Formation of various TiO₂ nanostructures from electrochemically anodized titanium

TiO₂ nanostructures, including nanowires, highly ordered nanotube arrays, and single crystalline nanoplates, were obtained from electrochemically anodized Ti foil in a fluorine-containing ethylene glycol electrolyte. The TiO₂ nanowires, formed by the electric field induced chemical splitting of TiO₂ nanotubes by fluoride ions during a lengthy anodization can be isolated from the nanotube arrays by mild ultrasonication. The highly ordered TiO₂ nanotubes buried underneath are thus exposed. The transformation of as-prepared amorphous TiO₂ nanotubes into the anatase or rutile phase as a function of annealing temperature was systematically scrutinized by Raman spectroscopic measurements in conjunction with TEM imaging. In the latter study, the ultramicrotomed sections of the samples clearly exhibited the formation of highly crystalline TiO₂. More importantly, crystalline TiO₂ nanotubes were mechanically broken into intriguing single crystalline TiO₂ nanoplates as a result of ultramicrotoming. The present study not only demonstrates a facile approach to produce high quality TiO₂ nanowires, nanotubes, and nanoplates in a simple manner, but also provides valuable insights into temperature dependent crystalline transformation in the anodic TiO₂ nanotube arrays.