NiS2@rGO Nanosheet Wrapped with PPy Aerogel: A Sandwich-Like Structured Composite for Excellent Microwave Absorption

To reduce electromagnetic pollution as well as increase the accuracy of high-precision electronic equipment, more attention has been paid to new electromagnetic wave (EMW) absorbing materials, which have the advantages of strong absorption, wide absorption bands, and a narrow thickness. In this study, a novel ternary type of the NiS2@rGO/polypyrrole (PPy) sandwich-like structured composites was synthesized via a facile two-step method, in which the hydrothermal method was used to prepare NiS2@rGO binary composites and then the in situ polymerization method was used to synthesize the PPy, which acted as the outer layer of the sandwich-like structure. The morphologies and electromagnetic absorption performance of the NiS2@rGO/PPy were measured and investigated. A sample with 6 wt% NiS2@rGO/PPy loading paraffin-composite obtained an outstanding reflection loss (RL) of −58.7 dB at 16.44 GHz under a thickness of 2.03 mm. Simultaneously, the effective electromagnetic wave absorption bandwidth for RL < −10 dB, which covered 7.04 to 18.00 GHz (10.96 GHz), was achieved by changing the thickness of the absorber from 2.0 to 3.5 mm. The results not only suggest that the NiS2@rGO/PPy composite has excellent performance in the field of EMW absorption but also prove that the novel sandwich-like structure can contribute to appropriate impedance matching through multiple relaxation and interfacial polarization processes.


Introduction
In recent years, with the development of advanced high-power electronic equipment, electromagnetic (EM) pollution, which has harmful effects on human life, has become significant [1][2][3]. In this regard, more attention has been focused on developing highly efficient and stable electromagnetic wave (EMW) absorbing materials to solve EM pollution [4]. The practical application of single-component absorbers, such as Fe 3 O 4 , Co, and polypyrrole (PPy), is restricted because they do not have the characteristics of small matching thickness, light weight, strong absorption features, and a wide EM absorption band. In contrast with single absorbers, multiple-component absorbers can better meet the requirements of being a highly efficient EMW absorber on account of their adjustable impedance matching and multilayer structures [5,6].
Reduced graphene oxide (rGO), a new type of two-dimensional material, has shown a promising application prospect in the field of EMW absorption [7,8]. On the one hand, a large specific surface area will increase the transmission intensity of electromagnetic waves in rGO-based composites and help EM

Synthesis of NiS 2 @rGO Binary Composite
The NiS 2 @rGO binary composite was prepared through a facile hydrothermal method [30,31]. Firstly, 0.05 g of GO powder was added into 50 mL of deionized water and then sonicated for 60 min at 20 • C (120 W, 40 KHz). After that, the GO dispersed solution continued to be magnetically stirred at 70 • C for further processing. Secondly, 0.9506 g of NiCl 2 ·6H 2 O dissolved in 30 mL of deionized water was added to the GO dispersed solution. A quantity of 0.213 g of sulfur powder was dissolved in 30 mL of deionized water under sonication for 10 min at 20 • C to form a S dispersed solution and then added into the GO mixture solution. Then, 1.6 g polyvinyl pyrrolidone (PVP-30,000) was dissolved in 10 mL deionized water and slowly added into the abovementioned GO mixture solution. After that the mixture solution magnetically stirred for 20 min at 20 • C to mix well. Subsequently, the mixture was placed in a 160 mL Teflon-lined stainless-steel autoclave and maintained at 200 • C for 12 h. After this reaction, the obtained product was washed with deionized water and ethanol several times and vacuum dried at 60 • C for 14 h.

Synthesis of NiS 2 @rGO/PPy Sandwich-Like Structure Composite
The NiS 2 @rGO/PPy sandwich-like structure composites were synthesized as described in the suit method [32]. Briefly, 40 mg of the NiS 2 @rGO binary composite synthesized in the previous step was dissolved in a mixture of 4 mL deionized water and 4 mL ethanol to form a NiS 2 @rGO dispersed solution and sonicated for 10 min. Then, 135 mg of pyrrole was slowly trickled into the aforementioned solution. After that, 2 mL of deionized water and 2 mL of absolute ethanol was used to fully dissolve 1.6 g FeCl 3 ·6H 2 O, and the FeCl 3 ·6H 2 O solution was stepwise injected into the NiS 2 @rGO/Py solution under swift mechanical stirring for 5 min. A fluffy black powder was obtained after the NiS 2 @rGO/Py solution had reacted for 24 h. Finally, the black NiS 2 @rGO/PPy composites were rinsed with absolute ethanol and distilled water several times until the filtrate was colorless, and then, they were dried in a vacuum at 50 • C for 24 h. In addition, the NiS 2 @rGO/PPy with 40, 60, and 80 mg of NiS 2 @rGO were used for comparative analysis, which are recorded as samples A, B and C, respectively.

