Shaping Nanometer-Scale Architecture Through Surface Chemistry

摘要

We have found that scrolled titania nano-tubes, axially anisotropic nano-objects, are shaped because of the formation of undercoordinated surface sites that fold layers of anatase titania into scrolled nanotubes with large curvatures. The additional driving force for self-coiling into tube-like structures is obtained by intralayer coordination of undercoordinated sites in scrolled structures. Addition of positively charged ions capable of penetrating the interlayer spacing causes charging of the undercoordinated sites and unscrol-ling of the nanotubes into sheet-like structures. In addition, unscrolling was found to change the crystalline structure from quasianatase in the nanotubes to quasirutile in the sheet-like structures. Reversible sheet-to-tube transitions, and associated lattice transformation, were obtained by deprotonation (dc-charging) of titania nanoshcets. By taking advantage of the different coordinations of Ti atoms on the surface of TiO_2 nano-objects in different environments, we gained control over the synthesis of axially anisotropic nano-objects, such as scrolled nanotubes and nanosheets, and compared them to nano-objects that have axial symmetry, such as nanoparticles and nanocubes. These findings demonstrate the importance of surface states in nanoscale regimes and open new synthetic avenues for control of nanoparticle shape through manipulation of surface chemistry.