The preparation process of VS2/GDC hybrids is schematically illustrated in Fig. 1, which includes the pre-preparation of stacked VS2 nanosheets and the subsequent synthesis of VS2/GDC hybrids. Briefly, NH4VO3 and CH3CSNH2 provide the sources of V and S elements for the preparation of stacked VS2 nanosheets in an alkaline environment as supplied by NH3·H2O, and PVP is selected as a surfactant. V4+ and S2− are combined by covalent bonds to form multilayer stacked hexagonal nanosheets with varying sizes as peeled by NH3·H2O, which are attributed to NH3 and NH4+ decomposed from NH3·H2O to weaken the agglomeration of VS2 nanosheets. Subsequently, the Glucose could be easily carbonized to the carbon shell like sugar coating under nitrogen atmosphere and form the VS2/GDC multi-interface heterostructures, which is conducive to providing a stable chemical environment for VS2 and enhancing the EMW attenuation through effective multiple loss mechanisms such as conductive loss, dipole polarization, interface polarization, multiple reflections and scattering. As a result, the 3D conductive network can be controlled by the amount of Glucose doping, carbonization temperature and carbonization time, which could promote the strong coupling effect between impedance matching and attenuation constants.

The XRD patterns of VS2, GDC-700 oC, VS2/15GDC-700 oC are presented to demonstrate the evolutions of phases of the hybrids after annealed at 700 oC for 2 h under nitrogen atmosphere, as shown in Fig. 2a. The diffraction patterns of VS2 have four obvious broad peaks located at 15.0°, 32.3°, 45.1°, 57.0°, corresponding to the (001), (100), (012) and (110) planes of crystal structure of VS2 (PDF#89-1640, P-3m1-164 lattice group), respectively [12]. There are several weak impurity peaks as observed in the patterns, confirming that the VS2 nanomaterials have good crystallinity. After the pyrolysis of glucose at 700 °C, the peak located at 20.0° can be well indexed to (002) plane of typical incompletely carbonized carbon which not only preserves the carbon skeleton but also introduces a moderate amount of carbon into the EMW absorption system to regulate the impedance matching. The peaks in the XRD patterns of VS2/15GDC-700 oC are similar to those of VS2 except the (011) peak. Since the TMD hybrid contains a large number of 1T phase, the shifts of peaks in the XRD patterns of VS2/15GDC-700 oC are due to the transition from the 1T phase to the 2H phase of VS2 [29]. Unfortunately, the peak of carbon is not visible in the XRD patterns of VS2/15GDC-700 oC, which is ascribed to the relatively low carbon contents that can ensure a suitable impedance matching for VS2/15GDC-700 oC. As shown in Fig. 2b, combined with Raman spectra of VS2/15GDC-700 oC, we can determine the existence of GDC and graphitization degree, which affect the micro morphology of porous structure conducive to EMW multiple scattering and reflection.

To further demonstrate the formation of effective multi-interface heterostructures between VS2 and GDC, the surface chemical compositions and electronic state of each element of VS2/15GDC-700 oC are determined by XPS analysis. As shown in Fig. 2c, the survey spectra of VS2/15GDC-700 oC indicate different chemical environments of constituent elements (S, C, V, O) observed at 163.4, 285.8, 516.7, 530.6 eV, respectively. The oxidation state of V is characterized in Fig. 2d. The coexistence of V 2p3/2 and V 2p1/2 in the VS2 crystal is evident by the distinctive main peaks located at 516.1 and 523.5 eV. In Fig. 2e, the S 2p spectrum exhibits two contributions (S 2p3/2 and S 2p1/2). The S-O peak located at a lower binding energy is attributed to the oxidized surface of VS2, which also confirms the presence of O element. Especially, the ratio of S-O peak area of VS2/15GDC-700 oC is significantly less than that of untreated VS2 (Figure S1), demonstrating that the fully encapsulated GDC provides a stable chemical environment for the system through conformal protection of VS2. The C 1s spectrum of VS2/15GDC-700 oC shown in Fig. 2f is mainly attributed to glucose treated by carbonation at 700 oC, where the peak at 284.3 eV could correspond to sp3-hybridized C (C–C). The XPS results strongly suggest that VS2 coexists with GDC within the EMW absorbing materials after the pyrolysis reaction, effectively forming a heterostructure of VS2/GDC.

