Building the conformal protection of VB-group VS 2 laminated heterostructure based on biomass-derived carbon for excellent broadband electromagnetic waves absorption

doi:10.21203/rs.3.rs-2071570/v1

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Building the conformal protection of VB-group VS 2 laminated heterostructure based on biomass-derived carbon for excellent broadband electromagnetic waves absorption

https://doi.org/10.21203/rs.3.rs-2071570/v1

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Although VB-Group transition metal disulfides (TMDs) VS2 nanomaterials with specific electronic properties and multiphase microstructures have shown fascinating potential in the field of electromagnetic wave (EMW) absorption, the efficient utilization of VS2 is limited by their environmental instability and sole electromagnetic (EM) loss mechanism. In order to fully exploit the maximal utilization values of VS2 nanomaterials for EMW absorption through mitigating the chemical instability and optimizing the EM parameters, biomass-based glucose derived carbon (GDC) like sugar-coating has been decorated on the surface of stacked VS2 nanosheets via a facile hydrothermal method, followed by high-temperature carbonization. As a result, the modulation of doping amount of glucose injection solution (Glucose) could effectively manipulate the degree of GDC coating on VS2 nanosheets, further implementing the EM response mechanisms of the VS2/GDC hybrids through regulating the conductivity and constructing multi-interface heterostructures, as reflected by the enhanced EMW absorption performance to a great extent. The minimum reflection loss of VS2/GDC hybrids could reach -52.8 dB with a thickness of 2.7 mm at 12.2 GHz. Surprisingly, compared with pristine VS2, the VS2/GDC hybrids have the effective absorption bandwidth (EAB) increased from 2.0 to 5.7 GHz, while their environmental stability is much enhanced because of GDC. Obviously, this work provides a promising candidate to realize frequency band tunability of EMW absorbers with exceptional performance and environmental stability.

VS2/GDC hybrids

multi-interface heterostructures

broadband absorption

environmental stability

The exposure of human being to electromagnetic (EM) irradiation resulting from the extensive utilization of EM devices is undesirable, since the interaction between electromagnetic waves (EMWs) and human body resulting from the transduction of EM energy is harmful to human health, which also interferes with the normal operation of electronic equipment. Thus, it is essential that the transduction of EM energy is safe to human society and is environmental-friendly [1-3]. Recently, the VIB-group two-dimensional (2D) transition metal disulfides (TMDs) such as MoS₂ [4-6] and WS₂ [7, 8] are of much interest for their applications in EMW absorption due to their peculiar electronic structures, large specific surface area, multiphase microstructures with the coexistence of 1T, 2H and 3R phases [9, 10] and abundant active sites. With the rapid development of electronic technology and increasing demand on EMW absorption, the VIB-group TMDs have exhibited some disadvantages in the field of EMW absorption, such as the narrow effective absorption bandwidth (EAB) that is caused by the finite active sites located only at the edges of their microstructures, and the difficulty in achieving appropriate impedance matching because of the limitations of their energy band structures [7, 8]. Therefore, extensive efforts have been devoted to the design of new-generation EMW absorbers with indispensable capability of EM energy transduction.

The VB-group TMDs VS₂ nanomaterials are novel EMW absorbing materials, which can be modulated to become either semiconductors or metallic materials through altering their crystal structures or the number of stacked layers. Particularly, VS₂ nanomaterials can be prepared with well-tuned synthesis routes to exhibit electrochemical active sites at the edges of their microstructures and inside their stacked layers, which could break the aforementioned bottleneck of VIB-group TMDs (such as WS₂ or MoS₂) for EMW absorption applications [11]. Zhang et al. have synthesized the coaxial stacking VS₂ nanosheets via a facile one-step hydrothermal route, which have the minimum reflection loss (RL_min) at 4 GHz as low as -57.0 dB [12], while other reported TMDs have to incorporate heterogeneous EMW absorbing materials with multiple loss mechanisms to achieve such exceptional EMW absorption performance. In order to further enhance the EMW absorption performance of VS₂ nanosheets which exhibit narrow bandwidth and chemical instability under an oxidation environment, it is an effective strategy that multi-components could be introduced into the VS₂ nanosheets to construct multi-interface heterostructures and protective layers of the nanosheets [13-16].

