Effects of Thickness and Adsorption of Airborne Hydrocarbons on

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Effects of Thickness and Adsorption of Airborne Hydrocarbons on Wetting Properties of MoS: An Atomistic Simulation Study 2

Mohammad Khalkhali, Hao Zhang, and Qingxia Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00481 • Publication Date (Web): 12 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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The Journal of Physical Chemistry

Effects of Thickness and Adsorption of Airborne Hydrocarbons on Wetting Properties of MoS2: An Atomistic Simulation Study Mohammad Khalkhali, Hao Zhang, and Qingxia Liu∗ Department of Chemical and Materials Engineering, University of Alberta, Edmonton, T6G 1H9, Canada E-mail: [email protected] Phone: +1 780 492 1119 . Fax: +1 780 492 2881

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Abstract 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◦ . Kozbial et al. have recently suggested that the freshly exfoliated MoS2 was intrinsically relative hydrophilic (WCA = 69.0 ± 3.8◦ ), however, it could become fairly hydrophobic after 1 day exposure to the ambient air (WCA = 89.0 ± 3.1◦ ). 1 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◦C), Gaur et al. showed that the WCA was a function of number of layers and changed from 98◦ in monolayer sample to 88◦ in a sample with eleven layers. 2 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.

Introduction Since the emergence of graphene, a single atomic plane of graphite, the research on twodimensional (2D) materials has developed without a break. 3 These materials are made through exfoliating or growing atomic layers to obtain a few or even mono layer structures. 4 Although graphene possesses superior properties such as the fastest charge transport, large stiffness and high thermal conductivity, its zero band gap limits its application in electronic 2


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devices. 5 This limitation of graphene has promoted development of transition metal dichalcogenides (TMDs) as potential alternative 2D materials that can offer potential solutions for nanoelectronic and optoelectronic applications. 6,7 Molybdenum disulfide (MoS2 ) is one of the most studied TMDs due to its distinctive electronic, optical, catalytic, and tribological properties. 8–11 Bulk MoS2 is a semiconductor with indirect band gap of 1.2 eV. 12 By decreasing the number of layers down to monolayer, the quantum confinement effect would transform this indirect band gap to a direct one with 1.8 eV. 13 This property along with comparative low cost of production make MoS2 a perfect candidate for transistors and photodetectors as well as flexible electronic devices. 14–16 In addition to the promising future in electronics, the monolayer of MoS2 has been recently proposed as a suitable nanoscale membranes. 17 This could have potential applications in biological sensors 18–21 and water distillation. 22,23 Moreover, MoS2 has been used as catalyst for processes such as hydrodesulfurization and hydrogen evolution 24–27 and long known as an excellent solid lubricant for tribological applications. 28 A clear understanding of surface wetting properties of MoS2 is essential for these applications. Wettability is also the key factor in extraction of MoS2 mineral, molybdenite, through froth flotation. Similar to other TMDs, the Mo and S atoms in MoS2 are bonded covalently within the layers, and layers are held together through weak van der Waals forces. This structure causes an anisotropic surface property in which the rupture of covalent bonds between Mo and S generates a high-energy edge surface while the basal plane has considerably lower excess energy. 29 While the properties of the edge surface are important in flotation and catalysis applications, the basal plane is the main player when 2D MoS2 is implemented in bio or optoelectronic applications. The basal plane of MoS2 has been generally believed to be hydrophobic. However, the wide range of water contact angles (WCAs) reported for this surface have caused an uncertainty about surface properties of MoS2 . WCA values as high as 98◦ , 2 92◦ , 30 and 85◦ 31 and as low as 76◦ 32 and 69.0 ± 3.8◦ 1 have been reported. Different sources may cause this dis-

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crepancy including different methods for contact angle measurements, different experiment conditions or adsorption of airborne hydrocarbon contaminants. The latter have already been confirmed to alter the WCA of pure gold, 33 graphene 34 and boron nitride nanotubes. 35 In separate studies, Chow et al. 36 and Kozbial et al. 1 have shown that airborne hydrocarbon contaminants have the same effect on wetting properties of MoS2 . Kozbial et al. showed that the intrinsic WCA of 69.0 ± 3.8◦ of freshly exfoliated MoS2 increases to 89.0 ± 3.1◦ after exposure to ambient air for a day. 1 The thickness of MoS2 thin film has been shown to affect the wetting properties as well. Gaur et al. studied effects of the surface structure and the number of layers on wettability behavior of MoS2 . 2 They showed that samples grown at low temperatures (550◦C) were hydrophilic (WCA∼23.8◦ ) due to their semi-crystalline nature and the presence of (001) planes vertically oriented to the substrate. 2 In highly crystalline samples grown at high temperature (900◦C), the WCA was a function of the number of layers changing from 98◦ in monolayer sample to 88◦ in a sample with eleven layers. 2 Molecular dynamics (MD) simulations offer suitable tools to complement experiment and provide deeper understanding of wetting properties, especially at the nanoscale. The solidliquid contact angle can be calculated directly from MD simulations of a liquid droplet on a solid surface. It is well established that contact angle of sessile drop is size dependent at the nanoscale. Therefore, it is a common practice to calculate the contact angle for droplets with different sizes and to evaluate the macroscopic contact angle by extrapolating results to r → ∞ using “modified Young’s equation”: 37,38

cos(θ) = cos(θ∞ ) −

τ , rγlv

(1)

where γlv is the liquid-vapor surface tensions, τ is the line tension, r is the radius of the contact line (it is assumed to be circular), and θ∞ stands for the contact angles at r → ∞ which is calculated through the Young’s equation 39 as

