The Morphological Development of Ordered Nanotube Structure Due to the Anodization of Ti Foil with Axial and Radial Current Flow

Abstract

Background: One-dimensional titania nanotubes (TNT) have attracted increasing scientific and technological attention due to their physical properties and their potential applications. Dimensionality and well-aligned ordered structure have a crucial role in determining the properties and performance of titania nanotubes. Therefore, an understanding of the transformation and growth mechanisms to explain the origin of this nanomaterial symmetry is of great importance. Objectives: The relationship between the direction of current flow and the morphology of the anodized foil was investigated to understand the influence of a compact oxide layer formation on the growth of nanotubes. Methods: To achieve the purpose, single (SA) and double-sided anodization (DA) were performed to control the direction of the current flow in this experiment by immersing one side and both sides, respectively in the electrolyte containing 0.6 wt% of NH4F, 1.0 wt% of H2O2, and 98.4 wt% of ethylene glycol (EG) at 60V. Results: It was found that the channeling of current flow into axial and radial directions influenced the effectiveness of oxygen species in the formation of an initial oxide layer. The field-assisted dissolution of the compact oxide layer resulted in a low-symmetry nanotube arrangement, whereas the growth at the interface, which is governed by the plastic flow mechanism, resulted in high-symmetry nanotube arrangement in a hexagonal form. These findings offer an integrated perspective when determining whether the plastic flow mechanism or field-assisted dissolution occurs during anodization. Octahedral titania crystals were also found on the surface of the anodized film, indicating the possibility of forming facet structures via anodization. Conclusion: This research successfully showed the influence of current flow via SA & DA on the growth of TiO2 nanotubes. An axial flow of current in Ti foil during SA resulted in disordered nanotubes, while the radial flow of current during DA stemmed the growth of nanotubes from the Ti-TiO2 interface to form well oriented hexagonal nanotube structures.