Carbon nanomaterials, such as graphene and CNTs, have become typical representatives of EMW absorbers operated in the high-frequency range due to their high conductivity, nanostructures with large specific surface area and abundant defects as well as the facilely tuned dielectric properties and impedance matching [17-20]. In most cases, the combination of dielectric-loss materials and carbon-based materials is an effective method in constructing EM attenuation systems with high performance, which can be further enhanced by defect engineering and tuning 3D conductive networks in the systems [21]. The multiple loss mechanisms, such as multiple internal reflections and scattering, dipole polarization caused by abundant functional groups and defects, as well as interfacial polarization presented at multi-interface heterostructures, could be essential for the attenuation of EM energy [22]. Although the carbon nanomaterials as represented by graphene and CNTs are expected to regulate the EMW absorption properties of VS₂ nanomaterials by virtue of their well-known outstanding environmental stability and dielectric properties, they can only form protective layers with porous or reticular structures onto the VS₂ nanomaterials, which may not fully encapsule VS₂ nanomaterials to improve their environmental and chemical stability. Being alternatives to conventional carbon-based materials, the environmental-friendly biomass derived carbon-based materials are considered to be promising resources in facilely synthesizing environmental-friendly and efficient EMW absorbers [23, 24]. In addition to the advantages of conventional carbon-based materials such as low density, high conductivity and high dielectric loss, they can be used as natural templates for the preparation of EMW absorbers with low cost, simple preparation process and good EMW absorption performance [25-27]. Furthermore, some laminated carbon materials, which are derived mostly from biomass, can be uniformly coated on the surface of VS₂ nanomaterials by combining solution coating with high-temperature carbonization. Dong et al. successfully prepared the core-shell SiC_w@C heterostructures with thickness-dependent EMW absorption in X-band and Ku-band by combining a hydrothermal synthesis route and carbonization processes, with glucose being the carbon precursor [28]. The RL_min value was as low as -61.2 dB with EAB located in the whole X-band and Ku-band, which was attributed to the dipole polarization, interfacial polarization, relaxation polarization and the matched impedance in the systems. The coating layer of carbon materials not only enhanced the environmental and chemical stability of the EMW absorbing materials, but also constructed a large number of multi-interface heterostructures, leading to a promising candidate for the development of efficient and stable EMW absorbing materials.

In this work, VS₂/glucose-derived carbon (GDC) hybrids were successfully prepared via a simple hydrothermal method followed by high-temperature carbonization, using glucose as carbon precursor. The prominent EMW absorption performance of VS₂/GDC hybrids, which could be optimized by the content of GDC doping, has been systematically investigated. Furthermore, the microstructures and morphological characteristics of the target EMW absorbing materials are investigated. The EMW absorption mechanisms are analyzed based on the EM properties, impedance matching and attenuation constant of the EMW absorbing materials. It is revealed that GDC can tune the microstructure of VS₂ nanomaterials and the VS₂/GDC hybrids show superior EMW absorption performances with strong EMW absorption, wide EAB at a small thickness and a lower content of glucose. Undoubtedly, this work could open a new avenue for the development of TMDs/biomass-based EMW absorbing materials with low cost, environmental friendliness and excellent EMW absorption performance.

2.1 Materials

Ammonium metavanadate (NH₄VO₃) was purchased from China Pharmaceutical Shanghai Chemical Reagent Company. Thioacetamide (CH₃CSNH₂) was purchased from Tianjin Guangfu Fine Co. Ltd.. Polyvinylpyrrolidone (PVP) was purchased from Xianshuigu Industrial Park, Jinan District, Tianjin. Ammonia (NH₃·H₂O) was purchased from Tianjin Kaitong Chemical Reagent Co. Ltd. The aforemntioned raw materials were of analytical grade and employed without further purification. Glucose injection solution (Glucose) was purchased from Henan Kelun Pharmaceutical Co. Ltd. (50% concentration), which could be used directly in experiments.

