Bamboo shoot nanotubes with diameters increasing from top to bottom: Evidence against the field-assisted dissolution equilibrium theory

Abstract

The formation mechanism of anodic TiO2 nanotubes remains controversial. To further explore the growth of nanotubes, we compared the difference between galvanostatic and potentiostatic anodizing processes, and discovered that the electric current kept dropping in the final phase of the potentiostatic process, whereas the voltage kept rising in the final phase of the galvanostatic process. Therefore, the so-called steady growth state of the field-assisted dissolution equilibrium model was not observed in either anodizing process. Furthermore, during galvanostatic anodization, we discovered for the first time that the diameters of the nanotubes gradually increased from top to bottom (from 134.9 nm to 202.0 nm). Bamboo shoot nanotubes with increasing diameters were fabricated through one-step and multi-step galvanostatic anodizations. These experimental observations cannot be explained by the field-assisted dissolution equilibrium theory. However, the electronic current model provides a reasonable explanation for the results obtained in this paper. Keywords: Anodization, TiO2 nanotubes, Dissolution equilibrium, Formation mechanism