Explaining improved photocatalytic activity of Double-walled TiO2 Nanotube: A hybrid density functional calculation

[Display omitted] •TiO2 NT constructed from (101) slab was studied using m-B3LYP for the first time.•The II-type band structure of double-walled NT was inferred from single-walled one.•The calculation identified the II-type and smaller band gap of double-walled TiO2 NT.•The mechanism of photocatalysis for double-walled TiO2 has been explained. The single-walled and double-walled TiO2 NTs rolled up with rectangular anatase (101) slab, have been investigated using modified hybrid density functional (m-B3LYP) with all-electron Gaussian basis set. We have considered single-walled and double-walled TiO2 NTs only with chirality of (0, n) and (0, m)@(0, n), respectively. Their equilibrium geometric structure, stability, electronic structure and band gap edge position relative to water splitting redox potential have been calculated. For single-walled and double-walled TiO2 NTs, the diameters and Ti-O bond lengths remain unchanged compared to TiO2 slab. To analyze the stability of TiO2 NTs, we have calculated the strain energies and formation energies which decrease with the increased diameters, and simulated the binding energies for double-walled ones to describe two walls interaction. when increasing the NTs diameters, the band gap widths of single-walled TiO2 NTs decrease with the valence band and conduction band moving downward. The double-walled TiO2 NTs exhibit II-type band structure characteristics, and their band gap widths are considerable smaller than that of single-walled ones, with the band gap edge straddling the water splitting redox potential. Based on these important findings, we can explain the mechanism of improved photocatalytic activity of double-walled TiO2 NTs as compared to the single-walled ones.