Electronic and optical properties of TiO2 nanotubes and arrays: a first-principles study

Abstract

Recently, the synthesis, properties, modifications, and applications of TiO₂ nanomaterials have attracted much research attention. Here, based on extensive density functional theory calculations, we explored the stability, electronic structures and optical absorption properties of single-walled TiO₂ nanotubes (SWTONTs) and TiO₂ nanotube arrays (TONTAs), which are constructed from anatase TiO₂(101) monolayers and bilayers, respectively. We obtained the stable D_nd (n = 3–5) and S_2n(−n, n) (n = 3–9) SWTONTs, and found that SWTONTs energetically prefer S_2n symmetry. Compared with S_2n(−n, n) SWTONTs, the calculated Young's moduli of D_nd(−n, n) SWTONTs are more stiff due to their relatively large strain energies. The band gaps of hexagonal TONTAs are not sensitive to their apertures, which are less than that of TiO₂ bilayers. The narrow band gaps of TONTAs originate from the edge states mainly contributed by the Ti and O atoms at the core region. The calculated optical absorptions of both SWTONTs and TONTAs display anisotropic features. These results clearly reveal that the electronic and optical properties of TiO₂ nanostructures are strongly associated with their symmetry, dimensions and morphology, which provide useful insights into the understanding of the related experimental observations.