Molybdenum disulfide
Molybdenum disulfide is an inorganic compound composed of molybdenum and sulfur. Its chemical formula is .
The compound is classified as a transition metal dichalcogenide. It is a silvery black solid that occurs as the mineral molybdenite, the principal ore for molybdenum. 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 dry lubricant because of its low friction and robustness. Bulk is a diamagnetic, indirect bandgap semiconductor similar to silicon, with a bandgap of 1.23 eV.
Production
is naturally found as either molybdenite, a crystalline mineral, or jordisite, a rare low temperature form of molybdenite. Molybdenite ore is processed by flotation to give relatively pure. The main contaminant is carbon. also arises by thermal treatment of virtually all molybdenum compounds with hydrogen sulfide or elemental sulfur and can be produced by metathesis reactions from molybdenum pentachloride.Structure and physical properties
Crystalline phases
All forms of have a layered structure, in which a plane of molybdenum atoms is sandwiched by planes of sulfide ions. These three strata form a monolayer of. Bulk consists of stacked monolayers, which are held together by weak van der Waals interactions.Crystalline exists in one of two phases, 2H- and 3R-, where the "H" and the "R" indicate hexagonal and rhombohedral symmetry, respectively. In both of these structures, each molybdenum atom exists at the center of a trigonal prismatic coordination sphere and is covalently bonded to six sulfide ions. Each sulfur atom has pyramidal coordination and is bonded to three molybdenum atoms. Both the 2H- and 3R-phases are semiconducting.
A third, metastable crystalline phase known as 1T- was discovered by intercalating 2H- with alkali metals. This phase has trigonal symmetry and is metallic. The 1T-phase can be stabilized through doping with electron donors such as rhenium, or converted back to the 2H-phase by microwave radiation. The 2H/1T-phase transition can be controlled via the incorporation of sulfur vacancies.
Allotropes
-like and buckyball-like molecules composed of are known.Exfoliated flakes
While bulk in the 2H-phase is known to be an indirect-band gap semiconductor, monolayer has a direct band gap. The layer-dependent optoelectronic properties of have promoted much research in 2-dimensional -based devices. 2D can be produced by exfoliating bulk crystals to produce single-layer to few-layer flakes either through a dry, micromechanical process or through solution processing.Micromechanical exfoliation, also pragmatically called "Scotch-tape exfoliation", involves using an adhesive material to repeatedly peel apart a layered crystal by overcoming the van der Waals forces. The crystal flakes can then be transferred from the adhesive film to a substrate. This facile method was first used by Konstantin Novoselov and Andre Geim to obtain graphene from graphite crystals. However, it can not be employed for a uniform 1-D layers because of weaker adhesion of to the substrate ; the aforementioned scheme is good for graphene only. While Scotch tape is generally used as the adhesive tape, PDMS stamps can also satisfactorily cleave if it is important to avoid contaminating the flakes with residual adhesive.
Liquid-phase exfoliation can also be used to produce monolayer to multi-layer in solution. A few methods include lithium intercalation to delaminate the layers and sonication in a high-surface tension solvent.
Mechanical properties
excels as a lubricating material due to its layered structure and low coefficient of friction. Interlayer sliding dissipates energy when a shear stress is applied to the material. Extensive work has been performed to characterize the coefficient of friction and shear strength of in various atmospheres. The shear strength of increases as the coefficient of friction increases. This property is called superlubricity. At ambient conditions, the coefficient of friction for was determined to be 0.150, with a corresponding estimated shear strength of 56.0 MPa. Direct methods of measuring the shear strength indicate that the value is closer to 25.3 MPa.The wear resistance of in lubricating applications can be increased by doping with Cr. Microindentation experiments on nanopillars of Cr-doped found that the yield strength increased from an average of 821 MPa for pure to 1017 MPa at 50% Cr. The increase in yield strength is accompanied by a change in the failure mode of the material. While the pure nanopillar fails through a plastic bending mechanism, brittle fracture modes become apparent as the material is loaded with increasing amounts of dopant.
The widely used method of micromechanical exfoliation has been carefully studied in to understand the mechanism of delamination in few-layer to multi-layer flakes. The exact mechanism of cleavage was found to be layer dependent. Flakes thinner than 5 layers undergo homogenous bending and rippling, while flakes around 10 layers thick delaminated through interlayer sliding. Flakes with more than 20 layers exhibited a kinking mechanism during micromechanical cleavage. The cleavage of these flakes was also determined to be reversible due to the nature of van der Waals bonding.
In recent years, has been utilized in flexible electronic applications, promoting more investigation into the elastic properties of this material. Nanoscopic bending tests using AFM cantilever tips were performed on micromechanically exfoliated flakes that were deposited on a holey substrate. The Young's modulus of monolayer flakes was 270 GPa, while the thicker flakes were stiffer, with a Young's modulus of 330 GPa. Molecular dynamic simulations found the in-plane Young's modulus of to be 229 GPa, which matches the experimental results within error.
Bertolazzi and coworkers also characterized the failure modes of the suspended monolayer flakes. The strain at failure ranges from 6 to 11%. The average yield strength of monolayer is 23 GPa, which is close to the theoretical fracture strength for defect-free.
The band structure of is sensitive to strain.
Chemical reactions
Molybdenum disulfide is stable in air and attacked only by aggressive reagents. It reacts with oxygen upon heating forming molybdenum trioxide:Chlorine attacks molybdenum disulfide at elevated temperatures to form molybdenum pentachloride:
Intercalation reactions
Molybdenum disulfide is a host for formation of intercalation compounds. This behavior is relevant to its use as a cathode material in batteries. One example is a lithiated material,. With butyl lithium, the product is.Applications
Lubricant
Due to weak van der Waals interactions between the sheets of sulfide atoms, has a low coefficient of friction. in 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 using a pin on disc tester at low loads give friction coefficient values of <0.1.is often a component of blends and composites that require low friction. For example, it is added to graphite to improve sticking. A variety of oils and greases use, 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, forms a composite with improved strength as well as reduced friction. Polymers that may be filled with include nylon, 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 -based lubricants include two-stroke engines, bicycle coaster brakes, automotive CV and universal joints, ski waxes and bullets.
Other layered inorganic materials that exhibit lubricating properties includes graphite, which requires volatile additives and hexagonal boron nitride.
Catalysis
is employed as a cocatalyst for desulfurization in petrochemistry, for example, hydrodesulfurization. The effectiveness of the catalysts is enhanced by doping with small amounts of cobalt or nickel. The intimate mixture of these sulfides is supported on alumina. Such catalysts are generated in situ by treating molybdate/cobalt or nickel-impregnated alumina with or an equivalent reagent. Catalysis does not occur at the regular sheet-like regions of the crystallites, but instead at the edge of these planes.finds use as a hydrogenation catalyst for organic synthesis. As it is derived from a common transition metal, rather than a group 10 metal, is chosen when price or resistance to sulfur poisoning are of primary concern. is effective for the hydrogenation of nitro compounds to amines and can be used to produce secondary amines via reductive amination. The catalyst can also effect hydrogenolysis of organosulfur compounds, aldehydes, ketones, phenols and carboxylic acids to their respective alkanes. However, it suffers from low activity, often requiring hydrogen pressures above 96 MPa and temperatures above 185 °C.