Anisotropic Wetting Characteristics of Water Droplets on Phosphorene

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Anisotropic Wetting Characteristics of Water Droplet on Phosphorene: Roles of Layer and Defect Engineering Shuai Chen, Yuan Cheng, Gang Zhang, Qing-Xiang Pei, and Yong-Wei Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10788 • Publication Date (Web): 06 Feb 2018 Downloaded from http://pubs.acs.org on February 15, 2018

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The Journal of Physical Chemistry

Anisotropic Wetting Characteristics of Water Droplet on Phosphorene: Roles of Layer and Defect Engineering Shuai Chen,1 Yuan Cheng,1 Gang Zhang,1,* Qingxiang Pei,1 Yong-Wei Zhang1 1

Institute of High Performance Computing, A*STAR, 138632 Singapore

ABSTRACT We study the wetting behavior of water droplet on pristine and defective phosphorene using molecular dynamics simulations. It is found that unlike prototypical two-dimensional materials such as graphene and MoS2, phosphorene exhibits anisotropic contact angle along armchair and zigzag directions. This anisotropy is tunable with increasing the number of layers and vacancy concentration. More specifically, the water contact angles decrease with increasing the number of layers, indicating the importance of water-substrate interaction. The contact angles along both armchair and zigzag directions increase with increasing vacancy concentration, and the anisotropy disappears when the defect concentration is high. For an in-plane pristine-defective phosphorene heterostructure, when the junction is zigzag-oriented, a spontaneous diffusion of water droplet from defective region to pristine region occurs; when the junction is armchairoriented, however, the spontaneous motion is suppressed. The energetic factor plays a role for the difference in the motion of water droplet along zigzag and armchair directions. Our work highlights the unique and fascinating directional wetting behavior of water droplet on phosphorene.

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1. INTRODUCTION Two-dimensional (2D) materials have attracted increasing attention due to their unique electrical1, optical2, thermal3 and mechanical properties4. Compared with many other 2D materials,

single-layer

black

phosphorus

or

phosphorene

possesses

several

unique

characteristics,5-7 such as a large direct band gap and high carrier mobility. In fact, few-layer phosphorene-based field-effect transistor has been recently demonstrated.8,

9

Besides,

phosphorene is also promising for biological applications due to its little disruption to protein10 and excellent performance in killing cancer cells11, 12. Moreover, liquid exfoliation has been shown to be able to produce high-quality phosphorene,13, 14 which paves the way for applications of phosphorene in biological engineering. Obviously, any biological applications of phosphorene require in-depth understanding of its interaction with biomolecules and fluids. In recent years, the wetting and diffusion behaviors of water in/on nanomaterials, such as graphene15-19, carbon nanotube20-22, boron-nitride23-25, WS2 and MoS226-28 have been extensively studied. We note, however, that the research on the interfacial behavior of water on phosphorene has just started.29,

30

Although the wetting and

diffusion behaviors of water droplet (with a fixed number of water molecules) on pristine phosphorene surfaces have been studied,29, 30 the macroscopic contact angle of water droplet on phosphorene remains unexplored. To obtain this intrinsic property, water droplet with different sizes should be used.31 In addition, since phosphorene possesses a strong structural anisotropy3234

due to its puckered honeycomb lattice, thus for practical applications, it is important to

understand its directional dependence of macroscopic contact angle. Furthermore, many properties, such as electronic band gap and atomic/molecular adsorption, are layer-dependent. However, the layer-dependent wetting behavior of water droplet on multilayer phosphorene 2


