(21a) Surface Enrichment and Step Structure On Yttria-Stabilized Zirconia: A Combined Monte Carlo and DFT Study

Yttria-stabilized zirconia (YSZ) is a mixed oxide with high oxygen ion conductivity due to the presence of Y³⁺ induced oxygen defects within the cubic phase of zirconia. The high oxygen conductivity of YSZ facilitates its application in a variety of applications such as oxygen sensors, solid oxide fuel cell (SOFC) membranes, and heterogeneous catalyst supports. The surface structure of YSZ influences its performance in each of these applications, motivating characterization of stable YSZ surfaces at the atomistic scale. Imaging of the YSZ(111) surface reveals a high concentration of monatomic height step edges, formed upon Ar-ion sputtering and vacuum annealing. To investigate surface segregation in yttria-stabilized zirconia (YSZ), DFT energies describing surface energy as a function of yttrium lattice position were used to parameterize a reactive-force field (ReaxFF). We used ReaxFF to perform Monte Carlo (MC) simulated annealing to sample structural configurations of flat YSZ (111) and vicinal YSZ (111) stepped surfaces. We evaluated yttrium surface segregation, oxygen vacancy position, and surface step composition for flat and stepped YSZ surfaces. It is thermodynamically favorable for yttrium atoms to segregate to the surface of YSZ, and specifically to step edge sites. Surface saturation of yttrium occurs at approximately 40% (40:60 Y:Zr ratio) whereas yttrium concentration at the step edge does not approach a saturation value, suggesting that steps on the YSZ surface are mainly yttria-terminated. It is thermodynamically favorable for oxygen vacancies to occupy positions in the subsurface layer of YSZ, and a higher fraction of vacancies occupy positions NN to Y than NN to Zr. Yttrium segregation to step edges on the YSZ surface does not lower the surface formation energy of the stepped surface below that of the flat (111) termination, and the formation of surface steps of the specific geometry considered herein is not thermodynamically favorable on pure zirconia or YSZ.