†Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, United States.
‡Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, United States.
ACS Nano. 2015 Mar 24;9(3):3023-31. doi: 10.1021/nn5072073. Epub 2015 Mar 12.
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.
最近，人们对石墨烯的浓厚兴趣和热情也引发了对其他二维（2D）材料和材料组合的关注，这些材料现在开始崭露头角。这些新兴 2D 材料家族之一是过渡金属二卤化物（TMD）。到目前为止，人们对“单层”TMD 材料的润湿性知之甚少。在这项研究中，我们通过 WO3 和 MO3 薄膜的热还原和硫化合成了大面积、连续的单层二硫化钨（WS2）和二硫化钼（MoS2）薄膜在 SiO2/Si 衬底上。与块状材料的约 90°相比，单层 TMD 薄膜的水接触角为约 83°。我们还制备了双层和三层 WS2 薄膜，并研究了水接触角随层数增加的转变。对于双层，水接触角增加到约 85°，对于三层膜，水接触角增加到约 90°。超过三层后，测量的水接触角没有明显变化。这种润湿转变表明，水在某种程度上通过单层 TMD 片与下面的二氧化硅衬底相互作用。实验观察到的润湿转变与 TMD 层的数量介于仅考虑范德华吸引力的一个连续体模型和仅考虑偶极-偶极相互作用的另一个模型之间。我们还研究了润湿随时间的变化。干净的单层 WS2 薄膜（没有空气中的污染物）表现出很强的亲水性，水的前进接触角约为 70°。然而，随着时间的推移，当空气中的碳氢化合物和水吸附到清洁的 WS2 片上时，样品会老化。大约 7 天后，老化过程完成，老化的单层 WS2 薄膜的前进水接触角稳定在约 83°。这些结果表明，干净（即未老化）的单层 TMD 是亲水材料。我们进一步表明，在老化的单层 WS2 和 MoS2 薄膜的晶格中用氧取代硫原子可以用来产生明确定义的“疏水性-亲水性”图案，这些图案在水冷凝和蒸发实验中优先在表面上积累并形成微滴阵列。
