Interactions of Water with Pristine and Defective MoS₂

Abstract

Molybdenum disulfide (MoS 2 ) is a lamellar solid lubricant often used in aerospace applications because of its extremely low friction coefficient (∼0.01) in inert environments. The lubrication performance of MoS 2 is significantly impaired by exposure to even small amounts of water and oxygen, and the mechanisms behind this remain poorly understood. We use density functional theory calculations to study the binding of water on MoS 2 sheets with and without defects. In general, we find that pristine MoS 2 is slightly hydrophilic but that defects greatly increase the binding affinity for water. Intercalated water disrupts the crystal structure of bulk MoS 2 due to the limited space between lamellae (∼3.4 Å), and this leads to generally unfavorable adsorption, except in the cases where water molecules are located on the sites of sulfur vacancies. We also find that water adsorption is more favorable directly below a surface layer of MoS 2 compared to in the bulk.

Spectroscopic Evaluation of Surface Chemical Processes Occurring in MoS2 upon Aging

“…This microstructural change is proposed to be a contributing factor to the observed changes in the extent of oxidation within the wear track for the case of MoS 2 samples that were pre-annealed and aged. This interpretation is supported by DFT calculations, which have shown that water adsorption is only thermodynamically favorable on Mo-terminated (100) edge planes 66 or defects such as sulfur vacancies or oxygen substitutions on (002) basal planes, 67 and indicated that sulfur-terminated (110) planes of MoS 2 are likely unfavorable for water adsorption. Moreover, flake oxidation studies have observed preferential oxidation with associated cleavage along (100) edge planes as opposed to (110) edge planes.…”

Water adsorption on MoS2 under realistic atmosphere conditions and impacts on tribology

“…To determine how much energy is associated with MoS 2 layer separation, we used DFT calculations (PBE functional, 520 eV, DFT details same as our previous work 37 ) on a 5 × 5 unit cell of MoS 2 with 4 layers in the 2H configuration (ESI Fig. 2 † ).…”

“…The MoS 2 structure was held fixed at the crystallographic minimum computed with density functional theory (DFT) (PAW_PBE pseudopotentials, 520 eV energy cutoff, and a 4 × 4 × 1 Monkhorst–Pack k -points grid) in our previous work. 37 The energy of separating layers of MoS 2 was computed using single point DFT calculations with the same parameters.…”

“…32,33 Previous work from our group has suggested that water adsorbs on the edges of MoS 2 flakes, disrupting the lamellar structure and resulting in higher friction. 34–37 Others have suggested increases in friction are due to MoS 2 oxidizing to MoO 3 , 27,28 hydrogen bonding from water, 38,39 or capillary effects. 40…”

Interfacial Liquid Water on Graphite, Graphene, and 2D Materials

“…Several techniques have been applied to study the interaction and properties of water with graphite and 2D materials surfaces. Chiefly, among them were water contact angle, − ,,,− X-ray reflectivity, ,, electron microscopy, − vibrational sum-frequency-generation spectroscopy, ,− X-ray spectroscopies, ,, AFM methods, − 3D AFM, ,,,,,− and impedance methods. ,,, The experimental techniques were complemented by a variety of theoretical and molecular dynamics methods. ,− ,− Nanofluidic channels made from two-dimensional crystals enabled the fabrication of devices that relied on the interfacial liquid water properties. , Those devices were also applied to study the interaction of water with graphene and hexagonal boron nitride surfaces. , …”