Growth of porous anodic TiO2 in silver nitrate solution without fluoride: Evidence against the field-assisted dissolution reactions of fluoride ions

Al, Ti, Zr and other metals can be anodized to form metal oxides with a porous structure or a nanotube structure. However, the mechanism of formation of oxide nanopores is still controversial. Anodic TiO2 nanotubes are usually obtained in an electrolyte containing fluoride ions, so a field-assisted dissolution reaction involving fluoride ions is considered to be the main reason for the formation of nanopores. In this paper, titanium was anodized at high current in a silver nitrate solution without fluoride, and a porous titanium oxide structure was obtained, which proves that a field-assisted dissolution reaction involving fluoride ions is not essential for the formation of nanopores. The electronic current leads to oxygen evolution, and the oxygen bubble mold effect is the real reason for the formation of nanopores. These interesting results show that the mechanism of formation of anodized TiO2 nanotubes is the same as that of porous oxide structures. Moreover, the growth rate of nanotubes in an aqueous solution of silver nitrate was much higher than that in a glycol solution of ammonium fluoride at the same voltage.