AB-INITIO STUDY OF CO AND H-2 INTERACTION ON ZNO SURFACES USING A SMALL CLUSTER MODEL

We have studied the adsorption of H-2 and CO molecules, as well as the dissociation of H-2, on the (ZnO)(6) cluster model using the ab initi o Hartree-Fock method. The effective core potential was used for Zn, C , and O atoms at double-zeta-type valence basis set level, whereas for H we used Dunning's basis set, We have also added polarization and di ffuse functions to the O, C, and H basis set. The CO molecule interact s with the lowest coordination zinc sites which are located on the edg e between the (0001) and (10 (1) over bar 0) surfaces. The decrease in CO bond length upon adsorption on ZnO surfaces is associated with the charge transfer from CO to the surface, Our calculations indicate the 5 sigma orbital from adsorbed CO stabilized to a 1.56 eV deeper energ y, Of all the configurations investigated, the molecular H-2 interacti on has the lowest binding energy with a decrease in H-2 bond strength, The H-2 molecule also dissociates on the zinc and oxygen sites of the ZnO cluster, and the preferential dissociation site is the oxygen whi ch has a coordination number of two. The H-2 dissociation shows a larg e stabilization energy for the most stable adsorption site which is th e lowest coordination site. Molecular CO and H-2 adsorption yields a s maller change in the estimated energy gaps and ionization potentials, We have also analysed the geometry of the adsorbed molecules, the Mull iken charge, the orbital SCF energies, and also the molecular orbital densities and contour plots. Our results are compared with the availab le experimental data, (C) 1997 Elsevier Science B.V.