The microstructure and morphology of VS2 nanosheets and VS2/15GDC-700 oC are further analyzed by SEM and TEM, as shown in Fig. 3. Figure 3a-b indicates that VS2 are composed of hexagonal nanosheets with different sizes, and the averaged diameter of VS2 nanosheets is about 1 µm. The rough surface of VS2 nanosheets has different degrees of protrusion, which is conducive to their combination with GDC. The microstructure of VS2 nanosheets consists of 1T phase (green circles) and 2H phase (yellow triangles), which can be associated with Fig. 3c. Figure 3d-e shows that with the carbonization of Glucose, the carbon layers with different thicknesses are coated on the surface of VS2 nanosheets, forming the sugar-like coating which can provide a stable chemical environment for VS2, and the tightly packed carbon layers could prevent VS2 nanosheets from being oxidized. It is suggested that the addition of GDC can maintain the original morphology of VS2 nanosheets to a large extent, and can facilitate the construction of multi-interface heterostructures with VS2 nanosheets.

In general, the EMW absorption performance can be evaluated by reflection loss (RL), according to the transmit-line theory, which can be expressed by the following equations [30–32]:

$$RL=20{\text{l}\text{o}\text{g}}_{10}\frac{\left|{Z}_{in}-{Z}_{0}\right|}{\left|{Z}_{in}+{Z}_{0}\right|}$$

1

$${Z}_{in}={Z}_{0}\sqrt{\frac{{\mu }_{r}}{{\epsilon }_{r}}}\text{tan}h\left[j\frac{2\pi fd}{c}\sqrt{{\mu }_{r}{\epsilon }_{r}}\right]$$

2

where Zin is the input impedance of the absorber, Z0 is the impedance of free space, εr is the complex permeability, µr is the permittivity, f represents the EMW frequency, d is the thickness of the absorber, c is the velocity of light in free space (3×108 m·s− 1). Obviously, the EM parameters εr (εr = ε' - jε") and µr (µr = µ' - jµ") are the main factors in evaluating the EMW absorption performance. To obtain the EM parameters for all samples, we tested the samples containing 60 wt.% paraffin wax by using an Anritsu MS4644A vector network analyzer in the frequency range of 2.0–18.0 GHz. For non-magnetic materials such as VS2 and GDC, µ' and µ" are 1 and 0, respectively, meaning that the negligible magnetic loss does not play a dominant role in the EMW absorbing system. Since the real part (ɛ') of the relative complex permittivity represents the electric energy storage, while the imaginary part (ɛ'') is associated with the capacity of electric energy loss, for the dielectric-based absorbers, the dielectric loss tangent tanδε (tanδε = ε" / ε') relates to the degree of dielectric loss, and εr is explained by the Debye theory [33–35]:

$${\epsilon }_{r}={\epsilon }^{{\prime }}-j{\epsilon }^{{\prime \prime }}={\epsilon }_{\infty }+\frac{{\epsilon }_{s}-{\epsilon }_{\infty }}{1+j2\pi f\tau }$$

3

where ɛ' and ɛ'' can be expressed by the following equations:

$${\epsilon }^{{\prime }}={\epsilon }_{\infty }+\frac{{\epsilon }_{s}-{\epsilon }_{\infty }}{1+{\omega }^{2}{\tau }^{2}}$$

4

$${\epsilon }^{{\prime }{\prime }}=\frac{{\epsilon }_{s}-{\epsilon }_{\infty }}{1+{\omega }^{2}{\tau }^{2}}\omega \tau +\frac{\sigma }{\omega {\epsilon }_{0}}$$

5

where ε∞ is the relative dielectric constant at infinite frequency, εs is the static permittivity, ω is the angular frequency, τ is the polarization relaxation time, σ is the electrical conductivity, and ε0 (ε0 = 8.854×10− 12 F·m− 1) is the vacuum dielectric constant. Generally, the relationship between ɛ' and ɛ'' can be inferred as follow:

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

6

Figure 4 displays the frequency-dependent ε', ε" and tanδε for VS2, GDC-700 oC, VS2/GDC-700 oC hybrids at 2-18 GHz.

