Metal Adatoms and Clusters on Ultrathin Zirconia Films

Nucleation and growth of transition metals on zirconia has been studied by scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. Since STM requires electrical conductivity, ultrathin ZrO₂ films grown by oxidation of Pt₃Zr(0001) and Pd₃Zr(0001) were used as model systems. DFT studies were performed for single metal adatoms on supported ZrO₂ films as well as the (1̅11) surface of monoclinic ZrO₂. STM shows decreasing cluster size, indicative of increasing metal-oxide interaction, in the sequence Ag < Pd ≈ Au < Ni ≈ Fe. Ag and Pd nucleate mostly at steps and domain boundaries of ZrO₂/Pt₃Zr(0001) and form three-dimensional clusters. Deposition of low coverages of Ni and Fe at room temperature leads to a high density of few-atom clusters on the oxide terraces. Weak bonding of Ag to the oxide is demonstrated by removing Ag clusters with the STM tip. DFT calculations for single adatoms show that the metal-oxide interaction strength increases in the sequence Ag < Au < Pd < Ni on monoclinic ZrO₂, and Ag ≈ Au < Pd < Ni on the supported ultrathin ZrO₂ film. With the exception of Au, metal nucleation and growth on ultrathin zirconia films follow the usual rules: More reactive (more electropositive) metals result in a higher cluster density and wet the surface more strongly than more noble metals. These bind mainly to the oxygen anions of the oxide. Au is an exception because it can bind strongly to the Zr cations. Au diffusion may be impeded by changing its charge state between -1 and +1. We discuss differences between the supported ultrathin zirconia films and the surfaces of bulk ZrO₂, such as the possibility of charge transfer to the substrate of the films. Due to their large in-plane lattice constant and the variety of adsorption sites, ZrO₂{111} surfaces are more reactive than many other oxygen-terminated oxide surfaces.