ANODIC UNDERPOTENTIAL DEPOSITION AND CATHODIC STRIPPING OF IODINE AT POLYCRYSTALLINE AND SINGLE-CRYSTAL GOLD - STUDIES BY LEED, AES, XPS, AND ELECTROCHEMISTRY

The oxidative chemisorption and cathodic stripping reductive desorption of iodine have been compared at smooth polycrystalline and well-defined Au(111) single-crystal electrodes. Experimental measurements were based upon cyclic voltammetry, thin-layer coulometry, X-ray photoelectron spectroscopy, Auger electron spectroscopy, and low-energy electron diffraction. The results indicate that iodide is oxidatively adsorbed as zerovalent atomic iodine at potentials between -0.4 V and +0.4 V (Ag/AgCl reference); at lower potentials, surface iodine is reductively desorbed as aqueous iodide, while at considerably more positive potentials, it is oxidized to aqueous iodate. Studies with the Au(111) electrode in dilute aqueous CsI solutions showed ordered adlayer structures at the selected potentials investigated. Below -0.4 V, the potential at which oxidative deposition of iodine starts to occur, a distinct (4 x 4) quarter-coverage CsI layer (THETA-Cs = GAMMA-Cs/GAMMA-Au = THETA-I = GAMMA-I/GAMMA-Au approximately 0.25) was formed. At -0.4 V < E < -0.2 V, THETA-I increased to 0.33; this increase was coupled with the loss of adsorbed Cs, and the structure of this adlattice was Au(111)(square-root 3 x square-root 3)R30-degrees-I. At E > -0.2 V, the I coverage reached 0.4, a value made possible by a compression of the original (square-root 3 x square-root 3)R30-degrees structure in one dimension to form a nearly hexagonal iodine adlattice with a (5 x square-root 3) unit cell. The amount of adsorbed iodine continues to increase as the potential is made still more positive until the surface is saturated with a monolayer of close-packed I atoms of coverage limited by van der Waals interactions; additional iodine forced into the already space-limited interfacial layer only leads to the formation of molecular iodine, which is evolved into the solution as I2(aq). The oxidative chemisorption process may be thought of as the oxidative underpotential deposition of I atoms, while the reductive desorption reaction may be viewed as the cathodic stripping of iodide ions.