Strongly Bound Surface Water Affects the Shape Evolution of Cerium Oxide Nanoparticles

The surface structure and composition of functional materials are well known to be critically important factors controlling the surface reactivity. However, when doped the surface composition will change and the challenge is to identify its impact on important surface processes and nanoparticle morphologies. We have begun to address this by using a combination of density functional theory and potential-based methods to investigate the effect of surface dopants on water adsorption and morphology of the technologically important material, CeO₂, which finds application as electrolyte in SOFCs, catalyst in soot combustion, and enzyme mimetic agent in biomedicine. We show that by mapping CeO₂ surface phase diagrams we can predict nanoparticle morphologies as a function of dopant, temperature and water partial pressure. Our results show that a low temperature, un-doped CeO₂ nanocubes with active {100} surface sites are thermodynamically stable. But at the typical high temperature operating conditions favors polyhedra where {100} surfaces are replaced by less active {111} surfaces by surface ion migration. However, on doping with trivalent cations, such as Gd³⁺, will increase binding of water on the {100} surfaces and hence act to preserve the cuboidal architecture, by capping the active surfaces. As surfaces tend to be decorated by impurities and dopants it is clear that their role should receive more attention and the approach we describe can be routinely applied to nanomaterials, morphologies and associated active/inactive surfaces.