EFFECT OF TRACE AMOUNTS OF CL- IN CU UNDERPOTENTIAL DEPOSITION ON AU(111) IN PERCHLORATE SOLUTIONS - AN IN-SITU SCANNING-TUNNELING-MICROSCOPY STUDY

The effect of small amounts of chloride, in the concentration range 10(-7)-10(-4) M, on adsorption behavior and adlayer structure in Cu underpotential deposition on Au(111) in HClO4 solutions was investigated by in-situ scanning tunneling microscopy and cyclic voltammetry. Depending on the Cl- concentration and on the potential two (quasi-) hexagonal adlayer structures - a non-primitive, commensurate (2 x 2) and an incommensurate ''(5 X 5)'' structure - with average Cu adatom spacings of 3.3 and 3.67 Angstrom, respectively, were observed. Both structures involve cooperative adsorption of Cu and Cl-, for the latter a CuCl(111)-like bilayer structure is proposed. This is the dominant structure in the major range of potential and Cl- concentration, the (2 x 2) exists only at potentials below 0.14 V versus SCE and if the Cl- concentration does not exceed a critical value. The transition between ''(5 X 5)'' and (2 X 2) structure proceeds by an island growth mechanism. ''(5 x 5)'' formation and (2 X 2)--> ''(5 X 5)'' transition are governed by diffusion limited transport of Cl- to the electrode. The data demonstrate that qualitatively different adsorption behavior and adlayer structures may result if the interface concentration of a (co-) adsorbing species drops below a critical value.