Permittivity-Regulating Strategy Enabling Superior Electromagnetic

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Functional Nanostructured Materials (including low-D carbon)

A Permittivity-Regulating Strategy Enabling Superior Electromagnetic Wave Absorption of Lithium-aluminum-silicate (LAS)/rGO Nanocomposite Siru Lu, Long Xia, Jiaming Xu, Chuheng Ding, Tiantian Li, Hua Yang, Bo Zhong, Tao Zhang, Longnan Huang, Li Xiong, Xiaoxiao Huang, and Guangwu Wen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00348 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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ACS Applied Materials & Interfaces

A

Permittivity-Regulating

Strategy

Enabling

Superior

Electromagnetic Wave Absorption of Lithium-aluminum-silicate (LAS)/rGO Nanocomposite Siru Lu.a Long Xia, a* Jiaming Xu,a Chuheng Ding,a Tiantian Li,a Hua Yang,b Bo Zhong,a Tao Zhang,a Longnan Huang,a Li Xiong,a Xiaoxiao Huangc and Guangwu Wend aSchool

of Materials Science and Engineering, Harbin Institute of Technology

(Weihai), Weihai 264209, China bSchool

of Science, Lanzhou University of Technology, Lanzhou, 730050, China

cSchool

of Materials Science and Engineering, Harbin Institute of Technology, Harbin

150001, China dSchool

of Materials Science and Engineering, Shandong University of Technology,

Zibo 255000, China *Corresponding author’s Email: [email protected]

ABSTRACT Lithium aluminum silicate (LAS) nanoparticles have been successfully loaded on graphene nanosheets through adding silane coupling agent KH-550 by sol-gel process, hydrothermal reaction and heat treatment process. Through regulating the complex permittivity of rGO by LAS nanoparticles and KH-550, LAS/rGO-KH-550 possesses excellent microwave absorption performance. The maximum reflection loss of LAS/rGO-KH-550 reaches -62.25 dB at 16.48 GHz with thickness of only 2.7 mm 1

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and the widest bandwidth is up to 6.64 GHz below -10 dB. The LAS/rGO-KH-550 has effective absorption (99.9 %) below -20 dB at all X and Ku band (8-18 GHz). And the adding quantity of composites in the paraffin matrix is only 20 wt%. The results demonstrate that the interfacial polarization, the Debye dipolar relaxation, the well-matched characteristic impedance and the quarter-wavelength matching all play important

roles

LAS/rGO-KH-550

on

improving

the

nanocomposites.

microwave

absorption

Consequently,

the

properties

of

LAS/rGO-KH-550

nanocomposites can be readily applied as ultra-wide band, light weight and ultra-high performance microwave absorbing material. KEYWORDS: Lithium aluminum silicate, graphene, microwave absorption, nanoparticles, complex permittivity, impedance matching 1. INTRODUCTION Currently, with the rapid development of information technology, especially microwave communication technology1-3, electromagnetic (EM) radiation pollution existing in our environments has become a nonnegligible issue4, which causes harm on human health and interferes with the electronic device operation5-8. In this regard, the EM wave-absorbing materials aroused great attention to scientists in the aim of solving these problems9-11. Carbon-based materials are the most widely used as EM wave absorbing material, due to the light weight, low price and excellent conductivity12,13. For example: carbon spheres14, porous carbon15, carbon nanotubes (CNTs)16, graphene17 and reduced graphene oxides (rGO)17.

