The effect of oxygen vacancies on water wettability of a ZnO surface.

In this study, the effect of oxygen vacancies on the water wettability of a hydrated ZnO(100) surface has been examined via molecular dynamics simulations with a reactive force field (ReaxFF). The results show that the oxygen vacancies on the ZnO surface change the structures of the ZnO surface and subsequently its water adsorption capability. While a 1 : 1 ratio of water to hydroxyl is observed for a water monolayer absorbed on ZnO(100) without oxygen vacancies, additional water adsorption as coordinate hydroxyl that resides on the vacancy site and bonds with three lattice zinc atoms is observed on the surfaces with oxygen vacancies. The results also show that the energy of the interaction per unit area between water and the hydrated ZnO surface is 55.1% higher in the presence of the oxygen vacancies than that without oxygen vacancies. This leads to a water contact angle of ~115° for the hydrated ZnO(100) surface in the absence of vacancies and ~21° with vacancies. The wetting kinetics of a water droplet on a ZnO(100) surface with and without oxygen vacancies are compared with the diffusion-limited reactive wetting and molecular kinetics models, respectively. In addition, the ordering of the vacancy sites is found not to significantly affect the wettability of the ZnO(100) surface.