Hydration of Rutile TiO₂: Thermodynamics and Effects on n- and p-Type Electronic Conduction

Abstract

The bulk conductivity of polycrystalline 1 mol-% Fe-doped rutile TiO2 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.

Facet-engineered TiO2 nanomaterials reveal the role of water–oxide interactions in surface protonic conduction

“…3 For T001 and T100 we expect that the chemisorbed layer is at full coverage and fully dissociated, so that the activation energies reect the enthalpy of proton charge mobility (diffusion). They are in overall agreement with the 58 kJ mol À1 (0.60 eV) found for bulk proton migration in polycrystalline rutile and within the range of proton diffusivities in the different directions of single crystal rutile, 40 knowing that surface transport has somewhat lower activation enthalpies than in bulk. The activation energy of T101 should, on the one hand, contain some dissociation enthalpy, but may on the other hand reect easier jumps between the loosely bonded water molecules on the surface of T101 than the more rigid and dissociated water on T001 and T100.…”

Generation of an electromotive force by hydrogen‐to‐water oxidation with Pt‐coated oxidized titanium foils

“…Diffusion of molecular H 2 through the pores is a result of Δ µ H . However, another transport channel may be opened by diffusion of atomic H along the TiO 2 surface 29. The room‐temperature movement of H atoms along the bridging O atoms has been previously shown in scanning tunneling microscopic studies on TiO 2 (110) surfaces 30.…”

Aluminum solubility in rutile (TiO2)

“…Reaction 2, the predominant mechanism under dry conditions (e.g., Gesenhues and Rentschler 1999) is not expected to play an important role due to its independence from f O 2 . Likewise, Reaction 3 is not expected to play a significant role due to the energetic favorability of OH defects over oxygen vacancies in rutile that has been grown under equilibrium hydrothermal conditions (Erdal et al 2010;Bjørheim et al 2010).…”

“…If anything, the formation of oxygen vacancies from freeing lattice 2 oxygen should have a positive volume change and would be increasingly unfavorable at higher pressures. Instead, under high pH 2 O, the underlying defect structure in rutile shifts from an oxygen vacancy-dominated structure to one of hydrogen defects (Erdal et al 2010;Bjørheim et al 2010). Moreover, OH concentrations in rutile rise sharply with the addition of Al (Fig.…”