4


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cos(θ∞ ) =

γsv − γsl , γlv

(2)

where γsl , γsv , and γlv are solid-liquid, solid-vapor, and liquid-vapor surface tensions, respectively. The contact angle can also be calculated indirectly from MD simulation results. In this approach, MD simulations are used to evaluate the components of the Young’s equation. Since calculating γsl and γsv from MD simulations are challenging, the contact angle is usually evaluated through the calculation of solid-liquid work of adhesion Wsl using the Young-Dupré equation:

Wsl = γlv (1 + cos(θ∞ )),

(3)

where Wsl is the solid-liquid work of adhesion. The indirect and direct approaches used in this study have been proven to be accurate WCA calculation methods. The indirect method used in this study is the dry-surface method of Leroy and Müller-Plathe. 40,41 The direct calculation approach used in this study is the one we recently proposed for calculating WCA directly from semi-spherical droplet simulations. 42 Accuracy of MD technique directly depends on the interatomic potentials (force fields) that describe interactions between all atoms in the system. 43 MD studies on wetting properties of MoS2 have delayed, mainly because accurate forcefields were not available to model MoS2 -water interactions. Luan and Zhou 17 modified the force field developed by Varshney et al. 44 to reproduce the experimental results of Gaur et al. 2 They used TIP3P water model and the monolayer of MoS2 . In a more comprehensive study, Leroy 45 proposed new sets of force field parameters to modify interactions between some of the most popular water models and two of the most cited MoS2 forcefields (force fields of Liang et al. 46,47 and of Varshney et al. 44 ). The forcefield parameters were modified to obtain WCA reported by Kozbial et al. 1 Leroy 45 also showed that electrostatic interactions between water and MoS2 (monolayer and bilayer) have negligible effect on the WCA of the basal plane. 45 Almost simultaneously,

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Govind Rajan et al. developed a new set of force field parameters for MoS2 and made the same conclusion about electrostatic interactions. 48 They also found that electrostatic interactions had negligible effect not only on the WCA, but also on the friction coefficient and slip length on the MoS2 basal plane. 48 The details of the forcefield parameters developed by this group were published in a later study. 49 Although wetting properties were not considered as an observable during the force field development, contact angles reported for water (a polar solvent) and diiodomethane (a nonpolar solvent) on basal plane of MoS2 were in a good agreement with experimental results. 49 In this study, we investigate the two main factors that are believed to affect the wetting properties of MoS2 , namely, adsorption of airborne hydrocarbon contaminants and number of layers, using atomistic simulations. At the first step, we examine the applicability of the force field developed by Sresht et al. 49 in evaluating WCA on basal plane of MoS2 . The effect of adsorption of airborne hydrocarbon contaminants and the number of layers on the WCA is then investigated. We show that MD simulations can successfully explain the change in WCA due to these factors. This study complements works of previous researchers finding that dispersion interactions govern wetting properties of MoS2 basal plane. 45,48 We confirm that effect of adsorption of airborne hydrocarbon contaminants and the number of layers can also be explained through change in the dispersion interactions between water and MoS2 . The results of this study can clarify the discrepancy in the wide range of WCA values reported in literature and provide an insight in wetting properties of MoS2 .

Methods In all of simulations in this work, the hexagonal 2H allotrope (space group P 63 /mmc) of MoS2 and SPC/E rigid water model were used. We utilized an orthorhombic cell of MoS2 instead of the hexagonal unit cell which was made according to the procedure explained by Varshney et al. 44 Please refer to the supporting information for more details and graphical

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representation of simulation setups.

Forcefield The forcefield parameters used for MoS2 in this study are those recently proposed by Sresht et al. 49 Table 1 represents all forcefield parameters used in this study. Table 1: Forcefield parameters used in this study. Nonbond σ σ + 4ij ( rij )12 − ( rij )6 √ √ ij = i .j , σij = σi .σj q (e) (kJ/mol) +0.5000 0.485 -0.2500 2.085 -0.8476 0.650 +0.4238 0.000 -0.2400 0.276 +0.0600 0.126 Bond Ub = 12 k(r − r0 )2 2 r0 k (kJ/(mol.Å )) 430.3 2.410 1.000 2846.0 1.090 Angle Ua = 12 k(θ − θ0 )2 θ0 (◦ ) k (kJ/(mol.rad2 )) 2050.0 83.80 1187.7 83.80 1187.7 83.80 0.000 0.000 109.0 276.0 107.8

Uij =

Mo S O H (H2 O) C H (CH4 )

Mo-S O-H C-H

Mo-S-Mo S-Mo-S (ψ) S-Mo-S (θ) S-Mo-S (ξ) H-O-H H-C-H

1 qi qj 4π0 r

σ(Å) 4.430 3.340 3.166 0.000 3.500 2.500

Since this forcefield is developed in OPLS-AA framework, nonbonded interactions between 1-2 and 1-3 bonded atoms were set to zero, and those between 1-4 bonded atoms were scaled by a factor of 0.5.

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Simulation details All molecular dynamics simulations were carried out in LAMMPS software. 50 Modified NoseHoover thermostat and barostat 51 were used for temperature and pressure control. A time step of 1 fs was used in all MD simulations. Constrains on the rigid water model were applied through SHAKE algorithm. The long-range Coulombic interactions were handled through particle-particle particle-mesh Ewald (PPPM) solver with the accuracy of 1.0e-4. Grand Canonical Monte Carlo (GCMC) simulations for CH4 adsorption on MoS2 basal plane were carried out in Sorption module of Materials Studio. GCMC simulations consisted of 1 × 106 equilibration and 2 × 106 production steps. Snapshots of 1000 most stable configurations were saved during the GCMC simulations for further analyses. The Metropolis Monte Carlo method was used with four types of operations for a grand canonical ensemble, namely, rotation, translation, creation, and deletion. Same weights were applied to all operators. Cut-off distance of 15 Å and Ewald solver accuracy of 1.0e-4 were used and all GCMC simulations were performed at 298 K.

