Local structure and phase transitions within ordered electrochemical adlayers: Some new insights from in situscanning tunneling microscopy

The application of in situatomic-resolution scanning tunneling microscopy (STM) to the elucidation of local structure and potential-induced phase transitions within ordered adlayers on monocrystalline metal electrodes is discussed, and illustrated primarily for compressed CO adlayers on Pt(111). The value of the additional structural information on adsorbate binding sites that can be obtained from infrared reflection-absorption spectroscopy (IRAS) as well as from potentiodynamic STM imaging tactics is emphasized. The atomic-level and nanoscale structural alterations wrought upon the Pt(111)/CO adlayer by bismuth predosing are described as a further illustration of the synergetic value of the spatial and surface-bonding information provided by STM and IRAS. A comparison is made between the potential-dependent adlayer characteristics of the compressed CO adlayers with iodide adlayers on gold and platinum low-index electrodes. Possible reasons for the marked differences in the phase-transition behavior of these nonmetallic adlayers are briefly discussed, along with comparisons with analogous adlayers in ultrahigh vacuum environments.