Tungsten disulfide

Tungsten disulfide is an inorganic chemical compound composed of tungsten and sulfur with the chemical formula WS2. This compound is part of the group of materials called the transition metal dichalcogenides. It occurs naturally as the rare mineral tungstenite. This material is a component of certain catalysts used for hydrodesulfurization and hydrodenitrification.
WS2 adopts a layered structure similar, or isotypic with MoS2, instead with W atoms situated in trigonal prismatic coordination sphere (in place of Mo atoms). Owing to this layered structure, WS2 forms inorganic nanotubes, which were discovered after heating a thin sample of WS2 in 1992.
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.
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 photooxidation reaction involving visible wavelengths of light readily absorbed by monolayer WS2 (< ~660 nm/ > ~1.88 eV). In addition to light of suitable wavelength, the reaction requires oxygen and water to proceed, with the water thought to act as a catalyst for oxidation. The products of the reaction include tungsten oxide 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) or 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.
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.