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remains unknown. Last but not least, it is known that atomic defects, such as vacancies, can be formed easily in phosphorene.35-37 However, their effect on the wettability of water droplet remains unknown. Clearly, answers to these questions are of great importance for water-related and/or biological applications of phosphorene. In this work, we first investigate the wetting behavior of a water droplet on phosphorene using molecular dynamics simulations, with emphasis on the effect of water droplet size and the number of phosphorene layers. Next, we examine the wettability of defective phosphorene, focusing on the effects of the defect distribution and concentration on the wettability. Finally, we study the diffusion behavior of water droplet on an in-plane heterostructure consisting of two domains with one being defective while the other being perfect. The findings revealed here may be useful for controlling the wettability of phosphorene, which may facilitate the applications of phosphorene in biological systems. 2. METHODS In the simulations, water molecules in the initial state were arranged regularly in the threedimensional cube (cf. Figure 1a). The in-plane dimensions of a single layer were fixed to be 60×60 Å2, while the out-of-plane dimension was varied, leading to different droplet sizes. The distance between nearest neighbor water molecules was 3 Å.38 To study the wetting behavior of water droplet with different sizes, models with different numbers of layers were constructed, in which the numbers of water molecules were 2000, 3000, 4000, 5000 and 6000, respectively. Initially, the bottom layer of water molecules was set to be 3 Å away from the phosphorene surface. For few-layer phosphorene, the number of layers was varied from 1 to 6, and the concentration of mono-vacancy was varied from 0 to 3.5%. The lateral dimensions of the singlelayer phosphorene were 198×198 Å2. The distance between neighboring phosphorene layers was 3



set to be 5.5 Å. All the simulations were performed with periodic boundary conditions in the lateral directions. The effect of the periodic image interactions on the behavior of the droplet was negligible due to the large size of the phosphorene model. All the simulations were performed by using LAMMPS package39. The simulations were performed for 3 ns with an integration time step of 1 fs. The O–H distance and the H–O–H angle were fixed by the SHAKE algorithm40. The phosphorous atoms were fixed during the simulations. The water droplet was kept at the temperature of 300 K, which was realized by the Nose-Hoover thermostat with the temperature damping parameter being 100 fs. The samples of the trajectories were stored every 1 ps. The water droplet was relaxed for 1 ns initially, and after that, the water droplet would reach an equilibrium configuration (cf. Figure 1b). The sample in the last 2 ns was used to measure the water contact angle (WCA). The positions of all atoms at each time step were acquired from the simulation output files. Three steps were taken to calculate the contact angle from the positions (cf. Figure 1c).31 First, a time-averaged water number density was calculated through a dense spatial mesh with a grid spacing of 0.5 Å (i.e. L=0.5 Å) at a certain azimuthal angle,30 which was determined by the output direction (armchair or zigzag). Then, the liquid−vapor interface was defined as the contour line with a density level at half of the bulk value, ρ0/2. Lastly, the cross-sectional profile of the liquid−vapor interface was fitted into an arc, where WCA was calculated as the contingence angle at the basal plane.

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Figure 1. (a) Initial structure and (b) equilibrium configuration of the simulation model. Red, white and tan spheres represent the oxygen, hydrogen and phosphorus atoms, respectively. (c) Distribution of water density in units of its bulk value, ρ0. Grid spacing L=0.5 Å. The liquid−vapor interface is defined as the contour line at ρ0/2 (the curved red line).

The Lennard−Jones (L−J) potential (cf. Equation 1) was used to describe the homo-interatomic interaction.

U

σ

ij

= 4ε ij[(

rij

ij 12

σ

) −(

ij 6

rij

) ], rij < rc

(1)

where, i and j are either oxygen (O), hydrogen (H) or phosphorus (P) atoms. σij is the distance at which the interatomic potential is zero and εij is the depth of the potential well. rij is the distance between two atoms. rc is the cutoff, which is equal to 20 Å. The hetero-interatomic potentials were obtained through the Lorentz－Berthelot mixing rules41:

σ

ij

ε

= (σ ii + σ jj ) / 2 ij

= (ε iiε jj )1/ 2

(2) (3)

The water model in this study was the rigid extended simple point charge potential (SPC/E)42, in which the charges on the oxygen and hydrogen atoms were -0.8476e and +0.4238e. The O−H distance was 1 Å and the H−O−H angle was 109.47°. The L−J potential parameters of oxygen, 5



