Recent Advances in MoS₂-Based Nanocomposites: Synthesis, Structural Features, and Electrochemical Applications

Abstract

This article presents a review of current research on the use of molybdenum disulfide (MoS₂) and its composites as promising materials for energy storage systems and functional coatings. Various MoS₂ morphologies, including nanoflowers, nanoplatelets, and nanorods, are considered, as well as their effects on electrochemical properties and specific capacity. Particular attention is paid to strategies for modifying MoS₂ using carbon nanomaterials (graphene, carbon nanotubes, porous carbon) and conductive polymers, which improve electrical conductivity, structural stability, and durability of electrodes. The important role of chemical vapor deposition (CVD), which allows the formation of uniform coatings with high purity, controlled thickness, and improved performance characteristics, is noted. A comparative analysis of advances in the application of MoS₂ in sodium-ion batteries, supercapacitors, and microwave absorbers is provided. It has been shown that the synergy of MoS₂ with carbon and polymer components, as well as the use of advanced deposition technologies, including CVD, opens new prospects for the development of low-cost, stable, and highly efficient energy storage devices.

1. Introduction

The remarkable progress in surface engineering and materials science has led to the development of advanced coating technologies capable of enhancing the performance, durability, and efficiency of engineering components. Among the diverse families of materials explored in this context, molybdenum disulfide (MoS₂) and its derivatives have emerged as one of the most promising classes of two-dimensional (2D) materials. Their unique layered crystal structure, high anisotropy, and exceptional tribological, electrical, and catalytic characteristics make MoS₂-based nanocomposites a focal point of modern research in nanotechnology, tribology, and surface modification [1,2,3].

Molybdenum disulfide belongs to the family of transition metal dichalcogenides (TMDs) and possesses a hexagonal crystal structure similar to that of graphene, where molybdenum atoms are sandwiched between two layers of sulfur atoms. These layers are held together by weak van der Waals forces, enabling easy sliding between them and resulting in a low coefficient of friction. This property, combined with high thermal stability and chemical inertness, explains the long-standing interest in MoS₂ as a solid lubricant and protective coating material [4,5]. However, pristine MoS₂ exhibits certain limitations, such as poor load-bearing capacity and moderate oxidation resistance, which restrict its practical application in extreme environments [6].

To overcome these drawbacks, researchers have developed MoS₂-based nanocomposites, in which MoS₂ is combined with metals, oxides, carbides, or other 2D materials. Such hybridization leads to synergistic effects that significantly enhance hardness, toughness, corrosion resistance, and thermal conductivity while maintaining the lubricating properties of MoS₂ [7,8]. For instance, incorporating titanium (Ti), nickel (Ni), or tungsten (W) into MoS₂ coatings improves their mechanical strength and wear resistance, while the addition of graphene or Al₂O₃ nanoparticles enhances oxidation stability and load-bearing capacity [9,10]. As a result, MoS₂-based nanocomposites are now being considered for high-performance applications in aerospace, energy, automotive, and microelectronic devices.

Among various deposition techniques, CVD has become one of the most powerful methods for synthesizing high-quality MoS₂ coatings and nanocomposites. Unlike traditional mechanical or chemical routes, CVD enables precise control over film thickness, stoichiometry, and microstructure at the atomic scale. The process involves a series of heterogeneous chemical reactions between gaseous precursors near or on a heated substrate, leading to the controlled growth of solid MoS₂ layers [11]. Depending on the precursor chemistry and process parameters, different forms of MoS₂ can be obtained—from few-layer nanosheets to dense nanocomposite coatings with tailored physical and chemical properties [12,13].

The advantages of CVD over other coating methods include its high purity, scalability, and the ability to deposit uniform films on complex geometries. This makes it particularly suitable for producing MoS₂ coatings for turbine blades, microelectronic chips, and energy conversion systems. Furthermore, the low defect density and strong adhesion of CVD-grown films contribute to their superior tribological and electronic performance compared to coatings produced by sputtering or electrodeposition [14]. For example, recent studies have demonstrated that CVD-grown MoS₂ coatings exhibit a friction coefficient as low as 0.03 under vacuum conditions and maintain structural stability up to 600 °C, highlighting their potential for use in aerospace mechanisms and high-temperature tribological systems [14].

In this work, they demonstrate a reconfiguring nucleation CVD strategy for the direct synthesis of twisted bilayer MoS₂ with twist angles varying from 0° to 120°. The resulting materials exhibit clearly discernible moiré periodicities, and the interlayer coupling strength is found to strongly depend on the twist angle. Furthermore, by optimizing the gas flow rate and the molar ratio of NaCl to MoO₃, we significantly enhance both the yield of TB-MoS₂ within the bilayer MoS₂ population (up to 17.2%) and the density of TB-MoS₂ domains (28.9 pieces/mm²). This reconfiguring nucleation approach offers a promising route toward the precise and scalable growth of TB-TMDCs, paving the way for systematic studies of twist angle-dependent physical properties and for potential applications in next-generation electronic and optoelectronic devices [15].

The structural and functional properties of MoS₂-based coatings synthesized by CVD are strongly influenced by process parameters such as deposition temperature, precursor type, carrier gas composition, and chamber pressure. At high temperatures (typically between 700 and 1000 °C), the formation of crystalline 2H-MoS₂ phases is favored, which exhibit semiconducting behavior and excellent tribological properties. In contrast, at lower temperatures, amorphous or metastable phases may form, which can be advantageous for catalytic or electrochemical applications [16]. In addition, doping MoS₂ films with transition metals (e.g., Co, Ni, Fe) or integrating them with dielectric matrices (SiO₂, Al₂O₃, TiO₂) has been shown to significantly enhance their corrosion resistance and hardness [17].

Recent advances in modified CVD techniques, such as metal–organic CVD (MOCVD), plasma-enhanced CVD (PECVD), and atomic layer deposition (ALD), have further broadened the capabilities of MoS₂ nanocomposite synthesis. MOCVD enables the precise control of chemical composition and crystal orientation using volatile organometallic precursors, while PECVD allows deposition at lower temperatures, which is crucial for thermally sensitive substrates. ALD, in turn, provides atomic-level control over film thickness and uniformity, enabling the fabrication of MoS₂-based nanolaminates and heterostructures with superior mechanical integrity and electronic functionality [18].

These developments have opened up new avenues for integrating MoS₂-based nanocomposites into next-generation electronic, tribological, and catalytic systems. For example, MoS₂/graphene hybrid coatings synthesized by PECVD demonstrate outstanding lubrication and conductivity, making them suitable for flexible electronics and sensors. Similarly, MoS₂-TiO₂ nanocomposites produced by MOCVD have shown enhanced photocatalytic efficiency under visible light, while MoS₂-Al₂O₃ coatings provide remarkable oxidation and wear resistance at elevated temperatures [19].

Furthermore, the exceptional electronic and optical properties of CVD-grown MoS₂ have attracted significant interest for use in semiconductor and optoelectronic devices. Its direct bandgap (≈1.8 eV for monolayer MoS₂) and high carrier mobility enable its use in field-effect transistors, photodetectors, and energy conversion devices. Integration of MoS₂ films with dielectric and metallic layers via CVD has also facilitated the development of vertically stacked heterostructures and tunneling transistors with tunable band alignment and reduced contact resistance [20,21].

