NiS2@MoS2 Nanospheres Anchored on Reduced Graphene Oxide: A Novel Ternary Heterostructure with Enhanced Electromagnetic Absorption Property

For the purposes of strength, military equipment camouflage, and protecting the health of organisms, electromagnetic wave absorbing materials have received a lot of attention and are widely studied. In addition to having a strong absorption intensity and a wide effective absorption bandwidth, materials that are lightweight, thermally stable, and antioxidative are also highly desirable. In this study, we fabricated core–shell structured NiS2@MoS2 nanospheres anchored on reduced graphene oxide (rGO) nanosheets (NiS2@MoS2/rGO) by a simple two-step hydrothermal method. The combination ratio was adjusted to achieve proper impedance matching. The electromagnetic parameters and the absorption performance were investigated in detail. A composite loaded with 30 wt.% of the sample achieved a minimum reflection loss (RL) value of −29.75 dB and the effective bandwidth (RL value of less than −10 dB) ranged from 4.95 GHz to 18.00 GHz (13.05 GHz), with a thickness ranging from 1.5 mm to 4.0 mm. This study proved that the generated significant interfacial polarization and synergetic interaction between components can result in NiS2@MoS2/rGO composites with enhanced electromagnetic absorption performance.


Synthesis of NiS 2 @MoS 2 /rGO Composites
The NiS 2 @MoS 2 /rGO composites were synthesized using a hydrothermal method. Firstly, 0.05 g of GO powder was added to 50 mL of distilled water under sonication for 60 min. Then, a certain amount of ammonium molybdate ((NH 4 ) 6 Mo 7 O 24 ·2H 2 O) was dispersed in 20 mL of distilled water and stepwise injected into the GO solution. After magnetic stirring for 20 min at room temperature, a certain amount of thiocarbamide, which was dispersed in 20 mL of distilled water, was added dropwise to the above solution and stirring was maintained for another 20 min at room temperature. Afterward, a certain amount of as-prepared NiS 2 nanospheres, which were prepared in the previous step, was added. The mixture solution continued to be magnetically stirred for 60 min at 70 • C, after which it was transferred to a Teflon-lined, stainless-steel autoclave (200 mL) and reacted at 200°C for 12 h. During this reaction, the pressure facilitated the hydrothermal reduction of GO [33]. Finally, the black composites were centrifuged, rinsed with distilled water and absolute ethanol several times, and dried in a vacuum at 50 • C for 15 h. For convenience, the NiS 2 @MoS 2 /rGO composites synthesized under different reagent ratios are denoted as sample A, sample B, and sample C. The detailed experimental reagent ratios are listed in Table 1.

Characterization and Measurement
The crystal structure of the samples was determined using a Bruker D8 Advanced X-ray diffractometer (XRD). The XRD patterns with Cu K α radiation (λ = 1.5406 Å) at 40 kV and 40 mA were recorded in the range of 2θ = 5-80 • . Field emission scanning electron microscopy (FE-SEM, Hitachi Nanomaterials 2019, 9,292 4 of 14 S4800) was used to determine the microstructure of the composite. Further characterization of the NiS 2 @MoS 2 /rGO composites was performed using transmission electron microscopy with energy dispersive X-ray spectroscopy (TEM/EDS, JEL-2100F, JEOL, Tokyo, Japan) and a high-resolution transmission electron microscope (HRTEM, Tecnai G 2 F20, FEI company, Hillsboro, America). X-ray photon spectroscopy (XPS) was performed on an XPS spectrometer (ESCALAB 250, Thermo, America).

EM Absorption Measurement
The measured samples used for EM absorption measurements were prepared by compounding NiS 2 @MoS 2 /rGO composites with different proportions of wax (20 wt.%, 30 wt.%, 40 wt.%) and fabricating the corresponding cylindrical shaped compacts (Φ out = 7.00 mm, Φ in = 3.04 mm). Then, the microwave scattering parameters were recorded at 2-18 GHz with a coaxial wire method using a vector network analyzer (Agilent N5224A PNA). The vector network analyzer (VNA) works by using a signal generator to scan the sample in a frequency band. Four scattering parameters (S11, S22, S12, and S21) of the two-port network were measured respectively and the electromagnetic parameters of the sample were derived based on the measured scattering parameters. The VNA used in this experiment is a two-port measurement, which usually has built-in grounding devices, without the need for additional earthing devices. In the measuring device, the angle of the incident electromagnetic wave is 90 • .

