Published: September 27, 2011 r2011 American Chemical Society 18264 dx.doi.org/10.1021/ja2054644 | J. Am. Chem. Soc. 2011, 133, 18264–18271 ARTICLE pubs.acs.org/JACS Initiation and Inhibition of Dealloying of Single Crystalline Cu 3 Au (111) Surfaces Aparna Pareek, † Sergiy Borodin, †,§ Asif Bashir, †,§ Genesis Ngwa Ankah, †,§ Patrick Keil, † Gerald A. Eckstein, † Michael Rohwerder, † Martin Stratmann, † Yvonne Gr€ under, ‡ and Frank Uwe Renner* ,† † Max-Planck-Institut f€ ur Eisenforschung, Max-Planck-Straße 1, D-40237 D€ usseldorf, Germany ‡ European Synchrotron Radiation Facility, BP220, F-38043 Grenoble, France b S Supporting Information 1. INTRODUCTION Many electrochemical processes still lack fundamental under- standing, which is often due to the complexity of the involved systems and the special challenges for in situ characterization. One prime example, dealloying, also termed as selective dissolution or leaching, is a specific corrosion process contributing to perilous materials deterioration. 1À5 About 25% of the huge cost to society because of corrosion could be saved by better corrosion knowledge and management. 1 However, this process is also of pivotal technological importance. Historically, dealloying was already used in ancient societies, 6 and in 1928, Raney invented the well- known dealloyed catalyst Raney nickel. 7 A similar methodology is used today for the most active catalyst particles for the oxygen reduction in fuel cells 8À10 which are prepared by dealloying of a CuÀPt nanoparticle precursor. Several further applications from catalytic materials to sensors 11À20 were recently proposed for nanoporous metals which are obtained by dealloying of bulk alloys. Despite the thus fundamental importance of dealloying, key features are, with first successful steps, not yet predictable and often unknown. Important issues are the structure, stability, and com- position of the forming passive-like surface films or the processes leading to their breakdown at the so-called critical potential (E c ). While several important approaches have been discussed, 21À25 only few atomistic simulations have been performed. 26 Also the critical pit diameter as well as related surface and interface energies were considered in a purely thermodynamic approach. 24,25 How- ever, also the role of kinetic effects and surface diffusion was often emphasized in dealloying. 27À32 Microscopic atomic-scale observa- tions, also needed to evaluate any simulation result, are still rare but will benefit the understanding of stability of structural materials as well as of catalyst alloy nanoparticles. The simplest scenario of dealloying occurs, if a binary alloy of elements with sufficiently different equilibrium potentials is exposed to an acidic electrolyte in which no stable bulk oxide is formed. Cu and Au are elements of largely different equilibrium potential values (E e ). Also they possess a large difference in respective lattice parameters (13%) which makes the initial processes of dealloying in this system naturally addressable by X-ray diffraction. Above E c , the entire crystal transforms into a nanoporous network of the nobler element. 33 Below E c , the more reactive element (here Cu) of a freshly exposed Cu 3 Au alloy initially dissolves, but a passive noble metal film is formed. 34 With the initial dissolution of Cu above E e , an epitaxial ultrathin Received: June 21, 2011 ABSTRACT: Dealloying is widely utilized but is a dangerous corrosion process as well. Here we report an atomistic picture of the initial stages of electrochemical dealloying of the model system Cu 3 Au (111). We illuminate the structural and chemical changes during the early stages of dissolution up to the critical potential, using a unique combination of advanced surface- analytical tools. Scanning tunneling microscopy images indicate an interlayer exchange of topmost surface atoms during initial dealloying, while scanning Auger-electron microscopy data clearly reveal that the surface is fully covered by a continuous Au-rich layer at an early stage. Initiating below this first layer a transformation from stacking-reversed toward substrate-oriented Au surface structures is observed close to the critical potential. We further use the observed structural transitions as a reference process to evaluate the mechanistic changes induced by a thiol-based model-inhibition layer applied to suppress surface diffusion. The initial ultrathin Au layer is stabilized with the intermediate island morphology completely suppressed, along an anodic shift of the breakdown potential. Thiol-modification induces a peculiar surface microstructure in the form of microcracks exhibiting a nanoporous core. On the basis of the presented atomic-scale observations, an interlayer exchange mechanism next to pure surface diffusion becomes obvious which may be controlling the layer thickness and its later change in orientation.