Molybdenum disulfide (MoS₂), a representative layered transition metal dichalcogenide, exhibits physical and chemical properties at the nanoscale that differ significantly from its bulk counterpart. With advances in nanofabrication techniques and two-dimensional materials research, nano-scale MoS₂ has attracted considerable attention in frontier fields including electronic devices, energy storage, catalysis, sensing, and biomedicine. This article systematically reviews the structural characteristics, synthesis methods, frontier applications, and industrialization challenges of nano-scale MoS₂.

Structural Characteristics of Nano-Scale MoS₂

### Dimensional Transition from Bulk to Nanoscale

Bulk MoS₂ crystallizes in the 2H phase, belonging to the hexagonal crystal system with space group P6₃/mmc. Its crystal structure consists of S-Mo-S triple layers stacked along the c-axis, with strong covalent bonding within layers and weak van der Waals forces between layers. When MoS₂ is thinned along the c-axis to monolayer or few-layer (≤5 layers) form, the material's optical, electrical, and catalytic properties undergo significant changes.

Bulk MoS₂ is an indirect-gap semiconductor with a bandgap of approximately 1.2 eV; monolayer MoS₂ transforms into a direct-gap semiconductor with an increased bandgap of approximately 1.8-1.9 eV. This dimensional effect endows nano-scale MoS₂ with unique advantages in photodetection and light-emitting devices. Furthermore, the dramatically increased specific surface area of nano-scale MoS₂ and the higher proportion of exposed edge active sites are of great significance for catalytic reactions and sensor responses.

### Phase Engineering: 2H and 1T Phases

In addition to the thermodynamically stable 2H phase (trigonal prismatic coordination), MoS₂ also exists in a metastable 1T phase (octahedral coordination). The 1T-phase MoS₂ is metallic, with electrical conductivity approximately 10⁷ times higher than the 2H phase. Chemical exfoliation (e.g., lithium intercalation) can partially convert 2H-phase MoS₂ to the 1T phase, and the resulting mixed-phase nanosheets exhibit excellent performance in electrocatalysis and supercapacitors. However, the 1T phase is thermodynamically unstable and prone to spontaneous phase transformation to the 2H phase, representing a key challenge for industrial applications.

Synthesis Methods for Nano-Scale MoS₂

### Liquid-Phase Exfoliation

Liquid-phase exfoliation involves delaminating bulk MoS₂ in solvents through ultrasonication or shear forces. Common solvents include N-methylpyrrolidone (NMP), dimethylformamide (DMF), and isopropanol, whose surface tensions match the surface energy of MoS₂ for efficient exfoliation. The sulfur content in the exfoliated dispersion can be measured following ASTM D4294 methods to indirectly assess exfoliation efficiency. This method is straightforward and scalable but produces nanosheets with non-uniform thickness and a low 1T-phase content.

### Chemical Vapor Deposition (CVD)

CVD enables the growth of large-area, high-quality monolayer or few-layer MoS₂ films on various substrates. A typical process uses molybdenum trioxide (MoO₃) and sulfur powder as precursors, reacting at 700-900°C in a tube furnace. By controlling the temperature gradient, carrier gas flow rate, and precursor dosage, the number of layers, domain size, and film uniformity of MoS₂ can be tuned. CVD-grown MoS₂ films possess high crystal quality suitable for electronic device research, though equipment costs are high and yields are limited.

### Hydrothermal/Solvothermal Synthesis

The hydrothermal method uses sodium molybdate (Na₂MoO₄) and thiourea (CH₄N₂S) or L-cysteine as precursors, reacting at 180-240°C in an autoclave for 12-48 hours to synthesize MoS₂ nanoflowers, nanosheets, or quantum dots. This method requires modest equipment, offers morphology controllability, and is suitable for batch preparation of nano-scale MoS₂ powders. According to GB/T 23274-2009 standards for MoS₂ content determination, hydrothermal products typically contain certain amounts of amorphous phases and impurity sulfides, requiring subsequent annealing to improve crystallinity.

### Lithium Intercalation Exfoliation

This method utilizes n-butyllithium (n-BuLi) to intercalate lithium ions between MoS₂ layers, weakening the interlayer van der Waals forces, followed by ultrasonication in water to produce monolayer nanosheets. Lithium intercalation can efficiently yield monolayer MoS₂ with a relatively high proportion of the 1T phase. However, this method uses flammable organolithium reagents, demanding strict operational safety, and the exfoliated nanosheets are susceptible to oxidation and phase degradation in air.

Frontier Application Research

### Electronic Devices and Logic Circuits

Monolayer MoS₂, with its direct bandgap and high on/off ratio (up to 10⁸), is regarded as one of the most promising two-dimensional semiconductor materials after graphene. Field-effect transistors (FETs) based on monolayer MoS₂ have achieved room-temperature carrier mobilities exceeding 200 cm²/(V·s) and current densities surpassing 480 μA/μm. The International Technology Roadmap for Semiconductors (ITRS) has included MoS₂ and other 2D materials as candidate channel materials for future transistor nodes.