2


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Recently, graphene has attracted widely concerning as one kind of microwave interference shielding materials due to its unique two-dimensional structure and outstanding properties, such as ultra-low density, high specific surface area and excellent environmental stability18. However, most of electromagnetic (EM) wave will experience reflection process when it is incident on graphene surface, due to the high permittivity of graphene compared with that of air, which means poor impedance matching19,20. Efficient EM wave absorbing material, having wide absorption bandwidth and strong absorption capacity21, can be achieved by improving impedance matching22,23 and strong attenuation characteristics24. One of the effective methods to improve microwave absorption (MA) performance of graphene is to decorate various magnetic particles because impedance matching can be modulated by the synergistic action of the dielectric loss and magnetic loss. Therefore, the magnetic lossy materials, such as Ni25-27, Co12,13, Fe3O44,28,29, CoFe2O430, NiFe2O431,32 and their mixtures33,34, were introduced into graphene. Another way to modify impedance matching can be achieved by the incorporation of EM transparent materials, possessing relatively low real part of the permittivity (ε′ =1-5) and low dielectric loss tangent (tan δ≤ 0.01)35. Ceramic materials with a low dielectric loss have been proven to be the appropriate matrix for EM materials, such as SiC36, SiO237 and amorphous Si3N438. Currently, EM transparent ceramic materials are used to be composited with graphene for EM wave absorption33,39-44. Liu et al.45 synthesized rGO/CoNi/SiO2 core-shell nanocomposites via liquid-phase reduction 3



reactions and a sol-gel route which exhibit excellent absorption properties with the best reflection loss (RL) of -46.3 dB at 6.2 GHz with a matching thickness of 4.2 mm. Fe-doped NiO@SiO2@graphene nanocomposites have been fabricated by Wang et al.46, whose RL can reach -51.2 dB at 8.6 GHz with a thickness of 4 mm and the absorption band of 4 GHz below -10 dB (7-11 GHz). Zhang et al.18 fabricated a novel quaternary nanocomposites consisting of α-Fe2O3@SiO2 nanoparticles, PANI and graphene nanosheets, which possesses the maximum RL about -50.06 dB at 14.4 GHz with thickness of 2.3 mm. Lithium aluminum silicate (LAS) glass-ceramic, one of the most important glass-ceramics systems, has been extensively studied over the past several years due to its ultra-low thermal expansion as well as high temperature stability and chemical durability.20,25,47. The primary crystalline phases of the LAS system include eucryptite (Li2O–Al2O3–2SiO2) and spodumene (Li2O–Al2O3–4SiO2). The β-spodumene phase has the Tetragonal-quartz like structure, where part of the Si4+ ions are replaced by Al3+ ions. The charge is balanced by the Li+ ions existed in the (Si, Al)O4 framework48. In our previous work, it has been proved that LAS also possesses excellent wave-transparent properties with ultra-low complex permittivity49 (ε′=2.78, tan 𝛿=0.01). After that, the Fe3O4@LAS/rGO composite materials with sandwich structure were synthesized34. The excellent impedance matching performance of LAS ensures that the incident EM wave penetrates LAS particles and arrives at LAS/Fe3O4 interface. The RL value could reach -65 dB at 12.4 GHz with a thickness of 2.1 mm and the absorption bandwidth is 4 GHz (< -10 dB). According to the above research, 4


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it can be concluded that LAS indeed enhance the wave absorption properties of Fe3O4/rGO to some degree. However, the enhancement of MA properties was mainly attributed to the interaction between LAS and Fe3O4. It is reasonable to speculate from both theoretical and experimental perspectives that the addition of LAS into graphene can also adjust the electromagnetic parameters and improve impedance matching. Furthermore, exploring a facile route to enhance MA performance of graphene that possesses ultralow density, strong electromagnetic wave absorption as well as wide bandwidth is urgently demanded for practical application. Herein, the LAS/rGO nanocomposites were designed to adjust impedance matching through changing the complex permittivity of rGO by LAS nanoparticles with ultra-low complex permittivity. Silane coupling agent KH-550 was added to change the electrification of LAS sol. Thus, the interfacial bonding between LAS nanoparticles

and

rGO

nanosheets

is

enhanced.

The

LAS/rGO-KH-550

nanocomposites were successfully fabricated and exhibited excellent microwave absorption properties such as low density, broadband and strong absorption at X and Ku band. This method establishes a new way to design graphene-ceramic materials, which possess ultra-high microwave absorption performance.

