COORDINATION CHEMISTRY OF CO AND NH3 ON ZNO(0001) - A MOLECULAR CLUSTER STUDY OF THE CO AND NH3 BONDING INTERACTION WITH A D10 ION

A detailed investigation of the electronic structure of CO chemisorbed on the ZnO(0001) polar surface has been carried out by using the local-density-functional molecular-cluster approach. A Zn22O22/CO model cluster is used to evaluate internuclear distances, adsorption energies and vibrational frequencies of the surface-CO and C-O vibrational modes. The chaining to the rest of the solid has been simulated with pseudo-atoms carrying a not-integer nuclear charge. A realistic description of the electronic structure of the Lewis acid site is shown to be crucial to obtain a correct chemisorptive interaction. In particular, it appears that removing half an electron from each surface Zn allows an excellent agreement between experiment and theory as a consequence of the absence of occupied dangling bonds on ZnO(0001). Preliminary results relative to NH3 chemisorption over the same surface are also presented and discussed. The bonding mechanism of CO to ZnO(0001) is dominated by a covalent interaction involving a donation from the highest occupied molecular orbital of CO into the empty levels of the coordinatively unsaturated Zn surface ions. The same kind of mechanism is active in NH3 chemisorption, even though the electrostatic interaction between the NH3 dipole moment and the field at ZnO(0001) is at least as important as the covalent one. Slight variations of the electronic structure of the surface greatly affect the bonding of CO to the surface itself as a consequence of a consistent donation into the CO lowest unoccupied molecular orbital from the partially occupied dangling bonds of the Zn unsaturated ions nearest to the Lewis acid site. Transition state calculations seem to indicate that the assignments of the ultraviolet photoelectron spectra of ZnO(0001) after exposure to CO or NH3 need to be slightly modified.