2.2 Synthesis of VS₂ nanosheets

Firstly, 1.3104 g of NH₄VO₃, 15.288 g of NH₃H₂O, 4.2056 g of CH₃CSNH₂ and 2.800 g of PVP were dissolved in 84 ml of deionized water under continuously stirring for 1 h to obtain a homogeneous solution. Then, the solution was transferred into a 140 ml polytetrafluoroethylene (PTFE) reactor and sealed in a stainless steel hydrothermal vessel that was heated at 190 ^oC for 12 h. After the reaction, the reactor was cooled to room temperature, and the precipitate was collected after centrifugation, washing by deionized water and absolute ethanol for several times, and drying in vacuum at 60 ^oC for 12 h. The obtained samples were denoted as VS₂ nanosheets.

2.3 Synthesis of GDC

The evenly stirred solution (69 ml of deionized water and 15 ml of Glucose was transferred into a stainless steel hydrothermal vessel with a 140 ml PTFE reactor, which was heated at 190 ^oC for 12 h. After the reaction, the reactor was cooled to room temperature, the precipitate was collected after centrifugation, washing by deionized water and absolute ethanol for several times, and drying in vacuum at 60 ^oC for 12 h. The dried product was transferred into a tube furnace and heated at 700 ^oC for 2 h (with a heating rate of 5 ^oC/min) under nitrogen atmosphere. Finally, the product was cooled to room temperature, which was denoted as GDC-700 ^oC.

2.4 Synthesis of the VS₂/GDC hybrids

The as-prepared VS₂ (1 g in mass), 69 ml of deionized water and 15 ml of Glucose were stirred evenly. Then, the mixture was transferred into a 140 ml polytetrafluoroethylene (PTFE) reactor, which was sealed in a stainless steel hydrothermal vessel for the reaction to occur at 190 ^oC for 12 h. After the reaction, the reactor was cooled to room temperature, and the precipitate was collected after centrifugation, washing by deionized water and absolute ethanol for several times, and drying in vacuum at 60 ^oC for 12 h. The dried product was transferred into a tube furnace and heated to 700 ^oC under nitrogen atmosphere with a heating rate of 5 ^oC/min. The reaction lasted for 2 h and the reactor was then cooled to room temperature. The obtained sample was denoted as VS₂/15GDC-700 ^oC. The total volume of the added deionized water and Glucose was 84 ml, and the volume of Glucose was 10 ml and 20 ml, corresponding to the samples denoted as VS₂/10GDC-700 ^oC and VS₂/20GDC-700 ^oC, respectively.

2.5 Characterization

The X-ray diffraction (XRD) patterns were recorded using Bruker-AXS D8 X-ray diffractometer with Cu Kα radiation (λ = 0.1546 nm). The XPS analysis was carried out on X-ray photoelectron spectrometer (ESCALAB250Xi, Thermofisher Co). The morphology and crystal structure of the as-synthesized products were characterized by scanning electron microscope (SEM, S-4300; Hitachi) with an accelerating voltage of 20 kV and transmission electron microscopy (TEM, Hitachi, H-7650). The electromagnetic properties of the samples were determined by the coaxial method with a vector network analyzer (VNA, MS-200644A Anritsu) in the frequency range of 2–18 GHz. The samples containing 60 wt. % paraffin wax were prepared.

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 2p_3/2 and V 2p_1/2 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 2p_3/2 and S 2p_1/2). 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]:

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

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

where Z_in is the input impedance of the absorber, Z₀ is the impedance of free space, ε_r is the complex permeability, µ_r 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 ε_r (ε_r = ε' - jε") and µ_r (µ_r = 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 tanδ_ε (tanδ_ε = ε" / ε') relates to the degree of dielectric loss, and ε_r is explained by the Debye theory [33–35]:

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

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

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

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

where ε_∞ is the relative dielectric constant at infinite frequency, ε_s 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:

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

Figure 4 displays the frequency-dependent ε', ε" and tanδ_ε 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, tanδ_ε 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 (M_Z) 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 M_Z, as follows [36, 37]:

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

where Z' is the real part of normalized input impedance. In ordinary circumstances, the value of M_Z 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]:

