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Wettability of Graphene-coated Surface: Free Energy Investigations using Molecular Dynamics Simulation Shih-Wei Hung, Pai-Yi Hsiao, Chien-Pin Chen, and Ching-Chang Chieng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp511036e • Publication Date (Web): 25 Mar 2015 Downloaded from http://pubs.acs.org on April 7, 2015

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The Journal of Physical Chemistry

Wettability of Graphene-coated Surface: Free Energy Investigations using Molecular Dynamics Simulation Shih-Wei Hung1,†, Pai-Yi Hsiao1, Chien-Pin Chen2, Ching-Chang Chieng1,3,* 1

Department of Engineering and System Science, National Tsing Hua University, Hsinchu, Taiwan

2

The University of Michigan – Shanghai Jiao Tong University Joint Institute, Shanghai Jiao Tong University, Shanghai, China 3

Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Kowloon, Hong Kong

AUTHOR INFORMATION Corresponding Author *[email protected] Present Addresses †The University of Michigan – Shanghai Jiao Tong University Joint Institute, Shanghai Jiao Tong University, Shanghai, China.

ACS Paragon Plus Environment

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ABSTRACT

A clear understanding of the wettability of graphene and graphene coated surfaces is of critical importance for the practical applications of graphene. The present study provides microscopic and thermodynamic perspectives into the wettability of graphene coated surfaces by molecular dynamics simulations along with free energy calculations utilizing the umbrella sampling. The water droplet adhesion process on graphene coated surface was characterized by the change in surface area, mean force, and free energy of the droplet. The thermodynamic landscape analysis reveals that the different contributions to the free energies from different underlying substrates induce different entropic resistances from graphene, which leads to the similarity in wettability for

graphene

coated

silicon

and

hydroxylated

silicon

dioxide

substrates.

KEYWORDS Graphene, Interfacial phenomena, Contact angle, Thermodynamics.


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Introduction Graphene, a flat monolayer of carbon atoms arranged in a two-dimensional honeycomb lattice, has already attracted considerable attention owing to its exceptional electrical, mechanical, and chemical properties.1–4A clear understanding of the wettability of graphene and graphene coated surfaces is very important for the practical applications of graphene. Therefore, increasing efforts have been made to study the interaction between water and graphene/graphene coated surfaces in recent years.5–17 The different degrees of wetting transparency of graphene coating on various underlying substrates were observed by experiments, simulations, and theory.18,19 Rafiee et al.6 proposed that the graphene did not alter the wetting behavior of underlying substrate, for which the wettability was dominated by van der Waals forces. However, the wetting transparency broke down on surfaces such as glass, where the wettability was controlled by the hydrogen bonding with water. Shih et al.5,9 showed that the monolayer graphene itself was not transparent to wetting. Besides, the wettability of graphene coated surface was determined by both water– graphene and water–underlying substrate interactions; their relative magnitudes affected the change in wettability on supported graphene relative to that on the bare underlying substrate, resulting in different degrees of wetting transparency. In other words, the wetting transparency broke down for superhydrophobic and superhydrophilic underlying substrates. Raj et al.8 reported that the underlying substrates did not affect the wettability of graphene coatings due to the large interlayer spacing between the graphene and the underlying substrate. Li et al.7 demonstrated that the partial wetting transparency of graphene was observed for clean graphene without hydrocarbon contamination. Driskill et al.10 investigated the wetting transparency of graphene in water and showed the enhanced attraction between water droplet and graphene due to the presence of water under the graphene layer. In recent studies5–10, theoretical models,


