Extraction of Tungsten Disulfide by Supercritical Carbon Dioxide

Supercritical carbon dioxide solvent is used to assist the extraction of tungsten disulfide and obtain less layers or single layer of tungsten disulfide. The extraction process is safe and free of pollution, which provides a reference for the preparation of other layered materials.

In nature, substance exists in three states, that is, solid, liquid and gas. Depending on the temperature and pressure, the state changes between these three. When the temperature is greater than a certain point, no matter how high the pressure is, the substance cannot pass from gas to liquid. The temperature value at this point is called the critical temperature (Tc). At the critical temperature, the minimum pressure required for the gas to become a liquid is called the critical pressure (Tp). When the temperature of the fluid is higher than Tc and the pressure of it is greater than Tp, we define it as a supercritical fluid.

Supercritical carbon dioxide (SC CO₂) is one of the most widely studied supercritical fluids. It has high dispersion coefficient, remarkable surface wettability, low interfacial tension and excellent transport performance, which make it the environmentally friendly solvent assisting the extraction of layered materials.

SC CO₂ is used by the researchers to assist the extraction of tungsten disulfide. Absolute ethyl alcohol and deionized water are also used during the process. The equipment for extraction consists of a stainless steel high-pressure reactor, a heating jacket and a temperature controller.

The extraction of tungsten disulfide is as follows:

1. Put 50 mg WS₂ powder in 10 mL mixed solution of absolute ethyl alcohol and water, and continuously ultrasound the mixture in the ice-water bath for 2 hours;

2. At the same time, heat the reactor to 40°C, then transfer the ultrasound-treated mixture to the reactor and turn on the stirring device;

3. The carbon dioxide is charged into the reactor until the pressure reaches 16 MPa. The temperature and pressure remained the same throughout the extraction process;

4. After the reaction with SC CO₂ for 3 hours, The carbon dioxide in it is discharged and the suspension is collected. The suspension is dispersed by ultrasound in the ice-water bath for 3 hours and further centrifuged at the rotate speed of 3000 rpm for 15 minutes.

5. The centrifugation is finished and the extraction of tungsten disulfide is completed.

The effect of extraction is best at 40% ethanol and less layers of or single layer of WS₂ is prepared in the end. The whole process is safe and free of pollution, which provides a reference for the preparation of other layered materials.

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