First-principles study of the single- and double-walled nanotubes of TiO2

The geometric and electronic properties of the TiO2 single- and double-walled nanotubes (SWNT and DWNT), constructed by rolling the hexagonal nanosheet along the armchair (n,n) and the zigzag (n,0) directions, have been investigated systematically using the methods based on the density functional theory. The SWNTs with size to n=20 have been modeled and studied. The strain energies of the SWNTs decrease monotonically as the radii of the nanotubes increase, regardless of the rolling direction. The band gaps of the SWNTs are increased with the increase of the n value, approaching that of the nanosheet. The stability of the DWNT in respect to their SWNT components is studied and the optimized distance between the walls has been determined. The band gap values of DWNTs are decreased significantly compared with that of SWNTs due to the offset of the bands of the two constitutive SWNTs. And the value of band gap is almost independent of the intershell distance. The band edges of nanotubes with respect to the redox potentials of water splitting are estimated. The band gaps of TiO2 nanotubes could cover the redox potentials of water splitting, by comparing the band gap position of the bulk anatase with respect to the redox potentials of water splitting.