Preparation and Microwave Absorption Properties of NixSy/PVDF Nanocomposites

Octahedral NixSy nanoparticles and NixSy/PVDF nanocomposites are successfully prepared by solvothermal method. The electromagnetic absorption performance of NixSy/PVDF with different filler content is investigated in the range of 2–18 GHz. An optimal reflection loss value of −32.75 dB at 10.48 GHz is achieved for the filler content of 20 wt%, and the bandwidth <−10 dB can reach up to 4.48 GHz with a thickness of 3 mm. Furthermore, fundamental mechanisms of electromagnetic absorbing is discussed, indicating that the interface polarization, electric dipole polarization, Debye relaxation and impedance matching are the main factors affecting the absorption performance of NixSy/PVDF composites.


INTRODUCTION
Electromagnetic pollution has been recognized as a new type of pollution in today's society due to the extensive use of electronic equipment and communication technology in commercial, civil and military fields Zhan et al., 2018;Zhao Z. et al., 2019). The electromagnetic radiation of various communication devices and electronic instruments seriously threatens the health of human beings, for examples reducing people's energy and physical strength, decreasing the ability of people to remember and think, inducing cancer, as well as producing brain tumors and cardiovascular diseases. And, electromagnetic waves will interfere with each other in space, which causing much damage to the communication system, control failure, and poor communication. In addition, electromagnetic wave leakage can also pose a safety hazard to state secrets (Dai et al., 2018;Feng et al., 2018;Li et al., 2018). In order to reduce the harm of electromagnetic radiation, an effective method is to use electromagnetic wave absorbing material . The traditional materials limit their practical application due to the high density. Therefore, the new generation absorbing materials should have the characteristics of small thickness, light weight, wide absorption frequency range and strong electromagnetic wave absorption characteristics (Liu et al., 2012;Wu et al., 2016). In recent years, many nanomaterials have been proved to exhibit excellent absorbing properties, such as Fe 3 O 4 (Jia et al., 2010), SiC (Kuang et al., 2019), RGO aerogels (Zhang M. et al., 2019), C@CoFeO@MnO 2 (Feng et al., 2020), Fe 3 O 4 @NPC (Xiang et al., 2019), and PbS (Pan et al., 2017). In addition, many studies have shown that most of the nanocomposites have synergistic effects, which will be very beneficial to the absorbing properties of the materials Jian et al., 2018;Ma et al., 2018). The typical composites are composed of matrix and filler. Polyvinylidene fluoride (PVDF) is a semi-crystalline material with CH 2 -CF 2 repeating unit. Due to its good piezoelectric properties (Mahato et al., 2015), PVDF has been widely studied and applied as a matrix of composites in the field of wave absorption (Meng et al., 2014;Naseer et al., 2019;Zhao B. et al., 2019).
Transition metal compounds are common absorbers in the field of electromagnetic wave absorption due to their diverse shapes. Lan et al. (2020) designed a novel binary cobalt nickel oxide hollowed-out spheres (Co 1.29 Ni 1.71 O 4 ) via facile hydrothermal and calcination method. The Co 1.29 Ni 1.71 O 4 has a spinel structure, which could improve the dielectric loss and wave absorbing performance of the material under high temperature processing. Sulfide is the most common type of transition metal compound with special band structure and good mechanical properties, which has been extensively applied in solar cells Nair et al., 2019), photocatalysis , lithium batteries , and sodium ion batteries (Peng et al., 2016) fields. In addition, transition metal sulfide nanomaterials have been widely used as a dielectric loss absorber in the field of absorbing wave due to their dielectric properties. For example, Ning et al. (2015) developed few-layered MoS 2 nanosheets (MoS 2 -NS) by the topdown exfoliation method, and the MoS 2 -NS composites showed an lower reflection loss (RL) of −38.42 dB when the thickness was 2.4 mm. However, the high density of a single transition metal sulfide material limits its application in real life. Toward this end, it needs to be compounded with other materials to reduce the density of the material and utilize synergy to improve the absorbing properties. Up to now, many composite materials with transition metal sulfide have been studied and used in absorbing wave field, such as PANI/CoS/CDs (Ge et al., 2016), MoS 2 /GN (Zhang D. et al., 2018), Ni/ZnS (Zhao et al., 2014), MoS 2 /PVDF  and so on. However, the microwave absorption properties of Ni x S y and its related inorganic-organic composite materials have not been reported. In this study, octahedral Ni x S y nanoparticles were synthesized by solvent thermal method and used as fillers to prepare Ni x S y /PVDF nanocomposite. The absorption electromagnetic wave performance of Ni x S y /PVDF nanocomposites was studied, and the absorbing mechanism was elaborated and analyzed.

