COPPER ADLAYER FORMATION ON AU(111) FROM SULFURIC-ACID ELECTROLYTE

We have used in situ scanning tunneling microscopy to examine submonolayer Cu deposition from a sulfuric acid electrolyte on Au (111), sampling over large areas of the surface. We describe the growth of three distinct superlattice phases of Cu on the Au(111) surface before the onset of bulk deposition of Cu. In the earliest stages of deposition, during an electrochemical current peak, we observe the rapid growth of a low height phase, with an apparent height of 0.46 +/- 0.10 angstrom. During this time, a second, monolayer-height phase forms as small islands. We identify these phases as the (square-root 3 X square-roor 3) R30-degrees and (1 X 1) Cu superlattices, respectively. A third phase appears at about 175 mV (versus Cu/Cu+2), and grows from nucleation points at the top edges of atomic steps and around the small (1 X 1) Cu islands. This phase, which we believe is a (5 X 5) Cu adlayer, slowly supplants the (square-root 3 X square-root 3) R30-degrees layer.