Lateral interactions in CO adlayers on prismatic ZnO faces: a FTIR and HRTEM study

In this paper the CO adsorption at 77 K on the ZnO (Kadox) surface is studied. CO adsorption occurs mainly on the (1010BAR) prismatic faces of ZnO microcrystals (one CO per Zn2+ ion), giving a complex IR spectrum in the 2190-2168 cm-1 interval. The CO frequency shifts with the coverage from 2190 cm-1 (theta almost-equal-to 0, singleton) to 2168 cm-1 (theta = 1), as a consequence of through-space interactions (either dynamic and static) among the oscillators. At least five discrete components appear and disappear in the 0 less-than-or-equal-to theta less-than-or-equal-to 1 interval. These features arise from the progressive stepwise filling of the surface Zn2+ sites, and correspond to different variants of the surrounding of the CO molecules. The first overtones of the CO stretching mode are also observed in the 4372-4335 cm-1 interval, while the combination mode between the CO and the M-C stretching vibrations are found in the 2415-2375 cm-1 interval. The erosion of localized lattice modes in the 750-600 cm-1 region, indicates that CO adsorption is accompanied by surface relaxation. Finally some considerations concerning the perturbation of the OH groups, upon CO adsorption, are included.