First Principles Study of Hydrated/Hydroxylated TiO2 Nanolayers: From Isolated Sheets to Stacks and Tubes

Periodic density functional calculations are carried out to investigate the structure and the stability of hydrated/hydroxylated TiO2 layered compounds, nanosheets, and nanotubes. Due to a very efficient interlayer hydrogen bonding, the ABA-stacked "step 3" H2Ti3O7 compound is found to be the most stable bulk phase, in agreement with the experiment. For single sheets in a water-rich environment other forms are instead favored, all close in energy, namely, "step 2" titanates, hydroxylized-anatase-like layers, and lepidocrocite-TiO2. Finally, it is shown that a lepidocrocite-TiO2 sheet, when hydroxylated only on one side, spontaneously forms a scroll-like nanotube. The nanotube diameter estimated from our models perfectly matches the similar to 3 nm value observed for