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combining the Young-Dupré equation20 and continuum models, have been implemented to examine the experimental and simulation observations. The effects of number of graphene layers and interlayer spacing between graphene and substrate on the wetting transparency have been investigated by the continuum model from the van der Waals interaction.5–9 In addition to van der Waals interaction, Driskill et al.10 included the electrostatic interaction between water molecules on both sides of graphene by introducing the dipole-dipole interaction to the continuum model to study the wettability of graphene with the water layer beneath the graphene. In the present study, the application of molecular dynamics (MD) simulations along with free energy calculations utilizing the umbrella sampling method21 was proposed to provide molecular and thermodynamic insight on the interaction between water and various solid surfaces. At first, equilibrium MD simulations were carried out to calculate the static contact angle of a water droplet on the selected solid surface. The umbrella sampling method was then implemented to study the change in surface area, mean force, and free energy of droplet during the adhesion process. In order to consider the change in shape of droplet, the net free energy of adhesion,22 which described the free energy required to separate a sessile droplet from a solid surface to form a free sphere, was selected instead of the work of adhesion calculated from the Young-Dupré equation. The approach of calculations was validated by the good agreement between simulated and theoretical values. Furthermore, the thermodynamic properties were decomposed into the contributions from graphene and the underlying substrate to understand the roles of graphene and underlying substrate played in the wettability of graphene coated surface.

Computational Methods Model System and Simulation Procedure


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MD simulations were performed as a direct approach to address various water-surface interactions. In addition to graphene surface, two substrates, silicon (Si) and hydroxylated silicon dioxide (SiO2-H), with different dominant water-substrate interactions were studied. The interaction between Si substrate and water is due to van der Waals interaction only because there is no difference in localized electron density for Si substrate. For SiO2-H substrate, the localized electron density distribution due to the difference in electron affinity of silicon and oxygen and the effects of bond polarization were represented by partial charges on the atoms.23 Thus, the interaction between SiO2-H and water involves both electrostatic and van der Waals interaction. The (100) and α termination quartz (1010) surfaces24 were chosen for Si and SiO2-H substrates, respectively. The flexible, extended simple point charge (SPCE-F) model25 was chosen as the main water model in the present study because the previous study26 has shown that the calculated surface tension using pressure tensor method with the SPCE-F model provided the best agreement with the experimental value. To understand the effect of different water models on the calculated free energy of adhesion, the widely used extended simple point charge (SPCE) model27 was also applied for comparison. Recent studies7,28 asserted that previously reported hydrophobic nature of graphitic surfaces was a consequence of hydrocarbon contamination from air. The watercarbon interaction for hydrophilic graphitic surfaces were also developed based on ab initio calculation data.29 In order to compare with previous studies5,6,8,9 of wetting transparency of graphene, which showed the hydrophobic behavior of graphene, the Werder model30 for hydrophobic graphitic surfaces was chosen in the present study to capture the characteristics of the surface. The van der Waals parameters of water-Si interaction was taken from the parameters of bulk Si used in the JA model.23,31 The partial charges and the van der Waals parameters of


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water-SiO2-H interaction were obtained from the JA model.23,31 All the atoms of solid surface were set to be fixed in all the simulations. All of the simulations were performed with the GROMACS 4.6 program32 and visualized with the software PyMOL.33 The simulations were performed in a canonical ensemble with integrating time steps of 1.0 fs, and the temperature was controlled at 300 K by a Nosé−Hoover thermostat34,35 with a time constant of 0.2 ps. For the non-bonded van der Waals (vdW) interactions, a cutoff distance of 2.0 nm was used. The electrostatic interaction was treated using particle mesh Ewald summation method36 with a real-space cutoff distance of 2.0 nm. Free Energy Calculation The free energy of adhesion of water droplet on surface was investigated using the umbrella sampling method.21 The umbrella sampling method along with mean force calculation is wellestablished and widely applied in biology. The details of the free energy calculation were described in our previous study37,38 and elsewhere.39,40 In the present study, the force constant of the harmonic potential applied in the umbrella sampling was chosen to be 2000 kJ/mol·nm2 to ensure sufficient overlap over the sampling windows (windows size of 0.1 nm). For each window, a 1 ns production run was conducted for data analysis. The systematic mean forces acting on the water droplet, which are the force contributed by all other atoms in the system averaged over all the configurations, are governed by the equilibrium free energy landscape of droplet adsorption onto the surface. Therefore, the free energy profile can be calculated by integrating the mean force, 〈 〉, along the direction perpendicular to the surface (z direction). This is the reason why the free energy profile is also called a potential of mean force: 41

