Adsorption of Au and Pd atoms on thin SiO2 films:: The role of atomic structure

The low-temperature deposition of Au and Pd atoms on thin SiO2 films grown on Mo(112) has been studied by scanning tunneling microscopy (STM), infrared reflection absorption spectroscopy, and CO adsorption. On the monolayer films, gold forms three-dimensional nanoparticles at temperatures as low as 10 K and does not affect the phonon spectra of the silica films. On the other hand, palladium exhibits structure-dependent behavior. Pd deposition leads to a red-shift of the phonon frequency of the monolayer and not of the multilayer films. The amount of CO that may adsorb on the Pd particles formed on the O-poor monolayer films is much smaller than that on the O-rich films (with an additional O-layer on the Mo surface) and multilayer films. These experimental findings have been rationalized on the basis of density functional theory calculations showing that the Pd atoms penetrate into the hexagonal rings of the silica network and bind at the SiO2/Mo interface, which in turn depends on the atomic structure of the film. This bonding is very strong on the O-poor films so that Pd is unable to bind CO, while on the O-rich films CO may even pull out the Pd atoms from the interface thus promoting aggregation and particle formation. The Pd migration into the film has been directly supported by low-temperature STM images.