Anionic or Cationic S-Doping in Bulk Anatase TiO 2 : Insights on Optical Absorption from First Principles Calculations

Using first principles calculations, we investigate the structural, electronic, optical, and energetic properties of S-doped anatase TiO 2 bulk systems. To ensure accurate band gap predictions, we use the HSE06 exchange correlation functional, and the absorption spectra are obtained with density functional perturbation (DFPT) theory by employing HSE06. Various oxidation states (anionic and cationic) of sulfur are considered depending on the location in bulk TiO 2: in interstitial position or in substitution for either oxygen or titanium atoms. Among the explored structures, two anionic and one cationic configurations induce an improved optical absorption response in the visible region as observed experimentally. Moreover, we undertake a thermodynamic analysis as a function of the chemical potential of oxygen and considering three relevant sulfur chemical doping agents (S 2, H 2 S, and thiourea). It highlights that cationic configurations (S 4+ and S 6+ ) are strongly stabilized in a wide range of oxygen chemical potential (including standard conditions), whereas anionic species are stabilized only at very low chemical potential of oxygen. The metastable cationic Ti (1–2 x ) O 2 S 2 x system involving the presence of S 4+ species in substitution for Ti 4+, with the formation of SO 2 units, should offer the best compromise between the thermodynamic conditions and the expected optical properties.