The key factor for fabricating through-hole TiO2 nanotube arrays: a fluoride-rich layer between Ti substrate and nanotubes

Through-hole TiO2 nanotube arrays (THTNA) are fabricated successfully by applying a large-voltage pulse (ΔV ≥ 40 V) at the end of anodization, and a mechanism is proposed that the fluoride-rich layer (FRL) between Ti substrate and nanotubes is the key factor for fabricating THTNA. In order to confirm the mechanism, the effects of temperature of voltage pulse on the morphology of the bottom of TiO2 nanotubes are explored. The results show the inner diameter of the bottom became larger with the temperature increasing due to the wall thickness of bottom of TiO2 nanotubes decreased and assisted by the increased fluorine content of the bottom, which is strong evidence for the mechanism proposed. What is more, the inner diameter of the bottom of THTNA can be manipulated via this novel mechanism.