Adsorption dynamics of CO on the polar surfaces of ZnO

Measurements of initial adsorption probabilities, S-0, as well as the coverage dependence of the adsorption probability, S(Theta(CO)), of CO on Zn-ZnO [ZnO(0001)] and O-ZnO [ZnO(000 (1) over bar)] are presented. The samples have been characterized by He atom scattering, He atom reflectivity measurements, LEED, and XPS. Samples with different densities of defects were examined, either by investigating different samples with identical surface termination (for O-ZnO) or by inducing defects by ion sputtering at low temperatures (for Zn-ZnO). The influence of kinetic energy and impact angle (for Zn-ZnO) as well as adsorption temperature on the adsorption dynamics have been studied. For both polar surfaces the shape of the coverage dependent adsorption probability curves are consistent with a precursor mediated adsorption mechanism. Adsorbate assisted adsorption dominates the adsorption dynamics for high impact energies and low adsorption temperatures, especially for Zn-ZnO. The He atom reflectivity measurements point to the influence of an intrinsic precursor state. In contrast to the Zn-ZnO surface, for O-ZnO a weak thermal activation of the CO adsorption was observed. Total energy scaling is obeyed for Zn-ZnO. The heat of adsorption for CO on both polar faces varies between 7 kcal/mol (low coverage) and 5 kcal/mol (high coverage). A comparison of He atom reflectivity with S(Theta(CO)) curves demonstrates that CO initially populates defect sites on both surfaces. For O-ZnO an increase in S-0 with decreasing density of defects was observed, whereas for the Zn-terminated surface S-0 was independent of the defect density within the range of parameters studied. (C) 2000 American Institute of Physics. [S0021- 9606(00)70239-X].