Interaction between Cu Nanoparticles and CeO₂(111) Film Surfaces

The morphology and electronic structure of vapor-deposited Cu nanoparticles on a fully oxidized CeO 2 (111) surface at 300 K have been investigated by X-ray photoelectron spectroscopy (XPS), synchrotron radiation photoemission spectroscopy (SRPES), and scanning tunneling microscopy (STM). Cu nanoparticles were found dispersed on the CeO 2 surface with a tendency to nucleate at step edges at very low coverage. The Cu atoms are oxidized to Cu + upon deposition on CeO 2 at 300 K, and this is accompanied by the partial reduction of the CeO 2 thin film, as evidenced by the Cu LVV and Ce 3d spectra. Deposition of Cu above 0.5 monolayer (ML) gives rise to neutral Cu atoms and to the growth of two-layer Cu particles. These results indicate the presence of electronic metal–support interaction at the Cu/CeO 2 interface, which involves electron transfer across the metal/support interface. Combining the SRPES and STM results, this charge transfer from Cu to CeO 2 was quantified. It was found that the average charge transfer is largest at 0.22 ML (particle containing about 94 Cu atoms), where approximately one electron is transferred per Cu atom from Cu to CeO 2, which agrees well with the Cu LVV result. Such quantitative insights will help to characterize the chemical state of active species and improve the understanding of particle size effects and the electronic metal–support interaction.