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_5c, 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_5c 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.