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

Abstract The thermodynamic and kinetic properties of nanocrystalline oxides, including nominally undoped TiO 2 (anatase) and Pr- and Cu-doped CeO 2 , are reviewed. The electrical properties of nominally undoped nanocrystalline TiO 2 and CeO 2 differ from conventional microcrystalline materials due to a greatly reduced specific grain boundary impedance and enthalpy of reduction. In TiO 2 , an uncommon domain of ionic conductivity is observed at high oxygen partial pressures, whereas at low P (O 2 ), the electronic conductivity increases strongly with a P (O 2 ) −1/2 dependence. Nanocrystalline CeO 2 , on the other hand, exhibits strongly enhanced oxygen nonstoichiometry and electronic conductivity over the whole P (O 2 ) range. Reduced defect formation energies at interface sites are proposed to be responsible for these properties. The apparent solubility of copper in nanocrystalline CeO 2 –Cu 2 O 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 CeO 2 –PrO x , with up to 70 mol% PrO x , 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 cm 2 /s) and the low activation energy (≈0.3 eV) suggest short circuiting diffusion paths via interfaces.