Modelling of alpha-Cr2O3 and ZnO crystal morphology and its relation to the vibrational spectra of adsorbed CO

A detailed discussion of the morphology of alpha-Cr2O3 and ZnO samples (obtained through combustion methods) as experimentally determined by high-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) is reported. The shape of the microcrystals is compared with the results of atomistic modelling, also after considering relaxation effects. In this way the distribution of the most commonly exposed faces is singled out. The electric field at the surface sites of the different relaxed and unrelaxed faces is then calculated and correlated with the experimentally observed stretching frequencies of adsorbed CO through relationships established by Hartree-Fock calculations. Most of the spectroscopic features observed for CO/alpha-Cr2O3 can be interpreted in terms of formation of Cr3+...CO bonds with predominant electrostatic character. The same does not hold for Zn2+...CO bonds, where the sigma overlap contribution plays a more important role. The methology followed in this paper, is designed to provide;a cross check between calculations and experimental interpretation. From this comparison a better idea of the utility and limitations of the force field methods in studying the surface properties of oxides is also obtained.