Characterization and Electromagnetic (EM) Parameters Measurement
X-ray diffraction (XRD) was performed using a Bruker D8 Advanced X-ray diffractometer (BRUKER, Karlsruhe, Germany) with Cu K α radiation (λ = 1.5406 Å) at 40 KV over the range of 2θ = 5-80 • . The Fourier transform infrared (FT-IR) spectra of the samples were characterized by a Nicolet IS10 FT-IR spectrometer with KBr and the field emission scanning electron microscopy (FE-SEM) data of the samples were analyzed using a Hitachi S4800 field emission scanning microscope. The further morphology, crystal structure, and element distribution properties of the NiS 2 @rGO/PPy composite were determined by transmission electron microscopy (TEM) with an energy-dispersive X-ray spectroscope (EDS) and a high-resolution transmission electron microscope (HRTEM), which were provided by JEL-2100F (JEOL, Tokyo, Japan) and Tecnai G 2 F20 (FEI Company, Hillsboro, OR, USA), respectively. X-ray photon spectroscopy (XPS) was performed on an ESCALAB 250 (Thermo, Waltham, MA, USA).

EM Absorption Measurement
The microwave scattering parameters of the samples composing of paraffin wax NiS 2 @rGO/PPy composite were studied using an Agilent N5224A PNA (Agilent Technologies Inc, Santa Clara, CA, USA) vector network analyzer (VNA) recorded at 2-18 GHz. Samples composed of wax and NiS 2 @rGO/PPy composite were fabricated into cylindrical compacts (Φ out = 7.00 mm, Φ in = 3.04 mm) with different proportions of NiS 2 @rGO/PPy (4, 6, and 8 wt%). The VNA operates based on the transmission line theory, and was used to calculate the electromagnetic parameters of the sample by measuring four scattering parameters (S 11 , S 22 , S 12 , and S 21 ) of the two-port network [33][34][35]. The electromagnetic parameter testing principle is that when an electromagnetic wave is incident on an absorbing material with a certain thickness, it will generate multiple reflections and transmits. The schematic diagram of the fixture and the test sample is illustrated in Figure S1.

Results and Discussion
The NiS 2 @rGO/polypyrrole (NiS 2 @rGO/PPy) sandwich-like structure composite was synthesized by a facile two-step method. Fitstly, the NiS 2 @rGO binary composite was prepared by a hydrothermal method. Secondly, an in suit method was used to synthesize the NiS 2 @rGO/PPy sandwich-like structure composites on the basis of NiS 2 @rGO binary composite. The synthetic procedure of the NiS 2 @rGO/PPy sandwich-like structure composite is illustrated in Scheme 1. measuring four scattering parameters (S11, S22, S12, and S21) of the two-port network [33][34][35]. The electromagnetic parameter testing principle is that when an electromagnetic wave is incident on an absorbing material with a certain thickness, it will generate multiple reflections and transmits. The schematic diagram of the fixture and the test sample is illustrated in Figure S1.

Results and Discussion
The NiS2@rGO/polypyrrole (NiS2@rGO/PPy) sandwich-like structure composite was synthesized by a facile two-step method. Fitstly, the NiS2@rGO binary composite was prepared by a hydrothermal method. Secondly, an in suit method was used to synthesize the NiS2@rGO/PPy sandwich-like structure composites on the basis of NiS2@rGO binary composite. The synthetic procedure of the NiS2@rGO/PPy sandwich-like structure composite is illustrated in Scheme 1. Scheme 1. The preparation process of the NiS2@rGO/polypyrrole (PPy) sandwich-like structure composite.

Characterization of Samples
In order to investigate the crystal structure of the NiS2@rGO/PPy ternary composite and the NiS2@rGO binary composite, the XRD patterns are presented in Figure 1a. The existence of NiS2 in the NiS2@rGO composite can be clearly confirmed, because all of the diffraction peaks correspond with the standard card of NiS2 (#88-1709). Furthermore, the XRD pattern of NiS2@rGO/PPy ternary composite is almost consistent with the standard card of NiS2 (#88-1709) and the standard C card (#75-0444), while the characteristic diffraction peaks are broad and weak due to the existence of amorphous PPy, which is indicative of poor crystallization. The XRD pattern of GO is presented in Figure S2. To verify the existence of PPy, the surface chemical states of the NiS2@rGO/PPy ternary composite were further investigated via FT-IR. The FT-IR spectra of PPy and NiS2@rGO/PPy are shown in Figure 1b. Figure 1b shows that for NiS2@rGO/PPy, the characteristic bands at 1546 and 1449 cm −1 are associated with symmetrical and anti-symmetrical ring-stretching modes, respectively, The preparation process of the NiS 2 @rGO/polypyrrole (PPy) sandwich-like structure composite.

