nanosheets. XPS was used to characterize the surface chemical compositions and electronic states of elements in permalloy@iron oxide/graphene nanocomposites. Four peaks with binding energies of 284.5, 532, 710 and 870 eV can be seen in the survey scan XPS spectrum of nanocomposites (Figure S4), indicating the existence of C, O, Fe and Ni elements. Compared to GO, the intensity ratio of O and C peaks (IO/IC) for nanocomposites is obviously reduced, suggesting that GO has been partially reduced.


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The high resolution Fe 2p spectrum of nanocomposites (Figure 3a) can be fitted by four peaks. The peaks at 708.0 and 721.9 eV are attributed to the Fe 2p3/2 and Fe 2p1/2 of Fe metal, respectively.44 While the other two peaks located at 710.4 and 731.0 eV are related to the Fe 2p3/2 and shakeup satellite Fe 2p1/2 of Fe3+.44 The high-resolution Ni 2p spectrum (Figure 3b) shows four deconvolution peaks. The two main spin-orbit peaks at 853.0 and 871.2 eV with an energy separation of 17.2 eV are related to the Ni 2p3/2 and Ni 2p1/2 of Ni metal, respectively.45-46 The peaks at 858.0 and 877.2 eV are the corresponding shakeup satellites.45-46 No peaks with binding energy of about 855 and 877 eV are found, suggesting the absence of Ni2+ in the nanocomposites.45-46 These results confirm that Fe elements are presented as Fe (0) and Fe (Ⅲ) and Ni elements are only existed as Ni (0) in the nanocomposites, which are accordant with the above TEM observations. Figure 3c and 3e show the C 1s spectra of nanocomposites and GO, respectively. The C 1s spectrum of GO (Figure 3e) can be fitted by three peaks with binding energies of 284.5, 286.5 and 287.4 eV, which are ascribed to the C-C/C=C in aromatic rings, C-OH and C-O groups, respectively.3 While the C 1s spectrum of nanocomposites (Figure 3c) has only two deconvolution peaks of C-C/C=C and C-N groups. Compared with GO, the oxygen-contained groups in nanocomposites are obviously decreased, further suggesting that GO was partially reduced during the formation of nanocomposites. Raman spectra of GO and permalloy@iron oxide/graphene nanocomposites (Figure S5) reveal that the ID/IG value of nanocomposites increases in comparison with GO, which also indicates that GO was partially reduced.47-49 The O 1s spectrum of GO (Figure 3f) reveals two



peaks located at 531.3 and 532.4 eV, which is accordant to the C-O and C=O bands.50-51 In comparison, the additional peak at 530.1 eV in O 1s spectrum of nanocomposites (Figure 3d) is associated to the lattice oxygen anions in Fe-O bond.52 Brunauer-Emmett-Teller (BET) surface areas (Figure S6) of GO and nanocomposites (S3) are 45 and 2 m2/g, respectively. The BET surface area of nanocomposites is nearly zero, suggesting that the adsorption capacity of N2 is negligible. The magnetic hysteresis loops measured at room temperature (Figure S7) reveal that the nanocomposites show reduced saturation magnetization (Ms) values, which is mainly originated from the nonmagnetic graphene. The bare nanoparticles reveal superparamagnetic behavior with nearly zero coercivity (Hc) value, while the nanocomposites show ferromagnetic feature with improved Hc value. This is probably due to the enhanced surface anisotropy which is increased by the well-dispersion of nanoparticles on graphene nanosheets. In practical applications, hydrophobic feature is an essential requirement for novel microwave absorbents because it can protect the electronic devices from corrosion by moisture in the wet environment. However, most of the graphene-based microwave absorbents were synthesized in aqueous solution or by using water-soluble GO as raw materials. The residual oxygen-contained groups make these graphene-based nanocomposites possess hydrophilic attribute. Herein, we use the conventional Fe3O4/graphene nanocomposites reported in Ref. 53 as an example. As displayed in Figure 4a, the conventional Fe3O4/graphene nanocomposites have an average water contact angle of 37o, revealing a typical hydrophilic characteristic.


