Study on the Absorbing Properties of V-Doped MoS₂

Abstract

This study employed a hydrothermal method to prepare V-doped MoS₂. The influence of varying filler ratios (30 wt%, 40 wt%, 50 wt%) on its absorption properties was analyzed. For annealing studies, a precursor powder with a 40 wt% filler ratio was heat-treated at 600 °C for 2 h. The results obtained through characterization and testing indicate that the unannealed 40 wt% filler sample demonstrates superior absorption performance, with minimum reflection loss (RL_min) of −32.24 dB, an effective absorption bandwidth (EAB) of 4.40 GHz, and 99.9% electromagnetic (EM) wave attenuation. However, upon subjecting the sample with a 40 wt% filling ratio to annealing treatment, a notable decrease in impedance matching degree was observed, and regions with impedance matching values close to 1 were no longer present. Consequently, it can be concluded that at a filling ratio of 40 wt%, the sample’s excellent attenuation coefficient in conjunction with its good impedance matching collectively contribute to its superior comprehensive absorption performance.

1. Introduction

Wireless communication technology has been extensively applied across various domains, including the Internet of Things, mobile communication, intelligent transportation, medical health, satellite communication, and quantum technology. Concurrently, EM pollution has emerged as a novel form of environmental contamination. Its adverse effects encompass interference with communication signals, potential hazards to human health, and disruption of military applications. The rapid advancement of 5G communication, coupled with the widespread adoption of portable smart devices such as mobile phones, computers, and wearable devices like bracelets, has significantly enhanced the convenience of modern life. However, the resultant EM radiation poses substantial risks. It not only presents harm to physical health but also interferes with the operation of precision instruments [1,2,3]. To address the growing issue of EM pollution, the development of high-performance EM microwave absorbing materials (MAMs) has emerged as a key research focus [4,5]. Molybdenum disulfide (MoS₂), a metal sulfide, is a popular candidate for the study of MAMs due to its 2D layered structure and excellent electrical properties [6,7,8]. However, the absorption performance of pure MoS₂ remains constrained in terms of broadband absorption and high absorption efficiency, primarily due to its poor impedance matching and insufficient dielectric loss. In recent years, researchers have significantly enhanced the EM wave absorption performance of MoS₂ through morphology control and composite strategies [9,10]. For example, Zhang et al. fabricated two-dimensional MoS₂/graphene hybrid nanosheets with isomorphic heterostructures through liquid-phase exfoliation and hydrothermal synthesis. By regulating the ratio of MoS₂/graphene, optimized sample achieves an EAB of 5.6 GHz at 2.2 mm thickness [11]. Qi and his group developed hollow carbon spheres@MoS₂ composites via template etching-hydrothermal methods. The introduced MoS₂ nanosheets significantly improved interfacial polarization loss ability of hollow carbon spheres@MoS₂ composites, thereby enhancing the overall EM wave absorption performance [12]. However, morphology control and composite strategies often involve complex synthesis procedures and high costs, which hinder the large-scale application of MAMs.

Doping is an effective strategy for tuning the physicochemical properties of materials. In MAMs, elemental doping can alter charge distribution, modulate electrical conductivity, and introduce additional polarization centers, thereby simultaneously enhancing dielectric loss and improving impedance matching [13,14]. For MoS₂, vanadium (V) is considered an ideal doping element due to its unique 3D orbital configuration and multivalent nature. On the one hand, the ionic radius of V is similar to that of Mo, allowing it to be stably incorporated into the MoS₂ lattice without compromising the structural integrity of the crystal. On the other hand, compared to other transition metal dopants (Fe, Co, Ni), V doping can enhance conduction loss without introducing magnetic responses, which is advantageous for precise regulation of the dielectric loss mechanism and enables single-variable analysis. Therefore, in this work, V-doped MoS₂ was selected as the research focus. This approach aims to investigate the dielectric properties and microwave absorption performance of V-doped MoS₂. The findings offer a new design strategy for developing high-performance two-dimensional MAMs.

