Anodic TiO2 nanotubes: double walled vs. single walled.

Electrochemical formation of self-organized TiO2 nanotube layers has been a highly active research field for more than 10 years. In the present manuscript we investigate the formation of two distinctly different anodic TiO2 nanotube morphologies, 'single walled' and 'double walled' tubes, which are formed mainly depending on the nature of the anodization electrolyte. While the widest used electrolytes are ethylene glycol (EG) based, forming double walled structures, tubes formed in dimethyl sulfoxide (DMSO) based electrolytes show a single tube walled morphology. Here we provide reasons for the formation of double walled tubes, characterize tubes for their composition, structure and certain properties, and give measures to suppress or minimize double wall formation. Except for the fact that in DMSO single walled tubes are formed, we also show that they grow sufficiently slowly to allow partial crystallization of the tubes during growth--this, in turn drastically influences their electronic properties. Finally we discuss the effects and potential consequences of double or single wall growth for TiO2 nanotube applications.