  Understanding the complex chemistry of functional nanomaterials is of
fundamental importance. Controlled synthesis and characterization at the atomic
level is essential to gain deeper insight into the unique chemical reactivity
exhibited by many nanomaterials. Cerium oxide nanoparticles have many
industrial and commercial applications,resulting from very strong catalytic,
pro-and anti-oxidant activity. However, the identity of the active species and
the chemical mechanisms imparted by nanoceria remain elusive, impeding the
further development of new applications. Here, we explore the behavior of
cerium oxide nanoparticles of different sizes at different temperatures and
trace the electronic structure changes by state-of-the-art soft and hard X-ray
experiments combined with computational methods. We confirm the absence of the
Ce(III) oxidation state at the surface of CeO2nanoparticles, even for particles
as small as 2 nm. Synchrotron X-ray absorption experiments at Ce L3and M5edges,
combined with X-ray diffraction (XRD), high-resolution transmission electron
microscopy (HRTEM) and small angle X-ray scattering (SAXS) and theoretical
calculations demonstrate that in addition to the nanoceria charge stability,
the formation of hydroxyl groups at the surface profoundly affects the chemical
performance of these nanomaterials
