Molecular dynamics simulation study on surface structure and surface energy of anatase

Molecular dynamics simulations were performed to investigate the relaxed structures and surface energies of perfect and pit anatase TiO₂ surfaces. It is shown that the slab containing more than two unit-cell layers away from the fixed layer expresses the surface characteristics of perfect anatase TiO₂ (1 0 1) and (1 0 0) surfaces well, while the slab containing more than one unit-cell layer away from the fixed layer expresses the surface characteristics of the (0 0 1) surface well. Their surface energies follow the sequence (0 0 1) < (1 0 1) < (1 0 0). Simulation results also indicate that the pit edges expose many undercoordinated atoms, and the more highly undercoordinated atoms exhibit the larger displacement vectors. Moreover, the surface energy of the pit surface is higher than that of the perfect surface. The surface energies of pit anatase (1 0 1) surfaces are linearly related to the pit sizes along the [11̄1̄] and [0 1 0] directions, and the changes in their surface energies are less than 0.05 J m^-2, while the surface energies increase sharply with the increase in pit depth within 1 nm. Therefore, for anatase (1 0 1) surface, in order to obtain a higher surface energy, one may increase the pit sizes, particularly along the [1 0 1] direction.