Nickel nanoparticle–decorated reduced graphene oxide via one-step microwave-assisted synthesis and its lightweight and flexible composite with Polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene polymer for electromagnetic wave shielding application

Nickel nanoparticle–decorated reduced graphene oxide nanocomposites (NiG) were prepared by a one-step microwave-assisted solvothermal method. The as-prepared NiG nanocomposite systems were further heated up to 800 °C under an inert atmosphere (named NiG-800) to modify their structural and electromagnetic properties. Thereafter, these developed NiG-800 nanocomposite systems of rGO and nickel nanoparticles (25 wt.%) were applied as nanofillers (50 wt.% and 70 wt.%) in a SEBS (Polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene) polymer matrix to create NiG-800(50)-SEBS and NiG-800(70)-SEBS nanocomposites. The addition of NiG-800 to SEBS led to an increase of Young’s modulus from 16 (SEBS) to 35 MPa (NiG-800(70)-SEBS) while the maximum elongation is still around 300%. The developed NiG-800(70)-SEBS nanocomposite exhibited high-performance electromagnetic wave absorption (minimum reflection loss RLmin ≈ –48.2 dB at 9.29 GHz) at a low thickness of 2.3 mm in the frequency range of 8.2−12.4 GHz. The prepared NiG-800(70)-SEBS nanocomposite has the potential of an electromagnetic wave absorber. The NiG-800(70)-SEBS nanocomposite reported here has total shielding efficiency > 10 dB at a thickness of 1 mm in the whole frequency range (X-band) with reflection ≈ 50% and absorption ≈ 40% which has the potential for electromagnetic wave absorber applications.


Introduction
In recent years, reduced graphene oxide (rGO)-based carbon nanomaterials have attracted immense research interest due to their thermal, mechanical, electronic, and optical properties and high surface area [1][2][3][4][5]. Reduced graphene oxide introduces the properties such as a large contact area provided by the planar structure of graphene sheets and the unique ability to promote fast electron-transfer kinetics for a variety of electroactive species [6,7] and consequently has a high potential to be used as an electromagnetic wave absorber or shielding material [8][9][10][11]. Decorating rGO with metal nanoparticles has opened new opportunities for the preparation of advanced materials and composites [12][13][14]. Preparation methods, such as a hydrothermal method, for these composites involve hydrothermal reactions of nickel salts with graphene oxide (GO) followed by calcination and reduction [15,16]. A different procedure published by Gotoh et al. [17] has introduced the preparation of Ni nanoparticle-decorated graphene sheets from cation-exchanged graphite oxide. The reaction of nickel chloride with hydrazine hydrate in GO water solution has been reported by a research group of Ji et al. [18]. Moreover, this research group further described a solvothermal reaction of graphite oxide with nickel acetylacetonate in triethylene glycol at 270 °C [19]. Solvothermal preparation of nickel nano-chains which were anchored on rGO has been reported by Xu et al. [20]. In addition, an interesting preparation procedure has been reported in the work of Li et al. [21]. The reported method is known as the Joule heating process and it is based on the application of direct current to the micro-Ni/rGO film mounted in a vacuum chamber. Furthermore, a facile synthesis for 3D graphenecarbon nanotube-nickel nanostructures based on the microwave decomposition of nickelocene in acetonitrile with the presence of thermally exfoliated reduced graphene oxide (TErGO) has been described by Bae et al. [22].
