Structure and Properties of Zirconia Nanoparticles from Density Functional Theory Calculations

The structure, stability, and electronic properties of a series of zirconia nanoparticles between 1.5 and 2 nm in size, (ZrO2 +/- x)(n) within the n = 13 to n = 85 range, have been investigated through density functional theory (DFT) based calculations. On the methodological side we compare results obtained with standard DFT functionals with the DFT+U approach and with hybrid functionals. As representative models, octahedral and truncated octahedral morphologies have been considered for the zirconia nanoparticles. Partly truncated octahedral nanoparticles with ZrO2 stoichiometry display the highest stability. On the contrary, nanoparticles with octahedral and cuboctahedral (totally truncated octahedral) shapes are less stable due to oxygen deficiency or excess, respectively. We show that the calculated formation energies scale linearly with the average coordination number of the Zr ions and converge to the bulk value as the particle size increases. The formation energy of a neutral oxygen vacancy in the nanoparticles has also been investigated. In comparison to the ZrO2(101) surface of tetragonal zirconia, we found that three- and four-coordinated O atoms have similar formation energies. However, the two-coordinated O ions on the surface of the nanoparticles have considerably smaller formation energies. In this respect the effect of nanostructuring can be substantial for the reactivity of the material and its reducibility. The low-coordinated sites create defective states in the electronic structure and reduce the effective band gap, which can result in enhanced interaction with deposited species and modified photocatalytic activity.