Correlation of nonstoichiometry and electrical conductivity of thin-film Ce1-xZrxO2-δ solid solutions
Given the wide range of oxygen nonstoichiometry δ of ceria-based materials (CeO2-δ), they are highly suited for application in three-way catalytic converters for exhaust gas aftertreatment of combustion engines [1, 2]. Addition of zirconia to ceria (Ce1-xZrxO2-δ; CZO) enables even higher reduction efficiency [2, 3]. Furthermore, due to the large active surface, in either nanostructured bulk or thin-film CZO the oxygen-storage capacity is expected to greatly enhance. The investigation of such films or nanostructures would, hence, provide deeper understanding of the defect chemistry and reduction kinetics in the CZO or even give new insights into the nonstoichiometry-related effects in them.
The electrical conductivity σ of Ce1-xZrxO2-δ (x = 0, 0.2, 0.33, 0.5, 0.67 and 1) is measured at 600-900 °C and oxygen partial pressures pO2 of 1e-24 to 0.2 bar. The conductivities are determined by 4-probe DC resistance measurements and impedance spectroscopy, and their values for bulk and thin film CZO are compared. The nonstoichiometry of Ce1-xZrxO2-δ at variable pO2 is determined by the resonant nanobalance method, in which the mass changes of CZO films deposited onto high-temperature piezoelectric resonator are proportional to the shift in resonance frequency of the latter [4, 5]. High values and evident dependence of electrical conductivity on pO2 (σ changing from ~1e-6 to ~1e-2 S/cm at 600 °C and from ~1e-3 to ~10 S/cm at 900 °C with decreasing pO2) were observed for pure CeO2, whereas for ZrO2 the σ was almost independent on pO2 and increased with temperature from ~1e-6 to ~1e-5 S/cm. At high pO2 (1e-5 – 0.2 bar) the oxide solid solutions exhibited higher electrical conductivities than pure CeO2 or ZrO2, but the decrease of electrical conductivity with increasing Zr content and its saturation at oxygen partial pressures below 1e-10 bar were observed. Likewise, a strong dependence of δ on pO2 was observed for CZO thin films below this pressure. However, the behavior of electrical conductivity of bulk Ce1-xZrxO2-δ only approximately correlates with the deviation of oxygen non-stoichiometry derived for CZO films. At 900 °C and pO2 of 1e-14 bar, the maximum electrical conductivity was attained for bulk CZO with x = 0.2-0.5, whereas maximum δ = 0.08-0.09 was observed for ceria-zirconia thin films with 50-67% zirconium. The analysis and tentative explanation of the observed phenomena in ceria-zirconia solid solutions are provided.