Molybdenum disulfide
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Molybdenum disulfide (or moly) is an inorganic compound composed of molybdenum and sulfur. Its chemical formula is Template:Chem2.
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.<ref name=ullmann>Sebenik, Roger F. et al. (2005) "Molybdenum and Molybdenum Compounds", Ullmann's Encyclopedia of Chemical Technology. Wiley-VCH, Weinheim. {{#invoke:doi|main}}</ref> Template:Chem2 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 Template:Chem2 is a diamagnetic, indirect bandgap semiconductor similar to silicon, with a bandgap of 1.23 eV.<ref name=band/>
ProductionEdit
Template:Chem2 is naturally found as either molybdenite, a crystalline mineral, or jordisite, a rare low temperature form of molybdenite.<ref>{{#invoke:citation/CS1|citation
|CitationClass=web }}</ref> Molybdenite ore is processed by flotation to give relatively pure Template:Chem2. The main contaminant is carbon. Template:Chem2 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.<ref>Template:Cite book</ref>
Structure and physical propertiesEdit
Crystalline phasesEdit
All forms of Template:Chem2 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 Template:Chem2. Bulk Template:Chem2 consists of stacked monolayers, which are held together by weak van der Waals interactions.
Crystalline Template:Chem2 exists in one of two phases, 2H-Template:Chem2 and 3R-Template:Chem2, 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.<ref>Template:Cite book</ref>
A third, metastable crystalline phase known as 1T-Template:Chem2 was discovered by intercalating 2H-Template:Chem2 with alkali metals.<ref>Template:Cite journal</ref> This phase has trigonal symmetry and is metallic. The 1T-phase can be stabilized through doping with electron donors such as rhenium,<ref>Template:Cite journal</ref> or converted back to the 2H-phase by microwave radiation.<ref>Template:Cite journal</ref> The 2H/1T-phase transition can be controlled via the incorporation of sulfur (S) vacancies.<ref>Template:Cite journal</ref>
AllotropesEdit
Nanotube-like and buckyball-like molecules composed of Template:Chem2 are known.<ref>Template:Cite journal</ref>
Exfoliated Template:Chem2 flakesEdit
While bulk Template:Chem2 in the 2H-phase is known to be an indirect-band gap semiconductor, monolayer Template:Chem2 has a direct band gap. The layer-dependent optoelectronic properties of Template:Chem2 have promoted much research in 2-dimensional Template:Chem2-based devices. 2D Template:Chem2 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 Template:Chem2 to the substrate (either silicon, glass or quartz); the aforementioned scheme is good for graphene only.<ref>Template:Cite journal</ref> While Scotch tape is generally used as the adhesive tape, PDMS stamps can also satisfactorily cleave Template:Chem2 if it is important to avoid contaminating the flakes with residual adhesive.<ref name="Castellanos-Gomez-2012">Template:Cite journal</ref>
Liquid-phase exfoliation can also be used to produce monolayer to multi-layer Template:Chem2 in solution. A few methods include lithium intercalation<ref>Template:Cite journal</ref> to delaminate the layers and sonication in a high-surface tension solvent.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Mechanical propertiesEdit
Template:Chem2 excels as a lubricating material (see below) 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 Template:Chem2 in various atmospheres.<ref name="Donnet-1996">Template:Cite journal</ref> The shear strength of Template:Chem2 increases as the coefficient of friction increases. This property is called superlubricity. At ambient conditions, the coefficient of friction for Template:Chem2 was determined to be 0.150, with a corresponding estimated shear strength of 56.0 MPa.<ref name="Donnet-1996" /> Direct methods of measuring the shear strength indicate that the value is closer to 25.3 MPa.<ref>Template:Cite journal</ref>
The wear resistance of Template:Chem2 in lubricating applications can be increased by doping Template:Chem2 with Cr. Microindentation experiments on nanopillars of Cr-doped Template:Chem2 found that the yield strength increased from an average of 821 MPa for pure Template:Chem2 (at 0% Cr) to 1017 MPa at 50% Cr.<ref name="Tedstone-2015">Template:Cite journal</ref> The increase in yield strength is accompanied by a change in the failure mode of the material. While the pure Template:Chem2 nanopillar fails through a plastic bending mechanism, brittle fracture modes become apparent as the material is loaded with increasing amounts of dopant.<ref name="Tedstone-2015"/>
