Photoabsorption of 1–2 nm molecular Ce-oxo nanoclusters <i>versus</i> ceria: intervalence charge transfer but no size effects

The absorption spectra of a series of Ce-oxo clusters (molecular nanoparticles) with sizes ranging from 3 to 100 Ce atoms per cluster are reported. The O 2p to Ce 4f charge-transfer absorption onset in these systems is unaffected by size and almost identical to that of 5 nm CeO_(2-<i>x</i>) particles and other Ce based materials such as the metal-organic framework Ce-UiO-66. This clearly demonstrates that in CeO₂ based systems, with highly localized 4f LUMO orbitals, quantum confinement effects are not influential on electronic structure. Importantly, this allows the use of ultrasmall ceria particles in (visible light) photochemical applications without detrimental band-gap enlarging size-effects. Changes in colour in CeO₂ materials and these Ce-oxo clusters are instead clearly attributed to the presence of (defective) surface sites. Surface Ce(iii) sites in mixed-valence clusters contribute a Ce(iii)/Ce(iv) intervalence charge transfer transition in the visible region, which affects their colour. Ce₂₄ clusters with a range of aliphatic and aromatic carboxylate ligands are studied, to compare the effect of surface ligands on the electronic structure. Ligand influence on the absorption spectra is only observed when highly conjugated (naphthyl) or strongly electron donating substituents (anisole or aniline) are present.