In recent years, electromagnetic (EM) radiation pollution has received growing attention due to its adverse effects on human health as well as on the working of electronic devices [23][24][25][26][27][28][29]. Therefore, high-performance electromagnetic interference (EMI) absorbing and shielding materials are required to prevent these issues [28,[30][31][32][33]. High-efficiency EM wave absorption materials for stealth technologies have also been found to escape the detection of radar effectively, which is believed to be crucial to ensure the survivability of weapons and equipment on the battlefield [25,34]. There are three typical mechanisms involved in the development of promising EMI absorbing and shielding materials [24,26,34]. The first one is an interaction with the EM fields by charge carriers, which requires materials with high electrical conductivity. The second mechanism is the absorption of EM radiation by the electric and/or magnetic dipoles interacting with radiation. The third mechanism is associated with multiple internal reflections from scattering centers and interfaces or defect sites in the EM wave absorber. Hence, a nanocomposite material that combines high electrical conductivity, magnetic loss, and multiple reflections is a promising candidate for EMI absorbing and shielding [24,26,34]. It has already been reported that carbon-based nanomaterials containing nickel nanoparticles exhibit EMI shielding properties [35]. For example, the multiwalled carbon nanotube/nickel-coated carbonized loofah fiber/ polyether ether ketone composite reported by Li et al. [36] manifested high EMI shielding efficiency. Nickel nanoparticles embedded in carbon foam or on graphene sheets have also been proposed as EMI shielding materials [37,38]. Recently, the flexible polymer-based nanocomposite containing nickel chains for EMI shielding has been reported by Wang et al. [39]. Moreover, the epoxy matrix-based composites containing graphene and nickel nanoparticles [40] or rGO and nickel chains [41] have been reported as promising candidates for electromagnetic wave shielding applications. Recently, the metal-organic frameworkderived nickel/carbon composites with optimized impedance showed potential to apply them as microwave absorbers [42]. It can be argued that nowadays the fast economical and environmentally friendly procedures for the synthesis of advanced shielding materials attracted high attention of researchers.
Herein, we report nickel nanoparticle-decorated reduced graphene oxide nanocomposite (NiG) prepared via a facile one-pot microwave solvothermal synthesis as advanced filler for flexible polymer-based composite with high electromagnetic shielding performance. The microwave-assisted reaction of graphene oxide with nickel acetylacetonate in diethylene glycol solvent with the presence dodecylamine leads to the reduction of nickel precursor and the assembling of metallic nickel nanoparticles on the reduced graphene surface. For further application in EMI shielding, the asprepared NiG nanocomposites were annealed at 800 °C in an inert atmosphere to carbonize the material and achieve good electromagnetic properties. Carbonized NiG nanocomposites were further mixed with Polystyrene-blockpoly(ethylene-ran-butylene)-block-polystyrene (SEBS) polymer to obtain a lightweight and highly flexible composite applicable as efficient EMI shielding material. The SEBS-based composites were prepared with 50 and 70 wt.% loadings of annealed NiG nanocomposite. It was demonstrated that the presented SEBS-based composites exhibited excellent mechanical properties and achieved high activity in EM wave absorption.

Preparation of graphene oxide
Graphene oxide (GO) was prepared by a modified Hummers method [43,44]. The typical synthesis was performed as follows: 3 g of graphite and 1.5 g of NaNO 3 were mixed with 75 ml of H 2 SO 4 (98%) in a 1000-ml flask at 0 °C. The prepared mixture was then stirred for 15 min, and 9 g of KMnO 4 was carefully added to the suspension with vigorous magnetic stirring for about half an hour. Then, the prepared mixture was stirred for 30 min. After that, the reaction temperature was fixed at room temperature for an additional 48 h. Afterward, 138 ml of H 2 O was added slowly to the pasty mixture with vigorous agitation for 10 min. Then, 420 ml of warm H 2 O and 30 ml of H 2 O 2 were added to this prepared yellow suspension. The yellow suspension was washed with an aqueous solution of H 2 SO 4 (6 wt.%) and H 2 O 2 (1 wt.%). Finally, the prepared suspension was washed with deionized water until the pH was neutral.

Synthesis of nickel-decorated reduced graphene oxide nanocomposite
Graphene oxide (0.510 g) prepared via the Hummers method [43] was used and added to DEG (50 ml) in a Teflon reaction vessel (Table S1, supplementary file). Then, Ni(II) (Acac) 2 (0.499 g, 1.94 mmol) and dodecylamine (1.01 g, 5.45 mmol) were mixed and stirred. A teflon-lined reaction vessel containing the precursor's suspension was positioned and closed tightly in the microwave reactor. Duration of the microwave synthesis was 15 min at microwave power 100% (600 W) and temperature reached 250 °C. The details of the microwave synthesis conditions can be seen in Fig. S1 (supplementary file). Furthermore, the product solution was cooled down to 50 °C spontaneously and centrifugated and washed with methanol twice. The black precipitate was dried at 80 °C in the oven. The achieved yield of the as-prepared sample represented as NiG was 0.504 g. To investigate the impact of annealing temperature on the structural and physical characteristics, the as-prepared NiG sample was carbonized in a tubular furnace in an atmosphere of argon at 800 °C for 2 h. The carbonized nickel-decorated reduced graphene oxide nanocomposite was labeled as NiG-800.

