# Tungsten disulfide

Tungsten disulfide is an inorganic chemical compound composed of tungsten and sulfur with the chemical formula WS$_2$. 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.
WS$_2$ adopts a layered structure similar, or isotypic with MoS$_2$, instead with W atoms situated in trigonal prismatic coordination sphere (in place of Mo atoms). Owing to this layered structure, WS$_2$ forms non-carbon nanotubes, which were discovered after heating a thin sample of WS$_2$ in 1992.

## Structure and physical properties

Bulk WS$_2$ forms dark gray hexagonal crystals with a layered structure. Like the closely related MoS$_2$, it exhibits properties of a dry lubricant.
Although it has long been thought that WS$_2$ is relatively stable in ambient air, recent reports on the ambient air oxidation of monolayer WS$_2$ have found this to not be the case. In the monolayer form, WS$_2$ 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 WS$_2$ (< ~660 nm; > ~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 MoS$_2$, has similarly been observed to occur in ambient light and atmospheric conditions.
WS$_2$ is also attacked by a mixture of nitric and hydrofluoric acid. When heated in oxygen-containing atmosphere, WS$_2$ converts to tungsten trioxide. When heated in absence of oxygen, WS$_2$ does not melt but decomposes to tungsten and sulfur, but only at 1250 °C.
Historically monolayer WS$_2$ 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. WS$_2$ also undergoes exfoliation by treatment with various reagents such as chlorosulfonic acid and the lithium halides.

## Synthesis

WS$_2$ 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 WS$_2$ 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 1×10$^{13}$ cm$^{−2}$. Other routes entail thermolysis of tungsten(VI) sulfides (e.g., (R$_4$N)$_2$WS$_4$) or the equivalent (e.g., WS$_3$).
Freestanding WS$_2$ films can be produced as follows. WS$_2$ 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 WS$_2$ film spontaneously peels off.

## Applications

WS$_2$ 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.
WS$_2$ 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 MoS$_2$, nanostructured WS$_2$ is actively studied for potential applications, such as storage of hydrogen and lithium. WS$_2$ also catalyses hydrogenation of carbon dioxide:
    CO$_2$ + H$_2$ → CO + H$_2$O

### Nanotubes

Tungsten disulfide is the first material which was found to form non-carbon nanotubes, in 1992. This ability is related to the layered structure of WS$_2$, and macroscopic amounts of WS$_2$ have been produced by the methods mentioned above. WS$_2$ nanotubes have been investigated as reinforcing agents to improve the mechanical properties of polymeric nanocomposites. In a study, WS$_2$ nanotubes reinforced biodegradable polymeric nanocomposites of polypropylene fumarate (PPF) showed significant increases in the Young's modulus, compression yield strength, flexural modulus and flexural yield strength, compared to single- and multi-walled carbon nanotubes reinforced PPF nanocomposites, suggesting that WS$_2$ nanotubes may be better reinforcing agents than carbon nanotubes. The addition of WS$_2$ 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. WS$_2$ nanotubes were embedded into a poly(methyl methacrylate) (PMMA) nanofiber matrix via electrospinning. The nanotubes were well dispersed and aligned along fiber axis. The enhanced stiffness and toughness of PMMA fiber meshes by means of non-carbon nanotubes addition may have potential uses as impact-absorbing materials, e.g. for ballistic vests.
WS$_2$ 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 WS$_2$ nanotubes with molten lead, antimony or bismuth iodide salt by a capillary wetting process, resulting in PbI$_2$@WS$_2$, SbI$_3$@WS$_2$ or BiI$_3$@WS$_2$ core–shell nanotubes.

### Nanosheets

WS$_2$ 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) is investigating use of WS $_2$ as a channel material in field effect transistors. The approximately 6-layer thick material is created using chemical vapor deposition (CVD).