= − 〈 〉 ,

(1)


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where F(z0) is arbitrarily chosen as a reference state at the reference position z0, so that only the relative value of F(z) is obtained. In the present study, z0 was chosen at the position where the water droplet begins to interact with surface and F(z0) was chosen to be zero. Because the forces acting on the water droplet are additive, i.e., = ∑ , where α is the component in the system, the mean forces can be decomposed into a sum of components and allows us to identify contributions originating from different interactions. For graphene coated surface, the free energy contributions from the graphene and the underlying substrate are of interest. Thus, the total free energy was decomposed into graphene and underlying substrate contribution according to

= + = − 〈 〉 + − 〈 〉 ,

(2)

where fgra and fus are calculated by summing over the forces between the water molecules of droplet and the atoms of graphene and the underlying substrate, respectively.

Results and Discussion Static Contact Angles on Different Surfaces The calculated static contact angles of the water droplet with varying number of water molecules, i.e. 2500, 5000, and 8000, on graphene and Si surface are shown in Figure 1(a). The static contact angle of a water droplet on various surfaces was calculated by a 3 ns equilibrium MD simulation run. The density profile of a water droplet was obtained by using a cylindrical binning, and the surface normal passing through the center of mass of the droplet was used as the reference axis, as described by Werder et al.30 The contact angle was measured by a circular best fit through the contour of density=500 kg/m3 (which is the middle value of the bulk liquid and vapor densities of water). The representative snapshots and the corresponding density profiles are shown in the inset of Figure 1. The results show good agreement with the previous studies,


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i.e. the simulated contact angles on graphene surface were ~90° 13 or in the range of 95−100° 17 and the measured contact angle in experiment on Si surface was 77°.42,43 The variation of the contact angle values in Figure 1a shows that the change of droplet size on the contact angle is small, which indicates the line tension effect is small. In order to avoid the effect of line tension on the contact angle of droplets, the largest size of water droplet, 8000 water molecules, was chosen to study the effect of the addition of graphene. Figure 1b shows the contact angle of a water droplet on Si and SiO2-H with and without graphene coating. The complete spreading of water on bare SiO2-H surface is observed, which is consistent with the previous study44 that the fully hydroxylated quartz surface is completely wetting (~0°). For both surfaces, the coating of graphene increases the contact angle to 83-85°, which is approaching the contact angle of a bare graphene surface. The observation matches well with the experimental and computational results8 that the contact angle of graphene coated surface is independent of the type of underlying substrate.

(a)


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(b) Figure 1. (a) Contact angles of water droplet on graphene and Si surfaces as a function of base radius. (b) Contact angles of water droplets on Si and SiO2-H surfaces with and without a graphene coating. Corresponding snapshots and equilibrium density profiles with circular fit and tangent line (red dash line) are shown in inset. Droplet Adhesion Process To gain insight of the droplet adhesion process, the total surface area (black line) and mean force (blue line) of a water droplet adhesion onto the graphene coated silicon surface at different separation distance (∆z) is presented in Figure 2a. The total surface area of droplet, including the interfacial surfaces of liquid-vapor (ALV) and solid-liquid (ASL), was calculated by integrating the contour of density=500 kg/m3, which is the same as the choice of contact angle calculation. The separation distance, ∆z, was computed by shifting the coordinate of the center of mass of the droplet such that the free energy is a minimum at ∆z = 0. Three stages can be distinguished in Figure 2a. In the first stage, the droplet is far enough from the solid surface so that there is no interaction between water droplet and solid surface. Thus, the mean force is zero and the shape of droplet is a sphere (Figure 2f). In the second stage, the mean force is monotonically increasing until it reaches a maximum at ∆z = 1.05 nm, while the total surface area of droplet remains


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constant. The shape of droplet is elongated in vertical direction, like a bullet (Figures 2d and 2e). In the third stage, the mean force diminishes dramatically to nearly zero at ∆z = 0 nm, while the surface area increases sharply. The droplet is spreading on the surface and the shape of droplet is hemispherical (Figures 2b and 2c). To further understand the water droplet adhesion process, the total surface area was divided into the interfacial surfaces of liquid-vapor (ALV) and solid-liquid (ASL) and the mean force was decomposed into the contributions from graphene (fgra(z)) and underlying substrate (fus(z)).

