Structure and Dynamics of the Liquid-Water/Zinc-Oxide Interface from Machine Learning Potential Simulations

Interfaces between water and metal oxides exhibit many interesting phenomena like dissociation and recombination of water molecules and water exchange between the interface and the bulk liquid. Moreover, a variety of structural motifs can be found, differing in hydrogen bonding patterns and molecular orientations. Here, we report the structure and dynamics of liquid water interacting with the two most stable ZnO surfaces, (10 (1) over bar0) and (11 (2) over bar0), by means of reactive molecular dynamics simulations based on a machine learning high-dimensional neural network potential. For both surfaces, three distinct hydration layers can be observed within 10 angstrom from the surface with the first hydration layer (nearest to the surface) representing the most interesting region to investigate. There, water molecules dynamically dissociate and recombine, leading to a variety of chemical species at the interface. We characterized these species and their molecular environments by analyzing the properties of the hydrogen bonds and local geometries. At ZnO(11 (2) over bar0), some of the adsorbed hydroxide ions bridge two surface Zn ions, which is not observed at ZnO(10 (1) over bar0). For both surfaces, adsorbed water molecules always bind to a single Zn ion, and those located in proximity of the substrate are mostly H-down oriented for ZnO(10 (1) over bar0) and flat-lying, i.e., parallel to the surface, for ZnO(11 (2) over bar0). The time scales for proton-transfer (PT) reactions are quite similar at the two surfaces, with the average lifetime of adsorbed hydroxide ions being around 41 +/- 3 ps until recombination. However, water exchange events, in which adsorbed water molecules leave the surface and enter the bulk liquid, happen more frequently at ZnO(11 (2) over bar0) than at ZnO(10 (1) over bar0).