2. EXPERIMENTAL 2.1. Materials Aluminum nitrate (Al(NO3)3 · 9H2O), lithiumnitrate (LiNO3), SiO2 sol, 3-aminopropyltriethoxysilane

(NH2(CH2)3Si(OC2H5)3, 5


KH-550),

ethyl

alcohol


absolute (C2H5OH, wt% ≥99.7 %), graphene oxide solution(GO, 0.94 wt%). All the chemicals were used as received. 2.2. Fabrication of LAS/rGO-KH-550 The LAS sol was synthesized by sol-gel method according to our previous work50. Initially, KH-550 was hydrolyzed in the solution of DI water and C2H5OH by stirring for 0.5 h at room temperature with the volume ratio of KH-550: H2O: C2H5OH at 1:1:4.25. Then, 4.52 g LAS sol was added into the mixture by stirring at 70 °C. After 0.5 h, 16.9 g GO solution and 50 mL C2H5OH were added with stirring for 1 h. Then, the LAS sol/GO-KH-550 mixture was treated using the solvothermal method in an 80 mL Teflon-line autoclave at 200 °C for 16 h to form LAS sol/rGO-KH-55051. After cooling to room temperature, the product was washed with ethanol for 3 times and dried in a vacuum oven at 65 °C for 24 h. The obtained powder LAS sol/rGO-KH-550 was sintered in a tube furnace under a nitrogen atmosphere at 700 °C for 2 h to further remove the residual oxygen functional groups. Finally, LAS/rGO-KH-550 resultants were obtained. The schematic illustration of LAS/rGO-KH-550 nanocomposites is shown in Fig. 1. The samples with different KH-550 contents of 0 mL, 0.7 mL, 1.5 mL, 2.1 mL and 2.8 mL were denoted LAS/rGO, K1, K2, K3 and K4, respectively. As a comparison, 16.9 g GO solution and 50 mL C2H5OH was transferred to 80 mL Teflon-line autoclave for solvothermal treatment at 200 °C for 16 h, washed with filtration and freeze-dried to get the pure rGO.

6


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Fig. 1. Schematic illustration of LAS/rGO-KH-550 nanocomposites.

2.3. Characterization 7



The morphologies and microstructures of as-obtained samples were investigated by a field emission scanning electron microscopy (FESEM, MX2600FE) equipped with energy dispersive X-ray spectroscopy (EDS) and high resolution transmission electron microscopy (HRTEM, Tecnai F30 FEG). X-ray diffraction (XRD, Rigaku D/Max 2000 VPC) was used to characterize the crystalline components of samples with a Cu Kα radiation source. The structures of as-obtained samples were determined by Raman spectra (Renishaw, RM-1000) using a 532 nm laser. The binding energy was performed by X-ray photoelectron spectroscopy (XPS) with Al-Kα radiation. Therrmogravimetric analysis (TGA) was measured with a thermal analysis mass spectrometry (Netzsch STAR449C) up to 700 °C with a heating rate of 10 °C/min in air. The complex permittivity and relative complex permeability were obtained by a network analyzer (VNA, N5245A, Agilent, U.S.A) at 8-18 GHz band. The samples were uniformly mixed into paraffin with mass ratios of 1:4 and pressed into coaxial clapper in a dimension of outer diameter of 7 mm and inter diameter of 3.04 mm. The resistivity ρ was measured by the four-probe method using RTS-8 digital four-probe tester (10-4~105 Ω·m), and the calculations allowed for the thickness W of the plate. 3. RESULTS AND DISSCUSSION SEM measurement was used to investigate the morphologies of LAS/rGO and LAS/rGO-KH-550s (Fig.2).The yellow and red arrows in Fig. 2 correspond to rGO sheets and LAS nanoparticles, respectively. In Fig. 2a, only a few LAS nanoparticles can be seen in LAS/rGO without KH-550 treatment, and the self-aggregation of LAS particles leads to poor connection between LAS and rGO sheets. Significantly, the 8


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increased density of LAS nanoparticles on rGO sheets can be clearly observed in the SEM images of LAS/rGO-KH-550s. The LAS nanoparticles are distributed uniformly on the surface of rGO in K1 and K2 (Fig. 2b and 2c). Further increasing the molar weight of KH-550, the single LAS nanoparticles decrease while they are clustered with each other to form grape-like bunches in K3 (Fig.2d). For K4, the LAS nanoparticles aggregate into larger clusters and rarely exist alone on the surface of rGO (Fig. 2e). In particular, all samples exhibit micro cracks (the circles in Fig. 2) on the surface of rGO, which is conducive to EM wave absorption due to enhanced polarization.

