On the contrary, an oxygen deficiency in the outermost layer could provide the opposite charge for the missing cations, resulting in compensation of the excess holes and loss of the metallic character in the slab as shown in Fig. . Hence, introducing Cu or Fe vacancies and interstitial oxygen can improve the PEC performance. Especially, Cu vacancies enhance the PEC efficiency by facilitating charge separation. The charge separation is created by the fast delocalization of photogenerated holes to Cu-3d and O-2p states. The stoichiometric and O-deficient terminated surfaces show a typical DOS plot for a semi- show similar values for Bi atoms. The net charges of the (001)-stoichiometric terminated surface in Fig. )-terminated surface is also found to be the preferred orientation in experimental studies using X-ray diffraction. In general, terminations containing less Cu contribute predominantly to the most stable surfaces for all surface facets. This trend can be seen in a previous study as well.