**3. Highly ordered TNAs and modified TNAs designed by two-step anodic oxidation**

Using one-step anodization, the produced TNAs usually present disparity in tube lengths and relatively rough surfaces. It has been recently demonstrated that a two-step anodization is possible for preparation of highly ordered and uniform TNAs [14, 15] or unique hierarchical top layer/TNAs with significantly better performances in dye-sensitized solar cells [21] and in photoelectrochemical (PEC) water splitting [6]. A typical process of two-step anodization includes (1) the one-step anodization for growth TiO2 nanotubes, (2) subsequent removal of the as-grown TNAs by ultrasonication, and (3) the two-step anodization to grow TiO2 NTs from the same substrate [14, 15].