Materials
All reagents were analytical grade and used without further purification. The preparation of Ni x S y is described in the following: 0.3490 g NiN 2 O 6 ·6H 2 O, 0.064 g S powder and 0.24 g PVP were added to 60 ml of ethylene glycol, and the mixture was stirred at room temperature for 1 h. After all the particles were dissolved, the stirring was stopped. And, the reaction kettle was placed in a kettle and placed in an oven. The reaction was carried out at 200 • C for 12 h. After cooled to room temperature, the product was washed with water and ethanol, dried for use.
In order to obtain Ni x S y /PVDF composite film, an appropriate amount of PVDF was firstly dissolved in 25 mL N-N dimethylformamide (DMF) and stirred for 30 min at room temperature. Then, Ni x S y octahedral particles were added, and ultrasonic stirring was performed. After all the solids were dissolved, the solution in the beaker was poured into a watch glass, placed in an oven and dried at 70 ( • C for 3 h. After the mixture was pressed at 210(C ( • C for 15 min at 4 MPa. A cylindrical composite film was obtained. In this study, the filler content of samples were set at 10 wt%, 20 wt%, and 30 wt%, respectively. The preparation process is shown in Figure 1.

Characterization
The size and morphologies of Ni x S y nanoparticles were tests by scanning electron microscopy (SEM, Quanta 250 FEG). The elemental composition was examined by field emission scanning electron microscopy (FE-SEM, JSM-7500F) with energy dispersive spectrometer (EDS) spectrum and copper grids. The nanomaterials X-ray diffraction (XRD) patterns were collected using a Shimadzu 6000 X-ray diffractometer with Cu Kα radiation (λ = 1.5416 Å) for the Ni x S y phase analysis.

Measurements of Electromagnetic Parameters
The cylindrical Ni x S y /PVDF with different concentrations were prepared by hot pressing method. The microwave absorption properties of Ni x S y /PVDF were measured by two-port vector network analyzer (Agilent N5244a) with coaxial wire setup. The range of measurement was 2-18 GHz. Figure 2 displays the SEM and FESEM photographs of the Ni x S y materials. As shown in Figure 2a, Ni x S y exhibits an octahedral nanoparticle with a particle size of about 450 nm. From Figure 2b, we can find that the elements of Ni and S are uniformly dispersed by scanning the distribution of the elements in the rectangular frame.

