Real Role of Fluoride Ions in the Growth of Anodic TiO2 Nanotubes

While porous anodic oxides have been extensively studied, there is limited research on the specific role of fluoride ions in the anodization. This study compares the morphology of titanium anodization products before and after the addition of ammonium pentaborate to the NH4F electrolyte. The current–time curve in anodization is analyzed to elucidate the real role of fluoride ions. The findings indicate that the real role of fluoride ions in the anodization is to form an anionic contaminated layer and cause the generation of electronic current, triggering the oxygen bubble mold effect and promoting nanotube growth. However, when pentaborate anions are introduced in the anionic contaminated layer, they hinder the production of electronic current, leading to a significant reduction in electronic current. Consequently, the growth rate of nanotubes decreases, resulting in the formation of bulb-shaped nanotube embryos. This study provides interesting evidence supporting the role of fluoride ions by combining the ionic and electronic current theory with the oxygen bubble mold effect. These pieces of evidence cast doubt on the simple chemical dissolution of fluoride ions in the traditional field-assisted dissolution theory. The experimental results significantly contribute to our understanding of the growth mechanism of porous anodic oxides.