Surface Science 430 ( 1999 ) L521–L526 www.elsevier.nl/locate/susc Surface Science Letters In-situ observation of an ordered sulfate adlayer on Au(100) electrodes M. Kleinert, A. Cuesta, L.A. Kibler, D.M. Kolb * Department of Electrochemistry, University of Ulm, 89069 Ulm, Germany Received 14 December 1998; accepted for publication 17 February 1999 Abstract An ordered adlayer of sulfate or bisulfate has been found on the unreconstructed Au(100) surface in 0.1 M H 2 SO 4 by in-situ STM. The ordered adlayer is formed concomitantly with lifting of the (hex)-reconstruction. The new structure consists of sulfate/bisulfate rows that run parallel to the edges of the islands that were created during the (hex)(1×1) transition. The distance between the adions within the rows is 4.0±0.3 A ˚ , and that between rows 10.4±0.7 A ˚ , corresponding to a ( 1.4±0.1 0 0 3.6±0.2 ) superstructure with a surface coverage of about 0.2. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Au(100) electrodes; Metal–electrolyte interfaces; Ordered adlayers; STM; Sulfate 1. Introduction existence of a large double-layer region, besides the easy preparation and handling, has been the main reason for gold electrodes being employed One of the most intensively pursued goals in as a model system for the study of interfacial today’s electrochemistry is to understand the struc- properties of solid electrodes [5]. ture and composition of the electrochemical double The report by Clavilier et al. [6,7] of an anoma- layer, in general, and to understand adsorption lous adsorption state in the voltammograms of a processes at the electrode/electrolyte interface, in well-defined Pt(111) electrode in sulfuric and per- particular (see, for example, [1,2]). Much work chloric acid solutions, which was attributed by has been carried out for the mercury/electrolyte these authors to hydrogen adsorption, but later interface [ 3 ], since the use of the dropping mercury shown by Al Jaaf et al. [8] to be due to specific electrode provides an easy way to expose a clean adsorption of anions, has stimulated the study of surface to the solution. Similarly well-defined con- sulfate adsorption on a number of single crystal ditions have been achieved in recent years for metal electrodes [9–15]. From the very beginning single crystal surfaces of noble metals [4]. The of their observation, current spikes have been attributed to structural transitions on the surface * Corresponding author. Fax: +49 731-502-5409. E-mail address: dieter.kolb@chemie.uni-ulm.de (D.M. Kolb) and in ionic adlayers [8,16 ]. 0039-6028/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0039-6028(99)00347-7