Preparation of Tungsten Disulfide by Sol-Gel Method

The sol-gel method is a classic approach for preparing nanomaterials through solution-based chemical reactions. It is valued for its ability to achieve molecular-level uniform mixing and control the microstructure and morphology of the resulting products, offering unique advantages in the synthesis of tungsten disulfide (WS₂) nanomaterials.

The preparation of tungsten disulfide by sol-gel method follows the basic principle that a precursor sol is formed in solution from the tungsten source and sulfur source, and then the target product is generated via gelation, drying, and heat treatment processes. Compared to the hydrothermal method, the sol-gel method places greater emphasis on the chemical reactions of precursors in solution and the formation of a gel network, typically requiring subsequent heat treatment to enhance the crystallinity of WS₂. This method is well-suited for preparing WS₂ thin films, nanoparticles, or porous structures and finds wide application in catalysis, sensors, and energy storage.

In terms of raw material selection, commonly used tungsten precursors include tungsten tetrachloride (WCl₄), ammonium tungstate ((NH₄)₂WO₄), or tungsten ethoxide (W(OC₂H₅)₆); typical sulfur sources include thiourea (CS(NH₂)₂), hydrogen sulfide (H₂S), or carbon disulfide (CS₂); solvents such as ethanol, isopropanol, or water are often used to prepare the precursor solution; and to promote sol formation and gelation, acids (e.g., hydrochloric acid) or bases (e.g., ammonia) may be added to adjust the pH.

Using tungsten ethoxide and thiourea as an example, the typical sol-gel synthesis process for WS₂ is as follows: First, prepare the precursor solution by dissolving 0.005 mol of tungsten ethoxide (approximately 2.05 g) in anhydrous ethanol with stirring until fully dissolved, then dissolving 0.01 mol of thiourea (approximately 0.76 g) in ethanol and slowly adding it to the tungsten solution while continuously stirring. The solution may become slightly turbid due to minor hydrolysis. Add about 0.1 mL of 1 mol/L hydrochloric acid to adjust the pH to 3-4, promoting hydrolysis and polycondensation reactions. Next, on a magnetic stirrer, heat the mixed solution at a constant temperature (e.g., 50°C) with stirring for 2 hours to form a sol, during which tungsten ethoxide hydrolyzes into a tungsten-oxygen network with thiourea dispersed within it. Then, transfer the sol to an open container and let it stand in a 60°C oven for 4-6 hours to gel, forming a light yellow, uniform wet gel. Dry the wet gel in a vacuum oven at 80°C for 12 hours to obtain a dry gel. Finally, place the dry gel in a tube furnace under a nitrogen or argon atmosphere, heat it to 600°C at a rate of 5°C/min, and hold it there for 2 hours. During this step, thiourea decomposes to release H₂S, which reacts with the tungsten-oxygen precursor to form WS₂. After natural cooling, retrieve the product—a black WS₂ powder or thin film (depending on the substrate).

The key steps in the sol-gel synthesis of WS₂ are precursor hydrolysis, gel network formation, and sulfidation. First, tungsten ethoxide hydrolyzes under trace water or acid catalysis to form hydroxylated tungsten-oxygen monomers, which then undergo polycondensation to create a three-dimensional network. During heat treatment, thiourea decomposes into H₂S. Finally, the tungsten-oxygen network reacts with H₂S to produce WS₂ crystals.

Process parameters significantly affect the sol-gel synthesis of WS₂. Regarding pH, acidic conditions (pH 3-5) facilitate hydrolysis and polycondensation, while alkaline conditions may cause rapid precipitation, compromising gel uniformity. For heat treatment temperature, a temperature too low (e.g., 400°C) may result in incomplete sulfidation, leaving WO₃ impurities, while a temperature too high (e.g., 800°C) may lead to excessive grain growth or loss of morphological control. The tungsten-to-sulfur ratio should be controlled between 1:2 and 1:3 to ensure sufficient sulfur supply and prevent oxide impurities. The solvent evaporation rate is also critical—rapid drying can cause the gel to crack, necessitating controlled temperature and humidity for slow evaporation.

Precautions for sol-gel synthesis of WS₂ include: ensuring an inert atmosphere during heat treatment to prevent oxidation of WS₂ to WO₃; conducting experiments in a fume hood due to the potential release of trace toxic gases (e.g., H₂S) from thiourea decomposition; and avoiding excessive water in the precursor solution preparation to prevent premature precipitation.

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