2. Experiment

The experimental preparation process is depicted in Figure 1. High-purity (99.999%) starting materials were employed, consisting of ammonium metavanadate (NH₄VO₃; 0.1755 g), sodium molybdate dihydrate (Na₂MoO₄·2H₂O; 0.3629 g), and thioacetamide (C₂H₅NS; 2.2539 g). The powder was dissolved in 30 mL of deionized water, followed by ultrasonic treatment for 1 h. The solution was transferred to a 100 mL hydrothermal reactor and placed it in an oven to heat at 220 °C for 24 h. Since C₂H₅NS decomposes under hydrothermal conditions to produce H₂S, this can effectively inhibit the formation of oxidation products. After cooling to room temperature, it was washed with deionized water and ethanol and then centrifuged. The filtered powder was then dried in a vacuum oven at 60 °C for 12 h. The as-synthesized V-doped MoS₂ sample was designated as VMS. The VMS sample was placed in a crucible and transferred to a vacuum tube furnace for annealing at 600°C for 2 h (heating rate of 5°C/min). Prior to annealing, Ar was introduced into the vacuum tube at a flow rate of 300 mL/min for one hour to remove oxygen. Continuous argon flow was maintained during both the heating and cooling stages to ensure an oxygen-free environment. The annealed sample was labeled as VMSA. The EM wave absorption properties of both VMS and VMSA were systematically characterized, with corresponding mechanisms elucidated.

3. Result and Discussion

3.1. The Phase Structure and Microstructure of VMS and VMSA Materials

Figure 2a presents the XRD patterns of both the unannealed VMS and annealed VMSA samples. Both samples exhibit distinct diffraction peaks, these peaks are consistent with the reference card PDF # 37-1492 (MoS₂). There are obvious diffraction peaks near the diffraction angles of about 14.33°, 33.02°, 39.67°, and 58.04°, which correspond to the (002), (100), (103), and (110) crystal planes. This result means that V mainly exists in MoS₂ in the form of doping. Then, it can be found that VMS and VMSA both exhibit similar characteristic peaks, indicating that annealing treatment did not destroy the crystal structure of MoS₂. Moreover, the lattice microstrain of VMS and VMSA can be calculated using the following formula [15]:

$${\beta }_{hkl}={\beta }_s+{\beta }_D$$

(1)

$${\beta }_{hkl}={\displaystyle \frac{k\lambda }{Dcos\theta }}+4\epsilon tan\theta$$

(2)

$${\beta }_s=4\epsilon tan\theta$$

(3)

In this equation, ${\beta }_{hkl}$ is the semi-peak width, and ${\beta }_s$ and ${\beta }_D$ are the semi-peak width caused by the microscopic morphology and the semi-peak width caused by the grain size, respectively. The values of D, $\lambda$, $\epsilon$, and $\theta$ are grain size, wavelength, strain constant, and peak position, respectively, with k being a constant of 0.89. As shown in Figure 2b, VMSA displays a greater microstrain than VMS, which may be attributed to the increased disorder within the MoS₂ crystal caused by the doping of V. To further confirm our results, the XPS was adopted. In Figure 2c, the peaks at 228.0 eV $\left(3{\mathit{d}}_{5/2}\right)$ and 231.2 eV $\left(3{\mathit{d}}_{3/2}\right)$ correspond to Mo⁴⁺ and no corresponding oxidation state of Mo⁶⁺ is found [16]. These peaks located at 514.8 and 522.1 eV are attributed to V $2{\mathit{p}}_{3/2}$ and V $2{\mathit{p}}_{1/2}$, indicating the V⁴⁺ chemical state of VMS (Figure 2d) [17]. The two peaks at 512.5 and 520.2 eV can be assigned to V²⁺, which can be attributed to the slight oxidation of V [18]. It is worth noting that the characteristic peak of V⁴⁺ is significantly enhanced while that of V²⁺ is weakened after annealing treatment, indicating that more V element has been doped into the MoS₂ crystal structure. Furthermore, the characteristic peaks of V⁴⁺ show a noticeable shift after annealing treatment, which proves the significant regulatory effect of V doping on the electronic structure of MoS₂ [16]. For the S $2\mathit{p}$, the two peaks at 161.1 and 162.4 eV are corresponding to S $2{\mathit{p}}_{3/2}$ and S $2{\mathit{p}}_{1/2}$ of S²⁻, respectively [19]. Overall, the XRD and XPS results effectively demonstrate that annealing treatment effectively improves V doping in the MoS₂ crystal structure, which has a significant modulation on the energy band and electrical structure of MoS₂.

