ELSEVIER Journal of Electroanalytical Chemistry 379 (1994) 223-231 Structures of copper and halides on Pt( 111)) Pt( 100) and Au( 111) electrode surfaces studied by in-situ scanning tunneling microscopy Hiroki Matsumoto, Junji Inukai, Masatoki Ito zyxwvutsrqponmlkjihgfedcbaZYX Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-I Hiyoshi, Kohoku-ku, Yokohama 223, Japan Received 6 January 1994; in revised form 8 April 1994 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSR Abstract Structures of copper and halide (chloride, bromide or iodide) in a sulfuric acid solution on Pt(lll), Pt(100) and Auclll) single-crystal electrode surfaces were systematically investigated by in-situ scanning tunneling microscopy. A (4 X 4) structure was imaged after the first cathodic current peak for Cu + Cl on Pt(ll1). Distorted (4 X 4) and (& X &)R30” structures were found after the first and the second peaks, respectively, for Cu + Br on Pt(ll1). For Cu + I on Pt(lll), a (6 x &)R30° structure was observed after the copper deposition peak. A ~(2 X 2) structure was obtained for Cu + Cl, Cu + Br and Cu + I on Pt(100). On a Au(ll1) surface, a (5 x 5) structure was observed for Cu + Cl after both the first and the second cathodic current peaks. For Cu + Br on Au(lll), (fi x fijR19.1” was found after the first peak, and (4 x 4) structures were imaged after both the second and third peaks. A (3 x 3) image was obtained for Cu + I after a single peak on Au(ll1). Structural models for the copper and the halide layers are discussed based on the adsorbate-substrate interaction at these surfaces. Coadsorption structures on platinum surfaces are understandable as those of halide adsorption on a pseudomorphic copper (1 X zyxwvutsrqponmlkjihgfedcbaZYXWV 1) layer on platinum, while on Au(ll1) the coadsorption layer is interpreted as an ultrathin crystal of CuX(X = Cl, Br, I). Keywords: Underpotential deposition; Coadsorption structure; Platinum; Gold; Copper; Halides; Scanning tunneling microscopy 1. Introduction The underpotential deposition KJPD) of foreign metals is an electrochemical phenomenon in which an adlayer of metal species in solution can be reversibly deposited onto a substrate electrode surface before bulk metal deposition. UPD has been studied exten- sively over the past two decades as a means of the formation of ultrathin metallic films [1,2]. For UPD, it is well documented that the anions in the electrolyte solutions have a pronounced influence, which must be considered fully for understanding its mechanism [3-51. The coadsorption structure of UPD copper and halides on Pt(ll1) has been extensively investigated as a typical example [3,6-131. These studies have estab- lished the “bilayer coadsorption structure” of copper and halides on a Pt(ll1) electrode surface: an ordered structure of halide ions is formed on an adlayer of copper deposited on the Pt(ll1) surface. In our previ- ous paper [ll], we reported an investigation of copper coadsorption structures with bromide and iodide on Pt(ll1) using in-situ scanning tunneling microscopy (STM). In both cases, the bilayer model was valid, with 0022-0728/94/$07.00 0 1994 Elsevier Science S.A. All rights reserved SSDI 0022-0728(94)03481-I halide structures on a copper monolayer deposited on platinum. Coadsorption of copper and chloride on Au(ll1) is of particular interest, and in-situ STM recently pro- vided the first information about the coadsorption structure of copper and chloride on a A~(11 1) elec- trode surface [11,14-181. All reports of STM studies show consistently a (5 x 5) structure; however, the in- terpretation of this structure is still not agreed. For example, combining the results of low energy electron diffraction (LEED) and X-ray photoemission spec- troscopy (XPS) [193, Batina et al. [18] concluded that the (5 X 5) structure originates from the copper adlayer structure on Au(lll), and stated that chloride cannot be seen on copper using STM. On the contrary, we concluded in our previous paper [l 11, on the basis of a systematic investigation of coadsorption of copper and halides on both Au(lll) and Pt(lll), that chloride is visible by STM and forms a (5 x 5) structure on the copper layer. However, we could not determine the structure of the copper underlayer on Au(ll1) during coadsorption with halides [ill. In this paper, we report further systematic investiga-