Therefore, the current review aims to provide a comprehensive overview of recent advances in the synthesis, modification, and application of MoS₂-based nanocomposites fabricated by CVD methods. The discussion focuses on the mechanisms of film formation, the influence of deposition parameters on structure and performance, and the relationship between microstructural evolution and functional behavior. Special attention is devoted to the integration of MoS₂ coatings into tribological, catalytic, and electronic systems, as well as to the prospects for scaling up CVD processes for industrial production. By summarizing the latest experimental and theoretical findings, this review seeks to highlight the central role of CVD in the design of advanced MoS₂-based materials for future high-tech applications.

2. Modification Techniques

The progress in materials science and nanotechnology has opened up new avenues for altering and improving material characteristics through surface treatments. Both physical and chemical methods are extensively utilized for applying and removing coatings. Physical techniques, such as high-velocity oxygen fuel (HVOF) spraying, plasma spraying, and magnetron sputtering, excel at creating coatings with strong adhesion and high density [22]. Magnetron sputtering, in particular, is well suited for producing thin, consistent, and high-quality layers. On the chemical front, methods like CVD and electrochemical deposition (electrolysis) are commonly employed to achieve uniform coatings on intricate shapes [23]. Additionally, sol–gel processing and laser-assisted coatings offer enhanced resistance to wear and corrosion [24].

CVD, specifically, allows for precise surface modification and functionalization. This is achieved by directly depositing thin films or creating functional coatings through chemical reactions involving gaseous precursors. The process involves stages of nucleation and growth, leading to conformal and uniform thin layers. For example, combining ammonia (NH₃) and silane (SiH₄) can yield silicon nitride (SiN_x) coatings. This method provides control over thickness, high uniformity, and the capability to produce both impermeable and selective layers suitable for membranes and barrier applications at the angstrom-to-nanometer scale. While traditional CVD necessitates high temperatures, vacuum environments, and potentially hazardous precursors, recent advancements have addressed these challenges, expanding its practical uses [25].

To address the insufficient photoresponse of MoS₂ photodetectors, the researchers implemented an in situ chemical doping technique using gold chloride hydrate. This method introduces chlorine atoms as n-type dopants into CVD-grown MoS₂, which helps to minimize the trapping of photoinduced electrons and amplify the photogating effect. Consequently, the doped devices exhibited responsivity and specific detectivity values of 99.9 A/W and 9.4 × 10¹² Jones, respectively, under low source-drain (V_DS = 0.1 V) and gate (V_GS = 0 V) voltages representing 14.6- and 4.8-fold improvements over pristine devices. This straightforward yet effective strategy presents a promising approach for enhancing the performance of MoS₂ photodetectors and other two-dimensional materials [26].

While wet-jet machining effectively reduces surface roughness and induces residual compressive stresses in CVD coatings, its effectiveness diminishes with high initial roughness due to reduced coating thickness. Spray polishing (SAP) can further refine the surface but has minimal impact on tensile stress (

Table 1. Modification techniques.

Chemical Modification Techniques
Method	Key Features	Applications	Advantages	Limitations	Ref.
CVD	Gas-phase deposition using precursors and reactants	Deposition of CNTs, oxides, anticorrosion coatings	High uniformity, good adhesion, high deposition rate	High temperatures, toxic precursors, high cost	[28]
ALD	Sequential precursor delivery, reaction occurs only on the surface	Nanostructures, membranes, electronics	Atomic-level thickness control, excellent uniformity and conformality	Slow process, expensive and highly reactive precursors	[29]
Grafting	Chemical attachment of functional groups or monomers to the surface	Polymer membranes, improved hydrophilicity	Introduction of new functionalities, high selectivity	Only applicable to functional polymers, scale-up difficulty, high cost	[30]
Sol–gel	Conversion of precursor solutions (metal alkoxides) into oxide films	Photocatalysis, filtration, TiO2 membranes	Simplicity, porosity control, potential for nanostructuring	Possible cracking during drying, sometimes low stability, long process	[31]
Physical Modification Techniques
Method	Key Features	Applications	Advantages	Limitations	Ref.
EPD	Deposition of charged particles on an electrode under an electric field	Ceramic coatings, membranes	Fast deposition, uniformity, thickness control	Requires conductive substrate, limited materials	[32]
PVD (Physical Vapor Deposition)	Vapor-phase reactions at high temperature	CNTs, oxides, anticorrosion coatings	Uniformity, high adhesion, fast deposition rate	High temperature, toxic precursors, high cost	[33]
Sputtering	Deposition of Ar+ ions from a target onto a substrate (PVD)	Thin films, electronics	Versatility, low temperature, no precursors	Low deposition rate, expensive equipment, sometimes poor adhesion	[34]
Dip coating	Immersion of substrate into solution followed by solvent evaporation	Polymer membranes, LbL assembly	Simplicity, reproducibility, multilayer capability	Non-uniform coating, low quality, dependent on conditions	[35]
Meyer rod coating	Deposition using a rod (doctor blade)	Gas separation membranes, MMM	Thickness control (100 nm–10 μm), high reproducibility	Requires viscosity optimization, defects at low viscosity	[36]
Spin coating	Uniform spreading of solution by spinning	GO, PDMS membranes	Simplicity, low cost, high uniformity	Only for small samples, not scalable	[37]
Spray coating	Suspension spraying under pressure	Ceramic and polymer membranes	Simplicity, minimizes pore penetration, no intermediate layer	Lower permeability, requires viscosity control	[38]
Thermal evaporation	Evaporation of material in vacuum and deposition onto substrate	Metals, organic coatings	Simplicity, cheaper than PVD	Difficult for membranes, limited adhesion, non-uniform on complex shapes	[39]
Thermal spraying	Spraying molten particles (plasma, HVOF, etc.)	Anticorrosion coatings, mechanical protection	High deposition rate, coating strength	High porosity, low adhesion, requires powerful energy sources	[40]
Blending	Mixing functional polymers and nanoparticles with a matrix	UF/MF membranes, composites	Simplicity, improves hydrophilicity, mechanical strength, antifouling	Affects bulk, not just surface; compatibility issues	[41]

). To overcome these limitations, a combined approach of wet blasting and SAP has been developed. This integrated process results in smoother surfaces while maintaining residual compression, preventing significant thinning of the coating, and improving the adhesion between the coating and the substrate [27].

Methods for modifying membrane and thin-film surfaces offer significant potential for tailoring both structural and functional properties. Chemical approaches, including CVD, ALD, grafting, and sol–gel techniques, provide atomic-level control, enable the introduction of specific functional groups, and allow the formation of structures with precisely defined morphologies, although they require complex equipment and stringent operating conditions. Physical methods, such as EPD, magnetron sputtering, dip-, bar-, spin-, and spray-coating, thermal evaporation, spraying, and blending, are generally simpler, more scalable, and cost-effective, but they offer less precise control over coating properties. Consequently, the selection of an appropriate surface modification method depends on the desired coating characteristics as well as technological and economic considerations. In this context, the combination of chemical and physical approaches appears particularly promising for achieving comprehensive improvements in the performance and functionality of membrane and thin-film materials.

