GAS-INDUCED RESTRUCTURING OF PALLADIUM MODEL CATALYSTS STUDIED WITH ATOMIC FORCE MICROSCOPY

The structure of thin Pd films evaporated onto planar SiO2 substrates changes dramatically during oxygen/hydrogen exposures in ultrahigh vacuum. In this work we have used an atomic force microscope (AFM), operated in the attractive mode, to obtain the three-dimensional morphology of the Pd surface for different film thicknesses and treatments, and compared the data with transmission electron microscopy (TEM) micrographs. During restructuring, a 100-angstrom film changes from being a smooth continuous film with cracks into metal clusters dispersed on the SiO2 support. In the 5-angstrom case the metal films are already well dispersed as fabricated. Here the gas exposure instead results in a clustering effect resulting in larger particles. The AFM gives results which are consistent with TEM micrographs but also gives additional information on metal particle shape which can lead to a further understanding of the restructuring process.