Quantum mechanical ab initio characterization of a simple periodic model of the silica surface

The electronic, structural, and vibrational properties of a thin silica film are studied by means of periodic ab initio techniques, both Hartree-Fock and density functional. The film is cut from bulk edingtonite, a tetragonal silica structure with five SiO2 groups per unit cell; the dangling bonds are saturated so as to obtain a fully hydroxylated surface. This model system is proved to possess a number of attractive properties which make it suitable for the study of some properties of silica surfaces: (1) the parent structure is rather stable, its energy per unit SiO2 being comparable to that of common zeolites such as faujasite; (2) the formation of the surface involves low energy, no reconstruction, and scarce relaxation; (3) the simplest two-dimensional structure contains only five SiO2 groups in the unit cell and can therefore be treated with limited computational effort; (4) the building block of the two-dimensional periodic structure is easily related to standard cluster models of silica; (5) the density of surface hydroxyls is comparable to that experimentally observed on dehydrated silica; (6) the stretching frequency of the OH group at the surface is very close to that obtained with the best cluster models and to the experimental value for isolated hydroxyls on amorphous silica; (7) a few variants of the fundamental structure are possible, and different low-energy surfaces can be cut from them, which allow the simulation of a number of local situations within the same basic model.