3. The Variety of CVD Methods and Their Role in Creating Functional Coatings

Modern CVD technologies have evolved significantly over the past decades, resulting in a wide variety of process modifications. These methods differ in pressure, energy source, precursor chemistry, and process temperature, all of which directly influence the structure, composition, and functional properties of the resulting coatings [42]. The versatility of these approaches allows the synthesis of materials with controlled morphology, purity, and adhesion strength for applications ranging from microelectronics and energy devices to protective coatings and catalysis.

Pressure-based CVD methods, including atmospheric (AP-CVD), low-pressure (LP-CVD), high-pressure (HP-CVD), and ultra-high-vacuum (UHV-CVD) systems, enable the growth of films with tailored uniformity and density. AP-CVD is suitable for large-area coatings such as graphene or oxide layers, while LP-CVD produces high-purity, uniform films ideal for semiconductor devices. HP-CVD allows the formation of dense selective layers with enhanced mechanical integrity, and UHV-CVD is used for epitaxial growth where atomic precision is critical [43]. Several of these methods have also been successfully applied to MoS₂, enabling high-quality bilayer and few-layer films for photodetectors and nanoelectronic devices [44].

Beyond the deposition of MoS₂ thin films, various surface modification and functionalization techniques have been employed to tailor the electronic, optical, and chemical properties of MoS₂ for specific applications. Chemical in situ doping, such as the introduction of chlorine atoms via gold chloride hydrate, has been shown to significantly improve the photoresponse of CVD-grown MoS₂ photodetectors by reducing electron trapping and enhancing the photogating effect, leading to remarkable increases in responsivity and detectivity [45].

Plasma treatments and energy-assisted modifications, including RF- and MW-PECVD, have been applied to MoS₂ to enhance crystallinity, surface uniformity, and interlayer coupling. These methods allow low-temperature processing, making them compatible with flexible substrates and enabling integration into optoelectronic devices [46]. Similarly, hybrid CVD approaches, such as MOCVD, LCVD, and iCVD, have been utilized to fabricate MoS₂ nanocomposites and heterostructures with controlled thickness, morphology, and electronic properties, demonstrating potential in nanoelectronics and catalytic systems [47,48].

In summary, the integration of chemical, plasma-assisted, and hybrid modification strategies with mechanical post-treatments provides a versatile and effective framework for enhancing the functional performance of MoS₂. By selectively tuning surface chemistry, crystallinity, and morphology, these methods support the development of MoS₂-based devices with optimized optoelectronic, catalytic, and mechanical properties, bridging the gap between material synthesis and practical applications (

Table 2. Modern CVD methods, their variations, advantages, limitations, and applications.

Method	Conditions/Features	Applications	Materials/Examples	Ref.
AP-CVD	~1 atm, high deposition rate	Graphene devices, turbine blades, electronics	Graphene zeolite Al2O3 SiO2	[49]
LP-CVD	Reduced pressure, heating without carrier gas	Uniform coatings, membranes, semiconductors	SLG SiC Al2O3 MoS2	[50]
HP-CVD	Pressure > 1 atm	Smooth layers, H2-selective membranes	SiO2 TMOS	[51]
UHV-CVD	Extremely low pressure, low T	Epitaxial growth of Si, SiGe	Si SiGe	[52]
AP-PECVD	Atmospheric pressure, no vacuum	Low-cost coatings, large samples	Polymer layers	[53]
DC-PECVD	Direct current plasma	Carbon coatings, fuel cells	a-C:H, steel	[54]
iPE-CVD	Initiator at low power	Polymer films, functional coatings	PNIPAAm, PHEMA	[55]
MW-PECVD	Microwave plasma, high power	Barrier layers, superhydrophobic coatings	SiNx:H, PDMS	[56]
RF-PECVD	Plasma at 13.56 MHz	DLC films, PVDF modification	DLC, PET, PVDF	[57]
VHF-PECVD	Frequency > RF, high deposition rate	Gas-impermeable coatings, solar cells	SiNx, Si	[58]
RI-PECVD	Radical injection (H)	Carbon nanowalls	CNWs Al2O3	[59]
AA-CVD	Aerosol precursors, ~1 atm	Use of non-volatile precursors	Oxides, metals	[60]
LCVD	Laser heating, high rate	Local coatings, mask repair	Re, Au, Pt	[61]
MOCVD	Metal–organic precursors, low T	Electronics, corrosion resistance	SnOx:F Cu MoS2 Ag	[62]
HFCVD	Hot filament (2000 °C), H2 + hydrocarbons	Diamond and carbon coatings	Diamond nanoparticles	[63]
OCVD	Monomer oxidation	Conductive polymers, sensors	PEDOT PANI, etc.	[64]
CDCVD	Diffusion of precursor and oxidizer toward each other	Silica membrane synthesis	SiO2 MoS2	[65]
iCVD	Radical polymerization at low T	Functional coatings on polymers	Polymer membranes	[66]

).

While the spontaneous creation of solid particles within a gaseous environment is generally viewed as detrimental, causing issues like inconsistent film thickness, wasted materials, and flaws in the final product, it can be harnessed for specialized laboratory applications. For instance, this phenomenon can be employed to initiate and precisely manage the growth of nanopowders, nanoparticles, or nanocomposite coatings in the gas phase. The resulting particle dimensions are dictated by the initial nucleation events within the reactor and the subsequent rate at which these particles enlarge.

Figure 1 illustrates a standard CVD apparatus. In this setup, a precursor gas flows into the reactor, encounters a heated substrate, and undergoes a surface reaction that yields a robust ceramic layer (such as Al₂O₃, Cr₂O₃, or SiC). Inert gases like argon are commonly introduced as diluents. It is important to recognize that the primary constraints for this technique are the deposition temperature and pressure [67,68].

There are reports on the direct selenization of transition metal foils, which allows the production of homogeneous and highly crystalline few-layer TMSes [70]. One of the best-known examples is the growth of MoSe₂ on Mo foil. Specifically, the material is synthesized by AP-CVD, where annealed Mo foil surfaces are subjected to direct selenization in selenium (Se) vapor at 550 °C for 60 min. Films obtained under these conditions reach sizes on the order of centimeters.

After synthesis, they can be easily transferred to various substrates by removing the Mo foil in a dilute ferric chloride (FeCl₃) solution. The thickness of the transferred thin films on a SiO₂/Si substrate (285 nm thick) ranges from 3.4 to 6 nm, as illustrated in detail in Figure 2a–k [71].

Figure 3 shows the central section of an UHVCVD reactor mounted in a 16-inch spherical stainless steel vacuum chamber. The heating station is mounted through the top flange, and the silicon wafer is fixed to a support beneath the heater with a gap that ensures uniform heating. The substrate temperature is monitored by a thermocouple located at a similar distance from the wafer.