Morphology is another important factor that affects the electrical structure and microwave absorption properties of materials. Figure 2f–i display SEM images of VMS and VMSA at 1.00 μm and 3.00 μm scales, the figure reveals that the two groups of samples have a similar flower-like structure, indicating that annealing treatment did not alter their original morphology. As previously reported works, such flower-like structures are beneficial for promoting multiple reflections/scatterings of EM waves, thereby enhancing the EM wave absorption performance of the samples [20,21,22]. However, in this work, the effect of morphology on VMS and VMSA is the same, so it is not the primary factor influencing their performance changes.

3.2. The Absorbing Properties of VMS and VMSA

As depicted in Figure 3, the complex dielectric constant $({\epsilon }_r={\epsilon }^{\prime }-j{\epsilon }^{″})$ of three distinct mixing ratios of the sample VMS and paraffin wax, as well as the complex dielectric constant image of a mixture comprising VMSA (with a filling amount of 40 wt%) and paraffin wax, were illustrated. During the process of sample re-preparation and detection, the mixture of the sample and paraffin was processed into a ring-shaped specimen using relevant abrasive tools. The VMS samples were mixed with paraffin at mass ratios of 30 wt%, 40 wt%, and 50 wt%, and were correspondingly labeled as VMS-30, VMS-40, and VMS-50. And the VMSA sample with a 40 wt% filler loading was designated as VMSA-40. As illustrated in Figure 3a, the dielectric constant $\left(\epsilon \right)$ exhibits a weakening trend with increasing frequency, which can be elucidated by the Debye relaxation theory. Specifically, as the frequency of the EM field increases, the dipole motion within the material cannot keep pace with the rapidly changing EM field, leading to a relaxation phenomenon. Consequently, the dielectric constant begins to vary with frequency, and its value gradually diminishes. The ${\epsilon }^{\prime }$ value of the VMS-30 sample is the lowest, while the ${\epsilon }^{\prime }$ values of the VMSA-40 and VMS-50 samples are essentially coincident and higher than those of the other samples. With increasing sample filling ratio, the ${\epsilon }^{\prime }$ of VMS gradually rises. When the sample filling ratio reaches 50 wt%, the ${\epsilon }^{\prime }$ range is between 33 and 31. The change trend of the ${\epsilon }^{″}$ for the four samples is analogous to that of ${\epsilon }^{\prime }$. However, the ${\epsilon }^{″}$ value of the VMSA-40 sample is higher than that of VMS-40, with a value range of 27–29. Based on the obtained XRD and XPS results, the doping of V as a donor center injects electrons into the conduction band of MoS₂, forming a high concentration of charge carriers, which leads to the enhancement of electromagnetic parameters [23]. In summary, as the filling amount of VMS increases from 30 wt% to 50 wt%, the dielectric constant significantly rises. This is attributed to the increased conductivity resulting from the higher filling rate, which in turn enhances the $\left(\epsilon \right)$. To evaluate the loss capacity of EM wave energy, the dielectric loss tangent value was examined, as shown in Figure 3b. Similar to the $\left(\epsilon \right)$, the dielectric loss tangent ($tan\left(\delta \epsilon \right)$) value increases with the filling rate, indicating that the dielectric loss capacity also rises with the filling rate and is further improved after VMS annealing. In order to further elucidate the dielectric loss mechanism of the sample, a Cole–Cole relationship curve was plotted based on the Debye relaxation theory. The relationship in the Cole–Cole plot is defined as follows:

$${\left[{\epsilon }^{\prime }-{\displaystyle \frac{1}{2}}({\epsilon }_s+{\epsilon }_{\infty })\right]}^2+{\left({\epsilon }^{″}\right)}^2={\displaystyle \frac{1}{4}}{({\epsilon }_s-{\epsilon }_{\infty })}^2$$

(4)

In the formula, ${\epsilon }_s$ and ${\epsilon }_{\infty }$ represent the static dielectric constant and optical dielectric constant. The ideal Debye relaxation model predicts a Cole–Cole diagram featuring a perfect semicircle centered on the real axis of the complex dielectric constant plane. However, this ideal behavior is rarely observed in natural materials, with only a few exceptions like water. After Maxwell–Wagner effect correction, the Cole–Cole diagram of the actual material is deviated from the standard semicircle, but an arc and/or a long straight line with a positive slope, where the arc represents the relaxation process, and the long straight line represents the conduction loss. As illustrated in Figure 3c–f, all four samples display distinct positive-slope linear segments, confirming the presence of conduction losses. Notably, while the VMS-40 sample exhibits multiple relaxation arcs suggesting complex polarization mechanisms, the annealed VMSA-40 shows only a single arc, demonstrating that thermal treatment significantly alters the dielectric relaxation behavior.

