1 Introduction

With the advent of the 5G era, the global intelligent manufacturing of electronic equipment has developed rapidly, providing great convenience for information dissemination, medical care and technology in human society [1,2,3,4]. However, at the same time as this great convenience comes, hidden dangers also come. Electromagnetic waves inadvertently endanger human health, the use of other electronic equipment, the operation of medical equipment and national defense security. Therefore, the study of electromagnetic wave absorbing materials has attracted great attention of scientific researchers. The working principle of electromagnetic wave absorbing materials is to attenuate and absorb the electromagnetic waves injected into the material, and then convert the electromagnetic wave energy into heat energy or other forms of energy through loss mechanisms such as dielectric loss and magnetic loss of the material. Emerging electromagnetic wave absorbing materials should not only have the characteristics of "light, thin, strong, and wide" of traditional electromagnetic wave absorbing materials but also have the characteristics of strong environmental adaptability, corrosion resistance, tensile, and compressive strength [5, 6]. According to their absorption mechanism, electromagnetic wave absorbing materials are usually divided into dielectric loss materials and magnetic loss materials [7, 8]. Dielectric loss materials include sulfides (CoS2 [9], MoS2 [10]), carbon materials (carbon nanotubes [11, 12], graphene [13],) and conductive polymers [14]; magnetic loss materials usually include metal oxides (Co3O4 [15], Fe3O4 [16]), single metal particles (Fe [17]) and alloys (FeNi [18]). Guo et al. used ZIF-67 MOFs nanosheet-anchored cotton fibers (ZIF-67@CF) as precursors to fabricate WS2/CoS2@carbonized cotton fibers (CCF) with a layered heterogeneous structure through tungsten etching, sulfidation, and carbonization processes. In addition to the synergistic effect of the electro-magnetic dual-loss mechanism, the layered heterostructure and multiple components of WS2/CoS2@CCF showed better impedance matching. In addition, the numerous W-S-Co bands and heterojunction interfaces of heterogeneous WS2/CoS2 facilitate additional interfacial/dipole polarization losses and conductive losses, thus improving the electromagnetic wave attenuation performance [19]. Feng et al. described the classification and preparation methods of MOFs, and analyzed their absorption mechanisms. And the current research progress was introduced in detail from the perspectives of single metal MOFs, multi metal MOFs, and the composite of absorbing materials derived from MOFs with other materials (including carbon materials, MXene, and conductive polymers), and their advantages and disadvantages were analyzed [20]. Looking to the future, new absorbing materials continue to emerge.

Transition metal dichalcogenides are traditional two-dimensional materials that are often used in batteries, optoelectronic devices, and supercapacitors due to their physicochemical properties. It is worth noting that transition metal dichalcogenides also play an important role in the field of electromagnetic wave absorbing materials [21]. Common transition metal dichalcogenides are divided into monometallic dichalcogenides and heterobimetallic dichalcogenides. CoS2 is a common monometallic disulfide. As a semiconductor material, CoS2 has excellent electrical conductivity and can greatly improve the electronic transition ability [22]. The local symmetry breaking of the surface atoms of CoS2 can induce polarization loss, thereby increasing the dielectric loss capability. Second, CoS2 has a cubic pyrite structure, ferromagnetism (Tc∼122 K), and semi-metallic properties, which can also induce weak magnetic losses [23].

Designing unique structures is an effective way to improve the performance of composite materials. Hou et al. successfully synthesized a three-dimensional porous nitrogen doped carbon matrix modified with molybdenum dioxide (MoO2) nanoparticles using a NaCl assisted template strategy. The unique design of the structure and the introduction of oxygen vacancy defects endow the MoO2/C composite material with excellent electrochemical performance [24]. Mu et al. prepared CoP nanoparticles coated with N, P doped carbon shells through supramolecular self-assembly method (CoP@PNC) Then, the amorphous phase is formed at the hot spot. Based on a reasonable nano/micro structure, when used as an anode electrode, the transmission paths of Li+ and e− are shortened, and the electronic conductivity is enhanced, resulting in excellent electrochemical performance [25]. In summary, in the fields of batteries and catalysis, unique structures are an effective way to improve performance [26,27,28,29,30,31].