Characterization of Samples
The synthesis processes of the NiS 2 microspheres and NiS 2 @MoS 2 /rGO composites are schematically depicted in Figure 1. Firstly, the NiS 2 nanospheres were synthesized via a simple hydrothermal method. Secondly, the NiS 2 nanospheres were added to the mixed solution of MoS 2 and GO, which were stirred well. Thirdly, the mixed solution was maintained with magnetic stirring for 60 min at 70 • C and then synthesized through a hydrothermal method at 200 • C for 12 h. During this process, the NiS 2 nanospheres and MoS 2 nanoplates formed core-shell structured NiS 2 @MoS 2 nanospheres due to homologous molecular affinity and GO was transformed into rGO via a redox reaction. At the same time, the NiS 2 @MoS 2 nanospheres were anchored onto reduced graphene oxide nanosheets. photon spectroscopy (XPS) was performed on an XPS spectrometer (ESCALAB 250, Thermo, America).

EM Absorption Measurement
The measured samples used for EM absorption measurements were prepared by compounding NiS2@MoS2/rGO composites with different proportions of wax (20 wt%, 30 wt%, 40 wt%) and fabricating the corresponding cylindrical shaped compacts (Φout = 7.00 mm, Φin = 3.04 mm). Then, the microwave scattering parameters were recorded at 2-18 GHz with a coaxial wire method using a vector network analyzer (Agilent N5224A PNA). The vector network analyzer (VNA) works by using a signal generator to scan the sample in a frequency band. Four scattering parameters (S11, S22, S12, and S21) of the two-port network were measured respectively and the electromagnetic parameters of the sample were derived based on the measured scattering parameters. The VNA used in this experiment is a two-port measurement, which usually has built-in grounding devices, without the need for additional earthing devices. In the measuring device, the angle of the incident electromagnetic wave is 90°.

