Electrochemical growth of vertically-oriented high aspect ratio titania nanotubes by rabid anodization in fluoride-free media

Vertically-oriented high aspect ratio titania nanotube bundles have been grown by a potentiostatic anodization of titanium sheet in fluoride-free electrolytes. The anodization conditions like the applied voltage were optimized for the synthesis of titania nanotubes in HClO ₄ and NaCl electrolyte. The resulting nanotubes have a length of about 30, outer diameter about 40 nm, inner pore size of about 10 nm and the aspect ratio was 750:1 by anodization in 0.1 M perchloric acid of pH ~ 1 at applied voltage of 20 V. While for nanotubes prepared in 0.3 M NaCl of pH 4.3, the length was above 50\ with the aspect ratio of 1250:1. A method to increase the uniformity of nanotube was demonstrated by pretreatment the titanium sheet by (4 wt% HF + 5 M HNO ₃) solution prior to anodization. Titania nanotubes were prepared, for the first time, by anodization in aqueous H ₂SO ₄ electrolyte alone with tube length above 500 nm. Annealing studies were performed, on high aspect ratio Titania nanotube layers produced in HClO ₄ electrolyte, in the temperature interval of 300 to 550 °C. The XRD patterns and TEM data confirmed the formation of single anatase phase after annealing at 450 °C with perfect nanoubular structure. While the rutile titania phase starts to emerege after annealing at about 500 °C and the evidence for the appearance of rutile phase due to the oxidation of the underlying Ti metal at the interface between nanotubeATi-metaI was given. On the other hand, the nanotubular structure starts to destroy upon annealing temperature of approximate 550 °C by tube flattening and losing of roll-up characteristics as indicated in SEM images. The superior morphology of these high aspect ratio nanotubes and their rapid growth rate foreshadow a bright future in wide applications like dye-sensitized solar cells, water photolysis and nanobiomedical.