hydrogen and phosphorus atoms were taken as εOO=0.1553 kcal/mol, σOO=3.166 Å, εHH=0, σHH=0, and εPP=0.4000 kcal/mol, σPP=3.33 Å.30 The electrostatic interaction was modeled using Coulomb’s law. The particle−particle particle-mesh (PPPM) algorithm43 with an accuracy of 10-4 was applied to minimize the error in the long-range interaction terms. 3. RESULTS AND DISCUSSION 3.1 Wetting behavior of water droplets with different sizes on pristine phosphorene The contact angles of water droplets with different sizes on a single-layer phosphorene surface along armchair and zigzag directions are shown in Figure 2a. The left-top and right-bottom insets in Figure 2a present the equilibrium configurations of the ensemble from the side view, where the number of water molecules is 2000 and 6000, respectively. The time-dependent process is shown in Supporting Information (movie-1, movie-2). The results indicate that the WCA along armchair direction decreases from 67.8° to 64.2° with increasing the number of water molecules from 2000 to 6000. Similarly, the WCA along zigzag direction decreases from 67.0° to 63.2°. A comparison between the WCAs of the same-sized water droplet on phosphorene along different directions shows that the contact angle in armchair direction is larger than that in zigzag direction. This anisotropic wettability could be explained by the anisotropic structure of phosphorene. It is known that phosphorene has a puckered structure, which exhibits a strong structural anisotropy. As a result, there is a larger pinning effect on water along armchair direction than zigzag direction. The larger pinning effect along armchair direction results in a larger restriction for wetting, causing it to be less hydrophilic. This structure-property based mechanism is evidenced by the fact that the droplet from the top view takes an ellipsoidal shape (cf. right-top inset in Figure 2a).

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The relationship between microscopic contact angle θ and macroscopic contact angle θ∞ is given by the modified Young’s equation,31 which relates the surface tensions of relevant phases (subscripts S, L and V for solid, liquid and vapor phase, respectively) to the line tension τ, the contact angle θ and the droplet base radius R:

γ SV = γ SL + γ LV cos θ + τ / R

(4)

For a macroscopic droplet, the value of τ/R approaches to zero. In this case, the macroscopic contact angle θ∞ is defined as:

cos θ ∞ = (γ SV − γ SL ) / γ LV

(5)

Combing Equation 4 with Equation 5, the relation between microscopic contact angle θ and macroscopic contact angle θ∞ is

cos θ ∞ = cos θ + τ / (γ LV R )

(6)

Therefore, cosine of the microscopic contact angle θ is a linear function of the droplet base curvature 1/R. As shown in Figure 2b, an obvious linear dependence of cosθ on 1/R is observed for both armchair and zigzag directions. Based on these two functions, the macroscopic contact angles in armchair and zigzag directions are predicted to be θarm=58.1°, θzig=56.7°, respectively.

7



Figure 2. (a) Contact angles of different-sized water droplets on the phosphorene surface along armchair and zigzag directions. The top and side views of the ensemble are also shown here. (b) Cosine of the contact angle θ as a function of the droplet base curvature 1/R.

3.2 Wetting behavior of water droplet on pristine phosphorene with different number of layers Variation of WCA with the number of phosphorene layers is shown in Figure 3a, in which the number of water molecules is 4000. It is seen that the WCA along armchair direction decreases from 65.5° to 64.1° with increasing the number of phosphorene layers from 1 to 3, and then converges to the value of 64.1° as the number of layers exceeds 3. Similarly, the WCA along zigzag direction decreases from 64.4° to 62.9°. Regardless of the number of layers, the WCA in armchair direction is always larger than that in zigzag direction, indicating that multilayer phosphorene also exhibits an anisotropic wettability. The time-averaged interaction potential energy between water molecules and phosphorene was calculated to analyze the mechanism of 8


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the variation trend of WCA (cf. Figure 3b). It is seen that the averaged interaction potential energy increases quickly when the number of phosphorene layer increases from one to three, and then converges to a constant when the number of layers is larger than three. These trends can also be explained based on structure-property relationship. The distance between two adjacent layers in few-layer phosphorene is about 5.5 Å. Therefore, the distance between the fourth layer and the water molecules is larger than 16.5 Å. The interaction between phosphorene and water molecule is described by Lennard-Jones potential, which decreases sharply when the atomic distance is larger than 10 Å. Therefore, 10 Å, 12 Å, or 15 Å are often used as the cutoff distance for Lennard-Jones potential in simulations.30,

27, 29

As a result, the interaction of the water

molecules with the fourth layer is so weak that it is virtually negligible when compared with that with the first three layers. That is the reason why the interaction potential energy reaches a constant value when the number of layer is greater than three, and the water contact angle decreases with increasing the number of phosphorene layers from 1 to 3, and then converges to a constant value when the number of layers exceeds 3.

