Simulation of anodizing current-time curves and morphology evolution of TiO₂ nanotubes anodized in electrolytes with different NH₄F concentrations

Highlights: • Total current curve could be separated into two parts: ionic current and electronic current. • Correlation between NH₄F concentration and morphology of TiO₂ films is elucidated. • Nanotube length is determined by the ionic current rather than the total current. • Surface morphology is related to the electronic current and the evolution rate of O₂. • We also propose a feasible method to predict nanotube length. - Abstract: Anodic TiO₂ nanotubes (ATNTs) have been investigated for many years. However, the kinetics of oxide growth still remains unclear as well as the relationship between structural features and anodizing parameters. Here, the simulation and separation of anodizing current-time curves are proposed to overcome this challenge. A series of constant voltage anodizing processes in different concentrations of NH₄F solutions have been compared in detail. The effect of NH₄F concentration on the morphological structure and length were systematically investigated. The morphology images show that ATNTs with lotus-root-shaped nanostructure can also be fabricated when the same voltage are adopted in the second-step anodization as the first-step anodization. We separate the total anodizing current into ionic current and electronic current according to a theoretical formula and find a linear relationship between nanotube length and steady-state ionic current. The interesting results indicated that, the growth of nanotubes is more dependent on the ionic current while the surface morphology of TiO₂ nanotubes is related to electronic current and high NH₄F concentration is beneficial to the growth of ribs around the nanotubes