(a)

(b)

(c)

(d)

(e)

(f)

Figure 2. (a) The profile of surface area (black line) and mean force (blue line) of water droplet at different separation positions; I indicates the first stage, II is the second stage, and III is the third stage. (b)-(f) The snapshots of the water droplet at different separation distances as black triangles marked in (a). (b) is the state that the sessile droplet attached on a surface completely, where the free energy is minimum. (f) is the state that no interaction between spherical droplet and surface, where the free energy is chosen to be zero in the present study.


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The changes in total surface area (∆A, black solid line), liquid-vapor interface (∆ALV, red dashed line), and solid-liquid interface (∆ASL, blue dotted-dashed line) of droplet adhesion process on bare graphene, Si/graphene, and SiO2-H/graphene are shown in Figures 3a, 3b, and 3c, respectively. The three profiles for different surfaces are in qualitative consistency by shape. The profiles of ∆ALV and ∆ASL indicate the liquid-vapor interface keeps decreasing while the solid-liquid interface remains increasing during the adhesion process. During the second stage, the rate of decrease in ∆ALV equals to the rate of increase in ∆ASL, so that ∆A keeps constant. In the third stage, ∆ASL increases dramatically but ∆ALV decreases slightly and reaches minimum at ∆z = 0 nm, which implies ALV stops the spreading of droplet (increase in ∆ASL) and determines the shape of droplet in equilibrium. The results show that ∆A is greater for the more hydrophilic surface due to the large increase in ∆ASL. The total free mean force profile for bare graphene surface is shown in Figure 4a. The Figures 4b and 4c display the total mean force (f(z), black solid line), graphene contribution (fgra(z), red dashed line) and underlying substrate contribution (fus(z), blue dotted-dashed line) during the droplet adhesion process for graphene coated surfaces. As Figures 4b and 4c show, the graphene provides repulsive force while the underlying substrate provides attractive force during the adhesion process. In the second stage, the repulsive force from graphene is much smaller than the attractive force from the underlying substrate. Thus, the total force increases monotonically in the second stage. However, the repulsive force from the graphene increases shapely in the third stage and equals to the attractive force from underlying substrate at ∆z = 0 nm, which reflects that the graphene resists the further spreading of droplet on solid surface. Although Si underlying substrate provides greater attractive force than the SiO2-H underlying substrate, the graphene generates greater repulsive force for Si underlying substrate


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as well. The similarity of total mean force for two graphene coated surfaces arises from a balance of opposing contributions from underlying substrate and graphene.

(a)

(b)

(c)


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Figure 3. The change in the total surface area (black solid line), liquid-vapor interface (red dashed line), and solid-liquid interface (blue dotted-dashed line) of water droplet adhesion on different surfaces. (a) Bare graphene surface. (b) Si/Graphene surface. (c) SiO2-H/ Graphene surface.

(a)

(b)


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(c) Figure 4. The total mean force (black solid line) and the graphene (red dashed line) and underlying substrate (blue dotted-dashed line) contributions acting on the water droplet during the adhesion process for different surfaces. (a) Bare graphene surface. (b) Si/Graphene surface. (c) SiO2-H/Graphene surface.

(a)


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(b)

(c) Figure 5. (a) The potential energy and (b) the mean force from graphene acting on the droplet for bare graphene surface (black), Si/Graphene surface (red), and SiO2-H/Graphene (blue) as a function of distance between droplet and graphene. (c) The potential energy function (black) and force (blue) as a function of distance between carbon (graphene) and oxygen (water).

