Liquid-phase exfoliation (LPE) and solvothermal synthesis are two prominent solution-based methods for producing two-dimensional (2D) transition metal dichalcogenides (TMDCs) such as MoS2 and WS2. These techniques offer scalable and cost-effective routes to synthesize high-quality 2D materials with controlled layer thickness and tailored properties. Unlike chemical vapor deposition (CVD), which requires high temperatures and vacuum conditions, solution-based methods enable large-scale production and compatibility with flexible substrates, making them attractive for applications in catalysis, electronics, and energy storage.

Liquid-phase exfoliation involves the dispersion of bulk layered materials in a solvent, followed by mechanical or chemical exfoliation to produce thin layers. The process begins with the selection of a suitable solvent that matches the surface energy of the target material to minimize aggregation and ensure stable dispersion. Common solvents include N-methyl-2-pyrrolidone (NMP), isopropanol, and dimethylformamide (DMF), which exhibit surface tensions close to that of TMDCs, promoting efficient exfoliation. Ultrasonication or shear mixing is then applied to overcome the van der Waals forces between layers, yielding monolayer or few-layer nanosheets. The exfoliation efficiency depends on parameters such as solvent properties, sonication time, and power. For instance, prolonged sonication can reduce lateral flake dimensions but may introduce defects.

Intercalation strategies are often employed to facilitate exfoliation by weakening interlayer interactions. Alkali metal intercalation, using compounds like n-butyllithium, inserts ions between the layers, expanding the interlayer spacing and reducing the energy required for exfoliation. Electrochemical intercalation offers a more controlled approach, where an applied potential drives ion insertion into the bulk material. After intercalation, mild agitation or sonication in a polar solvent leads to spontaneous exfoliation. The choice of intercalant affects the final material quality; for example, lithium intercalation can induce phase transitions in MoS2 from the semiconducting 2H phase to the metallic 1T phase, altering electronic properties.

Solvothermal synthesis, in contrast, involves the reaction of metal and chalcogen precursors in a sealed vessel at elevated temperatures and pressures. This method allows precise control over stoichiometry, crystallinity, and morphology. In a typical synthesis, precursors such as molybdenum chloride and thiourea are dissolved in a solvent like ethylene glycol or water, then heated to temperatures between 120 and 200 degrees Celsius. The high-pressure environment promotes the formation of well-defined 2D structures. Solvent selection is critical, as it influences reaction kinetics and product morphology. Polar solvents with high boiling points are preferred for their ability to dissolve precursors and sustain high-temperature reactions.

Layer thickness control in both LPE and solvothermal synthesis is achieved through optimization of process parameters. In LPE, centrifugation separates exfoliated flakes by size and thickness, with slower speeds isolating thicker aggregates and higher speeds collecting thinner nanosheets. In solvothermal synthesis, adjusting reaction time, temperature, and precursor concentrations can tune the lateral dimensions and layer numbers. For example, shorter reaction times tend to yield thinner nanosheets, while prolonged heating may result in multilayer growth. Additives such as surfactants or polymers can also stabilize thinner layers by preventing restacking.

The applications of solution-processed 2D chalcogenides are vast, particularly in catalysis and electronics. In catalysis, exfoliated MoS2 and WS2 exhibit enhanced activity for hydrogen evolution reactions (HER) due to their exposed edge sites and tunable electronic structures. The metallic 1T phase of MoS2, accessible through lithium intercalation, shows superior conductivity and catalytic performance compared to the 2H phase. Solvothermally synthesized TMDCs with controlled defects and doping further improve catalytic efficiency. These materials are also explored for photocatalytic applications, leveraging their visible-light absorption and charge carrier mobility.

In electronics, solution-processed TMDCs are integrated into flexible and printed devices. Thin-film transistors (TFTs) fabricated from exfoliated MoS2 demonstrate respectable mobilities, though typically lower than CVD-grown counterparts due to higher defect densities. However, the scalability and compatibility with roll-to-roll processing make solution-based methods appealing for large-area electronics. Hybrid systems combining exfoliated TMDCs with conductive polymers or nanoparticles enable tailored electronic properties for sensors and memory devices. For instance, MoS2-polyethyleneimine composites exhibit tunable resistance switching behavior, relevant for neuromorphic computing.

The distinct advantage of solution-based methods lies in their versatility for functionalization and composite formation. Covalent and non-covalent modifications can be introduced during or after synthesis to enhance solubility, stability, or performance. For example, thiol-terminated ligands grafted onto MoS2 edges improve dispersion in organic solvents and facilitate integration with polymers. Inks formulated from exfoliated TMDCs enable direct printing of conductive and semiconducting patterns, advancing wearable and flexible electronics.

Despite these advantages, challenges remain in achieving uniform flake sizes, minimizing defects, and scaling up production without compromising quality. Post-synthetic treatments such as annealing or chemical passivation can mitigate some defects, but further optimization is needed to rival the consistency of CVD-grown materials. Nevertheless, the ability to produce 2D chalcogenides at scale with tailored properties ensures continued interest in liquid-phase exfoliation and solvothermal synthesis for next-generation technologies.

In summary, liquid-phase exfoliation and solvothermal synthesis provide versatile pathways to produce 2D TMDCs with controlled layer thickness and functionality. Through careful solvent selection, intercalation strategies, and process optimization, these methods yield materials suitable for catalytic and electronic applications. While challenges in defect control persist, the scalability and adaptability of solution-based approaches position them as key enablers for advancing 2D material technologies beyond the limitations of conventional CVD growth.