Preferential location of zirconium dopants in cerium dioxide nanoparticles and the effects of doping on their reducibility: a DFT study

Abstract

Structural properties and reducibility of zirconium-doped cerium dioxide systems were studied using periodic plane-wave calculations based on density functional theory. A systematic analysis of the results for nanoparticles of two sizes, Ce_40−nZr_nO₈₀ ∼ 1.5 nm large and Ce_140−nZr_nO₂₈₀ ∼ 2.4 nm large, in comparison with slab model data for Ce_1−xZr_xO₂(111) surface has been performed focusing on specific nanoscale effects. Several loadings of Zr dopants ranging from 0.7 to 50 atomic metal percent have been considered. Subsurface cationic sites of ceria are calculated to be energetically most favourable for doping Zr⁴⁺ ions in all models. The system stability with several zirconium ions is defined by the relative stability of the occupied individual Zr⁴⁺ positions when only one zirconium ion is present. Data for the Ce₇₀Zr₇₀O₂₈₀ nanoparticle with an equal number of Ce⁴⁺ and Zr⁴⁺ cations reveal that atomic orderings of neither separated oxide (Janus-type) particles nor randomly intermixed ones are more stable than the distribution of Zr atoms occupying all cationic positions inside the nanoparticle to minimize the presence of surface zirconium. The basicity of surface oxygen centers in nanoparticles is predicted to be decreased when Zr dopants are located in surface positions. The presence of Zr⁴⁺ dopants in CeO₂ systems can notably lower the oxygen vacancy formation energy and shows interesting peculiarities at higher Zr loadings. Among them is the higher stability of inner oxygen vacancies in Zr-containing nanoparticles and enhanced oxygen mobility beneficial for application in catalysis and as a solid electrolyte with oxygen ions as charge carriers. Similar to pure ceria, Zr-doped ceria nanoparticles exhibit notably higher reducibility than the corresponding extended systems.