Anodised TiO2 nano-tubes: Voltage ramp influence on the nano-structured oxide and investigation of phase changes promoted by thermal treatments

TiO2 nano-tubes have been generated by anodising commercially pure (99.6%) titanium in a 1 M solution of Na2SO4 which contains a small amount of NaF (0.1-1 wt%). The use of an initial voltage ramp, prior to the application of a constant voltage to the cell, led to an increase in the thickness of the nano-tube layer. The thermal stability of the nano-tube layer has also been evaluated by annealing at a variety of temperatures, from 200 to 600°C Scanning Electron Microscopy (SEM) of the annealed nano-tubes indicate that they are stable up to 500°C Raman spectroscopy was performed to monitor and evaluate the changes in the crystal structure promoted by the thermal treatments. The as-prepared nano-tubes are amorphous, while anatase begins to form at 300°C and rutile is found at 550°C, a temperature at which the nano-tubes begin to collapse. Initial results obtained by characterising the as-prepared nano-tubes with Transmission Electron Microscopy (TEM) are also discussed. © 2007 WILEY-VCH Verlag GmbH and Co. KGaA.