Hydroxylation of TiO2-B: insights from density functional calculations

Density functional calculations are carried out to investigate the interaction of water with the low-index stoichiometric surfaces of the TiO2-B polymorph of titanium dioxide. Dissociative adsorption is predicted for the (100) surface, whereas mixed dissociative/molecular adsorption is favored on both the (010) and (110) surfaces. On the (001) surface, water is able to stabilize the type-II termination, which is metastable in a dry environment, by converting the oxo ions into hydroxyls. At high temperature, water desorption is likely to convert the hydroxylated type-II surface to a type-I termination, whereas the reverse type-I -> type-II transition is not allowed when re-adsorption occurs. This could explain the experimental observation that surface hydroxyls on TiO2-B surfaces are not fully regenerated upon successive heating and cooling cycles.