DFT Study on Oxygen-Vacancy Stability in Rutile/Anatase TiO2: Effect of Cationic Substitutions

In this study, a full-potential density functional theory was used to investigate the effects of Ti substitution by different cations. In both rutile and anatase, Ti atom was replaced by Ce, Au, Sn, Ag, Mo, Nb, Zr and Y. Phase stability, electronic structure and formation energy of oxygen vacancy were compared for rutile and anatase. The results indicated that substitution of Ce and Zr increases anatase stability through which photocatalytic activity is enhanced. It seems that the cationic capacity and size play a critical role in anatase to rutile phase transition, where with an equi- or higher valence than Ti, larger cations increase the stability of anatase phase. Oxygen vacancy concentration, as a second factor of photocatalytic activity, was also studied by calculating its stability due to the cationic substitution. The data revealed that Au, Ag, Y, Ce and Sn effectively reduce oxygen-vacancy formation energy. Of the studied cations, Au and Ag had maximum reduction in band gap, by creating defect states in the middle of the band gap resulting from the overlaps of d-orbitals of these elements and oxygen p-orbitals. Mo and Ce impurities did not have a significant effect on reducing gaps by creating defect states under the conduction band. Finally, Sn impurity also generated defect states in the middle of the gap merely with the lack of oxygen.