Water Dissociation and Further Hydroxylation of Perfect and Defective Polar ZnO Model Surfaces

ZnO is a high-band gap semiconductor material important for microelectronic and catalytic applications, such as water splitting among others. Although the nonpolar face of ZnO has been well-studied, its polar faces (Zn- and O-terminated) are less studied because of intrinsic difficulties to the model. Here, we combine density functional theory calculations and analytical modelling to determine the thermodynamics of the water molecule interaction with perfect ZnO polar model surfaces, (0001) and (0001) surfaces (p-Zn and p-O). Defects (oxygen vacancies, pits, and missing oxygen rows) are also investigated. Adsorption, dissociation, surface migration, and agglomeration are considered. We find that H2O preferentially adsorbs and dissociates on Zn atoms on p-Zn and at defects on p-O. At room temperature, water is found to spontaneously dissociate, except for p-O, in which dissociation is endothermic. After dissociation, the resulting protons either bind to surface oxygen atoms or to zinc atoms to form hydrides. Migration of H and OH is limited on p-Zn with moderate barriers and absent on p-O. Interestingly, further agglomeration or islanding of OH species is inhibited by repulsive OH-OH electrostatic forces. Consequently, although polar surfaces are highly reactive with water, they cannot sustain high OH coverages, unless highly defective. This limitation is one obstacle to ZnO catalytic activity, pointing to the need to tune temperature and pressure conditions.