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While the permalloy@iron oxide/graphene nanocomposites prepared in this work are highly hydrophobic with an average water contact angle of 136.5o. FTIR spectrum (Figure S8) indicates that the permalloy@iron oxide/graphene nanocomposites were modified by oleylamine, which can not only act as linker to connect permalloy@iron oxide nanoparticles and graphene, but also be used to modify graphene into hydrophobic. To further examine the water-resisting property of the nanocomposites, the permalloy@iron oxide/graphene nanocomposites were immersed in water for two weeks. Figure S9 shows the corresponding TEM images of nanocomposites (S3) which were immersed in water for two weeks. It is clearly seen that the nanoparticles still remain core-shell structure and are uniformly anchored on the surface of graphene without aggregation. There results suggest that the iron oxide shell in the nanocomposites is stable in water, which can protect the inner permalloy from oxidation. These results indicate that the nanocomposites meet the requirement of microwave absorbing materials in moisture environment. Based on the transmission line theory, the microwave absorption performance is evaluated by reflection loss (RL), which can be calculated by the following equation:54, 55 Z in  Z 0 Z in  Z 0

(1)

Z in Z 0 (  r  r )1 2 tanhj (2fd c)( r r)1 2 

(2)

RL  20 log

Where Z 0  0  0 , Z in , f, d, c, μr and εr are the impedance of free space, input impedance, frequency of microwave, thickness of absorber, velocity of light, relative complex permeability and complex permittivity, respectively. Figure 5 shows the



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three-dimensional representations of RL values versus frequency and thickness for the bare permalloy@iron oxide nanoparticles and permalloy@iron oxide/graphene nanocomposites with different amount of graphene. The RL value exceeding -10 dB (RL ≤ -10 dB), which means at least 90% microwaves are absorbed, is commonly used to estimate if the microwave absorbents can be used in practice. Obviously, the RL values

of

bare

permalloy@iron

oxide

nanoparticles

(Figure

5a)

and

oleylamine-modified GO nanosheets (Figure S10) cannot reach -10 dB in the whole frequency range, and the maximum RL value are only -5.6 and -3.56 dB, respectively. By decorating the permalloy@iron oxide nanoparticles onto hydrophobic graphene, the hydrophobic nanocomposites show extremely improved microwave absorption performance, as shown in Figure 5b-d. The strongest RL value of -71.1 dB can be obtained at 8.18 GHz for the nanocomposite of S3, when the thickness is 3.67 mm. A broad effective absorption bandwidth (RL ≤ -10 dB) can be achieved in the frequency range of 4.25-18 GHz, covering about 86% of the measured frequency range when the absorber thickness varies from 2.0 to 5.0 mm. Besides, the microwave absorption performance of permalloy@iron oxide/graphene nanocomposites can be tuned by changing the amount of graphene. Table S1 shows the comparison of microwave absorption performances between permallay@iron oxide/graphene and other reported magnetic carbon-based nanocomposites. Obviously, the hydrophobic permallay@iron oxide/graphene nanocomposites prepared in this work reveal relative high microwave absorption performance and broad effective bandwidth. The microwave absorption characteristics are mainly determined by the complex


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permittivity (εr = ε' - jε") and complex permeability (μr = μ' - jμ") of the absorbents. The frequency dependence of complex permittivity and permeability for the samples are displayed in Figure S11. Compared to the permalloy@iron oxide nanoparticles, the complex permittivity of nanocomposites is greatly improved by the addition of dielectric graphene. The complex permeability of nanocomposites slightly decreases, which is mainly due to the reduced Ms value (Figure S7). Figure S12 shows the comparison of complex permittivity and permeabillity for permalloy@iron oxide/graphene nanocomposites (S3) before and after immersed in water for two weeks. The difference between the treated and untreated nanocomposites can be ignored, which further demonstrates their anti-oxidative ability and stability in wet circumstance. Dielectric loss and magnetic loss indentify the loss capacities of microwave absorbents, which can be evaluated by dielectric loss tangent ( tan     ' '  ' ) and magnetic loss tangent ( tan     ' '  ' ), respectively. By the introduction of graphene, the nanocomposites show greatly improved dielectric loss tangent in the entire frequency range compared to the permalloy@iron oxide nanoparticles (Figure S13a). And the values slightly increase as the amount of graphene increases. The enhancement of dielectric loss is mainly attributed to the residual defects and functional groups of graphene, which can result in higher ε" and larger dielectric loss. Cole-Cole semicircles (Figure S14) reveal that the enhancement of dielectric loss is probably ascribed to the existences of dipole polarization and interface polarization in the nanocomposites. The frequency dependence of magnetic loss tangent for the four samples (Figure S13b) reveals that the magnetic loss tangent



