(297f) Probing Realistic Water-2D Material Interfaces Via Combined Quantum and Classical Simulations

Two-dimensional (2D) materials are gaining increasing attention for use in seawater desalination, osmotic power harvesting, and biological sensing devices. Because water comes into close contact with 2D material surfaces in these applications, it is important to understand water-2D material interactions at the molecular level. At the same time, 2D materials often have defects, such as vacancies and grain boundaries in them, and thus understanding their effect on interfacial properties is crucial to model such interfaces realistically. In this talk, I will highlight our recent work on the combined use of quantum-mechanical density functional theory (DFT) calculations, classical molecular dynamics (MD) simulations, and kinetic Monte Carlo (KMC) simulations to probe water-2D material interfaces. While DFT calculations can be used to predict the distribution of charge inside defective 2D materials, MD simulations can be used to simulate the thermodynamic and transport properties of interfacial water. Moreover, KMC simulations can be combined with DFT and graph-theoretic algorithms to predict the shapes of vacancy defects in 2D materials.

First, I will briefly outline how the combined application of KMC simulations, DFT calculations, and chemical graph theory has enabled the solution of the isomer cataloging problem for vacancy defects in 2D materials. I will demonstrate the excellent agreement of simulated vacancy defect shapes in two prototypical 2D materials – graphene and hexagonal boron nitride (hBN) – with experimental transmission electron microscopy images. Not only that, I will also briefly discuss the use of machine learning to develop structure-formation relationships for vacancy defects in graphene.

Second, I will discuss the effect of vacancy defects and surface roughness on the wettability and slip length of water on hBN. Indeed, at the molecular level, the no-slip boundary condition is violated, as quantified by the slip length of water on the surface. I will show that vacancies at a lower concentration of 0.082 nm^-2 do not affect the wettability of hBN, although they still affect the water slip length. On the other hand, vacancies at a larger concentration of 0.32 nm^-2 affect interfacial properties significantly. In fact, nitrogen vacancies at such concentrations can increase the slip length of water on hBN threefold to around 18 nm, presenting hBN as an alternative high-slip surface to graphene. I will also explain how surface roughness in hBN can explain the water contact angle of 66°and water slip length of 1 nm measured experimentally, simultaneously highlighting the prominent role played by electrostatic interactions in the interfacial properties of water on realistic hBN surfaces.

Third, I will present a study on the role of grain boundaries and interfacial electrostatic interactions in modulating water and ion transport via bicrystalline nanoporous hBN. I will discuss how grain boundaries alter the areas and shapes of nanopores in bicrystalline hBN, as compared to the nanopores in monocrystalline hBN. Although bicrystalline nanoporous hBN with a lower misorientation angle shows an improved water flow rate by ~30%, it demonstrates reduced Na⁺ ion rejection by ∼6%, as compared to monocrystalline hBN. I will also explain the role of the nanopore shape in water desalination, demonstrating that more elongated pores with smaller sizes can match water permeation through less elongated pores of slightly larger sizes, with a concomitant ~3–4% decrease in Na⁺ rejection. Finally, I will discuss how the water flow rate is affected by interfacial electrostatic interactions. In this regard, the water flow rate is the highest when altered partial charges on B and N atoms are determined using DFT calculations, as compared to when no partial charges or bulk partial charges (i.e., charged hBN) are considered.

Overall, these multi-scale investigations of thermodynamic and transport properties offer new insights into the wettability of defective 2D material surfaces and water flow on them, as well as into water and ion transport through nanopores in realistic 2D materials.