In-situ STM study of the initial stages of corrosion of Cu(100) electrodes in sulfuric and hydrochloric acid solution

Abstract

An in-situ scanning tunneling microscopy (STM) study of Cu(100) electrode surfaces in sulfuric and hydrochloric acid solutions in the potential range −0.6 to −0.1 V versus Ag AgCl (KCl sat) is presented, revealing the surface structure and dynamics in the double-layer region and providing detailed structural data on the initial stages of anodic Cu dissolution. After preparation by electropolishing in phosphoric acid, large, atomically flat terraces, separated by frizzy, almost randomly oriented steps are observed in H 2SO 4 solution in the double-layer region. Atomic-scale observations reveal a (1 × 1) surface lattice in the entire potential range, even during Cu dissolution. In HCl solution this surface morphology and atomic structure are visible only at potentials negative of −0.4 V, whereas above −0.4 V a c(2 × 2) Cl adlattice is observed, together with strong faceting of the steps along the [010] and [001] directions. At low etch rates, the dissolution of clean Cu surfaces proceeds solely by step-flow etching in both electrolytes. In H 2SO 4 solution the dissolution process is accompanied by strong fluctuations in the step positions and by an increase in step roughness. In HCl solution the [010]- and [001]-oriented steps are stabilized by the c(2 × 2) Cl adlayer and Cu dissolution proceeds by the subsequent removal of complete atomic rows consisting of primitive (√ a × √2)R45° units of the (2 × 2) adlattice along these steps. A mechanism is proposed where the dissolution of Cu atoms occurs at two slightly different, structurally well-defined kink sites in the c(2 × 2) lattice, which rapidly travel along the step edge (i.e. along [010] or [001]) during the dissolution process. Adsorbed impurities can locally pin Cu terraces, resulting in the formation of highly anisotropic islands, peninsulas and troughs, or induce the formation of monoatomic etch pits.