Fate of Water Molecules on (11-20) and (1-102) α-Alumina Surfaces: 2D Periodic Self-Consistent Charge-Density Functional Tight-Binding/Molecular Mechanics Molecular Dynamics Study

Abstract

Numerous studies on the adsorption and dissociation of water molecules on α-alumina surfaces, both experimentally and theoretically, have been presented within decades. However, the dynamical properties of water at the solid–liquid interface are still a challenging problem. To explain the dynamics of the adsorption and dissociation of water molecules on α-alumina surfaces, mainly (11-20) and (1-102) faces, we carried out a two-dimensional periodic self-consistent charge density-functional tight-binding/molecular mechanics molecular dynamics study. In this study, we used the same approach previously applied to the (0001) α-alumina surface. The adsorption of single, dimer, and monolayer water molecules was employed to study the adsorption and dissociation of water molecules at (11-20) and (1-102) α-alumina surfaces. The single water system showed that no significant changes occurred on both surfaces, which indicates that no water dissociation was observed. Dimer water molecules on both surfaces were dissociated to form new OsHw groups following a 1-4 dissociation channel. Monolayer water molecules showed dissociation that occurred on both surfaces. Protonation analysis showed that both surfaces form stable OH groups without attempting to form water molecules. At the 100 ps simulation period, for the (11-20) α-alumina surfaces, up to 1.93 OH groups/nm2 were formed. The (1-102) α-alumina surface showed higher reactivity with a formation rate of up to 5.0 OH groups/nm2. By contrast, 2.1 OH groups/nm2 were formed on the (0001) α-alumina surface. The formation of OH groups on the (11-20) and (1-102) α-alumina surfaces occurred in the equilibration step and continued during the sampling step. In studying these three surfaces using a similar approach, the (1-102) α-alumina surface is the most reactive with water.