Electrochemical and Scanning Tunneling Microscopic Study of Dealloying of Cu3Au

Dealloying of Cu~Au has been examined by in situ STM and several electrochemical methods. Three different regimes of behavior were noted. At low overpotentials, clustering of gold atoms occurs near sites of copper dissolution. This is essentially a two-dimensional process. The formation and smoothing of these clusters by capillary action, monitored in real time, demonstrated the highly mobile nature of the surface species. At higher potentials, the electrode is largely passivated by the enrichment of gold. However, there exist small localized regions of three-dimensional roughness which may be correlated to extended dealloying catalyzed by bulk solid-state defects. When the potential is increased above the critical potential (Ec), global surface roughening occurs. Correlating STM with chronoamperometric and chronopotentiometric results demonstrates that this transition occurs by nucleation and growth. Selective dissolution of copper depends on the exposure of fresh sites by the migration of passivating gold atoms. Adsorption can strongly influence this transport process, as manifest by changes in Ec. In comparison to sulfate media, chloride caused a decrease in Eo, while derivatization of CuaAu with an alkyl-thiol produced an increase in Ec. These shifts are consistent with the enhancement and inhibit ion of gold surface diffusion by the respective adsorbates. Dealloying is a phenomenon of great importance in general corrosion (1, 2), stress corrosion cracking (2, 3), and catalysis (4). In the case of a binary alloy A-B, such as Cu-Au, dealloying entails the selective dissolution of the less-noble element A, in this case, copper. A schematic of a typical potentiostatic polarization curve is given in Fig. 1. The curve exhibits a domain of very low potentialindependent current followed by a region of rapidly increasing current. The potential defining the transition between these regimes is known as the critical potential, Ec. Ec is a strong function of alloy composition (1, 2). At potentials below E~, dissolution of the active species, A, leads to an enrichment of the noble species B, in this case, gold, which suppresses the further dissolution of A. It is unclear if this blocking layer is pure B, or a B-enriched alloy. The critical potential is associated with the breakdown of this passivating overlayer and the onset of massive dealloying. At potentials greater than E~, gross surface roughening occurs and the near-surface of the alloy evolves into a fine, porous, noble-metal enriched network. This process has * Electrochemical Society Active Member. been described as a cellular-phase transformation (4). It has been suggested that the critical potential, Eo, defines the transition from a stable planar surface to a highly ramified interface (2). A variety of mechanisms have been proposed to explain the breakdown phenomenon that leads to massive dealloying. Modern discussions have largely concentrated on the relative importance of volume diffusion vs. surface transport processes (1). Pickering and Wagner (5) suggested that the rate of dealloying is controlled by solid-state diffusion of the less-noble metal via divacancies. Prior to this, Wagner (6) demonstrated that if volume diffusion controls the rate of dealloying, then geometrical instabilities will develop along the planar interface which will grow rapidly with time. This could account for the sponge-like morphology of the dealloyed layer. The critical potential was ascribed to a potential-dependent concentration of divacancies (2). The large increase in defect density would occur at high overpotentials where oxidation from highly coordinated surface sites becomes possible. This mechanism depends on the blocking layer being B-enriched as opposed to a pure B overlayer. An alternative dealloying mechaDownloaded 28 Jan 2009 to 146.6.143.190. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp J. Electrochem. Soc., Vol. 138, No. 11, November 1991 9 The Electrochemical Society, Inc. 3225