From Molecular Oxo-Hydroxo Ce Clusters to Crystalline CeO₂

Many studies report the synthesis and characterizations of CeO2 nanoparticles (NPs) due to their applications in catalysis, energy storage, or biomedical fields. In this study, we report a comprehensive interpretation of X-ray measurements on a series of oxo-hydroxo polynuclear Ce complexes whose atomic structures are resolved. The set of investigated samples represent the formation of CeO2 small nanoparticles from a few Ce ions. Such clusters might serve as good building blocks in nanoscale architectures, and many clusters together may perform as highly active small particles. Therefore, the set of Ce clusters with growing size (from 0.6 to 1.2 nm) and nuclearity (from 6 to 38 Ce atoms) were synthesized and characterized by single-crystal X-ray diffraction (XRD), high-energy X-ray scattering (HEXS), and high-energy-resolution fluorescence detection (HERFD) X-ray absorption spectroscopy (XAS) methods and compared to larger CeO2 NPs and bulk CeO2. Methods reveal consistent trends as the size of the system grows from molecular Ce-{n} 2, 6, 24, 38, and 40 clusters to bulk CeO2. HEXS reveals a broadening in distribution for the short Ce–O bonds for the small clusters. Concomitantly, the HERFD-XAS performed at the Ce LIII edge indicates a gradual splitting of the Ce 5d states as the core of the complexes becomes more CeO2-like. The experimental observations have been supported by electronic structure calculations, based on the crystallographic determination of the cluster structures. Theoretical simulations allow us to isolate the structural and electronic properties of individual Ce sites within clusters and mark the great difference between surface and core Ce atoms. It also shows how a combination of simulations from different sites results in the accurate reproduction of the corresponding experimental data. This approach based on the experimentally determined atomic coordinates of clusters was then successfully extended to model Ce LIII edge HERFD-XAS spectra for larger CeO2 NPs. By linking the atomic and electronic structures of Ce polynuclear complexes, CeO2 nanoparticles, and bulk CeO2, this work extends the fundamental knowledge of Ce oxide nanomaterials and supports a better understanding and predictability of their electronic structure.