Recent Developments in Synthesis Techniques and Microwave Absorption Performance of Materials Based on Molybdenum Disulfide (MoS₂) with Metal Oxides: A Review

Yusuf Sani ^1,2, Raba’ah Syahidah Azis ^1,3,*, Ismayadi Ismail ¹, Yazid Yaakob ³, Chen Soo Kien ^1,3, Mohd Mustafa Awang Kechik ³, Lim Kean Pah ³

1. Materials Synthesis and Characterization Laboratory (MSCL), Institute of Nanoscience and Nanotechnology (IONS), University Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia

2. Department of Pure and Applied Physics, Federal University Wukari, P.M.B 1020, Taraba State Nigeria

3. Department of Physics, Faculty of Science, University Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia

* Correspondence: Raba’ah Syahidah Azis

Academic Editor: Changming Fang

Received: October 11, 2024 | Accepted: December 25, 2024 | Published: January 26, 2025

Recent Progress in Materials 2025, Volume 7, Issue 1, doi:10.21926/rpm.2501003

Recommended citation: Sani Y, Azis RS, Ismail I, Yaakob Y, Kien CS, Kechik MMA, Pah LK. Recent Developments in Synthesis Techniques and Microwave Absorption Performance of Materials Based on Molybdenum Disulfide (MoS₂) with Metal Oxides: A Review. Recent Progress in Materials 2025; 7(1): 003; doi:10.21926/rpm.2501003.

© 2025 by the authors. This is an open access article distributed under the conditions of the Creative Commons by Attribution License, which permits unrestricted use, distribution, and reproduction in any medium or format, provided the original work is correctly cited.

1. Introduction

The widespread use of electronics and the swift advancement of wireless technologies are experiencing rapid expansion. This exacerbates electromagnetic pollution, which is the primary contributor to physical pollution and poses a substantial threat to public health [1,2]. Creating advanced microwave absorption materials is the greatest solution to this issue. These substances can convert electromagnetic energy into thermal energy without producing any additional pollutants [3]. When examining potential uses, the perfect microwave absorber should possess a broad absorption bandwidth, minimal weight, thin thickness, and corrosion resistance [4,5]. In contrast to traditional electromagnetic shielding materials, microwave absorbers are considered a more efficient approach for managing electromagnetic pollution [6]. When it comes to reducing electromagnetic interference, magnetic and dielectric losses make up the primary mechanisms in microwave absorbers [7]. Typical single-loss (dielectric or magnetic) microwave absorption materials often struggle to satisfy the demands of practical uses [8]. Therefore, there is a growing interest in microwave absorption regarding the utilization of materials that incorporate multiple loss mechanisms, whether magnetic or dielectric, to address this concern. The electromagnetic component of magnetic/dielectric composites may be artificially modified to increase microwave absorption [9] ammeters and impedance matching [10,11]. As per the fundamental equation, when both the permeability $\mu_r=\mu^{\prime}+j\mu^{\prime\prime}$ and the dielectric constant $\varepsilon_{r}=\varepsilon^{\prime}+j\varepsilon^{\prime\prime}$ are identical, the incident wave can penetrate the material entirely without any reflection. Additionally, the attenuation characteristics are influenced by the loss mechanism, which comprises magnetic loss, dielectric loss, and conductivity loss.

Nanocomposites made of different ferrites and molybdenum disulfide (MoS₂) have been thoroughly studied to solve these problems. High-quality absorbers may be made from two-dimensional materials with extremely thin layers because they have outstanding electrical characteristics and a large specific surface area [12]. Transition metal sulphide is thought to be the greatest material for electromagnetic wave absorption because of its very tiny band gaps, unusual shapes, and remarkable dielectric properties.

In recent years, scientists have learned that MoS₂ has excellent dielectric loss properties and is lightweight, making it a preferred material for the construction of portable electromagnetic absorbers. However, the material's high permeability and low permittivity make it hard to meet the standards for new wave-absorbing materials [13]. Recent studies have shown that some ferromagnetic and MoS₂ material blends perform very well in terms of microwave absorption. A hydrothermal process may be used to create high-purity MoS₂ [14]. MoS₂ is a significant component of 2D materials that has excellent interfacial polarisation and dielectric loss characteristics because of its high specific surface area and semiconductor characteristics. The single-atom thickness and multiple atomic layers of MoS₂’s surface offer the design framework and support for microwave absorption materials (MAMs) [15]. Additionally, van der Waal forces, which promote numerous microwave reflections, cause the layered MoS₂ to stack. The microwaves may penetrate and attenuate due to the small layer thickness. Furthermore, various controlled structures with broadband absorption may be designed using 2D materials as structural elements. Additionally, research has shown that eddy currents may be successfully mitigated through resonance shifts into the high-frequency region of plate structures. As a result, substantial microwave absorption properties are expected for a hierarchical flower-like MoS₂ that is created from an extensive number of sheet-like units [16].

When a conductor is exposed to a time-varying magnetic field, circulating electrical currents, referred to as eddy currents, are induced within it. The magnitude of these currents varies with frequency, with substantial currents at lower frequencies, resulting in Joule heating and corresponding energy dissipation. However, as the frequency increases, the skin effect confines eddy currents to the conductor’s surface, thereby minimizing their influence and the region through which they may flow. The fundamental framework for comprehending the behavior of eddy currents in conductive materials is provided by Faraday's Law. According to the skin effect idea, alternating currents, such as eddy currents, are limited to a smaller surface layer at higher frequencies, thereby reducing energy dissipation. A thorough understanding of this phenomenon is critical for analyzing eddy current behavior in high-frequency applications. Resonance at a specific frequency amplifies vibrations or currents within a structure, such as a plate, at that frequency. The magnitude of the generated eddy currents is reduced by shifting the resonance to higher frequencies, where the skin effect makes the eddy currents weaker. Higher frequency resonance shifts in plate structures minimize energy dissipation in conductive materials by increasing vibrational amplitudes and inhibiting eddy currents via the skin effect. The increase in inductive reactance and the reduction in eddy current resistance at higher frequencies limit current flow. This theory is often used in non-destructive testing methods and electromagnetic shielding. The amplification of unwanted eddy currents may be successfully reduced by shifting the resonance frequency of plate structures to higher bands, which is consistent with the approach of minimizing energy losses by resonance tuning.

This article offers a thorough synopsis of the developments in MoS₂-based materials in microwave absorption (MA). By discussing the microwave absorption (MA) traits of MoS₂ and its combinations, the study explored the most effective methods for improving the MA qualities of these composite materials. This review mainly focuses on studying the electromagnetic characteristics of MoS₂, especially how the inclusion of composite materials with either dielectric or magnetic components, or both, affects those properties. Finally, we outline potential future paths and opportunities for MoS₂-based MAM development.

1.1 Structures and Properties of MoS₂

MoS₂ nanostructures are pretty different from those of other nanoparticles that are often classified as quantum dots (0D), one-dimensional (1D), two-dimensional (2D), or three-dimensional (3D) forms. Furthermore, its properties and potential applications vary according to the dimension, encompassing the possibilities of being metallic, superconducting, or semiconducting. It is present in several levels and forms. Its three possible bulk (3D) structures are rhombohedral (R), tri-agonal (T), and hexagonal (H), where 2H MoS₂ represents a two-layer hexagonal MoS₂. Three main features (1-T, 2-H, and 3-R) are identified in Figure 1. 1T is located in an octagonal configuration structure, while 2-H and 3-R are arranged in a trigonal prismatic shape [17]. Table 1 displays the lattice constants for every structure. It is established that the first structure is metallic, whereas the remaining two exhibit semiconducting properties [18]. Hexagonal MoS₂ monolayers also exhibit semi-conductivity properties. The 2-H and 3-R components function as arid lubricants as well. Because of its nonlinear optical properties, the 3-R phase finds applications in biomedical and nonlinear optical mass detection in quantum measurements [19]. Unlike other materials constrained to specific structural forms such as dots, wires, sheets, or bulk frameworks, MoS₂ exhibits a range of atomic configurations due to its structural adaptability. For example, MoS₂'s many-phase materials might appeal to gas sensors that need rapid adsorption and high sensitivity [20]. Additionally, MoS₂'s polymorphic structures show a 1H phase. The single-layer hexagonal configuration of the 1H phase is distinguished by the trigonal prismatic coordination of Sulphur atoms with molybdenum atoms. The electrical characteristics of MoS₂ are defined by this phase and others, such as the 2H and 1T phases: the 1H phase is semiconducting, while the 1T phase is metallic. Among the various crystalline forms of MoS₂, the 1T phase with an orthorhombic structure and the 2H phase with a hexagonal structure are the most common. The 1T phase is thermodynamically metastable and show metallic behavior, whereas the 2H phases are thermodynamically stable and shows semiconductor properties. In certain circumstances, the 1T and 2H phases may transform into one another [21]. In contrast to 2H-MoS₂, 1T-MoS₂ would exhibit greater conductivity and a wider layer spacing, which might significantly increase the sample's polarization intensity and, thus, its electromagnetic wave loss [22]. The attenuation and electrical conductivity of electromagnetic waves need to be further improved, despite MoS₂'s powerful multiple scattering and reflection on electromagnetic waves. Additionally, preparing the 1T-MoS₂ metastable phase is challenging, which impedes further improvements to MoS₂'s microwave absorption performance [23]. Weak van der Waals interactions keep neighboring monolayers apart and keep bulk MoS₂ monolayers together. Strong covalent Mo-S bonds exist inside each monolayer. It is typically present as black powder or particles. A MoS₂ monolayer is a structure comprising two S atom layers sandwiched between a Mo atom layer. The free spacing between MoS₂ monolayers is 0.30 nm, and the interlayer space is 0.62 nm [24]. The saturation S atoms on the bottom plane (apart from the edges) not only form individual MoS₂ monolayers but also chemically stabilize the bulk MoS₂ [25]. Depending on atom-stacking configurations, these monolayers may form either the trigonal prismatic 2H phase or the octahedral 1T phase. The temperature and flakes' thickness determine the conductivity of nanoMoS₂, which rises with higher temperatures and falls with higher flakes' thicknesses until it approaches the bulk structure [26]. Furthermore, MoS₂ will unavoidably generate active sites throughout the preparation process, such as defects in structure or interfaces. Even if these flaws are just a few nanometers in size or even one atom, they will significantly alter the material's structure and electrical characteristics, impacting its usefulness [27]. The dielectric characteristics and microwave absorbing mechanism of MoS₂ could be effectively investigated. The application prospect of low-dimensional transition metal sulphide in the field of microwave absorption could finally be expanded if the microstructure of MoS₂ could be altered or designed in the real-world environment of two-dimensional materials and the structure-activity relationship of defects or interfaces could be introduced by means of qualitative characterization and analysis [28]. Additionally, single MoS₂ has low microwave absorption, which is why researchers are focused on exploring MoS₂ composite materials [29]. The microwave absorbance performance may be enhanced by composites made of magnetic or dielectric materials [30].

Figure 1 Distinct stacking and coordinating sequences for the MoS₂ structures 1T, 2H, and 3R [17].

Table 1 Comparison of several MoS₂ structures.

