IR-spectroscopy of CO adsorption on mixed-terminated ZnO surfaces

Abstract

The adsorption of CO on two different mixed-terminated, single-crystal surfaces of zinc oxide, ZnO(101−0) and ZnO(112−0), was investigated by employing infrared reflection absorption spectroscopy (IRRAS) using p- and s-polarized 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(101−0), the ν(C-O) band shows a red shift of 16cm−1 when increasing the CO coverage from 0.5 ML (2185cm−1) to 1 ML (2169cm−1). On the ZnO(112−0) surface, for isolated CO molecules, a frequency of 2192cm−1 was detected. The CO band shifts to 2170cm−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.