Abstract
Actinide research at the nanoscale is gaining fundamental interest due to environmental and industrial issues. The knowledge of the local structure and speciation of actinide nanoparticles, which possibly exhibit specific physico-chemical properties in comparison to bulk materials, would help in a better and reliable description of their behaviour and reactivity. Herein, the synthesis and relevant characterization of PuO$_2$ and ThO$_2$ nanoparticles displayed as dispersed colloids, nanopowders or nanostructured oxide powders, allow to establish a clear relationship between the size of the nanocrystals composing these oxides and their corresponding An(IV) local structure investigated by EXAFS spectroscopy. Particularly, the probed An(IV) first oxygen shell evidences an analogous behaviour for both Pu and Th oxides. This observation suggests that the often observed and controversial splitting of the Pu-O shell on the Fourier transformed EXAFS signal of PuO$_2$ samples is attributed to a local structural disorder driven by a nanoparticle surface effect rather than to the presence of PuO$_{2+x}$ species.