Critical parameters and factors in the formation of spaced TiO2 nanotubes by self-organizing anodization

Highlights: • For spaced TiO₂ NTs spacing is initiated in the very early stage of nanotube growth. • Spaced tube diameter can be varied from 45 nm to 215 nm. • Tube-to-tube spacing is maintained by small diameter tube stacks. • Tube spacing can be adjusted from 21 nm to 151 nm. • We introduce a model for the growth of spaced nanotubes. Self-organized TiO₂ nanotube arrays can be grown under a wide range of electrochemical conditions. In the present work, we evaluate the occurrence of spacing between tubes and the connection of this effect to organization of tubes on two-size scales. The results show that tube-spacing is initiated in the very early stages of anodization between individual pore morphologies. Furthermore, the spacing, as well as the organization on two-size scales can be controlled by changing the anodization conditions, e.g., electrolyte composition, applied voltage and temperature. Namely, adjustment of H₂O content, electrode temperature and voltage can lead to spaced nanotubes, and allow to control spacing. Finally, we draw conclusions on the possible mechanism relevant to the growth of spaced tubes.