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.