Also, the metallic 1T phase of MoS₂ has a much lower charge transfer resistance than the semiconductor 2H and 3R phases, which makes it a much better conductor. In particular, 1T MoS₂ exhibits conductivity around 10⁷ times greater than the 2H semiconducting phase [32]. Consequently, novel applications, including electrocatalysis, photocatalysis, supercapacitors, batteries etc., [33,34] are made possible by the difference in conductivity between 1T and 2H-phase MoS₂. Interestingly, the multiphase 1T/2H-MoS₂ can acquire both semiconductor and magnetic properties by incorporating magnetic 1T-MoS₂ into semiconducting 2H-MoS₂. In other words, 1T-phase-incorporated 2H-MoS₂ nanosheets can produce room-temperature magnetization behavior in non-magnetic 2H-MoS₂ semiconductors [35]. Therefore, researchers anticipate that it will be a successful method of refining the electromagnetic properties of MoS₂ and a fresh approach to modifying the absorbance of microwaves in MoS₂-based absorbers.

2. Microwave Absorption Mechanism of Materials Based on MoS₂

There are usually two processes involved in the material's microwave absorption process. First, microwaves completely penetrate the absorber. Permittivity and permeability are necessary for this procedure to meet the impedance matching requirement [36]. Subsequent conversion and dissipation of the microwave energy penetrating the absorber are associated with the material's internal loss process. Magnetic and dielectric losses are the primary loss mechanisms [37].

2.1 Process of Microwave Absorption and Attenuation

According to Maxwell's electromagnetic field theory, changes in the magnetic field induce changes in the electric field, while an ever-changing electric field causes a constantly varying magnetic field. Consequently, altering magnetic and electric fields mutually induce each other, generating synchronized waves propagating from nearby to distant regions at a specific velocity. This process gives rise to the generation of electromagnetic waves [38]. When electromagnetic waves come into contact with a solid material, various interactions take place, including absorption, reflection, secondary reflection, transmission, and retransmission. Absorption, reflection, and transmission are the three EM response behaviors that occur when a two-dimensional (2D) material is used instead of a bulk material [39]. Additionally, retransmission and secondary reflection disappear. Figure 2 shows a comparison between 2D materials and solid bulk materials [40]. Furthermore, certain 2D materials, like Ti₃C₂T_x, possess a layer-by-layer developed structure, which may lead to numerous reflections and scattering in addition to being advantageous for microwave absorption and attenuation. According to optical laws, electromagnetic waves are reflected, absorbed, and transmitted when they strike absorbing materials' surfaces. The conduction and displacement currents are the two distinct electrical currents that arise inside a non-magnetic dielectric material when electromagnetic waves strike its surface. Most charge carriers do not participate in the process of conducting dielectric materials. Because of this, the bonded charges may be shifted if the material is exposed to an extra electrical field [41]. Through a process known as polarization, displacing these electrical charges may result in the creation of a dipole field, which is the equivalent applied field. The dielectric tangent is the tangent of the angle at which the displacement phasors and the total current form. These bonded charges result in the displacement of electrical current, while a net flow of free charges produces the conductive current. For 2D materials, it is essential to measure the attenuation functions and one-layer electromagnetic wave responses. The incident waves are converted into transmission waves (T), absorption waves (A), and reflectance waves (R), as shown in Figure 2(a). The absorbing properties of a single-layer 2D material may be affected by high electrical conductivity, which converts electromagnetic waves into electrical energy, and polarization loss, which converts electromagnetic waves into thermal energy Figure 2(b) [42]. The dielectric loss may considerably attenuate and absorb waves in 2D materials like graphene. Both internal and extrinsic losses are included in the dielectric loss. The former is dependent on the material's crystal structure. The microstructure's defects, including microcracks, porosity, uneven crystalline orientation, and grain boundaries, are associated with extrinsic loss [43,44]. The relaxation and conduction losses, which are mostly influenced by the material's conductivity and polarisation, respectively, are included in the dielectric loss [45]. Polarisation generally results from certain functional groups, imperfections, and interfaces. Furthermore, intrinsic and hopping conduction are the two key factors influencing electrical conductivity; despite its low conductivity, the hopping conduction mechanism probably has the most impact [46].

Figure 2 Illustration of the incidence of electromagnetic waves (EM) through two types of materials: (a) solid and two-dimensional; (b) single-layer [47].

2.2 Theories of Microwave Absorption

The electromagnetic wave will follow three distinct processes when it enters the material's surface: transmission, absorption, and reflection [48]. The mismatch in the incident medium's surface impedance is the primary cause of electromagnetic wave reflection. The more electromagnetic waves are reflected, the greater the impedance difference between the surfaces. The inherent electromagnetic loss characteristics of the material determine how the incoming electromagnetic wave is absorbed. Attenuation constant ($\alpha$), complex permeability ($\mu_{r}$), and complex permittivity ($\varepsilon_{r}$) are often used to characterize a material's loss properties. Consequently, impedance matching and attenuation characteristics are two essential requirements that must be met to optimize the absorption of electromagnetic waves [49].

2.2.1 Impedance Matching

Enhancing an appropriate matched impedance between the incident wave and the absorbing substance at the interface within free space is critical to improving the entrance of electromagnetic waves into the substance. This factor, referred to as impedance matching characteristics, is essential [49]. Achieving perfect impedance matching is the primary factor determining how effectively the material absorbs an electromagnetic wave since it may only be lost once it enters the substance. When an electromagnetic wave (EM wave) propagates through a vacuum with impedance $Z_{0}$ and reaches the surface of a losing medium with an input impedance $Z_{in}$, it may encounter both transmission and reflection. A matched impedance is a vital component that determines the amount of electromagnetic wave transmission. One might compute the impedance matching degree ($\left|\Delta\right|$) using equation (1) [50]:

\[ |\Delta|\,=\,|sinh^2(K\mathrm{f}d)\,-\,M| \tag{1} \]

Properties of permittivity’s and permeabilities may be used to calculate the values of two variables, M and K.

\[ K\,=\,\frac{4\pi\sqrt{\mu^{\prime}\varepsilon^{\prime}}\sin\left(\frac{\varepsilon^{\prime\prime}/\varepsilon^{\prime}\mu^{\prime\prime}/\mu^{\prime}}{2}\right)}{c\,\cdot \,cos(\varepsilon^{\prime\prime}/\varepsilon^{\prime})\cos(\mu^{\prime\prime}/\mu^{\prime})} \tag{2} \]

\[ M\,=\,\frac{4\mu^{\prime}\varepsilon^{\prime}\,\cdot \,cos(\varepsilon^{\prime\prime}/\varepsilon^{\prime})\,\cdot \,cos(\mu^{\prime\prime}/\mu^{\prime})}{\left(\mu^{\prime}cos(\varepsilon^{\prime\prime}/\varepsilon^{\prime})\,-\,\varepsilon^{\prime}cos(\mu^{\prime\prime}/\mu^{\prime})\right)^2\,}+\,\left[tan\frac{\varepsilon^{\prime\prime}/\varepsilon^{\prime}\,-\,\mu^{\prime\prime}/\mu^{\prime}}{2}\right]^2\left(\mu^{\prime}cos(\varepsilon^{\prime\prime}/\varepsilon^{\prime})\,+\,\varepsilon^{\prime}cos(\mu^{\prime\prime}/\mu^{\prime})\right)^2 \tag{3} \]

In this case, ε′ and ε′′ stand for the real and imaginary components of $\varepsilon_{r}$, while c is the speed of light in a vacuum. μ' and μ'' also stand for the real and imaginary portions of $\mu_{r}$. Successful impedance matching is shown when the magnitude of Δ is close to zero for a given thickness. If it differs much from zero, impedance matching is considered insufficient. Furthermore, interferometric phase elimination may also reduce electromagnetic waves at specific wavelengths. In other words, absorbers may, under certain boundary circumstances, attenuate incoming electromagnetic waves. When the layer thickness d of the absorbing material corresponds to an odd integer of $\left(\frac{1}{4}\right)$λ of the incoming wave, the electromagnetic wave with a given frequency will be reflected [51]. Moreover, coherent destructive interference will be used to cancel out the waves. Equation (4) illustrates the quarter-wavelength wave theory [52]:

\[ \mathrm{d}\,=\,\left(\frac{n}{4}\right)\lambda_0/(|\varepsilon_r||\mu_r|)^{\frac{1}{2}}(\mathrm{n}\,=\,1,3,5,7\ldots) \tag{4} \]

Here, 'd' represents the layer thickness, 'f' for the equivalent frequency, and 'c' is the speed of electromagnetic radiation in a vacuum. The phase difference between the absorbing layer's surface and bottom may vary by changing the thickness. If the phase difference is an odd multiple of the half wavelength, the two waves have opposing phases. In this case, their mutual interference will successfully neutralize one another, resulting in the desired reduction of electromagnetic waves.

2.2.2 Attenuation Characteristics

The majority of electromagnetic waves may penetrate a material once the absorbing material and the impedance-matching concept come into contact. To minimize the emission of electromagnetic waves, it's crucial to depend on the intrinsic properties of the material described by $\varepsilon_{r}=\varepsilon^{\prime}-j\varepsilon^{\prime\prime}$ and $\mu_{r}=\mu^{\prime}-j\mu^{\prime\prime}$, which signify the material's loss characteristics. The real parts of permittivity’s and permeabilities represent a material's ability to preserve electrical and magnetic energies, while the imaginary parts represent a material's ability to dissipate electromagnetic energies, respectively. Therefore, $\varepsilon^{\prime\prime}$ and $\mu^{\prime\prime}$ are essential factors in determining how well EM waves attenuate [52]. Materials' loss capacity may be calculated using the tangent of dielectric losses and the magnetic losses tangent, which are shown in equations 5 and 6.

\[ tan\delta_\varepsilon\,=\,\frac{\varepsilon^{\prime\prime}}{\varepsilon^{\prime}} \tag{5} \]

\[ \tan\delta_\mu\,=\,\frac{\mu^{\prime\prime}}{\mu^{\prime}} \tag{6} \]

According to theory, a material's ability to reduce electromagnetic energy rises as its electromagnetic loss tangent and dielectric loss tangent values rise. On the other hand, expanding $\varepsilon^{\prime\prime}$ and $\mu^{\prime\prime}$ arbitrarily would increase the impedance matching degree and negatively impact absorption capacity overall.

