Atomic structures and dynamics of a Cu(100) electrode in dilute hydrobromic acid: An in situ STM study

The surface electrochemistry of Cu(100) in 10 mM hydrobromic acid electroly te has been studied by means of cyclic voltammetry and in situ STM. In the potential range between the onset of the anodic copper dissolution at posit ive and the hydrogen evolution at negative electrode potentials, the CV of Cu(100) in 10 mM HBr is characterized only by the double-layer charge. With in this potential regime a highly ordered (root2 x congruent to2)R45 degree s -superstructure is seen in the STM experiments assigned to specifically a dsorbed bromide anions. No desorption of the bromide adlayer has been found in these STM experiments even at extremely negative potentials at the onse t of hydrogen evolution. Therefore the bromide desorption potential is conc luded to lie within the potential regime of massive hydrogen evolution at e ven more negative potentials. Adsorbed bromide induces a drastic restructur ing and faceting of the surface topography depending on the applied potenti al. The driving force of this process is the formation of thermodynamically favored copper steps aligned parallel to close packed [100] directions of the bromide adsorbate. Dynamic processes like copper dissolution and deposi tion are also strongly influenced by the geometry of the (root2 x root2)R45 degrees bromide adlayer. Corrosion as well as deposition of copper materia l follows the close packed [100] directions of the bromide adsorbate. For m oderate reaction rates an additional anisotropy between the [001]- and [010 ]-direction is observed due to the nonequivalence of two different kinds of bromide stabilized copper steps. The origin of these two kinds of steps is the phase relation of close packed adsorbate rows of adjacent terraces. Th e deposition of copper material does not only start at the lower but unusua lly, also at the upper sites of step edges leading to the formation of micr ofacets. Not only the growth of monoatomically high islands is observed but also a double-layer and multilayer growth of copper. (C) 2001 Elsevier Sci ence B.V. All rights reserved.