Molecular Dynamics Simulation of First-Adsorbed Water Layer at Titanium Dioxide Surfaces

The behavior of the first-adsorbed water layer at titanium dioxide surfaces is critical to the fundamental understanding of titanium dioxide-based applications. Using classical MD simulations, we study the properties of first-adsorbed water layers at four TiO2 surfaces, including the density profile, the angular orientation distribution, the HB structural and dynamic properties, and the vibrational spectra of water molecules in the first-adsorbed water layer. The calculation results reveal the characteristics of water. (a) Rutile (110) has O-w atoms of water that are located at the top sites of Ti-sc, and two H atoms are facing away from the surface. (b) Rutile (011) has water molecules that lean on the surface with one H atom directed toward the surface O-2c atoms and the other one pointing toward the bulk water. (c) TiO2-B (100) has water that forms the "H-up" and "H-down" configurations. The "H-up" configuration has the O-w atoms atop the Ti-sc sites with two H atoms pointing toward the bulk water. The "H-down" configuration has both H atoms pointing toward the surface O-2c sites. (d) TiO2 B (001) has water that has a random distribution; yet, the in-layer HBs promote the formation of small water clusters near the surface. The vibrational spectra, the HB network strength, and the HB lifetime are also analyzed in this work. A significant red shift of the vibrational spectra suggests an enhanced HB network, which also results in a much longer HB lifetime. For the studied surfaces, the TiO2-B (100) has the most stable HB network, which is evidenced by the slowest decay of the HB lifetime.