Preparation of Tungsten Disulfide by Hydrothermal Method

The hydrothermal method is a commonly used technique for synthesizing tungsten disulfide (WS₂) nanomaterials. Due to its simplicity, low cost, and ability to effectively control the morphology and size of the resulting products, it has gained widespread application in the field of materials science in recent years.

The preparation of tungsten disulfide by hydrothermal method is based on the principle that the target product is generated through a chemical reaction between precursors in a high temperature and high pressure aqueous solution environment. Compared to methods like vapor deposition or mechanical exfoliation, the hydrothermal method offers the advantage of synthesizing nanostructures at relatively low temperatures, typically between 150-250°C, without requiring complex equipment. During the synthesis, tungsten and sulfur sources undergo a redox reaction in the aqueous solution to form WS₂ crystals, with the morphology controllable by adjusting reaction conditions such as temperature, time, pH, and the use of surfactants.

In terms of raw materials, commonly used tungsten sources include sodium tungstate (Na₂WO₄), tungsten chloride (WCl₆), or tungsten trioxide (WO₃); sulfur sources include thiourea (CS(NH₂)₂), thioacetamide (CH₃CSNH₂), and sodium sulfide (Na₂S); reducing agents include hydroxylamine hydrochloride (NH₂OH·HCl) or oxalic acid (H₂C₂O₄); and surfactants such as cetyltrimethylammonium bromide (CTAB) or polyethylene glycol (PEG) can be employed.

Using sodium tungstate and thiourea as raw materials, the hydrothermal synthesis of WS₂ follows this process: First, prepare the solution by weighing appropriate amounts of sodium tungstate and thiourea to ensure a tungsten-to-sulfur molar ratio of 1:2, dissolving each in 50 mL of deionized water with stirring until fully dissolved, then adding 0.01 mol of hydroxylamine hydrochloride as a reducing agent and stirring until uniform, resulting in a slightly yellow solution. To control morphology, approximately 0.1 mmol of CTAB can be added and stirred further. Next, transfer the mixed solution to a 100 mL Teflon-lined stainless steel autoclave. Place the autoclave in a constant-temperature oven for the reaction, during which thiourea decomposes to release H₂S, reacting with WO₄²⁻ to form WS₂. After the reaction, allow the autoclave to cool naturally to room temperature, then open it to retrieve the black precipitate. Wash the precipitate alternately with deionized water and ethanol via centrifugation, and dry it in a vacuum oven at 60°C to obtain WS₂ powder. To enhance crystallinity, the dried product can be annealed in a tube furnace at 600°C for 2 hours under argon or nitrogen protection.

The core of hydrothermal WS₂ synthesis lies in the chemical reaction between the tungsten and sulfur sources in a high-temperature, high-pressure aqueous environment. Taking sodium tungstate and thiourea as an example, the reaction process can be simplified as follows: First, thiourea decomposes at high temperature, CS(NH₂)₂ + 2H₂O → 2NH₃ + H₂S + CO₂, producing H₂S and ammonia gas. Then, WO₄²⁻ is reduced by the reducing agent to a lower oxidation state tungsten intermediate, which reacts with H₂S to form WS₂, i.e., WO₄²⁻ + 2H₂S + reducing agent → WS₂ + 2H₂O + oxidation products. Finally, WS₂ molecules nucleate in the solution and, with the aid of surfactants or extended reaction time, grow into nanosheets, flower-like structures, or rod-like structures.

Process parameters significantly influence the hydrothermal synthesis of WS₂. Regarding temperature, below 150°C, the reaction may be incomplete, leading to WO₃ impurities in the product, while above 250°C, morphological uniformity may be compromised. In terms of time, a short reaction duration (e.g., 6 hours) tends to produce smaller nanoparticles, while a longer duration (e.g., 24 hours) can yield layered or flower-like structures. For pH, an acidic environment favors H₂S release and WS₂ formation, whereas neutral or alkaline conditions reduce yield. Surfactants play a notable role: increasing CTAB concentration can shift WS₂ morphology from nanosheets to rods, while PEG tends to promote fibrous structures.

Precautions during hydrothermal WS₂ synthesis include: ensuring safe operation of the autoclave to avoid leaks under high temperature and pressure; properly handling waste liquid, which may contain trace H₂S, to prevent environmental pollution; and recording detailed parameters during experiments to facilitate optimization and reproducibility.

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