SummaryNon-stoichiometric ceria nanoparticles (NPs) were obtained by a gas aggregation source with a magnetron and were mass-selected with a quadrupole mass filter. By varying magnetron power, Ar gas flow, and the length of the aggregation tube, NPs with an average diameter of 6, 9, and 14 nm were synthesized and deposited onto a substrate, thus obtaining NP films. The morphology of the films was studied with scanning electron microscopy, while high resolution transmission electron microscopy was used to gain a deeper insight into the atomic structure of individual NPs. By using X-ray photoelectron spectroscopy we analyzed the degree of reduction of the NPs of different diameters, before and after thermal treatments in vacuum (reduction cycle) and in O2 atmosphere (oxidation cycle) at different temperatures. From this analysis we inferred that the size is an important parameter only at intermediate temperatures. As a comparison, we evaluated the reducibility of an ultra-thin ceria film with the same surface to volume ratio as the 9 nm diameter NPs film, observing that NPs are more reducible than the ceria film.
The functionality
of cerium oxide, and in particular its reactivity,
can be significantly altered by the addition of diluted cationic species
with different electronic properties as compared to cerium. We investigate
the modifications induced by Ag and Cu as modifier cations in cerium
oxide ultrathin epitaxial films. The reducibility is assessed by following
the modifications of the oxidation state of surface Ce ions by X-ray
photoemission spectroscopy, during thermal treatments in ultrahigh
vacuum and oxygen partial pressure. A significantly higher reducibility
is observed in Ag- and Cu-modified films as compared to pure CeO2 films of the same thickness. The thermal stability of the
cation modifier concentration and the changes of the surface structure
with the reducing treatments are also discussed. The modifications
induced in the material are explained by comparison with density functional
theory calculations, which indicate that the oxygen vacancy formation
energy is significantly modified by the addition of Ag or Cu in the
cerium oxide matrix. The obtained results are of help in view of a
rational design of catalysts with optimized performance.
We investigated the evolution of the electronic structure of cerium oxide ultrathin epitaxial films during reduction and oxidation processes using resonant inelastic X-ray scattering at the Ce L3 absorption edge, a technique sensitive to the electronic configurations at the 4f levels and in the 5d band thanks to its high energy resolution. We used thermal treatments in high vacuum and in oxygen partial pressure to induce a controlled and reversible degree of reduction in cerium oxide ultrathin epitaxial films of different thicknesses. Two dominant spectral components contribute to the measured spectra at the different degrees of oxidation/reduction. In ultrathin films a modification of the electronic properties associated with platinum substrate proximity and with dimensionality is identified. The different electronic properties induce a higher reducibility in ultrathin films, ascribed to a decrease of the surface oxygen vacancy formation energy.
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