Study of a family of 40 hydroxylated β-cristobalite surfaces using empirical potential energy functions

We present a study of a family of 40 unique hydroxylated β-cristobalite surfaces generated by cleaving the β-cristobalite unit cell along crystallographic planes to include a combination of several low Miller index surfaces. The surface silicon atoms are quantified as percentages of Q² and Q³ centers based on their polymeric state. We find that Q³ centers are, on average, three times more abundant than Q² centers. To study the surface properties, we use two different empirical potential energy functions: the multibody potential proposed by Fueston and Garofalini (J. Phys. Chem. 1990, 94, 5351) and the newly developed CHARMM potential by Lopes et al. (J. Phys. Chem. B 2006, 110, 2782). Our results for the surface water interactions are in good agreement with previous ab initio theoretical studies by Yang et al. (Phys. Rev. B 2006, 73, 146102) for the (100) surface. We find that the most commonly studied family of {100} surfaces is unique and is the only surface with 100% abundance of Q² centers, whereas there are nine examples of surfaces with 100% Q³ centers. The predominantly pure Q³ surfaces show no hydrogen bonding with the neighboring surface hydroxyl groups and weakly adsorb water overlayers. This is markedly different from the {100} pure Q² surface that shows strong hydrogen bonding within the surface groups and with water. As compared to all the surfaces studied in this work, we find that the {100} surfaces are not true representations of the overall β-cristobalite surfaces and their properties. We find that the two main factors that characterize the physical properties of silica surfaces are the polymeric state of the silicon atom and surface topography. Two types of pure Q³ crystallographic planes have been identified and are labeled as Q^3A and Q^3B based on the differences in their topological features. Using the {111} and {011} surfaces as examples, we show that the Q^3A surface adsorbs H ₂O that forms a stable monolayer, but the Q^3B surface fails to form a stable H₂O overlayer. Other crystallographic planes with different ratios of Q² to Q³ centers are contrasted by the differences in the hydrogenbonding network and their ability to form ordered H₂O overlayers.