Abstract
Borophene, a novel two-dimensional material unveiled in 1998, has garnered significant interest among researchers due to its distinct mechanical and electrical characteristics. Efforts to experimentally synthesize borophene continue to captivate researchers’ interest in recent years. Given the current lack of experimental studies on the interaction between water and the borophene surface, molecular dynamics simulation offers a valuable approach for predicting the substance’s reactivity with water. Additionally, such simulations can assess the hydrophilicity and hydrophobicity of borophene, providing valuable insights into its properties. In our current research, we utilized reactive molecular dynamics simulation to investigate the wetting behavior of borophene. Our findings reveal that the borophene surface exhibits hydrophobic characteristics, demonstrating anisotropic wettability. Specifically, the water contact angle was calculated to be 149.11° along the zigzag direction and 148.4° along the armchair direction. The contour map of the interaction energy between a water molecule and the borophene surface revealed a notable energy barrier in the zigzag direction. This barrier contributes to the asymmetric spreading of the water droplet on the surface. Density profiles and radial pair distribution function (RDF) diagrams of the water droplet on the borophene surface further corroborated the hydrophobic nature of borophene by indicating a significant distance between the water droplet and the surface. Moreover, analysis of the number of hydrogen bonds demonstrated that borophene efficiently utilizes nearly all its capacity to form hydrogen bonds. Additionally, we compared the wettability of borophene with that of other two-dimensional materials, such as various graphene allotropes and phosphorene, which have been subjects of recent investigation.
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Introduction
Borophene is a two-dimensional surface of boron atoms1,2,3. Borophene has been the focus of a large number of researchers due to its various applications in various sciences. In comparison to other two-dimensional materials, borophene is highly significant, with many of its allotropes predicted by quantum studies prior to their experimental synthesis2,4,5,6,7. Finally in 2007, a few years after the prediction of the first structure of borophene surface, this structure was modified, and more stable structures of it were introduced7,8,9. boron is the fifth element in the periodic table, and its ground-state configuration is [He]2s22p1, which makes this atom to not follow the octet rule because the number of atomic orbitals are largen than valance electrons. This causes the boron atom to use unconventional three-center-two-electron bonds (3c-2e) besides the conventional two-center-two-electron bonds (2c–2e) in order to reach an equilibrium structureuse; as a result, this fact leads borophene surface to the formation of various allotrops10,11.
During recent years, some allotropes of borophene surface were experimentally synthesized on metal substrates such as Ag5,12,13,14, Al15, Au16, Cu14, Ir17. Recently, some of the borophene's mechanistic features18,19,20, optical transparency20,21, thermal conductivity19,22,23,24,25, biomedical applications26,27, and electrical properties28 are investigated. Additionally, sundry studies convey that borophene surface is a great candidate to design batteries29,30,31,32 and gas sensors33,34,35. Despite the extensive research on borophene’s properties, its wettability characteristics remain unexplored.
First-hand information on a surface hydrophilicity or hydrophobicity can be valuable across various industries such as self-cleaning surfaces36,37, anti‐icing properties38, development of super waterproof coatings39,40, non-stick material41, anti-fouling42, vapor chamber43, thermal diodes44, Separation of oil from water45, anti-corrosion coating46,47, and so forth.
Calculating the contact angle experimentally is challenging because the existence of impurities and defects on the surface is inevitable; as a result, this changes the behavior of the water droplet48. Molecular dynamics simulations provide a robust framework for examining the wettability of two-dimensional materials. Molecular dynamics simulation not only enables researchers to accurately investigate changes at the molecular level, but also macroscopic parameters such as density, surface tension, viscosity, contact angle, and layering mechanism can be studied in more detail49,50,51.
In the present work, we will study the wettability of borophene with molecular dynamics simulation approach. For this purpose, first, the contact angle of a water drop will be calculated with the help of drawn contour maps. In the following, the energy contour map of the borophene surface, the density profile diagrams of the water droplet, the RDF diagram, the number of hydrogen bonds and the way the water droplet moves on the borophene surface will be studied and analyzed.
Computational method
Reactive force field
Reactive molecular dynamics simulation is a bridge between the world of quantum calculations and the experimental force field. The reactive force field can model chemical reactions based on the dynamic distribution of charges and the concept of bond length. Equation (1) shows the total energy that the reactive force field calculates for the system.