On the lower flange of the chamber, quartz windows are connected to a microscope objective and an LED, mounted symmetrically on 1.75-inch diameter nipples. Additionally, a portable camera is attached to the vertically oriented retractable chamber. The colored lines in the diagram indicate the paths of the light rays. These optical components, together with the control and data acquisition systems, form a monitoring system that enables real-time monitoring of the sample surface condition, including the detection of micro- and macrodefects, as well as roughness [72].

In MW-PECVD, plasma is excited by long-wavelength electromagnetic radiation, which ensures the formation of uniform and functionally stable coatings. For example, demonstrated the effectiveness of this approach for modifying polyethylene terephthalate (PET) by depositing a thin layer of hydrogenated amorphous silicon nitride (a-SiN_x:H), which significantly improved the moisture barrier properties of the membrane. The experimental setup included a process and loading chambers; at a power of 1500 W and using SiH₄ and NH₃ gases, a uniform thin film was obtained (Figure 4). Furthermore, the method showed potential for the formation of oxidation-resistant coatings, as well as optically transparent superhydrophobic surfaces based on PDMS with a water contact angle of up to ~170° [73,74].

Figure 5 schematically illustrates a typical LCVD pyrolysis setup, comprising a precursor gas delivery system, a heater, a reaction vacuum chamber, a direct laser writing system, and a computerized control unit responsible for platform motion and processing trajectory.

In this configuration, continuous-wave (CW) lasers such as CO and Nd:YAG lasers are employed as the energy source for localized pyrolysis. The optical transmission system incorporates a beam expander, an attenuator, and a focusing lens (or half lens), which concentrate the laser beam onto the substrate surface, inducing localized heating and enabling selective material deposition.

The laser scanning trajectory is precisely controlled by a galvanometer mirror system or a 3D movable stage, allowing the fabrication of micro- and nanoscale structures with complex geometries. The substrate temperature is continuously monitored using pyrometers or thermocouples, while the precursor gas flow rate is regulated by mass flow controllers to ensure stable process conditions [76].

To improve the clarity of presentation, the refined figure includes labeled optical paths, highlighted laser–substrate interaction zones, and simplified flow diagrams to better illustrate the functional relationships among system components.

Thus, CVD and its many variations comprise a wide range of technologies enabling the production of coatings and functional layers with tailored structural, mechanical, and optical properties. From atmospheric AP-CVD to UHV-CVD, from PECVD to specialized CDCVD and iCVD methods, each modification meets specific technological requirements and finds application in electronics, membrane technologies, energy, and nanoengineering. Key parameters remain pressure, temperature, precursor composition, and energy source, determining the uniformity, selectivity, and functionality of the resulting coatings.

4. Morphological Evolution and Structural Features of CVD-Grown MoS₂: The Influence of Growth Parameters, Substrate, and Holding Time

One promising approach to localized surface modification is the use of laser-induced deposition processes. In particular, LCVD enables the formation of coatings and micro- or nanostructures with high spatial selectivity by locally heating the substrate with laser radiation. Figure 6 shows a typical LCVD pyrolysis setup, including a precursor gas supply system, a heating element, a reaction vacuum chamber, an optical system for direct laser writing, and a control unit for platform movement and processing trajectory control. The nucleation and growth of MoS₂ nanorods were investigated using SEM analysis. As shown in Figure 6a, a tilted-view SEM image reveals the initial formation of nanorod seeds on the MoS₂ film surface, with arrows indicating the nucleation sites. A nanorod in its early stage of growth is highlighted by a yellow dotted line, illustrating the onset of vertical growth. Cross-sectional SEM imaging (Figure 6b) further confirms the vertical alignment of the nanorods and their uniform orientation. These observations demonstrate the controlled nucleation and subsequent growth of MoS₂ nanorods, providing insight into the early-stage morphology and growth mechanism of the nanostructures [77].

Chemically synthesized MoS₂ nanosheets demonstrate remarkable resilience in a typical lab environment (around 40% relative humidity), maintaining their integrity for roughly three years. Over time, however, a degradation process becomes apparent: nanorods, measuring several micrometers in length and mere nanometers in diameter, emerge from the original MoS₂ seed structures. This growth is attributed to the chemical reaction between the MoS₂ and oxygen and water vapor, leading to the formation of an amorphous molybdenum trioxide (MoO₃) layer. Density functional theory calculations corroborate the critical involvement of water and oxygen in this transformation. The calculated adsorption energy of oxygen on the MoS₂ surface (−1.09 eV) is substantially greater than that of water (−0.10 eV), suggesting that oxygen is preferentially adsorbed and initiates the oxidation of molybdenum atoms. Consequently, this research offers enhanced insight into the aging mechanisms of MoS₂ when exposed to oxygen and moisture, a significant constraint on their real-world utility [78].

Figure 7a presents a scanning electron micrograph of the pristine MoS₂ nanoparticles. The image reveals a dense, continuous arrangement of vertically aligned nanostructures, each 5–20 nm thick. These structures interlock to form a highly crystalline, interconnected network. Previous transmission electron microscopy investigations [78] have indicated that the exposed tips of these vertically oriented nanoparticles possess a sharp morphology, which is crucial for excellent field emission properties.

The development of the MoS₂ structure closely reflects its morphological transformation. SEM analysis (Figure 8) reveals a variety of crystal shapes that depend strongly on the growth conditions, confirming the polycrystalline nature of the synthesized films [79]. These morphological variations are primarily governed by the local concentration balance of molybdenum and sulfur precursors within the confined-space CVD environment. In this process, sulfur is introduced from solid sulfur powder placed in a separate upstream heating zone. The controlled heating of this zone produces sulfur vapor, which is transported by the carrier gas into the reaction chamber, where it reacts with the MoO₃ vapor to form MoS₂.

The proximity between the MoO₃ source and the substrate, as well as the relative temperatures of the sulfur and molybdenum zones, strongly influence the resulting Mo/S ratio during synthesis. At elevated temperatures (850–900 °C), variations in the sulfur vapor pressure alter the growth kinetics of the zigzag edges, which in turn determine the overall crystal morphology [80]. Due to the high chemical reactivity of these edge terminations, crystal growth proceeds preferentially along these directions rather than across the basal plane.

As a result, a diverse set of structural forms is observed, including triangular, three- and six-pointed star-like, and twinned crystals. The emergence of complex morphologies such as six-pointed stars is associated with the coalescence of misoriented grains exhibiting different lateral growth rates [81]. The occurrence of cyclic twins (Figure 8k) arises from the symmetrical intergrowth of adjacent crystal domains [82], while mirror twin boundaries form when MoS₂ flakes rotate by 180° within the plane, producing characteristic inversion domains [83].

The CVD process utilized SiO₂/Si, Si, and quartz glass as the base materials. Before deposition, these substrates underwent a thorough cleaning regimen involving ultrasonic treatment in deionized water, a detergent solution, alcohol, and sulfur powder. The source materials for the deposition were WO₃ and MoO₃ powders (sourced from Aladdin Industrial Co., Ltd., Pico Rivera, CA, USA). In a tube furnace (model BTF-400C from BEQ Co., Ltd., Newark, DE, USA), Si (weighing 0.3 g) was positioned above a mixture of MoO₃/WO₃ (0.1 g) within distinct heating zones (as illustrated in Figure 9). The substrates were then placed face-down onto a boat containing the MoO₃/WO₃ mixture.