Characterization of Samples
The synthesis processes of the NiS2 microspheres and NiS2@MoS2/rGO composites are schematically depicted in Figure 1. Firstly, the NiS2 nanospheres were synthesized via a simple hydrothermal method. Secondly, the NiS2 nanospheres were added to the mixed solution of MoS2 and GO, which were stirred well. Thirdly, the mixed solution was maintained with magnetic stirring for 60 min at 70 °C and then synthesized through a hydrothermal method at 200 °C for 12 h. During this process, the NiS2 nanospheres and MoS2 nanoplates formed core-shell structured NiS2@MoS2 nanospheres due to homologous molecular affinity and GO was transformed into rGO via a redox reaction. At the same time, the NiS2@MoS2 nanospheres were anchored onto reduced graphene oxide nanosheets. The phase identification and phase purity of the NiS2@MoS2/rGO composites were investigated by the XRD technique. The XRD spectrum of Sample B is shown in Figure 2 and the XRD spectrum of MoS2/rGO. The PDF card of NiS2 (73-0574), and the PDF card of MoS2 (75-1539) are shown for comparison. When compared with PDF 73-0574, the presence of NiS2 can be clearly confirmed. It can be seen that most of the diffraction peaks correspond with the NiS2 PDF; only a few of the peaks are weaker due to the effect of MoS2 and rGO. Through the comparison with the PDF of MoS2, we found The phase identification and phase purity of the NiS 2 @MoS 2 /rGO composites were investigated by the XRD technique. The XRD spectrum of Sample B is shown in Figure 2 and the XRD spectrum of MoS 2 /rGO. The PDF card of NiS 2 (73-0574), and the PDF card of MoS 2 (75-1539) are shown for comparison. When compared with PDF 73-0574, the presence of NiS 2 can be clearly confirmed. It can be seen that most of the diffraction peaks correspond with the NiS 2 PDF; only a few of the peaks are weaker due to the effect of MoS 2 and rGO. Through the comparison with the PDF of MoS 2 , we found that the XRD spectrum of MoS 2 /rGO matches the PDF of MoS 2 (PDF#75-1539) well. Four characteristic diffraction peaks at around 2θ = 14.1 • , 32.9 • , 37.1 • , and 58.7 • corresponding to the crystal faces (002), (100), (102), and (110), respectively, can be assigned to hexagonal MoS 2 . Due to the low crystallinity of rGO, it is difficult to see the characteristic diffraction peak from C (002) in the spectrum. However, it is difficult to find the diffraction peaks of MoS 2 and rGO in the XRD spectrum of NiS 2 @MoS 2 /rGO. This is because the crystallinity of MoS 2 and rGO is far weaker than that of NiS 2 and the diffraction peaks of NiS 2 and MoS 2 partly overlap [34].  (110), respectively, can be assigned to hexagonal MoS2. Due to the low crystallinity of rGO, it is difficult to see the characteristic diffraction peak from C (002) in the spectrum. However, it is difficult to find the diffraction peaks of MoS2 and rGO in the XRD spectrum of NiS2@MoS2/rGO. This is because the crystallinity of MoS2 and rGO is far weaker than that of NiS2 and the diffraction peaks of NiS2 and MoS2 partly overlap [34]. In order to study the chemical composition of NiS2@MoS2/rGO more thoroughly, the XPS patterns are provided in Figure 3. Mo 3d spectra are presented in Figure 3a, in which the two peaks at 229.55 eV and 229.0 eV are attributed to Mo 3d5/2 and the two peaks at 232.85 eV and 232.45 eV are assigned to Mo 3d3/2 [35]. The peak at 226.6 is due to the existence of a few S species [36]. The peaks of C 1s in Figure 3b located at 284.8 eV and 285.95 eV correspond to the C-C bonds of aromatic rings and C=O bonds of carbonyl, respectively. As shown in Figure 3d, Ni 2p consists of four main binding energy peaks [15]. The peaks at 854.2 eV and 873.0 eV are attributed to the core level of Ni 2p. The other two binding energy peaks are also assigned and correspond to the satellite features of Ni 2p at 857.35 eV and 880.8 eV [37]. From the S 2p spectrum (Figure 3c), it is easy to find that the S atom exists in NiS2@MoS2/rGO composites with two different phases (metallic 1T-phase and semiconducting 2Hphase). The peaks at 162.0 eV and 168.95 eV are attributed to the metallic 1T-phase NiS2@MoS2, while those at 162.8 eV and 163.75 eV correspond to the semiconducting 2H-phase NiS2@MoS2. In order to study the chemical composition of NiS 2 @MoS 2 /rGO more thoroughly, the XPS patterns are provided in Figure 3. Mo 3d spectra are presented in Figure 3a, in which the two peaks at 229.55 eV and 229.0 eV are attributed to Mo 3d 5/2 and the two peaks at 232.85 eV and 232.45 eV are assigned to Mo 3d 3/2 [35]. The peak at 226.6 is due to the existence of a few S species [36]. The peaks of C 1s in Figure 3b located at 284.8 eV and 285.95 eV correspond to the C-C bonds of aromatic rings and C=O bonds of carbonyl, respectively. As shown in Figure 3d, Ni 2p consists of four main binding energy peaks [15]. The peaks at 854.2 eV and 873.0 eV are attributed to the core level of Ni 2p. The other two binding energy peaks are also assigned and correspond to the satellite features of Ni 2p at 857.35 eV and 880.8 eV [37]. From the S 2p spectrum (Figure 3c), it is easy to find that the S atom exists in NiS 2 @MoS 2 /rGO composites with two different phases (metallic 1T-phase and semiconducting 2H-phase). The peaks at 162.0 eV and 168.95 eV are attributed to the metallic 1T-phase NiS 2 @MoS 2 , while those at 162.8 eV and 163.75 eV correspond to the semiconducting 2H-phase NiS 2 @MoS 2 . Nanomaterials 2019, 9, x 6 of 16 SEM and TEM images are presented in Figure 4 to characterize the microstructure and the morphologies of the materials in this study. Figure 4a   To visually demonstrate the distribution of elements in the NiS2@MoS2/rGO composites, energy dispersive X-ray spectrometry (EDS) ( Figure S2) and element maps of C, Ni, Mo, and S ( Figure 5) are displayed. In Figure 5b, the C element in red has a uniform distribution, which indicates that rGO acts as the base layer of the composites. Meanwhile, the Ni, Mo, and S elements are found on the core-shell structure, demonstrating that the NiS2 nanospheres (Ni and S elements) are evenly coated To visually demonstrate the distribution of elements in the NiS 2 @MoS 2 /rGO composites, energy dispersive X-ray spectrometry (EDS) ( Figure S2) and element maps of C, Ni, Mo, and S ( Figure 5) are displayed. In Figure 5b, the C element in red has a uniform distribution, which indicates that rGO acts as the base layer of the composites. Meanwhile, the Ni, Mo, and S elements are found on the core-shell structure, demonstrating that the NiS 2 nanospheres (Ni and S elements) are evenly coated by MoS 2 nanosheets. In order to validate the core-shell structure of NiS 2 @MoS 2 in detail, a HRTEM image and the selected area electron diffraction (SAED) pattern of the NiS 2 @MoS 2 /rGO composites are presented in Figure 6. As shown in Figure 6a

