IN-SITU STM STUDY OF THE INITIAL-STAGES OF CORROSION OF CU(100) ELECTRODES IN SULFURIC AND HYDROCHLORIC-ACID SOLUTION

An in-situ scanning tunneling microscopy (STM) study of Cu(100) electr ode surfaces in sulfuric and hydrochloric acid solutions in the potent ial range -0.6 to -0.1 V versus Ag/AgCl (KClsat) is presented, reveali ng the surface structure and dynamics in the double-layer region and p roviding detailed structural data on the initial stages of anodic Cu d issolution. After preparation by electropolishing in phosphoric acid, large, atomically flat terraces, separated by frizzy, almost randomly oriented steps are observed in H2SO4 solution in the double-layer regi on. Atomic-stale observations reveal a (1x1) surface lattice in the en tire potential range, even during Cu dissolution. In HCl solution this surface morphology and atomic structure are visible only at potential s negative of -0.4 V, whereas above -0.4 V a c(2x2) Cl adlattice is ob served, together with strong faceting of the steps along the [010] and [001] directions. At low etch rates, the dissolution of clean Cu surf aces proceeds solely by step-flow etching in both electrolytes. In H2S O4 solution the dissolution process is accompanied by strong fluctuati ons in the step positions and by an increase in step roughness. Zn HCl solution the [010]- and [001]-oriented steps are stabilized by the c( 2x2) Cl adlayer and Cu dissolution proceeds by the subsequent removal of complete atomic rows consisting of primitive (root 2x root 2)R45 de grees units of the (2x2) adlattice along these steps. A mechanism is p roposed where the dissolution of Cu atoms occurs at two slightly diffe rent, structurally well-defined kink sites in the c(2x2) lattice, whic h rapidly travel along the step edge (i.e. along [010] or [001]) durin g the dissolution process. Adsorbed impurities can locally pin Cu terr aces, resulting in the formation of highly anisotropic islands, penins ulas and troughs, or induce the formation of monoatomic etch pits. (C) 1998 Elsevier Science B.V.