Molecular Behavior of Water on Titanium Dioxide Nanotubes: A Molecular Dynamics Simulation Study

Understanding molecular behavior of water on titanium dioxide nanotubes (TiNTs) is of great interest in order to enable potential application of TiNTs in dye-sensitized solar cells, photocatalysis, and biomedical coatings. Using molecular dynamics simulations, we study the static and dynamic properties of water on TiNT with a diameter of ∼1.0 nm. The TiNT modified by a carbon nanotube (CNT) inside is built to investigate the effect of surface chemistry changes on the sorption and diffusion of water. The results show that the water molecules outside TiNT conform with the two-layer model for water on a planar surface. The difference is that the first water layer is further from Ti_5c as an effect of surface curvature, indicating the easier water desorption on the surface. This layer disappears for water inside TiNT, leaving a water layer with the hydrogen atoms pointing to the O_2c to form hydrogen bonds. The simulations also reveal that the orderly structure is destroyed in the carbon modified nanotubes. Diffusion of water contact with the outer TiNT surface is found to be slower than that of water inside tube. While the diffusion is highly improved of water confined in the tube covered by carbon, which is larger than that of water on the outer surface. (Chemical Equation Presented).