Morphology evolution of TiO2 nanotubes by a slow anodization in mixed electrolytes

Investigations of anodic TiO2 nanotubes (ATNTs) are generally based on NH4F electrolytes rather than mixed electrolytes and nanotube embryos are hardly derived by direct in situ methods. Herein, the morphology evolution of ATNTs grown by a slow anodization in mixed NH4F/H3PO4 electrolytes was studied. And the total anodizing current was quantitatively separated into ionic current and electronic current by a novel method. The addition of H3PO4 was found to extend the current increase stage but defer the quasi-equilibrium stage during anodization, which help to extend the formation process of nanotube embryos and explore it. H3PO4 addition decreased the nanotube length but increased the nanotube diameter. The relationship between current-time curves and nanotube geometry parameters (length and diameter) can be clarified based on the electronic current and oxygen bubble mould combined with oxide flow model. In mixed electrolytes, PO43 − anions hinder the migration of F− anions and cause the decrease of electronic current. Furthermore, PO43 − anions hinder the migration of O2- and Ti4 + ions, increase the thickness of barrier oxide, and further cause the decrease of ionic current and oxide growth rate. It is the first time to observe the existence of distinctive upper layer in mixed electrolytes, leading to no apparent ribs and high stability of nanotubes.