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 HClO4 and NaCl electrolyte. The resulting nanotubes have a length of about 30 microm, 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 approximaately 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 microm 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 HNO3) solution prior to anodization. Titania nanotubes were prepared, for the first time, by anodization in aqueous H2SO4 electrolyte alone with tube length above 500 nm. Annealing studies were performed, on high aspect ratio Titania nanotube layers produced in HClO4 electrolyte, in the temperature interval of 300 to 550 degrees C. The XRD patterns and TEM data confirmed the formation of single anatase phase after annealing at 450 degrees C with perfect nanoubular structure. While the rutile titania phase starts to emerege after annealing at about 500 degrees C and the evidence for the appearance of rutile phase due to the oxidation of the underlying Ti metal at the interface between nanotube/Ti-metal was given. On the other hand, the nanotubular structure starts to destroy upon annealing temperature of approximate 550 degrees 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.