Solute segregation, electrical properties and defect thermodynamics of nanocrystalline TiO2 and CeO2

The thermodynamic and kinetic properties of nanocrystalline oxides, includi ng nominally undoped TiO2 (anatase) and Pr-and Cu-doped CeO2, are reviewed. The electrical properties of nominally undoped nanocrystalline TiO2 and Ce O2 differ from conventional microcrystalline materials due to a greatly red uced specific grain boundary impedance and enthalpy of reduction. In TiO2, an uncommon domain of ionic conductivity is observed at high oxygen partial pressures, whereas at low P(O-2), the electronic conductivity increases st rongly with a P(O-2)(-1/2) dependence. Nanocrystalline CeO2, on the other h and, exhibits strongly enhanced oxygen nonstoichiometry and electronic cond uctivity over the whole P(O-2) range. Reduced defect formation energies at interface sites are proposed to be responsible for these properties. The ap parent solubility of copper in nanocrystalline CeO2-Cu2O of about 10 mol% i s much enhanced over that of coarse-grained ceria and is accommodated by se gregation of copper to the grain boundaries. Nanocrystalline CeO2-PrOx, wit h up to 70 mol% PrOx, is found to be single phase. The oxygen deficiency in this system attains large values (x > 0.1) with evidence for vacancy order ing. The chemical diffusivities (approximate to 10(-6) cm(2)/s) and the low activation energy (approximate to0.3 eV) suggest short circuiting diffusio n paths via interfaces. (C) 2000 Elsevier Science B.V. All rights reserved.