Fig. 2. SEM images of (a) LAS/GO, (b) K1, (c) K2, (d) K3 and (e) K4.

The microstructures and statistic fractions of diameters of LAS/rGO and LAS/rGO-KH-550s are displayed in Fig. 3. The yellow arrows and green circles in Fig.

3

correspond

to

rGO

sheets

and

LAS

nanoparticles,

respectively.

Low-magnification TEM images (Fig. 3a, 3d, 3g, 3j and 3m) show that the quantity of LAS nanoparticles increases initially and then decreases with the ratio increase of 9



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KH-550, which further confirms the results of SEM images. It can be clearly seen that the diameters of LAS nanoparticles are about 10-30 nm in all samples (Fig. 3b, 3e, 3h, 3k and 3n). The LAS particle size distribution of LAS/rGO is in the range of 10-40 nm (Fig. 3c) and mainly concentrated in the range of 20-30 nm. After adding KH-550, the

LAS

particle

size

distribution

concentrations

of

LAS/rGO-KH-550

nanocompostes are as follows: K2>K1>K3>K4 (Fig.3f, 3i, 3l and 3o). Evidently, K2 possesses the most uniform LAS particle size.

Fig. 3. Low-magnification TEM images, middle-magnification TEM images and LAS particle size distribution of (a-c) LAS/GO, (d-f) K1, (g-i) K2, (j-l) K3 and (m-o) K4.

The weight loss ratios (%) of LAS nanoparticles in LAS/rGO-KH-550s were measured by TGA characterization in air. As shown in Fig. S1 (supporting information), there are two drastic curves: the first step is attributed to the loss of the absorbed water and a few function groups of rGO in the range of 0-200 °C (the red arrow). The second step corresponds to the violent oxidation of carbon from 490 °C to 680 °C (the blue arrow). The content of LAS nanoparticles in K1, K2, K3 and K4

10


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is ~77.6 %, 79.9 %, 80.5 % and 81.7 % through calculation respectively, which confirms the results of SEM and TEM. Fig. 4a displays the X-ray diffraction (XRD) patterns recorded for rGO, LAS, LAS/rGO and LAS/rGO-KH-550s. A main broad diffraction peak of pure rGO at 2θ of ca. 24.4 º corresponds to (002) plane. It shows broad amorphous peaks in the pattern of pure LAS, indicating an amorphous structure of LAS. As for LAS/rGO and LAS/rGO-KH-550s, broad peaks from amorphous LAS are found, but no obvious characteristic diffraction peaks from rGO occur, which implies the stacking disordered of the rGO sheets caused by the amorphous LAS nanoparticles. To further investigate the microstructure of LAS nanoparticles and element distribution, the selected area electron diffraction (SAED), HRTEM, elemental maps and EDS pattern of K2 are shown in Fig. S2 (supporting information). The amorphous rings in Fig. S2b agree well with the XRD result. It reveals that the K2 is composed of C, O, Al and Si from the elemental maps (Fig.S2 c-g).

Fig. 4. (a) XRD patterns of samples and (b) Raman spectra of GO, LAS/rGO and K2.

11



The C1s of GO and the XPS survey spectra of K2 are shown in Fig. S3 (Supporting information). In Fig. S3a, the peaks of GO centered at ~284.7 eV, ~286.8 eV and ~288.6 eV can be observed, which are corresponding to C-C/C=C, C-O and C=O groups, respectively. The XPS spectrum of K2 (Fig. S3b) shows the characteristic peaks of Al2p, Li1s, Si2p, C1s, N1s and O1s at about ~74.7 eV, ~56.4 eV, ~102.9 eV, ~285.2 eV, ~400.1 eV and ~532.3 eV. For K2, C-C/C=C (~284.7 eV), C-N (~285.4 eV), C-O (~286.8 eV), C=O (~288.9 eV) groups can be found in the C1s peak (Fig. S3d). Evidently, the C-O group decreases while the C-N group appears in K2 compared to GO, confirming that the reduction process of GO and the introduction of KH-550 succeed. The peaks at 398.4 eV and 400.7 eV in the high resolution XPS spectrum of N1s region (Fig. S3c) are attributed to the –NH2 and amide (of ester) N respectively52, which further demonstrates the introduction of KH-550 and chemical connections between GO and LAS modified by KH-550. The Raman spectra of samples are shown in Fig. 4b. It is clearly observed that two evidence peaks at around 1353 cm-1 and 1595 cm-1 are characteristic for the D and G bands of graphitic carbon53,54 in all the samples. And more notably, there is 2D peaks (~2690 cm-1) of LAS/rGO and K2 indicates that the rGO sheets are within a few layers55. It is well known that the D peak can be attributed to the disorder and defects of graphitic carbon materials while the G peak corresponds to the internal vibration of the sp2-bonded carbon atoms56. Thus, the intensity ratio (ID/IG) is associated with the lattice-defect density in graphitic carbon materials57. Obviously, LAS/rGO and K2 possess higher ID/IG ratios than pure GO, which is ascribed to the 12


