Mechanistic aspects and growth of large diameter self-organized TiO2 nanotubes

The present work describes the anodic growth of self-organized TiO2 nanotube layers in glycerol/water/ammonium fluoride electrolytes. In these electrolytes, it is possible to grow nanotubes at potentials between 2 and 40 V leading to tube diameters ranging from 20 up to nearly 300 nm. Within the work we describe the growth stages of the nanotubes and their morphology, as well as the influence of critical electrochemical parameters on the tube growth, such as fluoride concentration and water content in the electrolyte. Furthermore, for nanotubes grown at 40 V, the presence of crystalline anatase structure can be observed. We show that there is a significant effect of even small additions of water (0.67 vol.%) into the glycerol/ammonium fluoride on the resulting geometry, that the growth rate determining step is fluoride ion diffusion to the tube bottom, and that during anodization permanently a high-field oxide layer (that accordingly increases in thickness with higher applied voltage) is present on the tube bottom.