Several parameters should be carefully considered in practical applications to develop microwave-absorbing materials that satisfy the needs of "thin, lightweight, wide-bandwidth, and strong absorption" [53]. The electromagnetic parameters must meet specific requirements for the material to have the necessary attenuation qualities. Equations (7) and (8) may be used to calculate the reflection loss (RL), which is a standard method of determining a material's absorbent capacity [54].

\[ \mathrm{RL}=20\mathrm{log}\left|\frac{Z_{in}-Z_0}{Z_{in}+Z_0}\right| \tag{7} \]

\[ Z_{in}=\sqrt{\frac{\mu_{r}}{\varepsilon_{r}}\mathrm{tanh}\left[j\left(\frac{2\pi fd}{c}\right)\sqrt{\mu_{r}\varepsilon_{r}}\right]} \tag{8} \]

\[ Z_0=\sqrt{\frac{\mu_0}{\varepsilon_0}} \tag{9} \]

In this context, $Z_{in}$ corresponds to the input impedance of the absorbing material, $Z_{0}$ represent the free space impedance 'd' signifies the thickness of the specified material, 'C' represents the speed of light, and '$f$' denotes the given frequency. The better a substance absorbs electromagnetic radiation, the higher its RL value. Effective absorption is 90% absorption when the RL value is less than -10 dB. The term "effective absorption bandwidth" (EAB) refers to the frequency range that an absorbent might effectively absorb. Apart from the RL value, the attenuation coefficient ($\alpha$) may also be used to express the material's ability to attenuate electromagnetic waves in accordance with the microwave transmission principle. Equation (10) is used to calculate this coefficient [55].

\[ \alpha=(\sqrt{2\pi f}/c)\times\sqrt{(\mu^{\prime\prime}\varepsilon^{\prime\prime}-\mu^{\prime}\varepsilon^{\prime})+\sqrt{(\mu^{\prime\prime}\varepsilon^{\prime\prime}-\mu^{\prime}\varepsilon^{\prime})^2+(\varepsilon^{\prime}\mu^{\prime\prime}-\varepsilon^{\prime\prime}\mu^{\prime})^2}} \tag{10} \]

Here, $c$ denotes the electromagnetic wave's velocity in a vacuum, while $f$ signifies the frequency of the incident wave. Theoretically, a material's ability to attenuate electromagnetic waves increases with its α value. In conclusion, the ability of a material to absorb electromagnetic waves is determined by its attenuation and impedance matching characteristics; often, the RL value is used to measure the absorption efficiency of the material.

2.3 Losses Mechanism

2.3.1 Magnetic Loss

The energy used by ferromagnets to resist the coercive force when magnetized within a magnetic field is commonly known as hysteresis loss in magnetic materials [56]. As a result, the intrinsic coercive strength of the magnetic material itself will influence its hysteresis loss [57]. Additionally, the term "eddy current loss" describes the energy dissipation produced by the eddy current when magnetic materials are electromagnetically inducted under an alternating magnetic field [58]. Furthermore, there is residual magnetism because the magnetic flux density changes more slowly than the applied magnetic field. The magnetic flux density of the internal magnetic material and the applied magnetic field are unequal [59,60]. Therefore, the residual loss is the energy loss produced to remove the remaining magnetic [61]. Furthermore, the resonance created by the magnetic material absorbing a significant quantity of energy from the surrounding electromagnetic field in order to preserve the stability of the interior magnetic flux density is known as natural resonance [62]. A specific vibration will take place at the magnetic domain wall of the magnetic substance as a result of the force received and the action of the magnetic field [63]. The domain wall will shake and suffer a specific loss when the vibration frequency matches the frequency of the external magnetic field. Exchange resonance produces a relaxation loss at the highest and ultrahigh frequencies [64].

Magnetic metals are often used in microwave absorption applications because of their relatively high saturation magnetization, which enhances permeability and is consistent with the Snoek limit [65]. However, at higher frequencies, their high electrical conductivity reduces permeability [66]. A practical approach for achieving enhanced microwave absorption properties (MAPs) involves leveraging the cooperative interaction between magnetic metals and MoS₂. Ferromagnetism in the non-magnetic semiconducting MoS₂ may be produced by cation doping or by introducing a metallic 1T phase into semiconducting 2H-MoS₂, according to theoretical predictions and experimental investigations [67]. Large saturation magnetization, complex permeability, and eddy current loss- especially from natural and exchange resonances-are characteristics of magnetic materials that effectively absorb microwaves by magnetic loss [68]. A highly effective approach to enhancing microwave absorption through combined magnetic and dielectric loss mechanisms is the integration of MoS₂ with magnetic nanostructures [69]. To maximize impedance matching and improve microwave attenuation, magnetic nanostructures may introduce magnetic losses into MoS₂-based systems and provide a synergistic effect. Magnetic materials are classified as metal oxides (spinal ferrites and hexagonal ferrites) and metals/alloys (Fe, Co, Ni, FeCo, and FeNi) based on their chemical makeup and structure. This variety offers a wide range of magnetic property adjustments to customize its microwave-absorbing performance. The synthesis, structure, and microwave absorption characteristics of MoS₂@magnetic material composites are covered in the sections that follow.

The in-situ development of magnetic metals on MoS₂ nanosheets promotes the production of magnetic-dielectric structures, which boost absorption performance via combined magnetic and dielectric features. One of the primary research studies in the field of electromagnetic (EM) wave absorption is the creation of composite materials that incorporate MoS₂ along with magnetic components [70]. One important metric that describes an object's capacity to produce a magnetic field in a certain orientation is its magnetic moment, which measures the substance's magnetic intensity and the direction of its magnetic field [71].

Wang et al. (2019) report that the magnetic properties of MoS₂ are generally enhanced when combined with metal oxides. Specifically, the saturation magnetization of MoS₂ increased from 0.53 emu·g^-1 for undoped MoS₂ to 0.93 emu·g^-1 upon doping with 3% nickel (Ni). The zigzag edges and changes in the quantity of voids in the material might cause this improvement. The synthesized nanocomposite achieved a maximum reflection loss of -58.08 dB at an optimal thickness of 2.05 mm [72]. 2D MoS₂ may acquire a magnetic moment by transition metal doping. Numerous recent investigations have demonstrated that incorporating transition metal elements into MoS₂ may result in magnetically active materials. For example, Schwingenschlögl et al. [73], computed the stability and magnetic characteristics of monolayer MoS₂ doped with IV, V, and VI transition metals using density functional theory (DFT). Their results showed that single Mn, Fe, Co, or Zn atoms may be used to replace Mo atoms in MoS₂ to create diluted magnetic semiconductors. The stability and electromagnetic characteristics of monolayer MoS₂ doped with V, Cr, Mn, Fe, and Co were examined in further detail by Yue et al. [74], who confirmed that these hybrid MoS₂ structures show notable stability.

Remaining losses, hysteresis/looping losses, and eddy current losses are the main methods for attenuating microwave radiation that leads to magnetic losses [14]. The term "eddy current loss" describes the material in the oscillating high-energy electromagnetic field as a result of eddy currents being electromagnetically induced. Figure 3(a) shows the contribution of Magnetic Loss Mechanisms for microwave absorption. To investigate the eddy loss process, the eddy current loss coefficient $C_{0}$ [75] is computed using the equation 11:

\[ C_0=\mu^{\prime\prime}(\mu^{\prime})^{-2}\mathrm{f}^{-1}=2\pi\mu_0d^2\delta \tag{11} \]

Figure 3 Microwave absorbing performance of MAMs showing the contribution of (a) Losses Mechanisms and (b) Polarisation Defects [76,77].

Where $f$ is the frequency, $d$ is the absorber's thickness, $\delta$ represents the conductivity, $\mu^\prime$ represents the permeability's real component, $\mu^{\prime\prime}$ is its imaginary part, and $\mu_{0}$ is the vacuum permeability [78]. The measured real and imaginary components of permeability may be substituted into Equation 10 to get the value of $C_{0}$ [79]. In general, it is possible to determine whether eddy loss or natural resonance loss is responsible for the material's magnetic loss by observing how the value of $C_{0}$ changes with frequency [80]. Natural resonance loss is the primary source of the material's magnetic loss if the $C_{0}$ value curve exhibits significant frequency fluctuations. Conversely, the eddy current loss is the primary source of the material's magnetic loss if the value of $C_{0}$ is almost constant [81].

This phase inconsistency between the magnetic field and induction causes the microwave energy to dissipate as thermal energy. Hysteresis loss occurs in magnetic materials due to irreversible magnetization phenomena such as domain wall displacement and rotation. Residual losses, on the other hand, result from a variety of mechanism-driven magnetization relaxation processes [82]. The magnetization relaxation process causes permeability's real and imaginary components to vary with frequency. When electrons and ions are out of equilibrium, magnetic hysteresis plays a significant role in the residual loss at low frequencies and weak magnetic fields. The bulk of the residual loss at high frequencies comprises domain wall resonance, dimensional resonance, and natural resonance. Figure 3 illustrates polarisation defects, magnetic loss mechanisms, and multiple scattering for MAMs.

2.3.2 Electrical Conduction Loss

Some carriers will continue to undergo directional drift under the operation of the microwaves since the accurate absorption medium is not a perfect insulator, which will result in the generation of a conduction current. The joule heat created by this electric current leads to the dissipation of microwave energy due to its thermal effect. The material's electrical conductivity affects this dissipation process. This process generally accounts for a large fraction of the behavior in high-conductivity materials [83]. MoS₂'s special structure helps build conductive networks when paired with conductive materials. These networks allow polarized electrons to migrate and jump between neighboring sites, resulting in excellent conductivity for microwave attenuation. Figure 3(b) illustrates the microwave behavior and loss mechanism for MAMs.

2.3.3 Loss as a Result of Polarisation Relaxation Defects and Defects Polarization

Polarisation is produced when a medium has asymmetric positive and negative charges in the presence of an electric field. Regarding composite component media, the Maxwell-Wanger effect-also referred to as interface, dipole, and defect-induced polarisation in the microwave frequency range-is the primary cause of the medium's polarisation [76]. Electromagnetic energy is transformed into other kinds of energy by the polarisation relaxation and damping processes that follow since polarisation cannot keep up with the frequency shift of the external electric field. Polarisation often encompasses many methods, including dipole, interface, atomic, and electron polarisation as shown in Figure 4. When carrier movement at the material interface is impeded and turns into a bound charge that develops within the contact, interfacial polarisation often occurs in the low-frequency region.

Figure 4 Schematic representation of various polarization mechanisms in microwave absorbing materials (MAMs).

The real component of the material's permittivity will rise as a result of the charge aggregation-induced field distortion. The primary polarisation mechanism in the gigahertz region of microwave frequencies is dipole polarisation. When an electromagnetic field is introduced, the originally disorderly configuration of dipoles will spin with a constant orientation and alter in response to the introduction of an electromagnetic field. Permittivity is affected by the relaxation process that often occurs along with dipole polarisation. Debye models may generally be used to examine the medium's relaxation process. The Havriliake-Negami dielectric polarisation relaxation relation yields the following formula [84]:

\[ \left(\varepsilon^\prime\,-\,\frac{\varepsilon_s\,+\,\varepsilon_\infty}{2}\right)^2\,+\,(\varepsilon^{\prime\prime})^2\,=\,\left(\frac{\varepsilon_s\,-\,\varepsilon_\infty}{2}\right)^2 \tag{12} \]

In this context, $\varepsilon_s$ stands for the stationary permittivity, $\varepsilon_\infty$ represents the comparative dielectric permittivity at higher frequencies, $\varepsilon^\prime$ denotes the real permittivity and $\varepsilon^{\prime\prime}$ represents the imaginary permittivity. When Debye relaxation behavior is present, the medium's ε′ and ε′′ plots will resemble a semicircle, sometimes referred to as a Cole-Cole semicircle. Each semicircle represents a Debye relaxation process [85].

