MODEL SYSTEMS IN ELECTROCATALYSIS - ELECTRONIC AND STRUCTURAL CHARACTERIZATION OF VAPOR-DEPOSITED PLATINUM ON THE BASAL-PLANE OF HIGHLY ORIENTED PYROLYTIC-GRAPHITE

Platinum vapor deposited at submonolayer coverages on the basal plane (bp) of highly oriented pyrolytic graphite (HOPG) in ultrahigh vacuum (UHV) was characterized in situ by X-ray photoelectron spectroscopy (X PS) and in air by scanning tunneling microscopy (STM). The effect of d efect sites on the electronic and morphological nature of these deposi ts was investigated by first sputtering the HOPG(bp) with Ar+, while b locking a section of the specimen with a mask and then exposing the en tire surface to Pt vapor. An analysis of the XPS spectra indicated tha t in the Ar+-sputtered region, the coverage, normalized by the Pt atom flux, was about 50% larger and the Pt binding energy ca. 0.2 eV highe r, i.e. slightly more oxidized, than in the pristine region. STM image s revealed discrete cluster-type features a few nm in average diameter and one to two atoms high, distributed fairly evenly within each regi on. The total amount of Pt contained within these clusters, as estimat ed from these images, however, could not account for more than 1-10% o f that detected by XPS. The density of clusters was larger and their a verage size somewhat smaller in the sputtered, compared to the pristin e regions, indicating that an increase in the density of defect sites promotes the growth of a larger number of clusters, albeit of smaller size.