Electronic and optical properties of anatase TiO2 nanotubes

The structural, electronic and optical properties of anatase TiO2 nanotubes are investigated using pseudopotential density-functional theory (DFT) calculations. Band structure and density of states (DOS) show discrete energy levels at the top of the valence band and immediately below the Fermi level. This groundwork of electronic structure calculations predicts a possible band gap modification of the TiO2 nanotube structure compared to the bulk. We observe significant electronic structural differences with the change in dimensions (radius) of the nanotube. The photon energy dependent imaginary part of the dielectric function further indicates the exact optical transitions from occupied valence bands to unoccupied conduction bands. All allowed optical transitions determine the actual optical band gap of anatase TiO2 nanotubes, which is higher than the direct band gap at the Γ point of the band structure. The result reveals that the band gap of TiO2 nanotube is not only dependent on the tube radius but also on other parameters such as the tube wall thickness and atomic arrangement in the wall of the tube. This result also shows the considerable optical anisotropy along the two axial directions of the nanotube.