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

Xiaoping Gao, Gregory J. Edens, Michael J. Weaver
1994 The Journal of Physical Chemistry  
Atomic-level structural and dynamical aspects of the electrode potentialinduced sequence of surface phases on ordered Au(ll0) in dilute aqueous alkali iodide electrolytes have been explored by scanning tunneling microscopy (STM) coupled with cyclic voltammetry. This system exhibits a potentially rich series of electroinduced phase transitions triggered primarily by increasing iodide adsorption towards more positive potentials, together with alkali cation coadsorption. Exploration of the
more » ... ce phase-transition dynamics, in particular, is facilitated by means of "potentiodynamic" STM image sequences obtained during potential steps or especially voltammetric sweeps, thereby providing a direct link to conventional (macroscopic) electrochemical phenomena. At the most negative potentials, beyond ca -0.7 V vs SCE, Au(ll0) forms a largely uniform (1 x 3) phase. The microscopic steps associated with the formation and iodide adsorbate-inddced removal of this reconstruction were examined by potentiodynamic STH. In cesium iodide electrolyte, "one-missing-row" (1 x 3) and (1 x 2) regions are seen to be formed by periodic row displacement en route to the final "three-missing-row" (1 x 3) phase. At higher electrode potentials, formation of a stable (1 x 1) substrate phase is triggered by disordered iodide adsorption. Above about -0.6 V vs SCE, several ordered iodide phases were detected. At the lowest potentials, a mixed Cs+-I" (2 x 4) adlayer was observed by STM, suggested to have the stoichiometry CsI 2 . While this phase is sharply (and reversibly) removed above -0.4 V, a pair of cation-independent ordered iodide adlayers were discerned between about -0.35 and 0.2 V. The first consists of a noncommensurate near-hexagonal phase, with one iodine row rotated slightly from the (110) perpendicular direction. This corrugated compressible structure ("adlayer I") transforms sharply above ca 0 V vs SCE to a slightly highercoverage, approximately (3 x 2) (01 -0.67), form ("adlayer II") with iodine rows wedged in between the (110) substrate "rails." The potential-induced adlayer I -II transition occurs less readily than the reverse transformation. Above 0.2 V, an unusual long-range, yet ordered, Au(ll0) restructuring develops featuring substrate fracturing into parallel alternating strings and ditches ca 25-40 A wide along the (110) direction, but with retention of the original (1 x 1) substrate unit cell. Disti Ibltioml Availability Codes Dit Avail aiidjor Dist Special
doi:10.1021/j100084a026 fatcat:q3av7vmqpzeylag6trzpza33hm