Figure 8. SEM images of MoS₂ crystals grown on SiO₂ substrate at a growth temperature of 780 °C with a MoO₃/S ratio of 1:20 and 25 mg NaCl. (a–d) SEM images of MoS₂ with low magnification. High-magnification SEM images of various shapes of CVD-grown MoS₂: (e) three-point star, (f) four-point star, (g) tilt boundary, (h) mirror boundary, (i,j) six-point star, (k) seven-point star, and (l) mirror twins [84].

Figure 8. SEM images of MoS₂ crystals grown on SiO₂ substrate at a growth temperature of 780 °C with a MoO₃/S ratio of 1:20 and 25 mg NaCl. (a–d) SEM images of MoS₂ with low magnification. High-magnification SEM images of various shapes of CVD-grown MoS₂: (e) three-point star, (f) four-point star, (g) tilt boundary, (h) mirror boundary, (i,j) six-point star, (k) seven-point star, and (l) mirror twins [84].

The synthesis of MoS₂ was carried out using a two-zone CVD system, in which the sulfur and molybdenum oxide (MoO₃) precursors were placed in separate temperature zones to ensure controlled vapor transport and reaction. During the process, the sulfur source was heated to 200 °C to generate sulfur vapor, while the MoO₃ and Si substrate zone was maintained at 650 °C for 45 min. After this stage, the system was held at these temperatures for an additional 5 min before being allowed to cool naturally (Figure 9b).

For the synthesis of WS₂, an analogous procedure was followed, with sulfur and tungsten oxide (WO₃) serving as the precursor pair. The sulfur zone was again heated to 200 °C, while the WO₃ and substrate zone was maintained at 1000 °C for 90 min. Throughout both processes, a constant nitrogen (N₂) flow of 40 sccm was used as the carrier gas to transport sulfur vapor into the reaction zone and facilitate the formation of the dichalcogenide films. This methodology ensured stable sulfur delivery, uniform reaction conditions, and reproducible film growth, as reported in [85].

Figure 9. SEM image of MoS₂ grown on (a) NFT SiO₂/Si; (b) PTCDA SiO₂/Si; (c) O₂ plasma-cleaned SiO₂/Si; (d) NFT Si; (e) PTCDA Si; (f) O₂ plasma-cleaned Si [85].

Figure 9. SEM image of MoS₂ grown on (a) NFT SiO₂/Si; (b) PTCDA SiO₂/Si; (c) O₂ plasma-cleaned SiO₂/Si; (d) NFT Si; (e) PTCDA Si; (f) O₂ plasma-cleaned Si [85].

Figure 10 shows SEM images of samples synthesized under different growth conditions, demonstrating a close correlation between the MoS₂ morphology and the process parameters. At low inert gas flow rates, high T_S (substrate temperature), and low T_G (growth temperature) values, dendritic growth is observed due to the increased sulfur concentration, which leads to the formation of irregularly shaped crystals with significant lateral dimensions (Figure 10a). Low TG promotes the activation of multiple growth centers. Decreasing the sulfur evaporation temperature reduces its partial pressure, causing a transition in geometry from dendritic to concave triangular (Figure 10b). With a further increase in temperature, growth leads to the formation of triangular and truncated triangular crystals. In general, increasing the temperature and/or increasing the Mo/S ratio contributes to greater compactness of the structure and a decrease in its concavity [86].

An analysis of literature data and SEM images revealed that the morphology of MoS₂ synthesized by CVD is closely dependent on process parameters, substrate type, and long-term storage conditions. Vertically oriented nanoparticles with a high degree of crystallinity can transform into nanorods over time, demonstrating structural evolution under environmental influences.

A comparison of various studies indicates that the Mo/S ratio, evaporation temperature, and growth temperature play a key role in the formation of MoS₂ crystals of various shapes—from simple triangles to complex multi-arm stars and twins. Increasing the growth temperature and optimizing the sulfur partial pressure promote the formation of more compact crystals with an ordered morphology. Furthermore, the use of different substrates (SiO₂/Si, Si, quartz glass) directly influences crystal nucleation and orientation, determining grain boundaries and the defect structure of the material. Thus, controlling growth conditions during CVD allows for targeted regulation of the morphology and structural characteristics of MoS₂, opening up prospects for its application in electronics, optoelectronics, and nanoenergy. Further research should focus on precisely correlating growth parameters, morphology, and the material’s functional properties to develop reproducible methods for producing high-quality single-crystal MoS₂ films.

5. XPS Investigation of Defect States, Oxidation, and Interfacial Chemistry in MoS₂ Heterostructure

X-ray photoelectron spectroscopy (XPS) was employed to examine the chemical states of molybdenum and sulfur in MoS₂ films synthesized at different temperatures. Figure 11 and Figure 12 display the XPS spectra for the Mo3d and S2p core levels, respectively. The Mo3d spectra were resolved into three distinct doublets. The first, with a Mo3d_5/2 binding energy (BE) of 229.1 eV and a spin–orbit splitting of 3.15 eV, corresponds to Mo⁴⁺ in 2H-MoS₂. The second doublet, at 232.5 eV, indicates Mo⁶⁺ in MoO₃, while the third, at 228.3 eV, is linked to Mo in substoichiometric MoS_x [87,88]. Notably, Mo⁶⁺ concentration remained consistently around 5 at.%, whereas the MoS_x fraction increased with deposition temperature from ~1.3 at.% to ~5.4 at.% [89].

These results highlight temperature’s critical role in forming substoichiometric MoS_x phases, which impacts structural and electronic properties. Understanding these correlations is vital for optimizing MoS₂ growth and its performance in applications like catalysis, electronics, and energy storage. This work systematically explores the effect of deposition temperature on the structural, chemical, and electronic characteristics of MoS₂ thin films.

Figure 13 illustrates the temperature dependence of the MoS_x concentration estimated from XPS data. According to [90], the BE of 228.3 eV for MoS_x corresponds to a [S]/[Mo] ratio of ~1.3. In stoichiometric MoS₂, each Mo atom is bonded to six S atoms, while in MoS_x this ratio implies that each Mo atom lacks, on average, two S neighbors. To avoid triple counting of missing S atoms, the concentration of sulfur vacancies (V_s) was calculated as [89]:

$$V_s\approx {\displaystyle \frac{2M{o}_{x+}}{3\left(M{o}_{x+}+M{o}^{4+}\right)}}\times 100\%$$

(1)

where Mo_x⁺ represents the MoS_x component intensity and Mo⁴⁺ the MoS₂ component intensity. The results indicate that V_s increases with deposition temperature, rising from ~0.9 at.% at 650 °C to ~3.6 at.% at 950 °C.

The [S]/[Mo] ratio, when calculated directly, remains remarkably stable at 2.0 ± 0.1 for every sample. Nevertheless, this direct calculation is unable to reveal subtle compositional variations close to the XPS sensitivity limit. Additionally, the S2p spectra undergo minor alterations with increasing temperature, including a small shift (approximately 0.1 eV) towards reduced binding energy [89].