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vacancy defects in rGO and the presence of amorphous LAS nanoparticles between rGO sheets. K2 shows the highest ID/IG due to more LAS nanoparticles attaching rGO sheets than LAS/rGO. The vacancy defects are caused by the removal of the carbon atoms bonding attaching to oxygen-containing functional groups in GO58-60, which are beneficial to enhance defect polarization60. The RL represents the EM absorption ability of MA materials. According to the transmission line theory, the RL values of samples can be calculated using the following equations61: 𝑍𝑖𝑛 = 𝑍0

𝜇

𝜀 tanh

(𝑗

2𝜋𝑓𝑑 𝜇𝜀 𝑐

)

𝑍𝑖𝑛 ― 𝑍0

RL (dB) = 20log10(𝑍𝑖𝑛 + 𝑍0)

(1) (2)

Here, ɛ and μ are the complex permittivity and permeability respectively, 𝑓 is the microwave frequency, d is the absorber thickness, Z0 is the impedance of free space, and Zin is the input impedance of absorber. The 3D RL and projection plots of rGO, LAS/rGO and LAS/rGO-KH-550s at X and Ku band are displayed in Fig. 5 and 6, respectively. It is obvious that pure rGO has no effective absorption in any thickness (Fig. 5a and 6a). After the addition of LAS, it can be seen that the maximum RL values of LAS/rGO reached to -15.95 dB at 8 GHz with thickness of 5.6 mm (Fig. 5b). It has the best bandwidth of 4.5 GHz and effective absorption below -10 dB at all frequency of 8-18 GHz (Fig. 6b). Additionally, it is clearly that all the LAS/rGO-KH-550 samples have effective absorption (99.9 %) below -20 dB at all frequency of 8-18 GHz. For the K1, it exhibits the maximum absorption of -50.48 dB at 12.08 GHz with thickness of 3.1 mm (Fig. 5c). It has bandwidth of 5.84 GHz below 13



-10 dB (Fig. 6c). K2 shows the maximum absorption of -62.25 dB at 16.48 GHz with thickness of only 2.7 mm (Fig. 5d) and exhibits the widest bandwidth of 6.64 GHz below -10 dB (Fig. 6d). Continue to increase the KH-550 ratios, the maximum RL value becomes lower than K2 but still better than LAS/rGO. For K3, the maximum absorption of -45.35 dB at 8.48 GHz with thickness of 4.2 mm and the widest bandwidth of 5.12 GHz below -10 dB can be observed in Fig. 5e and 6e. It may be due to the purple-like structure. K4 possesses the maximum absorption of -45.88 dB at 15.2 GHz with thickness of 2.7 mm and the widest bandwidth of 6.32 GHz below -10 dB (Fig. 5f and 6f). Table 1 shows different composites for microwave absorption in recent literatures.

14


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Fig. 5. 3D RL and projection plots of (a) rGO, (b) LAS/rGO, (c) K1, (d) K2, (e) K3 and (f) K4 at 8-18 GHz.

15



Fig. 6. 3D projection plots of (a) rGO, (b) LAS/rGO, (c) K1, (d) K2, (e) K3 and (f) K4 at 8-18 GHz. .

