MoS2 is a single-atom-thick membrane with hydrophilic sites

What is Tungsten Disulfide?

By studying the electron distribution in these crystals, Jamison gave another explanation for the good tribological properties of MoS2 and WS2. In their structure, six nonbonding electrons fill a band and are confined in the structure. These electrons create a net positive charge on the surface layer, promoting easy shear through electrostatic repulsion. WS2 is thermally more stable and oxid-resistant (about 50 to 100°C) than MoS2. The slow rate of oxidation of WS2 can be explained by the formation of tungsten trioxide (WO3), which is also known to provide a lower friction coefficient than molybdenum trioxide (MoO3). In a dry nitrogen environment, the steady-state friction coefficient of WS2 films grown by pulsed laser deposited on stainless steel against a steel counterface is about 0.04. A transfer film made of very thin WS2 sheets is observed on the pin side. Analyzed by SEM, the sheets are thin enough (60 nm) to be transparent to the electron beam. In this chapter, we investigated the tribological properties of WS2 or IF-WS2 coatings in an ultrahigh vacuum and at different temperatures (−130 to 200°C). Friction experiments were performed in an analytical ultra-high vacuum tribometer. This tribometer consists of a linear reciprocating pin-on-flat configuration installed directly inside an ultra-high vacuum chamber. The system uses traditional surface analysis techniques, X-ray Photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES). The pins were made of AISI 52100 steel with a radius of curvature of 4 mm. We used a normal load of 3 N on the pin leading to a maximum Hertzian contact pressure of 470 MPa. A very low friction coefficient was obtained with both types of coatings, indicating the very interesting tribological properties of WS2 coatings.

MoS2 is a single-atom-thick membrane with hydrophilic sites

MoS2 is a single-atom-thick mem. Vacancies can be easily introduced into the MoS2 monolayer, which suggests nanoporous MoS2 can be used in water desalination applications. In one study, it has been found by molecular dynamic simulations that a nanopore with a single-layer molybdenum disulfide can effectively reject ions and also increase the permeability of the membrane. More than 88% of ions were rejected by membranes having pore areas ranging from 20 to 60 Å2. In that same study, water flux was found to be increased around 2 to 5 times more than other nanoporous membranes. Another study reveals that MoS2 nanopores with “open” and “closed” states can be successfully regulated for water flow and ion filtering. Mechanical stretching of the filter was used to change the size of the nanopore effectively, and it has been found that both steric and electrostatic effects contribute to blocking the passage of ions. Graphene nanopores with functionalized hydroxyl groups can boost the membrane's permeability but reduce desalination efficiencies. The addition of accurate functional groups to the edges of the nanopores requires complex fabrication.

Inorganic fullerene-like nanoparticles of WS2 and MoS2

Hollow closed-cage carbon structures, the fullerenes (C60), and carbon nanotubes have been known for some time. Research into similar structures from other (inorganic) layered compounds started soon after. Thus, IF NP and INT of tungsten disulfide (WS2) first and subsequently of molybdenum disulfide (MoS2) were discovered in 1992 and have elicited considerable interest in this emerging field. This observation is surprising because the chemical bond is unstable beyond a few angstroms. Hence, structures with hollow spaces of a few nanometers and above were initially considered unfavorable. The formation of such hollow closed cages can be attributed to the inherent instability of the planar nanostructures of layered compounds. In graphite, the carbon atoms are bonded in flat sp2 bonds forming a hexagonal network (Figure 13.3A). The graphene sheets are stacked together via weak van der Waals forces.

In the case of MS2, where M stands for a metal atom like molybdenum or tungsten, the molecular sheet comprises a layer of M atoms sandwiched between two outer sulfur layers. Each M atom binds to six sulfur atoms forming a lattice with trigonal biprism (octahedral) coordination. In analogy to graphite, weak van der Waals forces are responsible for stacking the S–M–S layers together. Therefore, these compounds are highly anisotropic concerning their physical and chemical properties. The crystal's basal (van der Waals) surfaces, which are perpendicular to the c-axis, consist of sulfur atoms that form bonds to three underlying W/Mo atoms. These sulfur atoms are chemically inert. However, rim W (or Mo) and S atoms, that is, atoms on the edge of the layer, which are abundant in the nanostructure, are only four- and twofold bonded, respectively, making the planar form unstable and forcing it to fold and close on itself. Therefore, by folding the molecular sheet and stitching the rim atoms together, seamless and stable nanotubular (one-dimensional) and spherical (zero-dimensional fullerene-like) structures with all W/Mo and S atoms being six- and threefold bonded, respectively, are produced. Initially, only the transition metal chalcogenides of WS2 and MoS2 were known as closed-cage structures and nanotubes. However, over the years, this family has been expanded considerably, and it now encompasses a large number of other compounds like oxides, hydroxides, nitrides, chlorides, sulfides, selenides, and even pure elements like bismuth, arsine, and phosphorus.

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