Wetting and frictional properties of hexagonal boron nitride with atomic-scale defects and roughness

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Mtech(Res); 2023; Chemical engineering

Two-dimensional (2D) materials such as graphene, hexagonal boron nitride (hBN), and molybdenum disulfide (MoS2) have become materials of choice for applications spanning optoelectronics, atomically thin coatings, and high-throughput and high-selectivity membranes. In such applications, the exposure of 2D materials (e.g., hBN) to liquids underscores the importance of understanding 2D material-liquid interactions. Wettability is one of the important interfacial properties of 2D materials such as hBN, and understanding it is vital for designing devices for seawater desalination and osmotic power harvesting. The contact angle of water is the fundamental property measured in experimental investigations on wettability; so far, however, studies have not considered the effect of defects on the water contact angle on hBN surfaces. In this thesis, we simulated the wetting behaviour of water on monolayer and bulk hBN, in their pristine and defective forms, using classical molecular dynamics simulations supported by quantum-mechanical density functional theory calculations. We considered five defect topologies – the B, N, BN, B2N, and B3N vacancy defects – and also studied the effect of the defect concentration on the water contact angle to investigate more realistically the interfacial properties of defective hBN. We found that defects at a concentration of 0.082 nm-2 no longer affect the wetting properties of hBN surfaces. While bulk hBN, modeled as a stack of four monolayers, showed hydrophilic behavior, monolayer hBN exhibited hydrophobic behaviour. Additionally, hBN surfaces containing B and B2N vacancies exhibited increased hBN-water electrostatic interactions, especially at a higher vacancy concentration of 0.328 nm-2. We found 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. Additionally, the inclusion of surface roughness accurately predicts the experimental water slip length of ~1 nm on hBN. Our results underscore the importance of considering realistic 2D materials with surface roughness models while modeling nanomaterial-water interfaces in molecular simulations.