Based on the transmission line theorem [24], the RL performance of the sample can be calculated using Equations (5)–(9). In these equations, Z_in represents the incident impedance of MAMs, Z₀ denotes the impedance of free space, d is the thickness of the absorber, c is the speed of light, and f is the frequency. RL is commonly used to describe the intensity of EM wave absorption by materials and serves as a key metric for characterizing their absorbing properties [25,26,27,28,29,30,31]. A negative RL value indicates that lower reflection loss corresponds to higher absorption loss, suggesting that the material has better attenuation of the EM wave.

$${\epsilon }_r={\epsilon }^{\prime }-j{\epsilon }^{″}$$

(5)

$${\mu }_r={\mu }^{\prime }-j{\mu }^{″}$$

(6)

$$Z_{in}=Z_0\sqrt{{\displaystyle \frac{{\mu }_r}{{\epsilon }_r}}}tanh\left[j{\displaystyle \frac{2\pi fd}{c}}\sqrt{{\mu }_r{\epsilon }_r}\right]$$

(7)

$$RL=20log\left|{\displaystyle \frac{Z_{in}-Z_0}{Z_{in}+Z_0}}\right|$$

(8)

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

(9)

Figure 4 presents the three-dimensional reflection loss (RL) color maps for the four samples at various frequencies and coating thicknesses. As shown in Figure 4a–d, the RL_min of the VMS-30, VMS-40, VMS-50, and VMSA-40 samples, at thicknesses of 5.10 mm, 1.78 mm, 1.38 mm, and 1.45 mm, respectively, are −28.17 dB, −32.24 dB, −7.40 dB, and −9.64 dB. The VMS-30 and VMS-40 samples exhibit RL_min values below −10 dB, indicating that their absorption efficiency of EM waves exceeds 90%, with their absorption bands located in the 12–18 GHz range, which falls within the Ku band. Notably, the VMS-40 sample demonstrates the best wave absorption performance, achieving an RL_min of −32.24 dB, corresponding to an absorption efficiency of 99.9%. In contrast, RL_min of VMS-50 and VMSA-40 samples is greater than −10 dB, so these two samples are obviously difficult to meet the actual needs.

Another important factor in evaluating the absorption performance is the maximum EAB, so we calculated the EAB for the first two samples. As shown in Figure 5a,b, the EAB of the VMS-30 and VMS-40 samples are 2.4 GHz and 4.4 GHz, with corresponding coating thicknesses of 5.46 mm and 1.43 mm. To compare the comprehensive absorbing properties of the four samples more intuitively, we summarized their absorption performance parameters in

Table 1. Parameters of absorbing properties.

Sample	RLmin (dB)	d (mm)	f (GHz)	Bandwidth (GHz)	EAB (GHz)	d (mm)	RL (dB)	f (GHz)
VMS-30	−28.17	5.10	18.00	1.27	2.40	5.46	−25.09	16.81
VMS-40	−32.24	1.78	12.20	3.23	4.40	1.43	−26.22	15.60
VMS-50	−7.40	1.38	11.60	0.00	0.00	0.00	0.00	0.00
VMSA-40	−9.64	1.45	11.40	0.00	0.00	0.00	0.00	0.00

. According to Table 1, the RL_min of the VMS-30 sample is −28.17 dB, achieved at a layer thickness of 5.10 mm and a frequency of 18.00 GHz. The VMS-40 sample exhibits the best maximum RL value of −32.24 dB, at a layer thickness of 1.78 mm and a frequency of 12.20 GHz. The VMS-50 sample has a RL_min of −7.40 dB, at a layer thickness of 1.38 mm and a frequency of 11.60 GHz, while the VMSA-40 sample has a RL_min of −9.64 dB, at a layer thickness of 1.45 mm and a frequency of 11.40 GHz. The EAB of the VMS-30 sample is 2.40 GHz, with a corresponding layer thickness of 5.46 mm, an RL_min value of −25.09 dB, and a frequency of 16.81 GHz. The EAB of the VMS-40 sample is 4.40 GHz, with a corresponding layer thickness of 1.43 mm, an RL_min value of −26.22 dB, and a frequency of 15.60 GHz. The EAB of the VMS-50 and VMSA-40 samples in this frequency band is 0.00 GHz. In summary, the VMS-40 sample has the best comprehensive wave absorption performance among the four samples.

