Electrochemical <i>In Situ</i> Video-STM Studies of the Phase Transition of CO Adlayers on Pt(111) Electrodes
Jie Wei, Yanxia Chen, Olaf M. Magnussen
Abstract
The potential-dependent (2 × 2)-3CO → (√19 × √19)R23.4°-13CO phase transition of CO adlayers on Pt(111) in a 0.1 M H2SO4 electrolyte containing minor amounts of CO was studied on the subsecond time scale by in situ high-speed scanning tunneling microscopy. At a potential of 0.20 V versus Ag/AgCl, the (2 × 2)-3CO phase is converted into an apparent (1 × 1)-CO adlayer. Upon increasing the potential to 0.25 V, this adlayer first transiently exhibits both a local (1 × 1) lattice as well as a (√19 × √19)R23.4° superlattice before it is gradually transformed into a static (√19 × √19)R23.4°-13CO adlayer with a rotated and expanded adsorbate arrangement. In the transient structure, highly dynamic fluctuations in the adsorbate positions are observed. These results can be rationalized by a microscopic mechanism in which the removal of CO adsorbates leads to highly mobile defects with a local (1 × 1) structure in the (2 × 2)-3CO adlayer. With decreasing CO coverage, a densely packed arrangement of these (1 × 1) clusters with (√19 × √19)R23.4° symmetry is formed, which subsequently rearranges gradually into the (√19 × √19)R23.4°-13CO adlayer.