Lightweight excellent microwave absorption properties based on sulfur doped graphene

Journal Pre-proofs Original article Lightweight Excellent Microwave Absorption Properties Based on Sulfur Doped Graphene Lin Tan, Menghui Zhu, Xiaoyang Li, Huixia Feng, Nali Chen, Dan Zhao PII: DOI: Reference:

S1319-6103(19)30093-6 https://doi.org/10.1016/j.jscs.2019.08.005 JSCS 1079

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14 June 2019 22 August 2019 27 August 2019

Please cite this article as: L. Tan, M. Zhu, X. Li, H. Feng, N. Chen, D. Zhao, Lightweight Excellent Microwave Absorption Properties Based on Sulfur Doped Graphene, Journal of Saudi Chemical Society (2019), doi: https:// doi.org/10.1016/j.jscs.2019.08.005

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Lightweight Excellent Microwave Absorption Properties Based on Sulfur Doped Graphene Lin Tan*ab, Menghui Zhu a, Xiaoyang Li a, Huixia Feng*ab, Nali Chen a, and Dan Zhao a

a. School of Petrochemical Engineering, Lanzhou University of Technology, Lanzhou Gansu 730050, P. R. China. b. State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals, Lanzhou University of Technology, Lanzhou, Gansu 730050, P. R. China. E-Mail: [email protected]; [email protected]

Abstract Lightweight and high-efficiency electromagnetic wave (EMW) absorption sulfur doped graphene (S-GS) was fabricated by reducing and doping graphene oxide (GO) using chemical method. The obtained S-GS exhibits an extremely low reflection loss

(RL), wide effective absorption frequency bandwidth at a thin coating thickness (d), light weight and low cost. The minimum RL reaches -52.3 dB at 17.5GHz with a matching thickness of 1.22mm. More excitedly, the EM absorption properties could be double-adjusted. By changing the amount of dopant and the thickness of S-GS, The minimum RL touches -52.2 dB at 9.4GHz and -49.3 dB at 8.5GHz, respectively, even much lower than most kinds of GS based composites which combined with polymers, magnetic nanoparticles and so on. In a word, S-GS can be promised to be an ideal candidate for constructing novel MW absorber with lightweight, strong absorption characteristics, and thin matching thickness. Keywords: graphene, sulfur doped, microwave absorption, lightweight.

1. Introduction EMW absorbing materials have drawn increasing attention around the world with swift evolution of wireless communications and high frequency equipment. An ideal EMW absorber may usually possess features like lightweight, thin matching thickness, strong absorption and broad absorption frequency bandwidth, which are necessary to satisfy the ever-growing demand for practical applications[1-3]. In recent years, significant efforts have already been devoted to graphene as a potential candidate for EMW absorbing material because of its outstanding intrinsic properties including low density, large specific surface area, thermal and chemical stability[4]. However, sole graphene suffers from interfacial impedance mismatching due to the high conductivity[5]. Although, by optimizing the substrate thickness, permittivity and

wave incidence angle and depositing graphene on a dielectric substrate, K. Batrakov et al. demonstrated the enhanced microwave-to-terahertz absorption in graphene, which possesses 80% and 65% absorptance at 30 GHz and 1 THz, respectively[6]. The prepare condition is rigid and the application is limited, also. Usually, incorporation of other nanosized magnetic or dielectric materials, such as ZnO[7], SiO2[8], CuS[9], TiO2[10], MnO2[11], Fe3O4[12], Fe[13] and conductive polymer[14] et cetera, could improve its microwave absorption performance effectively[15]. However, compared with graphene, for graphene composite, no matter the higher density introduced by metal particles or the thicker matching thickness due to the addition of polymer, the introduction of other materials confines their practical application. Is it possible to build up lightweight, thin matching thickness microwave absorber just using graphene by changing graphene’s electromagnetic properties? As we know, the properties of graphene could be tuned by changing the reduction methods of GO[16] or heteroatom doping[17-18]. Chao Wang at al. have proved that after chemical reduction of GO by hydrazine, the excellent hexagonal “graphene” framework cannot be completely recovered due to the existence of residual groups and defects. That is exactly the reason why reduced graphene oxide (rGO) possesses MW absorption performance. Although the Reflection loss of rGO is just -7dB at 7 GHz[19], it give us an idea that the EM performance can be “created” by tuning the reductive degree using chemical reduction method. In addition, GO can also be reduced by L-ascorbic acid (L-AA) according to Bai’s work[20]. Using PEO as adhesive, the optimal reflection loss value is close to -13.0 dB with a coating layer of

