The Structures of Iodine and Carbon Monoxide Overlayers on Ir(111) Electrodes:  An In Situ Scanning Tunneling Microscopy Study

Cyclic voltammetry and in situ scanning tunneling microscopy (STM) were used to examine iodine and carbon monoxide (CO) chemisorbed at well-defined Ir(111) electrodes. Immersing an ordered Ir(111) electrode in 0.1 mM KI at 0.2 V resulted in oxidative adsorption of a monolayer of iodine, arranging in a well-ordered (√7 × √7)R19.1° structure. Adsorption from 1 mM KI solutions produced a bilayer of iodine comprising a lower (√7 × √7)R19.1° overlayer and a higher hexagonal layer. STM atomic resolution yielded detailed spatial structures of these iodine overlayers. Irreversible adsorption of CO from CO-saturated 0.1 M HClO4 solutions at 0.2 V rendered a monolayer of CO with unique island configurations. Most CO islands have 4-column arrangements, with each individual column containing 3, 4, 5, and 4 CO molecules. A (3√3 × 3√3)R30° structure was identified in a local area uniformly covered by identical CO islands. All of the protrusions in the STM images exhibit comparable intensity, implicating similar surface binding configurations of CO molecules. This result is consistent with the predominant terminal binding for CO at Ir(111), as indicated by IR spectroscopy.