Electrochemical adsorption of tin on Pt(111) was studied by cyclic voltammetry and scanning tunneling microscopy. At high coverages, Sn adlayers show only one broad oxidation peak at 0.8 V. With decreasing Sn coverage, a specific redox peak pair appears at around 0.61 V and achieves a maximum value of 76 mC cm À2 . Its decrease upon further dissolution of Sn is connected to the appearance of hydrogen and sulfate adsorption. The coexistance of the ' spike ' for sulfate adsorption with the Sn redox peaks at low coverages indicates the segregation of Sn into islands. Tin islands could only be observed by STM after coadsorption of CO or Cu-UPD. Besides decoration of steps, tin islands were randomly dispersed on the terraces of the Pt(111) electrode surface. After oxidation of CO the Sn domains are no longer visible at potentials in the hydrogen region. The reason is the high mobility of Sn on the Pt(111) surface.

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The effect of co-adsorption of CO on an underpotentially deposited (UPD) silver monolayer on a Pt(111) single crystal electrode in 0.05 M sulfuric acid is investigated for the first time by means of electrochemical scanning tunneling microscopy (EC-STM). Pure electrochemical experiments suggest that the co-adsorption of CO onto Pt single crystal electrodes previously modified by a monolayer of Ag, forces Ag atoms of the first UPD monolayer into a second adlayer. The present EC-STM studies reveal the formation of a large-area Ag network after the co-adsorption of CO. The resulting Ag nanostructures formed on wide Pt(111) terraces are approximately 0.5 nm high and 10 nm wide. The desorption of the newly formed second Ag adlayer, the oxidation of CO and the desorption of Ag atoms from the first adlayer are monitored by EC-STM and simultaneously detected in the corresponding CVs in three different oxidation peaks. EC-STM images recorded afterwards show the unchanged Pt surface. The presence of Ag on the surface leads to a downward shift of the onset of oxygen adsorption on the Pt(111) surface.