PROBING REDOX-INDUCED MOLECULAR-TRANSFORMATIONS BY ATOMIC-RESOLUTION SCANNING TUNNELING MICROSCOPY - IODIDE ADSORPTION AND ELECTROOXIDATION ON AU(111) IN AQUEOUS-SOLUTION

In-situ atomic-resolution scanning tunneling microscopy (STM) has been utilized on ordered Au(111) under electrode potential control in acidic aqueous solution to examine potential-dependent iodine adlayer structures formed by iodide electrooxidation as well as covalent electrosorption. In the potential region -0.3 to +0.4 V vs SCE, below where electrooxidation of solution iodide occurs, several distinct iodine adlayer structures were observed. At the most negative potentials, structures close to the hexagonal (square-root 3 x square-root 3)R30-degrees (theta(I) = 0.33) pattern are evident. The registry between the adlayer and the substrate lattice was deduced in part from composite-domain images created by stepping the electrode potential so as to alter markedly the adsorbate coverage during acquisition of a given STM image. At potentials between -0.2 and +0.2 V, adlayer patterns progressively closer to the (5 x square-root 3) (theta(I) = 0.4) structure became increasingly prevalent. This features a diminution in the iodine spacing along two of the three iodine rows, with a corresponding ca. 5-degrees shift from the R30-degrees direction. Above 0.2 V, the STM images indicate that predominant presence of more complex higher-coverage adlayers (theta(I) almost-equal-to 0.44) featuring long-range (19-22 angstrom) z corrugations rotated by 8-10-degrees from the hexagonal iodine adlattice. These corrugations arise from periodic alterations in the iodine-binding site, necessitated by I-I distances that approach the van der Waals diameter. Above 0.3 V, however, polyiodide chains were also observed, featuring shorter (2.8-3.2 angstrom) I-I distances compatible with adsorbate-adsorbate chemical bonding; these increasingly distort the monomeric iodine superlattice structure. At the onset of solution iodide electrooxidation, at 0.45 V, multilayer iodine films were observed to form, consisting of polyiodide strands growing outward from a (5 x square-root 3) surface template. This STM spatial information is compared with structural data obtained previously from potential-dependent surface Raman spectra. The prospects of utilizing in-situ electrochemical STM to explore redox-induced surface molecular transformations are noted in the light of these findings.