Interaction between Cu Nanoparticles and CeO2(111) Film Surfaces

The morphology and electronic structure of vapor-deposited Cu nanoparticles on a fully oxidized CeO2(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 CeO2 surface with a tendency to nucleate at step edges at very low coverage. The Cu atoms are oxidized to Cu+ upon deposition on CeO2 at 300 K, and this is accompanied by the partial reduction of the CeO2 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/CeO2 interface, which involves electron transfer across the metal/support interface. Combining the SRPES and STM results, this charge transfer from Cu to CeO2 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 CeO2, 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.