2.3.4 Dielectric Losses

In comparison to traditional carbon-based dielectric absorbers, the dielectric-dominated MoS₂ sheet absorbent for EMW absorption was initially demonstrated in 2015 and showed higher attenuation performance. Defect dipole polarisation from Mo and S vacancies, together with the nanomaterial's larger specific surface area, were credited with improved microwave absorption [86]. In microwave absorption, relaxation polarisation and conduction loss are the most frequent dielectric losses [87]. Any dielectric substance will create a leaky conduction current when exposed to an electric field, but this current will dissipate as heat, causing conduction loss [88]. Moreover, the current flowing through the material due to the dielectric's relaxation polarisation is what causes the relaxation polarisation loss [89]. The Debye relaxation theory ought to be the most popular and developed of the dielectric polarization theories. The real and imaginary parts of a material's permittivity are not independent, in accordance with Debye’s relaxation theory, and they have a connection like this (Eqn.11) [90]. Consequently, Cole-Cole curves at various frequencies or temperatures are the graphs that show the connection between the real and imaginary sections of the permittivity [91]. A semi-circle is comparable to the Cole-Cole correlation because of the conductivity loss inside the material [92]. Polarisation loss would be the primary source of the loss at this point, with each semicircle representing a relaxation polarisation process [93]. On the other hand, conductive loss could be the primary cause of losses if the Cole-Cole relation is a sloped straight line. Liang created 2D MoS₂ nanosheets using a straightforward hydrothermal process in 2016 in order to investigate their microwave absorption characteristics. The findings showed that the MoS₂ nanosheets' high dielectric loss and improved interfacial polarization made them very efficient electromagnetic (EM) absorbers [94].

3. Method of Synthesis of MoS₂

There are two ways to synthesize nanostructured MoS₂: top-down and bottom-up. Using a top-down method, commercially available MoS₂ bulk crystals are physically reduced to MoS₂ nanoparticle size [95]. In contrast, the bottom-up method uses hydrothermal or solvothermal techniques, chemical vapor deposition (CVD), and other small-molecule reactions to synthesize MoS₂ nanomaterials [96]. Furthermore, research has been published on MoS₂ monolayers, multiple layers, nanoparticles, and quantum dots [97]. In both top-down and bottom-up approaches, ongoing attempts have been documented to fabricate MoS₂ nanomaterials [98,99]. Figure 5 shows different synthesis techniques for producing MoS₂.

Figure 5 Schematic representation of the mechanisms for (a-b) mechanical and laser exfoliation, (c) anodic and (d) cathodic electrochemical exfoliation of MoS₂ nanosheets (e-g) main liquid-phase exfoliation [100,101,102,103]. Bulk MoS₂.

3.1 Top-Down Method

3.1.1 MoS₂ Exfoliation

The exfoliation approach may readily generate MoS₂ nanosheets because of the multilayered structure and van der Waals interactions. MoS₂ nanosheets have been synthesized via liquid-phase exfoliation, chemical exfoliation, mechanical exfoliation, and electromechanical exfoliation, as described [104,105]. For instance, in the mechanical exfoliation method, appropriate MoS₂flakes are delicately separated from the bulk MoS₂ crystal using adhesive tape and then transferred onto a designated substrate [106]. When the scotch tape is taken off, some MoS₂ remains on the substrate. Consequently, random-shaped and sized single- or few-layer MoS₂ nanosheets are produced. The exfoliation approach produces high-quality 2D materials that are useful for studying the purity of materials and device performance. Nevertheless, it is challenging to regulate the MoS₂'s thickness and size during this process, and the materials that are produced are unsuitable for large-scale manufacturing and scaled-up applications [107]. Li et al. [108], used the sticky tape approach to physically exfoliate monolayer- and multilayer MoS₂ nanosheets from SiO₂/Si. MoS₂ flakes were mechanically separated from their substrate, Si/SiO₂. An atomic force microscope (AFM) and a bright-field optical microscope were used to characterize the single-layer and multilayer MoS₂ materials. AFM investigations showed that the height of a single MoS₂ sheet was 0.8 nm, while the thicknesses of 2, 3, and 4 layers of MoS₂ nanosheets were 1.5, 2.1, and 2.9 nm, respectively. It was discovered that the van der Waals connections between MoS₂ and SiO₂ were significantly weakened. Because of its significant affinity for Sulphur, gold may be used as a substrate to exfoliate the MoS₂ nanosheets for this purpose. Owing to the powerful van der Waals connections between the Au and MoS₂ layers, gold can effectively separate the MoS₂ monolayer from the bulk material [109,110]. Huang et al. [111] used an Au-assisted exfoliation technique to create large-area MoS₂ nanosheets. A Ti or Cr adhesion-coated substrate was covered with a thin layer of Au in a conventional synthesis. It is necessary to pass a MoS₂ bulk crystal under high pressure in order for it to make excellent contact with an Au-covered substrate on tape. The large-area monolayer sheets were recovered from the Au surface when the tape was removed. Top-down methods are utilized to create single- and multilayer MoS₂ nanosheets, which have been used to study some of the fundamental properties of MoS₂ nanosheets.

3.1.2 Mechanical Exfoliation

One popular technique for creating MoS₂ nanomaterials is mechanical exfoliation, which is well-recognized for separating bulk material from thin, highly crystallinity-rich layers. This technique was first developed using sticky tape and is perfect for study since it successfully maintains the crystal structure of MoS₂. This technique, which involves using adhesive tape to remove thin MoS₂ sheets from a block of MoS₂, is also known as micro-mechanical exfoliation. Frindt et al. [112] initially introduced this technique in 1965, and it was effective in obtaining a large number of layers of MoS₂. Mechanical exfoliation remains the most effective technique for producing the purest, most crystalline, and precisely thin nanosheets from layered materials to date [113]. Despite the high degree of crystallinity of MoS₂ produced through this approach, large-scale applications remain challenging due to the inefficiencies and lack of reproducibility in the preparation process. Consequently, researchers have investigated ultrasound-assisted exfoliation techniques. Utilizing 1-Dodecanethiol, Liu et al. [114] successfully synthesized graphene-like MoS₂ materials via ultrasonic exfoliation (Figure 5a). The highest concentration of graphene-like MoS₂ was accomplished when the volume ratio of 1-dodecanethiol to trichloromethane was 1:1, and the ultrasonication duration was maintained at 12 hours. Additionally, dual-solvent ultrasonic exfoliation was facilitated by incorporating chloroform Bulk crystals, which were pushed onto 3M scotch tape and folded several times in the conventional mechanical exfoliation dichalcogenides (TMD) process (Figure 5b). To ensure that the tape containing the MoS₂ samples adheres effectively to the substrate, an appropriate amount of pressure is applied. Upon removal of the Scotch tape, the surface of the SiO₂/Si substrate is coated with numerous TMD flakes. This technique can yield nearly all high-quality monolayers of two-dimensional TMDs [115]. Radisavljevic et al. utilized this technique in 2011 to exfoliate monolayer MoS₂, which was employed as the channel material in the fabrication of an interbond tunneling field-effect transistor (FET) [116]. This kind of transistor uses less power than conventional transistors. The development of applications for 2DTMD single crystals is hindered by a significant limitation of the conventional mechanical exfoliation (ME) method, which yields small crystal sizes ranging from tens to hundreds of microns, along with a low yield and the potential for adhesive residue [117]. To address these challenges, several modified ME techniques have been developed to produce monolayer TMD crystals.

3.1.3 Electrochemical Exfoliation

Electrochemical exfoliation of MoS₂ is an effective technique for generating high-quality nanosheets suitable for microwave absorption applications. The resultant materials exhibit increased surface area and tunable electromagnetic properties, rendering them ideal candidates for diverse technologies in microwave engineering and materials science. Future research efforts may optimize exfoliation conditions and investigate hybrid materials to further improve microwave absorption efficiency [118]. Electrochemical exfoliation has been reported to produce MoS₂ nanoflakes in their pure 2H phase without altering their intrinsic structure. This contrasts with the commonly used chemical exfoliation and hydrothermal synthesis methods for nanoscale MoS₂ production, which tend to facilitate the formation of the 1T phase [119]. Techniques for producing MoS₂ nanosheets via electrochemical exfoliation can be categorized into anode exfoliation, cathode exfoliation, and other electrochemical exfoliation methods, depending on the configuration of the exfoliation electrodes. Anode exfoliation is performed using diluted H₂SO₄ [120] or Na₂SO₄ [121] solution with bulk MoS₂ as the anode and platinum wire or foil as the cathode. The procedure is comparable to graphene exfoliation by anode [122]. The oxidation of electrolyzed water generates oxygen-containing radicals (OH and O) that congregate around sizable MoS₂ crystals when a voltage is applied to the working electrode material. The dW force between the two layers is weakened when intercalated SO4^2- enters the gap between the MoS₂ layers as a result of the oxygen radicals attacking the edges of the bulk MoS₂ crystals. Moreover, the distance between layers of MoS₂ is further increased by the emission of O₂ produced by oxygen-containing free radical oxidation and SO₂ gas produced by sulphate ion reduction.

Ultimately, the gas's eruption force separates the 2D MoS₂ flakes from the main MoS₂ crystal, leaving them floating in the electrolyte (Figure 5b) [100]. The two primary components for achieving anodic exfoliation are the generation of gases and oxygen-containing free radicals. In contrast to anodic exfoliation, cathodic exfoliation is employed to produce MoS₂ nanosheets using bulk MoS₂ crystals as cathodes and platinum wires or foils as anodes, either in an aqueous solution of inorganic salts (such as (K₂SO₄ or KCl)) or in an organic solution of long-chain ammonium salts. This exfoliation's mechanism and procedure are comparable to those of graphene's electrochemical cathode exfoliation. Tetra-n-butylammonium bisulfate (TBA.HSO₄) is a common example of cathodic exfoliating bulk MoS₂. Zhang et al. employed it in propylene carbonate as an electrolyte to create 2D MoS₂ flakes by cathodic electromechanical exfoliation [101]. When voltage is applied, the TBA⁺ cation (TBA · HSO₄ $\rightleftharpoons$ TBA⁺ + HSO₄^-) intercalation penetrates the negative charged MoS₂ crystal interlayer and causes the interlayer distance to increase to 0.89 nm. Hydrogen bubbles are then created by the migration and reduction of H⁺ ions (HSO₂^- $\rightleftharpoons$ SO₄^2- + H⁺, 2H⁺ + 2e^- → H₂), which widens the space between neighboring layers even more. Ultimately, under the influence of gas eruption, the MoS₂ nanosheets are separated and suspended in the electrolyte. Thus, the primary criteria for accomplishing cathode exfoliation are the choice of an appropriate electrolyte to accomplish the intercalation of cathode ions and the production of gas (Figure 5c). In addition, the development of unstable 2H-MoS₂ due to the oxidation of the anode exfoliation is prevented since the cathode exfoliation is mostly conducted in an organic solvent. include the use of organic solvents and electrolytes. The process is simple to use and safe for the environment. Therefore, it is anticipated that this technique will be used to exfoliate MoS₂ on a wide scale. You et al. (2014) demonstrated how to create single-layer or multiple layers of nanosheets of MoS₂ (with lateral sizes up to 20 μm) in a diluted H₂SO₄ electrolyte through anodic exfoliation. During this process, oxygen free radicals and SO₄^2- ions intercalate into bulk MoS₂ crystals, generating SO₂ or O₂ bubbles. These bubbles create a jetting force that facilitates the exfoliation of MoS₂ nanosheets [123]. Although MoS₂ exfoliation was accomplished in this study, information on the size of MoS₂ nanosheets and the exfoliation yield was not provided. Then, using anode exfoliation in 0.5 M Na₂SO₄ solution, Niu et al. effectively created MoS₂ nanosheets with a wide area (lateral size of 5-50 μm range), molecular layer thickness (7% of MoS₂ tiny flakes are one-layer and more than 70% of 2-5 layers), as well as low oxidation [100].

