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

In situ scanning tunneling microscopy (STM) combined with infrared reflecti on absorption spectroscopy (IRAS) has been used to investigate the coverage -dependent structural changes of CO adlayers adsorbed on Ni(111) in solutio n. 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 tha t a (root3 x root3)R30 degrees structure with a coverage of 0.33 ML started to form, where all the CO molecules occupy the bridge sites. A further inc rease in the CO coverage gave rise to the formation of a new c(4 x 2) struc ture with a coverage of 0.5 ML until adlayer saturation. The combined in si tu TM and IRAS results showed equal occupations of the on-top and bridge CO species in the c(4 x 2) phase. (C) 2000 Elsevier Science B.V. All rights r eserved.