Adsorption of an Au atom and dimer on a thin Al₂O₃/NiAl(100) film: Dependence on the thickness of the Al₂O₃ film

With calculations based on density-functional theory (DFT) we investigated the adsorption of a single Au atom and a dimer on thin Al₂O₃(001) films supported on NiAl(100). The interaction of the Au adsorbates with the surface was shown to depend on the thickness of the film. The adsorption energy for an Au atom on Al₂O₃(001)/NiAl(100) of film thickness ≤four atomic layers was significantly enhanced - over three times that on a bulk Al₂O₃(001) surface, and accompanied with a shortened Au-oxide bond and an uplifted Au-binding Al. The strong Au-surface interaction involved a decreased work function of Al₂O₃(001)/NiAl(100) and consequently drove charge to transfer from the substrate to the adsorbed Au atom; the charge was transferred from NiAl, through alumina, on monolayer Al₂O₃(001)/NiAl(100), but directly from alumina on thicker layers. For an Au dimer, both upright (end-on) and flat-lying (side-on) geometries existed. The flat-lying dimer was preferred on mono- and tri-layer alumina films, having a greater adsorption energy but a weakened Au-Au bond, whereas the upright geometry prevailed for films of other thickness, having a weaker adsorption energy and being less charged, similar to that on a bulk Al₂O₃(001) surface. The results imply an opportunity to control the properties and morphologies of metal clusters supported on an oxide film by tuning its thickness.