Influence of uniaxially incommensurate adlayers on the surface morphology: Iodide on Cu(100)

The potential dependent phase behavior of specifically adsorbed iodide on Cu(I 0 0) in 10 mM HClO4/1 mM KI has been studied using cyclic voltammetry in combination with in situ STM. At positive electrode potentials near the onset of the copper dissolution reaction the electrode surface is covered by a uniaxially incommensurate iodide adlayer showing an additional one-dimensional long-range height modulation superimposed on the atomic corrugation. The atomic distances and the wave length of the striped superstructure change with the applied electrode potential indicative for an electrocompression process. The iodide layer close to the saturation coverage can be understood in terms of a uniaxially expanded c(2 x 2)-I structure. The resulting unit cell can be described as c(p x 2)-I with p > 2a(Cu). At saturation coverage a nearest neighbor distance of 3.7 Angstrom is found which is significantly smaller than the iodine van-der-Waals diameter. In contrast to a uniformly expanded iodide layer at positive potentials, regular domain wall structures are observed at more negative potentials. At a certain potential close to the onset of the hydrogen evolution reaction a first order phase transition leads to the formation of a c(6 x 2)-I phase which remains stable even under massive hydrogen evolution. Dynamic STM measurements exhibit a preferential direction for this phase transition along the substrate <011> directions. The role of defects such as step edges for this phase transition is elucidated by high resolution STM images. (C) 2002 Elsevier Science B.V. All rights reserved.