Underpotential deposition of Cu on iodine-modified Au(111):: an in situ scanning tunneling microscopy study

The electrochemical deposition of Cu on iodine-modified Au(111) surfaces has been investigated by in situ electrochemical scanning tunneling microscopy (ECSTM) and cyclic voltammetry (CV) in sulfuric acid solutions. In situ ECSTM studies reveal different iodine adlayer structures before and during the process of copper underpotential deposition (UPD). At the beginning of the cathodic scan and for potentials higher than the onset of UPD a c(p x root 3R-30 degrees) iodine structure is observed on wide terraces. For lower potentials this iodine structure transforms to a more compact (3 x 3) structure characterized by two different structural variations (symmetric and asymmetric) sometimes observed coexisting in the same terrace. Charge transfer analysis from CV measurements reveals that the amount of copper deposited at these potentials is not sufficient to account for this structure in the framework of a hard-ball structural model. During the UPD process itself other iodine structures are also observed as a function of copper deposition, together with an additional compression of the iodine adlayer associated with the formation of a Cul bilayer, in agreement with previously reported X-ray diffraction data. At the end of the UPD process a Cu(1 x 1) monolayer is formed with a lattice parameter equal to that of Au(111). The same course of structural changes was also observed during the anodic scan where stripping of the copper layer takes place, returning to the initial iodine c(p x root 3R-30 degrees) structure. Our results strongly suggest that the iodine adlayer is constantly present as the top layer during the process of electrodeposition and stripping of Cu with no noticeable loss of iodine in the process. The observed structures are discussed in terms of iodine-copper interactions. (C) 2001 Elsevier Science B.V. All rights reserved.