Structural variations of CO adlayers on a Pt(100) electrode in 0.1 M HClO4 solution: an in situ STM study

In the present study, we have investigated structures of a CO adlayer on a well-defined Pt(100) electrode surface in 0.1 M HClO4 aqueous solutions saturated with N2, 1% CO/He and 100% CO by using in situ STM. The in situ STM images with molecular resolution demonstrated that highly ordered structures of the CO adlayer, denoted (2 × n) - 2(n - 1)CO with CO coverages of (n - 1)/n, dynamically varied with the electrode potential and the CO partial pressure in solution. As the CO partial pressure increased, more compressed structures of the CO adlayer formed on the electrode surface. In each solution, a phase transition of the CO adlayer on the terrace site was observed to be triggered by increasing the electrode potential, accompanied by a partial desorption of surface CO without charge transfer. A series of in situ STM images revealed transient local structures during the phase transition of the CO adlayer. Specifically, unique structures were found to appear in the vicinity of monoatomic steps in N2- and 1% CO/He-saturated solution, but not in 100% CO-saturated solution.