ELECTROCHEMICAL AND ULTRA-HIGH-VACUUM CHARACTERIZATION OF RHODIUM ON PT(111) - A TEMPERATURE-DEPENDENT GROWTH MODE

Using cyclic voltammetry as a probe of surface morphology combined with AES and LEED, a detailed study is presented of the growth of well-defined rhodium films on Pt(111). Deviations from strictly layer-by-layer growth, as predicted by theoretical plots of the rhodium 302 eV AES peak-to-peak intensity versus time, were elucidated by voltammetry. It was shown that the structure of the rhodium film was temperature dependent. For T less than or equal to 400 K,''pseudo''-Frank-van der Merwe behaviour is observed whereby the second rhodium monolayer begins forming prior to the completion of the first with both layers giving rise to pseudomorphic (1 x 1) islands. At higher substrate temperatures, the ratio of the amount of second layer to first layer rhodium (for a fixed exposure) increases and at the highest temperatures investigated, evidence for the existence of bulk rhodium microcrystallites was deduced from LEED and voltammetry. The behaviour of rhodium contrasts significantly with palladium on Pt(111) which previously was shown to display good layer-by-layer growth irrespective of temperature or coverage, in agreement with expectations based on lattice mis-match and surface energy criteria. The differences between rhodium and palladium may be understood in terms of the increased strain associated with rhodium films and more especially, the larger surface energy of rhodium as compared to platinum.