So far, various morphologies of CoS2 have been successfully fabricated, including microspheres [32], nanocrystals [33], and hollow microspheres [34]. Li et al. prepared bimetallic nickel cobalt oxide/carbon (NiCo2O4/C) composite materials in the shape of sea urchins using a hydrothermal method, and studied their absorption performance by adjusting different morphologies by changing the calcination temperature. The various advantages of the synthesized NiCo2O4/C composite material contribute to achieving satisfactory absorption characteristics [35]. Through investigation, it is found that CoS2 with hollow sea urchin morphology has not been studied in the field of electromagnetic wave absorption. Therefore, CoS2 hollow sea urchin derived from Co(CO3)0.5(OH)·0.11H2O was selected as the substrate and the electromagnetic wave absorbing material of its composite was studied in this paper. FeCoS2 is a heterobimetallic disulfide. Heterobimetallic dichalcogenides have become a research hotspot due to their diverse chemical compositions, rich spatial structures, and unique optical, electrical, and magnetic properties. Notably, synergistic effects can be induced between bimetallic ions compared to monometallic sulfides [36].

In this work, a composite of heterobimetallic disulfide nanoparticles dispersed in CoS2 hollow sea urchins was developed. The large specific surface area of CoS2 provides support for the distribution of nanoparticles and greatly improves the dispersion effect of nanoparticles. The dispersion modification of nanoparticles greatly increases the specific surface area of the material and generates multiple heterogeneous interfaces, which trigger the excellent interface polarization effect of the material, thereby enhancing the electromagnetic wave loss capability. The special hollow sea urchin structure can trigger multiple scatterings of electromagnetic waves, which also consume part of the incident electromagnetic waves to a certain extent. The constructed heterobimetallic disulfide nanoparticles and hollow sea urchins constitute a three-dimensional conductive network, which greatly improves the electronic conductivity and enhances the conductance loss capability. In this study, a high-performance electromagnetic wave absorbing material was developed, which provides ideas for designing new structures of high-performance advanced electromagnetic wave absorbing materials.

2 Experimental section

2.1 Materials

Co(NO3)2·6H2O, Fe(NO3)3·9H2O, NH4F, CH4N2O (urea), absolute ethanol (EtOH), sulfur powder. All reagents are of analytical grade and can be used directly without further purification. Deionized water is used throughout the experiment.

2.2 Synthesis of Co(CO3)0.5(OH)·0.11H2O precursors

The Co(CO3)0.5(OH)·0.11H2O hollow sea urchin precursor was prepared according to the previous process with slight modifications. First, appropriate amounts of Co(NO3)2·6H2O, NH4F, and CH4N2O were dissolved in 50 mL of distilled water in this order, followed by stirring for 1 h. The uniform solution is moved into a Teflon-lined autoclave and kept at 200 °C for 7 h. The reacted precipitate was washed repeatedly with absolute ethanol and deionized water, and then dried under vacuum overnight. The dried black powder is Co(CO3)0.5(OH)·0.11H2O hollow sea urchin.

2.3 Synthesis of CoS2

During this process, Co(CO3)0.5(OH)·0.11H2O was made into CoS2 by two anneals. Co(CO3)0.5(OH)·0.11H2O was first placed in a muffle furnace and annealed at 400 °C for 2 h in air to obtain Co3O4 powder. Then, the uniformly mixed Co3O4 and sulfur powder were annealed at 400 °C for 2 h in an Ar atmosphere to obtain CoS2.

