Ab initio modeling of TiO2 nanosheets

We present density functional calculations on 1-6 monolayer (ML) thick TiO2 films peeled off from the main low-index surfaces of anatase. The structure of the films is optimized both by constraining the lattice constants to those of bulk anatase, and by allowing them to relax. It is found that the stability order of the films does not follow that of the surfaces from which they are derived, and does not increase monotonously with film thickness. Furthermore, relaxing the lattice constants can induce large modifications in the film structure. In particular, two anomalously stable films are found. One derives from the 2 ML (001) film, and rearranges to a lepidocrocite-TiO2 nanosheet. The other one derives from a 4 ML (101) film, and gives rise to a novel phase, where all the Ti ions are fivefold coordinated.