The role of fluoride and phosphate anions in the formation of anodic titanium dioxide nanotubes

Researches on the application of anodic TiO2 nanotubes (ATNTs) have received extensive attention, but the formation mechanism of ATNTs is still controversial. Here, different from the pure NH4F electrolyte, the role of PO43- anion in the formation of ATNTs is systematically studied based on the NH4F/H3PO4 mixed electrolyte. In the NH4F/H3PO4 mixed electrolyte containing the same concentration of fluoride ions, it is found that no nanotubes are formed but oxide films with cavities are observed. This interesting result contradicts the traditional field-assisted dissolution reaction of fluoride ions. Based on the oxygen bubble mold and the ionic current and electronic current theory, explanation is that PO43- hinders the migration of F-, thereby inhibiting the generation of electronic current and oxygen gas. Oxygen bubbles released are not enough to play a role as mold, in which case oxides cannot grow up around the oxygen bubbles to form nanotubes. Moreover, it is also found that the formation mechanisms of two types of holes on the surface are quite different. The dissolution reaction of fluoride ions can only result in fake holes on the surface. While the real pores connected to nanotubes are formed by oxygen bubble mold under enough electronic current.