2.4 Synthesis of FeCoS2@CoS2

Heterobimetallic sulfides are doped by wet impregnation. Firstly, 0.2 mmol Co(NO3)2·6H2O and Fe(NO3)3·9H2O were dissolved in 2 mL of deionized water, and then 1 mmol CoS2 was added. The well-stirred solution was dried under vacuum. The mixture was ground and mixed with sulfur powder, followed by annealing at 400 °C for 2 h in an H2/Ar environment to obtain FeCoS2@CoS2 powder. The molar ratios of FeCoS2 and CoS2 were controlled to 1/1, 2/1, 4/1, 8/1, and the samples were named 1/1, 1/1, 4/1, and 8/1.

2.5 Characterization

The powder X-ray diffraction (XRD) was measured and recorded on a Rigaku Ultima IV with Cu-Ka radiation (k = 0.154178 nm). The morphological characteristics of all products were observed and characterized by field emission scanning electron microscopy (F-SEM; JEOL JSM-7800F) and transmission electron microscopy (TEM; JEOL JEM-2100). The surface elemental composition of all products was measured by the X-ray photoelectron spectroscopy (XPS) on a Thermo Fisher ESCALAB 250Xi spectrometer with an Al Ka X-ray source (1486.6 eV).

2.6 Electromagnetic parameters

The FeCoS2@CoS2 hybrids were mixed with paraffin to form a ring model (Φin: 3.04 mm, Φout: 7.00 mm), and its weight ratio was controlled to 7/3. A vector network analyzer (N5234A; Agilent, USA) was exploited to measure the complex permeability (\({\mu }_{r}={\mu }^{\prime}-j{\mu }^{"}\)) and complex permittivity (\({\varepsilon }_{r}={\varepsilon }^{\prime}-j{\varepsilon }^{"}\)) in the frequency range of 2.0–18.0 GHz.

3 Results and discussion

Scheme 1 clearly illustrates the synthesis process of the hollow sea urchin Co(CO3)0.5(OH)·0.11H2O precursor and FeCoS2@CoS2 composite. The first is the hydrothermal preparation of Co(CO3)0.5(OH)·0.11H2O precursor [37]. During this process, urea is decomposed at high temperatures to produce ammonium and carbonate, ammonium provides an alkaline environment for cobalt deposition to form hydroxide, and carbonate acts as an intercalating anion. Ammonium fluoride plays a role in adjusting the morphology. It can be observed from the schematic diagram that different hydrothermal times lead to the generation of different morphologies. In response to this phenomenon, some verification experiments were done to reveal its growth process and speculate its growth mechanism. Based on the Ostwald ripening process and experimental data, the growth mechanism model Fig. S1. was successfully constructed [38]. In the early stage (reaction time of 1 h), many small-sized nanoparticles were formed. After 3 h of reaction, solid spheres with surface-loaded nanoneedles were obtained. As the reaction time was extended to 6 h, the inner core was completely dissolved, just to form a homogeneous sea urchin-like hollow sphere. Further increasing the reaction time to 10 h, the sea urchin-like structure collapsed and the nanoneedles were distributed chaotically. Finally, when the reaction time was increased to 24 h, the morphology and structure became solid spheres assembled by nanosheets. Co(CO3)0.5(OH)·0.11H2O with hollow sea urchin morphology was selected as the precursor, which was converted into CoS2 after the subsequent two-step annealing. The required nitrate was doped into CoS2 by wet impregnation method, and then mixed with sulfur powder for high-temperature vulcanization in an Ar atmosphere to obtain FeCoS2@CoS2, where the role of sulfur powder is to ensure that CoS2 is not reduced [39].

Scheme 1
scheme 1

Schematic diagram of the synthesis process of Co(CO3)0.5(OH)·0.11H2O and FeCoS2@CoS2 composites