WCA calculation methods In this study, a direct and an indirect calculation approaches were utilized to evaluate WCA from MD simulations. The direct method is the one we recently developed to calculate WCA from MD simulation of a partial spherical droplet on a solid surface. 42 In this method, the contact angle is calculated along the contact line through a convex hull triangulation of the liquid droplet and is represented as an angular distribution. The indirect method used in this study is the dry-surface method of Leroy and MüllerPlathe, 40,41 which calculates the solid-liquid work of adhesion, Wsl . This quantity is calculated through thermodynamic integration between the actual solid-liquid interface and a state in which average solid-liquid interaction energy per unit area is reduced to a value of the order of 0.01 mJ/m2 . The Lennard-Jones contribution (USL,LJ ) to the total solid-liquid potential energy (USL ) is defined as 8


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USL,LJ (α) = α

NW X

( 4M oO .

i=1

N Mo X j=1

"

σM oO rij

12

−

σM oO rij

6 #

" 12 6 #) NS X σSO σSO + 4SO . , − r r ij ij j=1 (4)

where NW , NM o , and NS are the number of water molecules, molybdenum, and sulfur atoms, respectively. r refers to the distance between two interacting atoms and the parameter α is a scaling factor. It will be shown later that the Coulombic interaction between water and MoS2 is relatively negligible. As a results, it is safe to apply the change in interfacial energy through the modification of the Lennard-Jones potential acting between water and MoS2 . This modification is applied by altering the scaling parameter α defined in Equation 4. So, the solid-liquid work of adhesion, Wsl , is calculated as 1 Wsl = A

Z

α

α0

USL,LJ (α0 ) α0

dα0 ,

(5)

T,V,N,α0

where A is the the area of the solid-liquid interface and < ... > denotes the ensemble average in the canonical ensemble. 45 Calculating Wsl , WCA can be derived indirectly from Equation 3.

Droplet simulations Details of preparation of the initial configuration and WCA calculation algorithm were outlined in our recent study. 42 The area of basal surface of MoS2 used in droplet simulations was 21.98 × 22.08 nm2 . A vacuum gap of 18 nm was added in z direction to avoid interactions of water droplet and MoS2 with their periodic images. Seven water droplets with radii of 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0 nm were made through cutting partial spheres from the pre-equilibrated water structure. The initial contact angle of water droplets on MoS2 surface was set to 100◦ . In the first step, an energy minimization were performed to cure any possible close-contact between water molecules or between water molecules and the MoS2

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surface. Then, water droplets were relaxed through a NVT simulation at 298 K for 5 ns. Oscillation of the center of mass of water droplet was monitored to ensure equilibrium. WCA distributions were calculated from snapshots of the last 1 ns of simulations, which were saved every 1 ps.

Dry-surface simulations The dimensions of MoS2 basal surface in dry-surface simulations were 4.40 and 4.44 nm in x and y direction, respectively. A water film consisting of 9541 water molecules (approximate thickness of 14.6 nm) was relaxed at 298 K and added to the vacuum gap above the MoS2 basal surface. The dimension of simulation cell in z direction was 45 nm. The system was simulated in NVT ensemble for 3.5 ns and the last 500 ps of simulations were used to calculate the average solid-liquid interaction energy. The oscillation of solid-liquid interaction energy were monitored to ensure equilibrium.

Methane adsorption A four-layer MoS2 crystal with the same basal surface dimensions as those used in dry-surface method were used in GCMC simulations of methane adsorption. The length of vacuum gap above the basal surface was ∼3 nm. An empty box with the same volume as the vacuum above the basal surface were used to calculate the equilibrium loading of CH4 in the reference system (n0 in Equation 7). In Figure 1, the results of these simulation are compared with the experimentally developed Peng-Robinson equation of state for methane. 52 Figure 1 shows a very good agreement between experimentally developed Peng-Robinson equation of state and results of GCMC simulations. Density profile of gas was calculated through meshing the length of gas volume in z direction with the mesh size of dz = 0.1 Å (plots are provided in supporting information). Methane molecules reside in the first adsorption layer were recognized through the first peak in the density profile. The thickness of the first adsorption layer obtained from GCMC simulations is ∼ 0.5 nm, which is in a 10


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60 Peng-Robinson GCMC

40 P (bar)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

0 0

1

2

3

3

(mmol/cm )

Figure 1: Comparison between equation of state for methane calculated though GCMC simulations and in experiment. 52 good agreement with the airborne contaminant thickness on graphene and MoS2 reported by Kozbial et al. 1 According to this agreement, we assumed the adsorption of methane on MoS2 was stronger than the hydrophobic effect that would force the methane molecules to gather and form a methane droplet. To calculate the surface coverage of gas, the projections of hydrogen and carbon atoms of methane molecules reside in the first adsorption layer on the basal surface of MoS2 were constructed. Each atom was represented as a circle with the van der Waals radius of the corresponding atom. The area on the surface which covered by these circles was then calculated through a fine mesh (dx = dy = 0.1 Å).

