Defect Chemistry of Rutile TiO2 from First Principles Calculations

In the current contribution, we study the defect chemistry of bulk rutile TiO2 through first principles defect calculations in combination with thermodynamic modeling. We apply three levels of theory, PBE, PBE+U, and the HSE hybrid functional. The defect calculations reveal that the included isolated defects, v(T nu)(q) v(O)(q) Ti-i(q), OHOq, and H-O(q), display shallow thermodynamic charge transition levels, whereas oxygen interstitials, O-i(q), display a deep (0/-2) transition level. The formation energies of the complexes between v(Ti)(4/), v(O)(center dot center dot), and OHO center dot are as low or even lower than those of the isolated defects, and all included complexes display exothermic binding enthalpies, indicating their possible presence under thermodynamic equilibrium. Through thermodynamic modeling, we show that the finite temperature defect structure of rutile TiO2 largely is dominated by v(Ti)(4/), OHO center dot, and their mutual complexes under wet oxidizing conditions and lower temperatures and by v(Ti)(4/), v(O)(center dot center dot), and their complexes at higher temperatures. Hence, we indicate that TiO2 cooled from high temperatures or grown at low temperatures will display defect structures very different from previously known, which together with previous results for defects in surfaces gives a new basis for understanding for instance photocatalytic properties of TiO2.