In Eq. (1), the first term calculates the bond energy, and the second term calculates the Lone pair electron energy. In the following, the third and fourth terms calculate over-coordination and under-coordination energies for the system, respectively. Also, expressions Eval and Epen are used to calculate angular energy and penalty energy, respectively. Ecoa is the term related to three-body conjugation energy calculation. Ec2 and Etriple are used to calculate the energy correction for the C2 molecule and the triple bond energy, respectively. Moreover, Etors is the term related to the calculation of torsion angle energy and Econj corresponds to the conjugation effect term. Other terms are related to the calculation of hydrogen energy and include the contribution of hydrogen bond, van der Waals, and Coulomb energies respectively. To know more details about the Rexx force field, refer to references52,53. In the current work, Reax force field reference54 is used to study interatomic interactions.
Simulation details
In the present work, we investigate the wetting behavior of the two-dimensional borophene surface with the helping hand of molecular dynamics simulation. All the simulations have been performed with a laerge-scale atomic/molecular massively parallel simulator (LAMMPS) package55, and structures illustrated and photos taken by Visual Molecular Dynamics (VMD) software56. Periodic boundary conditions were considered periodic in all directions except the Z direction. Boron atoms are considered fixed during the simulation. The temperature of this simulation is 300 K in which the Nose–Hoover thermostat has been used in order to keep the temperature constant during the simulation. It goes without saying that the simulation is performed using an NVT ensemble with a time step of 0.2 fs. Besides, the total simulation time was 1.5 ns; this time was enough to the droplet to reach equilibrium. Also, a droplet with 500 water molecules was used in this simulation.
Details of borophene and water drop configuration
In this section, borophene surface and the initial configuration of the water drop on this surface will be introduced. The two lattice constants of this structure of borophene are 1.614 Å and 2.866 Å and its out-of-plane height (h) in the unit cell is 0.911 Å57. Figure 1 shows the surface and unit cell of borophene.
Structure of borophene surface along the Zig-zag and Armchair direction and the position of atoms in the unit cell.
According to Fig. 1, the borophene surface consists of two sub-layers, top and bottom. In this figure, the light and dark purple spheres represent atoms in the top and bottom sublayer, respectively. Also, the X and Y direction of the borophene surface has an armchair and zigzag structure, respectively.
Among the two-dimensional structures, phosphorene and penta-graphene are examples that, be close resemblance to borophene, are composed of sub-layers. The arrangement of atoms, the out-of-plane height, and the distance between atoms in each of these structures are different. For example, the out-of-plane height of phosphorene is 2.246 Å and penta-graphene is 1.20 Å.
To study the wettability of the borophene surface, a drop of water is placed in the center of this surface. Figure 2 shows the initial configuration of a water droplet on the borophene surface.
Initial configuration of water drop on borophene surface from (a) top view, (b) zig-zag direction, and (c) armchair direction of borophene. (Purple, red, and white spheres represent boron, oxygen, and hydrogen atoms, respectively).
Figure 2a shows the initial configuration of a water droplet on the borophene surface from the top view. In this figure, the radius of the water drop is 16 Å and the size of the borophene surface is 79 × 79 Å2. On the other hand, the water droplet is 23 Å away from the edges of the borophene surface. Figure 2b and c show the initial configuration from the zigzag and armchair directions of the surface. In these images, the distance of the water drop from the surface of borophene is 3 Å.
Contact angle measurement methods
Wettability is a substantial indicator to identify the properties of solid materials. In fact, wettability shows the degree of interaction and adhesion of a liquid such as water in contact with the surface of a solid. Calculating the contact angle of a liquid drop on a solid surface determines the degree of wetting of the solid surface. The Yang equation is used to measure the contact angle in order to describe the wettability properties58:
In Eq. (2), γsv, γsl, and γlv represent the surface tensions of the solid–vapor, solid–liquid, and liquid–vapor surfaces, respectively. θ indicates the contact angle of the droplet on the solid surface.
