Unravelling the charge state of gold on partially reduced ceria thin films

The charge state of vapor-deposited Au nanoparticles on partially reduced CeO_x(111) thin films (1.8 ≤ x < 2) at 300 K was systematically studied by synchrotron radiation photoemission spectroscopy (SRPES), resonance photoemission spectroscopy (RPES), work function measurements, and infrared reflection absorption spectroscopy (IRAS). The results show that Au grows as three-dimensional (3D) particles on the well-ordered CeO_1.85(111) surface. A pronounced decrease in the resonance enhancement ratio and a simultaneous increase in the work function at low Au coverages (<0.15 monolayer (ML)) indicate substantial electron transfer from the ceria support to Au, forming negatively charged Au^δ- species. As Au coverage exceeds 0.15 ML, both the Ce³⁺ concentration and work function stabilize, revealing that further deposited Au remains in the metallic state. Complementary IRAS analyses confirm a redshift in CO stretching frequencies for Au/CeO_x compared with Au/CeO₂, consistent with enhanced electron back-donation from electron-rich Au nanoparticles. These findings provide direct experimental evidence that Au nanoparticles supported on reduced ceria initially acquire negative charge via interfacial electron transfer before transitioning to a metallic state with increasing coverage.