The zinc (Zn) atoms are tetrahedrally coordinated by oxygen (O) atoms in wurtzite structure. The wurtzite surface is a low-index surface [39,40]. Each surface Zn cation is missing an O anion, and each surface O anion is missing the nearest zinc cation. The simulated ZnO slab was composed of 14 layers with a thickness of more than the cutoff value of the interactions between atoms.