Adsorption dynamics of CO on the polar surfaces of ZnO

Measurements of initial adsorption probabilities, S-0, as well as the cover age 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 h ave been characterized by He atom scattering, He atom reflectivity measurem ents, LEED, and XPS. Samples with different densities of defects were exami ned, either by investigating different samples with identical surface termi nation (for O-ZnO) or by inducing defects by ion sputtering at low temperat ures (for Zn-ZnO). The influence of kinetic energy and impact angle (for Zn -ZnO) as well as adsorption temperature on the adsorption dynamics have bee n studied. For both polar surfaces the shape of the coverage dependent adso rption probability curves are consistent with a precursor mediated adsorpti on mechanism. Adsorbate assisted adsorption dominates the adsorption dynami cs 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 sca ling is obeyed for Zn-ZnO. The heat of adsorption for CO on both polar face s 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 inc rease in S-0 with decreasing density of defects was observed, whereas for t he Zn-terminated surface S-0 was independent of the defect density within t he range of parameters studied. (C) 2000 American Institute of Physics. [S0 021- 9606(00)70239-X].