3.1.4 Liquid-Phase Exfoliation

In the liquid-phase exfoliation process, bulk crystals are transformed into single- or multi-layer 2D structures by applying external pressures, such as shear forces or ultrasonic stress, within water or organic solvents. These solutions can produce sheets and composites with potential scalability and can be made in large numbers using industrial processes. Under ultrasonic action, the solvent molecules may cause the material layers to separate or strip into separate layers by weakening their connection [124]. At the nanoscale, ultrasound also aids in the division of these layers into tiny sheets. There are now around two methods for stripping MoS₂ in solution, and the weak van der Waals forces between its layers allow it to be broken down into multiple or single layers by acoustic process. The first approach involves physical methods such as shearing, bubbling, grinding, ultrasonic treatment, and stirring [125]. Although it is entirely physical, certain chemical components could still be involved. To stop the shed flakes from recombining, for instance, surfactants like chitosan and sodium deoxycholate might be added to the solution [126]. Sometimes bubbles are created by electrolysis, and tiny bubbles may get into the material's interface and cause it to slip off. Since various methods employ different processes, liquid exfoliation is really just a broad idea. Ion intercalation, ion exchange, and sonication-assisted exfoliation are the primary liquid exfoliation techniques, as seen in Figure 5(d-f), and may be carefully categorized [100]. Liquid exfoliation appears relatively straightforward; All that is required to create nanosheets is to immerse the bulk materials in solvents and apply the appropriate ultrasonic treatments. However, selecting the right solvent is crucial since there are so many options available. The standard solution theory states that the polarizability and ionization of various components may be used to assess the stability of a mixed solution. Israelachvili [127] described the behavior’s using Eq. (13):

\[ V_{(r)}\,=\,\frac{\sqrt{3I}}{4}\frac{a^6}{r^6}\frac{(n_A^2\,-\,n_B^2)^2}{(n_A^2\,+\,2n_B^2)^{3/2}} \tag{13} \]

where r is the distance between two atoms, $I$ is their ionization potential, an is their polarizability, and $V_{(r)}$ is the dispersive interatomic potential. Refractive indices $n_{A}$ and $n_{B}$ are used to characterises the solute and solvent. The solution will be homogenous and evenly distributed, and there will be no solute accumulation, according to the Hamaker constant [128] A, which is comparable to Eq. (14). In order to further characterises the solution, the Flory-Huggins theory [129] developed the Flory-Huggins parameter, v, taking into account the effects of solvent to solute and solvent to solvent.

\[ A\,=\,\frac{3\mathrm{l}}{16\sqrt{2}}\frac{(n_A^2\,-\,n_B^2)^2}{(n_A^2\,+\,2n_B^2)^{3/2}} \tag{14} \]

As can be seen, for Eqn. (13) and (14), the V(r) and A will approach zero when $n_{A}$ approaches $n_{B}$, indicating that the system's total energy will be at its lowest. Thus, the solution will be homogeneous and evenly distributed, and there won't be any solute aggregation. Considering the interaction of solute to solute and solvent to solvent, the Flory-Huggins theory [129] established the Flory-Huggins parameter, v, to characterise the solution more correctly.

\[ \chi\,=\,\frac{z}{2}\frac{(2\varepsilon_{AB}\,-\,\varepsilon_{AA}\,-\,\varepsilon_{BB})}{KT} \tag{15} \]

where $\varepsilon$ is the intensity of the intermolecular communication pairwise interaction energy and z is the coordination of the solvent and solute. Additionally, the letters A and B stand for solute and solvent, respectively. The solute will spread well in the solvent if χ < 0, indicating significant interactions between the solute and solvent. The solute molecules will be drawn to one another and form a mass to deposit if ꭓ > 0.

The theoretical analysis allows for the selection of the best solution for various materials. There is a wide variety in the surface energy of various solvents. Therefore, a few solvents with comparable surface energies for various materials may save costs and increase the likelihood of a successful sample preparation. Naturally, the surface energy matching hypothesis mentioned above is just one component. If required, further deuterogenic hypotheses will be presented for the experimental procedure described in the next sections. Coleman et al. [130] used high-temperature solvents; nevertheless, 2-D nanomaterials are prone to agglomeration as the solvent evaporates, and high boiling point solvents with a low volatility cannot be eliminated. Zhou et al. [131] found that combining volatile solvents in an optimal ratio could create a stable, well-dispersed 2D nanosheet suspension, effectively overcoming this limitation.

3.2 Bottom-Up Method

3.2.1 Chemical Vapor Deposition (CVD)

MoS₂ and high-quality transition metal dichalcogenides may be synthesized on a large scale using chemical vapour deposition (CVD) technology, mainly for the creation of MoS₂ films [132]. It is a bottom-up method that maintains the material's shape, flaws, and crystallinity. Vapour combines with the substrate in a large-scale chemical process called CVD to create thin films. Direct vaporization is used in the CVD process, also known as the vapour-solids deposition method. It produces a superior monolayer on the substrate with fewer microscopic flakes. It is possible for precursors to initiate chemical reactions at high temperatures because of their charge and sublimation into a gaseous state. Because strong sediments accumulate on the matrix surfaces during the process of condensation, it is often used to produce single-layer composite structures and multilayered transition metal MoS₂ with great control over the number of layers. For the production of large-area monolayer MoS₂ films, S powder is employed as a Sulphur source and MoO₃ powder as a molybdenum source. Monolayer films develop uniformly and consistently; however, the consistency of the material is reduced due to faults that arise during the growing process [133]. High-quality semiconductor materials are often synthesized using the CVD process, which has a lengthy history. In a conventional CVD method of MoS₂ nanosheets, solid Mo or MoO₃ powder precursors serve as Mo sources, while solid S powder or H₂S gas may serve as S sources [134,135]. In a low-pressure container, the vaporized S and solid MoO₃ react to create the nuclei necessary for the development of MoS₂ [134]. After that, when the carrier gas flows across the substrates, MoS₂ gradually expands and increases in size.

The chemical vapor deposition (CVD) technique presents both significant advantages and notable limitations for synthesizing MoS₂ nanosheets and other nanomaterials. CVD is advantageous for producing large-area, high-crystallinity MoS₂ layers with excellent uniformity, essential for advanced applications in electronics and optoelectronics. By maintaining precise control over variables such as temperature, pressure, and precursor flow, CVD can yield atomically thin, defect-free MoS₂ nanosheets with tailored electronic properties [136]. Additionally, the scalability of CVD facilitates industrial-scale production, allowing for consistent coverage over large substrates to meet commercial demands for cost-effective, high-volume manufacturing [137]. The flexibility of the CVD process also supports the fine-tuning of MoS₂’s properties, as parameters like layer number and crystal orientation can be adjusted through the precise regulation of temperature, precursor ratios, and substrate choice [138]. However, CVD demands rigorous control of growth parameters, where slight deviations can compromise product consistency and quality. Establishing an optimal environment-requiring precise substrate treatment and the selection of metal precursors-adds complexity and elevates operational costs. The high temperatures and expensive precursors further contribute to CVD’s elevated cost structure, with potential material wastage from unused precursors also posing environmental concerns [139]. As CVD scales to larger production volumes, maintaining a defect-free structure becomes increasingly challenging, as even minor inconsistencies during growth can introduce defects, thus affecting the electronic performance and reliability of the final MoS₂ nanosheets. Overall, while CVD is effective for synthesizing high-quality MoS₂ nanosheets, its cost, complexity, and defect management present challenges that continue to guide research efforts toward refining this method.

The growth of MoS₂ in the CVD process typically occurs at temperatures ranging from 700 to 1000°C and often involves the use of a metal catalyst like gold (Au). Zhang et al. [140], made modifications to the preparation, resulting in MoS₂ triangular monolayers with grain sizes up to 150 µm. Alharbi et al. [141,142], produced the largest island-density MoS₂ film in the field by using a chemical vapour deposition technique. The various synthesis methods for synthesizing MoS₂ are shown in Figure 6.

Figure 6 Different synthesis techniques for producing MoS₂ [143].

3.2.2 Hydrothermal/Solvothermal Method

MoS₂, which has a two-dimensional sheet structure, has been widely studied because of its large molybdenum and Sulphur vacancies for polarisation properties and a high specific surface area [144]. It has gained recognition as a competitive microwave absorption material due to its excellent electric conductivity, remarkable interfacial polarisation effect, and simplicity of structural design. The hydrothermal method is one of the most frequently discussed techniques for creating nanomaterials with various morphologies and thicknesses [145]. This method involves an uneven reaction with aqueous solutions of solvents or mineralizers under high pressure and low temperatures in enclosed stainless-steel vessels called autoclaves. The goal is to dissolve and recrystallize minerals that are normally quite intractable. If water is employed as a solvent, this process is referred to as hydrothermal; organic solvents are utilized in the solvothermal approach [146]. This technique may synthesize nanoparticles with high crystallinities, desirable crystalline facets, variable morphologies, and controllable sizes by varying experimental parameters like temperatures, solvent, time, etc. Hydrothermal chemical synthesis is a widely used process because it uses very little energy and, more significantly, is ecologically pleasant [147]. An example of an MoS₂ synthesis is shown in Figure 7a. Non-aqueous solvents are used as intermediates, mineralizers, and pressure carriers in the solvothermal process. It is possible to use multiple non-aqueous solutions with different properties at the same time, producing nanomaterials with different properties. Bottom-up synthesis is used in both solvothermal and hydrothermal synthesis processes. Core-shell composites may be made using solvothermal techniques. MoS₂ nanomaterials produced using different bottom-up techniques exhibit diverse sizes, forms, morphologies, and thicknesses, making them suitable for many applications [148]. Using a hydrothermal technique and a chemical vapor deposition (CVD) approach, Yang et al. [149] achieved an RLmin of -53.03 dB at 7.86 mm.

Figure 7 Schematic representation of the synthesis of MoS₂ using (a) solvothermal, (b-c) hydrothermal, and (d) CVD methods [150,151,152].

Ma et al. [153] produced MoS₂-PANI nanocomposites with high microwave absorption characteristics using a conventional hydrothermal method. The MoS₂-PANI nanomaterials exhibit an optimum reflection loss value of -59.79 dB at 9.68, an efficient EAB of 3.1 GHz, and a corresponding thickness of 3.44 mm within the C- and X-band frequencies. Based on their experiment, they found three things that explain why MoS₂-PANI nanocomposites are so good at absorbing microwaves: excellent electrical conductivity, smaller particle size, and improved specific surface area. Liang et al. [154] evaluated the hydrothermally produced pure MoS₂ nanosheets' microwave absorption characteristics. The results show that the significant dielectric loss produced an effective microwave reflection loss of -47.8 dB at 12.8 GHz and an efficient absorption bandwidth of 5.2 GHz. Spinel ferrites have been studied extensively in recent years because they have traditionally been used to absorb microwaves.

