The F doping converts some Ti4+to Ti3+originated from the charge compensation caused by the replacement of F−ions with O2−ions in the lattice [40].
TheTi3+surface states in TiO2form a donor level between the bandgaps of TiO2, which would enhance the visible light absorption[40]. Furthermore, the F doping promotes the creation of oxygenvacancies (F and F+centers) due to the replacement of surfaceOH groups, which also can induce the enhancement of the visiblelight absorption [30,41]. Such an enhancement in absorbance can increase the number of photo-generated electrons