EARLY STAGES OF PLATINUM ELECTRODEPOSITION ON HIGHLY ORIENTED PYROLITIC GRAPHITE - SCANNING-TUNNELING-MICROSCOPY IMAGING AND REACTION PATHWAY

The early stages of Pt electrodcposition (0.675-0.620 V vs RHE) on hig hly oriented pyrolytic graphite (HOPG) from chloroplatinic acid at 25- degrees-C have been studied by ex-situ STM and SEM imaging complemente d with electrochemical data. Nucleation and 3D growth of Pt initiate a t HOPG surface defects. Large Pt agglomerates containing flat crystall ites with well-defined geometries are found around HOPG steps. Pt crys tallites formed by 1-2 nm size clusters become more compact as the ele ctrodeposition potential is shifted negatively or the Pt electrodeposi ted charge is increased. High-resolution STM imaging reveals large unc overed HOPG areas with the nearest-neighbor C-C distance d = 0.24 +/- 0.02 nm and flat hexagonal Pt crystallites. Electrochemical data combi ned with STM imaging can be interpreted in terms of a diffusion-contro lled Pt(IV) to PT(II) reaction at HOPG and a surface reaction leading to Pt(0) at HOPG defects.