Electromagnetic Absorption Property
The EM attributes (μr and εr) of the as-prepared samples loaded with different fillers were measured, where μr is the complex permeability, μr = μ′ − jμ″; εr is the complex permittivity, εr = ε′ − jε″; the results are shown in Figure 7 and Figure 8. In general, ε′ and μ′ account for the amount of polarization in the samples and represent the ability to store electric and magnetic energy, while ε′′ and μ′′ denote the dissipated electric and magnetic energies. Note that ε′ and ε′′ increase upon increasing the filler loading, indicating a strong dielectric loss. However, the relatively high permittivity is not beneficial to the impedance match and could result in more reflection on the surface. Because the dielectric polarization failed to approach the alternating EM field in the highfrequency range, the values of the real part of the permittivity show a decreasing trend upon increasing the frequency within 2-18 GHz, generally. The curves in Figure 7b and Figure 7g present

Electromagnetic Absorption Property
The EM attributes (µ r and ε r ) of the as-prepared samples loaded with different fillers were measured, where µ r is the complex permeability, µ r = µ − jµ"; ε r is the complex permittivity, ε r = ε − jε"; the results are shown in Figures 7 and 8. In general, ε and µ account for the amount of polarization in the samples and represent the ability to store electric and magnetic energy, while ε and µ denote the dissipated electric and magnetic energies. Note that ε and ε increase upon increasing the filler loading, indicating a strong dielectric loss. However, the relatively high permittivity is not beneficial to the impedance match and could result in more reflection on the surface. Because the dielectric polarization failed to approach the alternating EM field in the high-frequency range, the values of the real part of the permittivity show a decreasing trend upon increasing the frequency within 2-18 GHz, generally. The curves in Figures 7b and 7g present a complicated frequency-dependent behavior. Note that in the frequency ranges of 15-17 GHz (Figure 7b) and 10-14 GHz (Figure 7g), we find strong resonance peaks, which are related to high conductivity, electronic spin, skin effects, and charge polarization due to the polarized centers and point effects. The permeability curves of all samples exhibit similar values in the whole testing frequency range. For the µ and µ value (Figure 8a,b,h), the obvious resonance peaks can be attributed to the natural resonance. Furthermore, the negative value could be due to the motion of charges in an EM field.  The electromagnetic absorption (EA) performance of the composites can be evaluated by the values of reflection loss (RL). A smaller RL suggests highly efficient absorption. The tested frequency range is from 2 to 18 GHz (λ = c/f decreases from 150 to 17 mm); thus, this measurement process can be considered under far-field conditions because the source-to-shield distance is much larger than the free-space wavelength [38]. According to the transmission line theory, the RL at the interface can be expressed using the following equations: where ƒ is the frequency, d is the thickness, and c is the velocity of light in the free space. Z 0 is the free space of impedance matching. When the RL is lower than −10, −20, or −30 dB, it means more than 90%, 99%, or 99.9% of the EM energy is absorbed, respectively. It has been acknowledged that a bandwidth lower than −10 dB can be regarded as an effective EA bandwidth. The electromagnetic absorption (EA) performance of the composites can be evaluated by the values of reflection loss (RL). A smaller RL suggests highly efficient absorption. The tested frequency range is from 2 to 18 GHz (λ = c/f decreases from 150 to 17 mm); thus, this measurement process can be considered under far-field conditions because the source-to-shield distance is much larger than the free-space wavelength [38]. According to the transmission line theory, the RL at the interface can be expressed using the following equations: where ƒ is the frequency, d is the thickness, and c is the velocity of light in the free space. Z0 is the free space of impedance matching. When the RL is lower than −10, −20, or −30 dB, it means more than 90%, 99%, or 99.9% of the EM energy is absorbed, respectively. It has been acknowledged that a bandwidth lower than −10 dB can be regarded as an effective EA bandwidth. The thickness-dependent EA performance of the three as-fabricated composite samples was investigated and the results are shown in Figure 9 and Figure 10. In Figure 9f, we find that high filler loading leads to poor EA performance. This is because the extremely high values of ε′ and ε′′ cause an impedance mismatch and thus lead to EM waves reflecting on the surface rather than being absorbed. Comparing sample A and sample C, it is clear that the composite loaded with 30 wt% of sample B has the best EA performance for each thickness. The minimum RL value is −29.75 dB when the thickness is 4.0 mm and a broad effective bandwidth (RL value of less than −10 dB) can cover from 4.95 GHz to 18.00 GHz (13.05 GHz) with a thickness ranging from 1.5 mm to 4.0 mm. It can be concluded from the results that the EA properties of the as-prepared materials are substantially influenced by altering the component ratio given in the experimental section. Taking advantage of The thickness-dependent EA performance of the three as-fabricated composite samples was investigated and the results are shown in Figures 9 and 10. In Figure 9f, we find that high filler loading leads to poor EA performance. This is because the extremely high values of ε and ε cause an impedance mismatch and thus lead to EM waves reflecting on the surface rather than being absorbed. Comparing sample A and sample C, it is clear that the composite loaded with 30 wt.