Nickel-decorated reduced graphene oxidebased SEBS nanocomposite preparation
Nickel-decorated reduced graphene oxide-based SEBS nanocomposite, NiG-800(70)-SEBS, containing 70 wt.% of NiG-800 material was prepared as follows: 202.6 mg of NiG-800 material was mixed with 86.9 mg SEBS polymer in a beaker (Table S2, supplementary file). Then, 5 ml of toluene was added. The mixture was homogenized in an ultrasonic bath and under vigorous stirring, a black viscous solution was formed. This solution was poured into a Petri dish and left to dry in a hood at room temperature. After drying, the solid and flexible composite was obtained. In addition, the SEBS-based composite labeled as NiG-800(50)-SEBS containing 50 wt.% of the NiG-800 nanocomposite filler was prepared in the same manner as described in the case of the NiG-800(70)-SEBS.

Characterization techniques
Nickel nanoparticle-decorated rGO was synthesized in PTFE-lined microwave reactor ERTEC Magnum II (600 W; 2.45 GHz). For the product separations, EBA 21 Hettich centrifuge was used. Powder XRD patterns were recorded on Rigaku MiniFlex 600 equipped with a CoK α (λ = 1.7903 Å) X-ray tube (40 kV, 15 mA). Data processing and crystal size calculations were done with Rigaku PDXL2 software. The average crystalline size of the crystallites was evaluated using Scherrer's formula, d = Kλ/β cos θ, where d is the crystalline size, K is a grain shape-dependent constant (0.9), λ is the wavelength, θ is a Bragg reflection angle, and β is the full-width halfmaximum. The heating of products was carried out in a tube furnace Nabertherm R50/250/12 with a B410 controller connected to a nitrogen atmosphere source. TGA analyses were performed on Setaram LabSys Evo with TG/ DSC sensor (heating ramp 5 °C min −1 , up to 1000 °C, airflow 60 ml min −1 ). SEM images were collected on Nova NanoSEM (FEI) with Schottky field emission electron source (0.02-30 keV) and TLD, and CBS detectors. Raman spectroscopy was performed on a Thermo Raman microscope Nicolet DXR equipped with a He-Ne laser with emission wavelength 633 nm (power 1 and 4 mW).
The spectra were recorded from 50 to 2000 cm −1 . Transmission electron microscopy (TEM) was carried out on a JEOL JEM 2100 microscope operated at 200 kV (LaB 6 cathode, point resolution 2.3 Å) equipped with an OLYM-PUS SYS TENGRA camera (2048 × 2048 pixels). X-ray photoelectron spectroscopy measurements were carried out on a Kratos Analytical Axis Supra spectrometer equipped with monochromatized Al-Kα radiation (1486 eV). The pressure in the analysis chamber was around 10 −6 Pa. The analyzed area was approximately 1.4 mm 2 and the pass energy was set at 150 eV. The C 1 s peak of carbon has been fixed to 284.8 eV to set the binding energy scale [45]. The obtained data was executed with the CasaXPS program (Casa Software Ltd, UK) and spectra were deconvoluted with the least-squares fitting routine carried by the software with a Gaussian/Lorentzian (85/15) product function after subtraction of a nonlinear baseline [46]. The content of nickel in the NiG-800 nanocomposite was determined by ICP-OES spectroscopy performed on an iCAP 6500 spectrometer by Thermo Scientific (nickel spectral line λ = 221.647 nm). The complex permittivity, complex permeability, and scattering (S) parameters were measured on a PNA-L network analyzer (Agilent N5230A) using an X-band waveguide operating within the frequency range of 8.2 −12.4 GHz. The techniques of Nicolson − Ross − Weir are utilized, which provide a direct assessment of complex permittivity and permeability using input S-parameters. The magnetization measurements were performed with a Vibrating Sample Magnetometer (Lake Shore 7407, US) at room temperature (300 K) in an air atmosphere in a magnetic field range from −760 up to + 760 kA m −1 (−10,000 to 10,000 Oe). The developed composites were pressed to achieve uniform thickness for testing mechanical and shielding properties. Mechanical characteristics of SEBS nanocomposites were studied by utilizing a Testometric universal-testing machine of type M 350−5CT (Testometric Co. Ltd., Rochdale, UK).