Using the hydrothermal technique, Liang et al. [94] created MoS₂ nanosheets with remarkable dielectric losses and microwave absorption capabilities. The resultant MoS₂ nanosheets, with their significant dielectric loss and higher interfacial polarisation, are highly desirable as an electromagnetic (EM) absorber. The findings demonstrate that MoS₂ produced at 180°C has more significant real and imaginary parts of permittivity than other samples. MoS₂ nanosheets produced at 180°C have an optimum reflection loss (RL) value of -47.8 dB at 12.8 GHz owing to their excellent electric conductivity and polarisation effect. Additionally, it has been shown that MoS₂ has an effective absorption bandwidth EAB of 5.2 GHz (<-10 dB) with absorbance thicknesses of 2.0 mm. The findings showed that, at thin thicknesses, MoS₂ nanosheets may accomplish a wide efficient absorption bandwidth EAB, making them a promising choice for microwave absorption. MoS₂ possesses adaptable semiconductor properties and a configurable band gap because electrons in the valence band are readily propelled to the conduction band by heat or electric-field activation [155]. Since activated electrons can travel like free electrons and result in substantial conduction loss, MoS₂ may be classified as a dielectric loss-type MAM. Sheet-like MoS₂ is thought to be a good candidate for absorbing microwaves because it has an extensive specific surface area and a lot of active sites that can interact with EM radiation. A schematic depiction of the synthesis of MoS₂ using hydrothermal, solvothermal, and CVD techniques is shown in Figure 7(a-d). The various synthesis methods, precursors, and applications are presented in Table 2.

Table 2 Different synthesis techniques, precursors, and their application.

4. MOS₂ As MAMs

Recently, molybdenum disulfide (MoS₂), generally found as single- or few-layer nanosheets, has garnered much interest for use as electromagnetic wave absorbers [164]. The dielectric loss is largely responsible for MoS₂'s unique geometrical architecture, peculiarities, and non-magnetism, which all influence its ability to absorb electromagnetic waves. Therefore, MoS₂ has enormous potential for microwave absorption. A diagram of the sample fabrication process for Fe@MoS₂ composites is shown in Figure 8. Wang et al. [165] research revealed the exceptional microwave absorption properties of exfoliated MoS₂, using Stober process. For the three-dimensional nano-flower structure composite, an optimal reflection loss (RL) coefficient of -25.0 dB to -33.0dB was discovered with a material thickness increases from 1.5 mm to 2.1 mm. The highest adequate bandwidth (RL ≤ -10) was 4.96 GHz, which covered 8.08–11.76 GHz.

Figure 8 Schematic representation of (a) the sample synthesis procedure, (b) the quarter-wavelength matching thickness (mm), (c) microscopic SEM plates, and (d) microwave reflection loss (RL) of porous coin-like Fe and Fe@MoS₂ composites [166].

Su et al. [167] successfully synthesized MoS₂ nanospheres through the hydrothermal method. The synthesized samples exhibited a distinctive hollow, flower-like structure. The synthesized MoS₂ nanospheres had a matching thickness of 2.0 mm and a peak reflectivity loss (RL) of -45 dB at 8.3 GHz. The dielectric loss and electromagnetic wave destructive interference within the MoS₂ nanospheres are responsible for the significant broadening of the adequate absorption bandwidth observed in all samples in the X-band. In light of this, a flower-shaped MoS₂ nanosphere is a potential thin-layer, broadband, multiband, and light-weight absorber in the 8.2-12.4 GHz frequency range. Mixed-dimensional van der Waals force hybrid structures, including MoS₂, were created by Sun et al. [168]. These heterostructures included the 2-1 form (2D MoS₂/1D carbon nanotubes), the 2-3 type (2D MoS₂/3D carbon layers), and the 2-0 type (2D MoS₂/zero-D Ni nanostructures). For 0D, the optimum refraction loss (RL) is -19.7 dB at 2.92 GHz; for 1D, it is -47:9 dB at 5.60 GHz; and for 3D, it is -69:2 dB at 4.88 GHz. The MoS₂ composites have excellent microwave absorption capabilities, as can be demonstrated. Nonetheless, a little study has been done on how the spherical shape affects pure MoS₂'s ability to absorb microwaves. Zhang et al. [169] developed a novel design of MoS₂/PANI-NDs composite by fusing PANI nanoneedles (PANI-NDs) with the MoS₂ nanosheet matrices through in-situ oxidative polymerization techniques. The produced composite's distinct design offered improved dielectric qualities, efficient electron transit, and adjustable electric conductivity. These beneficial characteristics produced outstanding electro-responsive results at extensively used electric field intensities. The greatest reflection loss (RL) value of -44.4 dB at 11.48 GHz for the MoS₂ nanosheets was obtained at 3 mm thickness. In mechanical automation and aerospace, a new approach to building innovative materials has been made possible by the distinctive structure of the MoS₂/PANI-NDs composite.

Zhang et al. [166] synthesized MoS₂-NS, H-MoS₂ and HH-MoS₂ nanocomposites with varied hierarchical and porous configurations using the hydrothermal technique. The absorbed microwave Interfacial polarisation and dipole orientation polarisation are used to execute the as-prepared sample polarisation, as shown in Figure 8. Additionally, the resulting composites with 30 weight percent loading have a matching bandwidth with efficient attenuation (RL < -10 dB) up to 3.32 GHz and an optimal reflection loss (RL) value of -44.67 dB at a thin thickness of 1.4 mm on the MoS₂/C display. This work offers a fresh perspective on the mechanism of MoS₂-based materials' microwave absorption and a unique understanding of new microwave absorbents with lower thicknesses, more reflection loss, and a broader absorption frequency range. Numerous initiatives have been made to improve the architectural advantages of MoS₂ absorbers, including interface design, the development of novel nanomaterials, and multilayer structural designs that optimize impedance. In general, MoS₂ is considered a popular method for enhancing the potential microwave absorption capability when combined with additional dielectric and magnetic materials. Gai et al., [170]. A simple approach for synthesizing lightweight and inexpensive materials was developed, and the latter demonstrated a narrow absorption bandwidth and poor reflection loss (RL) performance. Strong impedance matching in the composite and a variety of loss pathways, including conductive loss, dipole polarisation, and interfacial polarization, are most likely to blame for this. The greatest reflection loss (RL < -10 dB) value of -49.1 dB was obtained using a filter thickness of 2.5 mm at 6.1 GHz and a wide bandwidth of 6.4 GHz. The findings indicate that the PPy@MoS₂ composite in its current state is a promising option for high-performance, light-weight, wide-bandwidth absorbers [170].

Su et al. [171] synthesized molybdenum disulfide polypyrene (MoS₂@Ppy) nanocomposites using an in-situ polymerization technique and a microwave-assisted hydrothermal process. MoS₂'s surface was evenly covered with PPy nanoparticles, and some tiny particles resembling flowers were embedded in the folds of the MoS₂. Ppy's extraordinary conductivity greatly enhanced the complicated permittivity of composites. The microwave absorbance properties of MoS₂@Ppy nanocomposite materials were thoroughly investigated. The MoS₂@Ppy nanocomposite demonstrated robust microwave energy absorption, a thin thickness, and a wide absorption bandwidth, achieving an optimum reflection loss (RL) of -61.1 dB at 11.2 GHz with a thickness of 2.28 mm. In comparison to similar materials, the MoS₂@Ppy composite outperformed comparable materials. As very effective microwave absorbers, the MoS₂@Ppy nanocomposites offer many possible applications.

4.1 Microwave Absorption Performance of MoS₂ with MAMs

Researchers currently opt for 2D materials such as MoS₂ due to their substantial surface area, robust electrical properties, and favorable surface chemical activity [170,171]. However, these 2D materials typically feature a single-loss process. Composite treatment with other substances has enhanced the electromagnetic wave absorption capabilities of 2 dimensional-based microwave absorbers. Magnetic metals, in particular, have drawn a lot of interest in the field of EM wave absorption because of their excellent magnetic permeability and significant magnetic loss tangent [172]. Nevertheless, metals have limited practical applications owing to their superb density and susceptibility to oxidation [10]. By combining magnetic metallic materials with 2D molybdenum sulfide (MoS₂), which offers adjustable resistivity, it becomes possible to control the permittivity and optimize the composite's overall impedance matching level. Additionally, molybdenum sulfide nanoparticles have been incorporated with other magnetic and dielectric materials to create composite fillers that interact positively with incoming radiation.

Li et al. [173], synthesized RGO/MoS₂@Fe₃O₄ nanocomposite and observed a maximum reflection loss of -49.43 at 3.0mm. According to Zhang et al., Magnetic nickel nanoparticles are embedded in MoS₂ nanosheets to create a Ni/MoS₂ nanocomposite. Their findings indicate that introducing a magnetic loss mechanism has significantly enhanced the absorption capacity of Ni/MoS₂ composites compared to pristine MoS₂. With a filler loading of 60%, the RLmin value might reach -55 dB, and the adequate absorption bandwidth (EAB) can extend to 4.0 GHz. Recent studies show that coupling magnetic materials with MoS₂ nanosheets improves microwave absorption effectiveness while raising magnetic loss [91,174]. Fe₂O₄ nanoparticles (NPs) have become extensively employed as MAMs due to their remarkable chemical resistance and inexpensive cost. Nevertheless, they have very low microwave permeability. In contrast, magnetic metal (like Fe) NPs have higher permeability [175] but oxidize extremely fast in moist air or at high temperatures. To enhance MoS₂'s absorption capabilities, core-shell Fe@Fe₂O₄ nanocomposites are a viable alternative. It is possible to prevent this oxidation by covering Fe NPs with an oxide layer. Spinel ferrite absorbers are only moderately helpful in practice because of their great density, little flexibility, singular magnetic loss, and inadequate corrosion resistance [176]. Spinel ferrites have the aforementioned limitations, so combining them with dielectric materials like graphene, MoS₂, and other conductive polymers to form magnetic/dielectric hybrids has become a typical solution [144,177]. Lately, there has been more interest in making unique ferrites with different microstructures in this field. Zhang et al. [178] added clusters of NiFe₂O₄ to RGO to change the composites' electromagnetic properties as they were being made. MoS₂ cannot, however, fully match all impedances with a single dielectric loss method. This suggests that the impedance-matching capabilities of MoS₂ might be balanced with a magnetic component.

Because of their unique qualities, saturation magnetization, excellent complex permeability, and low-cost cost-conventional ferrite materials have found extensive application in the field of microwave absorption [179]. Spinel ferrites have attracted considerable interest in high-frequency devices because they have strong electromagnetic properties, effective absorption intensity, wide absorption bandwidth, thin thickness, light weight, significant magneto crystalline anisotropy, low coercivity, high permeability, low dielectric loss, high magnetic saturation, high resistivity, high squareness ratio, and low coercivity. Consequently, they have a sizeable magnetic loss, making them potential microwave absorbers [180,181]. The only known material that can absorb microwaves better than any other is spinel ferrite. Spinel ferrite's main benefits are its low weight and thinness, which give it a wider range of absorption and make it useful for things like stealth and space technology. On the other hand, ferrites alone have some flaws, such as low absorption, a high frame density, minimal dielectric losses, and a narrow absorption band [182].

