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

Abstract

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 NH 4 F 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.

Relationship between the Growth Rate of Nanotubes and the Current–Time Curve of the Anodizing Process

“…TiO 2 nanotubes prepared through electrochemical anodization have found wide applications in catalyst carriers, supercapacitors, biomedicine, and other cutting-edge fields. − However, the formation mechanism of anodic TiO 2 nanotubes remains highly controversial. − The classical field-assisted dissolution theory evolved from porous anodic alumina is still dominant. According to the field-assisted dissolution theory, the dissolution reaction of fluoride ions on titanium oxide is primarily responsible for pore formation in nanotubes. − In recent years, new theories such as the viscous flow model, electric field equilibrium model, and oxygen bubble model have emerged, challenging the classical field-assisted dissolution theory. − Traditional field-assisted dissolution theory fails to explain the three phases of the current–time curve or establish a relationship between the nanotube growth rate and the dissolution rate with respect to anodizing current. − Despite emphasizing a balance between oxide growth and dissolution, few studies have reported on the growth rate and dissolution rate of TiO 2 nanotubes or examined their relationship with fluoride ion concentration. − …”