Underpotential deposition of Cu on iodine-modi®ed Au1 1 1): an in situ scanning tunneling microscopy study A. Mart  õnez-Ruiz a,b , J. Valenzuela-Benavides c, * , L. Morales de la Garza c , N. Batina d a Facultad de Ciencias, Universidad Aut onoma de Baja California, A.P. 1820, Ensenada, BC 22800, Mexico b Centro de Investigaci on Cient  õ®ca y de Educaci on Superior de Ensenada, A.P. 2732, Ensenada, BC 22800, Mexico c Centro de Ciencias de la Materia Condensada, Universidad Nacional Aut onoma de M exico, A.P. 2681, Ensenada, BC 22800, Mexico d Universidad Aut onoma Metropolitana-Iztapalapa, A.P. 55-534, C.P. 09340, Mexico, DF, Mexico Received 12 May 2000; accepted for publication 7 December 2000 Abstract The electrochemical deposition of Cu on iodine-modi®ed Au1 1 1) surfaces has been investigated by in situ electro- chemical scanning tunneling microscopy ECSTM) and cyclic voltammetry CV) in sulfuric acid solutions. In situ ECSTM studies reveal dierent iodine adlayer structures before and during the process of copper underpotential de- position UPD). At the beginning of the cathodic scan and for potentials higher than the onset of UPD a cp  p 3R- 30° iodine structure is observed on wide terraces. For lower potentials this iodine structure transforms to a more compact 3  3 structure characterized by two dierent 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 sucient 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 CuI bilayer, in agreement with previously reported X-ray diraction data. At the end of the UPD process a Cu1  1 monolayer is formed with a lattice parameter equal to that of Au1 1 1). 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  p 3R-30° 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. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Electrochemical methods; Scanning tunneling microscopy; Chemisorption; Surface structure, morphology, roughness, and topography; Copper; Gold; Iodine; Solid±liquid interfaces 1. Introduction In situ electrochemical scanning tunneling mi- croscopy ECSTM) has become a powerful tool in modern electrochemistry in the past decade, with Surface Science 476 2001) 139±151 www.elsevier.nl/locate/susc * Corresponding author. Address: CCMC-UNAM, P.O. Box 439036, San Ysidro, CA 92143-9036, USA. Tel.: +52-61-74-46- 02; fax: +52-61-74-46-03. E-mail address: valenzue@ccmc.unam.mx J. Valenzuela- Benavides). 0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0039-602801)00691-4