THE STRUCTURE OF PT-AGGREGATES ON A SUPPORTED THIN ALUMINUM-OXIDE FILM IN COMPARISON WITH UNSUPPORTED ALUMINA - A TRANSMISSION ELECTRON-MICROSCOPY STUDY

Platinum has been deposited from the vapor onto a sample consisting of a thin aluminum oxide him grown on a NiAl(110) substrate wedge. At th e tip of the wedge small gamma-alumina crystallites are formed which h ave also been platinum decorated. The size distribution of the Pt aggr egates on the thin aluminum oxide sim is almost identical to the one f ound for the support-free alumina crystallites. It is also identical t o the size distribution observed with STM under UHV conditions in earl ier experiments [Th. Bertrams et al., Surf Sci. 331-333 (1995) 1515]. The present structural analysis reveals that after PI decoration the s tructure of the substrate remains unchanged. The Pt forms flat, epitax ial islands on both substrates. The nearest neighbor distances within the platinum aggregates have been determined for the supported oxide a s a function of particle size. For large particles with radii greater than 20 Angstrom the nearest neighbor distance of bulk Pt is observed. In the case of the smallest particles with radii close to 10 Angstrom a contraction of up to 10% is detected. The contraction of the intera tomic distance appears not to be isotropic. The observed Moire pattern s comply with the assumption that the substrate lattice underneath the Pt aggregates does not change. (C) 1997 Elsevier Science B.V.