Periodic Density Functional Theory Study of Water Adsorption on the α-Quartz (101) Surface

Plane wave density functional theory (DFT) calculations have been performed to study the atomic structure, preferred H2O adsorption sites, adsorption energies, and vibrational frequencies for water adsorption on the alpha-quartz (101) surface. Surface energies and atomic displacements on the vacuum-reconstructed, hydrolyzed, and solvated surfaces have been calculated and compared with available experimental and theoretical data. By considering different initial positions of H2O molecules, the most stable structures of water adsorption at different coverages have been determined. Calculated H2O adsorption energies are in the range -55 to -65 kJ/mol, consistent with experimental data. The lowest and the highest O-H stretching vibrational bands may be attributed to different states of silanol groups on the water-covered surface. The dissociation energy of the silanol group on the surface covered by the adsorption monolayer is estimated to be +80 kJ/mol. The metastable states for the protonated surface bridging O atoms (O-br), which may lead to hydrolysis of siloxane bonds, have been investigated. The calculated formation energy of a Q(2) center from a Q(3) center on the (101) surface with 2/3 dense monolayer coverage is equal to +70 kJ/mol which is in the range of experimental activation energies for quartz dissolution.