The effect of growth conditions and point defects such as isolated vacancies and dimer vacancy (DV) on the electronic, optical, and photocatalytic properties of the ZnO (10 (1) over bar0) surface was investigated using the first principles method. We found that the properties of this surface are very sensitive to growth conditions, and to the concentration of surface vacancies. Our results show that the formation of the 0 deficient surface is more probable under Zn-rich conditions, while, the formation of the Zn-deficient surface is dominant under O-rich conditions. For oxygen chemical potential values between these two limits, the formation of DV is dominant. On the other hand, we found that both isolated and dimer vacancies induce an important changes in the optical absorption spectrum of the ZnO (10 (1) over bar0) surface notably in the visible and UV regions. The obtained results show that the O-deficient ZnO (10 (1) over bar0) surface is more suitable than the Zn-deficient surface for photocatalysis and also for UV photodetector construction. We have also examined the effect of vacancies on the work function of the ZnO (10 (1) over bar0) surface. (C) 2016 Elsevier B.V. All rights reserved.