Results and Discussion WCA on the fresh MoS2 basal plane In this section, we use the forcefield parameters developed by Sresht et al. 49 to evaluate the WCA on the fresh basal plane of bulk MoS2 . There are some uncertainties in WCA values 11



reported by the developers of this forcefield, which convinced us to calculate WCA using more accurate methods. Sresht et al. used two different approaches to extract the WCA from droplet simulations. 49 The results of these two approaches did not agree with each other and the one reported as the WCA was chosen because it showed better agreement with the WCA reported by Kozbial et al. 1 In another work by the same research group, the work of adhesion of water on the basal plane of MoS2 were calculated using two different methods, namely, Young-Dupré and Bennett’s acceptance ratio methods. 48 In the former method, Wsl was obtained through WCAs calculated using droplet simulations at different temperatures, while in the latter Wsl is calculated through a free-energy route. Young-Duprée and Bennett’s acceptance ratio methods gave Wsl values of 76.6 and 92.0 mJ/m2 , respectively. Considering Equation 3 and the surface tension of SPC/E water model (60.6 mJ/m2 53 ), these values for work of adhesion lead to WCAs of 74.7◦ and 58.8◦ , respectively. Not only these values do not match, but also do not agree with the WCA reported by Kozbial et al. 1 (69.0 ± 3.8◦ ). Due to these uncertainties in WCA values reported for the forcefield used in this study, we decided to repeat the WCA calculations using two different methods. The first method is the one we recently developed to calculate WCA directly from liquid droplet simulations. 42 Due to the possible size dependence of contact angle at the nanoscale, nanodroplets with different sizes were simulated and the WCA was evaluated through extrapolating results to r → ∞. The solid-liquid work of adhesion were also calculated using the dry-surface method of Leroy and Müller-Plathe. 40,41 Leroy applied this method to modify MoS2 -water interaction parameters for two of the most cited MoS2 forcefields. 45 Figure 2 shows WCA and Wsl values evaluated on the fresh basal plane of MoS2 . To calculate Wsl through Equation 3, surface tension values for SPC/E and real water were taken from work of Vega and de Miguel. 53 According to experimental investigation of Gaur et al., the wetting behavior of single crystal MoS2 film with 11 layers is equivalent to the perfect bulk structure of MoS2 . 2 Thus, we used a large system consisting of 12 single layers to avoid possible thickness effect. Sresht et al. proposed the cut-off distance (rc ) of 12 Å for

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the potential used in this study. In addition to this cut-off distance, we also evaluated the WCA and Wsl with the same forcefield parameters but rc = 10 Å. 80 100

Droplet Method Dry-surface Method

70

Wsl (mJ/m )

Experiment

2

Experiment

WCA ( )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

90

80

70 50

10

12

10

cut-off (Å)

12 cut-off (Å)

(a)

(b)

Figure 2: Wetting properties of MoS2 modeled using Sresht et al. forcefield 49 with two different cut-off distances (12 and 10 Å). (a) and (b) show results for the contact angle and work of adhesion of water on the basal plane of MoS2 , respectively. Wetting properties were evaluated from MD simulation results using two methods, namely, droplet simulation and dry-surface methods. Experimental value of WCA were taken from work of Kozbial et al. 1 Error bars represent limits of the 95% confidence interval for WCA calculated from droplet simulations. When calculating the contact angle indirectly from work of adhesion, the uncertainty is inherently low. This is because the work of adhesion is an averaged value, which is calculated from a thermodynamic property of system, internal energy. Internal energy does not oscillate significantly once system reaches equilibrium and so the work of adhesion. It can be seen in Figure 2a that WCAs calculated using droplet and dry-surface methods are in good agreement with each other. This confirms that the usage of accurate WCA calculation methods can enhance the reliability of MD wetting studies. This figure also shows that using the cut-off distance originally proposed by Sresht et. al results in WCAs smaller than those reported experimentally. However, decreasing the cut-off distance from 12 to 10 Å significantly enhances the WCA obtained by this forcefield. On the other hand, Figure 2b shows that the solid-liquid work of adhesion is closer to the experimental value when rc = 12 Å. The difference between Wsl measured experimentally and in simulations is mainly because of the difference between the surface tension of real water (γlv = 71.7mJ/m2 ) 13



and that for SPC/E water model (γlv = 60.6mJ/m2 ). Here, we should emphasize that the WCA calculated from MD simulations should be treated with care, especially, when it is used as an observable to optimize solid-liquid forcefield parameters. This is simply because the liquid surface tension (γlv ) calculated using most of water models does not agree with the value calculated experimentally. As accurately pointed out by Leroy before, 40,41,45 "comparing simulations and experiments in this situation is equivalent to comparing the wetting behavior of two different liquids as far as surface tension is concerned.". This means that the WCAs calculated through MD simulations are water-model-dependent since changing the water model would change γlv . Having this in mind, one should consider that in this work, the wettability of MoS2 in different conditions is studied in a comparative manner. This means that we are interested in the difference between WCAs rather than their absolute values. As a result, we focus on WCA change while we are aware that the solid-liquid work of adhesion may not be in agreement with the experimental value.