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of nanocomposites is slightly decreased. This is mainly due to the addition of lightweight nonmagnetic graphene, which can gently reduce the value of μ". C0-f curves (Figure S15) reveal that the magnetic loss of samples is caused by nature/exchange resonances. Impedance matching condition, which represents the complementarily capability between dielectric loss and magnetic loss, is an important factor for the microwave absorption performance of materials, for it determines the entering ability of microwaves in the absorbers. The impedance matching condition can be described by the following equation:



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



(3)

Generally, the impedance matching condition is used to evaluate if the incident electromagnetic waves can enter into the microwave absorbers. When the impedance matching condition ( Zin / Z0 ) is closer to 1, the incident electromagnetic waves can more easily enter into the absorbers. Figure 6a shows the impedance matching condition of permalloy@iron oxide nanoparticles and nanocomposites. The impedance matching values of permalloy@iron oxide nanoparticles are far away from 1, while the values of nanocomposites are closed to 1 in the entire frequency range. Especially, the values of S3 and S4 are much closed to 1, which suggests good impedance matching condition, indicating that microwaves can more easily enter into the absorbers. In addition to the impendence matching condition, microwave attenuation capacity is another key parameter for microwave absorption materials. The microwave


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attenuation capacity can estimate the intrinsic attenuation capacity of microwaves by the absorbents, which can be evaluated by attenuation constant (α) as follows:56, 57 

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

(4)

Figure 6b shows the frequency dependence of α for the four samples. Obviously, by introducing graphene, the α values of nanocomposites are greatly enhanced in the whole frequency range, and the values increase as the amount of graphene increases, which is probably due to the well-dispersion of nanoparticles and excellent dielectric loss of graphene.58, 59 Moreover, S3 and S4 show the largest microwave attenuation constants. Combined the independence matching conditions and microwave attenuation abilities, the introduction of graphene can not only make the incident microwave more easily enter into the nanocomposites, but also improve the attenuation abilities of entering microwaves. These results can explain the superior microwave absorption performance of S3 and S4. The thickness and frequency are of great significance for microwave absorbers, since they can determine the practical application fields of absorbents. According to the quarter-wavelength matching (λ/4) model, the thickness of absorber (tm) and frequency of peak (fm) should satisfy the following formula:3 tm 

n nc  4 4 f m r  r

(n= 1, 3, 5, ...)

(5)

If the thickness of absorber is appropriately equal to λ/4, interference dissipation of microwaves occurs. The microwaves reflected from the front and back interface of absorbers have a phase difference of 180 o, which can form standing wave. Then a cancellation effect generates at the front interface of absorbers, leading to a



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consumption of microwaves. Figure 7a, 7c and 7e show the RL values versus frequency at different thickness for the nanocomposites. The corresponding simulations between absorber thickness and peak frequency under λ/4 model are displayed in Figure 7 b, 7d and 7f, respectively. The matching thicknesses achieved exp

from the corresponding RL curves (Figure 7a, 7c and 7e) are denoted as t m

(red

stars). Obviously, the red stars are located around the λ/4 curves, indicating that the relationship between absorber thickness and peak frequency of the nanocomposites satisfies λ/4 model. Furthermore, the λ/4 theory can provide a direct guide for the practical application fields of microwave absorbers. Once the expected band of applications is confirmed, the required thickness of absorbers can be simulated based on the λ/4 theory. Based on the above discussions, the improved impedance matching condition and high microwave attenuation are the key points for the enhanced microwave absorption, which are originated from the synergistic effect between magnetic nanoparticles and dielectric graphene. A feasible microwave absorption mechanism of permalloy@iron oxide/graphene nanocomposites was shown in Figure 8. Under the radiation of microwaves, electrons migrate in the graphene layers and transmit across the defects or neighboring graphene layers, which may causes a charge redistribution and dissipative current on the graphne nanosheets. The rGO nanosheets may form the capacitor-shape, leading to a conductive loss.37,

38, 60-62

At the same time, the

permalloy@iron oxide nanoparticles with core-shell structure may be polarized under microwave radiation, resulting in a conductive loss as well. Besides, more dipoles and


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polarized center can be formed in the graphene nanosheets due to the defects, functional groups, interfaces in the core-shell nanoparticles and interfaces between the nanoparticles and graphene sheets.63,

