  Grain boundaries (GBs) can be used as traps for solute atoms and defects, and
the interaction between segregants and GBs is crucial for understanding the
properties of nanocrystalline materials. In this study, we have systematically
investigated the tendency of Pt to segregate as well as the interaction between
Pt and oxygen vacancies at the sigma 3/(111) GB of ceria (CeO2). The Pt atom
has a stronger tendency to segregate to the sigma 3/(111) GB than to the (111)
and (110) free-standing surfaces, but the tendency is weaker than to the (100)
free-standing surface. Mechanic contributions (lattice distortion) play a
dominant role in the strong tendency of Pt to segregate. At the
Pt-segregated-GB (Pt@GB), oxygen vacancies prefer to form spontaneously near Pt
in the GB region. However, at the pristine GB (no Pt and no vacancies), oxygen
vacancies form only under O-poor conditions. Thus, Pt segregation to the GB
promotes the formation of oxygen vacancies, and their strong interactions
enhance the interfacial cohesion. We propose that GBs fabricated close to the
surfaces of nanocrystalline ceria can trap Pt from the bulk or other types of
surface, resulting in the suppression of the accumulation of Pt on the surface
under redox reactions, especially under O-poor conditions.
