Structure and properties of self-organized TiO₂ nanotubes from stirred baths

Self-organized porous nanotubular TiO₂ was anodically formed on titanium in 1M Na₂SO₄ electrolyte containing 0.5 wt pct NaF. The oxidation was carried out for 0.5, 1, 2, and 4 hours at 20 V with the baths stirred using (1) magnetic pellet and (2) ultrasonic vibration. During the initial stages of oxidation, a barrier type of oxide film is formed, which gives rise to the formation of pores beneath it. With increasing time of oxidation, the pores self-organize to tubular structure. Of the two types of agitation studied, ultrasonics help in earlier complete removal of the barrier layer. The nanotubes have single-pore diameter of 50 to 90 nm under the magnetic pellet conditions and 55 to 110 nm under the ultrasonic agitation condition. The porosity was of the order of 24 to 30 pct for both types. The charges that have flown onto the coatings are greater under the ultrasonated condition. The as-oxidized coatings were amorphous for both types. The coatings obtained by using magnetic pellet had lower tensile adhesion strengths than those obtained using ultrasonic agitation. Heat treatment for 2 hours at 500 °C maintained the tubular morphology and converted the amorphous coatings to anatase TiO ₂. These anatase containing coatings were analyzed for texture by X-ray pole figures, and it was found that (101) and (200) poles were randomly oriented.