Results and discussion
The nanocomposite of reduced graphene oxide (rGO) and metallic nickel nanoparticles was prepared via a one-step microwave-assisted reaction of graphene oxide and Ni(II) (Acac) 2 in DEG with the addition of dodecyl amine as both reduction agent and surfactant. Microwave-assisted heating was carried out in a monomodal reactor at the temperature of 250 °C for 15 min. DEG solvent was chosen due to its high permittivity (ɛ = 32), enabling its high dissolving capacity for polar inorganic and a variety of organic compounds [47]. An additional advantage of DEG is also its high polarizability which makes it an excellent microwave-absorbing agent [48]. The composition of the prepared NiG sample and the values of nickel content are given in Table S1 (supplementary file). The content of nickel in NiG sample determined by the ICP-OES technique was 25 wt.%. (200) crystal phases [18,49,50]. It is worth mentioning that metallic nickel nanoparticles are already formed within a one-step MW-assisted approach which confirms that the reduction process takes place during the synthesis in the MW reactor. The apparent size of nickel crystallites contained in NiG and NiG-800 samples derived from Scherrer's formula is about 16 and 93 nm, respectively. Other intense diffraction peaks found at lower 2 theta degrees are ascribed to the GO and rGO structure [18,49,50]. Figure 1b depicts the Raman spectra of NiG, NiG-800, and NiG-800(70)-SEBS samples. The Raman spectra of the samples gave rise to two Raman bands around 1340 and 1587 cm −1 , which is indicative of the vibration modes of the disordered carbon (D-band) and inplane vibrations (E 2g ) of ordered graphitic carbon (G-band), respectively [18,21,51]. These bands are also noticed in the Raman spectra of NiG-800(70)-SEBS nanocomposite. Besides D and G bands, Raman bands for the SEBS matrix were observed around 1000 and 1450 cm −1 .

X-ray photoelectron spectroscopy
An X-ray photoelectron spectroscopy (XPS) was employed to thoroughly investigate the bonding relations between elements in both as-prepared NiG and NiG-800 samples. XPS survey scans over the whole binding energy region are given in supplementary information (Fig. S2, supplementary file). As displayed in the high-resolution C 1 s XPS spectrum (Fig. 2a), a major contribution is assigned to graphitic C-(C, H) groups (284.8 eV). In particular, the asymmetry of the C 1 s peak in the high binding energy (B. E.) side could be understood as the simultaneous presence of oxidized carbon species (C-O, C = O, and COO-moieties) and graphitic-like compounds, Fig. 1 a Powder XRD pattern of as-prepared NiG sample and NiG-800 sample using a CoK α (λ = 1.7903 Å). b Raman spectra of NiG, NiG-800, and NiG-800(70)-SEBS samples the latter presenting intrinsic asymmetry of the C 1 s peak [52][53][54]. The Ni 2p 3/2 XPS spectrum of the as-prepared NiG sample displayed in Fig. 2b shows contributions for Ni 0 states of metallic nickel nanoparticles and Ni 2+ states that originate from surface oxidation of nickel species or residual nickel precursor species [53,55,56]. In the O 1 s XPS spectrum (Fig. S3, supplementary file), the presence of Ni 2+ species corresponds to the contribution of Ni-O species found at 529.9 eV [57].
