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 I 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 degrees C. Scanning Electron Microscopy (SEM) of the annealed nano-tubes indicate that they are stable up to 500 degrees 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 degrees C and rutile is found at 550 degrees 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. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.