Characterization of Samples
In order to investigate the crystal structure of the NiS 2 @rGO/PPy ternary composite and the NiS 2 @rGO binary composite, the XRD patterns are presented in Figure 1a. The existence of NiS 2 in the NiS 2 @rGO composite can be clearly confirmed, because all of the diffraction peaks correspond with the standard card of NiS 2 (#88-1709). Furthermore, the XRD pattern of NiS 2 @rGO/PPy ternary composite is almost consistent with the standard card of NiS 2 (#88-1709) and the standard C card (#75-0444), while the characteristic diffraction peaks are broad and weak due to the existence of amorphous PPy, which is indicative of poor crystallization. The XRD pattern of GO is presented in Figure S2. To verify the existence of PPy, the surface chemical states of the NiS 2 @rGO/PPy ternary composite were further investigated via FT-IR. The FT-IR spectra of PPy and NiS 2 @rGO/PPy are shown in Figure 1b. Figure 1b shows that for NiS 2 @rGO/PPy, the characteristic bands at 1546 and 1449 cm −1 are associated with symmetrical and anti-symmetrical ring-stretching modes, respectively, whereas the peaks located at 1300 and 1035 cm −1 correspond to C-N stretching vibrations and the C-H band in-plane vibration of PPy rings, respectively [36]. At the same time, the doping state of PPy can be verified by the characteristic peaks at 1166 and 897 cm −1 , and the existence of PPy can be proved by the peaks located at 1091 and 965 cm −1 [37]. These characteristic peaks in the FT-IR spectra of NiS 2 @rGO/PPy correspond well with the FT-IR spectra of PPy, which could be associated with the successful coating of PPy.  The chemical composition of the NiS2@rGO/PPy ternary composite was also determined by XPS ( Figure 2). The peaks of four elements (Ni, N, C, and S) are clearly displayed in Figure 2a, which further confirms that NiS2@rGO/PPy ternary composite was successfully synthesized. In addition, although the characteristic peaks of N and C are obvious, the characteristic peaks of Ni 2p and S 2p are weak because the NiS2 nanospheres coated with rGO and PPy were placed in the innermost layer of the composite structure, which is difficult to characterize and analyze accurately using XPS. As shown in Figure 2b, in the spectrum of C 1 s, the major peaks at 284.5 and 286.4 eV are attributed to the C-C bonds of the aromatic rings and the C=O bonds of the carbonyl, respectively. The intensity of the C=O bonds is subdued, which could indicate that the GO was reduced [38]. N 1s spectra are exhibited in Figure 2c, in which the peaks located at 399.65 and 400.9 eV are assigned to the C-N/N-H bonds and the N-C=O bonds, respectively [39,40]. The chemical composition of GO was determined by XPS ( Figure S3a), and the core-level spectrum of C 1S in GO is displayed in Figure  S3b. The phase analysis combining XRD, FT-IR and XPS spectra accurately proves that NiS2, rGO, and PPy successfully formed a new type of ternary composite without doping with any other substances. In this study, to investigate the microstructures and morphologies of the composites, typical The chemical composition of the NiS 2 @rGO/PPy ternary composite was also determined by XPS ( Figure 2). The peaks of four elements (Ni, N, C, and S) are clearly displayed in Figure 2a, which further confirms that NiS 2 @rGO/PPy ternary composite was successfully synthesized. In addition, although the characteristic peaks of N and C are obvious, the characteristic peaks of Ni 2p and S 2p are weak because the NiS 2 nanospheres coated with rGO and PPy were placed in the innermost layer of the composite structure, which is difficult to characterize and analyze accurately using XPS. As shown in Figure 2b, in the spectrum of C 1 s, the major peaks at 284.5 and 286.4 eV are attributed to the C-C bonds of the aromatic rings and the C=O bonds of the carbonyl, respectively. The intensity of the C=O bonds is subdued, which could indicate that the GO was reduced [38]. N 1s spectra are exhibited in Figure 2c, in which the peaks located at 399.65 and 400.9 eV are assigned to the C-N/N-H bonds and the N-C=O bonds, respectively [39,40]. The chemical composition of GO was determined by XPS ( Figure S3a), and the core-level spectrum of C 1S in GO is displayed in Figure S3b. The phase analysis combining XRD, FT-IR and XPS spectra accurately proves that NiS 2 , rGO, and PPy successfully formed a new type of ternary composite without doping with any other substances.