Effect of cut-off distance on WCA In atomistic simulations, it is customary to define a cut-off beyond which the van der Waals (vdW) interactions can be ignored. This is a reasonable approximation as vdW interactions have a short range nature, i.e., they decays rapidly for long distances. Depending on which material properties are considered as observables in the potential development process, the cut-off distance may be included in the optimizing parameters or treated as a constant. For most of the applications, cut-off distance of 10 to 15 Å is adequate. However, choosing an accurate cut-off is crucial for some material properties. The effect of truncation and the necessity of long-range corrections (LRC) to vdW interactions has been well-established for accurate two-phase simulations and the study of the interfacial properties. 54 Although the treatment of LRCs is well understood for bulk systems and has been extended to planar liquid-vapor interfaces, no development has been

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suggested for solid-liquid interactions. 41 As shown in the previous section, decreasing the cut-off distance just for 2 Å can enhance the WCA calculated from the forcefield developed by Sresht et al. greatly. The effect of cut-off distance on the solid-liquid work of adhesion in water-graphite system was also reported by Leroy et al. 41 As explained before, the number of layers is one of the parameters believed to affect wetting properties of MoS2 . Although using the cut-off distance of 10 Å leads to a better WCA on the bulk MoS2 , shortening the cut-off distance may lead to mask the thickness effect in molecular dynamics calculations. This is probably the reason why Leroy did not observe a significant difference between mono- and bilayer films when studied the wetting properties of MoS2 using different forcefields. 45 Using van der Waals-corrected density functional theory, Guo et al. observed a thickness dependent surface energy in ultrathin MoS2 films. 55 They attributed this effect to the thickness dependence of WCA on the basal plane of MoS2 films reported by Gaur et al. 2 Relations between solid surface energy and the liquid-solid contact angle is explained through the Young’s equation (Equation 2). In addition to the surface energy of MoS2 (γsv ), thickness may also affect the interaction energy between MoS2 and water, which alters γsl . This effect can not be studied by density functional theory (DFT) simulations as it is not yet practical to model the bulk water in DFT. To find the suitable cut-off distance, we calculated the solid-solid and solid-liquid interaction energies for MoS2 -water system using different cut-off distances. A large MoS2 system made of 12 layers were used. Figure 3 shows the total interaction energy that consists of var der Waals, which is in the form of Lennard-Jones potential in the forcefield used here, and Coulombic interactions. One important result of Figure 3 is that solid-solid (USS ) and solid-liquid (USL ) interactions indeed depend on the cut-off distance of the interatomic potential (rc ). USS becomes insensitive to the cut-off distance when rc ≥ 30 Å while USL reaches a plateau when rc > 50 Å. Figure 3 also shows that Coulombic interactions play significant role in USS while their effect is negligible in USL . This observation about the effect of Coulombic interactions in 15



cut-off (Å)

cut-off (Å)

10

20

30

40

50

60

70

10

20

30

40

60

70

Coulombic

-20

Lennard-Jones

-0.5

(MJ/mol)

-40

-60

-80

-1.0

U

SL

SS

(MJ/mol)

50

0.0

0

U

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 35

-100 -1.5 -120

-140 -2.0

(a)

(b)

Figure 3: (a) solid-solid and (b) solid-liquid interaction energies calculated using different cut-off distances. MoS2 has also been reported previously by Leroy 45 and Rajan et al. 48 According to Figure 3, the minimum cut-off distance in order to study the effect of number of layers on wetting properties of MoS2 accurately is rc = 50 Å. If the cut-off distance is smaller than 50 Å, it will affect the solid-liquid interaction and the WCA change would not be the pure effect of change in thickness. However, one should be careful about changing the cut-off distance. Since the forcefield parameters were originally developed with rc = 12 Å, changing the cut-off distance without modifying the forcefield parameters will result in wrong wetting properties. As a result, we need to modify the forcefield parameters in order to reproduce the experimental WCA when using rc = 50 Å. Modifying forcefield parameters for rc = 50 Å In the forcefield used in this study, water and MoS2 are interacting through 12-6 LennardJones and Coulombic pair potentials. As in the water model used in this study, i.e., SPC/E, there is no Lennard-Jones parameter for hydrogen and charges of oxygen and hydrogen are constant, the interaction between MoS2 and water can be tuned by altering Mo-O and SO Lennard-Jones parameters. Leroy used the same idea to modify MoS2 -water interaction

16


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parameters in some of the available forcefields for MoS2 in order to improve the wetting properties. 45 Following the work of Leroy, 45 we used a scaling parameter, α, to tune the Lennard-Jones interactions between MoS2 and water (see Equation 4). Simulations performed with α = 1 refer to the usage of Lennard-Jones parameters originally developed by Sresht et al. 49 Through the dry-surface method, work of adhesion and the WCA can be calculated for different value of α (Figure 4).

0.766

140

120

rc=50 Å rc=10 Å

WCA = 22.7

100

2

Wsl (mJ/m )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


WCA = 70.6 80

60

40

20

0

0.0

0.2

0.4

0.6

0.8

1.0

Figure 4: Variation of the solid-liquid work of adhesion (Wsl ) and water contact angle for bulk (12-layers) MoS2 and water system with respect to the scaling parameter α. The red and blue circles correspond to the cut-off distances of 10 and 50 Å, respectively. Solid lines represent nonlinear fits to the scatter data points with the fitting function Aα5/4 − Bα. Figure 4 confirms that cut-off distance of 50 Å results in a very small contact angle (22.7 ◦

) if Sresht et al. forcefield parameters are used without modification. Following the work of

Leroy, we used the fitting function in the form of Aα5/4 − Bα. 45 It is an arbitrary function and is chosen only because it gives good results with root mean square errors of 0.99 and 0.18 for rc =10 and 50 Å, respectively. As shown in Figure 4, if the scaling parameter of α = 0.766 is applied to the water-MoS2 Lennard-Jones when using rc =50 Å, the same WCA calculated using rc =10 Å can be achieved. 17



