Hydrothermal synthesis has emerged as a powerful and versatile method for producing high-quality two-dimensional (2D) nanocrystals, particularly transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2). This technique leverages high-temperature and high-pressure aqueous environments to facilitate the crystallization of layered materials, offering precise control over morphology, size, and phase composition. The process is particularly advantageous for scaling up production while maintaining the structural integrity and functional properties of the resulting nanocrystals.

The synthesis begins with the selection of appropriate precursors, typically metal salts and chalcogen sources. For MoS2, common precursors include ammonium molybdate and thiourea, while for WS2, sodium tungstate and thioacetamide are frequently employed. These precursors dissolve in a solvent, often water or a mixture of water and organic solvents, to form a homogeneous solution. The choice of precursors and their molar ratios directly influence the stoichiometry and crystallinity of the final product. Under hydrothermal conditions, temperatures ranging from 120°C to 250°C and pressures exceeding ambient conditions drive the reaction kinetics, promoting the formation of layered structures.

The chemistry of layered precursor transformation under hydrothermal conditions involves several key steps. Initially, the metal and chalcogen precursors undergo hydrolysis and condensation reactions, forming intermediate clusters. These clusters then assemble into layered structures through a combination of Ostwald ripening and oriented attachment mechanisms. The high-pressure environment suppresses the volatility of chalcogens, ensuring their incorporation into the crystal lattice. The presence of reducing agents, such as hydrazine or sodium borohydride, can further modulate the reaction by controlling the oxidation state of the metal centers, leading to the formation of semiconducting 2H-phase or metallic 1T-phase TMDs.

Exfoliation during hydrothermal synthesis occurs in situ, distinguishing it from post-synthesis mechanical or chemical exfoliation methods. The growth of 2D nanocrystals is facilitated by the intercalation of solvent molecules or ions between the layers, reducing van der Waals forces and promoting layer separation. For instance, the use of ammonia or hydrazine can lead to the intercalation of ammonium ions, which expand the interlayer spacing and weaken the interactions between adjacent layers. Simultaneously, the high-temperature environment provides the energy required to overcome these weakened interactions, resulting in the formation of ultrathin nanosheets. The thickness and lateral dimensions of the nanocrystals can be tuned by adjusting parameters such as reaction time, temperature, and precursor concentration.

The resulting 2D TMD nanocrystals exhibit exceptional electronic properties, making them suitable for a wide range of applications. In electronics, MoS2 and WS2 nanosheets serve as channel materials in field-effect transistors (FETs) due to their high carrier mobility and tunable bandgaps. The semiconducting 2H phase exhibits a direct bandgap in monolayer form, enabling efficient light-matter interactions for optoelectronic devices such as photodetectors and light-emitting diodes. The metallic 1T phase, on the other hand, shows promise as a conductive electrode material or as an interlayer in vertical heterostructures.

Catalysis is another area where hydrothermally synthesized 2D TMD nanocrystals excel. Their high surface area and exposed edge sites make them highly active for hydrogen evolution reactions (HER). For example, MoS2 nanosheets with sulfur vacancies or doped with transition metals exhibit enhanced catalytic activity, approaching the performance of platinum-based catalysts. The basal planes of these materials are generally inert, but the edges provide abundant active sites for proton adsorption and reduction. In addition to HER, these materials are effective for hydrodesulfurization in petroleum refining and for the reduction of nitrogen oxides in environmental remediation.

The hydrothermal method also enables the synthesis of heterostructured nanocrystals, where two or more 2D materials are combined to create interfaces with novel properties. For instance, MoS2-WS2 heterostructures can be synthesized by sequential or co-precipitation of precursors, leading to type-II band alignment that facilitates charge separation for photovoltaic or photocatalytic applications. The ability to control the composition and interface quality at the atomic level is a unique advantage of hydrothermal synthesis compared to other methods.

Despite its advantages, hydrothermal synthesis requires careful optimization to avoid common pitfalls such as incomplete crystallization or phase impurities. The pH of the reaction medium, for example, plays a critical role in determining the stability of intermediates and the final product morphology. Acidic conditions may favor the formation of amorphous aggregates, while alkaline conditions promote crystalline growth. Similarly, the choice of solvent can influence the nucleation rate and crystal habit, with organic additives like ethylene glycol sometimes used to modulate reactivity and surface energy.

In summary, hydrothermal synthesis offers a robust and scalable route to 2D TMD nanocrystals with controlled properties. The method’s ability to integrate precursor chemistry, crystallization, and exfoliation into a single step simplifies production while maintaining high material quality. The resulting nanocrystals find applications in next-generation electronics and catalysis, where their unique structural and electronic properties enable breakthroughs in performance and functionality. Continued advancements in understanding the growth mechanisms and kinetics will further enhance the precision and versatility of this synthesis approach.