First-principle Study of the TiO₂ Single-walled Nanotubes for Photocatalysis Application

The geometric and electronic properties of the TiO₂ single-wall nanotubes, constructed by rolling the most stable nanosheet along the (n,0) and (n,n) directions, have been investigated systematically. The strain energies of the nanotubes decrease monotonically as the radii of the nanotubes increase, regardless of the rolling direction. The band gaps of the nanotubes are increasing with the increase of the n value, approaching the value of the nanosheet. However, there is one nanotube significantly different from the others, i.e., the (6,0) nanotube. The substantial structural change of (6,0) nanotube causes a reduction of the band gap. Then, the isovalent sulfur (S) substitution and adsorption with the (6,0) nanotube have been studied. The band gaps are further reduced by 35% for S substitution and 22% for S adsorption, respectively. Aligning the band edge to that of bulk TiO₂, it shows that the (6,0) nanotube with S substitution and adsorption both cover redox potentials of water splitting.