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Three dimensional valency mapping in CeO$_{2-x}$ nanocrystals

Using electron tomography in combination with spatially resolved electron energy-loss spectroscopy at high energy resolution, we are able to map the valency of the Ce ions in CeO$_{2-x}$ nanocrystals. Our three-dimensional results show a clear facet-dependent reduction shell at the surface of ceria nanoparticles; {111} surface facets show a low surface reduction, whereas at {001} surface facets the cerium ions are more likely to be reduced over a larger surface shell. The novelty of this generic tomographic technique is that it allows a full three dimensional datacube to be reconstructed, containing a full electron energy-loss spectrum in each voxel. The ability to extract an electron energy-loss spectrum in each point of a reconstructed datacube, enables the three-dimensional investigation of a plethora of material-specific physical properties such as valency, chemical composition, oxygen coordination or bond lengths. These experiments will trigger the synthesis of nanomaterials with improved properties and the design of nanostructures with novel functionalities.