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The crystal structure of the 1/1 FeCoS2@CoS2, 2/1 FeCoS2@CoS2, 4/1 FeCoS2@CoS2, 8/1 FeCoS2@CoS2 and Co(CO3)0.5(OH)·0.11H2O samples was characterized by X-ray diffraction (XRD). In the XRD pattern of Fig. 1a, five diffraction peaks of CoS2 (JCPDS No.41-1471) appeared at 27.9°, 32.3°, 36.2°, 39.8° and 54.9°, corresponding to (111), (200), (210), (211) and (311) planes. Four diffraction peaks of FeCoS2 (JCPDS No. 75-0607) appeared at 30.7°, 35.2°, 36.4° and 54.6°, corresponding to the (100), (101), (102) and (110) crystal planes, respectively. The XRD curves of Co(CO3)0.5(OH)·0.11H2O (JCPDS No.48-83) and pure Co3O4 (JCPDS No.74-2120) and CoS2 can be seen in Fig. S2. This guarantees the precursor's successful synthesis of the body. It is worth noting that in Fig. 1a, no other diffraction peaks were found in the XRD curves of 1/1 and 2/1, indicating that there are no other impurities and the product is of high purity. But with the increase of iron nitrate and cobalt nitrate content, a diffraction peak appeared around 51.1°, which may be attributed to the formation of other sulfides of iron and cobalt. The formation of other sulfides indicates that only appropriate levels of iron and cobalt salts can ensure the stable formation of the final product.

Fig. 1
figure 1

XRD patterns a of 1/1 FeCoS2@CoS2, 2/1 FeCoS2@CoS2, 4/1 FeCoS2@CoS2, 8/1 FeCoS2@CoS2; Typical XPS spectra taken from the 2/1 FeCoS2@CoS2, showing b the survey scan and the c Fe, d Co, e S peaks

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As shown in Fig. 1b, the full view of the XPS spectrum of 2/1 has three peaks of Fe, Co, and S elements. As shown in Fig. 1c, the Fe 2p band consists of five sub-peaks, which are attributed to satellite peaks (719.6 eV), Fe2+ (723.9 eV and 710.7 eV), and Fe3+ (725.2 eV and 712.8 eV). The Co 2p (Fig. 1d) band consists of six sub-peaks attributed to satellite peaks (784.6 eV and 802.4 eV), Co2+ (781.3 eV and 796.5 eV), and Co0 (778.6 eV and 793.5 eV). The detailed S 2p (Fig. 1e) spectrum shows that the S 2p3/2 and 2p1/2 peaks are located at ~ 161.9 eV and 163.1 eV, and the peak at 168.6 eV represents the oxidized sulfur group, which indicates that the surfaces of all samples are slightly oxidized.

Detailed structural and morphological characterization of Co(CO3)0.5(OH)·0.11H2O, oxide Co3O4, sulfide CoS2 and composites 1/1 FeCoS2@CoS2, 2/1 FeCoS2@CoS2, 4/1 FeCoS2@CoS2, 8/1 FeCoS2@CoS2 are shown in Fig. 2. It can be seen from Fig. 2a that the morphology of Co(CO3)0.5(OH)·0.11H2O is an irregular sphere under 1 h hydrothermal time. It can be seen from Fig. 2b that the morphology of Co(CO3)0.5(OH)·0.11H2O is a sea urchin with a size of about 10 μm under 3 h hydrothermal time, and the morphology and size are uniform. As the reaction time increased to 6 h (Fig. 2b, c), the inner urchins were dissolved and then the hollow sea urchin morphology was formed, and the hollow sea urchins were uniformly dispersed and uniform in size. It is worth noting that the appearance and size did not change, which verifies the correctness of the growth mechanism. As the time continued to increase to 10 h, the internal dissolution continued to cause the sea urchin to collapse to form randomly scattered needles. As can be seen from Fig. 2d, the diameter of the needles is 100 nm, which is the same size as the needles of the hollow sea urchin (Fig. 2c). The above proves that the needles are indeed formed by the collapse of the hollow sea urchin. As the reaction time increased to 24 h (Fig. 2e), the morphology became a hydrangea composed of nanosheets, possibly related to ammonium fluoride. The reason for choosing (Fig. 2c) the hollow sea urchin as the precursor is that the internal hollow structure and large specific surface area can promote the loss of electromagnetic waves [40, 41]. Co3O4 (Fig. 2f) and CoS2 (Fig. 2g) are the oxidation products and sulfide products of the hollow sea urchin shown in Fig. 2c, respectively. It can be seen from the figure that the morphology did not change much after two calcinations. Melting leads to size reduction. It can be seen from Fig. 2h–k that FeCoS2 is uniformly dispersed on the surface of the CoS2 hollow sea urchin. From elemental mapping (Fig. 2l–o), it can be found that Fe, Co, and S elements are uniformly distributed, which again indicates that FeCoS2 is uniformly dispersed on the surface of CoS2.

