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

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.