Abstract
TiO2 nanotubular films prepared using the anodic oxidation process applied to various forms of metal titanium are promising materials for photocatalytic applications. However, during successive anodizations in batch-anodization cells, the chemical composition of the NH4F- and water-based ethylene glycol electrolyte changes with each subsequent anodization, which greatly affects the final photocatalytic properties of the annealed TiO2 nanotubular films. In the present study, 20 titanium discs (Φ 90 mm) were sequentially anodized in the same anodization electrolyte. The chemical composition of the electrolyte was measured after each anodization and correlated with the anodization current density, temperature, electrical conductivity, and pH of the electrolyte and with the morphology, structure, composition, and photocatalytic activity of the resulting TiO2 nanotube films. It was found that the length of the TiO2 nanotubes decreased with the age of the electrolyte due to its lower conductivity. The subsurface chemical composition was evaluated by time of flight secondary ion mass spectrometry (ToF SIMS) analyses, and the integrated ToF SIMS signals over a depth of 250 nm for the TiO2 nanotube films showed that the concentration of F– in the annealed TiO2 film increased with each subsequent anodization due to the increased pH value of the electrolyte. As a consequence, the concentration of the OH– and O2– species decreased, which is a major reason for the reduced photocatalytic activity of the TiO2 films. It is proposed that the length of the TiO2 nanotubes does not play a decisive role in determining the photocatalytic activity of the TiO2 nanotube films. Finally, the best measured degradation results of 60% for caffeine were thus achieved for the first anodized titanium discs. After that the efficiency gradually decreased for each subsequent anodized disc.
Highlights
Anodic oxidation is a well-known process that was first used for the protection of aluminum in the early 20th century
The anodic oxidation of the titanium was monitored by measuring the electrical current density, temperature, and electrical conductivity of the electrolyte during the anodization
All these parameters greatly influence the growth of the TiO2 nanotubes, their morphology, and properties
Summary
Anodic oxidation is a well-known process that was first used for the protection of aluminum in the early 20th century. Article cations at the oxide/electrolyte interface.[14] This knowledge enables the synthesis of an immobilized TiO2 photocatalyst with specific morphological characteristics, which can be used for various applications.[15,16] in order for the anodic oxidation of titanium to be used for the industrial production of TiO2 nanotube films, a comprehensive understanding of the changes in the electrolyte’s composition during the anodization process is needed. Most of the studies reported in the literature focused primarily on the influence of the electrolyte’s aging on the morphological changes to the TiO2 nanotube films. The concentration of negative ions in an annealed TiO2 film up to a depth of 250 nm was measured, which is essential for explaining the photocatalytic activity of the anodized films
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