The widely used method of micromechanical exfoliation has been carefully studied in Template:Chem2 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.<ref>Template:Cite journal</ref>
In recent years, Template:Chem2 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 Template:Chem2 flakes that were deposited on a holey substrate.<ref name="Castellanos-Gomez-2012" /><ref name="Bertolazzi-2011">Template:Cite journal</ref> The Young's modulus of monolayer flakes was 270 GPa,<ref name="Bertolazzi-2011" /> while the thicker flakes were stiffer, with a Young's modulus of 330 GPa.<ref name="Castellanos-Gomez-2012" /> Molecular dynamic simulations found the in-plane Young's modulus of Template:Chem2 to be 229 GPa, which matches the experimental results within error.<ref>Template:Cite journal</ref>
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 Template:Chem2 is 23 GPa, which is close to the theoretical fracture strength for defect-free Template:Chem2.<ref name="Bertolazzi-2011" />
The band structure of Template:Chem2 is sensitive to strain.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Chemical reactionsEdit
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 reactionsEdit
Molybdenum disulfide is a host for formation of intercalation compounds. This behavior is relevant to its use as a cathode material in batteries.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> One example is a lithiated material, Template:Chem2.<ref>Template:Cite book</ref> With butyl lithium, the product is Template:Chem2.<ref name=ullmann/>
ApplicationsEdit
LubricantEdit
Due to weak van der Waals interactions between the sheets of sulfide atoms, Template:Chem2 has a low coefficient of friction. Template:Chem2 in particle sizes in the range of 1–100 μm is a common dry lubricant.<ref>Template:Citation</ref> Few alternatives exist that confer high lubricity and stability at up to 350 °C in oxidizing environments. Sliding friction tests of Template:Chem2 using a pin on disc tester at low loads (0.1–2 N) give friction coefficient values of <0.1.<ref name="MiesslerTarr2004">Template:Cite book</ref><ref name="ShriverAtkins2006">Template:Cite book</ref>
Template:Chem2 is often a component of blends and composites that require low friction. For example, it is added to graphite to improve sticking.<ref name=moly/> 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, Template:Chem2 forms a composite with improved strength as well as reduced friction. Polymers that may be filled with Template:Chem2 include nylon (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 Template:Chem2-based lubricants include two-stroke engines (such as motorcycle engines), bicycle coaster brakes, automotive CV and universal joints, ski waxes<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and bullets.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Other layered inorganic materials that exhibit lubricating properties (collectively known as solid lubricants (or dry lubricants)) includes graphite, which requires volatile additives and hexagonal boron nitride.<ref>Template:Cite book</ref>
CatalysisEdit
Template:Chem2 is employed as a cocatalyst for desulfurization in petrochemistry, for example, hydrodesulfurization. The effectiveness of the Template:Chem2 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 Template:Chem 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.<ref>Template:Cite book</ref>
Template:Chem2 finds use as a hydrogenation catalyst for organic synthesis.<ref name=Shigeo>Template:Cite book</ref> As it is derived from a common transition metal, rather than a group 10 metal, Template:Chem2 is chosen when price or resistance to sulfur poisoning are of primary concern. Template:Chem2 is effective for the hydrogenation of nitro compounds to amines and can be used to produce secondary amines via reductive amination.<ref>Template:Cite journal</ref> The catalyst can also effect hydrogenolysis of organosulfur compounds, aldehydes, ketones, phenols and carboxylic acids to their respective alkanes.<ref name=Shigeo /> However, it suffers from low activity, often requiring hydrogen pressures above 96 MPa and temperatures above 185 °C.
