Uniaxial compression of iodide adlayers on Cu(1 1 1) studied under electrochemical conditions

The potential-dependent phase behavior of specifically adsorbed iodide on Cu(1 1 1) in 10 mM HClO4 containing either 0.1 or 0.05 mM KI has been studied using cyclic voltammetry in combination with in situ scanning tunneling microscopy. The adsorption of iodide on Cu(1 1 1) at negative electrode potentials close to the onset of the hydrogen evolution reaction leads to the formation of a (√3×√3)R30°-I structure. However, at more positive electrode potentials a transition to a uniaxially incommensurate (UIC) iodide adlayer takes place caused by a unidirectional compression of the (√3×√3)R30°-I phase. This compressed adlayer shows an additional one-dimensional long-range height modulation superimposed on the atomic corrugation. The process of compression can be described rather by the insertion of super-heavy domain-walls (shdw) into the (√3×√3)R30°-I phase than by a uniform and continuous compression of the adsorbate adlayer. Within the UIC phase regular (√3×√3)R30°-I regions are separated by broad and ‘fuzzy’ shdw instead of sharp dislocation lines as expected for ideal shdw. This observation points to a partial relaxation of these walls. Within these domain-walls the smallest nearest neighbor distance (NND) of 3.7 Å is significantly smaller than the van-der-Waals diameter of the uncharged iodine particle (3.9 Å) while the NND within adjacent commensurate (√3×√3)R30°-regions exceeds the ‘Pauling’ diameter of the single charged iodide (I−) by 1.85%. Both the NND at domain-walls and the wave-length of the striped superstructure change with the applied potential.