Manipulation of dopant distribution in yttrium-doped ceria particles.

Yttrium is one of the most widely used dopants (e.g., in yttrium-doped ceria (YDC) and yttria-stabilized zirconia) for improving performance of solid electrolytes utilized in solid oxide fuel cells. Oxygen ion conductivity is known to be a function of yttrium concentration in the electrolytes and thus it is important to further understand the factors that influence such concentration distribution. In this study, we examine the change in the distribution of yttrium concentration after annealing in YDC particles. The geometry and morphology of YDC particles, synthesized by spray pyrolysis, were characterized using transmission electron microscopy, and the distribution of yttrium was investigated by X-ray energy-dispersive spectroscopy (XEDS) considering electron interaction volume and geometry of the particle in the XEDS profile analysis. In addition, relative concentrations of Ce(lll) ions, which are thought to be proportional to the concentrations of the oxygen vacancies in the YDC particles, were examined using X-ray photoelectron spectroscopy to correlate with yttrium concentration distributions. It was found that in the as-prepared hollow spherical particles the concentration of yttrium increases linearly from the inner surface towards the outer surface. After annealing the distribution of yttrium becomes non-linear and the dopant is found to migrate to the inner and outer surfaces of the particle. The decrease of the concentration of Ce(lll) ions upon annealing followed by the formation of the dopant concentration gradient is likely due to the increase of the size of the crystallites in the particles.