There are assorted methods for calculating the contact angle of a water drop, one of them is fitting a circle. In this method, first of all, the water drop is meshed into small cubes, and then, the number of water molecules in each mesh is calculated. Accordingly, the density of water molecules at any point of the drop can be calculated. After calculating the density of water molecules in the droplet, parts of the water drop with a density of 0.4 g/cm3 are considered as the boundary between the liquid and vapor phases59. Then a circle is fitted on these points as the boundary between the two phases; and the angle of the circle with the surface is calculated. It should be noted that to follow this method, the coordinates of the atoms in the last 300 frames; where the water droplet reached equilibrium, were taken as output from the simulation, and then an average of their coordinates was used to calculate the contact angle.
Results and discussions
In this section, the wetting behavior of the borophene surface is investigated. For this purpose, first, in section “Contact angle”, the contact angle of the water droplet will be examined with the help of a density contour map. Then, in section “Energy contour map of borophene surface”, the surface energy contour map will be studied, and in the following sections, the density profile, the number of hydrogen bonds, and the RDF graph will be studied respectively.
Contact angle
In order to calculate the contact angle of the water drop on the borophene surface, we need to know how the atoms are placed in the equilibrium at the end of the simulation. Figure 3 shows the arrangement of water molecules on the borophene surface in an equilibrium state.
Equilibrium configuration of the water droplet on Borophene surface from (a) up view, (b) zigzag direction, and (c) armchair direction. (Purple, red, and white spheres represent boron, oxygen, and hydrogen atoms, respectively).
Figure 3 shows snapshots of the water droplet on the borophene in the equilibrium state. In order to be able to calculate the contact angle of the water drop on borophene, it is necessary to draw the density contour map of water molecules. In this regard, the water droplet has meshed with the size of 1.5 Å2, and then the number of water molecules in each meshing is calculated. Finally, each mesh of water droplet with the same density is marked with the same color. Figure 4 shows the density contour map of the water droplet.
Density contour map of water droplet from the (a) armchair direction and (b) zigzag direction.
According to the color bar in Fig. 4, the accumulation of particles in the center of the drop is more than in other areas, and by moving from the center of the drop to the outer shell of the it, the density of the particles decreases. In essence, according to the color bar in Fig. 4, the density of the water droplet in the center is 1, but as it moves away from the center of the drop, the density decreases and finally reaches zero.
The red circle, which is drawn from the density of 0.4 g/cm3 of water, in Fig. 4 defines the boundary between the liquid and the vapor phase. After that, the angle between the red circle and the borophene surface is calculated. This angle is 149.11° in the zigzag direction and 148.4° in the armchair direction. The contact angle of the water drop shows that the borophene surface is hydrophobic; to put it simply, water molecules do not have much tendency to spread on the borophene surface. The hydrophobicity of the borophene surface was also reported in previous studies60, but the exact contact angle of the water droplet was not investigated.
Additionally, density contour maps in Fig. 4 show that the distribution of water molecules near the surface is different along the zigzag and armchair directions. According to Fig. 4, in the armchair direction, the water molecules have a regular distribution near the surface, while in the zig-zag direction, have taken on a zig-zag structure to some extent. This behavior has not been observed in the central parts of the drop, which have a greater distance from the borophene surface. It seems that the reason for the erratic distribution of water molecules in the zigzag direction of the borophene surface is that the energy barrier in this direction is greater than The energy barrier in the armchair direction30. This causes the water droplet to be elliptical; so the contact angle of the water molecules in the zigzag and armchair directions is different.
Previous studies have shown that phosphorene, similar to borophene, has anisotropic wettability. unlike borophene, the contact angle of the water droplet on phosphorene in the zigzag direction is lower than in the armchair direction. Based on this study, the contact angle of a water drop on phosphorene61 in the zigzag and armchair direction is 130° and 132°, respectively. Phosphorus, vice versa borophene surface, has the barrier energy in the armchair direction. In layman's terms, the tendency of the water drop to spread in the armchair is less than the zigzag direction. The contact angle of water drop on penta-graphen62 for both directions is reported to be 134.63°, which shows that this surface does not have anisotropic behavior. Comparing the wetting behavior of the three surfaces of borophene, phosphorene, and penta-graphene shows that the most hydrophobic surface is borophene, phosphorene, and penta-graphene respectively.