Fig. 2
figure 2

SEM pattern of Co(CO3)0.5(OH)·0.11H2O with different shapes a microspheres, b solid sea urchin microspheres and c hollow sea urchin microspheres, d needles, e hydrangea), f Co3O4, g CoS2, h 1/1 FeCoS2@CoS2, i 2/1 FeCoS2@CoS2, j 4/1 FeCoS2@CoS2, k 8/1 FeCoS2@CoS2, l–o) Elemental mapping of 2/1 FeCoS2@CoS2; The low magnification TEM pattern of (p, q) 2/1 FeCoS2@CoS2, the HRTEM pattern of (r, s, u and v) 2/1 FeCoS2@CoS2

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The TEM of 2/1 FeCoS2@CoS2 is shown in Fig. 2p–v. It can be seen from Fig. 2p that the morphology of CoS2 is a hollow sea urchin microsphere, which corresponds to the SEM description. Figure 2s is an enlarged view of Fig. 2p, from which the observation can again prove that Co(CO3)0.5(OH)·0.11H2O still retains the basic morphology after three calcinations. Figure 2r shows the lattice spacing of 0.28 nm, pointing to the (200) plane of CoS2. And it can be found that the crystal lattice is obvious, indicating the high crystallinity of CoS2, which corresponds to the sharp peak of XRD. Figure 2v shows the lattice spacing of 0.20 nm, pointing to the (102) plane of FeCoS2. The above TEM data may indicate successful compounding of the material.

The RLmin value of FeCoS2@CoS2 composites can directly reflect the electromagnetic wave absorption performance of the absorber, which can be calculated by transmission line theory [42]:

$${\text{RL}}=\, \text{(dB)}=20\,\log\,\left|\frac{{\mathrm{Z}}_{\mathrm{in}}-{Z}_{0}}{{\mathrm{Z}}_{\mathrm{in}}+{Z}_{0}}\right|$$
(1)
$${Z}_{in}={Z}_{0}\sqrt{\frac{{\mu }_{r}}{{\varepsilon }_{r}}}\mathrm{tanh}\;\left(j\frac{2\pi fd}{c}\sqrt{{\mu }_{r}{\varepsilon }_{r}}\right)$$
(2)

\({Z}_{in}\) represents the normalized input impedance of the absorber, and \({Z}_{0}\) represents the impedance of free space. \({\varepsilon }_{r}\) and \({\mu }_{r}\) represent the relative complex permittivity and permeability, respectively. \(f\) is the frequency of the electromagnetic wave, \(d\) is the thickness of the absorber, and \(c\) is the speed of light of the electromagnetic wave in free space. Figure 3 shows the 3D and 2D RL map of all samples.

Fig. 3
figure 3

The 3D and 2D images of RLmin values of a 1/1 FeCoS2@CoS2, b 2/1 FeCoS2@CoS2, c 4/1 FeCoS2@CoS2, d 8/1 FeCoS2@CoS2

