Rapid growth of TiO2 nanotubes under the compact oxide layer: Evidence against the digging manner of dissolution reaction

The growth mechanism of porous anodic oxides still remains controversial. The field-assisted dissolution theory suggests that the growth of nanotubes is driven by a “top-down” digging manner. Here, a three-tier structure (nanotubes + compact layer + nanotubes) was prepared by three steps anodization. The anodizing conditions of the first-anodized nanotubes and the third-anodized nanotubes were exactly the same, but the length and diameter of the third-anodized nanotubes were larger than those of the first-anodized nanotubes. These are all unexplainable by the digging manner of the field-assisted dissolution. Based on the theoretical expression of voltage–time curves under constant current anodization, the total anodizing current can be divided into ionic current and electronic current. Through fitting results, it is found that the electronic current of the third anodization appears as a large value at the beginning and the ionic current of the third anodization is higher than that of the first anodization, which is consistent with the experimental phenomena. In other words, the reason why the length and diameter of the third-anodized nanotubes are larger than those of the first-anodized nanotubes is due to the large ionic current during the third anodization.