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

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, M_Z, α and the contour map for the corresponding 3D RL plots for VS₂ and GDC. As shown in Figure 5a-c, RL_min 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, M_Z with a thickness of 5.50 mm is also much close to 1 corresponding to the value of RL_min, and α reaches the maximum value (α_max) of 155.5. Figure 5 The (a) RL, (b) contour map for the corresponding 3D RL plots, (c) M_Z and α of VS₂; The (d) RL, (e) contour map for the corresponding 3D RL plots, (f) M_Z 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 RL_min value of GDC-700 ^oC is -12.3 dB at a thickness of 5.50 mm at 18.0 GHz corresponding to the M_Z value lower than 1, and the α_max 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 RL_min 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 M_Z is close to 1, α_max reaches 187.2, which proves that VS₂/15GDC-700 ^oC has good impedance matching and strong EM attenuation. The RL_min 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 RL_min 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 RL_min is about -16.5 dB at 18.0 GHz corresponding to the M_Z value with a thickness of 5.50 mm, and α_max 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 RL_min of VS₂/15GDC-700 ^oC demonstrate that excellent EMW absorption performance can be achieved by the synergistic effect of various factors that adjust the M_Z 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.

In summary, the VS₂/GDC hybrids are successfully prepared by a facile hydrothermal method followed by high-temperature carbonization processes, which are EMW adsorbing materials with a wide EAB. The encapsulation of the stacked VS₂ nanomaterials by GDC can be modulated through controlling the doping amount of Glucose, optimizing the conductivity of the EMW absorption system and the construction of multi-interface VS₂/GDC heterostructures that are beneficial for the environmental stability and EMW absorption performance of the system. Because of the coupling effect of impedance matching and attenuation constant, the RL_min value of VS₂/15GDC-700 ^oC is -52.8 dB with a thickness of 2.7 mm at a higher frequency of 12.2 GHz, which has an EAB of 5.7 GHz at a thickness of 2.5 mm, in comparison with those of pristine VS₂ nanomaterials. The coating of sugar-like biomass-derived GDC on the surface of stacked VS₂ nanosheets is assisted by the well-tuned microstructures of VS₂/GDC hybrids and defect engineering, which could enhance the conductive loss, dipole polarization, interface polarization, multiple reflections and scattering in the EMW absorbing materials. This work provides an in-depth understanding on the EMW absorption mechanisms of VB-group TMDs and opens a new avenue for the development of TMDs/biomass-based EMW absorbing materials with low cost, environmental friendliness and excellent EMW absorption performance.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (52102368, 52072192 and 51977009), Regional Joint Fund for Basic Research and Applied Basic Research of Guangdong Province (No. 2020SA001515110905)

Electronic Supplementary Material: Figure S1 (a) XPS wide scan (survey) and (b) high-resolution narrow scans on S in VS₂/GDC hybrids.

Conflicts of interest

The authors declare no conflict of interest. H. H. Wang and H. B. Zhang contributed the central idea, analyzed most of the data, and wrote the initial draft of the paper. H. H. Wang, H. B. Zhang, and T. T. Liu performed the research and carried out additional analyses. H. H. Wang and H. B. Zhang performed the characterization experiments for the samples and analyzed the data. Prof. J. Y. Cheng, Prof. S. R. Zhai, Prof. D. Q. Zhang, Prof. G. P. Zheng, and Prof. M. S. Cao contributed to refining the ideas and finalizing this paper. All the authors discussed the results and commented on the manuscript. The authors declare no competing financial interests.

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No competing interests reported.

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Building the conformal protection of VB-group VS 2 laminated heterostructure based on biomass-derived carbon for excellent broadband electromagnetic waves absorption

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental

2.1 Materials

2.2 Synthesis of VS₂ nanosheets

2.3 Synthesis of GDC

2.4 Synthesis of the VS₂/GDC hybrids

2.5 Characterization

3. Results And Discussion

4. Conclusions

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1

Building the conformal protection of VB-group VS 2 laminated heterostructure based on biomass-derived carbon for excellent broadband electromagnetic waves absorption

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental

2.1 Materials

2.2 Synthesis of VS2 nanosheets

2.3 Synthesis of GDC

2.4 Synthesis of the VS2/GDC hybrids

2.5 Characterization

3. Results And Discussion

4. Conclusions

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1

2.2 Synthesis of VS₂ nanosheets

2.4 Synthesis of the VS₂/GDC hybrids