Growth of Pt crystallites supported on gamma-Al2O3 studied by atomic forcemicroscopy

The growth behavior of Pt crystallites vacuum-deposited on flat gamma- and alpha -Al2O3 substrates was investigated by atomic force microscopy (AFM). In this study, we fabricated a thin layer of gamma -Al2O3 on a planar sapph ire (single crystalline alpha -Al2O3) substrate by Al+- and O+- ion implant ation and annealing at 900 degreesC. AFM observation revealed that gamma- a nd alpha -Al2O3 substrates have a negligible roughness compared with the si ze of the supported Pt crystallites deposited on them. The crystal structur e was identified as a gamma (111) plane for the gamma -Al2O3 substrate, but an alpha (0001) plane for the sapphire substrate by Rutherford backscatter ing, spectroscopy-channeling analysis (RBS-C) and X-ray diffraction analysi s (XRD). We then observed the surface morphology of Pt crystallites on gamm a- and alpha -Al2O3 substrates, which were fabricated by sputtering of a Pt target. The as-deposited Pt thin film changed into Pt crystallites dispers ed on the substrate with heat treatment at 800 degreesC in an oxidative atm osphere. It was found that the growth of Pt crystallites on gamma -Al2O3 wa s less, although the particle growth of Pt on alpha -Al2O3 was remarkable. The growth process of the Pt crystallites was also discussed along with the surface structure of the Al2O3 substrates. We attribute the inhibition of Pt crystallite growth on the gamma -Al2O3 substrate to the defective struct ure of,the gamma -Al2O3 compared with the stable structure of gamma -Al2O3.