Exploring Rutile (110) and Anatase (101) TiO₂ Water Interfaces by Reactive Force-Field Simulations

We have investigated static/structural as well as dynamical properties of anatase (101) and rutile (110) TiO₂ interfaces with liquid bulk water by reactive force fields (ReaxFF). Layered, well-organized structure of water in the interface region was clearly observed within 6.5 Å of the surfaces. The first-hydration layer molecules adsorbed to unsaturated surface Ti atoms undergo spontaneous dissociation leading, rather controversially, to full coverage of O_2c/O_b by H⁺ and partial coverage of Ti_5c by OH^–. Expected large variations of intrinsic electric field on the interfaces, and drop of electrostatic potential, were detected. Interfacial water was found to be heavily confined with a self-diffusion constant of 2 orders of magnitude lower than 2.28 × 10^–9 m²/s measured in the bulk water region. Moreover, the rotational movement of adsorbed water molecules was found to be considerably hindered as well. On the other hand, the calculated hydrogen-bond lifetime on the interface was shorter than in bulk water for both surface types. Finally, the IR spectra obtained from collective-water-dipole variations in the interfacial region revealed stronger effects on stretching vibrations on anatase (101) than on rutile (110); however, description of liquid-water bond-stretching vibrations generally suffers from lack of accuracy in the applied reactive potential.