Continuous variation of iodine adlattices on Ag(111) electrodes: In situ STM and ex situ LEED studies

The structure of the iodine adlattice formed on Ag(111) in aqueous solutions containing iodide ions has been determined as a function of the electrode potential by in situ scanning tunneling microscopy (STM) and ex situ low energy electron diffraction (LEED). In KI solutions buffered with KF-KOH at pH 10, continuous compression of the iodine adlattice from square (root 3 x root R-30 degrees), via (root 3qR beta degrees x root 3R-30 degrees), to (root 3 x root 3)R30 degrees was observed as the electrode potential was increased; this was then followed by an abrupt phase transition into a rotated hexagonal (root 3r x root 3r)R(30 degrees + alpha degrees) phase. Coadsorption of K was observed by Auger electron spectroscopy (AES) at potentials negative of that for the phase transition. In acid solutions (0.1 mM HI) the continuous compression from (root 3 x root 3)R30 degrees, via c(p x root 3R-30 degrees), into (root 3r x root 3r)R(30 degrees + alpha degrees) was observed. The Moire pattern observed for the (root 3r x root 3r)R(30 degrees + alpha degrees) adlattice, as well as the flatness of other adlattices observed in STM images, is explained in terms of lattice registry on the basis of a ''hard ball contact model''.