The force between droplet and graphene is attractive for bare graphene surface. On the other hand, the forces are mainly repulsive between droplet and graphene for graphene coated surfaces. To explain this observation, the profiles of energy and mean force from graphene as a function of distance between droplet and graphene are shown in Figures 5a and 5b. For the bare graphene surface, the interaction between droplet and graphene reaches the equilibrium state. For the


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graphene coated surfaces, the interactions between droplet and graphene no long stay in the equilibrium state due to the additional interaction from underlying substrate, resulting in the slight attraction in energy and great repulsion in force. Because of the addition interaction from underlying substrate, the height of the droplet on the graphene coated surface is lower than that on the bare graphene surface. Thus, the graphene atoms exert repulsive forces, along the positive z direction, on the droplet to maintain it at its equilibrium position. Besides, the difference between

energy and force can be understood by the potential function between carbon of graphene and oxygen of water. As shown in Figure 5c, when the C-O distance is smaller than the distance at which the potential reaches its minimum, i.e. 0.358 nm, the potential energy is still attractive while the force is repulsive and increases dramatically with the decreasing C-O distance. Previous studies45,46 have indicated that the preferred orientation of interfacial water molecules changes with different interactions from surface in order to optimize hydrogen bonding. The difference in the preferred orientation of interfacial water molecules results in the different force between droplet and graphene for bare graphene and graphene coated surfaces. The solid black lines in Figure 6 show the total free energy profile (F(z)) of a water droplet attached to the various surface. The components of total free energy from the graphene (Fgra(z)) and the underlying substrate (Fus(z)) are depicted in Figures 6b and 6c as red dashed line and blue dashed-dotted lines, respectively. The negative values of F(z) with decreasing separation distance indicates that droplet adhesion is a spontaneous process when the droplet is sufficiently close to the solid surface. The profile of Fus(z) implies that the underlying substrate promotes the adhesion between the water droplet and surface during the process. On the other hand, the graphene prevents the droplet from attaching to the surface. The contribution from the graphene is small in the second stage. Thus, the major contribution to the total free energy is from the


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underlying substrate. The dramatically increasing positive values of Fgra(z) in the third stage counters the negative values of Fus(z), and F(z) reaches the minimum at ∆z = 0 nm. Without the underlying substrate (Figure 6a), the adhesion between water droplet and graphene is favorable. With the addition of underlying substrate, the interaction between water droplet and graphene becomes unfavorable (Figures 6a and 6b). As the underlying substrate provides larger attraction, the graphene provides larger repulsion as well, which in turns leads to the similarity of free energy of the two graphene coated surfaces.

(a)

(b)


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(c) Figure 6. The profiles of free energy of adhesion of water droplet at different separation positions (black solid line) for different surfaces. The contributions from the graphene and the underlying substrate are shown in a red dashed line and a blue dotted-dashed line, respectively. (a) Bare graphene surface. (b) Si/Graphene surface. (c) SiO2-H/Graphene surface. Thermodynamic Properties of Droplet Adhesion The calculated net free energy of adhesion was then examined with the theoretical formula. Note that the case of bare SiO2-H surface was not included, because the free energy cannot be estimated using the umbrella sampling method for the completely wetting situation. In order to consider the change in shape of droplet adhesion process, the net free energy of adhesion22, which was derived to describe the free energy required to separate a sessile droplet attached on a solid surface to a free sphere, was selected for comparison with the simulation results. Assuming the volume of drop is constant during the process, the net free energy difference, ∆F, can be expressed as ∆ = − = ! " 0

!,

− $ ! "

!,

− %! − % "%

, &

=

,

1 ( ! 4*+ , - − . ! 2* 2345 1 − cos9 − %! − % *+ , :, (3)


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where FF is the free energy of the free spherical droplet, FA is the free energy of the sessile droplet attached on a surface. γLV, γSV, and γSL are the liquid-vapor, solid-vapor, and solid-liquid surface tensions, respectively. RF is the radius of the spherical drop, RA is the base radius of the sessile drop, and θ is the contact angle between the sessile drop and surface. Because the volume of drop is constant during the process, we have + = 4;/=