ResearchEdit
Template:Chem2 plays an important role in condensed matter physics research.<ref name="Wood-2022">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Hydrogen evolutionEdit
Template:Chem2 and related molybdenum sulfides are efficient catalysts for hydrogen evolution, including the electrolysis of water;<ref name="KibsgaardJaramillo2014">Template:Cite journal</ref><ref>Template:Cite journal</ref> thus, are possibly useful to produce hydrogen for use in fuel cells.<ref name="Sandia12517">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Oxygen reduction and evolutionEdit
Template:Chem2@Fe-N-C core/shell<ref>Template:Cite journal</ref> nanosphere with atomic Fe-doped surface and interface (Template:Chem2/Fe-N-C) can be used as a used an electrocatalyst for oxygen reduction and evolution reactions (ORR and OER) bifunctionally because of reduced energy barrier due to Fe-N4 dopants and unique nature of Template:Chem2/Fe-N-C interface.
MicroelectronicsEdit
As in graphene, the layered structures of Template:Chem2 and other transition metal dichalcogenides exhibit electronic and optical properties<ref name="nano">Template:Cite journal</ref> that can differ from those in bulk.<ref name=promising>Template:Cite journal</ref> Bulk Template:Chem2 has an indirect band gap of 1.2 eV,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> while [[Transition metal dichalcogenide monolayers|Template:Chem2 monolayers]] have a direct 1.8 eV electronic bandgap,<ref name= Splendiani>Template:Cite journal</ref> supporting switchable transistors<ref name="Radisavljevic" /> and photodetectors.<ref>Template:Cite journal</ref><ref name=promising /><ref>Template:Cite journal</ref>
Template:Chem2 nanoflakes can be used for solution-processed fabrication of layered memristive and memcapacitive devices through engineering a Template:Chem2/Template:Chem2 heterostructure sandwiched between silver electrodes.<ref name="flexible_memristor">Template:Cite journal</ref> Template:Chem2-based memristors are mechanically flexible, optically transparent and can be produced at low cost.
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.<ref name=rad1409>Template:Cite news</ref>
Template:Chem2 has been investigated as a component of flexible circuits.<ref name=UT>Template:Cite journal</ref><ref name="Chang-2015">Template:Cite journal</ref>
In 2017, a 115-transistor, 1-bit microprocessor implementation was fabricated using two-dimensional Template:Chem2.<ref>Template:Cite journal</ref>
Template:Chem2 has been used to create 2D 2-terminal memristors and 3-terminal memtransistors.<ref>Template:Cite news</ref>
ValleytronicsEdit
Due to the lack of spatial inversion symmetry, odd-layer MoS2 is a promising material for valleytronics because both the CBM and VBM have two energy-degenerate valleys at the corners of the first Brillouin zone, providing an exciting opportunity to store the information of 0s and 1s at different discrete values of the crystal momentum. The Berry curvature is even under spatial inversion (P) and odd under time reversal (T), the valley Hall effect cannot survive when both P and T symmetries are present. To excite valley Hall effect in specific valleys, circularly polarized lights were used for breaking the T symmetry in atomically thin transition-metal dichalcogenides.<ref>Template:Cite journal</ref> In monolayer Template:Chem2, the T and mirror symmetries lock the spin and valley indices of the sub-bands split by the spin-orbit couplings, both of which are flipped under T; the spin conservation suppresses the inter-valley scattering. Therefore, monolayer MoS2 have been deemed an ideal platform for realizing intrinsic valley Hall effect without extrinsic symmetry breaking.<ref>Template:Cite journal</ref>
Photonics and photovoltaicsEdit
Template:Chem2 also possesses mechanical strength, electrical conductivity, and can emit light, opening possible applications such as photodetectors.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Template:Chem2 has been investigated as a component of photoelectrochemical (e.g. for photocatalytic hydrogen production) applications and for microelectronics applications.<ref name="Radisavljevic">Template:Cite journal</ref>
Superconductivity of monolayersEdit
Under an electric field Template:Chem2 monolayers have been found to superconduct at temperatures below 9.4 K.<ref name=APL2012>Template:Cite journal</ref>
See alsoEdit
ReferencesEdit
External linksEdit
- {{#invoke:citation/CS1|citation
|CitationClass=web }} Template:Sister project Template:Molybdenum compounds Template:Sulfides