|Jmol interactive 3D||Image|
|Molar mass||160.07 g/mol|
|Melting point||1,185 °C (2,165 °F; 1,458 K) decomposes|
|Solubility||decomposed by aqua regia, hot sulfuric acid, nitric acid
insoluble in dilute acids
|Band gap||1.23 eV (2H)|
|hP6, space group P6
3/mmc, No 194 (2H)
a = 0.3161 nm (2H), 0.3163 nm (3R), c = 1.2295 nm (2H), 1.837 (3R)
|Trigonal prismatic (MoIV)
|Safety data sheet||External MSDS|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
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.
Structure and physical properties
2 usually consists of a mixture of two major polytypes of similar structure, 2H and 3R, with the former being most abundant. In 2H-MoS
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)) include graphite, which requires volatile additives, and hexagonal boron nitride.
Two-dimensional, single- or few-layer MoS
2, is a two-dimensional semiconductor, with the band structure very sensitive to strain.
- 2 MoS
2 + 7 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.
2 is often a component of blends and composites that require low friction. 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 consist 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 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, supporting the intimate mixture on alumina. Such catalysts are generated in situ by treating molybdate/cobalt or nickel-impregnated alumina with H
2S or an equivalent reagent.
MoS2 also finds some use as a hydrogenation catalyst for organic synthesis. Being derived from a common transition metal, rather than group 10 metal like many alternatives, MoS2 is chosen when catalyst price or resistance to sulfur poisoning are of primary concern. MoS2 is effective for the hydrogenation of nitro compounds to amines and can be used produce secondary amines via reductive alkylation. The catalyst can also can effect hydrogenolysis of organosulfur compounds, aldehydes, ketones, phenols, and carboxylic acids to their respective alkanes. The catalyst suffers from rather low activity however, often requiring hydrogen pressures above 95 atm and temperatures above 185°C.
Much research is focused on unusual morphologies of MoS2. Multilayer sheets are produced by liquid phase exfoliation. 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.
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, MoS
2 monolayers have a direct 1.8 eV electronic bandgap, allowing the production of switchable transistors and sensitive photodetectors. As a transition metal di-chalcogenide, MoS
2 possesses some of graphene's desirable qualities (such as mechanical strength and electrical conductivity), and can emit light, opening possible applications such as photodetectors.
The sulfur group on MoS
2 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 100,000 times less energy 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.
2 nanoflakes can be used for solution-processed fabrication of layered memristive and memcapacitive devices through engineering a MoO
2 heterostructure sandwiched between silver electrodes. The MoS
2-based memristors are mechanically flexible, optically transparent and can be produced at low cost.
In 2014 a two-dimensional, MoS
2-based semiconductor material for biosensing was announced. Compared to graphene, it offers higher sensitivity, better scalability and lends itself to high-volume manufacturing. The wide bandgap of MoS
2 prevents leakage and results in more sensitive and accurate readings.
The sensitivity of a graphene field-effect transistor (FET) biosensor is fundamentally restricted by the zero band gap of graphene, which results in increased leakage and reduced sensitivity. In digital electronics, transistors control current flow throughout an integrated circuit and allow for amplification and switching. In biosensing, the physical gate is removed, and the binding between embedded receptor molecules and the charged target biomolecules to which they are exposed modulates the current.
The demonstration biosensors provide protein sensing with a sensitivity of 196 even at 100 femtomolar, similar to one drop of milk dissolved in one hundred tons of water. A MoS2-based pH sensor achieved sensitivity of 713 for a pH change by one unit over a pH range of 3–9. In November 2015, researchers reported that sheets of MoS2 made with nanopores are the most efficient way found thus far to desalinate seawater, requiring less energy than the currently commercialized reverse osmosis process which utilizes plastic sheets. The efficiency is better for two main reasons: (1) less pressure is required since MoS2 sheets are thinner than reverse osmosis plastics, and (2) the molecules of MoS2 are arranged such that the Mo attracts water molecules into a pore while the S2 repels it out of the pore.
|Wikimedia Commons has media related to Molybdenum disulfide.|
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