Defect chemistry and transport properties of nanocrystalline cerium oxide

Simultaneous conductivity and thermopower measurements were performed on partially sintered nanocrystalline cerium oxide (∼15 nm grain size) over 10^-3 to 1 atm of oxygen pressure at 450 to 550°C. Although electronic conduction predominates, mixed conduction is apparent in the oxygen pressure dependence of the conductivity. The electronic mobility is activated with a small polaron hopping energy similar to that of bulk cerium oxide (∼0.5 eV). The oxygen vacancy migration energy is ∼1.6 eV, substantially larger than in bulk ceria, but not inconsistent with grain boundary values or bulk values at low temperatures (due to vacany-dopant association). The enthalpy of reduction of nanoceria is 1.84 eV, less than half of the corresponding value in bulk ceria. Nanoceria therefore has much larger defect populations (vacancies, electrons) if undoped and electron populations if doped (i.e., with trivalent cations). These results are discussed in terms of the role played by the high surface/grain boundary areas in the defect/transport properties of nanocrystalline oxides.