Fabrication of periodically aligned vertical single-crystalline anatase TiO₂ nanotubes with perfect hexagonal open-ends using chemical capping materials

A vertically aligned anatase TiO₂ (A-TiO₂) nanotube array has been fabricated by coating a ZnO nanorod (NR) template with a TiO₂ precursor solution. After coating, the ZnO NR cores were selectively etched in an acidic environment to form TiO₂ nanotubes (NTs). More specifically, after growing the ZnO NRs via a hydrothermal method, one drop of the TiO₂ precursor solution was cast to coat the ZnO NRs, the tops of which were previously covered with chemical capping materials by electrostatic interaction, and then the sample was sintered. Finally, the sample was immersed in an acidic solution resulting in selective etching of the ZnO NR cores. Thus, only TiO₂ NTs remained on the substrate. The capping material is effectively used to create a perfect, hexagonal open-ended TiO₂ NT array, which interestingly extends onset absorption towards the visible region.