Figure 14 presents a detailed characterization of MoS₂ grown on untreated GaN substrates. Panel (a) shows an optical image of the MoS₂ flakes, revealing lateral sizes of approximately 5 µm. The Raman spectra in panel (b) compare MoS₂/GaN with pure MoS₂ and GaN, showing characteristic GaN peaks at E₂(high) = 570.1 cm⁻¹, A₁(low) = 736.5 cm⁻¹, and Al₂O₃ E₁(g) = 418.2 cm⁻¹, alongside the MoS₂ E₂(g) (~385.8 cm⁻¹) and A₁(g) (~403.2 cm⁻¹) modes. For the MoS₂/GaN heterostructure, the A₁(g) peak shifts to 405.5 cm⁻¹, indicating a blue shift upon interaction with the substrate. The peak separation Δk between E₂(g) and A₁(g) increases from 17.4 cm⁻¹ in pure MoS₂ to 19.7 cm⁻¹ in the heterostructure, while an additional Si-related peak is observed at 520.7 cm⁻¹.

Panels (c–e) show XPS analysis of the core levels of Mo (3d), S (2p), and Ga (3d). Charge correction was applied using C 1s at 284.6 eV. The Mo (3d) spectrum displays peaks at 229.6 eV (Mo 3d_5/2) and 232.8 eV (Mo 3d_3/2), confirming Mo-S bonding in the Mo⁴⁺ state, with minor features at 233.8 eV and 236.3 eV corresponding to Mo⁶⁺ states (Mo-O). The S (2s) peak at 226.8 eV further supports Mo-S bonding. Deconvolution of the S (2p) spectrum reveals a spin–orbit doublet at 165.1 and 166.3 eV (Δ = 1.2 eV), consistent with the formation of MoS₂. Using these data, the sulfur stoichiometry and vacancy concentration were estimated [90].

These results collectively demonstrate that the MoS₂/GaN heterostructure maintains the intrinsic chemical composition of MoS₂ while exhibiting substrate-induced modifications in vibrational properties, highlighting the role of substrate interactions in tuning the structural and electronic characteristics of the films.

$${\displaystyle \frac{S(\mathit{at}.\%)}{\mathit{Mo}(\mathit{at}.\%)}}={\displaystyle \frac{\frac{I_S(2s)}{{\sigma }_S(2s)}}{\frac{I_{\mathit{Mo}}3d\left(\frac{5}{2}\right)}{{\sigma }_{\mathit{Mo}}3d\left(\frac{5}{2}\right)}}}$$

(2)

Atomic percentages of sulfur and molybdenum are indicated by S (atomic %) and Mo (atomic %), respectively. The integrated peak intensities for S (2s) and Mo (3d_5/2) are designated as IS (2s) and IMo 3d(5/2). The parameters σS(2s) and σMo3d(5/2) are obtained from well-regarded scholarly publications [91]. Analysis of the Ga (3d) level in the heterostructure, as shown in Figure 14e, demonstrates three identifiable components after peak deconvolution. The most significant component is associated with Ga-N bonds, while the other two are attributed to Ga-O bonds and the N (2s) state. This interpretation is in agreement with existing literature [92].

Figure 14. (a) Optical Image of MoS₂ on untreated GaN (b) Raman spectra of bare GaN, bare MoS₂ and the heterostructure. (c) XPS Mo-3d Spectra for MoS₂/GaN compared with bare MoS₂. (d) XPS core level spectra of bare MoS₂, (e) Mo-3d and S 2p, and (f) Ga-3d for the heterostructure [92].

Figure 14. (a) Optical Image of MoS₂ on untreated GaN (b) Raman spectra of bare GaN, bare MoS₂ and the heterostructure. (c) XPS Mo-3d Spectra for MoS₂/GaN compared with bare MoS₂. (d) XPS core level spectra of bare MoS₂, (e) Mo-3d and S 2p, and (f) Ga-3d for the heterostructure [92].

To thoroughly investigate the atomic makeup, chemical characteristics, and the influence of NaCl during the formation of MoS₂, XPS was employed on MoS₂ crystals produced at 780 °C (Figure 15).

Figure 15a presents the overall spectrum of the MoS₂ sample. Detailed analysis of the Mo 3d region (Figure 15b) revealed binding energy (BE) peaks at 229.68 and 232.80 eV. These correspond to the 3d_5/2 and 3d_3/2 orbitals of Mo in a +4 oxidation state (Mo⁴⁺), which is the expected state for molybdenum within the MoS₂ crystal structure [93]. A minor peak observed at 235.98 eV is likely attributable to molybdenum oxides, possibly MoO₃. This could stem from unreacted MoO₃ starting materials, a common occurrence in MoS₂ synthesized via CVD, and would manifest as peaks for Mo in a +6 oxidation state (Mo⁶⁺) [94,95].

A Mo 3d_5/2 peak at 226.99 eV suggests some degree of sulfur oxidation. The S 2p spectra (Figure 15c) displayed spin–orbit split peaks at 162.62 and 163.80 eV, representing the 2p_3/2 and 2p_1/2 states, respectively, with an energy difference of 1.18 eV. For calibration purposes, the C 1s peak was found at 284.28 eV (Figure 15d), and the O 1s peak was centered at 532.78 eV, likely originating from an oxide layer on the silicon substrate (Figure 15e). Furthermore, the Na 1s spectrum showed a peak at 1071.69 eV (Figure 15f), confirming the presence of sodium. This indicates that sodium from the NaCl precursor may persist under the MoS₂ crystals as intermediate growth compounds, such as Na₂Mo_xO_x [96] and Na₂SO₄ [97].

Figure 15. XPS analysis of MoS₂ grown at 780 °C: (a) XPS survey. (b) Peaks for Mo 3d orbitals 3d5/2 and 3d3/2 at 229.68 and 232.80 eV, respectively. Sulfur oxidation peak at 226.99 eV. A peak of low intensity observed at 235.98 eV representing MoO₃. (c) Orbitals of S 2p, 2p3/2 and 2p1/2, observed at 162.62 and 163.80 eV, respectively. (d) Carbon calibration peak at 284.28 eV. (e) O 1s peak at 532.78 eV representing the oxide layer of the Si substrate. (f) Na 1s peak observed at 1071.69 eV representing Na₂Mo_xO_x or Na₂SO₄ [96,97].

Figure 15. XPS analysis of MoS₂ grown at 780 °C: (a) XPS survey. (b) Peaks for Mo 3d orbitals 3d5/2 and 3d3/2 at 229.68 and 232.80 eV, respectively. Sulfur oxidation peak at 226.99 eV. A peak of low intensity observed at 235.98 eV representing MoO₃. (c) Orbitals of S 2p, 2p3/2 and 2p1/2, observed at 162.62 and 163.80 eV, respectively. (d) Carbon calibration peak at 284.28 eV. (e) O 1s peak at 532.78 eV representing the oxide layer of the Si substrate. (f) Na 1s peak observed at 1071.69 eV representing Na₂Mo_xO_x or Na₂SO₄ [96,97].

