H-Bond Features of Fully Hydroxylated Surfaces of Crystalline Silica Polymorphs: A Periodic B3LYP Study

Different models of hydroxylated surfaces of quartz, cristobalite, and tridymite have been studied with the hybrid B3LYP functional using the Gaussian basis set of polarized double-zeta quality with periodic boundary conditions. Starting from the optimized bulk structures of the polymorphs, 2D stabs exhibiting low (hkl) crystallographic planes have been cut, dangling bonds healed by hydroxyl groups, and the final structures fully optimized. The H-bond pattern at a given surface depends on the (hkl) plane and on the OH group density, exhibiting isolated, weakly interacting pairs, short chains, or strings extending through the whole surface. Cases in which no H-bonds are present envisage either a very low OH density or slab structural rigidity which hinders the OH groups to establish H-bond contacts. The thermodynamics of surface hydroxylation of the considered polymorphs has been shown to correlate with the strength of the H-bonds formed at the surfaces measured by the bathochromic shift of the nu(OH) stretching frequency with respect to the value for a free surface OH group. Simulation of the vibrational spectra in the OH stretching region for all Surfaces of each polymorph showed a general good agreement with the experimental spectra recorded on polycrystalline powdered samples validating the present surface models for further studies on molecular adsorption.