Effects of Thickness and Adsorption of Airborne Hydrocarbons on Wetting Properties of MoS2: An Atomistic Simulation Study

M Khalkhali and H Zhang and QX Liu, JOURNAL OF PHYSICAL CHEMISTRY C, 122, 6737-6747 (2018).

DOI: 10.1021/acs.jpcc.8b00481

Molybdenum disulfide (MoS2) has attracted great attention due to its distinctive electronic, optical, chemical, and mechanical properties. In almost all of these applications, having a clear understanding about the wetting properties of MoS2 is essential. The basal plane of MoS2 has been generally believed to be hydrophobic with water contact angle (WCA) around 90 degrees. Kozbial et al. have recently suggested that the freshly exfoliated MoS2 was intrinsically relative hydrophilic (WCA = 69.0 +/- 3.8 degrees); however, it could become fairly hydrophobic after 1 day exposure to the ambient air (WCA = 89.0 +/- 3.1 degrees) (Kozbial et al. Understanding the Intrinsic Water Wettability of Molybdenum Disulfide (MoS2). Langmuir 2015, 31, 8429-8435). They contributed this change in wetting properties to the adsorption of airborne hydrocarbons. The number of layers is another important factor that is believed to affect the wetting properties in ultrathin MoS2 films. For highly crystalline samples grown at high temperature (900 degrees C), Gaur et al. showed that the WCA was a function of number of layers and changed from 98 degrees in monolayer sample to 88 degrees in a sample with 11 layers (Gaur et al. Surface Energy Engineering for Tunable Wettability through controlled synthesis of MoS2. Nano Lett. 2014, 14, 4314-4321). In this work, we study the effects of these two parameters, namely, adsorption of airborne hydrocarbon contaminants and the number of layers, on MoS2 wetting properties using atomistic simulations. Results of our simulations confirm that both of these factors can affect the wetting properties of MoS2 through altering van der Waals interactions between MoS2 and water. We also show that the contributions of both energy and entropy of adhesion should be considered to understand the wetting properties of MoS2. Results of this work improve our understanding about the wetting properties of MoS2 and other transition metal dichalcogenides.

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