High-resolution N 1 s XPS spectrum nitrogen exhibited the peak of amine function (Fig. S4, supplementary file) [58]. The high-resolution C 1 s XPS spectrum of NiG-800 sample is illustrated in Fig. 3a. The most intense contribution at 284.8 eV is characteristic of C-(C, H) groups of graphene or graphitic-like materials. The asymmetry of C 1 s peak in the high B. E. side could be associated with the simultaneous presence of oxidized carbon species and graphitic-like compounds [52,54,59]. Further, the C 1 s shoulder component with B. E. 283.3 eV could be associated with the Ni-C bond [60]. The Ni 2p 3/2 XPS spectrum of NiG-800 sample (Fig. 3b) revealed contributions of Ni 0 states of metallic nickel nanoparticles and Ni 2+ multiplet that can be assigned to oxidized surface Ni species [53,56]. This observation correlates with the O 1 s XPS spectrum (Fig. S5, supplementary file) that shows the component of Ni-O species at 529.8 eV   [57]. The content of nickel on the surface of NiG-800 sample determined by the XPS method is 3.3 wt.%. The N 1 s XPS spectrum (Fig. S6, supplementary file) revealed the most intense contributions of pyridinic and graphitic nitrogen atoms [54,58]. It is proposed that nitrogen atoms are present as a consequence of dodecylamine addition within the preparation of the NiG sample. Figure 4 displays a scanning electron microscopy (SEM) image of NiG and NiG-800 samples. It can be noticed that the samples exhibited the morphology of rGO sheets with nickel nanoparticles embedded on the surface and between the sheet layers. Further, transmission electron microscopy (TEM) images of both NiG and NiG-800 samples, as shown in Fig. 5, revealed the structure of carbon-based nanosheets with anchored nanoparticles. It can be noticed that in the case of NiG sample, there are agglomerates of small nanoparticles (Fig. 5e), whereas the NiG-800 sample exhibited the presence of bigger, single nanoparticles of metallic nickel with the size of approximately 100 nm. This observation complements the results derived from Scherrer's formula. As illustrated in the HRTEM image in Fig. 5e, nanocrystals with lattice spacing of about 0.2 nm were observed in the case of NiG sample. This value corresponds to the (111) crystal plane of Ni nanocrystals [55]. Electron diffraction patterns of NiG and NiG-800 samples are displayed in Fig. 5f and g, respectively [61]. The content of nickel in NiG-800 nanocomposite sample determined by the SEM-EDX technique is 29.6 wt.%. It can be argued that this value corresponds to the nickel content determined by the ICP-OES method (Table S1). SEM images of NiG-800 composite with SEBS copolymer (NiG-800(70)-SEBS), as shown in Fig. 6, revealed the polymer-based matrix with incorporated NiG-800 material. The rGO-based nanosheet structures decorated with nickel particles are visible and it can be concluded that the NiG-800 nanocomposite is tightly anchored in the SEBS matrix. Based on the results from the elemental analysis of NiG-800 sample, it can be concluded that the content of nickel nanoparticles in the final composite material NiG-800(70)-SEBS is 17 wt.%.

Thermogravimetric analysis
To study the thermal behavior, TG analyses of prepared samples were performed (Fig. S7, supplementary file). TG curves of NiG and NiG-800(70)-SEBS samples recorded in

Magnetic properties
The magnetic properties of the NiG and NiG-800 were investigated by utilizing a vibrating sample magnetometer (VSM) at room temperature in an applied magnetic field up to 10 kOe. respectively. The increased value of saturation magnetization of NiG-800 nanocomposite sample is associated with an increase in crystallinity in nickel nanoparticles after annealing at high temperatures [62]. In comparison, the other reported rGO/Ni nanocomposite containing Ni nano-chains prepared from nickel chloride and hydrazine hydrate exhibited saturation magnetization values of about 3 emu g −1 [20]. As reported here, with the NiG and NiG-800 nanocomposites prepared via microwave-assisted approach, we achieved higher saturation Oe, respectively. The larger M s and lower H c values are supportive of the enhancement of the initial permeability, which can further improve the magnetic loss and consequently EM wave absorption characteristics [63,64]. Therefore, NiG-800 sample can provide higher EM wave absorption.

Electromagnetic interference shielding effectiveness
Electromagnetic interference shielding refers to the attenuation of the transmitting electromagnetic waves by the shielding material. A high value of electromagnetic interference (EMI) shielding effectiveness (SE) means less energy is transmitted through the shielding material. Either reflecting or absorbing the interfering electromagnetic wave will result in electromagnetic interference (EMI) shielding [65]. Although metal is the best material for electromagnetic radiation protection, the reflected wave may still interfere with the electronic component inside the enclosure or close by. Through the absorption mechanism, the electromagnetic interference shield functions on the basis that it absorbs the interfering electromagnetic wave by converting the wave energy into thermal energy and then reduces the interference to a very low value. However, absorption shielding necessitates a number of unique design requirements for the absorber's optimization, and this shielding mechanism is frequency-dependent. Measuring the portion of the incident (I 0 ) electromagnetic wave, transmitted (I T ) through and reflected (I R ) by the sample, scattering parameters S 21 and S 11 , respectively, was directly obtained as follows in Eqs.