XPS studies showed that the chemical state and stoichiometry of MoS₂ depend significantly on both the synthesis temperature and storage conditions. Mo 3d spectra confirm the presence of three components: Mo⁴⁺ in the 2H-MoS₂ structure, 6+ as MoO₃ impurities, and a nonstoichiometric MoS_x phase. With increasing synthesis temperature, the MoS_x concentration increases, indicating an increase in the number of sulfur vacancies (V_s) in the material. Additionally, a shift and change in the shape of the S 2p peaks is observed, indicating defects and a change in the chemical environment of sulfur [98,99,100].

Long-term aging of the samples in air and moisture leads to increased oxidation of MoS₂, manifested by an increase in the intensity of the MoO₃ peaks and the formation of S-O bonds. These results are consistent with Raman spectroscopy data, which reveal signatures of MoO₃. For MoS₂/GaN heterostructures, XPS and Raman analysis reveal strong chemical interactions at the interface, including shifts in the positions of characteristic peaks, indicating the influence of sulfur defects and interfacial bonds [101,102,103].

Thus, XPS analysis confirmed the key role of growth temperature and storage conditions in the formation of sulfur vacancies, oxidized phases, and interfacial interactions. These factors directly affect the electronic and optical properties of MoS₂ and should be considered when developing functional heterostructures and devices.

6. Investigation of DC Conductivity, Gas Sensing, and Electrochemical Behavior of MoS₂ Nanocomposites

A wide variety of studies have investigated the application potential of MoS₂-based materials in fields such as gas sensing, electrochemical energy storage, and battery technology, demonstrating the material’s multifunctionality and adaptability. However, no unified criterion was applied in selecting these references; instead, this section presents representative examples illustrating the broad range of MoS₂ functionalities reported in the literature.

To explore the electrical and sensing performance of MoS₂ and its polymer composites, several studies have employed four-probe conductivity measurements under controlled environments. For instance, the DC conductivity and ammonia sensing behavior of MoS₂, polypyrrole (PPy), and PPy/MoS₂ nanocomposites were examined using a four-probe setup coupled with a PID-controlled furnace [104]. As shown schematically in Figure 16, this configuration enabled precise temperature control and reliable data acquisition, establishing a foundation for understanding the interrelation between material structure and sensing response.

Figure 17 illustrates the standard four-probe method employed to ascertain the initial direct current (DC) conductivity of MoS₂, PPy, and PPy/MoS₂ nanocomposites. Room temperature measurements revealed conductivities of 2.2 × 10⁻⁶ S·cm⁻¹ for MoS₂, 2.03 S·cm⁻¹ for PPy, and 8.33 S·cm⁻¹ for PPy/MoS₂. The incorporation of MoS₂ into PPy resulted in a remarkable surge in the nanocomposite’s conductivity. This significant enhancement is attributed to the interaction between the lone pair electrons of nitrogen in polypyrrole and the molybdenum centers of MoS₂, which is proposed to generate additional holes in PPy, thereby elevating its electrical conductivity [104].

The electrochemical performance of MoS₂/BC₃ nanocomposite, pristine BC, and pure MoS₂ nanosheet electrodes is depicted in Figure 18 through cyclic voltammetry curves. These were recorded in 1 M H₂SO₄ electrolyte within the 0–0.5 V potential window at scan rates from 20 to 100 mV·s⁻¹. For improved clarity and comparison, the figure has been refined with consistent colors, normalized current scales, and clear scan rate labels [105,106,107].

Quasi-rectangular CV profiles were observed for the MoS₂/BC₃ NC and pristine BC electrodes, signifying the presence of both electric double-layer capacitance and pseudocapacitance. The MoS₂ NS electrode, however, displayed distinct faradaic redox peaks, characteristic of purely pseudocapacitive behavior [108,109,110].

The integration of the porous BC₃ framework with MoS₂ nanosheets in the composite resulted in a substantially larger CV area and a higher specific capacitance (34 F·g⁻¹ at 20 mV·s⁻¹), confirming enhanced charge storage due to synergy. The improved Figure 17 clearly highlights these electrochemical differences and the superior capacitive capabilities of the MoS₂/BC₃ nanocomposite [111].

Figure 18. CV profiles of (a) pristine MoS₂ NSs, (b) pristine BC [111].

Figure 18. CV profiles of (a) pristine MoS₂ NSs, (b) pristine BC [111].

Figure 18a,b presents the cyclic voltammetry curves for electrodes incorporating MoS₂/BC₃ nanocomposites. For comparison, the CV profiles of unmodified BC and MoS₂ nanosheets are also shown. These measurements were conducted within a potential range of 0–0.5 V, employing scan rates from 20 to 100 mV·s⁻¹ in a 1 M H₂SO₄ electrolyte. Electrodes featuring the MoS₂/BC₃ NC and those based on BC alone demonstrated CV shapes that were nearly rectangular. This suggests that their capacitive behavior arises from a combination of electric double-layer capacitance and pseudocapacitive processes. In contrast, electrodes made with MoS₂ NS exhibited distinct faradaic redox peaks, indicative of pseudocapacitance.

The synergistic integration of the highly porous BC framework with the layered MoS₂ NS within the MoS₂/BC₃ NC led to a larger CV area, which in turn translated to a higher specific capacitance. At a scan rate of 20 mV·s⁻¹, the composite material achieved a specific capacitance of 34 F·g⁻¹, substantially exceeding the capacitance values of the individual MoS₂ and BC components. At slower scan rates, the capacitance further improved, attributed to more extensive ion penetration into the porous structure.

These findings underscore a cooperative interaction between MoS₂ and BC, which enhances the specific surface area, electrical conductivity, and overall capacitive performance. This highlights the potential of MoS₂/BC₃ NC as a cutting-edge material for hybrid supercapacitor applications [112].

Given the growing demand and escalating prices of lithium-ion batteries (LIBs), sodium-ion batteries (SIBs) have attracted increasing attention as a cost-effective and sustainable alternative for large-scale energy storage. To achieve the commercial viability of SIBs, further progress in electrode materials, electrolytes, and current collector design remains essential. In particular, the development of advanced electrode fabrication techniques and strategies aimed at enhancing specific capacity and cycling stability can accelerate their integration into practical systems.

Among various electrode candidates, MoS₂-based materials have demonstrated promising electrochemical properties due to their layered structure, high surface area, and efficient ion intercalation capability. Numerous studies have reported on MoS₂ composites and heterostructures tailored for improved performance in SIBs.

Table 3. Comparison of LIB performances using different MoS₂/carbon materials [114].