In this study, to investigate the microstructures and morphologies of the composites, typical SEM and TEM images of the NiS 2 @rGO/PPy ternary composite were made and are presented in Figure 3. Figure 3a-c show SEM images of the NiS 2 @rGO/PPy ternary composite at different resolutions, and the EDS pattern has been presented in Figure S4b. It can be clearly observed that the PPy nanospheres uniformly coated the NiS 2 @rGO binary composites while maintaining the integrity of the binary composites, forming a complete sandwich-like composite structure. Meanwhile, at the edge of the sandwich-like structure, there is a slight aggregation of PPy nanospheres. As revealed by Figure 3b,c, the average thickness of the sandwich-like NiS 2 @rGO/PPy ternary composite is about 100 nm, which is mainly comprised of PPy nanospheres. The SEM images of NiS 2 @rGO is presented in Figure S5. The TEM pattern presented in Figure 3d accurately shows the microstructure of the inner layer of the sandwich-like NiS 2 @rGO/PPy composite, in which the NiS 2 nanospheres are well encased in rGO nanosheets. The diameter of the inner NiS 2 nanospheres is approximately 200 nm while the thickness of the out-layer rGO nanosheets is about 20 nm (Figure 3e). Such unique structures allow the NiS 2 nanospheres to fully interact with rGO nanosheets to significantly adjust the dielectric properties of the whole composite. In order to further validate the existence of NiS 2 nanospheres, the HRTEM pattern of NiS 2 @rGO/PPy composite is displayed in Figure 3f. The lattice planes were calculated to be 0.325 and 0.28 nm, which match well with the (200) and (311) planes of NiS 2 , respectively. The distribution of elements in the sandwich-like NiS 2 @rGO/PPy ternary composite is shown by the EDS spectra in Figure 4, from which C, N, Ni, and S were measured. The distributions of the measured Ni and S elements validate the hypothesis that the NiS 2 nanospheres are encased in rGO nanosheets; meanwhile, the distributions of the detected C and N elements further verify the unique sandwich-like structure. The morphology analysis, which combined SEM, TEM and EDS spectra, further validated that the new ternary composite composed with NiS 2 , rGO and PPy was synthesized with a unique sandwich-like structure to obtain an excellent EMW performance. The chemical composition of the NiS2@rGO/PPy ternary composite was also determined by XPS ( Figure 2). The peaks of four elements (Ni, N, C, and S) are clearly displayed in Figure 2a, which further confirms that NiS2@rGO/PPy ternary composite was successfully synthesized. In addition, although the characteristic peaks of N and C are obvious, the characteristic peaks of Ni 2p and S 2p are weak because the NiS2 nanospheres coated with rGO and PPy were placed in the innermost layer of the composite structure, which is difficult to characterize and analyze accurately using XPS. As shown in Figure 2b, in the spectrum of C 1 s, the major peaks at 284.5 and 286.4 eV are attributed to the C-C bonds of the aromatic rings and the C=O bonds of the carbonyl, respectively. The intensity of the C=O bonds is subdued, which could indicate that the GO was reduced [38]. N 1s spectra are exhibited in Figure 2c, in which the peaks located at 399.65 and 400.9 eV are assigned to the C-N/N-H bonds and the N-C=O bonds, respectively [39,40]. The chemical composition of GO was determined by XPS ( Figure S3a), and the core-level spectrum of C 1S in GO is displayed in Figure  S3b. The phase analysis combining XRD, FT-IR and XPS spectra accurately proves that NiS2, rGO, and PPy successfully formed a new type of ternary composite without doping with any other substances. In this study, to investigate the microstructures and morphologies of the composites, typical SEM and TEM images of the NiS2@rGO/PPy ternary composite were made and are presented in Figure 3. Figure 3a-c show SEM images of the NiS2@rGO/PPy ternary composite at different resolutions, and the EDS pattern has been presented in Figure S4b. It can be clearly observed that the PPy nanospheres uniformly coated the NiS2@rGO binary composites while maintaining the integrity of the binary composites, forming a complete sandwich-like composite structure. Meanwhile, at the edge of the sandwich-like structure, there is a slight aggregation of PPy nanospheres. As revealed by Figure 3b,c, the average thickness of the sandwich-like NiS2@rGO/PPy ternary composite is about 100 nm, which is mainly comprised of PPy nanospheres. The SEM images of NiS2@rGO is presented in Figure S5. The TEM pattern presented in Figure 3d accurately shows the microstructure of the inner layer of the sandwich-like NiS2@rGO/PPy composite, in which the NiS2 nanospheres are well encased in rGO nanosheets. The diameter of the inner NiS2 nanospheres is approximately 200 nm while the thickness of the out-layer rGO nanosheets is about 20 nm (Figure 3e). Such unique structures allow the NiS2 nanospheres to fully interact with rGO nanosheets to significantly adjust the dielectric properties of the whole composite. In order to further validate the existence of NiS2 nanospheres, the HRTEM pattern of NiS2@rGO/PPy composite is displayed in Figure 3f. The lattice planes were calculated to be 0.325 and 0.28 nm, which match well with the (200) and (311) planes of NiS2, respectively. The distribution of elements in the sandwich-like NiS2@rGO/PPy ternary composite is shown by the EDS spectra in Figure 4, from which C, N, Ni, and S were measured. The distributions of the measured Ni and S elements validate the hypothesis that the NiS2 nanospheres are encased in rGO nanosheets; meanwhile, the distributions of the detected C and N elements further verify the unique sandwich-like structure. The morphology analysis, which combined SEM, TEM and EDS spectra, further validated that the new ternary composite composed with NiS2, rGO and PPy was synthesized with a unique sandwich-like structure to obtain an excellent EMW performance.