As shown in Fig. 4, ε' and ε" decrease with increasing frequency, which could be caused by the increase in ω due to the polarization relaxation in the low-frequency range. Compared with that of VS2, ε' of VS2/GDC-700 oC hybrids fluctuates with a slight decrease, while ε' is decreased by a large margin with increasing content of GDC-700 oC and is generally higher than that of GDC, as shown in Fig. 4a. As shown in Fig. 4, GDC encapsulates the surface of VS2, where GDC has a low value of ɛ' and VS2 has a high value of ɛ'. As a consequence, higher values of ɛ' for the hybrids could be inhibited with increasing amount of GDC added in the hybrids. Such effect of GDC on ɛ' of hybrids is more significant when the amount of GDC is high. As shown in Fig. 4b, VS2, VS2/10GDC-700 oC and VS2/15GDC-700 oC have significantly higher ε" values than GDC-700 oC and VS2/20GDC-700 oC, which are strongly dependent on the amount of GDC. It is known that ɛ'' is determined by the relaxation loss and conductivity. The excessive doping of GDC leads to a large increase in the conductivity of the hybrid, which is not conducive to the effective absorption of EMW since high conductivity of system could cause the reflection of incident EMWs into free space. On the contrary, a moderate amount of GDC provides numerous conductivity paths for electrons hopping, enhancing the conductive loss. Furthermore, a large number of defects on the surface of GDC in VS2/15GDC-700 oC will promote the interfacial polarization, relaxation loss and scattering response, which facilitate the dipole polarization. Clearly, as shown in Fig. 4c, tanδε of VS2/15GDC-700 oC is the highest among the samples, suggesting that the introduction of GDC in the hybrid would enhance the dielectric loss of VS2/15GDC-700 oC. In addition, multiple interfacial polarizations in VS2/15GDC-700 oC are beneficial for the enhanced EMW absorption performance. As described by Eq. (6), the plots of ε' versus ε" should be a semicircle corresponding to a Debye relaxation process, which is called the Cole-Cole semicircle. The Cole-Cole semicircles for all samples are shown in Fig. 4d. The VS2, VS2/10GDC-700 oC and VS2/15GDC-700 oC samples show the semicircles in larger ranges of the plots than GDC-700 oC and VS2/20GDC-700 oC samples, which could correspond to more significant polarization relaxation in the samples. The results demonstrate that the appropriate amount of GDC can promote the occurrence of multiple dielectric polarization relaxation processes in the samples. When the content of GDC is increased, the mechanism of relaxation loss in the pristine VS2 system could be changed to that of conductive loss in the multi-component hybrid system, confirming our view that the conductive loss is an essential factor in improving the EMW absorption performance.

The impedance matching (MZ) and attenuation constant (α) are two important parameters that determine the EMW absorption performance of absorbers. To achieve zero reflection on the absorbers’ surfaces, the characteristic impedance of the absorbers needs to be equal or close to that of the free space. The degree of impedance matching can be validated by the measure of MZ, as follows [36, 37]:

$${M}_{Z}=\frac{2{Z}_{in}^{{\prime }}}{{{|Z}_{in}|}^{2}+1}$$

7

where Z' is the real part of normalized input impedance. In ordinary circumstances, the value of MZ tends to be close to 1 for the absorbers with good impedance matching, which means that the incident EMW enters into the absorber as much as possible. The second important factor that measures the EMW absorption performance of absorbers is the attenuation constant α, which can be expressed by the following equation [38, 39]:

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

8

The electromagnetic attenuation capacity of the absorbers is determined by the conductive loss, dielectric loss and relaxation loss. The higher the value of α, the stronger the ability to attenuate the EMW inside the absorbers.

Figure 5 shows RL, MZ, α and the contour map for the corresponding 3D RL plots for VS2 and GDC. As shown in Figure 5a-c, RLmin of VS2 is about -34.0 dB with a thickness of 5.50 mm at a lower frequency of 4.2 GHz, which has an effective bandwidth of 2.0 GHz at a thickness of 3.0 mm. At the same time, MZ with a thickness of 5.50 mm is also much close to 1 corresponding to the value of RLmin, and α reaches the maximum value (αmax) of 155.5. Figure 5 The (a) RL, (b) contour map for the corresponding 3D RL plots, (c) MZ and α of VS2; The (d) RL, (e) contour map for the corresponding 3D RL plots, (f) MZ and α of GDC-700 oC.

The VS2 sample with better EMW absorption performance has a thicker thickness and narrower effective bandwidth due to its low conductivity and sole absorption mechanism. To improve the EMW absorption performance of VS2, GDC is introduced into VS2 to improve its conductivity loss and dielectric loss. In order to analyze the changes in the absorbers before and after the addition of GDC, GDC-700 oC is chosen as a reference in comparative studies. As shown in Fig. 5d-f, GDC-700 oC at all thicknesses except for 5.50 mm do not show effective EMW absorption. The RLmin value of GDC-700 oC is -12.3 dB at a thickness of 5.50 mm at 18.0 GHz corresponding to the MZ value lower than 1, and the αmax value is 58.3. The GDC-700 oC exhibits the worst EMW absorption performance, which is attributed to the high electrical conductivity of the single carbon component that causes the reflection of most of the incident EMWs into free space, and a small amount of EMWs entering the absorber cannot be absorbed effectively.

