Theoretical investigation on water adsorption conformations at aqueous anatase TiO2/water interfaces

The interaction of water with TiO2 plays an important role in electrochemistry, geochemistry and environmental science. Yet, whether water spontaneously dissociates on a defect-free TiO2 surface is still in dispute. To solve this apparently simple but longstanding question, we present a systematic study of aqueous TiO2 interfaces with low-index stoichiometric anatase (101), (001) and (100) surfaces, using a combination of density functional theory based molecular dynamics (DFTMD) and free energy perturbation (FEP) methods. Our simulations show that on a defect-free (101) surface, molecular adsorption is thermodynamically stable due to the more acidic Ti2OH+ site than the TiOH2 site. In contrast, a mixed molecular-dissociative conformation is adopted on both (001) and (100) surfaces because of the existence of two kinds of Ti2OH+ sites with markedly different pK(a) values for the (001) surface and the comparable pK(a) values of TiOH2 and Ti2OH+ sites for the (100) surface, respectively. In addition, an interesting finding is that the pK(a) values for TiOH2 sites are approximately equal on these three surfaces and thereby we regard that the water adsorption state on anatase TiO2 surfaces is mainly dependent on the acid-base chemistry of the Ti2OH+ site. Kinetic analysis further verified the water adsorption features by the calculations of energy barriers for water dissociation and revealed that it is favorable for water dissociation into a terminal hydroxyl with the help of intermediate water molecules, which act as a water proton relay to reduce strain in the transition state.