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

The thermodynamic and kinetic properties of nanocrystalline oxides, including nominally undoped TiO2 (anatase) and Pr- and Cu-doped CeO2, are reviewed. The electrical properties of nominally undoped nanocrystalline TiO2 and CeO2 differ from conventional microcrystalline materials due to a greatly reduced 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(O2), the electronic conductivity increases strongly with a P(O2)−1/2 dependence. Nanocrystalline CeO2, on the other hand, exhibits strongly enhanced oxygen nonstoichiometry and electronic conductivity over the whole P(O2) range. Reduced defect formation energies at interface sites are proposed to be responsible for these properties. The apparent solubility of copper in nanocrystalline CeO2–Cu2O of about 10 mol% is much enhanced over that of coarse-grained ceria and is accommodated by segregation of copper to the grain boundaries. Nanocrystalline CeO2–PrOx, with 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 ordering. The chemical diffusivities (≈10−6 cm2/s) and the low activation energy (≈0.3 eV) suggest short circuiting diffusion paths via interfaces.