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From the application and production point of view, the critical value of RLmin is -10 dB. Effective absorption means that the material can absorb more than 90% of the electromagnetic wave energy, and the effective absorption value is the reflection loss less than the critical value [43, 44]. It can be seen in Fig. 3. that 2/1 FeCoS2@CoS2 has the smallest RL value and the widest EAB at a thinner thickness compared to other materials. Specifically, the RLmin values of 1/1 FeCoS2@CoS2, 2/1 FeCoS2@CoS2, 4/1 FeCoS2@CoS2 and 8/1 FeCoS2@CoS2 are -57.21 dB at 14.88 GHz and 2.10 mm, -75.23 dB at 16.88 GHz and 1.87 mm, -48.90 dB at 14.56 GHz and 6.30 mm, and -46.29 dB at 9.68 GHz and 9.30 mm, respectively (Fig. 3a–d). Notably, 2/1 FeCoS2@CoS2 has the largest EAB at 5.76 GHz and a thickness of 2.12 mm. In addition, the maximum EAB of 1/1 FeCoS2@CoS2, 4/1 FeCoS2@CoS2 and 8/1 FeCoS2@CoS2 are 5.20 GHz at 2.40 mm, 2.24 GHz at 7.00 mm and 3.20 GHz at 7.00 mm, respectively. The above data show that 2/1 FeCoS2@CoS2 has good electromagnetic wave absorption performance, which may be attributed to the phenomenon of agglomeration and uneven distribution of excess FeCoS2, which affects the impedance matching effect. \({\mu }_{r}\) and \({\varepsilon }_{r}\) largely determine the electromagnetic wave absorption properties of materials. According to the electromagnetic field theory, \({\varepsilon }^{\prime}\) and \({\varepsilon }^{\prime\prime}\) represent the storage and loss of electrical energy, while \({\mu }^{\prime}\) and \({\mu }^{{\prime}{\prime}}\) represent the storage and loss of magnetic energy.

The complex permittivity of all materials fluctuates significantly in Fig. 4a, b, and frequency dependence can be clearly observed. The \({\varepsilon }^{\prime}\) of 8/1 FeCoS2@CoS2 (5.92–6.73) and the \({\varepsilon }^{\prime\prime}\) of 4/1 FeCoS2@CoS2 (0.52–1.60) are the lowest values, while the ε' (6.59–9.44) and \({\varepsilon }^{\prime\prime}\) (2.62–4.74) of 1/1 FeCoS2@CoS2 is the maximum value. Large \({\varepsilon }^{\prime\prime}\) means high conductivity, however too high conductivity can lead to unbalanced impedance matching. Unbalanced impedance matching means that most of the electromagnetic waves cannot enter the interior of the material but are reflected off the surface of the material. It is shown above that an appropriate \({\varepsilon }^{\prime}\) and \({\varepsilon }^{\prime\prime}\) (for example, 2/1 FeCoS2@CoS2) can ensure the impedance matching balance, thereby causing strong absorption of electromagnetic waves. The above again verify the reason why 2/1 FeCoS2@CoS2 has excellent electromagnetic wave absorption performance. From Fig. 4e–f, it can be seen that the trend of changes in the imaginary part and tangent values of magnetic permeability of all materials is the same, with a fluctuation peak in the range of 8-11 GHz, indicating the existence of resonance loss in the material, thereby improving its electromagnetic wave absorption performance.

Fig. 4
figure 4

a Real part and b imaginary part of permittivity, d real part and e imaginary part of permeability, c tanδe, and f tanδm of all examples

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The magnetic loss of electromagnetic wave absorbing materials generally consists of three parts: eddy current loss, natural resonance and exchange resonance. Natural resonance usually occurs in the low frequency region, while exchange resonance is generally the resonance in the high frequency region. Eddy current losses are usually described by the following formula [45]:

$${C}_{0}={\mu }^{{\prime}{\prime}}{\left({\mu }^{\prime}\right)}^{-2}{f}^{-1}=2\pi {\mu }_{0}{d}^{2}$$
(3)

In the formula, \({\mu }_{0}\) represents the vacuum permeability, and d represents the electrical conductivity of the material. If the magnetic losses of the material include eddy current losses, the \({C}_{0}\) value remains the same.

