The anodization voltage influence on the properties of TiO₂ nanotubes grown by electrochemical oxidation

A systematic study of titanium dioxide (TiO₂) nanotubes (NTs) grown by electrochemical anodization in NH₄F +glycerol electrolyte has been carried out in a broad range of anodization voltage of 5-350V and acid concentration of 0.1-0.7wt%. It is found that NTs can be grown in the voltage range from 10 to 240V. The maximum NH₄F acid concentration at which NTs can be formed decreases with the anodization voltage (V_a). The maximum NH₄F acid concentration is 0.7% for V_a<60V, and it decreases to 0.1% at V_a = 240V. Glancing angle x-ray diffractometer (GAXRD) measurements show that as-grown amorphous TiO₂ transforms to the anatase phase when annealed at 400 °C, and further transforms to the rutile phase at annealing temperatures higher than 500 °C. The transition temperature from anatase to rutile phase depends on the anodization conditions. The electrical resistivity of the NT increases by eight orders of magnitude when V_a increases from 10 to 240V.