Wetting of Mono and Few-Layered WS₂ and MoS₂ Films Supported on Si/SiO₂ Substrates

Abstract

The recent interest and excitement in graphene has also opened up a pandora's box of other two-dimensional (2D) materials and material combinations which are now beginning to come to the fore. One family of these emerging 2D materials is transition metal dichalcogenides (TMDs). So far there is very limited understanding on the wetting behavior of "monolayer" TMD materials. In this study, we synthesized large-area, continuous monolayer tungsten disulfide (WS2) and molybdenum disulfide (MoS2) films on SiO2/Si substrates by the thermal reduction and sulfurization of WO3 and MO3 thin films. The monolayer TMD films displayed an advancing water contact angle of ∼83° as compared to ∼90° for the bulk material. We also prepared bilayer and trilayer WS2 films and studied the transition of the water contact angle with increasing number of layers. The advancing water contact angle increased to ∼85° for the bilayer and then to ∼90° for the trilayer film. Beyond three layers, there was no significant change in the measured water contact angle. This type of wetting transition indicates that water interacts to some extent with the underlying silica substrate through the monolayer TMD sheet. The experimentally observed wetting transition with numbers of TMD layers lies in-between the predictions of one continuum model that considers only van der Waals attractions and another model that considers only dipole-dipole interactions. We also explored wetting as a function of aging. A clean single-layer WS2 film (without airborne contaminants) was shown to be strongly hydrophilic with an advancing water contact angle of ∼70°. However, over time, the sample ages as hydrocarbons and water present in air adsorb onto the clean WS2 sheet. After ∼7 days, the aging process is completed and the advancing water contact angle of the aged single-layer WS2 film stabilizes at ∼83°. These results suggest that clean (i.e., nonaged) monolayer TMDs are hydrophilic materials. We further show that substitution of sulfur atoms by oxygen in the lattice of aged monolayer WS2 and MoS2 films can be used to generate well-defined 'hydrophobic-hydrophilic' patterns that preferentially accumulate and create microdrop arrays on the surface during water condensation and evaporation experiments.

Diameter-dependent wetting of tungsten disulfide nanotubes

“…The interaction of water with bulk and single-layer (graphenelike) WS 2 and MoS 2 surfaces has been recently studied (21)(22)(23). To learn about the interaction of the WS 2 nanotubes with water, the contact angle (θ) of water on the (outer) surface of individual INT-WS 2 was directly measured inside an ESEM by condensing water on the nanotubes at ∼4°C (Fig.…”

Understanding the intrinsic water wettability of graphite

“…297 Surface contamination is a serious concern for any solid surface, especially atomically thin materials, since contaminants can affect water wettability. A classic example is the adsorption of airborne contaminants onto gold rendering the hydrophilic surface to appear hydrophobic.…”

Vertically Oriented Arrays of ReS2 Nanosheets for Electrochemical Energy Storage and Electrocatalysis

“…[8][9][10][11][12][13][14][15][16][17][18][19] Here we report that while MoS 2 and WS 2 films tend to grow parallel to the substrate, under similar CVD growth conditions, ReS 2 nanosheets show a remarkable tendency to bend and stand vertically on the growth substrate. In fact, the propensity for ReS 2 to orient itself perpendicular to the growth substrate is independent of the substrate material used or the packing density (i.e.…”

“…This approach has been used in the literature [8][9][10][11][12][13][14][15][16][17][18][19] to produce a variety of monolayer TMD materials including MoS 2 , WS 2 , ReS 2 , MoSe 2 , and WSe 2 . Supplementary Figure S1 shows typical monolayer WS 2 films grown by this approach in our Lab (the TMD growth is always parallel to the growth substrate).…”