Tungsten disulfide

Structure and physical properties

Bulk WS2 forms dark gray hexagonal crystals with a layered structure. Like the closely related MoS2, it exhibits properties of a dry lubricant. We need: Crystalline phases section: WS2, just like mos2 can exist in different phases. Recently (2019) the 1T phase has been stabilised. Allotropes: The beginning paragraph talks of inorganic fullerenes, we should add some more details here. WS2 IFs were the first IFs other than C60!

Have another section for chemical reactions with acids etc.

Synthesis needs to add: WS2 was also looked at very early on in scotch tape exfoliation studies of Novoselov and Geim. Should get a mention and link --

Although it has long been thought that WS2 is relatively stable in ambient air, recent reports on the ambient air oxidation of monolayer WS2 have found this to not be the case. In the monolayer form, WS2 is converted rather rapidly (over the course of days in ambient light and atmosphere) to tungsten oxide via a photo-oxidation reaction involving visible wavelengths of light readily absorbed by monolayer WS2 ( ~1.88 eV). In addition to light of suitable wavelength, the reaction likely requires both oxygen and water to proceed, with the water thought to act as a catalyst for oxidation. The products of the reaction likely include various tungsten oxide species and sulfuric acid. The oxidation of other semiconductor transition metal dichalcogenides (S-TMDs) such as MoS2, has similarly been observed to occur in ambient light and atmospheric conditions.

WS2 is also attacked by a mixture of nitric and hydrofluoric acid. When heated in oxygen-containing atmosphere, WS2 converts to tungsten trioxide. When heated in absence of oxygen, WS2 does not melt but decomposes to tungsten and sulfur, but only at 1250 °C.

Historically monolayer WS2 was isolated using chemical exfoliation via intercalation with lithium from n-butyl lithium (in hexane), followed by exfoliation of the Li intercalated compound by sonication in water. WS2 also undergoes exfoliation by treatment with various reagents such as chlorosulfonic acid and the lithium halides.

Synthesis

WS2 is produced by a number of methods. Many of these methods involve treating oxides with sources of sulfide or hydrosulfide, supplied as hydrogen sulfide or generated in situ.

Thin films and monolayers

Widely used techniques for the growth of monolayer WS2 include

• chemical vapor deposition (CVD)
• physical vapor deposition (PVD)
• metal organic chemical vapor deposition (MOCVD)

Though most current methods produce sulfur vacancy defects in excess of 1×1013 cm−2.

Other routes entail thermolysis of tungsten(VI) sulfides (e.g., (R4N)2WS4) or the equivalent (e.g., WS3).

Freestanding WS2 films can be produced as follows: WS2 is deposited on a hydrophilic substrate, such as sapphire, and then coated with a polymer, such as polystyrene. After dipping the sample in water for a few minutes, the hydrophobic WS2 film spontaneously peels off.

Applications

WS2 is used, in conjunction with other materials, as catalyst for hydrotreating of crude oil. In recent years it has also found applications as a saturable for passively mode locked fibre lasers resulting in femtosecond pulses being produced.

Lamellar tungsten disulphide is used as a dry lubricant for fasteners, bearings, and molds, as well as having significant use in aerospace and military industries., which have extremely low coefficient of friction of 0.03.

WS2 can be applied to a metal surface without binders or curing, via high-velocity air impingement. The most recent official standard for this process is laid out in the SAE International specification AMS2530A.

Research

Like MoS2, nanostructured WS2 is actively studied for potential applications, such as storage of hydrogen and lithium. WS2 also catalyses hydrogenation of carbon dioxide: : CO2 + H2 → CO + H2O

Nanotubes

2/WS2 core–shell nanostructure.]] -- Tungsten disulfide is the first material which was found to form non-carbon nanotubes, in 1992. The addition of WS2 nanotubes to epoxy resin improved adhesion, fracture toughness and strain energy release rate. The wear of the nanotubes-reinforced epoxy is lower than that of pure epoxy.

WS2 nanotubes are hollow and can be filled with another material, to preserve or guide it to a desired location, or to generate new properties in the filler material which is confined within a nanometer-scale diameter. To this goal, non-carbon nanotube hybrids were made by filling WS2 nanotubes with molten lead, antimony or bismuth iodide salt by a capillary wetting process, resulting in PbI2@WS2, SbI3@WS2 or BiI3@WS2 core–shell nanotubes.

Nanosheets

WS2 can also exist in the form of atomically thin sheets. Such materials exhibit room-temperature photoluminescence in the monolayer limit.

Transistors

Taiwan Semiconductor Manufacturing Company (TSMC) as of 2019 is investigating use of as a channel material in field effect transistors. The approximately 6-layer thick material is created using chemical vapor deposition (CVD).

References

References

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