3.0 mm. Huang et al. explored the influence of graphene reduction degree on electromagnetic parameters of rGO/cobalt ferrite composites[21]. On the other hand, the conductivity and surface structure of graphene also can be tuned by doping heteroatoms[22-24]. As we known, the electronic properties of graphene can be tailored by interacting with other molecules, which either donate or withdraw free electrons[25-26]. After doping graphene with various heteroatoms (oxygen, boron, nitrogen, phosphor, sulfur, etc.), the graphitic carbon atoms are substituted

or

covalently

bonded

by

foreign

atoms.

Usually,

improved

electromagnetic interference (EMI) properties arise and are beneficial for particular applications depending on the type of dopants and their bonding configurations. Anchal Srivastava at al. have developed a microwave assisted route for synthesis of B, N Co-Doped reduced graphene oxide, which results good EMI shielding ability due to the improved conductivity results from the electrons introduced by N and holes provided by B[27]. By thermal annealing of GO, Min Koo at al. revealed the significant potential of sulfur-doped reduced graphene (S-GS) as EMI shielding materials firstly[33]. Because of the n-type doping contribution of the S atom to the doped graphene, not only the electrical conductivity of S-GS but also the EMI shielding property heightened compared with undoped rGO. In another work, mushroom-based sulfur compound was also thermal annealed to develop another kind of S-GS which exhibited 52% larger conductivity and 61% EMI shielding effectiveness compared with undoped rGO[34].Usually, in order to improve the efficiency of doping ， vigorous conditions is often provided. It looks as if

heteratom-doped graphene also hardly be used as MWA materials independently due to their excellent conductivity. Li at al. fabricated CoNi/N-doped greaphene hybrids which show a maximum reflection loss of -22 dB at 10 GHz with a matching thickness of 2.0 mm[30]. The only exceptional case is about fluorinated GO (FGO), whose enhanced S and X-band microwave absorption ability has been reported by M. R. Anatharaman[31]. The reflection loss (RL) is -37 dB (at 3.2 GHz with 6.5 mm thickness), -31 dB (at 2.8 GHz with 6 mm thickness) in the S band and -18 dB (at 8.4 GHz with 2.5 mm thickness) in the X band, respectively. It is presumed that fluorine is electronegative, the C-F bond gives rise to high polarity. And the absorption mainly comes from the dielectric loss because of polarization and the defects present in fluorinated graphene. Considering the toxicity of fluorine and the harsh preparation condition, is there any substitutional method to prepared heteroatom-doped graphene which possesses excellent EMW absorption just using mild condition to fabricate? We suppose, choosing appropriate reducing agent (heteroatom precursors) to chemical reduce GO and, at the same time, to dope rGO, could probably fabricate heteroatom doped rGO with excellently EMW performance. There is a traditional view that the residual defects and groups in heteroatom doped rGO would degrade the device performance[16] due to the decrease of conductivity. However, our research demonstrates that the residual defects and groups may be beneficial for the EEM absorption, which provides a new strategy to fabricate ultralight, tunable, EMW absorbers based only by graphene.

We synthesized S-GS by using Na2S as both the reductant and S source during wet chemical treatment of GO solution[32], which shows strong MWA capability at thin thickness. More importantly, the dielectric and EMW absorption properties can be tuned by different amount of dopant. Considering the relatively mild wet chemical preparation method, the S-GS in this work can be easily tailored and even modified by other materials. Therefore, this work also may provide a new strategy to design graphene-based EMW absorbing composites..

