Water adsorption on hydroxylated silica surfaces studied using the density functional theory

We present an ab initio investigation of water adsorption on ordered hydroxylated silica surfaces, using the density functional theory within the ultrasoft pseudopotentials and generalized-gradient approximation. The (100) and (111) surfaces of the hydroxylated cristobalite are used as substrates to adsorb water clusters and overlayers. Water adsorbs through hydrogen bonds formed between water and surface hydroxyl groups on the beta(alpha)-cristobalite (100) surface. A large enhancement of the hydrogen bonding in the adsorbed water dimer is observed, which can be inferred from the shortened hydrogen-bond (H bond) length, the vibrational spectra from the molecular dynamics simulation and the redistribution of electron density. At one monolayer (ML) coverage, a "tessellation ice," with characteristic quadrangular and octagonal hydrogen-bonded water rings, is formed. It has two types of H bonds and can exist on two different adsorption sites with two different OH orderings in a surface supercell. Our study is further extended to the beta-cristobalite (111) surface. Based on these studies, we find that the water-silica bond, which comprises several H bonds, is usually stronger than other associative water-surface interactions. The H bonds between water and surface usually differ in strength-and hence, in vibrational spectra-from those between adsorbed water molecules. Because the (100) and (111) surfaces sustain different silanol groups (geminal and isolated silanols), a well-defined two-dimensional tessellation ice phase can be observed only on the cristobalite (100) surface. On beta-cristobalite (111) surface, however, isolated water molecules, hydrogen-bonded to the surface hydroxyls, are formed, even at 1 ML coverage.