The geometric and electronic properties of the TiO_2 single- and double-walled nanotubes (SWNT &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 withsize to n=20 have been modeled and studied. The strain energies of the SWNTs decrease monotonically as the radiiof the nanotubes increase, regardless of the rolling direction. The band gaps of the SWNTs are increased with theincrease of the n value, approaching that of the nanosheet. The stability of the DWNT in respect to their SWNTcomponents is studied and the optimized distance between the walls has been determined. The band gap values ofDWNTs are decreased significantly compared with that of SWNTs due to the offset of the bands of the twoconstitutive SWNTs. And the value of band gap is almost independent of the intershell distance. The band edges ofnanotubes with respect to the redox potentials of water splitting are estimated. The band gaps of TiO_2 nanotubescould cover the redox potentials of water splitting, by comparing the band gap position of the bulk anatase withrespect to the redox potentials of water splitting.
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