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Recently,the synthesis,properties,modifications,and applications of TiO2 nanomaterials have attracted much research attention.Here,based on extensive density functional theory calculations,we explore the stability,electronic structures and optical absorption properties of single-walled TiO2 nanotubes(SWTONTs)and TiO2 nanotube arrays(TONTAs),which is constructed by anatase TiO2(101)monolayer and bilayer,respectively.We obtain the stable Dnd(n=3-5)and S2n(-n,n)(n=3-9)SWTONTs,and find that the SWTONTs energetically prefer the S2n structure.Compared with S2n(-n,n)SWTONTs,the calculated Youngs modulus of Dnd(-n,n)SWTONTs are more stiff due to their relative large strain energies.The band gaps of hexagonal TONTAs are not sensitive to their apertures,which are less than that of TiO2 bilayer.The narrowing 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 feature.These results clearly reveal that the electronic and optical properties of TiO2 nanostructures are strongly associated with their symmetry,dimension and morphology,which provide useful insights into the understanding of the related experimental observations.