Surface Roughness Explains the Observed Water Contact Angle and Slip Length on 2D Hexagonal Boron Nitride

Abstract

Hexagonal boron nitride (hBN) is a two-dimensional (2D) material that is currently being explored in a number of applications, such as atomically thin coatings, water desalination, and biological sensors. In many of these applications, the hBN surface comes into intimate contact with water. In this work, we investigate the wetting and frictional behavior of realistic 2D hBN surfaces with atomic-scale defects and roughness. We combine density functional theory calculations of the charge distribution inside hBN with free energy calculations using molecular dynamics simulations of the hBN−water interface. We find that the presence of surface roughness, but not that of vacancy defects, leads to remarkable agreement with the experimentally observed water contact angle of 66°on freshly synthesized, uncontaminated hBN. Not only that, the inclusion of surface roughness predicts with exceptional accuracy the experimental water slip length of ∼1 nm on hBN. Our results underscore the importance of considering realistic models of 2D materials with surface roughness while modeling nanomaterial−water interfaces in molecular simulations.

Anisotropic Interfacial Force Field for Interfaces of Water with Hexagonal Boron Nitride

“…Before conducting the simulations, we integrated the developed AIP into the widely used LAMMPS code, which allows us to accurately model the interactions between water molecules and h -BN at a large scale (see further information in Section S4). We considered two models: atomically flat pristine h -BN (Figure a) and a nanostructed h -BN surface (Figure c) with atomic step edges . These models allowed us to investigate the effects of edge steps on the wetting characteristics of water molecules on h -BN.…”

“…In these applications, there are intimate and non-negligible interactions between water and h -BN nanopores, nanochannels, and films, necessitating a comprehensive understanding of h -BN’s wettability. However, the intrinsic wettability of h -BN has remained elusive over the past few decades, as evidenced by the wide range (40°–165°) of reported water contact angles (WCAs) for monolayer and bulk h -BN in experiments. ,− Several factors contribute to this complexity, including surface contamination, surface roughness, and the presence of defects within h -BN layers. ,, Recent experimental endeavors have focused on reducing the influence of airborne contamination by measuring WCAs of high-quality h -BN films under controlled conditions, revealing a hydrophilic nature with WCAs falling within the range of 52°–67°. ,,, It is worth noting that experimental high-quality h -BN films may not be perfectly atomically flat and can contain surface defects or steps. , These imperfections have the potential to significantly influence the wettability of h -BN. ,, …”

“…However, the accuracy of these simulations relies heavily on the chosen force fields. In the case of describing the van der Waals (vdW) interaction between water molecules and h -BN, the commonly used isotropic Lennard-Jones (LJ) potential ( V ( r ) = 4ϵ­[(σ/ r ) 12 – (σ/ r ) 6 ]) has been widely adopted. ,− ,− These simulations predict a wide range of WCA for h -BN (0°–86°). − ,− The typical parameters describing the interactions between oxygen (O) atoms in water (H 2 O) and boron (B)/nitride (N) atoms in h -BN, for example, ε BO and ε NO , fall within the range of 2.16–6.13 meV and 4.33–8.97 meV, respectively. ,,, The wide range of LJ parameters leads to inconsistent WCAs, indicating that the LJ potential is not suitable for studying the wetting properties of h -BN.…”

“…However, this prediction contradicts the experimental observations. Kumar Verma and Seal et al. , claimed that this inconsistency may be attributed to the presence of defects or surface roughness/steps in experimental h -BN samples. Their further investigations confirmed that the surface steps of h -BN significantly reduce the calculated WCA, leading to a hydrophilic nature that aligns with the experimental results.…”

Water-Structure-Specific Entropic Dominance in the Filling of Boron Nitride Nanotubes

“…As a result, polarizability is accounted for by considering the fluctuations in atomic charges that capture the variations in molecular dipole moment induced by variations in molecular geometry and environment. , However, unlike other implementations such as the Drude polarizable model, the atomic polarization is not explicitly modeled in the present simulations. Of course, most recent studies attempting to accurately model the water–hexagonal boron nitride (hBN) interfacial interactions have not considered these explicit atomic polarizabilities; − despite that, these studies have been successful in capturing the wetting properties and the binding energy of water on hBN. Thus, we expect our results to be of reasonable accuracy and the trends in the calculated thermodynamic quantities to represent reality at least qualitatively.…”