2. Experimental section 2.1 Materials Graphite power (325 meshes) was purchased from Kaitong Co., Ltd (Tianjin, China). Analytical-grade Na2S•9H2O, P2O5, K2S2O8, KMnO4, NaNO3, and 35% HCl, 98% H2SO4, 80% N2H4•H2O and 30% H2O2 aqueous solution were purchased from Aladdin (China) and used directly without further purification. Ultrapure water (18MΩ) was produced by a Millipore System (Millipore Q, U.S.A.). 2.2 Synthesis of Graphene oxide GO was prepared from natural graphite via a modified Hummers method[28]. Firstly, 2.0 g of graphite was put into a mixture of 10 mL concentrated H2SO4, K2S2O8 (2.0 g) and P2O5 (2.0 g). The reaction mixture was maintained at 80 oC and kept stirring for 4 h and terminated by adding 500 mL of deionized (DI) water. Then the mixture was filtered and dried in vacuum for 12h. And then, 2.0 g of the preoxidized graphite, 1.0 g of NaNO3 and 50 mL of concentrated H2SO4 were mixed and stirred for 15 min within an ice bath. After that, 6.0 g of KMnO4 was slowly

added to the above suspension solution, kept stirring for 2 h under 20 oC then at 35 oC for 2 h. And then, 100mL of H2O was slowly added with vigorous agitation. The reaction temperature was rapidly increased to 98 oC and maintained for 30 min and terminated by adding 200 mL of deionized water. Then, 10 mL of 30 % H2O2 was added to the mixture. For purification, the mixture was washed by deionized water and 5% HCl for several times, and metal ions were removed by dialysis membranes for a week. GO film was obtained after drying in a vacuum for 35 h. 2.3 Synthesis of S-GS S-GS was fabricated using wet chemical reduction method mentioned by Yan[27]. At first, 300.0 mg of GO powder was dispersed into 50 mL of water under the assistant of powerful ultrasound for 45 min (100W KQ3200DE, Kunshan UltrasonicInstrument Co., China). And then, Different amount of Na2S•9H2O (8.1, 16.2, 24.3, 32.4, 48.6 mmol) was introduced as reducing agent to prepare S-GS, respectively. The mixture was heated at 95 oC under stirring for 3 h. After filtration, the obtained solid was washed with a large amount of deionized water several times. After freezed-drying (FD-1B-50, Shanghai Binlon Instrument Co., China), different kinds of black powder of S-GS were obtained and labeled as S-GS1, S-GS2, S-GS3, S-GS4 and S-GS5, respectively. 2.4 Synthesis of hydrothermal reduced GO (H-GS) and hydrated nitrile reduced GO (N-GS) H-GS and N-GS were prepared for the comparative study. To acquire H-GS, the 60 mL 1mg/mL GO suspension were transferred into 100 mL Teflon-lined stainless-steel

autoclave, and treated hydrothermally at 180 oC for 12 h; And for N-GS, N2H4•H2O (2.45 mL, 80%) was gradually added into the 200 mL suspension of GO (1 mg/mL), The mixture was then heated at 95 oC under stirring for 3 h.

The two kinds of solid

product was filtrated, and washed with water and ethanol several times before vacuum drying for 12 h, respectively. 2.5 Characterization The crystal structure of the as-synthesized samples was identified by X-ray diffraction (XRD) technique using an X-ray diffractometer (D/max-2400, Hitachi Co., Japan) equipped with Cu Kα as a radiation source (λ=1.5406 Å). The morphologies of the samples were carried out by a transmission electron microscope (TEM, Tecnai G2 FEI, U.S.A.) at 100kV. Raman spectra were recorded with a LabRam HR Evolution, Raman microscope (Horiba, France) operating at 532 nm. X-ray photoelectron spectroscopy (XPS) was performed on a VG ESCA-3000 scientific spectrometer (UK). The chemical structures of samples were confirmed by Fourier transform infrared (FT-IR850, Tianjin Gangdong SCI. & TECH. Development Co., LTD, China) spectroscopy.

