Stabilization Effect of Surface Impurities on the Structure of Ultrasmall ZrO2 Nanoparticles: An Ab-Initio Study

Abstract

Real nanoparticles can hardly be considered as isolated objects in vacuum with pure stoichiometric chemical composition. It can be expected that some surface stabilizing mechanisms may occur in real systems, and they could induce more or less pronounced effects on the structure of nanoparticles. In this work, we analyzed the stabilizing effect of surface impurities on the structure of ultrasmall ZrO2 nanoparticles using ab initio simulation methods. The study was performed on ZrO2 cluster models, with a diameter around 1.25 nm, using the Car–Parrinello molecular dynamics method combined with standard structural optimization methods. After the study of the naked cluster, the stabilizing effect was investigated by adding chemisorbed water molecules at the particle surface, with a covering degree ranging from 15% to 100%. The results clearly indicated that energy minimizations leaded invariably to highly disordered structures in the case of naked nanoparticles. The structure of the models got more ordered only with increasing water coverage. The comparison, via the pair distribution functions (PDF), to the experimental data obtained from a sample of nanoparticles of comparable size showed a good agreement for a high (but not total) water coverage, indicating that a surface stabilizing effect is a sound hypothesis to explain the experimental results. Surface stabilizing effects play a crucial role in the case of ZrO2, in contrast with recent calculations performed on other chemical systems (CeO2 or TiO2, for example); thus, for ZrO2 a stabilizing mechanism, such as the one proposed in this paper, must be inevitably included in the model, and this fact markedly complicates the problem of solving the ZrO2 nanocrystal structure.