SENSITIVITY OF ELECTROCHEMICAL ADLAYER STRUCTURE TO THE METAL CRYSTALLOGRAPHIC ORIENTATION - POTENTIAL-DEPENDENT IODIDE ADSORPTION ON AU(100) IN COMPARISON WITH OTHER LOW-INDEX SURFACES

The potential-dependent ordered atomic structures formed for iodide ad sorption on Au(100) from dilute aqueous alkali metal iodide electrolyt es as discerned by means of in-situ scanning tunneling microscopy (STM ) are reported and compared to the corresponding behavior of the other two low-index faces, Au(111) and (110). As in related studies, emphas is is placed on linking the microscopic structural information to the voltammetric and other macroscopic electrochemical response, including the use of ''potentiodynamic'' STM tactics where rear-space images ar e acquired during appropriate electrode potential steps or sweeps. Whi le relatively low iodide coverages, theta(I) similar to 0.1, are suffi cient to lift the hexagonal Au(100) reconstruction, yielding large ord ered (1 x 1) domains by ca. -0.6 V vs SCE, ordered iodide adlayers are formed only above -0.2 V. A compressible incommensurate (2 root 2 X p root 2) phase (theta(I) approximate to 0.46-0.49) is observed between ca. -0.2 and 0 V, which reverts to a distinct (2 root 2 X root 2)R45 degrees structure (theta(I) = 0.5) featuring uniform binding in 2-fold bridging sites. Interestingly, while the latter structure is retained locally at higher potentials, it is interspersed with narrow (3-4 ato m wide) strips having a rotated hexagonal pattern. The latter becomes increasingly dense, and eventually dominant, toward higher potentials. This microscopically nonuniform (or ''spatially modulated'') phase is noteworthy as well as unusual; the corresponding iodine phases on Au( 111) and (110) display, as is common, structural uniformity at a given potential. A related distinction concerns the potential-induced phase transitions: while they display sharp first-order behavior on Au(111) and (110), the phase transformations on Au(100) are diffuse. These mi croscopic-level differences in the adlayer phase structure and dynamic s as seen by STM are also reflected in the macroscopic behavior as dis cerned by cyclic voltammetry and ac impedence measurements. Neverthele ss, closely similar surface concentration-potential (T-E) behavior is seen for the ordered adlayers on all three low-index gold surfaces. Th e various adsorbate-adsorbate interatomic forces responsible for the o bserved potential-induced adlayer compressibilities are briefly outlin ed.