Oxygen vacancies in self-assemblies of ceria nanoparticles

Abstract

Cerium dioxide (CeO₂, ceria) nanoparticles possess size-dependent chemical properties, which may be very different from those of the bulk material. Agglomeration of such particles in nanoarchitectures may further significantly affect their properties. We computationally model the self-assembly of Ce_nO_2n particles (n = 38, 40, 80) – zero-dimensional (0D) structures – in one- and two-dimensional (1D and 2D) nanoarchitectures by employing density-functional methods. The electronic properties of 1D Ce₈₀O₁₆₀ and 2D Ce₄₀O₈₀ resemble those of larger 0D crystallites, Ce₁₄₀O₂₈₀, rather than those of their building blocks. These 0D, 1D and 2D nanostructures are employed to study the size dependence of the formation energy of an oxygen vacancy, E_f(O_vac), a central property in ceria chemistry. We rationalize within a common electronic structure framework the variations of the E_f(O_vac) values, which are computed for the Ce_nO_2n nanostructures with different sizes and dimensionalities. We identify: (i) the bandwidth of the unoccupied density of states projected onto the Ce 4f levels as an important factor, which controls E_f(O_vac); and (ii) the corner Ce atoms as the structural motif essential for a noticeable reduction of E_f(O_vac). These results help to understand the size dependent behaviour of E_f(O_vac) in nanostructured ceria.