Furthermore, van der Waals heterostructures constructed from MoS₂ and hexagonal boron nitride (h-BN) exhibit novel functionalities in tunneling FETs, photodetectors, and memory devices. By tuning the number of MoS₂ layers and the stacking angle, band engineering and spin-orbit coupling can be precisely controlled.

### Electrocatalytic Hydrogen Evolution Reaction (HER)

Edge sites of nano-scale MoS₂ exhibit excellent hydrogen evolution catalytic activity, with the hydrogen adsorption free energy (ΔG_H) of edge Mo atoms approaching the ideal 0 eV value of platinum (Pt), making it a candidate electrocatalyst to replace the noble metal Pt. According to ASTM F76 standard test methods, few-layer MoS₂ in acidic electrolytes can achieve overpotentials as low as 150-200 mV (vs RHE, at 10 mA/cm²) with Tafel slopes of approximately 40-60 mV/dec.

Through defect engineering (sulfur vacancy introduction), phase engineering (1T phase modulation), and composite strategies (combining with conductive supports such as graphene and carbon nanotubes), the catalytic activity of MoS₂ can be further enhanced. Sulfur vacancy concentrations in the 5%-15% range yield optimal catalytic performance, a conclusion validated by density functional theory (DFT) calculations.

### Lithium-Ion Batteries and Supercapacitors

As an anode material for lithium-ion batteries, nano-scale MoS₂ has a layered structure favorable for Li⁺ intercalation and deintercalation, with a theoretical specific capacity of 670 mAh/g (far exceeding the 372 mAh/g of graphite). However, MoS₂ suffers from volume expansion (approximately 100%), poor conductivity, and polysulfide shuttling during charge-discharge cycles, leading to unsatisfactory cycling and rate performance.

By compositing MoS₂ nanosheets with carbon materials (graphene, carbon nanotubes, porous carbon) to construct three-dimensional conductive networks, volume expansion can be effectively mitigated and electron conduction improved. According to GB/T 18287 standard test methods, MoS₂/graphene composite anodes can achieve capacity retention above 80% after 100 cycles at 0.5 C rate.

In the supercapacitor field, 1T-phase MoS₂ exhibits excellent pseudocapacitive energy storage performance due to its high conductivity and interlayer ion accessibility. In aqueous electrolytes, 1T-MoS₂ electrodes can achieve area-specific capacitances of 2-5 F/cm², with both energy density and power density surpassing those of conventional carbon-based supercapacitors.

### Gas and Biosensors

The high specific surface area and strong adsorption capability for gas molecules make nano-scale MoS₂ promising for gas sensing applications. Research has shown that monolayer MoS₂ can achieve ppb-level detection limits for NO₂ and NH₃ at room temperature. MoS₂ FET-based sensors achieve gas identification by monitoring channel current changes, with response times as short as several seconds.

In biosensing, MoS₂ nanosheets have been used for the detection of glucose, DNA, proteins, and tumor markers. The fluorescence quenching property of MoS₂ enables label-free nucleic acid detection with detection limits at the pM level. Additionally, the photothermal conversion capability of MoS₂ nanosheets (near-infrared absorption) shows therapeutic potential in tumor photothermal treatment.

Industrialization Challenges and Development Directions

### Scalable Production and Quality Control

Although various synthesis methods for nano-scale MoS₂ have been demonstrated in laboratories, scalable production remains challenging. Liquid-phase exfoliation struggles to control nanosheet layer distribution; CVD offers limited yields at high costs; and hydrothermal products suffer from insufficient crystallinity. Standardized quality testing systems (e.g., particle size distribution, layer number statistics, phase ratio, defect density) are not yet well-established, and inter-batch product consistency needs improvement.

### Stability and Environmental Impact

Nano-scale MoS₂ is prone to oxidative degradation in air, particularly under high-temperature and humid conditions, where MoS₂ surfaces form MoO₃ and SO₂. The thermodynamic instability of 1T-phase MoS₂ also limits its long-term use. Regarding environmental impact, research on the aquatic toxicity and bioaccumulation of nano-MoS₂ is still in early stages, requiring systematic ecotoxicological assessment following OECD test guidelines.

### Cost and Market Competition

The production cost of nano-scale MoS₂ is significantly higher than that of micron-scale industrial MoS₂ powder (typically priced at tens to hundreds of yuan per kilogram). In traditional applications such as lubrication and friction modification, the performance advantages of nano-scale products are insufficient to offset the cost gap. The industrialization breakthrough for nano-scale MoS₂ lies in high-value-added fields such as flexible electronics, precision sensors, and efficient catalysts.

Conclusion

Nano-scale molybdenum disulfide, leveraging the unique optoelectronic, catalytic, and sensing properties brought by dimensional effects, occupies an important position in frontier new materials research. From synthesis method optimization to application scenario expansion, related research is advancing from laboratory to pilot-scale and industrialization. With the maturation of scalable production technologies, the establishment of quality control standards, and the deepening of application validation, nano-scale MoS₂ is expected to achieve commercial deployment in high-value-added fields such as electronic devices, clean energy, and precision sensing, opening new growth directions for the traditional MoS₂ industry.