Interaction of Au with Thin ZrO₂ Films: Influence of ZrO₂ Morphology on the Adsorption and Thermal Stability of Au Nanoparticles

The model catalysts of ZrO₂-supported Au nanoparticles have been prepared by deposition of Au atoms onto the surfaces of thin ZrO₂ films with different morphologies. The adsorption and thermal stability of Au nanoparticles on thin ZrO₂ films have been investigated using synchrotron radiation photoemission spectroscopy (SRPES) and X-ray photoelectron spectroscopy (XPS). The thin ZrO₂ films were prepared by two different methods, giving rise to different morphologies. The first method utilized wet chemical impregnation to synthesize the thin ZrO₂ film through the procedure of first spin-coating a zirconium ethoxide (Zr­(OC₂H₅)₄) precursor onto a SiO₂/Si­(100) substrate at room temperature followed by calcination at 773 K for 12 h. Scanning electron microscopy (SEM) investigations indicate that highly porous “sponge-like nanostructures” were obtained in this case. The second method was epitaxial growth of a ZrO₂(111) film through vacuum evaporation of Zr metal onto Pt(111) in 1 × 10^–6 Torr of oxygen at 550 K followed by annealing at 1000 K. The structural analysis with low energy electron diffraction (LEED) of this film exhibits good long-range ordering. It has been found that Au forms smaller particles on the porous ZrO₂ film as compared to those on the ordered ZrO₂(111) film at a given coverage. Thermal annealing experiments demonstrate that Au particles are more thermally stable on the porous ZrO₂ surface than on the ZrO₂(111) surface, although on both surfaces, Au particles experience significant sintering at elevated temperatures. In addition, by annealing the surfaces to 1100 K, Au particles desorb completely from ZrO₂(111) but not from porous ZrO₂. The enhanced thermal stability for Au on porous ZrO₂ can be attributed to the stronger interaction of the adsorbed Au with the defects and the hindered migration or coalescence resulting from the porous structures