A TANDEM MICROSCOPIC-ELECTROCHEMICAL EXAMINATION OF A CHARGE-INDUCED SURFACE PHASE SEQUENCE - ORDERED AU(110) IN AQUEOUS IODIDE ELECTROLYTES AS PROBED BY POTENTIODYNAMIC SCANNING-TUNNELING-MICROSCOPY

Atomic-level structural and dynamical aspects of the electrode potenti al-induced sequence of surface phases on ordered Au(110) in dilute aqu eous alkali metal-iodide electrolytes have been explored by scanning t unneling microscopy (STM) coupled with cyclic voltammetry. This system exhibits a rich series of electroinduced phase transitions triggered primarily by increasing iodide adsorption toward more positive potenti als, together with alkali metal cation coadsorption. Exploration of th e real-space phase transition dynamics, in particular, is facilitated by means of ''potentiodynamic'' STM image sequences obtained during po tential steps or especially voltammetric sweeps, thereby providing a d irect link to conventional (macroscopic) electrochemical phenomena. Th ese tactics also enable the adsorbate binding sites to be obtained dir ectly and related to the observed corrugation in the adlayer STM image s. At the most negative potentials, beyond ca. -0.7 V vs SCE, Au(110) forms a largely uniform (1 X 3) phase. The microscopic steps associate d with the formation and iodide adsorbate-induced removal of this reco nstruction were examined by potentiodynamic STM. In cesium iodide elec trolyte, ''one-missing-row'' (1 X 3) and (1 x 2) regions are seen to b e formed by periodic row displacement en route to the final ''three-mi ssing-row'' (1 x 3) phase. At higher electrode potentials, formation o f a stable (1 X 1) substrate phase is triggered by disordered iodide a dsorption. Above about -0.6 V vs SCE, several ordered iodide phases we re detected. At the lowest potentials, a mixed Cs+-I- (2 x 4) adlayer was observed by STM, suggested to have the stoichiometry CsI2. While t his phase is sharply (and reversibly) removed above -0.4 V, a pair of cation-independent ordered iodide adlayers were discerned between abou t -0.35 and 0.2 V. The first consists of a noncommensurate near-hexago nal phase, with one iodine row rotated slightly from the (1(1) over ba r0$) perpendicular direction. This corrugated compressible structure ( ''adlayer I'') transforms sharply above ca. 0 V vs SCE to a slightly h igher-coverage, approximately (3 X 2) (theta(I) = 0.67), form (''adlay er II'') with iodine rows wedged in between the (1(1) over bar0$) subs trate ''rails''. The potential-induced adlayer I --> II transition occ urs less sharply than the reverse transformation. Above 0.2 V, an unus ual long-range, yet ordered, Au(110) restructuring develops, featuring substrate fracturing into parallel alternating strings and ditches ca . 25-40 Angstrom wide along the (1(1) over bar0$) but with retention o f the original (1 x 1) substrate unit cell.