Energy contour map of borophene surface
Since the unevenness of the borophene surface can affect the placement of water molecules, the energy profile of the borophene surface has been investigated in this section. Figure 5 shows the interaction energy contour map of a water molecule on the surface of borophene.
Interaction energy contour map of a water molecule on the borophene surface along the zigzag and armchair direction.
To draw the contour map shown in Fig. 5, a water molecule was scanned on the borophene surface and its interaction with each boron atom in the substrate is calculated. According to Fig. 5, different positions of the borophene surface have different interaction energies with water molecules. According to the color bar in Fig. 5, the interaction of a water molecule was strong in red regions and weak in blue regions.
As it is clear in this figure, the borophene surface has an energy barrier in the zigzag direction. Similar to previous studies, there are two different energy barriers in the zigzag direction, both of which are stronger than the energy barrier along the armchair direction30. This energy barrier has caused water molecules to have less tendency to spread in a zigzag direction. The difference in the energy barrier along the zigzag and armchair directions has led to the anisotropic wetting behavior of the borophene surface. This behavior is consistent with the contact angle studies that were done in the previous section. In the previous section, it was observed that the contact angle of the water drop on the borophene surface along the zigzag direction is lower than it is along the armchair direction.
Generally, spreading water droplets on two-dimensional surfaces is affected by their structure. For example, the diagram of the interaction energy of a water molecule on the phosphorene surface shows that the water molecule has a strong interaction with the phosphorus atoms in the armchair direction. To make it clearer, the energy barrier in the armchair phosphorene direction is higher than the energy barrier in the zigzag direction. As a result, the presence of this energy barrier in phosphorene gives less movement freedom to water molecules in this direction. Finally, this has caused the reduction of the contact angle along the armchair direction. Unlike borophene and phosphorene, in penta-graphene, the distance between the energy barriers is large, so the water drop can overcome the energy barrier and spread evenly in two directions.
Density profile
The equilibrium configuration of water drop on the borophene surface is shown in Fig. 3. Figure 6 shows the density profile diagram of this water droplet according to its height in the Z direction.
Density profile of water droplet on borophene surface along the Z direction.
In this figure, the highest atom of the substrate is considered as a reference atom for drawing the density profile diagram. According to Fig. 6, there is no separate layer structure of water drops on the borophene surface. The layering of water drops is tangled and overlapping. The height of the water drop on the borophene surface is about 27 Å. Moreover, the first peak of the density profile of water molecules on the borophene surface has been seen at 4 Å. In essence, the water drop is at a distance of 4 Å from the surface. Penta-graphene density profile diagrams62 show that water layers are completely separate from each other and the number of layering is much more than the layering of a water drop on the borophene surface.
Hydrogen bond
One of the most important parameters in determining the spatial structure of a water drop is calculating the number of hydrogen bonds. This parameter can be calculated both experimentally and using quantum mechanics. The number of hydrogen bonds was calculated according to the method reported in reference63. There is a hydrogen bond between two molecules i and j if first of all, the distance between the two oxygen atoms is less than 3.5 Å, second, the distance between the hydrogen atom i and the oxygen atom j is less than 2.45 Å, and finally, the angle between oxygen i and the OH bond Molecule j be less than 30°. Figure 7 shows the number of hydrogen bonds between water molecules on the borophene surface in the last 300 fs.
Number of hydrogen bond between water molecules relative to the time.
Figure 7 shows the number of hydrogen bonds in a water drop during 1.5 ns. The number of hydrogen bonds is affected by the surface structure. To make it clearer, water molecules form about 835 hydrogen bonds on the borophene surface. Since the number of hydrogen bonds that a water molecule can have is almost 2, it can be seen that the water drop on the borophene surface has almost used its full power to form hydrogen bonds. A water droplet with a higher number of hydrogen bonds between its molecules is more cohesive and spherical compared with other water droplets; as a result, its contact angle will increase. The high number of hydrogen bonds between water molecules indicates the hydrophobicity of the borophene surface.
The number of hydrogen bonds reported for a water molecule on the phosphorene surface is approximately 80061. On the other hand, the diagrams of hydrogen bonds of penta-graphene62 show that the water drop on penta-graphene has used a quarter of its container to form hydrogen bonds. Needless to say, the presence of more hydrogen bonds in the drop increaces the contact angle of the water drop with the surface63. This fact is evident in the contact angle of water drops on borophene, phosphorene, and penta-graphene.