The conductivity of H-GS, N-GS and S-GS were also explored by the

four-point probe system (ST2253, Suzhou Jingge Electronic Co., China). To evaluate the microwave absorption properties of S-GS, the electromagnetic parameters of mixtures composed by 40 % samples and 60 % paraffin were measured at 2-18 GHz with a network analyzer (N5244A PNA, Agilent, U.S.A.). The RC of the samples was calculated according to the transmission line theory, using the measured data of relative complex permeability and permittivity.

3. Result and discussion Figure 1(a) shows the XRD patterns of pristine graphite powder, GO and S-GS. Graphite powder shows a sharp (002) peak at 26.4o with a typical d-spacing of 3.37 Å. The diffraction peak of GO is observed at 2θ = 11.02o, indicating the distance between atomic layers of graphite (002) is expanded to 7.83 Å. It is also seen that the corresponding peak disappears after reduction. The broad diffraction peaks at 2θ = 24o could be recognized as (002) diffraction peak of graphene, which indicating that Na2S could be used to form S-GS successfully[34-35]. Raman spectra of the GO, S-GS are shown in Figure 1(b). GO has two bands peaking at 1355 cm-1 (D band) and 1584 cm-1 (G band), which indicates the defect or edge planes in the structure and the vibration of sp2 hybridized carbon, respectively. While after reduction of GO by Na2S, the corresponding Raman spectrum of S-GS also contained both D and G bands at 1352 and 1590 cm-1, respectively. And it also exhibited an increased D/G intensity ratio relative to that of GO. This change may suggest an increase in the number of smaller graphene domains after reduction and it also confirms the formation of new graphitic domains after the reduction process[27-28]. Furthermore, all kinds of S-GS with different doping level have been detected by XPS (Figure 2a). The sample exhibits S 2s and S 2p peaks at about 164 and 225 eV. And the sulfur signals reveal the successful incorporation of sulfur atoms into the graphene networks. The high-resolution S2p is split into four peaks composed of sulfur (164.0 eV), thiophene (165.2 eV), sulfate (169.1 eV) and sulphide (162.2 eV) (Figure 2b) [36]. There are two features could be observed obviously. That is, with the

increasing amount of dopant, the S atomic concentration decreased firstly and then, increased gradually (from S-GS2 to S-GS4, the atomic concentration of S atom on graphene surface is only from 0.18 to 0.61). However, at the same time, the proportion of functional groups of S changed apparently, the amount of sulphate on the graphene surface increases and becomes the dominant functional S group. Considering the facts (which will discuss soon), that S-GS2, S-GS3 and S-GS4 possess excellent MWA properties, an interesting hypothesis was proposed. If graphene was doped with low atomic concentration of S and the main functional S group was sulphate, the S-GS posseses excellent MWA properties. The hypothesis also can be used to explain the facts that why other S-GS mentioned by other work

[26, 27],

which

with high amount of S atom on graphene surface and reduced well, have no MWA properties. The TEM images (Figure 3 (a-b)) clearly show the morphologies of GO and S-GS2, respectively (considering all kinds of S-GS appears similarly pattern, only the TEM data of S-GS2 demonstrated in this paper). In Figure 2a, the numerous thin flake structures confirm that GO are not aggregated. And after reduced by Na2S, large S-GS2 nanosheets can be observed with wrinkled structures, too (figure 2b). Of course, some kind of agglomerating appeared. The FT-IR spectra of GO and S-GS are shown in Figure 3c. It is found that the FT-IR spectrum appears a GO absorption peak around 1718 cm-1, which attributed to the skeletal stretching vibration of C=O, the vibration peaks at 3413 and 1400 cm-1 are associated with the O-H stretching vibration, the peak appearing at 1618 cm-1 is due to the stretching vibration of