% of sample B has the best EA performance for each thickness. The minimum RL value is −29.75 dB when the thickness is 4.0 mm and a broad effective bandwidth (RL value of less than −10 dB) can cover from 4.95 GHz to 18.00 GHz (13.05 GHz) with a thickness ranging from 1.5 mm to 4.0 mm. It can be concluded from the results that the EA properties of the as-prepared materials are substantially influenced by altering the component ratio given in the experimental section. Taking advantage of the heterostructure constructed by ternary components, proper matching behavior, as well as the induced intensified interfacial polarization, were achieved with Sample B. The RL peaks of the samples all shift toward lower frequency bands with increasing thickness-a phenomenon that can be explained by the formula f = c/2πdµ . Therefore, the location of the RL peaks can be adjusted for certain applications with different frequencies by manipulating the thickness of the composites. the heterostructure constructed by ternary components, proper matching behavior, as well as the induced intensified interfacial polarization, were achieved with Sample B. The RL peaks of the samples all shift toward lower frequency bands with increasing thickness-a phenomenon that can be explained by the formula f = c/2πdμ′′. Therefore, the location of the RL peaks can be adjusted for certain applications with different frequencies by manipulating the thickness of the composites.  Two key factors should be taken into consideration when designing high-performance EA materials. One is the impedance match and the other is the attenuation capability. The first one requires that the values of permeability and permittivity are roughly equal, which allows the EM wave to enter the materials as much as possible. The second one requires strong EM propagation loss in the interior of the materials. The impedance matching ratio Z, calculated by Z = Zin/Z0 (Zin is normalized input impedance and Z0 is the impedance of free space), can reveal the degree of impedance matching. Actually, when |Zin/Z0| = 1, the waves can be ensured to enter the absorbents instead of reflecting back to the air, and the impedance matching is optimized. The EM wave attenuation inside the absorber is another important factor for the absorber, and the attenuation constant α of the as-prepared samples is calculated according to the following equation: Two key factors should be taken into consideration when designing high-performance EA materials. One is the impedance match and the other is the attenuation capability. The first one requires that the values of permeability and permittivity are roughly equal, which allows the EM wave to enter the materials as much as possible. The second one requires strong EM propagation loss in the interior of the materials. The impedance matching ratio Z, calculated by Z = Z in /Z 0 (Z in is normalized input impedance and Z 0 is the impedance of free space), can reveal the degree of impedance matching. Actually, when |Z in /Z 0 | = 1, the waves can be ensured to enter the absorbents instead of reflecting back to the air, and the impedance matching is optimized. The EM wave attenuation inside the absorber is another important factor for the absorber, and the attenuation constant α of the as-prepared samples is calculated according to the following equation: where c is the velocity of light in a vacuum. Figure 11a,b show the frequency dependence of the attenuation constant α, as well as the impedance matching ratio Z for the as-prepared samples with 20 wt.%, 30 wt.%, and 40 wt.%. It should be noted that the values of both Z and α for sample B are located in the middle position among these three samples. As mentioned earlier, impedance matching and attenuation capability should be taken into consideration simultaneously, not their unilateral superior performances. Therefore, the moderate values of Z and α can explain the enhanced EA performance for sample B. Apart from the suitable impedance matching and high attenuation ability of Sample B, its enhanced EA performance can also be attributed to its unique heterostructure formed by ternary components. For the purpose of strength, camouflage of military equipment, and protecting the health of organisms, EM wave absorbing materials have received lots of attention and are widely studied. In addition to a strong absorption intensity and wide effective absorption bandwidth, materials that are lightweight, thermally stable, and antioxidative are highly desirable. In this study, we fabricated core-shell structured NiS2@MoS2 nanospheres anchored on rGO nanosheets (NiS2@MoS2/rGO) by a simple two-step hydrothermal method. The combination ratio was adjusted in order to realize appropriate impedance matching. The EM parameters and the absorption performance were investigated in detail. A composite loaded with 30 wt% of the sample can achieve a minimum RL value of −29.75 dB, and the effective bandwidth (RL value less than −10 dB) can cover between 4.95 GHz and 18.00 GHz (13.05 GHz) with a thickness ranging from 1.5 mm to 4.0 mm. This study proves that the generated significant interfacial polarization and synergetic interaction between the components can result in NiS2@MoS2/rGO composites with enhanced EM absorption performance.