While S 11 gives a portion of reflected radiation, S 21 provides direct information on transmitted radiation through the sample of a given thickness. The EMI shielding properties in the frequency range from 8.2 to 12.4 GHz (X-band) were determined and expressed by S 21 parameter for the SEBS matrix as well as for concentrations 50 and 70 wt.% of NiG-800, respectively, at thickness of t = 1 mm, as depicted in Fig. 8. According to Fig. 8a, SEBS matrix transparency in a given region is around 94%, i.e., its shielding efficiency in a composite without nanoparticles is around S 21 = −0.5 dB, which is negligible. Significantly higher shielding efficiency S 21 = −6 dB was reached for nanocomposite with 50 wt.%. One of the key parameters influencing shielding efficiency

Reflection loss (RL)
For protection in a microwave or radio frequency band, absorbents like a microwave absorber (MAM) or a radar absorber (RAM) are used [65]. One of the most crucial requirements for microwave absorbers is that the absorbent substance minimizes front face reflection and matches the air's impedance to the absorption interface. By having high dielectric and magnetic loss values, the material must also increase the absorption of electromagnetic waves. Additionally, the dielectric properties of the material such as permittivity and magnetic properties such as permeability affect how well it absorbs electromagnetic waves. The electromagnetic proportions of these materials can be improved to achieve the maximum absorption of electromagnetic energy by incorporating dielectric and magnetic materials. According to a generalized transmission line theory, the reflection loss (RL) can be estimated from the measured magnetic and dielectric characteristics and the radio-absorber (RA) thickness by utilizing the following equations [67,68]: wherein the normalized input impedance can be calculated on the basis of the transmission line theory. When the impedance of free space and the composite material is balanced, maximum EM wave absorption implies minimum reflection loss (RL min ) [69][70][71].
To satisfy the minimum reflection loss, according to Eq. (3), the perfect matching condition is [68,72,73], Z is the input impedance of the layer; Z 0 is the wave impedance of the free space; f is the frequency; c is the speed of light; d is the layer thickness; ε* and μ* are complex relative permittivity and permeability. If condition Z = 1 is fulfilled, the reflection from the RA is absent. To find these conditions in radio-absorbing materials (RAMs), a numerical procedure close to a graphical procedure is usually applied to find the matching frequency f 0 and the matching thickness d 0 [67]. The complex permittivity (ε′ and ε″) and the complex permeability (μ′ and μ″) are important EM parameters for evaluating microwave absorption performance. The ε′ and ε″ are related to the dielectric properties and the μ′ and μ″ are associated with the magnetic properties [74].
In general, a lower RL value than −10 dB signifies the consumption of more than 90% of EM waves [75]. The reflection loss of developed nanocomposites, namely NiG-800(50)-SEBS and NiG-800(70)-SEBS, is shown in Fig. 9, in the frequency range of 8.2 to 12.4 GHz. Figure 9a ( Electromagnetic parameters such as complex relative permittivity and complex relative permeability have been investigated to understand the possible electromagnetic wave absorption mechanism of the developed nanocomposites. The real part of the permittivity and permeability is associated with the storage ability of electromagnetic energy; however, the imaginary permittivity and permeability are associated with energy loss of electromagnetic energy. Figure 10a shows the real and imaginary parts of the complex relative permittivity ( ′ and ′′ ) for developed nanocomposites. The ′ and ′′ exhibit a downward trend for both developed nanocomposites in the frequency range of 8.2 to 12.4 GHz. For NiG-800(70)-SEBS nanocomposite, the value of ′ varies from 35.8 to 26.7; however, ′′ fluctuates from 28.6 to 20.7. Further, for NiG-800(50)-SEBS nanocomposite, the value of ′ changes from 15.2 to 13.3, while, ′′ alters from 8.6 to 7.2. Various polarization behaviors, such as dipole polarization, interface polarization, and electric polarization, were linked to the fluctuation of imaginary permittivity [76]. Electric polarization could be abolished because it occurred in the 10 3 -10 6 GHz frequency range. Therefore, the primary dielectric loss pathways should be the interface polarization and the dipole polarization. Interface polarization may result in a greater variation of permittivity than dipole polarization. Noteworthy, ′ and ′′ increase with an increase of NiG-800 filler content from 50 to 70 wt.