aromatic C=C, and the peaks at 1253 and 1074 cm-1 can be attributed to the epoxy C-O stretching vibration and the alkoxy C-O stretching vibration, respectively[7]. The small peaks at 2920 cm-1 and 2852 cm-1 indicate the presence of C-H bonds[37]. Compared with GO, the peak area at around 3414 cm-1 of S-GS is much smaller, which indicate the successful reduction of GO. As we know, reduced GO always appears different properties by different reduce methods. For comparison, the hydrothermal reduced GO (H-GS) and hydrated nitrile reduced GO (N-GS) were also prepared. From figure 3d, we can easily find that the conductivity of H-GS is much lower than that of N-GS. And for S-GS, the values of the conductivity change between those of H-GS and N-GS according to the difference amounts of Na2S added as reducing agent. Generally speaking, the relative complex permittivity (εr=ε'-jε"), the relative complex permeability (μr=μ'-jμ") and proper matching between them are significant to determine EMW absorbing properties[10]. The relative complex permittivity values are obtained through coaxial method. According to the Debye equations, the real permittivity (ε') and the real permeability (μ') determine the ability of storing electric and magnetic energy within the medium, respectively. While the imaginary permittivity (ε") and the imaginary permeability (μ") represent their corresponding loss capabilities. Besides, dielectric loss tangents tanδε=ε"/ε' represents the dielectric relaxation properties[38]. And the attenuation constant, α, can be used to evaluate the absorption property of an absorber toward EMW (equation (1)) [39]. 𝛼=

2𝜋ƒ 𝑐

× (𝜇"𝜀" ― 𝜀′ 𝜇′) + (𝜇"𝜀" ― 𝜀′ 𝜇′)2 + (𝜇′𝜀" + 𝜇"𝜀′)2

(1)

Figure 4a-d exhibits the measured ε', ε", tanδε and α values for H-GS, N-GS and S-GS, respectively. The same tendency appears that, no matter ε', ε", tanδε or α, H-GS always shows the minimum value while N-GS presents the maximum value, and S-GS appears tunable tendency between H-GS and N-GS changed by the amount of reduce agent. Since all the samples we discussed are nonmagnetic material, the complex relative permeability is equal to (1, 0). As we know, the value of tanδε is usually used to estimate the loss ability of microwave. And higher values of tanδε usually imply higher dielectric losses. On the other hand, the higher α value also suggests stronger attenuation ability for EM wave. That is, increasing dielectric loss is beneficial to increase the absorption coefficient. However, the problem is that, with the increase of dielectric loss, the permittivity increases, and the EM reflection increases consequently. To obtain improved EM absorption properties, absorbing materials need to have a suitable permittivity and dielectric loss. It is revealed that the materials with high EM absorption capability require a low real part of permittivity and an appropriately high electrical conductivity[40]; That is way H-GS, which possesses much higher ε', ε", tanδε and α then N-GS, present weak EM absorption capability like N-GS (Figure 5a)[41]. Traditionally, in order to meet the objective of minimizing RL and maximizing effective absorption bandwidth, the EM absorbing materials are usually designed by combining a low permittivity phase and a high electrical conductivity phase[15, 42, 43]. Nevertheless, S-GS exhibits extraordinary EM absorption properties (Figure 5b-f). The optimum frequency can be tuned to achieve MW absorb from C band to Ku band

by adjusting the amount of Na2S which was added to reduce GO and dope S element in the same time. Moreover, S-GS samples also show extraordinary high RL value with thin matching thickness (Table 1).