Conclusions
In summary, core-shell structured NiS2@MoS2 nanospheres anchored on rGO nanosheets (NiS2@MoS2/rGO) were successfully synthesized by a simple two-step hydrothermal method. As the permittivity varies, the as-prepared NiS2@MoS2/rGO composites made with the reagent ratio of sample B demonstrate the best EM wave absorbing performance, because various dielectric loss and magnetic loss mechanisms of the composites lead to optimal impedance matching. A composite loaded with 30 wt% of the sample can achieve the minimum RL value of −29.75 dB and the effective bandwidth (RL value less than −10 dB) can cover from 4.95 GHz to 18.00 GHz (13.05 GHz) with a thickness ranging from 1.5 mm to 4.0 mm. These results indicate that the obtained NiS2@MoS2/rGO composites have a good EM wave absorbing performance with a wide absorbing frequency, strong absorption, good compatibility, and low density. For the purpose of strength, camouflage of military equipment, and protecting the health of organisms, EM wave absorbing materials have received lots of attention and are widely studied. In addition to a strong absorption intensity and wide effective absorption bandwidth, materials that are lightweight, thermally stable, and antioxidative are highly desirable. In this study, we fabricated core-shell structured NiS 2 @MoS 2 nanospheres anchored on rGO nanosheets (NiS 2 @MoS 2 /rGO) by a simple two-step hydrothermal method. The combination ratio was adjusted in order to realize appropriate impedance matching. The EM parameters and the absorption performance were investigated in detail. A composite loaded with 30 wt.% of the sample can achieve a minimum RL value of −29.75 dB, and the effective bandwidth (RL value less than −10 dB) can cover between 4.95 GHz and 18.00 GHz (13.05 GHz) with a thickness ranging from 1.5 mm to 4.0 mm. This study proves that the generated significant interfacial polarization and synergetic interaction between the components can result in NiS 2 @MoS 2 /rGO composites with enhanced EM absorption performance.

Conclusions
In summary, core-shell structured NiS 2 @MoS 2 nanospheres anchored on rGO nanosheets (NiS 2 @MoS 2 /rGO) were successfully synthesized by a simple two-step hydrothermal method. As the permittivity varies, the as-prepared NiS 2 @MoS 2 /rGO composites made with the reagent ratio of sample B demonstrate the best EM wave absorbing performance, because various dielectric loss and magnetic loss mechanisms of the composites lead to optimal impedance matching. A composite loaded with 30 wt.% of the sample can achieve the minimum RL value of −29.75 dB and the effective bandwidth (RL value less than −10 dB) can cover from 4.95 GHz to 18.00 GHz (13.05 GHz) with a thickness ranging from 1.5 mm to 4.0 mm. These results indicate that the obtained NiS 2 @MoS 2 /rGO composites have a good EM wave absorbing performance with a wide absorbing frequency, strong absorption, good compatibility, and low density.
Author Contributions: X.S. conceived and designed the experiments; Z.Z., X.L. and Y.C. performed the experiments; P.Z., M.S. and H.L. analyzed the data; Z.Z. and X.S. wrote the paper.

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