% in SEBS matrix. It can be associated with the presence of high conductive reduced graphene oxide and its introduction increases with the addition of NiG-800 fillers in the SEBS matrix [77]. According to the free electron theory [78]: where ρ, f, and o are the resistivity, frequency, and dielectric constant of a vacuum, respectively, the improved ′′ can be attributed to increased electrical conductivity. In addition, a low value of S 11 (−2 dB) indicates a high conductivity of the materials, which agrees with permittivities, mainly with loss part ( ′′ ≈ 25). Further, Fig. 10b shows the variation of ac with frequency for developed NiG-800(50)-SEBS and NiG-800(70)-SEBS nanocomposites. The ac values are in the range of 0.039-0.048 S/ cm and 0.12-0.15 S/cm for NiG-800(50)-SEBS and NiG-800(70)-SEBS, respectively. To profoundly investigate the dielectric characteristics of developed nanocomposites, the relaxation mechanism, which can be expressed by the Cole-Cole semicircle, is studied. According to Debye dipolar relaxation, ′ and ′′ have the following relation (5) [78]: where ∞ is the optical dielectric constant, s is the stationary dielectric constant, f is the frequency, and is the relaxation time. Hence, the ′ and ′′ have the following relational expression (6) [79]: Based on the above relation, the ′ -′′ plot should be single semicircle, represented as a Cole-Cole semicircle, and every semicircle denotes one Debye relaxation process [80]. Figure 10c depicts the Cole-Cole plots of the developed NiG-800(50)-SEBS and NiG-800(70)-SEBS nanocomposites. The presence of various Cole-Cole semicircles signifies the existence of complex dipole and interfacial polarization in the developed nanocomposites [81]. Figure 11a depicts the dependence of the real part ( ′ ) of permeability and imaginary part ( ′′ ) of permeability of NiG-800(70)-SEBS nanocomposite. The ′ value varies in the range of 0.89-1.27 and ′′ value in the range of 0.15-0.53 for NiG-800(70)-SEBS. The dielectric loss ( tan = �� ∕ � ) which signifies the dielectric loss ability of the composite material is depicted in Fig. 11b for NiG-800(70)-SEBS nanocomposite. Its value ranges from 0.69 to 0.80. Further, the magnetic loss ( tan = �� ∕ � ), which represents the magnetic loss ability of the material, is shown in Fig. 11c. The value of tan varies in the range of 0.015-0.066 for NiG-800(70)-SEBS nanocomposite. Magnetic loss typically consists of eddy current loss, exchange resonance, and natural resonance in the GHz frequency range [82]. Nature resonance typically occurs at low frequencies, whereas exchange resonance is the main source of resonance peaks at high frequencies [83]. The two resonance peaks below 10 GHz can therefore be attributed to nature resonance, while the peak at high frequency might be the result of exchange resonances [84]. In order to further investigate the magnetic loss of NiG-800(70)-SEBS nanocomposite, the frequency (f) dependence �� � −2 f −1 curve is studied, as shown in Fig. 11d. The �� � −2 f −1 value remains stable or fluctuates slightly in the range of 10.1 GHz to 10.5 GHz, Fig. 10 Frequency dependence on a real and imaginary part of the complex permittivity, b ac conductivity, and c Cole-Cole plots of nanocomposites 10.8 GHz to 11.0 GHz, and 11.4 GHz to 12.0 GHz, which is attributed to the eddy current loss [85]. The developed NiG-800(70)-SEBS nanocomposite exhibited a higher value of tan in comparison with tan , which indicates that the reflection loss is associated with an electrical loss instead of the magnetic loss [80,86]. Figure 12 depicts an EM shielding mechanism diagram of rGO-Ni-SEBS nanocomposites. When EM wave is incident on developed nanocomposite, EM waves interact with rGO and Ni nanoparticles inside the nanocomposite [20]. The presence of rGO sheets provides the dielectric loss capability of the nanocomposites; however, Ni nanoparticles provide magnetic loss. Dipole polarization, interfacial polarization, and conductance loss can typically cause a significant dielectric loss in rGO/Ni conductive networks under the influence of alternating EM waves, consuming the EM energy. Due to the different electrical characteristics on each side of the interface, the Ni nanoparticles attached to the rGO nanosheets cause an