As shown in Fig. 6a-c, with the introduction of 10 ml glucose, the EMW absorption performance of VS2/10GDC-700 oC has been slightly improved. Compared to that of VS2, the RLmin value of VS2/10GDC-700 oC reaches -37.4 dB at 4.9 GHz, which has an EAB of 3.7 GHz at a thickness of 2.0 mm. However, there is no obvious fluctuation in thickness and frequency band, suggesting that the addition of GDC with a low content does not influence the EMW absorption system. When the content of glucose reaches 15 ml in the system, VS2/15GDC-700 oC shows excellent EMW absorption performance with almost all sample thicknesses, as shown in Fig. 6d-f. When MZ is close to 1, αmax reaches 187.2, which proves that VS2/15GDC-700 oC has good impedance matching and strong EM attenuation. The RLmin value of VS2/15GDC-700 oC is -34.4 dB with a thickness of 2.50 mm at a higher frequency of 13.3 GHz, which has an EAB of 5.7 GHz at a thickness of 2.5 mm. The results thus demonstrate that appropriate amount of GDC is effective in adjusting the impedance matching of the absorber and improving the attenuation of EMWs, especially in regulating the thickness and frequency band of the hybrid. In order to further explore the EMW absorption performance of VS2/15GDC-700 oC, the EMW absorption performance of VS2/15GDC-700 oC with a thickness of 2.50-2.95 mm is analyzed in detail, as shown in Fig. 6j-k. The RLmin value of VS2/15GDC-700 oC reaches -52.8 dB with a thickness of 2.70 mm at 12.2 GHz. It can be observed that VS2/15GDC-700 oC achieves the best EMW absorption performance that could be ascribed to the synergistic effect of various factors. The multi-component EMW absorption mechanisms are also attributed to the multi-interface heterostructures of the system, which is conducive to the multiple reflection and scattering of EMWs. As shown in Fig. 6g-i, when the amount of glucose in the system is increased to 20 ml, the EMW absorption of VS2/20GDC-700 oC decreases sharply, which is similar to that of GDC-700 oC. The VS2/20GDC-700 oC sample can absorb EMWs effectively only with the thickness of 5.50 mm. Its RLmin is about -16.5 dB at 18.0 GHz corresponding to the MZ value with a thickness of 5.50 mm, and αmax does not exceed 75.7. When a large number of GDC with high conductivity are coated on the surface of VS2, the good EMW absorption performance of VS2 could be inhibited to exhibit a high value of α. Moreover, when the multi-interface structure of the coaxial stacking VS2 is masked by the coating of GDC, the interfacial polarization and multiple reflection could be suppressed. As a result, the highest values of EAB and RLmin of VS2/15GDC-700 oC demonstrate that excellent EMW absorption performance can be achieved by the synergistic effect of various factors that adjust the MZ and α values of the systems with strong EMW absorption, wide bandwidth and thin thickness, as shown in Fig. 6l.

The EMW absorption mechanisms of VS2/GDC hybrids are illustrated in Fig. 7. The VB-group TMDs VS2 nanomaterials with GDC coating break the barrier in EMW absorption that previously reported EMW absorbing materials have narrow EAB and rigorous requirements on EM parameters for the continuously tunable frequencies, which are attributed to the synergistic effect of multiple loss mechanisms. First, the abundant active sites located at the edges of microstructures and inside the VS2 nanomaterials promote the occurrence of dipole polarization under the alternating EM fields. Meanwhile, the GDC coating as obtained by high-temperature carbonization accelerates the construction of defect structures, which could enhance EMW absorption by coupling with the polarization loss. Second, lattice distortions and vacancies could occur in the microstructures of stacked VS2 nanomaterials as prepared by a well-tuned synthesis route, providing abundant defects for the EMW attenuation and EM energy conversion. Third, the multi-interface heterostructures are constructed by virtue of the full encapsulation of VS2 nanomaterials with GDC, which could result in the rearrangement of local charges between the VS2 nanomaterials and GDC coating with different conductivities, and thus the interface polarization. Fourth, the intervention of GDC adjusts the conductivity of the EM attenuation system, which can not only optimize the impedance matching for the EMWs that enter the absorbers as much as possible, but also generate plenty of freely moving electrons hopping between different energy levels of VS2/GDC heterostructures in the system, leading to the enhanced conductive loss. Last but not least, the multi-interface VS2/GDC heterostructure and uniform distribution of VS2 nanosheets with hexagonal stacking structure can also provide multiple reflection and scattering sites for the EMW absorption, ensuring that the EMWs can be effectively absorbed. To sum up, the excellent EMW absorption performance of VS2/GDC hybrids with strong absorption, wide EAB, thin thickness could be made possible by the synergistic effect of the above-discussed factors [40–42], which undoubtedly contribute to the in-depth understanding of EMW absorption mechanisms and the development of VB-group TMDs nanomaterials in the field of EMW absorption.