A TiO2 nanostructure transformation: from ordered nanotubes to nanoparticles.

Transformation of TiO2 nanotubes (NTs) to truncated tetragonal bipyramidal shape nanoparticles (NPs) was observed upon thermally annealing titanium dioxide (TiO2) ordered nanotube arrays in fluorine ambient, resulting from the reaction of fluorine ions (F(-)) from the electrolyte residues in long nanotubes grown by anodization in ethylene glycol+NH4F electrolyte. The size of the TiO2 nanoparticles formed depends on the fluorine concentration and can be controlled from 20 to 500 nm. The crystal and optical properties of the nanoparticle layers are superior compared with those of nanotube arrays which are also annealed but without undergoing a morphology transformation, as was shown by means of x-ray diffraction and photoluminescence spectroscopy measurements. The NT-NP transformation mechanism was studied by analyzing the initial stages of the NT-NP transformation. This was achieved by terminating the annealing process in F ambient after 1-5 min. It was found that amorphous NTs first contract, then break down, and finally merge and crystallize to form NPs.