Hydration of rutile TiO₂: Thermodynamics and effects on n- and p-type electronic conduction

The bulk conductivity of polycrystalline 1 mol-% Fe-doped rutile TiO ₂ has been measured as a function of p_H2O and temperature under oxidizing and reducing conditions. From the p_H2O-dependency of the conductivity, it is concluded that protons are significant positive defects and, furthermore, that mixed p-type electronic and protonic, and n-type electronic conduction dominate under oxidizing and reducing conditions, respectively. H₂O/D₂O isotope exchange confirmed that protons are significant charge carriers under wet oxidizing conditions below approximately 600°C. Thermodynamic parameters for the hydration reaction were obtained by modeling the experimental p_H2O and temperature dependencies, assuming that the acceptor dopant (Fe³⁺) is charge compensated by protons and oxygen vacancies, and from ab initio density functional theory (DFT) calculations. The experimental data yield standard enthalpy changes of hydration of -130 ± 16 kJ/mol, whereas the calculated values are somewhat more negative; -155 to -162 kJ/mol. Based on such favorable thermodynamics of hydration, it is concluded that protons will be the dominating positive defect in TiO₂ under most conditions of practical interest.