Electromagnetic Parameters of the NiS2@rGO/PPy Composite
To investigate the EMW absorption properties of the NiS2@rGO/PPy composite, real parts of complex permittivity (ε′), imaginary parts of complex permittivity (ε″), real parts of complex permeability (μ′), and imaginary parts of complex permeability (μ″) of samples A, B, and C with filler loadings of 4, 6, and 8 wt% were measured at room temperature. The real parts (ε′ and μ′) and imaginary parts (ε″ and μ″) of complex permittivity and permeability represent the storage ability of microwave energy and the dissipation ability of electric and magnetic energy, respectively [15]. Figure 5 shows that the value of μ′ and μ″ are appropriately constant and close to 1 and 0, respectively. There are no significant changes in the numerical values of the frequency-dependent complex permeability values (μr) of the samples calculated by the real and imaginary parts of complex permeability (μr = μ′ − jμ″), which is associated with the non-magnetic characteristics of the as-prepared materials. On the contrary, for the dielectric properties of the as-prepared materials, the frequency dependent complex permittivity values (εr, εr = ε′ − jε″) of the NiS2@rGO/PPy composite in paraffin matrix was measured at 2-18 GHz, as shown in Figure 5. In fact, numerous reports have indicated that in order to obtain a fine impedance match, a relatively moderate dielectric constant is needed, and the relatively higher or lower permittivity will cause the EMW to be reflected out of the absorber [16]. With the increase in the content of rGO-based binary composites, the high permittivity of PPy was effectively regulated, which is in favor of EMW absorption of NiS2@rGO/PPy. According to the free-electron theory, the relationship between the electrical resistivity (ρ) and ε″, and the relationship between electrical conductivity (σ) and skin depth (δ) can be expressed using the following equation [15]: where ƒ is the frequency of EMW, ε0 is the dielectric constant in vacuum (ε0 = 8.854 × 10 −12 F m −1 ). It can be found that lower ε″ bring about higher resistivity of NiS2@rGO/PPy composite, and lower conductivity could signifies a big skin depth, eventually increase transmittance and reduce reflection of the EM wave. Thus, a higher electrical resistivity will obtain a better electromagnetic absorption performance; and a relatively lower ε″ might be required. With the increase of filler loading, the value of ε′ and ε″ of the three samples also show an increasing trend. By contrast, with the increase of NiS2@rGO content, the value of ε′ and ε″ show an downward trend in every filler loading (4, 6, and 8 wt%). As shown in Figure 5, the ε″ values of all samples decrease with an increase in frequency

