Energetics of Cu Adsorption and Adhesion onto Reduced CeO2(111) Surfaces by Calorimetry

The morphology and interfacial energetics of vapor-deposited Cu on slightly reduced CeO2(111) surfaces at 300 K have been studied using single crystal adsorption calorimetry (SCAC), He+ low-energy ion scattering spectroscopy (ISS), X-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED). Copper grows as three-dimensional nanoparticles with a density of ∼1013 particles/cm2 on CeO2–x (111) (x = 0.05, 0.1, and 0.2). The initial heat of adsorption of Cu decreased with the extent of reduction, showing that stoichiometric ceria adsorbs Cu more strongly than oxygen vacancies. On CeO1.95(111), the heat dropped quickly with coverage in the first 0.1 ML, attributed to nucleation of Cu clusters on stoichiometric steps, followed by the Cu particles spreading onto less favorable sites (step vacancies and terraces). Above ∼0.1 ML (>0.8 nm in diameter), the Cu adsorption energies showed no variation with extent of ceria reduction: the heat of adsorption increased slowly with coverage (particle size) due to the formation of more Cu–Cu bonds per adatom as the size grows, finally approaching the heat of sublimation of bulk Cu by 3.5 ML (2.5 nm). The adhesion energy of Cu­(solid) to CeO1.95(111) was found to be 3.52 J/m2 for 2.2 nm diameter particles, decreasing slightly with the extent of reduction. The Ce 3d XPS line shape showed an increase in the Ce3+/Ce4+ ratio with Cu coverage, corresponding to donation of at most ∼0.17 and 0.06 electrons per Cu atom to CeO1.95(111) and CeO1.8(111), respectively.