(,>=?@25A?@2B 52345

C/B

+ ,

(4)

The relation between interfacial tensions and contact angle can be described by the Young’s equation %! − % = ! cos9,

(5)

Substituting eqs 4 and 5 into eq. 3 and defining D = E2/1 + cos9F − cos9,

(6)

We have ∆ = ! *+ , $2D/sin9,/= − D&,

(7)

According to eq 7, ∆F is related to *+ , $2D/sin9,/= − D& in a linear manner, and the slope is γLV. The relationships between ∆F and *+ , $2D/sin9,/= − D& using SPCE-F and SPCE models are shown in Figure 7. The slopes of SPCE-F and SPCE models are 48.05±0.45 (R2=0.99) and 42.42±0.20 (R2=0.99) kJ/mol/nm2, respectively. The intercepts are chosen to be zero because there is no interaction between water and surface when the water droplet is not in contact with the surface. The results match well with the experimental value 43.25 kJ/mol/nm2 (71.84 mN/m) of water surface tension at 300K.47 Although the values of calculated surface tension differ from that obtained by calculating the pressure tensor, i.e. 38.048/36.626 kJ/mol/nm2 for SPCE model and 42.2926 kJ/mol/nm2 for SPCE-F model, the results using different approaches are in qualitative agreement that the value using SPCE-F model is greater than that


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using SPCE model. As can be seen in Figure 7, the difference between the calculated free energy using different water models is slight. Thus, the key factor determining the surface tension in the present approach is the droplet geometry, i.e. base radius and contact angle. Because the SPCE model provides better agreement with experiment in terms of bulk liquid density than SPCE-F model,26,48 the value of calculated surface tension in the present approach with SPCE model matches better with the experimental value. The good agreement between results from theoretical formula and simulation indicates the validation of calculated free energy.

Figure 7. The calculated net free energies of adhesion of water droplet with different water models on different surfaces as a function of πR K , $2a/sinθ,/= − a&. The open symbols indicate the use of SPCE model, and the solid symbols indicate the use of SPCE-F model. Figure 8 shows the thermodynamic properties of water droplet adhesion on graphene coated Si and SiO2-H surfaces. The contributions from the graphene (colored in red) and the underlying substrate (colored in blue) are also displayed to indicate distinctly the effect of the graphene and the underlying substrate on the wettability of graphene coated surfaces. Because the values of contact angle of both graphene coated surfaces are similar (Figure 1b), the values of total free energy of adhesion are similar as well. The contribution from graphene is unfavorable (positive


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values), whereas the contribution from the underlying substrate is favorable (negative values). This result reveals that the graphene forms a resistance between water and he underlying substrate. The different underlying substrates provide different free energies to promote water droplet adhesion onto the surface. However, the more favorable contribution from the underlying substrate also countered with more unfavorable contribution the graphene produces, as observed in Figure 8a. The end result is that the free energy of adhesion for graphene coated surface is similar because the free energy from the graphene cancels out the free energy of the underlying substrate. Recent experimental results8 supported the current observations. The free energy contribution is further split into energetic and entropic components (Figure 8b). The change in internal energy, ∆U, is the sum of the changes of the potential energy between water droplet and other components in the system. The change in entropy, ∆S, can be obtained according to the thermodynamic relation, ∆F=∆U-T∆S, where T is the temperature. The energetic contributions are favorable while the entropic contributions are unfavorable for the droplet adhesion. This observation is consistent with previous studies that the entropy loss of system when a liquid-vapor interface turns to liquid-solid interface.49,50 It also shows that the surface with more energy gain also lose more entropy, which coincides with the thermodynamic investigation of the substrate strength dependence of the wettability.49 The thermodynamic properties is decomposed into the contributions from the graphene (Figure 8c) and the underlying substrate (Figure 8d)). Because of the stability of the shape of droplet, the mean force resulting from water-water interaction acting on the droplet is zero during the adsorption process. The value of ∆Fww, where ww denotes to the water-water interaction, is zero, which implies the energy-entropy compensation of water-water interaction, i.e. ∆Uww=T∆Sww. Thus, the contribution from water-water interaction was not considered in the present study. The


