Electrochimica Acta 56 (2011) 1694–1700 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta In situ scanning tunneling microscopy study of selective dissolution of Au 3 Cu and Cu 3 Au (0 0 1) Frank Uwe Renner a , Gerald Andreas Eckstein b , Liverios Lymperakis a , Andrea Dakkouri-Baldauf b , Michael Rohwerder a,∗ , Jörg Neugebauer a , Martin Stratmann a a Max-Planck-Institut für Eisenforschung, Max-Planck-Str.1, Düsseldorf 40237, Germany b Universität Erlangen-Nürnberg, Martensstrasse 7, D-91058 Erlangen, Germany article info Article history: Received 1 July 2010 Received in revised form 16 September 2010 Accepted 17 September 2010 Available online 25 September 2010 Keywords: Corrosion Alloys Scanning probe microscopy Dealloying abstract We present an electrochemical study of Au 3 Cu (0 0 1) single crystal surfaces in 0.1 mol dm -3 H 2 SO 4 and 0.1 mol dm -3 H 2 SO 4 + 0.1 mmol dm -3 HCl, and of Cu 3 Au (0 0 1) in 0.1 mol dm -3 H 2 SO 4 . The focus is on in situ scanning tunneling microscopy experiments. The changes of the surface morphology, which are time- and potential-dependent, have been observed, clearly resolving single atomic steps and mono- atomic islands and pits. Chloride additives enhance the surface diffusion and respective morphologies are observed earlier. All surfaces have shown considerable roughening already in the passive region far below the critical potential. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Electrochemical dealloying of metallic alloys can be regarded as a detrimental (wet) corrosion process related e.g. to stress cor- rosion cracking [1,2]. In this context dealloying has been studied in stainless steels and brass [3–5]. Noble metal alloys like Ag–Au or Cu–Au have been investigated as model systems to gain basic understanding of this corrosion process [6–10]. On the other hand a dealloying process is used to produce Raney Nickel, an important catalyst in the chemical industry [11]. Recently catalytic activity has also been discovered in nanoporous gold produced by dealloying of Ag–Au alloys, and more applications of dealloyed, nanoporous structures have been demonstrated from actuators to biomedical sensors [12–15]. Noble metal alloys have moved thus more directly into the focus of applied research. Nevertheless, many fundamental questions remain unsolved. The term “dealloying” refers to the selective dissolution of one or more elements of a metallic alloy, enabled by a relative large dif- ference in standard electrode potentials of their components [16]. A century ago, Tammann [17] observed that with a high content of the more noble metal in a binary noble metal alloy no pronounced (bulk) dealloying can be observed; he called the threshold con- centration the “parting limit”. With a noble metal concentration ∗ Corresponding author. Tel.: +49 (0)211 6792 442; fax: +49 (0)211 6792 218. E-mail address: m.rohwerder@mpie.de (M. Rohwerder). above the parting limit the alloy is similar to the noble element in its corrosion and dissolution behaviour. Below the parting limit, first a passive region with a very low dissolution current caused by the accumulation of noble metal species is observed, followed by a breakdown of passivity and massive dissolution of the reac- tive element above the so-called critical potential E c . At this stage porous bulk-dealloyed structures are formed with ligament sizes as low as a few nanometers (in the case of Pt or Pd) or a few tens of nanometers (Au). While bulk dealloying and the formation of the structures above E c has been more frequently studied [18–20], mechanistic stud- ies of the selective dissolution on the atomic scale have received less attention. With the demonstration that the scanning tunneling microscope can also be applied on surfaces immersed in liquids a powerful technique was available to study selective electrochemi- cal dissolution. As the measurement time for in situ STM imaging is usually longer than the fast structural changes taking place during dissolution above E c , the method is limited to address the sur- face morphology below E c . In addition many experimenters chose compositions of sample alloys well below the parting limits of the respective materials to limit the dissolution to a few atomic mono- layers. The main systems studied so far were Ag–Au and Cu–Au [8–10]. The passive potential region for Cu–Au is larger due to the lower standard electrode potential for Cu compared to Ag and a similar (but still different) E c compared to Ag–Au. Also, the Cu–Au system should exhibit some influence of strain that develops at the interface of forming Au-rich layers to the substrate due to the 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.09.061