Cation Mono- and Co-Doped Anatase TiO2 Nanotubes: An Ab Initio Investigation of Electronic and Optical Properties

The structural, electronic, and optical properties of metal (Si, Ge, Sn, and Pb) mono- and co-doped anatase TiO2 nanotubes are investigated, to elucidate their potential for photocatalytic applications. It is found that Si-doped TiO2 nanotubes are more stable than those doped with Ge, Sn, or Pb. All dopants lower the bandgap, except the (Ge, Sn) co-doped structure, the decrease depending on the concentration and the type of dopant. Correspondingly, a redshift in the optical properties for all kinds of dopings is obtained. Even though a Pb mono- and co-doped TiO2 nanotube has the lowest bandgap, these systems are not suitable for water splitting, due to the location of the conduction band edges, in contrast to Si, Ge, and Sn mono-doped TiO2 nanotubes. On the other hand, co-doping of TiO2 does not improve its photocatalytic properties. The findings are consistent with recent experiments, which show an enhancement of light absorption for Si- and Sn-doped TiO2 nanotubes.