文摘
The bulk conductivity of polycrystalline 1 mol-% Fe-doped rutile TiO2 has been measured as a function of pH2O and temperature under oxidizing and reducing conditions. From the pH2O-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 pH2O 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.