The adsorption of CO on two different mixed-terminated, single-crystal surfaces of zinc oxide, ZnO(1010) and
ZnO(1120), was investigated by employing infrared reflection absorption spectroscopy (IRRAS) using p- and spolarized
light. For both surfaces, one negative CO band was observed only for p-polarized light, indicating
that CO is bound to the surface Zn2+ sites in a nearly upright geometry. In addition, we observed a substantial
coverage-induced frequency shift for CO adsorption on both ZnO surfaces. For ZnO(1010), the ν(C-O) band
shows a red shift of 16 cm−1 when increasing the CO coverage from 0.5 ML (2185 cm−1) to 1 ML
(2169 cm−1). On the ZnO(1120) surface, for isolated CO molecules, a frequency of 2192 cm−1 was detected.
The CO band shifts to 2170 cm−1 at full monolayer. On both surfaces, the coverage-dependent frequency shift
is attributed to a combination of dynamic and substrate-mediated static adsorbate–adsorbate interactions.