Graphene and MoS2 interacting with water: A comparison by ab initio calculations

Abstract

Although very similar in many technological applications, graphene and MoS 2 bear significant differences if exposed to humid environments. As an example, lubrication properties of graphene are reported to improve while those of MoS 2 to deteriorate: it is unclear whether this is due to oxidation from disulfide to oxide or to water adsorption on the sliding surface. By means of ab initio calculations we show here that these two layered materials have similar adsorption energies for water on the basal planes. They both tend to avoid water intercalation between their layers and to display only mild reactivity of defects located on the basal plane. It is along the edges where marked differences arise: graphene edges are more reactive at the point that they immediately prompt water splitting. MoS 2 edges are more stable and consequently water adsorption is much less favoured than in graphene. We also show that water-driven oxidation of MoS 2 layers is unfavoured with respect to adsorption.

Understanding the Independent and Interdependent Role of Water and Oxidation on the Tribology of Ultrathin Molybdenum Disulfide (MoS2)

“…[33,34] Furthermore, the atomic structure of pristine-MoS2 also prevents water from intercalating between the layers due to the small interlayer distance, the lack of strong interlayer bonding sites for the water molecules and native oxides at the 6 edges. [15,40] Both pristine-MoS2 and graphene physically interact with water at the basal plane [35] and recent studies have suggested the formation of ice like water on the basal plane of other 2D materials. [41] Therefore the small increase in thickness of pristine-MoS2 by ~0.4nm is attributed to a single layer of water adsorbing on the pristine-MoS2 surface.…”

“…MoS2 has a tri-layer atomic structure, where the Mo/S atoms can polarize more strongly than the monolayer carbon structure of graphene to physisorb water on to the surface. [35] The absorption energy of a water molecule on defect free monolayer MoS2 is reported to be -0.18eV compared to -0.15eV on graphene. [35] The adsorption energy is reported to further increase for thicker and defective MoS2.…”

“…[35] The absorption energy of a water molecule on defect free monolayer MoS2 is reported to be -0.18eV compared to -0.15eV on graphene. [35] The adsorption energy is reported to further increase for thicker and defective MoS2. [35] Water contact angle (WCA) measurements show a lower contact angle of ~80 on pristine-MoS2 as compared to ~91°on graphene (Supporting information; Figure S4).…”

“…[34] The interaction of water molecules is relatively weak with the basal plane of 2D-MoS2 but they can interact strongly at defects and edges, where oxidation is understood to initiate. [35,36] Though the literature generally agrees that the oxidized state of MoS2 as MoO3 at the microscale is detrimental to its lubricious behavior, recent studies have questioned this frictional behavior of oxidized 2D-MoS2. [37,38] For instance, theoretical DFT simulations have reported easier shearing between 2D-MoS2/2D-MoO3 than 2D-MoS2/2D-MoS2.…”

2D nanoporous membrane for cation removal from water: Effects of ionic valence, membrane hydrophobicity, and pore size

“…By tuning the pore creation process, Jang et al 24 have demonstrated nanofiltration membranes that reject small molecules but offer high permeance to water or monovalent ions. Also, studies have shown how defects, oxidation, and functionalization can affect the ionic blockage [25][26][27] All of these studies point to a near future where 2D membranes will have a major impact on desalination processes. In this work, we address the issue of the selectivity of the porous membrane.…”