9



Figure 3. (a) The variation of WCAs with the number of phosphorene layers along armchair and zigzag directions. (b) The interaction potential energy between water molecules and phosphorene with different number of layers.

3.3 Wetting behavior of water droplet on defective phosphorene surfaces with different defect distributions and concentrations Since phosphorene has a puckered structure with two sub-layers, there are two types of single vacancy based on its location (cf. Figure 4a): One is at the top sub-layer, which is near the droplet, and the other is at the bottom sub-layer, which is slightly away from the droplet. Figure 4b shows the WCA as a function of concentration of single vacancy in both the top sub-layer and bottom sub-layer. Two representative concentrations of single vacancy located at the top sublayer are shown in Figure 4c (0.1% vacancy) and Figure 4d (3.5% vacancy), respectively, in which the distance between neighboring vacancies along armchair and zigzag directions were identical. It is interesting to see that the WCAs along both armchair and zigzag directions 10


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increase with increasing defect concentration. The difference between WCA along these two directions decreases continuously with increasing the defect concentration and disappears when the vacancy concentration reaches 3.5%. The dark green line (marked by triangular symbols) and light green line (marked by circular symbols) represent the WCAs along armchair and zigzag directions, respectively when the single vacancies are located at the bottom sub-layer. A similar trend is also observed for the vacancies located at the top sub-layer (see the dark and light blue lines). A comparison between (dark and light) blue and (dark and light) green curves suggests that the vacancies in the top sub-layer have more effect on WCA than the vacancies in the bottom sub-layer. The underlying mechanism for the increase in the WCA with the increase in defect concentration could also be explained based on structure-property relationship. The atomic density of the sheet decreases with increasing the defect concentration, which results in a decrease in the interaction between the sheet and water, especially for the sheet with vacancies located in the top sub-layer. Therefore, the surface hydrophobicity increases with the decrease in the interaction. In addition, the different contact angles along zigzag and armchair directions arise from the difference in the pinning effect of the puckered structure along the armchair and zigzag directions. The difference in the pinning effect is gradually suppressed with increasing the defect concentration, which diminishes the difference in the contact angle between zigzag and armchair directions.

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Figure 4. (a) Locations of single vacancy at the top or bottom sub-layers of phosphorene. Top and bottom sub-layers are colored by dark and light tan, respectively. (b) Variation of WCA with different defect concentrations along armchair or zigzag direction when single vacancies are located at top or bottom sublayers. Phosphorene configurations for the concentrations of single vacancy of (c) 0.1% and (d) 3.5% at the top sub-layer.

3.4 Diffusion behavior of a water droplet on pristine and defective phosphorene surfaces It has been reported that the gradient in surface wettability is able to drive water droplet to move from hydrophobic region to hydrophilic region on a fluorinated silane surface.44 The WCAs in Figure 4b indicate that phosphorene surfaces with different concentrations of vacancy exhibit different levels of wettability. It is expected that water droplet may diffuse on an in-plane heterostructure consisting of two domains, with one being defective while the other being perfect. To explore this phenomenon, we constructed two in-plane heterostructures, and denoted them as zigzag-oriented junction (cf. Figure 5a) and armchair-oriented junction (cf. Figure 5b), 12


