Debunking the formation mechanism of nanopores in four kinds of electrolytes without fluoride ion

Abstract

A highly controversial topic, the mechanism of the anodization of the formation of porous structures of metals is still believed by many researchers to be field-assisted dissolution by fluoride ions. In this paper, evidence against this mechanism is provided. Using SEM images of titanium oxides anodized in four fluoride-free electrolytes—NaNO3, KNO3, H2SO4, and H3PO4 solutions—and the corresponding current−time curves, this paper supports the newly grown oxides growing upward around the oxygen bubble mold in a viscous flow mode to form nanotubes. The electronic current is high enough to ensure the continuous release of oxygen gas, which is necessary to form nanotubes, such as in NaNO3 and KNO3 solutions. When the electronic current is small enough, the oxygen bubbles can only form porous structure similar to those found in bread, such as in H2SO4 and H3PO4 solutions.

Keywords

• Nanotubes
• Anodization
• Electronic current
• Fluoride-free electrolytes
• Current−time curve