The double-walled nature of TiO2 nanotubes and formation of tube-in-tube structures - a characterization of different tube morphologies

•Effect of anodization parameters and post-growth treatments on nanotube morphology.•Double-walled NTs consisting of inner and outer shells form at room temperature.•Optimized chemical treatment of double-walled NTs leads to single-walled tubes.•A significant thickening of NT walls is observed when anodizing at low temperatures.•Low T anodization represents a precursor state to a tube-in-tube configuration.

In the present work we show how to achieve different nanotube morphologies (namely, not only single- and double-walled tubes, but also a tube-in-tube configuration) by combining specific anodization parameters. We characterize as-grown tube layers in terms of morphology, chemical composition and properties. As-grown tubes exhibit a double-walled morphology, that is, they consist of an inner and an outer shell. The low quality inner shell can be removed by an optimized chemical treatment thus leading to nanotubes of a higher quality oxide and with a single-walled nature. Double-walled tubes grown at low electrolyte temperature provide a thick inner shell that after adequate annealing can form a unique tube-in-tube morphology.