Regular arrays of palladium and palladium-gold clusters supported on ultrathin alumina films: stability under oxygen

Grazing Incidence Small Angle X–ray Scattering (GISAXS) studies were conducted to determine the effect of oxygen on regular arrays of mono (Pd)– and bimetallic (Pd–Au) clusters. Unlike bimetallic Pd–Au particles, pure palladium clusters are not stable at a partial oxygen pressure of 10−6 mbar. A comparison of the experimental data with Fourier transforms of known distributions reveals that Pd clusters move and that their aspect ratio increases. These results are explained by the adsorption of oxygen that changes the equilibrium shape of the Pd particles and reduces the adhesion energy. In the case of bimetallic Pd–Au clusters, the presence of Au on the surface of the particle prevents adsorption of oxygen.