Effect of the number of layers on WCA Using the modified forcefield parameters and the cut-off distance of 50 Å, we studied the change in WCA with respect to the number of monolayers (from 1 (monolayer) to 12 (bulk)). N

layers

1

2

3

4

5

6

7

8

9

10

11

12

78 3

N

= 1

N

= 2

N

= 12

layers

layers

layers

76

Coulombic

-10

Lennard-Jones

2

0

2

(mJ/m )

72

( )

0

-20

SL

1

30

60

90

U

74

Probability (%)

WCA ( )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 35

120

-100

70 -110

0

2

4

6 N

8

10

12

layers

(a)

(b)

Figure 5: Variation of (a) WCA and (b) solid-liquid interaction energies with respect to the number of layers in a MoS2 film. In (a) WCAs were calculated using the dry-surface method and inset plot shows the WCA distribution obtained from direct simulation of 10 nm droplets. Dash line is a guide to the eye. In Figure 5a, the WCA calculated using the dry-surface method increases from 70.6◦ in 12-layer MoS2 to 76.6◦ in the monolayer. Similar results also observed in WCA distributions calculated using the droplet method. Gaur et al. reported the same trend by changing the number of layers from 1 to 11. 2 They showed that WCAs of 98◦ in mono- and 94◦ in bilayer MoS2 change to value of 88◦ in bulk. It is interesting that the contact angle difference between mono- and bilayer MoS2 thin films is similar in our simulation and in experimental values reported by Gaur et al. 2 The difference between contact angles of monolayer and bulk MoS2 are also in good agreement. Figure 5b shows that WCA changes with the number of layers mainly because of the change in van der Waals interactions between MoS2 and water. This is true as overall Coulombic interaction between MoS2 and water is negligible and the solid-liquid interaction is mainly due to the Lennard-Jones potential. As mentioned before, this is a well established observations reported by other researchers as well. 45,48,49 18


Page 19 of 35

The work of adhesion calculated through the dry-surface method is the free-energy change per unit area corresponding to the separation of liquid and solid phases from the state that they are in complete contact to the one that they do not interact. Thus, Wsl can be defined as

Wsl =

1 (∆U − T ∆S) A

(6)

where ∆U/A and ∆S/A are energy and entropy of adhesion, respectively. If we consider that changes in energy and entropy are independent of the temperature (which is acceptable assumption in low temperatures and small temperature changes), ∆U/A and ∆S/A can be evaluated through the temperature dependency of Wsl . Using the dry-surface method, we calculated the temperature dependency of Wsl around 298 K.

90 bulk

85

monolayer

2

1.48x10 -2.25x10 2

Wsl (mJ/m )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1

T

80 75 2

1.29x10 -1.86x10

-1

T

70 65

280

290

300

310

320

T (K)

Figure 6: Temperature dependency of Wsl in monolayer and bulk MoS2 . Solid lines represent linear fits. From linear fits in Figure 6, the ratio of entropy component (T ∆S/A) to energy component (∆U/A) is 0.45 and 0.43 in bulk and monolayer MoS2 , respectively. Through investigating experimentally derived data from two-component, three-phase solid-liquid-vapor 19



systems, Weber and Stanjek showed that the majority of excess entropies of adhesion fall in a narrow range between 0.1 and 0.2 mJ/m2 K. 56 They also showed that the entropic contributions make up 30-50% of the total work of adhesion at 25 ◦C and can not be neglected. 56 Both of these observation are in close agreement with the temperature dependency of the work of adhesion derived from our simulation data. Govind Rajan et al. also mentioned that the contribution of entropy is quite significant and the effect of entropy can not be neglected. 48 Transforming MoS2 from bulk to monolayer state causes a decrease in work of adhesion, which is ∆Wsl = 6.66mJ/m2 . At 298 K, contributions of energy and entropy components in this decrease are 18.4 and 11.8 mJ/m2 , respectively. This means that effect of entropy change, which is due to the layering and ordering of water in the vicinity of solid-liquid interface, is as important as the effect of energy change. This is expected as changing the number of layers affects the solid-liquid interaction energy and both energy (or enthalpy in case of NPT ensemble) and entropy of adhesion directly depend on USL . Taherian et al. showed that the entropy of adhesion could be calculated from the fluctuations of the water-surface dispersion energy and represent the configurational bias imposed on the water molecules by the attractive external potential of a solid wall. 57 By investigating thermodynamics of atomistic and coarse-grained models of water near nonpolar surfaces, Abraham and Leroy has recently confirmed that both energy and entropy of adhesion are dominated by the solidliquid interactions. 58 While ∆U/A is directly connected to the average of the solid-liquid interactions, ∆S/A is related to the fluctuations of these interactions. 58

Effect of adsorption of airborne hydrocarbons on WCA Kozbial et al. reported that an intrinsic WCA of 69.0 ± 3.8◦ for freshly exfoliated MoS2 increased to 89.0 ± 3.1◦ after 1 day exposure to the ambient air. 1 Through ATR-FTIR and ellipsometry, they showed that this increase in WCA was due to the adsorption of airborne hydrocarbons on the MoS2 surface. So far, we have shown that the Sresht et al. forcefield 49 is 20