Especially for S-GS2 and S-GS4, the RL

values even can touch to -52.41 dB, -52.30 dB and -49.26 dB with a matching thickness of 2.18 mm, 1.22 mm and 2.44 mm, respectively. Looking around all single materials which were used to absorb MW, it is hard to find one which could fulfill all of these properties we mentioned above just like S-GS. Table 2 summarizes the phase composition, optimum thickness, optimum frequency, and RL of graphene based absorption materials in the recent literature. We can easily find that S-GS even shows much better MWA properties than most of the graphene based composites. Especially, the thickness of S-GS is need only about 2 mm. Additionally, it is generally believed that one of the efficient ways to improve MWA properties of absorbers is to construct multiple interface structures, which can greatly enhance interface polarization. And the main purpose of the designed experiment is to enhance interface polarization through the introduction of sulfur atom. In order to prove this viewpoint, the plots of ε' versus ε" for the obtained samples are presented. According the Debye relaxation expression, ε' and ε" satisfy the following equation:

(ε′

―

𝜀𝑠 + 𝜀∞ 2 2

2

) + (ε") = (

𝜀𝑠 ― 𝜀∞ 2 2

)

(2) The above equation is defined as the Cole-Cole semicircle and is also characteristic of the Debye relaxtion process. As presented in figure 6, the plots of ε' versus ε" for

S-GS exhibits several relatively big semicircles. The result indicates that the introduction of sulfur greatly enhance multirelaxations, which results from the existence of interfacial polarizations. As an ideal MW absorbing material with good performance, two requirements should be meet. One is high attenuation constant which have been talked above. And another one is impedance matching, which means that the incident electromagnetic wave can enter into the absorbing material as much as possible[44]. Only the absorbing material meets the two requirements at the same time, and optimum absorbing properties can be reached. Based on the transmission line theory[45], when single layer material is used as absorbing medium, the normalized characteristic impedance values of the EM wave absorbers are calculated according to the following equation: Z = |𝑍𝑖𝑛/𝑍𝑜| = |𝜇𝑟/𝜀𝑟| tanh[𝑗(2πƒd/c) 𝜇𝑟𝜀𝑟

(3)

where the notations are defined as the input impedance (Zin), the free-space impedance (Z0), complex permeability (μr) and permittivity (εr), frequency of microwave (f), thickness of absorber (d) and the velocity of light (c). It is well-known that when Zin/Z0 value is 1, implying that no EM wave reflection happens on the absorber surface, which means well-matched impedance is achieved. Figure 6 (a-d) shows the normalized characteristic impedance values of N-GS, H-GS, S-GS2 and S-GS4, respectively. For N-GS, the input impedance is much lower than free-space impedance (figure 7 (a)). And totally opposite situation appears for H-GS, which possesses much higher input impedance compared with free-space impedance (figure 7 (b))[46]. Interestingly, reduced by different amount of Na2S, the Z values of S-GS are

tuned and for S-GS2 and S-GS4, it is close to 1 (figure 7(c-d)), which means most of the EMW can easily incident into the absorbers with minimum reflection at the air-absorber interface and then EMW will be converted to heat energy or dissipated by interference effect. These phenomena have never been reported by other rGO. Figure 7 (e, g) and figure 7 (f, h) indicate the RL curves and dependence of matching thickness (tm) on matching frequency (fm) of S-GS2 and S-GS4, respectively. It can be seen that RL curves show compatible tendency and the absorption peaks shift to lower frequency with increasing layer thickness. This tendency can be interpreted by quarter-wavelength cancellation [9]. That is, at the appropriate thickness and frequency, two forming reflected waves from air-sample and metal-sample interfaces could be out of phase by 180o, leading to an extinction of them at the air-sample interface. In this model, the relationship between matching thickness (tm) and frequency (fm) is shown as follow (equation (4)): tm = nc/(4ƒm |μ||ε|) (n = 1,3,5….)

