Causes for the Formation of Titania Nanotubes During Anodization

Titania nanotube arrays were prepared in the electrolyte containing dimethyl sulphoxide and HF through anodization method and the morphology and composition of the nanotube arrays were characterized through scanning electron microscopy, X-ray photoelectron spectroscopy, and Auger electron spectroscopy. The causes for the formation of nanotubes have been discussed according to the experimental results. Nanopores are formed firstly at the early stage of anodization, O^2- ion needed to oxidize the titanium metal below the pore wall must diffuse inward from both sides of the pore wall. Because of the different diffusion resistances, O^2- concentrations are different at different positions of the interface between titanium metal and pore wall, leading to different oxide compositions. As a result, the surface of pore wall is mainly composed of high valence oxide TiO₂, while the middle of pore wall is mainly composed of suboxides, such as Ti₂O₃ and TiO. The pore wall would crack easily at the middle low strength suboxides due to temperature changes during anodization, which results in the conversion of nanopores into nanotubes. The selective dissolution of suboxides in the electrolyte leads to the formation of gaps between nanotubes.