16


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Table 1．Different composites for microwave absorption in recent literatures. Absorber

Matrix

Absorber

Maximum

Optimum

RL K2 > K1 > K3> K4 > LAS/rGO. The tan δ𝐸 represents the material’s ability to convert the 19



Page 20 of 40

microwave radiation into other forms of energy57,64. LAS/rGO shows the lowest tan δ𝐸 due to the existence of LAS. With the increasing KH-550 ratios, tan δ𝐸 increases first and then decreases. K1 and K2 possess a higher tan δ𝐸 than K3 and K4 in constant,

demonstrating

the

interfaces

between

uniform

distribution

LAS

nanoparticles and rGO are beneficial to improve the material’s microwave dissipation ability. Compared with rGO and LAS/rGO, LAS/rGO-KH-550s balance the relation between tan δ𝐸 and tan δ𝑀 excellently at X and Ku band, implying better EM matching. The α value of samples calculated by Eq. (1) are shown in Fig.8c. Significantly, the rGO possesses the highest α value, while LAS/rGO shows the lowest α value. Afterward, compared to LAS/rGO, the α value of LAS/rGO-KH-550 is improved, leading to their excellent MA. It is evident that LAS and KH-550 cooperate with each other to get best EM matching, resulting in excellent MA properties.

20


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Fig. 8. (a) The dielectric loss tangent, (b) the magnetic loss tangent and (c) the attenuation constants α of samples at 8-18 GHz. The Cole-Cole circle represents Debye relaxation process which includes polarization and conduction loss. ε′, ε′′ and the relationship between them can be described according to Debye theory as follows65: ε′ = 𝜀∞ + (𝜀𝑠 ― 𝜀∞) (1 + 𝜔2𝜏2) (𝜀 ― 𝜀∞) (1 + 𝜔2𝜏2) + 𝜎 𝜔𝜀0 ε′′ = ωτ 𝑠

(

ε′ ―

𝜀𝑠 + 𝜀∞ 2 2

)

+ (ε′′)2 = (

𝜀𝑠 + 𝜀∞ 2 2

)

(4) (5) (6)

Fig. S4 shows the Cole-Cole plots of LAS/rGO-KH-550s. Many Cole-Cole semicircles can be seen in the curves of LAS/rGO-KH-550s, which highly demonstrates that several dielectric relaxation processes exist in LAS/rGO-KH-550s. The circles in 10 and 15.76 GHz correspond to the peaks in Fig. 7a and 7b. Notably, 21



K1 and K2 possess much more semicircles than K3 and K4, which enhance the attenuation of EM wave66,67. It can be explained by the self-aggregation of LAS nanoparticles of K3 and K4, which decreases the capacitor-like structure and the interfaces between LAS and rGO. The relaxation processes caused by multiple interfacial polarizations in samples improve the attenuation of EM wave68,69. Z (Zin/Z0) represents the ability of microwave to enter into the absorber and convert to other types of energy, or be attenuated through interference70, which can be calculated by eq. (1) and eq. (2). It is beneficial for improving EM wave absorption if the Z value is close to 1. Fig. S5 shows the Z with different frequencies and different thicknesses at corresponding peak RL of rGO, LAS/rGO and LAS/rGO-KH-550. It can be clearly observed that the Z value of rGO is stable at ~0.37, exhibiting very poor impedance matching characteristic. The Z value of LAS/rGO fluctuates around ~1.3 and the LAS/rGO possesses better impedance matching performance than pure rGO. However, after adding KH-550, the Z value of LAS/rGO-KH-550 is close to 1 which results in a strong EM absorption. Obviously, the Z value of K1 and K2 are closer to 1 than the Z value of K3 and K4 on the whole, which demonstrates that K1 and K2 possess better impedance matching performance. The results prove that LAS nanoparticles extremely improve the impedance matching of rGO, and KH-550 further regulates the impedance matching through enhancing the interfaces between LAS nanoparticles and rGO. In addition, the self-aggregation of LAS particles in K3 and K4 makes the impedance matching inferior to K1 and K2.

