A Two-step anodization to grow high-aspect-ratio TiO2 nanotubes

High-aspect-ratio TiO2 nanotubes with small diameter are favored in dye-sensitized solar cells for large dye loading provided by high surface areas. However, long TiO2 nanotubes with small diameter are difficult to grow under usual anodizing conditions due to unavoidable chemical dissolution of the top portion of the as-fashioned tubes. In the present work, two kinds of double-layered TiO2 nanotube arrays were prepared by changing voltage from high to low (i.e., from 30 V to 15 V) or from low to high (i.e., from 15 V to 30 V). It is found that the top layer can serve as a sacrificial layer to protect the continuous growth of the bottom layer from chemical dissolution. Accordingly, the two-step anodization from high voltage to low voltage is proposed to produce high-aspect-ratio TiO2 nanotubes with small diameter underneath a sacrificial top layer.