Formation mechanism of anodic titanium oxide in mixed electrolytes

- Compact anodic titanium oxide (CATO) and anodic TiO2 nanotubes (ATNTs) were compared.
- A distinct transform from CATO films into the porous ATNTs was observed firstly.
- FESEM evidences for oxygen bubble mould effect were provided firstly.
- Formation mechanisms for different titanium oxide films are confirmed consistent.

The formation mechanism of anodic TiO2 nanotubes (ATNTs) remains mysterious and complicated. Usually, ATNTs are fabricated in pure NH4F electrolytes rather than mixed electrolytes and Ti anodizations are carried out under potentiostatic rather than galvanostatic conditions. In this work, anodizations under the potentiostatic and galvanostatic conditions are conducted. The formation processes of titanium oxide film in different mixed electrolytes are explored in detail. By changing the mass ratios of oxalic acid and ammonium fluoride, different kinds of anodic titanium oxide films are fabricated: the compact anodic titanium oxide (CATO) films, transient-state anodic titanium oxide films and porous films (ATNTs). A distinct transform from CATO films into ATNTs was observed. Cavities within oxide and hemispheric bottoms are found, confirming the presence of oxygen bubble mould. Based on the electronic current and ionic current model, a consistent formation mechanism for three kinds of anodic titanium oxide films in mixed electrolytes is proposed.

172