# “Ultrasmall” ZrO2 Nanoparticles: Disentangling Core and Surface Contributions to Structural and Electronic Properties through First-Principles Modeling

## TL;DR

This study uses advanced modeling to understand how the structure and electronic properties of zirconia nanoparticles change with size and surface passivation.

## Contribution

The paper introduces a method to distinguish core and surface contributions in ultrasmall ZrO2 nanoparticles using first-principles calculations.

## Key findings

- Zr–O bond lengths vary from core to surface, enabling separation of core and surface regions.
- Core atoms in larger NPs resemble the cubic phase of zirconia, while surface atoms resemble the monoclinic phase.
- Quantum confinement effects in ZrO2 NPs are moderate and deviate from theoretical predictions.

## Abstract

We resort to first-principles molecular dynamics (FPMD)
and density
functional theory (DFT) calculations at the PBE and PBE0 levels of
theory to examine the structure, stability, and electronic properties
of zirconia nanoparticles (NPs) with diameters ranging from 0.9 to
2.0 nm. A procedure based on the use of water molecules and an appropriate
MD thermal annealing cycle is developed to generate [ZrO2]
n
models with different sizes (n = 14, 16, 43, 80, and 141) and different surface passivation
states. It is shown that the rate of passivation has a significant
influence on the NP structure and that NP models corresponding to
saturated passivation exhibit the best structural characteristics,
featuring close agreement with experimental atomic pair distribution
functions (PDFs). It is also found that the Zr–O bond length
varies as a function of the position of Zr and O atoms from the core
to the surface of NPs, providing a descriptor capable of separating
core and surface regions in ZrO2 NPs. A core–shell
structure has been demonstrated for NP models as small as 1.3 nm,
while for even smaller NPs, no separation between the core and shell
is possible. For the largest NP models, the core atoms show local
environments closer to the cubic phase of zirconia, while the local
structure of atoms close to the surface shows a large similarity with
the monoclinic phase. Finally, the study of electronic properties
has shown that ZrO2 NPs exhibit very moderate quantum confinement
effects. Moreover, the evolution of the band gap as a function of
size does not correspond well with the d
–2 trend expected from the effective mass approximation model. These
differences can only be partly attributed to the shell atoms, which
induce a slight decrease in the band gap compared to the contribution
of the core atoms.