(4)

When n is equal to 1, the λ/4 cancellation values of S-GS2 and S-GS4 are presented in figure 7 (g) and (h), which displays the simulated curves of tm (marked as tmsim) and the experimental points of tm (marked as tmexp) versus fm, respectively. Notably, the experimental scatter symbols are opportunely located on simulated curves of tm, indicating that S-GS obeys quarter-wavelength cancellation model[8]. For S-GS2, at the matching thickness of 1.22 mm and 2.18 mm, the maximum RL can be acquired at 17.5 GHz and 9.4 GHz, respectively. For S-GS4, at the matching thickness of 2.44 mm, the maximum RL can be acquired at 8.5 GHz. And at the same time, the

corresponding Z values are all almost close to 1. Therefore, the well-matched characteristic impedance facilitates the enhanced EMW absorbing performance. The EMW absorption mechanism of S-GS has been proposed. First, S-GS has a large specific surface area and appropriate conductivity, forming a conductive network. It is prone to charge migration and hopping along the interface layers. Second, the residual defects and functional groups on the surface of S-GS caused by incomplete reduction lead to introducing dipole polarization. Third, abundant interfacial polarization occurs due to the doping sulfur atoms.

4. Conclusion In terms of microwave absorption, dielectric performance acts vital but negative characters in attenuation and impedance matching. In this study, S-GS have been fabricated through a simple and valid chemical reduction method. By changing the molar ratio of the reducing agent, the permittivity of the S-GS can be adjusted to balance the energy conservation and impedance matching. We found that, by choosing appropriate reducing agent (heteroatom precursors) to chemical reduce GO and, at the same time, to dope graphene, could probably fabricate S-GS with excellently EMW performance. The residual defects and groups in S-GS may be beneficial for the EM absorption, which provides a new strategy to fabricate ultralight, tunable, MW absorbers based only by graphene. And more importantly, from N-GS, S-GS to H-GS, the reported progressive change regularities of dielectric and magnetic properties could help us to optimize the combination strategy with other materials to satisfy the different demand of practical applications.

Acknowledgements We are grateful for financial support from the National Natural Science Foundation of China (NSFC. 21664009, 51063003), the Ministry of Science and Technology project (No. 2009GJG10041), the Fundamental Research Funds for the Universities of Gansu (No. 1105ZTC136), the Natural Science Foundation of Gansu Province (No. 17JR5RA135).

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Figure caption Figure 1 (a) X-ray diffraction (XRD) patterns of pristine graphite powder, GO and S-GS; (b) Raman spectra of GO and S-GS; (c) XPS survey spectrum; (d) S2p spectra of S-GS Figure 2 XPS survey spectrum of S-GS; (b-f) S2p spectra from S-GS1 to S-GS5 Figure 3 TEM images of (a) GO and (b) S-GS2; (c) FT-IR spectra of GO and S-GS; (d) conductivity of H-GS, N-GS and S-GS Figure 4 Frequency dependence of (a) real, (b) imaginary parts of relative complex permittivity, (c) dielectric loss tangents and (d) attenuation constant of H-GS, N-GS and S-GS dispersed in paraffin with 40 wt%. Figure 5 RL of (a) H-GS and N-GS at a sample thickness of 2.0 mm, (b) S-GS1, (c) S-GS2, (d) S-GS3, (e) S-GS4 and (f) S-GS5 dispersed in paraffin as a function of frequency. Table 1 Conductivity, permittivity and EM Absorption Properties of GS, hydrothermal reduced GO, hydrated nitrile reduced GO and S-GS Table 2 Summary of EM Absorption Properties Reported in Recent Papers Figure 6 Cole-Cole plots of (a) S-GS1, (b) S-GS2, (c) S-GS3, (d) S-GS4 and (e) S-GS5 Figure 7 The normalized characteristic impedance curves of (a) N-GS; (b) H-GS; (c) S-GS2; (d) S-GS4; (e, g) RL curves and dependence of matching thickness on matching frequency of S-GS2; (f, h) RL curves and dependence of matching thickness on matching frequency of S-GS4

Figure 1 (a) X-ray diffraction (XRD) patterns of pristine graphite powder, GO and S-GS; (b) Raman spectra of GO and S-GS

(a)

Figure 2 (a) XPS survey spectrum; (b) S2p spectra of S-GS

Figure 3 TEM images of (a) GO and (b) S-GS2; (c) FT-IR spectra of GO and S-GS2; (d) conductivity of H-GS, N-GS and S-GS

Figure 4 Frequency dependence of (a) real, (b) imaginary parts of relative complex permittivity, (c) dielectric loss tangents and (d) attenuation constant of H-GS, N-GS and S-GS dispersed in paraffin with 40 wt%.

