Electronic structure and optical properties of transition metal doped titanate nanotubes

The electronic structure and optical properties of transition metals （TM）-doped （Fe, Co or Ru） titanate nanotubes （TNTs） have been investigated by using the plane-wave ultrasoft pseudopotential method. TM atom intercalation into the interlayer region of the layered TNTs structure has great influence on the geometry of the original structure and the formation energy of Co-doping or Ru-doping is lower after geometry optimizations. Doped atoms bond with the surrounding O atoms to form a kind of solid solution. The results show that, Fe,Co or Ru intercalation reduces the band gap of TNTs and introduces new energy levels in the band gap, extending the absorption edge of the doped TNTs well into the visible region, which is due to the appearance of the b1g（dx2－y2） and a1g（dz2） states; partial impurity bands are in semi-filled states, which act as the capture centers of holes to reduce the recombination of electrons and holes; the valence bands of the doped TNTs move towards low energy, making the holes more oxidative. Finally, the absorption spectrum of the doped TNTs shows that Ru-doped TNTs has a stronger absorptive capacity in the visible region.