The underpotential deposition (UPD) of Ag on Pt( 11 1) surfaces has been investigated in aqueous sulfuric acid solutions. Two sets of well defined UPD peaks in the cyclic voltammogram have been observed on a well ordered Pt( 11 1) surface. The total charge consumed for the first and second UPD processes has been found to be nearly equal to 240*20 pC cm-2, suggesting that the first and second adlayers of Ag are close-packed monolayers. The atomic structures of the first, second and bulk deposited Ag adlayers have been resolved in solutions using an electrochemical scanning tunnelling microscope. The observed interatomic distances were all nearly the same, within experimental error, indicating that the Ag layers are grown rotationally commensurate with respect to the underlying lattice. The first and second layers have been found to grow layer-by-layer in the UPD on Pt in sulfuric acid solutions. It has been demonstrated that the bulk deposition of Ag also occurs epitaxially.

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