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respectively. Figure 5a shows snapshots of the diffusion process of the water droplet on the zigzag-oriented junction. It is seen that the difference in wettability between the domains was able to drive the water droplet to move from defective region to perfect region (see movie-3 in Supporting Information). Figure 5b shows snapshots of the diffusion process of the water droplet on the armchair-oriented junction. It is seen that the difference in wettability between the two domains was unable to drive the droplet to diffuse from defective region to perfect region (movie-4 in Supporting Information). The displacements of the water droplet at every 0.01 ns in the diffusion process along zigzag and armchair directions were plotted in Figure 5c. At every 0.01 ns, the coordinates of all water molecules were recorded and the mass center of the water molecules was used to represent the position of the droplet, i.e. the average value of the coordinates. The displacement curve along zigzag direction indicates that the water droplet moved from defective region to pristine region; while the displacement curve along armchair direction demonstrates that the water droplet fluctuated around its initial position and could not move along the wettability gradient. To understand the underlying mechanism that results in such a different behavior along zigzag and armchair directions, we performed further analysis on the potential energy profiles of the water droplet in the diffusion processes. In the analysis, the average potential energies at every 0.05 ns were calculated and shown in Figure 5d. The energies corresponding to the snapshots in Figure 5a and 5b were also marked on the curves. It is seen that the potential energies of the water droplet at the initial A0 and Z0 position were the same, which were -856×103 kJ/mol. Because phosphorene possesses a puckered structure, it exhibits a larger pinning effect on water droplet along armchair direction than along zigzag direction. The larger pinning effect along armchair direction results in a larger restriction not only for wetting, but also for diffusion. Therefore, the 13



energy barrier for the diffusion along armchair direction is larger than that along zigzag direction, which can also be verified from the potential energy curves in Figure 5d (compare the curve from A2 to A3 with that from Z2 to Z3). These factors lead to the difference in the motion of water droplet along zigzag and armchair directions.

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Figure 5. The diffusion process of a water droplet on the junction composed of pristine and defective phosphorene domains along (a) zigzag and (b) armchair directions. (c) Displacements and (d) potential energies of the water droplet in the diffusion processes along zigzag and armchair directions.

4. CONCLUSION The present study provided a molecular-level understanding of the wetting and diffusion behavior of a water droplet on pristine and defective phosphorene sheets. Our study showed that phosphorene exhibited anisotropic wettability along armchair and zigzag directions, and the WCA decreased with increasing the droplet size in both directions. In addition, the macroscopic WCAs of water droplet on phosphorene along zigzag and armchair directions were predicted. The results for few-layer phosphorene indicated that the WCA decreased with increasing the number of layers from 1 to 3 and converged to a constant value when the number of layers was larger than 3. The results of defective phosphorene demonstrate that the WCA along both armchair and zigzag directions increased with increasing defect concentration. The difference between these two directions decreased with increasing defect concentration. It was also found that the effect of single vacancies located at the top sub-layer on WCA was larger than those located at the bottom sub-layer. Finally, the wettability difference along zigzag direction was able to drive the droplet to diffuse from defective region to pristine region. However, the wettability difference along armchair direction was unable to drive the droplet to diffuse due to the presence of a larger energy barrier. ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. 15



Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported in part by a grant from the Science and Engineering Research Council (152-70-00017). The authors gratefully acknowledge the financial support from the Agency for Science, Technology and Research (A*STAR), Singapore and the use of computing resources at the A*STAR Computational Resource Centre, Singapore. REFERENCES (1) Liu, Y.; Weiss, N. O.; Duan, X.; Cheng, H. C.; Huang, Y.; Duan, X. Van der Waals Heterostructures and Devices. Nat. Rev. Mater. 2016, 1, 16042. (2) Duan, X.; Wang, C.; Pan, A.; Yu, R.; Duan, X. Two-Dimensional Transition Metal Dichalcogenides as Atomically Thin Semiconductors: Opportunities and Challenges. Chem. Soc. Rev. 2015, 44, 8859-8876. (3) Zhang, G.; Zhang, Y. W. Thermal Properties of Two-Dimensional Materials. Chin. Phys. B 2017, 26, 034401. (4) de Wijn, A. S. Nanoscience: Flexible Graphene Strengthens Friction. Nature 2016, 539, 502-503. (5) Li, L.; Yu, Y.; Ye, G. J.; Ge, Q.; Ou, X.; Wu, H.; Feng, D.; Chen, X. H.; Zhang, Y. Black Phosphorus Field-Effect Transistors. Nat. Nanotechnol. 2014, 9, 372-377. (6) Liu, H.; Neal, A. T.; Zhu, Z.; Luo, Z.; Xu, X.; Tománek, D.; Peide, D. Y. Phosphorene: an Unexplored 2D Semiconductor With a High Hole Mobility. ACS Nano 2014, 8, 4033-4041.

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Anisotropic Wetting Characteristics of Water Droplets on Phosphorene

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