As shown in Fig. 5a, the \({C}_{0}\)-f curves of 1/1 FeCoS2@CoS2, 2/1 FeCoS2@CoS2, 4/1 FeCoS2@CoS2 and 8/1 FeCoS2@CoS2 fluctuate at high and low frequencies, indicating the presence of materials Magnetic losses due to exchange resonance and natural resonance. The above proves again that FeCoS2 and CoS2 have magnetic loss in the electromagnetic wave loss mechanism. At 8–11 GHz, all curves remain largely unchanged, indicating the presence of eddy current losses in the magnetic losses produced by the material [46].

Fig. 5
figure 5

a The \({\mathrm{C}}_{0}\)-\(f\) curves of all examples. b The α curves of all examples. The Cole-Cole curves of c 1/1 FeCoS2@CoS2, d 2/1 FeCoS2@CoS2, e 4/1 FeCoS2@CoS2 and f 8/1 FeCoS2@CoS2

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According to the transmission line theory, the electromagnetic wave absorption capacity of the material is also related to the attenuation coefficient [47, 48]. A high attenuation coefficient means that the electromagnetic waves entering the absorber can be attenuated or dissipated to the maximum extent [49, 50]. The attenuation coefficient α is usually characterized by the following equation [51]:

$$\alpha =\frac{\sqrt{2}\pi f}{c}\times \sqrt{\left({\mu }^{{\prime}{\prime}}{\varepsilon }^{{\prime}{\prime}}-{\mu }^{\prime}{\varepsilon }^{\prime}\right)+\sqrt{{\left({\mu }^{{\prime}{\prime}}{\varepsilon }^{{\prime}{\prime}}-{\mu }^{\prime}{\varepsilon }^{\prime}\right)}^{2}+{\left({\mu }^{\prime}{\varepsilon }^{{\prime}{\prime}}-{\mu }^{{\prime}{\prime}}{\varepsilon }^{\prime}\right)}^{2}}}$$
(4)

The microstructure and topography of the absorber can trigger different degrees of electromagnetic wave reflection or scattering, resulting in different degrees of attenuation coefficients. It can be seen from Fig. 5b that the attenuation coefficient of 1/1 FeCoS2@CoS2 is the highest, not 2/1 FeCoS2@CoS2. However, according to the 3D images of RL values in Fig. 3, 2/1 FeCoS2@CoS2 has the best electromagnetic wave absorption performance. The above situation shows that the attenuation coefficient can affect the electromagnetic wave absorption performance, but the electromagnetic wave absorption performance is ultimately determined by the synergistic effect of multiple absorption mechanisms [48, 52, 53].

Debye dipole relaxation is also a factor that must be discussed that affects dielectric loss. Debye dipole relaxation is generally characterized by the following equation [54,55,56]:

$${\varepsilon }^{\prime}={\varepsilon }_{\infty }+\frac{{\varepsilon }_{s}\mathop{-}{\varepsilon }_{\infty }}{1\mathop{+}{\left(2\pi f\right)}^{2}{\tau }^{2}}$$
(5)
$${\varepsilon }^{\prime\prime}=\frac{2\pi f\tau \left({\varepsilon }_{s}\mathop{-}{\varepsilon }_{\infty }\right)}{1\mathop{+}{\left(2\pi f\right)}^{2}{\tau }^{2}}$$
(6)
$${\varepsilon }_{r}={\varepsilon }_{\infty }+\frac{{\varepsilon }_{s}\mathop{-}{\varepsilon }_{\infty }}{1\mathop{+}j2\pi f\tau }={\varepsilon }^{\prime}-j{\varepsilon }^{\prime\prime}$$
(7)

where \(f\), \(\tau\), \({\varepsilon }_{s}\) and \({\varepsilon }_{\infty }\) represent frequency, polarization relaxation time, static permittivity and high-frequency limit permittivity, respectively.

