Vertical and lateral order in adsorbed water layers on anatase TiO ₂(101)

The structure and energetics of thin water overlayers on the (101) surface of TiO ₂-anatase have been studied through first-principles molecular dynamics simulations at T = 160 K. At one monolayer coverage, H ₂O molecules are adsorbed at the 5-fold Ti sites (Ti _5c), forming an ordered crystal-like 2D layer with no significant water-water interactions. For an adsorbed bilayer, H ₂O molecules at both Ti _5c and bridging oxygen (O _2c) sites form a partially ordered structure, where the water oxygens occupy regular sites but the orientation of the molecules is disordered; in addition, stress-relieving defects are usually present. When a third layer is adsorbed, very limited parallel and perpendicular order is observed above the first bilayer. The calculated energetics of multilayer adsorption is in good agreement with recent temperature-programmed desorption data.