Effect of Dense Oxide Film Thickness on the Growth of Lower Layer Nanotubes

There are two opposite mechanisms for the growth of anodic TiO 2 nanotubes. One is the field-assisted dissolution and ejection theory, the other is the oxygen bubble mold. In order to prove the latter mechanism is right, we constructed a three-layer nanotube structure (upper layer nanotubes + dense oxide film + lower layer nanotubes), which was obtained by three steps anodization. The upper and lower layers nanotubes are separated by a dense oxide film. When the dense oxide film is thin, the lower layer nanotubes is longer than the upper layer nanotubes. On the contrary, when the dense oxide film is thick, the lower layer nanotubes is shorter than the upper layer nanotubes. In either case, the anodization conditions of the upper and lower layer nanotubes are the same. This could not be explained by field-assisted dissolution theory. By analyzing the curve, we introduce a new method to decide the time of entering the third stage of the anodization, which is the main stage of nanotubes growth. With the second anodization voltage increases, the time edge entering the third stage become longer, which means the time to growth the nanotubes becomes shorter, and finally leading the different length of the nanotubes.