Titanium nanotubes grown by titanium anodization

In this work we investigated the structural and electrical properties of titanium dioxide (TiO₂) nanotubes (NTs) grown by electrochemical anodization of Ti metal sheets in NH₄ F+ H₂ O+glycerol electrolyte at different anodization voltages (V_a) and electrolyte composition. Our results revealed that TiO₂ NTs can be grown in a wide range of anodization voltages from 10 to 240 V. The maximum NH₄ F acid concentration, at which NTs can be formed, decreases with the anodization voltage, which is 0.7% for V_a <60 V, and decreases to 0.1% at V_a =240 V. Addition of water to the electrolyte results in an increase in NT growth rate and modification of NT film morphology. Glancing angle x-ray diffraction experiments show that as-grown amorphous TiO₂ transforms to anatase phase after annealing at 400 °C and further transforms to rutile phase at annealing temperatures above 500 °C. Samples grown in 30-120 V voltage range have higher crystal quality as seen from anatase (101) peak intensity and reduced linewidth. The electrical resistivity of the NTs varies with V_a concentration and increases by eight orders of magnitude when V_a increases from 10 to 240 V. The resistivity is also greatly dependent on H₂ O concentration increasing with its concentration up to nine orders of magnitude. Temperature dependent I-V and photoinduced current transient spectroscopy were employed to analyze electrical properties and defect structure on NT samples.