The Cole-Cole semicircle can reflect the Debye relaxation process and use it to express the relationship between \({\varepsilon }^{\prime}\) and \({\varepsilon }^{{\prime}{\prime}}\), which can be described by this equation [57, 58]:

$${\left({\varepsilon }^{\prime}\mathop{-}\frac{{\varepsilon }_{s}\mathop{+}{\varepsilon }_{\infty }}{2}\right)}^{2}+{\left({\varepsilon }^{{\prime}{\prime}}\right)}^{2}={\left(\frac{{\varepsilon }_{s}\mathop{-}{\varepsilon }_{\infty }}{2}\right)}^{2}$$
(8)

The Cole-Cole semicircle represents the Debye relaxation. As can be seen from Fig. 5c–f, semicircles appear in all the curves, which indicates the possible existence of Debye relaxation induced by defects, large specific surface area and heterointerfaces [59]. A long tail is seen from the 1/1 FeCoS2@CoS2 (Fig. 5c), 2/1 FeCoS2@CoS2 (Fig. 5d), 4/1 FeCoS2@CoS2 (Fig. 5e) and 8/1 FeCoS2@CoS2 (Fig. 5f) curve, again proving the existence of conduction losses [60,61,62].

Impedance matching characteristics are an important factor in studying and evaluating the absorption mechanism of materials. As mentioned above, excellent impedance matching ensures that electromagnetic waves are maximally allowed into the material rather than reflected off the surface. The standard for measuring the impedance matching of electromagnetic wave absorbing materials is \(Z=\left|{Z}_{in}/{Z}_{0}\right|\). The closer the \(Z\) value is to 1, the better the impedance matching effect of the material [63, 64]. From Fig. 6, it can be seen that the \(Z\) value corresponding to the minimum reflection loss of 2/1 FeCoS2@CoS2 is closest to 1 compared to other samples. From the above results, it is again proved that the impedance matching characteristics of 2/1 FeCoS2@CoS2 are extremely excellent.

Fig. 6
figure 6

Reflection loss and normal characteristic impedance Z of the as-synthesized composites (at 1.87 mm)

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The electromagnetic wave absorption mechanism of this material is shown in Fig. 7. In summary, the excellent electromagnetic wave absorption properties of the material mainly come from the synergistic effect of dielectric loss and magnetic loss. The detailed electromagnetic wave absorption mechanism is as follows: 1. The interface polarization triggered by multiple heterogeneous interfaces and large specific surface area greatly increases the dielectric loss, which leads to a strong electromagnetic wave attenuation ability. 2. The existence of the cavity structure fundamentally reduces the complex permittivity and greatly stabilizes the impedance matching, which means that electromagnetic waves can enter the material to the maximum extent instead of being reflected by the surface of the material. 3. High-temperature calcination can generate a large number of defects that cause dipole polarization and eventually trigger dielectric loss. 4. The surface heterobimetallic sulfide particles and the CoS2 hollow sea urchin form a three-dimensional conductive network, which significantly improves the conductivity and finally enhances the conductive loss strength of the material.

Fig. 7
figure 7

The EM wave absorption mechanism of FeCoS2@CoS2

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4 Conclusions

In conclusion, we prepared a novel electromagnetic wave absorbing FeCoS2@CoS2 composite by wet impregnation method and three calcinations. Co(CO3)0.5(OH)·0.11H2O hollow sea urchin spheres were first prepared by soft template method, and then calcined twice to obtain CoS2 with no obvious change in morphology. Then, FeCoS2@CoS2 composites were obtained by wet impregnation on CoS2 hollow sea urchin spheres, sulfided heterobimetallic, and calcined. Under the coordination of multiple absorption mechanisms, the composite exhibits remarkable electromagnetic wave absorption performance at a thickness of 1.97 mm at 16.88 GHz with an RLmin value of -75.23 dB. Furthermore, a maximum EAB of 5.76 GHz is achieved at 2.12 mm. The reason for the excellent electromagnetic wave absorption ability may be caused by the synergistic effect of the dielectric loss and magnetic loss of the material, as well as the conductance loss and multiple refraction. The innovative ideas and remarkable achievements of this research provide a feasible strategy for the construction and development of heterobimetallic sulfide electromagnetic wave absorbing materials.