Electromagnetic Parameters of the NiS 2 @rGO/PPy Composite
To investigate the EMW absorption properties of the NiS 2 @rGO/PPy composite, real parts of complex permittivity (ε ), imaginary parts of complex permittivity (ε"), real parts of complex permeability (µ ), and imaginary parts of complex permeability (µ") of samples A, B, and C with filler loadings of 4, 6, and 8 wt% were measured at room temperature. The real parts (ε and µ ) and imaginary parts (ε" and µ") of complex permittivity and permeability represent the storage ability of microwave energy and the dissipation ability of electric and magnetic energy, respectively [15]. Figure 5 shows that the value of µ and µ" are appropriately constant and close to 1 and 0, respectively. There are no significant changes in the numerical values of the frequency-dependent complex permeability values (µ r ) of the samples calculated by the real and imaginary parts of complex permeability (µ r = µ − jµ"), which is associated with the non-magnetic characteristics of the as-prepared materials. On the contrary, for the dielectric properties of the as-prepared materials, the frequency dependent complex permittivity values (ε r , ε r = ε − jε") of the NiS 2 @rGO/PPy composite in paraffin matrix was measured at 2-18 GHz, as shown in Figure 5. In fact, numerous reports have indicated that in order to obtain a fine impedance match, a relatively moderate dielectric constant is needed, and the relatively higher or lower permittivity will cause the EMW to be reflected out of the absorber [16]. With the increase in the content of rGO-based binary composites, the high permittivity of PPy was effectively regulated, which is in favor of EMW absorption of NiS 2 @rGO/PPy. According to the free-electron theory, the relationship between the electrical resistivity (ρ) and ε", and the relationship between electrical conductivity (σ) and skin depth (δ) can be expressed using the following equation [15]: where ƒ is the frequency of EMW, ε 0 is the dielectric constant in vacuum (ε 0 = 8.854 × 10 −12 F m −1 ). It can be found that lower ε" bring about higher resistivity of NiS 2 @rGO/PPy composite, and lower conductivity could signifies a big skin depth, eventually increase transmittance and reduce reflection of the EM wave. Thus, a higher electrical resistivity will obtain a better electromagnetic absorption performance; and a relatively lower ε" might be required. With the increase of filler loading, the value of ε and ε" of the three samples also show an increasing trend. By contrast, with the increase of NiS 2 @rGO content, the value of ε and ε" show an downward trend in every filler loading (4, 6, and 8 wt%). As shown in Figure 5, the ε" values of all samples decrease with an increase in frequency from 2  To obtain new types of EMW absorptive materials with strong absorption and wide absorption bands, not only should we think about the complex permittivity (εr) and the complex permeability (μr) of the as-prepared composites, but we also need to calculate the attenuation constant (α) and the impedance matching ratio (Z) [41]. In terms of the transmission line theory, the relationship between α and the corresponding matching frequency can be expressed through the following equation [42]: where ƒ is the frequency of EMW, and c is the velocity of light in a vacuum. Meanwhile, the impedance matching ratio (Z) can be described by the following equations [16]: where Z0 ≈ 378 Ω is the free space of impedance matching, Z1 is the impedance value of the absorber, εr and μr are the complex permittivity and permeability, respectively. On the one hand, it is obvious in Figure 6a that the overall trend of α decreased as the content of NiS2@rGO composites increased (αA > αB > αC). On the other hand, Figure 6b shows that the integral tendency of Z increased as the content of NiS2@rGO composites increased (ZC > ZB > ZA). Although Sample A possesses the best attenuation property and Sample C possesses the optimal impedance matching characteristic, the worst Z performance was shown by Sample A, and the poorest value of α was exhibited by Sample To obtain new types of EMW absorptive materials with strong absorption and wide absorption bands, not only should we think about the complex permittivity (ε r ) and the complex permeability (µ r ) of the as-prepared composites, but we also need to calculate the attenuation constant (α) and the impedance matching ratio (Z) [41]. In terms of the transmission line theory, the relationship between α and the corresponding matching frequency can be expressed through the following equation [42]: where ƒ is the frequency of EMW, and c is the velocity of light in a vacuum. Meanwhile, the impedance matching ratio (Z) can be described by the following equations [16]: where Z 0 ≈ 378 Ω is the free space of impedance matching, Z 1 is the impedance value of the absorber, ε r and µ r are the complex permittivity and permeability, respectively. On the one hand, it is obvious in Figure 6a that the overall trend of α decreased as the content of NiS 2 @rGO composites increased (α A > α B > α C ). On the other hand, Figure 6b shows that the integral tendency of Z increased as the content of NiS 2 @rGO composites increased (Z C > Z B > Z A ). Although Sample A possesses the best attenuation property and Sample C possesses the optimal impedance matching characteristic, the worst Z performance was shown by Sample A, and the poorest value of α was exhibited by Sample C. It is worth noting that an excellent EMW absorber should consider both impedance matching and energy conservation at the same time. For the Sample B, the value of the impedance matching ratio and attenuation constant are both in the middle of the acceptable level; thus, we predict that sample B will exhibit the most significant EMW absorption performance.
Nanomaterials 2019, 9, x FOR PEER REVIEW 9 of 13 Nanomaterials 2019, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/nanomaterials C. It is worth noting that an excellent EMW absorber should consider both impedance matching and energy conservation at the same time. For the Sample B, the value of the impedance matching ratio and attenuation constant are both in the middle of the acceptable level; thus, we predict that sample B will exhibit the most significant EMW absorption performance. To investigate the EMW absorption properties of the NiS2@rGO/PPy composite with different doping ratios, the measured complex permittivity and permeability were used to calculate the reflection loss values of the as-prepared composites by the following equation [43]: where Zin is the input characteristic impedance, Z0 ≈ 378 Ω is the free space of impedance matching, ƒ is the frequency of EMW, c is the velocity of light in a vacuum, d is the thickness of the composites, and εr and μr are the complex permittivity and permeability, respectively. When the reflection loss value is lower than −10, −20 and −30 dB, more than 90%, 99%, and 99.9% of the EM energy is absorbed, respectively [44]. It is probable that bandwidths lower than −10 dB are effective EMW absorption bandwidths. Figure 7 shows a plot of the RL versus the frequency of the samples with different filler loading ratios at various thicknesses. For