RESULTS AND DISCUSSION
In order to obtain the crystal structure of the Ni x S y nanoparticles, we tested the samples by XRD. Figure 3 shows the XRD diffraction pattern of Ni x S y nanoparticles. The peaks at the 2q of the asterisk in the figure are 30.34 • , 34.88 • , and 46.16 • , which correspond to the standard card PDF#02-1280 of NiS . The peaks at the remaining 2q angles correspond exactly to the standard card PDF#11-0099 of the cubic structure of NiS 2 (Zhu et al., 2019).
Supplementary Figure S1 shows the fine-scanned Ni 2p and S 2p XPS spectra of Ni x S y . As shown in Supplementary Figure S1A, the peaks at 855.7 and 853.5 eV are assigned to Ni 2p 1/2 and Ni 2p 3/2 of Ni 2+ , 871.9 and 874.2 eV were assigned to Ni 2p 3/2 and Ni 2p 1/2 of Ni 3+ . The Ni 2p spectra evidence the formation of NiS 2 (Yang et al., 2016). Supplementary Figure S1B shows the S 2p XPS spectra. The peaks at 161.54 and 162.4 eV are attributed to S 2p 3/2 and S 2p 1/2 of Ni-S bonding, while the peaks at 163.5 and 167.8 eV correspond to the S 2 2− and sulfates with high oxidation state. The XPS analyses point out the formation of Ni x S y .
In order to study the absorbing properties of Ni x S y /PVDF nanocomposites, the samples with different filler content were tested by the coaxial method in the range of 2-18 GHz. The permittivity and magnetic permeability of each sample can be obtained. Figure 4A shows the RL curves of four  samples at a thickness of 3 mm. It can be seen that the RL value of Ni x S y /PVDF composite is −9.8 dB at 6.1 GHz. By comparing the RL values of paraffin composites and Ni x S y /PVDF composite materials, we can see that Ni x S y /PVDF have much better absorbing effect than that of paraffin composites. The Ni x S y /PVDF nanocomposites with a filler content of 20 wt% have the optimal RL value of −32.75 dB and a broad effective bandwidth of 4.48 GHz. Figure 4B shows the RL of the material at different thicknesses when the filler content is 20 wt%. It is clear that the thickness is also a key factor affecting the absorbing properties of the material. Figures 4C,D show three-dimensional maps of Ni x S y /PVDF nanocomposites with filler content of 10 and 30 wt%, respectively. Compared with Ni x S y /wax, the absorbing properties of Ni x S y /PVDF are significantly improved, further demonstrating that compounding with PVDF can increase the electromagnetic wave absorption properties of the material. Figures 5A,B show the variation of the dielectric real (ε') and imaginary (ε") of the four materials with frequency. It can be seen that the ε' and ε" ratios of the nanomaterials composited with PVDF are larger than the ε' and ε" of the paraffin-composited materials, and the real and imaginary dielectric parts of the sample compounded with PVDF enhance with the increase of filling concentration. As the number of Ni x S y nanoparticles increases, more dielectric polarization occurs inside the material. And polarization causes polarization relaxation, so more polarization leads to an increase in ε". As the frequency increases, the real and imaginary parts of the dielectric remain almost stable, because the increase in frequency causes the rotational speed of the internal electric dipole of the material to keep up with the frequency of the external alternating electromagnetic field (Ohlan et al., 2010).
As known, Ni x S y nanoparticles are semiconductor materials, so there is only dielectric loss in the material without magnetic loss, and interfacial polarization exists between adjacent two phases with different dielectric constant and conductivity (Dai et al., 2017). Due to the complexation of Ni x S y nanoparticles with PVDF, there is interfacial polarization in the material (Figure 6). The relaxation phenomenon caused by the polarization of the interface converts the electromagnetic energy into thermal energy, which contributes to the improvement of the absorbing ability of the material (Roy and Bera, 2012). In addition, PVDF is a polar material because it contains F atoms. The positive and negative charge centers of polar molecules do not coincide, so there is an electric dipole moment (Xu et al., 2018). When an alternating electric field is applied to such a polar material, a galvanic couple occurs in the material. Polarization of the poles would further contribute to the loss of electromagnetic energy (Chen et al., 2009). When the filler content increases, that is, the surface of the Ni x S y nanoparticles in the polymer increase, the interfacial polarization increases, and the real and imaginary parts of the dielectric constant increase. However, the dielectric constant is too high to allow  the material to meet the dielectric matching, so that the electromagnetic wave cannot completely enter the interior of the material and affect its absorption of electromagnetic waves. As a result, the composite with 20% filler content achieves the best absorbing property.
In addition, Debye relaxation is another important dielectric loss mechanism. As the electric field in the medium changes, the delay of molecular polarization causes the relaxation process to occur. If the Debye relaxation behavior is considered, the relative complex permittivity can be expressed as follows (Dong et al., 2008;Shi et al., 2016): Frontiers in Materials | www.frontiersin.org Where f is the frequency, ε s is the dielectric constant in the DC dielectric, and ε ∞ is the relative dielectric constant of the high frequency limit. Based on Equations (2) and (3), the relationship between ε' and ε" can be inferred as follows: Among them, we find that the curve ε'-ε" is a semicircle. If Debye relaxation occurs, a semicircle will appear in the image. It can be seen from Figures 7A-D that there are two half rings in the ε'-ε" curve of the four materials, which proves the existence of the Debye relaxation loss mechanism. For the Ni x S y /wax composites, the relaxation peak mainly results from dipolar polarization . And two semicircles illustrate the double dielectric relaxation processes in Ni x S y /PVDF composites. When the filler content increases, the Cole-Cole plot is close to a straight line, which means that the conductivity loss plays the main role in the dielectric loss process and the polarization no longer occupies the first status. The Debye relaxation process has been hidden because of the enhanced electrical conductivity. The higher conductivity can lead to skin effect that inhibits the further incidence of microwaves into the absorbers (Zhu et al., 2019), which is the reason that high filler rate will reduce the absorbing performance of the composite.
Based on these results, it can be concluded that the mechanism of Ni x S y /PVDF may be interface polarization, electric dipole polarization, Debye relaxation and impedance matching. When the electromagnetic wave entering the absorber, the polyhedral structure of Ni x S y nanoparticles causes interfacial polarization at its interface with PVDF, and the relaxation caused by interfacial polarization improves the absorbing performance of the composite. In addition, there is a dipole in the polar material PVDF, which causes galvanic polarization after the electromagnetic wave is incident. Debye relaxation is another important dielectric loss pathway caused by the delay in molecular polarization.
The synergistic effect of different mechanisms increases the dielectric loss of the composite material, so that Ni x S y / PVDF composite material has excellent absorption properties. Table 1 summarizes electromagnetic wave absorption properties of various composite materials including transition metal compound nanomaterials reported in literature. Ni x S y /PVDF prepared in this work has the advantages of low filler content, thinner thickness and broadening efficient absorption bandwidth.

CONCLUSION
Transition metal sulfide nanomaterials have been widely used and studied as a dielectric loss absorbing material in the field of absorbing wave due to their dielectric properties. The high density of a single transition metal sulfide material limits its application in real life. For this reason, it needs to be compounded with other materials to reduce the density of the material, and the synergy between different materials is used to improve the absorbing properties. In this paper, octahedral Ni x S y nanoparticles with excellent dispersibility were prepared by solvothermal method and combined with PVDF. The Ni x S y /PVDF nanocomposites with different filler content were measured by coaxial method in the range of 2-18 GHz. The measurement results show that the material has the best absorption performance when the filler content is 20 wt%, the maximum reflection loss is up to −32.75 dB, and the absorption frequency band width is 4.48 GHz. Through the analysis of dielectric constant and absorption mechanism, the composite material of octahedral Ni x S y and PVDF has interface polarization, electric dipole polarization, Debye relaxation, and appropriate impedance matching.

DATA AVAILABILITY STATEMENT
All datasets generated for this study are included in the article/Supplementary Material.

AUTHOR CONTRIBUTIONS
Y-HD and NG: data curation and writing-original draft preparation. LW: conceptualization, methodology, visualization, and investigation. M-DL and G-SW: supervision.