Page 20 of 35

Page 21 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


able to reproduce the WCA reported by Kozbial et al. on freshly exfoliated MoS2 after mild modifications. Since this forcefield is developed in OPLS-AA framework, it has the potential to include organic components in the simulation. We used this forcefield in combination with OPLS-AA to study the effect of adsorption of a nonpolar small hydrocarbon, i.e. methane, on wetting properties of MoS2 . The applicability of OPLS-AA in predicting thermodynamic properties of normal alkanes has been confirmed before. 59 In the case of graphite, it has been shown that the adsorbed species can be removed by a thermal annealing in vacuum without introducing significant defects. 34 The interaction between the hydrocarbon and the MoS2 substrates are likely physisorption as well. The weak binding of the contaminant causes significant challenges in their structural characterization. Therefor, no structural information about the contaminants has been reported so far. 60 Our assumption of using a nonpolar hydrocarbon can be justified by the fact that adsorption of hydrocarbon contaminants has been shown to promote hydrophobicity. Anthropogenic increases in methane production over the past 200 years have resulted in a present-day methane concentration of more than 1800 parts per billion by volume. 61 In real world, this magnitude is considered catastrophic and responsible for global warming. In atomistic simulations scale, however, this concentration is so small to model. As a result, the densities modeled in this study are much larger than those reported for airborne hydrocarbons. Using the Grand Canonical Monte Carlo method, we first modeled the adsorption of methane on the basal plane of MoS2 and calculated the adsorption isotherm. Surface excess concentration of methane was evaluated as

Qads =

1 < n − n0 > A

(7)

where < ... > denotes the ensemble average, n is the number of mole of gas in the GCMC simulation, and n0 is the number of mole of gas in the reference system with the same pressure and volume but no adsorbent. n0 was calculated by GCMC simulation of the CH4 in an empty box. As shown in Methods section, n0 values calculated from these simulations were 21



in an excellent agreement with an experimentally developed Peng-Robinson equation of state for methane. Through density profile of methane, adsorption layers can also be identified. After identifying CH4 molecules reside in the first adsorption layer, the percentage of surface area covered by these molecules was also calculated through a fine 2D mesh applied on the surface. Figure 7 represents the results of GCMC simulations of CH4 adsorption on MoS2 .

80

10 Q

60

6 40 4 20

2 0

Surface Coverage (%)

ad

Surface Coverage

ad

(

2

mol/cm )

8

Q

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 35

0 0

10

20

30

40

50

P (bar)

Figure 7: The adsorption of methane on the basal surface of MoS2 modeled by GCMC simulations. Dash lines are used as guides to the eye. Figure 7 shows that both surface excess and surface coverage by CH4 molecules increase with a large rate up to P = 20 bar. After this pressure, the surface coverage reaches a plateau, meaning the CH4 concentration in the first adsorption layer does not change significantly when P > 20 bar. Qad , however, continues to increase when P > 20 bar but with a smaller rate compared to P < 20 bar. This can be explained by formation of the second adsorption layer which can also be identified through the gas density profiles. As the focus of this study is not CH4 adsorption on MoS2 , more details about adsorption simulations and isotherms are provided as supporting information. Density profiles calculated from GCMC simulations show that the thickness of the first 22


Page 23 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


adsorption layer is ∼ 0.5 nm. This is in a good agreement with the airborne contaminant thickness on graphene and MoS2 reported by Kozbial et al. 1 As a result, it is safe to assume that CH4 molecules reside in the first adsorption layer are those strongly attached to the surface and this attachment is stronger than the hydrophobic effect that would force the methane molecules to gather and form a methane droplet. According to this assumption, we fixed the CH4 molecules in the first adsorption layer and calculated the contact angle for basal surfaces with different surface coverage of methane. Dry-surface method was performed for hybrid MoS2 - CH4 system by applying the scaling parameter α on both MoS2 -water and CH4 -water Lennard Jones interactions. Due to the low concentration of methane compared to MoS2 and water, we assumed that electrostatic interactions between methane and water have negligible effect on the interaction energy between MoS2 - CH4 hybrid system and water. To test the accuracy of the dry-surface method in calculating the WCA on this hybrid surface, the WCA on the surface structure obtained from GCMC simulation at P = 10 bar was also calculated using the droplet method. According to Figure 7, about ∼26% of this surface is covered by methane. The results of WCA calculation on this surface are shown in Figure 8 along with those for the fresh basal plane. Note that the angular distributions shown in Figure 8 were calculated from 10 nm water droplet simulations. The WCAs calculated through simulation of a 10 nm semi-spherical water droplet are in a good agreement with those calculated through dry-surface method. Thus, Figure 8 confirms the applicability of dry-surface method in predicting the WCA change as a result of CH4 adsorption on MoS2 basal surface. Figure 8a clearly shows that the work of adhesion of water on basal surface of MoS2 significantly drops after adsorption of CH4 . This transforms the weakly hydrophilic nature of this surface to a hydrophobic one. Confirming the applicability of the dry-surface method in predicting wetting properties of the MoS2 basal surface after CH4 adsorption, we used this method to calculate the WCA for surfaces with different CH4 coverage. As explained before, different surface coverages obtained through fixing the CH4

23



90

3.5 WCA = 70.6

80

WCA = 67.2

CH4 adsorbed Fresh

3.0

adh

4

adsorbed

Probability (%)

CH

60

Fresh

2

(mJ/m )

70

W

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 35

50 40

WCA = 115.8

30 20

WCA = 111.3

2.5

2.0

1.5

1.0

10 0.5

0 -10

0.0

0.0

0.2

0.4

0.6

0.8

1.0

0

30

60

90

120

150

180

( )

(a)

(b)