sample A (Figure 7a-c), it can be seen that the minimum reflection loss was −20.56 dB at 8.28 GHz with a thickness of 4.0 mm, and a bandwidth of RL below −10 dB is about 3.12 GHz (7-10.12 GHz), for which the thickness was too thick and the effective EMW absorption bandwidth was narrow. As shown in Figure 7g, h, and i, the lowest reflection loss value of Sample C was −19.05 dB at 15.36 GHz with a thickness of 2.0 mm, and an bandwidth of RL below −10 dB is about 5.48 GHz (12.52-18.00 GHz). Compared with sample A, the effective EMW absorption thickness was much thinner and the effective EMW absorption bandwidth was greatly improved, but the maximum EMW absorption intensity was not satisfactory. This is because of the weak attenuation capability caused by the relatively low dielectric constant. Nevertheless, it must be mentioned that a satisfactory result among the three NiS2@rGO/PPy composites was shown not only for the characteristics of the maximum EMW absorption intensity and the corresponding thickness but also for the effective EMW absorption bandwidth of sample B. When the filler loading ratio reached 6 wt%, the lowest reflection loss was −47.15 dB at 16.15 GHz with a thickness of 2 mm, and the broadest effective bandwidth, from 10.44 to 16.48 GHz (6.04 GHz), was obtained when the thickness was 2.5 mm. The effective EMW absorption bandwidth with a To investigate the EMW absorption properties of the NiS 2 @rGO/PPy composite with different doping ratios, the measured complex permittivity and permeability were used to calculate the reflection loss values of the as-prepared composites by the following equation [43]: where Z in is the input characteristic impedance, Z 0 ≈ 378 Ω is the free space of impedance matching, ƒ is the frequency of EMW, c is the velocity of light in a vacuum, d is the thickness of the composites, and ε r and µ r are the complex permittivity and permeability, respectively. When the reflection loss value is lower than −10, −20 and −30 dB, more than 90%, 99%, and 99.9% of the EM energy is absorbed, respectively [44]. It is probable that bandwidths lower than −10 dB are effective EMW absorption bandwidths. Figure 7 shows a plot of the RL versus the frequency of the samples with different filler loading ratios at various thicknesses. For sample A (Figure 7a-c), it can be seen that the minimum reflection loss was −20.56 dB at 8.28 GHz with a thickness of 4.0 mm, and a bandwidth of RL below −10 dB is about 3.12 GHz (7-10.12 GHz), for which the thickness was too thick and the effective EMW absorption bandwidth was narrow. As shown in Figure 7g, h, and i, the lowest reflection loss value of Sample C was −19.05 dB at 15.36 GHz with a thickness of 2.0 mm, and an bandwidth of RL below −10 dB is about 5.48 GHz (12.52-18.00 GHz). Compared with sample A, the effective EMW absorption thickness was much thinner and the effective EMW absorption bandwidth was greatly improved, but the maximum EMW absorption intensity was not satisfactory. This is because of the weak attenuation capability caused by the relatively low dielectric constant. Nevertheless, it must be mentioned that a satisfactory result among the three NiS 2 @rGO/PPy composites was shown not only for the characteristics of the maximum EMW absorption intensity and the corresponding thickness but also for the effective EMW absorption bandwidth of sample B. When the filler loading ratio reached 6 wt%, the lowest reflection loss was −47.15 dB at 16.15 GHz with a thickness of 2 mm, and the broadest effective bandwidth, from 10.44 to 16.48 GHz (6.04 GHz), was obtained when the thickness was 2.5 mm. The effective EMW absorption bandwidth with a reflection loss of below −10 dB was 10.96 GHz (7.04-18.00 GHz) with a narrow thickness of 2.00-3.50 mm. In this work, the NiS 2 @rGO/PPy sandwich-like structure composites exhibited outstanding EMW absorption performance, which can be ascribed to the following reasons: firstly, the intensified polarization of the NiS 2 @rGO/PPy interface is conducive to EMW absorption; secondly, the sandwich-like structure formed by the PPy coating can increase the multiple losses of the incident EMW and effectively improve the impedance matching of the NiS 2 @rGO/PPy composites.  Regarding the thickness of the composites, many previous studies have mentioned am important phenomenon whereby the thickness of the composites is an important index of EMW absorption because it significantly affects the RL value and the maximum absorption frequency [15,41]. Hence, to further evaluate the optimal EMW absorption properties of the NiS2@rGO/PPy composite, the RL curves of sample B with a thickness from 1.80 to 2.45 mm at a filler loading ratio of 6 wt% are shown in Figure 8a. Meanwhile, 3D representations of RL and the corresponding contour maps are presented in Figure 8b and c to illustrate the relationship between EM absorption frequency and the reflection loss intensity with thicknesses of 1-5 mm. As Figure 8 displays, when the thickness of sample B with a filler loading of 6 wt% was between 1.95 and 2.3 mm, all of the lowest RL values were lower than −30 dB and the corresponding maximum absorption frequency appeared in the Ku frequency range. The minimum reflection loss was -58.7 dB at 16.44 GHz with a thickness of 2.03 mm, and the bandwidth of RL below -10 dB is about 4.32 GHz (13.68-18.00 GHz) with a thickness of only 2.03 mm. It is obvious that the NiS2@rGO/PPy composite has an excellent performance in the field of EMW absorption. This is because the effective EMW absorption bandwidth of the as-prepared composite almost covers the entire Ku frequency range, and the characteristics of strong absorption and thin thickness make it an advanced EMW-absorbing material. Regarding the thickness of the composites, many previous studies have mentioned am important phenomenon whereby the thickness of the composites is an important index of EMW absorption because it significantly affects the RL value and the maximum absorption frequency [15,41]. Hence, to further evaluate the optimal EMW absorption properties of the NiS 2 @rGO/PPy composite, the RL curves of sample B with a thickness from 1.80 to 2.45 mm at a filler loading ratio of 6 wt% are shown in Figure 8a. Meanwhile, 3D representations of RL and the corresponding contour maps are presented in Figure 8b and c to illustrate the relationship between EM absorption frequency and the reflection loss intensity with thicknesses of 1-5 mm. As Figure 8 displays, when the thickness of sample B with a filler loading of 6 wt% was between 1.95 and 2.3 mm, all of the lowest RL values were lower than −30 dB and the corresponding maximum absorption frequency appeared in the Ku frequency range. The minimum reflection loss was -58.7 dB at 16.44 GHz with a thickness of 2.03 mm, and the bandwidth of RL below -10 dB is about 4.32 GHz (13.68-18.00 GHz) with a thickness of only 2.03 mm. It is obvious that the NiS 2 @rGO/PPy composite has an excellent performance in the field of EMW absorption. This is because the effective EMW absorption bandwidth of the as-prepared composite almost covers the entire Ku frequency range, and the characteristics of strong absorption and thin thickness make it an advanced EMW-absorbing material.

