  Developing characterization techniques and analysis methods adapted to the
investigation of nanoparticles (NPs) is of fundamental importance considering
the role of these materials in many fields of research. The study of actinide
based NPs, despite their environmental relevance, is still underdeveloped
compared to that of NPs based on stable and lighter elements. We present here
an investigation of ThO2 NPs performed with High-Energy Resolution Fluorescence
Detected (HERFD) X-ray Absorption Near-Edge Structure (XANES) and with ab
initio XANES simulations. The first post-edge feature of Th L3 edge HERFD XANES
disappears in small NPs and simulations considering non-relaxed structural
models reproduce the trends observed in experimental data. Inspection of the
simulations from Th atoms in the core and on the surface of the NP indeed
demonstrates that the the first post-edge feature is very sensitive to the
lowering of the number of coordinating atoms and therefore to the more exposed
Th atoms at the surface of the NP. The sensitivity of the L3 edge HERFD XANES
to low coordinated atoms at the surface stems from the hybridization of the
d-Density of States (DOS) of Th with both O and Th neighboring atoms. This may
be a common feature to other oxide systems that can be exploited to investigate
surface interactions.