Radial distribution function diagram
One of the other parameters that help to have a better understanding of droplet behavior is the radial distribution function diagram. Accordingly, we can understand how the atoms are arranged and placed in relation to each other. The diagram in Fig. 8 shows the radial distribution function of B–O, O–O, and O–H atoms.
Radial distribution function (RDF) analysis. (a) Full RDF curves for Boron-Oxygen (B-O) and Boron–Hydrogen (B–H) interactions. (b) Zoom-in of the initial part of the RDF curves from (a), and (c) RDF curves for Oxygen–Oxygen (O–O) and Oxygen-Hydrogen (O–H) interactions.
The radial distribution function is one of the most important features of determining the structure of matter. Based on this function, the probability of finding an atom around a hypothetical central atom can be calculated. According to Fig. 8a, which is related to boron-oxygen and boron hydrogen atoms, shows that the oxygens are far away from the boron atom. This issue is consistent with the water droplet distance seen in the density profile graphs. There are two minor peaks in the initial part of this diagram for B–O and B–H; the magnification of these peaks is shown in Fig. 8b. The place of appearance of peaks B–H and B–O is almost the same, but the peak intensity of B–O is almost twice the peak of B–H. This shows that the first layer of water molecules is formed at this distance from the borophene surface, and also the water molecules in this layer are facing the borophene surface from their oxygen side.
Three main peaks have been observed for the radial distribution function of oxygen-hydrogen atoms in Fig. 8c. The first peak of this graph occurred at 0.9 Å. This peak shows the bond length of oxygen-hydrogen atoms in the water molecule. The second peak of this graph occurred approximately at 1.9 Å, representing the length of the hydrogen bonds between water molecules. The third peak is related to the hydrogen bond of molecules that are located at a further distance from the hypothetical central atom.
Two relatively sharp peaks and one small peak have also been observed in the red diagram, which is related to oxygen–oxygen atoms. The first peak appeared approximately at 2.9 Å. This means that water molecules are facing each other by their hydrogen atoms. The second peak of this diagram is located at a distance of 3.4 Å from the hypothetical central atom. The presence of hydrogen bonds between water molecules makes oxygen atoms unable to move freely.
The large distance of oxygens from boron atoms causes low interaction between oxygen and boron atoms, and as a result, the surface structure has a delicate effect on water molecules layering. This indicates the hydrophobicity of the borophene surface which was studied in the previous sections. The small out-of-plane height of the borophene structure, which is introduced in sections “Introduction–Results and discussions”, prevents water molecules from being placed between the grooves in the structure. As a result, water molecules are inevitably placed at higher heights. The large distance of the water droplet from the surface of borophene is clearly evident in the graphs of Figs. 6 and 8.
Conclusion
Borophene is a new two-dimensional material that can be a more suitable candidate for industrial applications due to its unique characteristics compared to other two-dimensional materials. For better and more appropriate use of borophene in the industry, it is necessary to have a deep understanding of the surface properties. For this purpose, in the present work, the wettability of borophene surface has been studied with the helping hand of molecular dynamics simulation. Our results show that the borophene surface is hydrophobic and anisotropic. For this reason, two different contact angles have been calculated along the zigzag and armchair directions, which are 149.11° and 148.4°, respectively. The analysis of the energy contour map of the borophene surface showed that there is an energy barrier along the zigzag direction, and this has prevented water molecules from spreading along this direction. The density profile graph showed that the water layers on the borophene surface are not separate from each other and peaks overlap each other overlap. Besides them, the RDF diagram showed that the water drop is located at a great distance from the surface. Finally, the calculation of the number of hydrogen bonds between water molecules revealed that the water drop on the borophene surface maximizes its hydrogen bonding potential. To sum up, all the parameters that studies showed that the borophene surface is hydrophobic.
Data availability
Data is provided within the manuscript.
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Foroutan, M., Sababkar, M. & Bavani, B.M. Exploring hydrophobicity or hydrophilicity of borophene surface via reactive molecular dynamics simulation. Sci Rep 14, 21436 (2024). https://doi.org/10.1038/s41598-024-71793-6
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DOI: https://doi.org/10.1038/s41598-024-71793-6