Xing et al. [15] Used a simple hydrothermal process to create a range of MoS₂/FeS₂ composites with local heterojunctions by varying the Mo/Fe molar ratio. The MoS₂/FeS₂ heterojunction area created a compact ohmic contact and a local electric field, significantly increasing the interface polarisation loss capacity, as shown by electron holography. Thus, with a thickness of only 2 mm, an adequate absorption bandwidth of up to 6.48 GHz (from 11.52 to 18 GHz) and a maximum reflection loss value of 60.2 dB at 8.08 GHz were successfully attained. These promising results provide a path towards the logical design of 2D material-based high-performance microwave absorbers. Their distinctive structure and believable mechanisms make it possible to produce sophisticated MA materials based on 2D composites.

Liao et al. [183] synthesized the MgFe₂O₄/MgO/C@MoS₂ composites by hydrothermal, carbonization, and electrospinning methods. The addition of a MoS₂ layer resulted in an enhancement of the dielectric constant of the composites. These prepared composites, MgFe₂O₄/MgO/C@MoS₂, demonstrated a remarkable maximum RL value of -56.94 dB at 9.5 GHz with an absorber thickness of 2.7 mm and an adequate absorption bandwidth (EAB) of 3.9 GHz (8.08 to 11.98 GHz). The remarkable EM wave absorption performance of the composites may be attributed to the right impedance matching achieved by modifying the composition of the MA material, the interfacial and dipole polarisation provided by many interfaces, and the high attenuation capabilities of a 1D structure. Hence, it suggests that MgFe₂O₄/MgO/C@MoS₂ composites hold promise for creating lightweight and highly efficient MA materials.

Bi et al. [184] used a hydrothermal technique and the MOF self-template approach to create CoZn/C@MoS₂@PPy absorbers with a 2D structure, low weight, and good performance. The primary aspects impacting the absorption performance of the CoZn/C@MoS₂@PPy composites include improved impedance matching, conduction loss, and exceptional dielectric loss when MoS₂ sheets and PPy are added. The produced composites exhibit exceptional microwave absorption capabilities due to enhanced impedance matching and increased microwave attenuation. With an absorber thickness of 1.5 mm, the results showed a maximum reflection loss value of -49.18 dB at 4.56 GHz. It has been demonstrated that CoZn/C@MoS₂@PPy composites might emerge as a popular choice due to their excellent microwave absorption capabilities and lightweight nature. MoS₂-based material categories for efficient microwave absorption are summarized in Figure 9. Table 3 shows the microwave absorption performance of MoS₂ with other MAMs.

Figure 9 Summary of MoS₂-based material categories for efficient microwave absorption [185].

Table 3 Microwave Absorption Performance of MoS₂ with other MAMs.

4.2 Absorption Performance of MoS₂ with Magnetic Metals Oxide

The superior magnetic loss characteristics of magnetic metallic materials, including Co, Fe, and Ni, as well as their alloys, have made them popular for microwave absorbers [204]. However, poor impedance matching makes it challenging to achieve effective microwave absorption with a single magnetic metallic material with magnetic loss [205]. When combined with magnetic metallic materials, molybdenum disulfide (MoS₂), a transversal semiconductor material with adaptive resistivity, may efficiently control the permittivity and maximize the impedance matching [156,203]. In conclusion, magnetic materials enhance the absorption performance of MoS₂-based microwave absorbers by contributing to both magnetic loss and optimized impedance matching. The tables below present several MoS₂-based microwave absorption composites incorporating magnetic elements, varying performance among magnetic components. Specific composites achieve superior microwave absorption at reduced coating thicknesses within the same component, likely due to differences in microstructure resulting from distinct preparation procedures, which increase electromagnetic wave reflection and loss within the composite materials.

4.2.1 Absorption Performance of MoS₂ with Iron Oxide

The semiconductor characteristics of MoS₂ may be readily adjusted due to its tunable band gap. This is because heat or electric field activation can easily excite electrons in the valence band of MoS₂ to the conduction band [206]. MoS₂ may be categorized as a dielectric loss-type MAM because activated electrons can spread like free electrons and result in significant conduction loss. Specifically, MoS₂ in the form of sheets is regarded as a potential material for microwave absorption because of its enormous specific surface area, which offers a multitude of active sites that may interact with electromagnetic radiation. Research has recently shown that MoS₂ nanosheets can be hybridized with magnetic substances to improve their microwave absorption performance [172]. This results in an increase in magnetic loss and an improvement in the impedance match. Fe₃O₄ nanoparticles (NPs) have been employed as magnetically activated materials (MAMs) in many applications due to their inexpensive cost and strong chemical stability [207]. Despite this, the NPs have extremely poor microwave permeability. On the other hand, magnetic metal nanoparticles (such as Fe) have a greater permeability [208], but they oxidize readily in moist air or at high temperatures. Fe nanoparticles may be coated with an oxide layer to halt such oxidation; one example is Fe@Fe₃O₄ NPs, a promising option to improve the microwave absorption efficiency of MoS₂ NSs. Table 4 summarizes the microwave absorption performance of MoS₂ with Fe oxide. The performance of MoS₂ with iron oxide in terms of microwave absorption is shown in Table 4.

Table 4 Microwave absorption performance of MoS₂ with iron oxide.

4.2.2 Absorption Performance of MoS₂ with Nickel Oxide

Due to their significant impedance-matching properties and permittivity, metal oxides, ferrites, and 2D-based materials are considered excellent absorption materials. The superior morphological, structural, physiochemical, electromagnetic, and dielectric characteristics may be able to reduce losses in a higher frequency range. Creating a thin, lightweight absorber made of a highly effective material with a strong absorption capacity is necessary.

A core-shell PPy@MoS₂ heterostructure nanocomposite resembling a nanotube was synthesized by Gai et al. [220] using hydrothermal techniques and chemical oxidation polymerization. Due to its distinct heterostructures, the suggested composite exhibits significantly improved electromagnetic wave-absorbing capabilities. At 6.1 GHz, the produced PPy@MoS₂ composite demonstrated outstanding reflection loss (RL) performance with a value of -49.1 dB. The as-synthesized nanocomposite also showed a wide absorption bandwidth, which could be caused by its excellent impedance matching and variety of loss mechanisms, such as conductive loss, dipole polarisation, and interfacial polarization. Remarkably, PPy@MoS₂'s broadest bandwidth of 6.4 GHz at 2.5 mm was obtained. The recently created PPy@MoS₂ may be an effective option for mitigating electromagnetic pollution due to its carefully managed preparation procedure and significantly improved performance. The microwave absorption performance of MoS₂ with nickel oxide is shown in Table 5.

Table 5 Microwave absorption performance of MoS₂ with Nickel oxide.

4.2.3 Absorption Performance of MoS₂ with Cobalt Oxide

MoS₂'s electrical performance may be easily regulated, and its microwave absorption characteristic is enhanced by the abundance of geometrical structures with distinctive surface and interface features, such as flake and flower-like structures, that are efficiently designed [226]. The topic of microwave absorbers has seen a surge in attention due to the discovery of composites based on MoS₂. To enhance their overall performances, Long et al. [227] created core-shell-structured Co_0.6Fe_2.4O₄@MoS₂NCs by designing and synthesizing them using a simple two-step hydrothermal process. The as-prepared core-shell-constructed Co_0.6Fe_2.4O₄@MoS₂NCs demonstrated excellent absorption, thin thickness, wide absorption bandwidth, and good stability. The maximum reflection loss of -79.9 dB at 11.2 GHz was obtained with a 2mm thickness and an adequate absorption bandwidth of 5.96 GHz.

Zhu et al. [228] used a simple hydrothermal method for fabricating a flower-like CoS₂@MoS₂ composite implanted within RGO. It is shown that the as-synthesized composites' exceptional attenuation capacity, improved impedance matching, distinctive surface shape, and microstructure are responsible for the enhanced microwave adsorption capabilities. The lowest reflection loss (RL) of -58.0 dB was achieved at 13.76 GHz with a 2.4 mm thickness. The corresponding effective absorption bandwidth reaches 6.24 GHz, from 11.76 to 18 GHz (Figure 10). Based on these findings, the as-synthesized composites appear to be desirable choices for outstanding performance and are thin, broadband, and adaptable microwave absorbers.

Figure 10 Microwave absorption performance of CoS₂@MoS₂ composites [228].

The microwave absorption performance of MoS₂ with cobalt oxide is shown in Table 6. Figure 9 illustrates the CoS₂@MoS₂ composites' microwave absorption capability.

Table 6 microwave absorption performance of MoS₂ with Cobalt oxide.

4.3 Absorption Performance of MoS₂ Combined with Various Metal Oxides

Prominent 2-dimensional metallic oxides include perovskite oxide nanosheets such as Bi₄Ti₃O₁₂, LaNb₂O₇, (Ca, Sr)₂Nb₃O₁₀, and CaLaNb₂TiO₁₀, as well as titania-based nanosheets (Ti_0.87O₂). The oxide oxides above are appropriate for MAMs because they typically have exceptionallyhighly complex permittivity (usually >100). However, the literature on this subject is limited because of their great permittivity, which makes it difficult to achieve impedance matching. Their use is increasingly common in microwave dielectric materials [236,237]. A common two-dimensional substance in transitional-metal chalcedony is MoS₂. Because of their qualities as semiconductors, exfoliated MoS₂ nanosheets are thought to be a viable option for employment in microwave absorption. Greater impedance matching with free space may be achieved more easily with these transitional-metal chalcogenides because they have a lower relative complicated permittivity and a smaller conductivity. Ning et al. [238] synthesized few-layered MoS₂ nanosheets (MoS₂-NS) using bulk MoS₂ via the top-down exfoliation approach. The EAB reached up to 4.1 GHz (9.6-13.76 GHz), while the highest reflection loss (RL) value of the prepared material is -38.42 dB. The imperfections causing significant dielectric relaxations, which are advantageous for enhancing the imaginary part of the relative permittivity (ε″), are demonstrated in the absorption mechanism. Also, MoS₂-NS has a large specific surface area, which makes it more likely for more transmission channels to form in microwave absorbers. The nanosheet nanoparticles cause conductive loss and have many conductive paths. Liang et al. [239] used a basic hydrothermal process to produce MoS₂ nanosheets of material. Their research indicates that the outermost areas of the resulting material may vary depending on the hydrothermal temperature. Compared to the other samples, it shows that the MoS₂ synthesized at 180°C has a greater permittivity and a greater surface area of 40.48 m²/g. When MoS₂ nanosheets are 2.2 mm thick, their lowest relative sensitivity (RL) is -47.8 dB, and they have a broad frequency-dependent 4.5 cover (11-15.5 GHz).

Dai et al. [240] synthesized flower-shaped MoS₂@Bi₂Fe₄O₉ microspheres with a hierarchical structure as an electromagnetic wave absorber using a simple two-step hydrothermal method. With a sample thickness of 2.8 mm, the produced nanocomposite showed a maximum reflection loss value of -52.3 dB at 12.4 GHz. It also demonstrated an adequate absorption bandwidth of 5.0 GHz (10.4-15.4) with a reflection loss below -10 dB. The study found that MoS₂ nanosheets and Bi₂Fe₄O₉ MPLs work synergistically to create flower-like MoS₂@Bi₂Fe₄O₉ MPs that can take advantage of the advantages of both multi-interfaces created by multi-polarisation and reflection. The development of a hierarchical structure consisting of a multiferroic core and a dielectric shell shaped like a flower offers a promising avenue for the discovery of a novel ultralight electromagnetic wave absorber candidate. The MoS₂@Bi₂Fe₄O₉ MPs' SEM pictures, HRTEM pictures, SAED pictures, reflection loss curves, and microwave absorption processes are shown in Figure 11.

