Effect of heat treatment on the crystal phase composition, semiconducting properties and photoelectrocatalytic color removal efficiency of TiO₂ nanotubes arrays

TiO₂ nanotube films were grown by potentiostatic anodization at 30 V during 2 hours in 0.05 M NH₄F in ethylene glycol (1% water) electrolyte. TiO₂ anodic films were heat treated at different temperatures during 0.5 h, and during different time length at the same temperature (600 °C). Crystal structure of the film was characterized by XRD measurements showing the appearance of anatase phase at 325 °C. The anatase formed in these films was preferably oriented towards [001] direction, which seems to be the origin of its thermal stability even at elevated temperatures, such as 800 °C. On the other hand, rutile was detected at temperatures higher than or equal to 600 °C, simultaneously with the thickening of the barrier layer. Glancing angle X-ray diffraction (GAXRD) measurements showed that rutile is mainly formed in the inner part of the film, meanwhile anatase phase remains in nanotube crystals (outer part), even at 700 °C. Indigo carmine decoloration was improved by increasing the heat treatment temperature which provided a better electron transport through the film; however, the appearance of rutile in the film drastically decreased the photoelectrochemical performance. Mott-Schottky measurements showed that rutile containing films have a more negative flat band potential than those only composed of anatase, which allowed us to propose that rutile film formed in the base of the tubes by thermal oxidation acts as a barrier for the transport of photogenerated electrons towards titanium substrate increasing the recombination in the film.