IN-SITU ATOMIC-FORCE MICROSCOPE STUDY OF PB UNDERPOTENTIAL DEPOSITION ON AU(111) - STRUCTURAL-PROPERTIES OF THE CATALYTICALLY ACTIVE PHASE

Atomic resolution atomic force microscope (AFM) images of Pb underpotential deposition on Au(111) electrodes are presented. Pb grows on the Au(111) surface by forming islands which are initially one Pb adatom high (0.30 nm) but which rapidly grow in height to 0.53 nm through association of the Pb with solvent or hydroxide. The Pb aggregates have a hexagonal close-packed atomic structure exhibiting an atom-atom spacing of 0.35 +/- 0.03 nm. The islands can be easily swept away by the AFM tip when the tip sample force is set at 80 nN, but Pb deposited at step edges is not swept away under these conditions. This suggests that the Pb (island)-Au interaction is relatively weak. The full monolayer of Pb adatoms form a hexagonal close packed layer on Au(111) surfaces with an 0.35 +/- 0.02 nm atom-atom spacing, the same as found atop the islands. The Pb overlayer is rotated 4.5-degrees relative to the Au(111) substrate and a Moire pattern arising from interference between the Pb overlayer and the substrate is observed. The highest level of catalytic activity for peroxide electroreduction is observed when the surface is maximally covered by the Pb islands.