Figure 11 Illustration of SEM pictures (a-d), HRTEM photographs (e-f), SAED pictures (g), reflection loss curves (h), and microwave absorption processes (i) of MoS₂@Bi₂Fe₄O₉ MPs [240].

4.4 Absorption Performance of Core-Shell Structure MoS₂ Composite

Despite its widespread use in military and commercial applications, creating microwave absorbers with adjustable and highly efficient electromagnetic absorbing capability is stillchallengingoday [228]. In order to develop matrices that include microwave-absorbing materials, single solid particles such as ferrites, oxide ceramics or alloys, and metallic magnets are often used as fillers [241]. However, their practical uses are considerably limited due to their constant high density, poor stability, and large loading content. MoS₂ features a direct energy gap and a non-centrosymmetric structure. Consequently, MoS₂'s outstanding electrical qualities as a semiconductor material make it a viable option for microwave absorption applications [242]. MoS₂ sheets are helpful as high-frequency absorbers because of their various advantages, which include reduced density, improved chemical durability, and special telecommunications transporting qualities [243]. Liao et al. [244] used hydrothermal processes and modified Stöber methods to effectively synthesize a core-shell Fe₃O₄@SiO₂@MoS₂ binary structure with dual shell layers. The electromagnetic characteristics of the composites were discovered to be efficiently altered when two separate layers of dielectric materials, SiO₂ and MoS₂, were wrapped around the outer layer of Fe₃O₄ nanostructures. The as-synthesized composite materials show excellent microwave absorption performance with a maximum reflection loss value of -62.98 dB at a 5.44 GHz thickness of 1.83 mm. Additionally, its highest EAB of 5.75 GHz readily satisfies the lightweight criteria. The results indicate that the ternary core-shell composite is a viable option for high-performance microwave absorbers. Zhang et al. [82] successfully synthesized a core-shell hetero-structure MoS₂/RGO hybrid exhibiting improved microwave absorption characteristics and an extended bandwidth using a straightforward one-step hydrothermal technique. The as-synthesized composite's maximum reflection loss value of -48.41 dB was obtained at an 8.16 GHz thickness of 2.52 mm with a wide band absorption bandwidth of 4.19 GHz. The findings provide a new avenue for creating superior MA materials for real-world uses.

4.5 Applications

Under typical conditions, MoS₂-based microwave-absorbing nanomaterials are primarily employed in electronic gadgets for the following purposes:

4.5.1 For General Public Usage

These materials may be primarily integrated within electronic devices such as communication cabinets, laptop computers, and mobile phones for general consumer applications [245]. Additionally, they may be used to lessen the noise and radiation from different electronic devices [246]. Furthermore, they can mitigate internal electromagnetic interference, including resonance and leakage within barrier frames, as well as interference from low-frequency echo and low-frequency coupling conductance radiation [247]. Additionally, they might be used between the cooling modules and the semiconductor [248]. Furthermore, as the 5G era develops quickly, new technologies like artificial intelligence have significantly enhanced the quality of life. The use of microwave absorption materials, for example, precisely eliminates the pervasive electromagnetic wave pollution interference in devices like printed circuit boards, adaptable circuit boards, noncontact sensors of liquid crystal polymers, and no-crosstalk piezoelectric sensors. Furthermore, the performance of microwave-absorbing materials has critical implications in medical fields, particularly for therapeutic applications. These applications are progressively expanding across multidisciplinary domains, including electronics, life sciences, chemistry, structural engineering, medical practice, and materials science.

4.5.2 For Broad Accessibility Across Diverse User Groups and Applications

2D-MoS₂ is the most promising and appropriate material for flexible electronics because of its exceptional optoelectronic and excellent mechanical qualities, such as its outstanding stiffness and breaking stress. However, challenges such as limited thermal stability, inconsistent doping, ambient stability, and production costs remain. To achieve favorable results, flexible MoS₂ is integrated with an epoxy-based photoresist (SU-8) as an encapsulation layer and employed as a non-degenerate n-type dopant [249]. Two-dimensional MoS₂ serves as an active layer in heterojunction solar devices with substrates such as aluminum (Al), silicon (Si), and silver (Ag), making it valuable in photovoltaic applications. Studies have reported an efficiency of 5.23% [250]. Furthermore, MoS₂ has demonstrated a power conversion efficiency of 8.11% when applied directly in solar cells, attributed to its dangling bonds, chemical reactivity, and unique band structure.

With an energy conversion efficiency of 3.69%, MoS₂-based nanocomposites have recently been used as an alternative electrode in dye-sensitized solar cells (DSSCs) [251]. When combined with other materials, the MoS₂-forming hybrid phototransistor is thought to offer new optoelectronic possibilities. A phototransistor with a photovoltaic efficiency of 3.03% has been developed as a result of the hybridization of 2D materials, such as MoS₂ quantum dots and indium selenide (lnSe) nanosheets [252].

4.5.3 For Use in Defense Applications

Stealth materials, also known as microwave absorption materials, are critical components in modern military technology, contributing to weaponry and defensive systems [253]. To get the greatest immediate military advantages, stealth materials might not only boost aggressiveness but also decrease detection rates and raise their own survival rates [254]. Stealth materials are now commonly employed in explosives, missiles, naval vessels, aircraft, and various ground combat vehicles [255]. In ground-based weaponry, they play a crucial role in preventing detection by aerial radar, infrared sensors, radar-guided missiles, and laser-guided munitions [256]. Primarily, stealth technology is essential in shielding combat aircraft from detection by infrared machinery, aerial fire control radar, and aeronautical early warning aircraft radar [257].

4.6 Summery

In summary, we have thoroughly examined current developments in 2-D MoS₂-based microwave-absorbing materials and their uses. Continued investigation into the properties of microwave-absorbing materials is essential to meet the demands of increasingly complex electromagnetic environments.

We examine the following elements of research significance:

1. Further research into the novel absorbing loss process within MoS₂-based absorbent materials is essential [258]. The use of the electron holographic of visual magnetic resonance, density functional theory to compute charge distribution and conduction, and finite element analysis to assess the contribution of the macroscopic structure are some of the new methods used to further explore the loss mechanism inside the material at the micro level [259]. The material's internal and external absorption processes are examined from a micro and macro perspective, and their impact on performance is evaluated.
2. Materials derived from MoS₂ that exhibit microwave absorption must be designed with multifunctionality in mind. For instance, thermal conductivity-absorbing materials are a novel kind of multipurpose electromagnetic protection material; when electronic equipment develops quickly, thermal conductivity-absorbing indicators' performance needs rise [260]. One of the primary focus areas for future research will be the development of innovative composite materials that exhibit high thermal resistance and effective electromagnetic wave absorption capabilities. There are several possible uses for flexible and compressible microwave-absorbing materials in wearable devices to safeguard human health [261,262]. In the future, architectural surfaces may be coated with hydrophobic microwave-absorbing materials that exhibit low thermal conductivity. This would integrate multiple functionalities, including thermal insulation, self-cleaning properties, and the mitigation of electromagnetic pollution [263,264].
3. The foundational principle for developing microwave-absorbing materials remains 'thin, lightweight, broad-spectrum, and robust. Multiband compatibility for stealth materials is required due to the multiband nature of battlefield detection systems [265]. As a result, the material must work with a variety of frequency ranges [266]. Several MoS₂-based microwave-absorbing materials now exhibit high absorption, although broad band development is still pending [267].
4. Numerous studies have shown that hollow fibers, hollow microspheres, core-shell structures, and porous architectures enhance absorption performance, increase multiple reflections, and reduce material density [268]. Therefore, further research into other wave-absorbing processes as well as the design and preparation of novel wave-absorbing nanomaterials with multicomponent composites, multilayered structures, and complex microstructures are potentially possible avenues for advancement.

5. Conclusions

Developing MAMs with low density, broad bandwidth, and excellent performance remains one of the most promising areas of research in order to address and overcome these difficulties and issues efficiently. Following years of rigorous study, 2D materials are now progressively playing a significant role in MAMs. Relevant efforts in the last several years have shown the superior MA characteristics of MoS₂. Because of its thin layer property and high surface area, MoS₂ offers a lot of interfaces that may be used to reduce inner scattering losses and enhance interfacial polarisation in microwave radiation. A range of regulated structures with wideband absorption capabilities, including layered stacking and core-shell siding, has been designed using the 2D MoS₂ materials as a component of the structure structure component. Furthermore. Due to their superb processability, 2D materials may be coupled with other materials to create multi-functional, multi-band electromagnetic absorbers.

MoS₂ also has a reduced permittivity due to its lesser electrical conductivity, which enables improved impedance matching to free space. MoS₂ has a few issues as well. (1) extremely high density. (2) Greater material loading is required as a result of low conductivity. (3) Conductivity has an impact on conduction losses as well. Furthermore, its conductivity affects its conduction loss. Appropriate microscopic structures and heterojunctions based on 2-dimensional MoS₂ may be constructed to solve the drawbacks. Additional advancements in the absorbers' electromagnetic attenuation and impedance matching properties are required in the search for more effective absorbers. Additional alternatives could potentially arise within the structure, linking the components simultaneously. A straightforward and sustainable synthesis preparation procedure is needed for the material preparation process. MoS₂ is a 2D compound material; further in-depth research is required to understand the absorption process. More information is required to know how the components work together to increase absorption performance, particularly in multi-component MoS₂-based composites.

In conclusion, more discussion is necessary on the correlation between the microstructure as well as the morphology of MoS₂ and its composites since this will aid in the development of modern, adaptable electronic devices. This research delivers a comprehensive evaluation of MoS₂-based microwave absorption materials (MAMs), mainly focusing on enhanced absorption properties achieved through metal oxide integration. Findings reveal that MoS₂-metal oxide composites significantly improve microwave absorption efficiency, with in-depth insights provided into the mechanisms responsible for this optimized performance across various applications. In a nutshell, despite the many obstacles and problems present in the current generation of MoS₂-based composite absorbing materials, we think that by improving hybridization and building hierarchical structures, these absorbers may become great MAMs. Furthermore, novel structure, technology, mechanism, multifunctionality, and other factors should be the focus of future developments in microwave absorber materials.

Acknowledgments

The authors are grateful for access to the measurement facilities provided by the Institute of Nanoscience and Nanotechnology (ION2) and the Department of Physics, Faculty of Science, Universiti Putra Malaysia. Additional gratitude is extended to the Fundamental Research Grant Scheme (FRGS), Ministry of Higher Education (MOHE), Grant No. (FRGS/1/2023/STG07/UPM/02/8) for funding.

Author Contributions

Yusuf Sani: Writing – Original Draft; Raba’ah Syahidah Azis: Supervision, Writing, Review & Editing; Ismail Ismayadi: Supervision, Investigation & editing; Yaakob Yazid: Supervision, review & editing; Chen Soo Kien: Review & Editing; Mohd Mustafa Awang Kechik: Review & Editing; Lim Kean Pah: Review & Editing.

Competing Interests

The authors have declared that no competing interests exist.