|Jmol-3D images||Image 1|
|Molar mass||160.07 g/mol|
|Melting point||1185 °C decomp.|
|Solubility in water||insoluble|
|Solubility||decomposed by aqua regia, hot sulfuric acid, nitric acid
insoluble in dilute acids
|Crystal structure||Hexagonal, hP6, space group P6
3/mmc, No 194
|Trigonal prismatic (MoIV()
|EU Index||not listed|
|Other anions||Molybdenum(IV) oxide|
|Other cations||Tungsten disulfide|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
|(what is: / ?)|
Molybdenum disulfide is the inorganic compound with the formula MoS
2. The compound is classified as a metal dichalcogenide. It is a silvery black solid that occurs as the mineral molybdenite, the principal ore for molybdenum. MoS
2 is relatively unreactive. It is unaffected by dilute acids and oxygen. In appearance and feel, molybdenum disulfide is similar to graphite. It is widely used as a solid lubricant because of its low friction properties and robustness.
Molybdenite ore is processed by flotation to give relatively pure MoS
2, the main contaminant being carbon. MoS
2 also arises by the thermal treatment of virtually all molybdenum compounds with hydrogen sulfide or elemental sulfur. It can also be produced by metathesis reactions from molybdenum pentachloride.
The natural amorphous form is known as the rarer mineral jordisite.
Structure and physical properties
2, each Mo(IV) center occupies a trigonal prismatic coordination sphere that is bound to six sulfide ligands. Each sulfur centre is pyramidal and is connected to three Mo centres. In this way, the trigonal prisms are interconnected to give a layered structure, wherein molybdenum atoms are sandwiched between layers of sulfur atoms. Because of the weak van der Waals interactions between the sheets of sulfide atoms, MoS
2 has a low coefficient of friction, producing its lubricating properties. Other layered inorganic materials exhibit lubricating properties (collectively known as solid lubricants (or dry lubricants)) including graphite, which requires volatile additives and hexagonal boron nitride.
Molybdenum disulfide is stable in air, consistent with its existence as a common mineral. It reacts with oxygen upon heating forming molybdenum trioxide:
- 2 MoS
2 + 9 O
2 → 2 MoO
3 + 4 SO
- 2 MoS
2 + 7 Cl
2 → 2 MoCl
5 + 2 S
2 with particle sizes in the range of 1–100 µm is a common dry lubricant. Few alternatives exist that confer high lubricity and stability at up to 350°C in oxidizing environments. Sliding friction tests of MoS
2 using a pin on disc tester at low loads (0.1–2 N) give friction coefficient values of <0.1.
Molybdenum disulfide is often a component of blends and composites where low friction is sought. A variety of oils and greases are used, because they retain their lubricity even in cases of almost complete oil loss, thus finding a use in critical applications such as aircraft engines. When added to plastics, MoS
2 forms a composite with improved strength as well as reduced friction. Polymers filled with MoS
2 include nylon (with the trade name Nylatron), Teflon, and Vespel. Self-lubricating composite coatings for high-temperature applications have been developed consisting of molybdenum disulfide and titanium nitride, using chemical vapor deposition.
Examples of applications of MoS
2-based lubricants include two-stroke engines (e.g., motorcycle engines), bicycle coaster brakes, automotive CV and universal joints, ski waxes, and even some bullets.
2 is employed as a cocatalyst for desulfurization in petrochemistry; e.g., hydrodesulfurization. The effectiveness of the MoS
2 catalysts is enhanced by doping with small amounts of cobalt or nickel and the intimate mixture is supported on alumina. Such catalysts are generated in situ by treating molybdate/cobalt or nickel-impregnated alumina with H
2S or an equivalent reagent.
Nanotubes and buckyball-like molecules composed of MoS
2 exhibit unusual tribological and electronic properties. MoS
2 has been investigated as a component of photoelectrochemical (e.g. for photocatalytic hydrogen production) applications and for microelectronics applications. MoS
2 and other transition metal dichalcogenides form bulk crystals composed of two-dimensional layers stacked in the vertical direction. Such two-dimensional layers are similar in form to graphene and express diverse electronic and optical properties that can differ from those in bulk. Whereas bulk MoS
2 has an indirect band gap of 1.2 eV, single layers of MoS
2 have a direct 1.8 eV electronic bandgap allowing the production of switchable transistors and sensitive photodetectors. The sulfur group on MoS
2's surface interacts with noble metals, including gold. The bond between MoS
2 and gold nanostructures was found to act as a highly coupled gate capacitor with a reduced carrier-transport thermal-barrier and increased thermal conductivity.
Calculations indicate that MoS
2 transistors would consume on the order of less power than silicon transistors while in the "off" state. For molybdenum disulfide sheets, Raman spectra cannot be fully explained as phenomena involving certain characteristic vibrations of the crystal network.
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