Figure 8: Water contact angles evaluated on MoS2 basal surface before and after adsorption of CH4 . In (a) and (b) water contact angles are calculated using dry-surface and droplet methods, respectively. In (a) dash line are used as a guide to the eye and solid horizontal lines correspond to the WCA calculated from dry-surface method. In (b) dash lines show the mean values of angular distributions calculated from simulations of a 10 nm water droplet on fresh and contaminated MoS2 basal planes. molecules reside in the first adsorption layers formed at different pressures. Figure 9 shows the WCA on the MoS2 basal surface as a function of the pressure at which the CH4 adsorption happened (P∗ ). As expected, the WCA in Figure 9 shows the same trend as the surface coverage in Figure 7. Similar to the surface coverage of CH4 , the rate of increase in WCA decreases as the pressure increases until it reaches a plateau when P∗ ≥ 20 bar. This is mainly because the adsorption layer of CH4 reached its maximum concentration limit and its structure does not change when P∗ ≥ 20. More interestingly, Figure 9 shows that even the smallest methane surface coverage (surface coverage of 6.7% for the adsorption layer formed at 1 bar) can increase the water contact angle for about 15◦ . This is close to the WCA increase reported by Kozbial et al.(∼20◦ ) for freshly exfoliated MoS2 after 1 day exposure to the ambient air. 1 Similar to the fresh MoS2 basal surface, Coulombic interactions are negligible in CH4 -contaminated surfaces. Similar to Figure 6, temperature dependency of Wsl was calculated for methane-contaminated 24


Page 25 of 35

140 130 120 0

110 (mJ/m )

-20

2

100

-40

-60

SL

90

U

WCA ( )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-80

80

Coulombic

-100

Lennard-Jones

70

0

10

20

30

40

50

*

P (bar)

60 0

10

20

30

40

50

*

P (bar)

Figure 9: Water contact angle (WCA) and solid-liquid interaction energies as functions of the pressure at which the CH4 adsorption happened (P∗ ). Dash lines are used as a guide to the eye. MoS2 basal surfaces. Note that in Figure 10, we assumed that temperature change from 280 to 320 K would not affect the surface coverage of methane. We also assumed that the work of adhesion has a linear temperature dependency as Equation 6. Figure 10 shows that similar to the fresh MoS2 basal surfaces, both energy and entropy of adhesion are important in determining wetting on the methane-contaminated surface. Although changing the methane surface coverage affects energy of adhesion more, the change in the entropy is still significant. At 298 K, change in the entropy component is 32% of the change in the energy component. Figures 9 and 10 show that adsorption of hydrocarbons affects the wetting properties of MoS2 through reducing the dispersion energy between MoS2 and water. As mentioned before, this reduction in dispersion energy affects both energy and entropy of adhesion.

25



75 *

P = 1 bar

70

2

Wsl (mJ/m )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 35

65

*

P = 35 bar

2

1.31x10 -2.16x10

-1

T

60 25 1

20 15

6.77x10 -1.49x10

280

290

-1

T

300

310

320

T (K)

Figure 10: Temperature dependency of Wsl in methane-contaminated MoS2 basal surfaces. Solid lines represent linear fits.

Conclusions Using atomistic simulations, we investigated two main factors believed to affect the wetting properties of MoS2 , namely, adsorption of airborne hydrocarbon contaminants and number of layers. The change in wetting behavior at different conditions were studied through calculating the water contact angle on the basal plane of MoS2 . An indirect and a direct approach were utilized to calculate the contact angle from molecular dynamics simulations. The indirect method used in this study is the dry-surface method of Leroy and MüllerPlathe 40,41 in which WCA is evaluated through calculating solid-liquid work of adhesion. The direct calculation approach used in this study is the one we recently proposed for calculating WCA from semi-spherical droplet simulations. 42 We showed that after a mild modification, MoS2 forcefield parameters recently developed by Sresht et al. 49 could reproduce the WCA reported by Kozbial et al. for freshly exfoliated MoS2 . 1 We studied the effect of cut-off distance on the solid-liquid interactions energies. We showed that the minimum cut-off distance of 50 Å was needed fo solid-liquid interactions to 26


Page 27 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


be independent of cut-off distance. The forcefield parameters for MoS2 -water iteration were modified according to this cut-off distance. Using the modified forcefield parameters, we studied the effect of number of layers on the wetting behavior of MoS2 . Similar to experimental observation of Gaur et al., 2 molecular dynamics simulation showed that decreasing the number of layers down to the monolayer increased the WCA on the basal plane. The WCA difference between mono- and bilayer is considerably bigger than the one between bi- and trilayer. Increasing the number of layers more than three did not have a significant effect on WCA. Changing the number of layers affected wetting properties of MoS2 through altering the van der Waals iterations between MoS2 and water. We also showed that adsorption of airborne hydrocarbons could transform the intrinsically hydrophilic basal surface (WCA ∼ 70 ◦ ) to a hydrophobic one. Methane adsorption at pressures as low as 1 bar caused a 15◦ increase in WCA. This change in water contact angle was shown to be directly related to the change in van der Waals interactions between water and MoS2 . Since both energy and entropy of adhesion are directly related to the solid-liquid interactions, changing the van der Waals interactions between MoS2 and water affects both components. It showed that contribution of entropy change in changing wetting properties of MoS2 is significant and can not be neglected.

Acknowledgement The authors gratefully acknowledge the support of The Canadian Centre for Clean Coal/Carbon and Mineral Processing Technologies (C5 MPT). Computational resources are provided by WestGrid. The authors also gratefully acknowledge Dr. Frédéric Leroy’s help and constructive discussions, leading to the accurate implementation of the dry-surface method in this study.

27



Supporting Information Available The following file is available free of charge. • supplementary.pdf: More details about simulation setups and the GCMC simulations of methane adsorption on the basal plane of MoS2 are provided in this document. This material is available free of charge via the Internet at http://pubs.acs.org/.

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Effects of Thickness and Adsorption of Airborne Hydrocarbons on

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