Understanding the wetting of transition metal dichalcogenides from an ab initio perspective

Transition metal dichalcogenides (TMDs) are a class of two-dimensional (2D) materials that have been widely studied for emerging electronic properties. In this paper, we use computational simulations to examine the water adsorption on TMDs systematically and the wetting property of tungsten diselenide ) specifically. We start with density functional theory (DFT) based random phase approximation (RPA), assessing the performance of exchange-correlation functionals and comparing water adsorption on various TMDs. We also perform ab initio molecular dynamics (AIMD) simulations on ${\mathrm{WSe}}_2$, from which we find that the distribution of interfacial water is sensitive to the exchange-correlation functional selected and a reasonable choice leads to a diffusive contact layer where water molecules prefer the “flat” configuration. Classical molecular dynamics (MD) simulations of water droplets on surfaces using appropriately parameterized water-surface interaction further confirm the dependence of water contact angle on the interaction and the interfacial water structure reproduced by different DFT functionals. Our study highlights the sensitivity of wetting to the water-substrate interaction and provides a starting point for a more accurate theoretical investigation of water-TMD interfaces.

Figure 1

Top and side views of stable adsorption configurations on the (a) W-top, (b) H-center, and (c) Se-top sites of ${\mathrm{WSe}}_2$, where green, grey, pink, and red balls represent the selenium, tungsten, hydrogen, and oxygen atoms. (d) Adsorption energy of water monomer on the sites of X-top (blue star), H-center (red square), and M-top (green triangle) of ${\mathrm{MX}}_2$ computed with SCAN. (e) Adsorption energy of water monomer on the H-center site of ${\mathrm{WSe}}_2$ (upper panel with green lines) and ${\mathrm{MoS}}_2$ (lower panel with blue lines) using different methods. The horizontal black dashed line located at –107 meV and –168 meV corresponds to the RPA results of ${\mathrm{WSe}}_2$ and ${\mathrm{MoS}}_2$ respectively.

Figure 2

[(a)–(g)] Top and side views of the lowest energy adsorbed ${\mathrm{H}}_2\mathrm{O}$ structures identified on ${\mathrm{WSe}}_2$. (h) The adsorption energy as defined in Eq. (1) for 2–8 water molecules in several configurations, in which the red stars and blue circles represent the chain-like and cyclic structures [see Figs. S3– S9 within SM [25] for other structures not presented in (a)–(g)]. (i) The total and decomposed interaction energies as a function of the number of ${\mathrm{H}}_2\mathrm{O}$ (denoted as $n$). The total interaction, i.e., the adsorption energy is shown in the red curve, the water-substrate and water-water interactions are shown in the green and blue curves, respectively. (j) The averaged distance of oxygen atoms (${\mathrm{r}}_{\mathrm{OO}}$) and hydrogen bonds (${\mathrm{r}}_{\mathrm{OH}}$) between the nearest ${\mathrm{H}}_2\mathrm{O}$ as a function of $n$, as depicted with green circles and green square.

Figure 3

AIMD simulations of liquid water on ${\mathrm{WSe}}_2$. (a) A snapshot 2 from AIMD simulation of interfacial water obtained with OVITO [54]. (b) Water density profiles as a function of distance from the substrate as results of PBE, SCAN and rev-vdW-DF2. The zero of height refers to the average height of the top planar of Se atoms. (c) Water distributions of the contact layer within $\sqrt{3}\times \sqrt{3}\times 1$ hexagonal supercell from rev-vdW-DF2 (left) and SCAN (right). The grey and green circles show the averaged position of surface W and Se atoms respectively. Angle distribution profiles of (d) dipole orientation $\theta$ and (e) leg rotation $\varphi$ for the contact layer water. The blue and green curves in (e) correspond to the water within contact layer with water orientation of $0^{\circ }\le \theta \le {180}^{\circ }$ (all) and ${70}^{\circ }\le \theta \le {110}^{\circ }$ (flat orientation) respectively. The insets of (d) and (e) are the illustrations of dipole orientation (red arrow) and leg rotation H-H direction (blue arrow) with respect to the surface normal (black arrow) respectively.

Figure 4

MD simulations. (a) Water density profiles obtained with MD and AIMD. (b) Snapshots of liquid droplets from MD simulations, corresponding to the same water density profiles of panel (a). The three settings of MD simulations include the following parameters, (i) $D_0=40\mathrm{meV}$; $\alpha =0.8{\AA }^{-1}$; $r_0=4.00\AA$, and (ii) $D_0=146\mathrm{meV}$; $\alpha =0.8{\AA }^{-1}$; $r_0=3.59\AA$, and (iii) $D_0=340\mathrm{meV}$; $\alpha =0.8{\AA }^{-1}$; $r_0=3.28\AA$.