DFT modeling of anatase nanotubes

TiO2 nanotubes constructed from anatase TiO2 layers were investigated with DFT methods employing the periodic CRYSTAL code. Films of thickness from one to three TiO2 layers (1-3 ml) have been considered. The dependence of strain energies, structural and electron properties on the tube diameter was investigated in the 10-70 angstrom range. Relative stabilities have also been considered. We found that the most stable nanotubes are in the region of D > 50 angstrom: lepidocrocite, fluorite-1 ml and 001-3 ml nanotubes differ in energy by less than 0.1 eV/TiO2. This is in agreement with experimental observations of tubes that have a size that range between 50 and 100 angstrom. At D < 20 angstrom, nanotubes with a 1 ml thickness (fluorite and 101 nanotubes) show higher stability. In addition, present calculations indicate that anatase films with a thickness of 1 to 3 ml only single walled nanotubes can be constructed. All investigated nanotubes possess a high (similar to 5-5.5eV) band gap compared to bulk TiO2 phases (4.3 eV for anatase calculated with the same functional and basis set) that differs by less than 0.1-0.3 eV from the corresponding flat slab and approaches smoothly this reference value.