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

Abstract

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.