  Intrinsic properties of a compound (e.g., electronic structure,
crystallographic structure, optical and magnetic properties) define notably its
chemical and physical behavior. In the case of nanomaterials, these fundamental
properties depend on the occurrence of quantum mechanical size effects and on
the considerable increase of the surface to bulk ratio. Here, we explore the
size dependence of both crystal and electronic properties of CeO2 nanoparticles
(NPs) with different sizes by state-of-the art spectroscopic techniques. X-ray
diffraction, X-ray photoelectron spectroscopy, and high-energy resolution
fluorescence-detection hard X-ray absorption near-edge structure (HERFD-XANES)
spectroscopy demonstrate that the as-synthesized NPs crystallize in the
fluorite structure and they are predominantly composed of CeIV ions. The strong
dependence of the lattice parameter with the NPs size was attributed to the
presence of adsorbed species at the NPs surface thanks to Fourier transform
infrared spectroscopy and thermogravimetric analysis measurements. In addition,
the size dependence of the t2g states in the Ce LIII XANES spectra was
experimentally observed by HERFD-XANES and confirmed by theoretical
calculations.