Conclusions
To sum up, a novel ternary type of the NiS2@rGO/PPy sandwich-like structure composites was synthesized by using a facile two-step method. The influence of the proportions of components on the EMW absorbing performance was investigated. The as-prepared compound with 60 mg NiS2@rGO binary composite added exhibited some distinctly strong EMW attenuation properties. From a practical point of view, this novel NiS2@rGO/PPy sandwich-like structure composite showed an effective EMW absorption bandwidth (RL < −10 dB) of about 10.96 GHz (7.04-18.00 GHz) with a thickness of 2.00-3.50 mm. The optimal reflection loss was −58.7 dB at 16.44 GHz for a sample with 6 wt% NiS2@rGO/PPy at a thickness of 2.03 mm. The superior performance of the as-prepared composites in the field of EMW absorption could be attributed to the novel sandwich-like structure and multiple relaxation and interfacial polarization processes, which contribute to the optimal compatibility of impedance matching. Therefore, it can be reasonably pointed out that the NiS2@rGO/PPy composites obtained in this work have the advantages of strong absorption capacity, absorption bandwidth, and narrow thickness, and has a strong application significance in preventing electromagnetic wave pollution.

Conclusions
To sum up, a novel ternary type of the NiS 2 @rGO/PPy sandwich-like structure composites was synthesized by using a facile two-step method. The influence of the proportions of components on the EMW absorbing performance was investigated. The as-prepared compound with 60 mg NiS 2 @rGO binary composite added exhibited some distinctly strong EMW attenuation properties. From a practical point of view, this novel NiS 2 @rGO/PPy sandwich-like structure composite showed an effective EMW absorption bandwidth (RL < −10 dB) of about 10.96 GHz (7.04-18.00 GHz) with a thickness of 2.00-3.50 mm. The optimal reflection loss was −58.7 dB at 16.44 GHz for a sample with 6 wt% NiS 2 @rGO/PPy at a thickness of 2.03 mm. The superior performance of the as-prepared composites in the field of EMW absorption could be attributed to the novel sandwich-like structure and multiple relaxation and interfacial polarization processes, which contribute to the optimal compatibility of impedance matching. Therefore, it can be reasonably pointed out that the NiS 2 @rGO/PPy composites obtained in this work have the advantages of strong absorption capacity, absorption bandwidth, and narrow thickness, and has a strong application significance in preventing electromagnetic wave pollution.
Author Contributions: Z.Z. and L.L. conceived and designed the experiments; Z.Z., Q.L. and Y.C. performed the experiments; H.L., H.Y. and G.C. analyzed the data; Z.Z. and X.S. wrote the paper.

Conflicts of Interest:
The authors declare no conflict of interest.