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Fig. 9a and 9b shows the maximum RL value and the corresponding Z with different frequencies and thicknesses of K2. As shown in Fig. 9b, the whole Z curve is especially close to 1, implying excellent EM wave absorption ability of K2. To better understand the absorption mechanism of LAS/rGO-KH-550, the quarter-wavelength matching model has been proposed to analyze the EM wave absorption of samples. In the model, the relationship of matching thickness (𝑡𝑚) and corresponding peak frequency (𝑓𝑚) of the maximum RL value can be described by the following equation71: 𝑛𝑐

𝑡𝑚 = 4𝑓𝑚

1

|𝜀𝑟||𝜇𝑟|; n =

1,3,5, …

(7)

where |𝜀𝑟| and |𝜇𝑟| stand for the modulus of 𝜀𝑟 and 𝜇𝑟 at 𝑓𝑚, respectively. If the calculated coincides with the experimental matching thickness, it indicates that the EM wave absorption performance agrees well with the model. Fig. 9c exhibits the experimental matching thicknesses and thicknesses calculated according to eq. (7). It is clear that all the 𝑡𝑒𝑥𝑝 dots fit well with the calculated 𝑡𝑓𝑖𝑡 𝑚 𝑚 curve, confirming that the excellent EM wave absorption of LAS/rGO-KH-550 can be interpreted by the quarter-wavelength matching model.

23



Fig. 9. (a) The maximum RL value, (b) the corresponding Z with different frequencies and thicknesses and (c) experimental and calculated matching thicknesses with different frequencies of K2 at 8-18 GHz..

The EM wave absorption mechanisms of LAS/rGO-KH-550 are illustrated in Fig. 10. Firstly, the defects and a small amount residual functional groups formed on the rGO surface during reduction process could generate dipole polarization and related relaxation loss. Secondly, the different resistivity of LAS and rGO (Table S1) results in aggregation and rearrangement of local charge, which causes interfacial polarization in alternating electromagnetic fields. The interfacial polarization plays a significant role in EM attenuation, such as interfaces between LAS nanoparticles, interfaces between rGO and LAS nanoparticles, and capacitor-like structures in the contact sites of rGO. The Maxwell-Wagner-Sillars (MWS) effect frequently occurs in 24


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the heterojunction structures as a result of the cumulation of charges at the interfaces as well as the formation of diploes on particles or clusters72. The LAS nanoparticles are decorated on the surfaces of the rGO, constructing a capacitor-like structure at the interfaces between LAS nanoparticles and rGO due to their disparate dielectric properties and resistivity. The capacitor-like structure can attenuate the power of incident wave, which has been reported by many literatures73-75. All these could improve dielectric properties and significantly enhance EM wave absorbing properties. Thirdly, the LAS nanoparticles on rGO possess excellent wave permeability and the KH-550 enhances the interfaces between LAS nanoparticles and rGO to regulate the complex permittivity, leading to a better impedance matching of LAS/rGO-KH-550 nanocomposites. This enables the reduction of reflection of incident waves, enhancing absorption of EM waves.

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Fig. 10. The electromagnetic wave absorption mechanisms of LAS/rGO-KH-550.

4. CONCLUSIONS In this work, LAS/rGO-KH-550 nanocomposites are successfully fabricated with a three-step method. The interfaces between LAS and rGO improved by the addition of KH-550 and the appropriate uniform single LAS nanoparticles result in the best MA properties of LAS/rGO-KH-550. Table 1 shows different composites for microwave absorption in recent literatures. It can be observed that the effective absorption bandwith of LAS/rGO-KH-550 is reached to 6.64 GHz and the absorber possesses not only strong EM wave absorption ability (RL=-62.25 dB), but also low filler loading of only 20 wt%. Due to its ultra-broad bandwidth, superior wave absorption property and light weight, we believe this work will open up new approaches to develop high performance wave absorption materials for various emerging applications. ASSOCIATED CONTENT Supporting Information. TGA profiles of LAS/rGO-KH-550s, HRTEM, element mapping and EDS of K2, XPS spectra, the cole-cole plots of LAS/rGO-KH-550s and the conductivity value of samples are supplied. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 51621091, 51372052, 51772060 and 51302050), Key Laboratory of

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Advanced Structural-Functional Integration Materials & Green Manufacturing Technology, Harbin Institute of Technology, Harbin, 150001, China.

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