1-the oxygen vacancy mean the absence of oxygen atom from the crystal lattice, negative charge carriers will be withdrawn to the vacancy site and increase the local charge density . So, the energy level of oxygen vacancy can serve as electron donor states to contribute to bandgap narrowing for visible-light harvest.
2- the oxygen vacancy plays a very important role in mediating photogenerated electron-hole separation and surface charge transfer in photocatalytic processe
3- surface oxygen vacancy also serves as excellent sites for molecular O2 adsorption and activation