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decomposition of entropy is not as straightforward as that of energy because it contains cross correlation terms that couple two or more of the components.51,52 The examination of previous study showed that the contributions to the entropy which arise from single components are significant larger that the cross correlation term.52 Therefore, the entropy, analogous to the energy, can be decomposed into contributions arising from individual components, as stated in the review paper of applications of entropy calculation.53 It can be observed from Figure 8c that graphene provides the same favorable energetic contributions for both Si and Si2O-H surfaces. The difference of the unfavorable contribution to the free energy for different underlying substrates is due to the entropy. On the other hand, for the underlying substrate, Figure 8d shows that the values of energetic contribution are small, indicating the graphene shields the majority of the energetic contribution from the underlying substrate, which agrees with the previous observations.5,8 Thus, the favorable contribution to the free energy from underlying substrate is driven by the entropy. The bare SiO2-H substrate shows strong hydrophilic (0˚ in Figure 1b) because the major interaction is the hydrogen bonding with water. The smaller value of ∆Uus for SiO2-H/graphene than that for Si/graphene implies that the presence of graphene at the water/SiO2-H interface disrupts the formation of hydrogen bonding between water and SiO2-H surface, as Rafiee et al.6 proposed. It is generally accepted that the entropy is related to the arrangement of water molecules.54 The arrangement of water molecules is determined by the interplay of graphene and underlying substrate. The different contributions to free energy from different underlying substrates (∆Fus in Figure 8d) affect the arrangement of water molecules, resulting in different entropy losses from graphene (-T∆Sgra in Figure 8c). There is no contact between water droplet and underlying substrate, so the entropy gain for the underlying substrate (-T∆Sus in Figure 8c) does not violate


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the assumption that establishing solid-liquid contacts from a liquid-vapor interface always leads to entropy loss. Our results indicate the shape of water droplet resulting from both watergraphene and water-underlying substrate interactions is entropically favorable for underlying substrate.

(a)

(b)

(c)


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(d) Figure 8. (a) Changes in free energy (black) of total system and contributions from graphene (red) and underlying substrate (blue). (b) Changes in free energy (black), internal energy (green), and entropy (cyan) of total system. (c) Changes in free energy (red), internal energy (green), and entropy (cyan) origin from graphene. (d) Changes in free energy (blue), internal energy (green), and entropy (cyan) origin from underlying substrate.

Conclusions To develop graphene assisted surface engineering for practical applications, such as heat transfer and microfluidics device, the understanding of the interaction between water and graphene/graphene coated surfaces is of critical importance. In order to elucidate the contentious concept of the effect of the underlying substrate on the wettability of graphene coated surface, MD simulations combined with free energy calculations were performed to provide microscopic and thermodynamic perspectives into the wettability of graphene coated surfaces. In summary, the values of calculated contact angle and free energy landscape show the similarity of wettability for graphene coated Si and SiO2-H substrates. Three stages of droplet adhesion process can be distinguished from the surface area and mean force profiles. By dividing surface area into liquid-vapor and solid-liquid interfaces, it can be shown that the liquid-vapor interface


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prevents the droplet from expanding the solid-liquid interface. The component analysis of mean force and free energy reveals that the graphene resists further spreading of water droplet on the graphene coated surface, so that the values of contact angle for different graphene coated surfaces are approaching that of a bare graphene surface. The thermodynamic analysis shows that the different interactions from underlying substrate induce different entropic resistance from graphene. In addition, the energetic contribution from underlying substrate is screened by the addition of graphene. This fundamental study of wettability using free energy calculation provides a thorough understanding of the interaction between water and graphene coated surface.


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AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The authors thank the National Center for High-Performance Computing, Taiwan for computing resources. The authors appreciate the financial support from National Science Council, Taiwan under Grant No. NSC-102-3113-P-007-014.


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Wettability of Graphene-Coated Surface - American Chemical Society

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