3.3. Analysis of Absorbing Mechanism

Attenuation characteristics are a key factor influencing the absorbing properties of materials [32]. Specifically, the ability of a material to efficiently convert EM waves into thermal energy and other forms of energy through conduction loss and dielectric loss is referred to as EM wave attenuation characteristics. As shown in Figure 6a, VMSA exhibits higher electrical conductivity than VMS, which is attributed to the annealing treatment resulting in increased V doping concentration and reduced oxidized impurities with low-conductivity. The high conductivity of VMSA also leads to an increase in its conduction loss capability. Additionally, the total attenuation capacity of EM waves can be quantified by the attenuation coefficient as shown in Equation (9). Generally, a higher attenuation coefficient indicates a stronger capacity for EM wave absorption [33]. As depicted in Figure 6b, the attenuation coefficients of the VMS-30, VMS-40, VMS-50, and VMSA-40 samples increase with frequency in the range of 2–18 GHz. Moreover, the attenuation coefficient rises with increasing filler content. For example, the loss capacity of VMS-50 is stronger than that of the other samples, while VMS-30 exhibits the weakest loss capacity. Notably, when the filler content is 40 wt%, the attenuation coefficient of the annealed sample (VMSA-40) is higher than that of the unannealed sample (VMS-40), indicating that annealing enhances the material’s loss capacity. However, the absorption performance of VMS-50 and VMSA-40 is not ideal, primarily because impedance matching is another critical factor in determining comprehensive absorption performance. Impedance matching refers to the degree of similarity between the material’s characteristic impedance and that of free space [34]. It is worth noting that the impedance matching ($|Z_{in}/Z_0|$) discussed herein is derived from transmission line theory and calculated based on the complex permittivity and permeability, rather than measured through electrochemical impedance spectroscopy (EIS). When the material’s characteristic impedance (evaluated using Equation (7)) is closer to the impedance of free space, EM waves can reflect less at the interface and penetrate deeper into the material. Ideally, when the coating’s characteristic impedance matches that of free space, EM waves can enter the material directly without reflection [35,36]. Therefore, excellent absorbing materials should possess both strong attenuation characteristics and good impedance matching [37]. As shown in Figure 6c–f, it is evident that the impedance matching of VMS-30 is the largest in the area close to 1, and the impedance matching of VMS-40 is close to 1. However, after annealing at a filling ratio of 40 wt%, the impedance matching decreases, and no areas close to 1 are observed. When the filling ratio is further increased to 50 wt%, areas with impedance matching close to 1 are virtually absent. Consequently, the comprehensive absorption performance of VMS-50 and VMSA-40 is the worst, with RL_min values higher than -10 dB. Although VMS-30 has good impedance matching, its loss capacity is weaker than that of VMS-40, resulting in lower comprehensive absorption performance. In summary, the excellent attenuation ability and good impedance matching of the VMS-40 sample collectively determine its superior comprehensive wave absorption performance.

4. Conclusions

In this study, V-doped MoS₂ powders were synthesized via the hydrothermal method with filling ratios of 30 wt%, 40 wt%, and 50 wt%. The results indicate that the RL_min values of the VMS samples with filling ratios of 30 wt% and 40 wt% are below −10 dB, which corresponds to an absorption efficiency of EM waves as high as 90%. It is notable that the VMS sample with a filling rate of 40 wt% exhibits the best comprehensive absorption performance, with EAB of 4.40 GHz at a thickness of 1.43 mm and RL_min value of −32.24 dB at a thickness of 1.78 mm. This outstanding performance is attributed to the sample’s excellent attenuation capacity and good impedance matching at a 40 wt% filling ratio. To further investigate the influence of annealing on VMS composites, the VMS samples were annealed at 600 °C for 2 h. It was found that annealing deteriorated the impedance matching characteristics, leading to a decrease in absorption performance. After further studying the microwave absorption mechanism of the sample, it is found that the attenuation coefficient increases with the increase in the filling rate of the filler. When the filling ratio is 30 wt%, the impedance matching is the largest in the area close to 1, followed by the area close to 1 when the filling ratio is 40 wt%. When the filling ratio is 40 wt%, the impedance matching degree decreases after annealing treatment, and there is no area close to 1. When the filling ratio is further increased to 50 wt%, the impedance matching area close to 1 is basically absent. Therefore, the excellent attenuation capacity and good impedance matching of the VMS sample at a 40 wt% filling ratio are key factors in its superior comprehensive microwave absorption performance.