Figure 5 RL of (a) H-GS and N-GS at a sample thickness of 2.0 mm, (b) S-GS1, (c) S-GS2, (d) S-GS3, (e) S-GS4 and (f) S-GS5 dispersed in paraffin as a function of frequency.

Table 1 Conductivity, permittivity and EM Absorption Properties of GS, hydrothermal reduced GO, hydrated nitrile reduced GO and S-GS

Table 2. Summary of EM Absorption Properties Reported in Recent Papers

Figure 6 Cole-Cole plots of (a) S-GS1, (b) S-GS2, (c) S-GS3, (d) S-GS4 and (e) S-GS5

Figure 7 The normalized characteristic impedance curves of (a) N-GS; (b) H-GS; (c) S-GS2; (d) S-GS4; (e, g) RL curves and dependence of matching thickness on matching frequency of S-GS2; (f, h) RL curves and dependence of matching thickness on matching frequency of S-GS4

Table 1 EM absorbing material ZnO/NPC/rG O graphene/SiO2 RGO/γ-Fe2O3 Graphene/PA NI RGO CoNi/N-doped graphene FGO Graphene/Fe3 O4 CoFeAl-LDH/ G Fe3O4/graphe ne RGO/Fe Fe3O4/GCs SGN/Fe3O4 β-LiFe5O8/GN

wt %

optimum frequenc y (GHz)

optimum thickness (mm)

min RL value (dB)

frequency range (GHz) (RL>10dB)

ref

40

12.8

2.6

-50.5

7.4

5

45

12.0 10.09

2.11 2.5

-42 -59.65

6 10

20

12.9

2.5

-45.1

12

7

2

-7

16

10

2.0

-22

5.2

25

3.2

6.5

-37

0.7

26

2.08

-49.05

30

39

50

14.2

2.5

-23.8

7.36

40

30

5.3

3.0

-27

1.50

41

40 70 50 60

5.64 8.76 16.5 16.3 9.4 17.5 8.5

3.0 3.5 2.0 9.0 2.18 1.22 2.44

-36.5 -32 -41 -28 -52.2 -52.3 -49.3

1.45 4.5 5.3 2.3 2.28 2.40 2.23

42 43 44 45

S-GS2

40

S-GS4

40

this work

Table 2

sampl e

S-GS1

Amount of Na2S (mmol)

8.1

Cond uctivi ty (S/cm ) 2.86

S-GS2

16.2

3.17

S-GS3

24.3

3.53

S-GS4

32.4

4.92

S-GS5

48.6

5.04

H-GS

-

0.50

N-GS

-

6.74

ε'

13.9 -10. 4 18.8 -13. 2 17.9 -12. 2 17.6 -12. 4 22.5 -14. 1 8.36.4 26.3 -15. 7

thic knes s (mm )

RLmin (dB)

Band width (GHz )

Absorb band

ε"

freque ncy (GHz)

6.03.9

4.48

5.00

-31.23

1.82

C

9.35 14.02 17.52 10.64

2.18 1.50 1.22 2.00

-52.41 -39.44 -52.30 -30.15

2.28 3.49 2.40 2.73

X Ku Ku X

14.70

1.50

-30.01

3.95

Ku

8.43.9

8.57

2.44

-49.26

2.23

X

12.8 -7.6

12.89

1.50

-22.04

3.28

Ku

1.61.8

15.80

2.00

-10.18

-

-

21.7 -7.7

8.83

2.00

-11.89

-

-

9.15.6 10.1 -5.7