New explanation on formation mechanism of anodic TiO₂ nanotubes

Anodic TiO₂ nanotubes (ATO) have attracted considerable scientific interests due to their various applications and mysterious growth mechanism. In particular, it has puzzled scientists for decades why ATO displays the distinct nanotubular morphology. For comparison, anodic Al₂O₃ (AAO), another important anodic oxide, features nanochannels. Different theories have been proposed, but the tube-formation mechanism is not clearly understood. Here, the top surface and cross-section morphologies of ATO obtained under different experimental conditions are investigated, and a new evolution mechanism of ATO is proposed. The formation, flow, and decomposition of a highly defective metastable sublayer in-between the neighboring pores plays an essential role in the formation of the characteristic morphology features, such as the “cap” layer at the ATO top surface, intertubular gaps, double-layered tube walls, nano-scaled ridges connecting the tube outer walls. The “cap” layer is due to the absence of the metastable substance at the ATO film top surface. The double-wall structure results from the different reaction environment and products at the upper and lower parts of the barrier layer. The intertubular gaps and ridges between neighboring tubes arise from decomposition and volume shrinkage of the metastable sublayer at the tube outer walls. This study offers insights to the growth kinetics of the ATO and other anodic oxides.