ongoing accumulation of free charges, which causes interface polarization. During the reduction reaction, rGO nanosheets are prone to defects that result in defect dipoles. Consequently, dipole polarization occurs when the vector sum of the pole moments of the dipoles in an alternating electric field is not zero [87]. Ni nanoparticles and rGO sheets would combine to create a conductive network with finite resistance, which is advantageous to increase conduction loss and EM energy attenuation [76]. Meanwhile, under an alternating electromagnetic field, magnetic natural resonance, and magnetic exchange resonances, eddy current loss can form as a result of the action of magnetic metal Ni nanoparticles. This results in a strong magnetic loss that dissipates EM energy. The majority of the EM energy can be attenuated and transformed into thermal or other types of energy under the synergistic loss

Mechanical properties
Mechanical properties of SEBS filled by NiG-800 nanoparticles were studied by tensile test as shown in stress-strain curves of NiG-800(50)-SEBS, NiG-800(70)-SEBS, and SEBS in Fig. 13. Neat SEBS polymer exhibits typical elastomeric behavior with very high maximum strain (796%) and low Young modulus (15.7 MPa). The addition of NiG-800 nanocomposite does not change the elastomeric nature of the matrix although it led to stiffening (34 MPa, 35 MPa) of the material while lowering elongation (306%, 286%) with 50 wt.% and 70 wt.%, respectively. These results show good dispersion of the filler in the matrix which agrees with Fig. 6.
It can be argued that the combination of nanofiller and polymer matrix properties leads to the overall performance of the material. In the case of NiG-800(70)-SEBS sample with a thickness of 1 mm, the electromagnetic properties of nickel-decorated rGO filler led to the high shielding efficiency of the composite (S 21 = −10.6 dB). With the utilization of a polymer matrix based on SEBS polymer, we were able to prepare highly flexible composite material with excellent mechanical properties. There are several works describing synthesis and EM shielding properties of nickel/ carbon/polymer composites [20,35,40,41]. However, in these reports, epoxy resin or PVFD are used as polymer matrices. Contrary to the reported works, we developed an easy and fast approach to decorating rGO surface with nickel nanoparticles based on microwave-assisted synthesis followed by the easy incorporation of the NiG-800 nanocomposite into the SEBS polymer matrix.

Conclusion
SEBS-based nanocomposites consisting of nickel nanoparticle-decorated reduced graphene oxide nanocomposite particles as nanofillers (50 wt.% and 70 wt.%) were developed and investigated for EM wave shielding/absorption. Nickel nanoparticle-functionalized reduced graphene oxide nanocomposites (NiG) were prepared via one-step microwave-assisted solvothermal approach. The as-prepared NiG nanocomposite particles were additionally annealed at 800 °C under an inert atmosphere (labeled NiG-800) to improve its structure and EM characteristics. The NiG-800 nanocomposite material contained 25 wt.% of nickel. It was demonstrated here that via mixing the NiG-800 nanocomposite with SEBS polymer we were able to easily prepare highly flexible composite material. The combination of properties of the NiG-800 nanofiller and SEBS polymer matrix resulted in the composite with promising potential for its application as EM shielding material. The prepared NiG-800(70)-SEBS nanocomposite displays minimum reflection loss RL min ≈ −48.2 dB at 9.29 GHz at a low thickness of 2.3 mm in the frequency range of 8.2 −12.4 GHz. The total shielding efficiency of this material (thickness 1 mm) is > 10 dB in the whole frequency range (X-band) with reflection ≈ 50% and absorption ≈ 40%. Better EM wave absorption performance is achieved by the reduced graphene oxide's homogeneously distributed Ni nanoparticles and multilevel heterogeneous interfaces, which are all beneficial to the optimization of impedance matching and attenuation behavior, which further comes from the synergistic loss mechanism between dielectric loss of rGO sheets and magnetic loss of Ni nanoparticles. This research develops a quick and simple method for creating advanced nanocomposites with magnetic and dielectric components that are optimized for high-performance electromagnetic wave absorption. The present work could be beneficial in the preparation of lightweight, flexible, and high-performance EM wave absorber/shielding material.