64

Moreover, the shell thickness of amorphous

iron oxide is only 1.56 nm, which makes the electrons easily tunnel between the core permalloy and graphene nanosheets due to the quantum tunneling effect (Figure S16).65, 66 This may also contribute dielectric loss ability. Therefore, the distinguished enhanced microwave absorption properties of nanocomposites are mainly caused by conductive loss, electron tunneling, dipole polarization, interface polarization, improved impedance matching, and synergistic effect between magnetic nanoparticles and dielectric graphene. Conclusion Crystalline-amorphous

permalloy@iron

oxide

core-shell

nanosparticles

were

uniformly adhered onto oleylamine-modified graphene nanosheets through an one-pot thermal decomposition route. The nanocomposites show a broad effective absorption bandwidth of 4.25-18 GHz with a thickness of 2.0-5.0 mm, which covers nearly 86% of the measured frequency range. The microwave absorption characteristic can be tuned by controlling the amount of hydrophobic graphene, and an ultrahigh RL value of -71.1 dB at 8.18 GHz can be obtained when the thickness is 3.67 mm. More importantly, compared to the commonly hydrophilic graphene-based hybrid materials, the nanocomposites show excellent hydrophobic and water resistant feature with a water contact angle of 136.5o. These lightweight and hydrophobic permalloy@iron oxide/graphene nanocomposites should be promised to be a novel kind of potential



microwave absorbents for practical applications, which can greatly improve the environmental adaptability and lifespan of electric devices, especially in the moist environment. Supporting Information The material and related structure characterization. TEM images of S1, S2, S3 and S4. TEM images of S3 before and after staying in water for a week. XRD images of GO and S3. Size distribution of core and shell in S3. Raman and FTIR spectra of GO and S3. Microwave absorption performance of oleylamine-modified GO nanosheets. Magnetic characterization and N2 adsorption-desorption isotherms of S1 and S3 composites. Electromagnetic parameters of all samples. Dielectric loss tangent and magnetic loss tangent of all samples. Schematic illustration of quantum tunneling effect. Comparison of the microwave absorption parameters between permallay@iron oxide/graphene and other reported magnetic carbon-based nanomaterials.

Acknowledgment This work was supported by the National Natural Science Foundation of China (11504293, 51572218, 51571104 and 51771085), Scientific Research Program Funded by Shaanxi Provincial Education Department (18JK0786), China Postdoctoral Science Foundation (2015M580870 and 2016T90942) and Young Talent Fund of University Association for Science and Technology in Shaanxi, China (20170605).

References (1) Liu, Q. H.; Cao, Q.; Bi, H.; Liang, C. Y.; Yuan, K. P.; She, W.; Yang, Y. J.; Che, R. C. CoNi@SiO2@TiO2 and CoNi@Air@TiO2 Microspheres with Strong Wideband Microwave Absorption. Adv. Mater. 2016, 28, 486-490.


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Figure Captions Figure 1. (a) Images of the disperse state of GO, oleylamine-decorated GO and permalloy@iron oxide/graphene nanocomposites in hexane and water, respectively; (b) Schematic illustration for the formation of permalloy@iron oxide/graphene nanocomposites. Figure 2. (a) TEM and (c) HAADF-STEM images of permalloy@iron oxide/graphene nanocomposites (S3); (b) HRTEM image of permalloy@iron oxide nanoparticles; (d) HAADF-STEM image of a single permalloy@iron oxide nanoparticle; (e-h) EELS elemental mapping of the single nanoparticle in Figure 2d; EELS spectra of (i) area 1 and (j) area 2 of the nanoparticle in Figure 2d. Figure 3. (a) Fe 2p, (b) Ni 2p, (c) C 1s and (d) O 1s XPS spectra of permalloy@iron oxide/graphene nanocomposites (S3); (e) C 1s and (f) O 1s XPS spectra of GO. Figure 4. Photographical images of a water droplet on the surface of (a) Fe3O4/graphene nanocomposites reported in Ref. 53 and (b) permalloy@iron oxide/graphene nanocomposites (S3) prepared in this work Figure 5. Three-dimensional representations of RL values for samples of (a) S1, (b) S2, (c) S3 and (d) S4, respectively. Figure 6. Frequency dependence of (a) impedance matching condition and (b) microwave attenuation constants for the samples Figure 7. RL values versus frequency at different thickness for (a) S2, (c) S3 and (e) S4. Simulations of the absorber thickness versus peak frequency under the λ/4 model for (b) S2, (d) S3 and (f) S4



Figure 8. Scheme of microwave absorption mechanism of the permalloy@iron oxide/graphene nanocomposites


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Figure 1



Figure 2


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Figure 3



Figure 4


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Figure 5



Figure 6


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Figure 7



Figure 8


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Graphical Abstract


Crystalline-Amorphous Permalloy@Iron Oxide Core-Shell

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