Structure	Electrode Materials	Fabrication Methods	Capacity	Ref.
Nanocable webs	MoS2 + Graphene	Unique side to face contact mode	1150 mA h g−1	[114]
Core–shell microsphere	MoS2 + N-doped carbon	One pot hydrothermal synthesis	856 mA h g−1	[115]
Nanobundle	MoS2 + MWCNT	Dry grinding process using a mortar and pestle	1214 mA h g−1	[116]
Nanobowl	MoS2 + C	Solvothermal synthesis	1164.4 mA h g−1	[117]
Flowerlike nanosheet	MoS2 + 3D Graphene foam	Hydrothermal synthesis	1127 mA h g−1	[118]
Hollow microsphere with onion-like solid nanosphere	MoS2 + C	One step hydrothermal synthesis	1107 mA h g−1	[119]
Hierarchical nanofiber	MoS2 + active carbon cloth	Simple dissolution and sintering method	971 mA h g−1	[120]
Flowerlike network	MoS2 + rGO + MWCNT	Hydrothermal synthesis	1275 mA h g−1	[121]
Sandwich like nanosheet	MoS2 + N-doped carbon	Self-polymerizing and carbonization process	1239 mA h g−1	[122]
Nanowires	1D−2D NiMoO4 Nanowires + Metallic 1T MoS2 Composite	Hydrothermal method using blade and spray coating.	941 mA h g−1	[123]
Nanosheets	Vertically aligned 1T-MoS2 on a graphene frame	Solvothermal method	666 mAh g−1	[124]
Nanotubes	Metallic MoS2	Solvothermal method	1150 mAh g−1	[125]

provides a comparative summary of recent research efforts on MoS₂-based battery systems, highlighting the diversity of synthesis approaches, structural modifications, and electrochemical characteristics investigated across different studies [113].

Studies of the electrical conductivity and electrochemical properties of composites based on MoS₂ and carbon-containing materials (PPy, BC, PANI, graphene, etc.), as well as thin films of gold nanorods and nanoparticles, demonstrated a significant improvement in the functional properties of the materials when forming hetero- and nanostructures [126,127,128]. The introduction of MoS₂ into the polymer matrix led to a sharp increase in electrical conductivity due to the interaction of nitrogen-containing centers with molybdenum, while multilayer structures of gold nanorods provided a dramatic reduction in sheet resistance and an increase in conductivity [129,130].

Electrochemical studies confirmed the synergistic effect between MoS₂ and carbon materials, manifested in improved capacitive behavior and an increase in specific capacitance, making these composites promising for the development of hybrid supercapacitor electrodes. Furthermore, microwave absorbers based on MoS₂-PANI demonstrated high absorption of electromagnetic radiation due to their increased specific surface area and improved electrical conductivity. A comparative analysis of lithium- and sodium-based batteries using MoS₂/carbon composite materials revealed that optimization of morphology (nanocables, nanobundles, nanocups, nanotubes, etc.) and synthesis methods (hydrothermal, solvothermal, carbonization, etc.) provides a significant increase in specific capacity-up to 1275 mA h g⁻¹. This confirms that modified MoS₂ structures play a key role in the creation of new generations of energy storage devices and functional materials.

Thus, the development of MoS₂-based composites opens up broad prospects for sensors, supercapacitors, microwave absorbers, and next-generation batteries.

7. Challenges and Future Directions

Despite considerable advancements in creating and utilizing MoS₂-based nanocomposites, several fundamental and practical hurdles persist. Overcoming these limitations is vital to fully unlock the capabilities of MoS₂ thin films and coatings for next-generation systems in energy, electronics, and protection.

• Managing Structural Imperfections and Layer Consistency.

Achieving consistent, defect-free MoS₂ layers is a primary challenge for large-scale production. Discrepancies in sulfur content, grain boundaries, and stacking faults significantly impact MoS₂’s electronic and catalytic effectiveness. Future investigations should prioritize real-time monitoring during CVD growth to precisely control layer thickness, nucleation density, and defect formation.

• Scaling Up and Ensuring Consistency in Deposition Techniques.

While CVD and its variations yield high-quality films, achieving scalability without sacrificing crystal perfection remains difficult. Hybrid deposition systems, such as plasma-assisted or low-temperature CVD, could bridge the gap between laboratory-scale production and industrial application. Furthermore, understanding the thermodynamics of precursor breakdown and gas flow dynamics is crucial for improving process reliability.

• Optimizing Interfaces and Adhesion.

Robust interfacial bonding between MoS₂ and various substrates (metals, ceramics, or polymers) is essential for mechanical resilience and long-term performance. Poor adhesion frequently leads to separation or cracking under thermal or mechanical stress. Future studies should explore strategies for modifying interfaces, including gradient coatings, intermediate adhesion layers, or surface functionalization methods.

• Ensuring Durability in Various Environments and Operating Conditions.

MoS₂-based coatings can degrade through oxidation, phase changes, or breakdown when exposed to high temperatures, humidity, and corrosive chemical environments. Developing passivation techniques, protective encapsulation layers, or compositional modifications (e.g., MoS₂/graphene or MoS₂/h-BN hybrids) could significantly enhance their longevity and dependability in demanding conditions.

• Integrating Analysis and Data-Driven Refinement.

The intricate relationship between MoS₂ synthesis, its structure, and its properties necessitates sophisticated characterization and modeling approaches. Combining in situ spectroscopy, machine learning algorithms, and high-throughput simulations will expedite the discovery of optimized synthesis methods and performance predictions for specific applications.

• Tailoring Designs for Specific Applications and Identifying Niche Uses.

While MoS₂ has been extensively researched for catalysis and energy storage, its potential in protective coatings, flexible electronics, tribology, and thermal management remains largely unexplored. Future research should focus on identifying these specialized areas where MoS₂ thin films can offer unique advantages, such as low friction or radiation resistance.

8. Conclusions

This review evaluated the key factors governing the synthesis and properties of MoS₂, with an emphasis on CVD while also considering the broader context of alternative fabrication methods. Although CVD remains the most versatile approach for producing wafer-scale and high-quality MoS₂ films, the analysis shows that material characteristics cannot be fully understood without comparing CVD results to exfoliation, hydrothermal growth, sulfurization of Mo/MoO₃ layers, and plasma-assisted techniques. These methods differ significantly in defect density, grain size, surface uniformity, and scalability, and their omission may lead to an incomplete understanding of structure-property relationships.

Raman, XPS, and PL characterizations consistently confirmed the formation of layered MoS₂, but the comparison of different synthesis strategies revealed that the stability of vibrational modes, oxidation tendency, and excitonic responses strongly depend on growth route and post-growth environment. In particular, sulfur deficiency and surface oxidation remain common challenges across all synthesis techniques, not only CVD-derived films. This underscores the need for improved passivation strategies and controlled atmosphere processing.

Importantly, the review highlights that the performance of MoS₂ is shaped by three interconnected factors:

(1)

Synthesis method and associated growth conditions;

(2)

Defect chemistry and stoichiometric deviations;

(3)

Environmental stability during storage and operation.

A critical comparison of recent studies shows that optimizing only one of these factors is insufficient; reliable device functionality requires an integrated approach that accounts for all three.

Looking ahead, future progress should focus on unifying experimental and theoretical efforts. This includes using in situ diagnostics, kinetic modeling, and surface engineering to control defect formation mechanisms and interfacial processes with higher precision. A more systematic comparison of CVD with alternative growth techniques is also essential for establishing clear guidelines for selecting synthesis routes based on target applications.

Such a framework will support the rational design of MoS₂-based materials for electronics, optoelectronics, catalysis, and protective coatings, enabling a more consistent transition from laboratory demonstrations to practical technologies.