Adlayer structures of CO adsorbed on Ni(111) electrode surfaces studied by in situ STM combined with IRAS

In situ scanning tunneling microscopy (STM) combined with infrared reflection absorption spectroscopy (IRAS) has been used to investigate the coverage-dependent structural changes of CO adlayers adsorbed on Ni(111) in solution. Two different structures were obtained in a CO-containing Na2SO4 (pH 3) solution depending on the CO coverage. At low CO coverage, it was found that a (3×3)R30° structure with a coverage of 0.33 ML started to form, where all the CO molecules occupy the bridge sites. A further increase in the CO coverage gave rise to the formation of a new c(4×2) structure with a coverage of 0.5 ML until adlayer saturation. The combined in situ STM and IRAS results showed equal occupations of the on-top and bridge CO species in the c(4×2) phase.