Department of Chemistry, Indian Institute of Technology Hyderabad, Sangareddy, Telangana 502285, India.
J Phys Chem B. 2021 Jul 15;125(27):7527-7536. doi: 10.1021/acs.jpcb.1c03344. Epub 2021 Jul 2.
The structure of black phosphorous (BP) is similar to the honeycomb arrangement of graphene, but the layered BP is found to be buckled and highly anisotropic. The buckled surface structure affects interfacial molecule mobility and plays a vital role in various nanomaterial applications. The BP is also known for wettability, droplet formation, stability, and hydrophobicity in the aqueous environment. However, there is a gap concerning the structural and dynamical behavior of water molecules, which is available in abundance for other monoatomic and polyatomic two-dimensional (2D) materials. Motivated by the technological importance, we try to bridge the gap by explaining the surface anisotropy-facilitated behavior of water molecules on bilayer BP using classical and first principles molecular dynamics (MD) simulations. From our classical MD study, we find three distinct layers of water molecules. The water layer closest to the interface is L1, followed by L2 and L3/bulk perpendicular to the BP surface. Water molecules in the L1 layer experience some structural disintegration in hydrogen bond (HB) phenomena compared to the bulk. There is a loss of HB donor-acceptor count per water molecule. The average HB count decreases because of an elevated rate of HB formation and deformation; this would affect the dynamic properties in terms of HB lifetime. Therefore, we observe the reduced lifetime of HB in the layer in close contact with BP, which again complements our finding on the diffusion coefficient of water molecules in distinct layers. Water diffuses relatively faster with diffusion coefficient 3.25 × 10 m s in L1, followed by L2 and L3. The BP layer shows moderate hydrophobic nature. Our results also indicate the anisotropic behavior as the diffusion along the x-direction is faster than that along the y-direction. The gap in the slope of the x and y components of mean-squared displacement (MSD) complements the pinning effect in an aqueous environment. We observe blue-shifted and red-shifted libration and O-H stretching modes from the calculated power spectra for the L1 water molecules compared to the L2 and L3 molecules from first principles MD simulations. Our analysis may help understand the physical phenomena that occur during the surface wetting of the predroplet formation process observed experimentally.
黑磷（BP）的结构类似于蜂窝状排列的石墨烯，但层状 BP 被发现是褶皱的，具有各向异性。褶皱的表面结构影响界面分子的迁移率，在各种纳米材料的应用中起着至关重要的作用。BP 还具有亲水性、液滴形成、在水介质中的稳定性和疏水性。然而，关于水分子的结构和动力学行为的信息还存在差距，而其他单原子和多原子二维（2D）材料中则有大量的水分子。受技术重要性的启发，我们试图通过使用经典和第一性原理分子动力学（MD）模拟来解释双层 BP 上水分子的表面各向异性促进行为来填补这一空白。从我们的经典 MD 研究中，我们发现了三层水分子。最接近界面的水分子层是 L1，其次是 L2 和 L3/垂直于 BP 表面的体相。与体相比，L1 层中的水分子在氢键（HB）现象中经历了一些结构解体。每个水分子的 HB 供体-受体计数减少。由于 HB 形成和变形的速率增加，平均 HB 计数减少，这会影响 HB 寿命方面的动态特性。因此，我们观察到与 BP 紧密接触的层中 HB 的寿命缩短，这再次补充了我们在不同层中水分子扩散系数的发现。水分子在 L1 中扩散相对较快，扩散系数为 3.25×10 m s，其次是 L2 和 L3。BP 层表现出中等疏水性。我们的结果还表明了各向异性行为，因为沿 x 方向的扩散速度快于沿 y 方向的扩散速度。x 和 y 方向的均方根位移（MSD）分量斜率之间的差距补充了水介质中钉扎效应。与第一性原理 MD 模拟相比，我们从 L1 水分子的计算功率谱中观察到蓝移和红移的摆动和 O-H 伸缩模式。我们的分析可能有助于理解在实验观察到的预液滴形成过程的表面润湿过程中发生的物理现象。
