Scanning tunneling microscopy images of ruthenium submonolayers spontaneously deposited on a Pt(111) electrode

Scanning tunneling microscopy (STM) was used to examine spontaneously depos ited ruthenium adlayers on the well-defined Pt(111) electrode. Stable and S TM discernible structures were obtained after a brief cyclic voltammetric t reatment of the ruthenium deposit. As demonstrated previously, the electroc hemically stabilized Pt/Ru electrodes are active catalysts in methanol elec trooxidation. The STM data indicate that the deposit is arranged in nanomet er size islands of which the detailed structural characterization is presen ted. Maximum ruthenium coverage is no higher than 20%, which confirms our p revious results obtained by the use of Auger electron spectroscopy. While m ost of the islands are monatomic, a fraction of the islands, approximately 10% of the total ruthenium coverage, displays a second monolayer deposit ov er the first monolayer. This is a new discovery showing an unexpected tende ncy of the spontaneously deposited ruthenium at such a low coverage to nucl eate in a bilayer configuration. The hydrogen underpotential deposition pro cess does not affect the spatial distribution of the islands, but ruthenium is reductively removed from the surface under hydrogen evolution condition s.