Double-walled structure of anodic TiO₂ nanotubes in H₃PO₄/NH₄F mixed electrolyte

Normally, the well-ordered anodic TiO₂ nanotubes (ATNTs) are obtained in NH₄F electrolyte, after annealing, the double-walled structure of nanotubes will appear. Here, after adding H₃PO₄ into NHF₄ electrolyte, we got the double-walled structure of nanotubes by anodizing without annealing, which means the direct existence of anion-contaminated layer in ATNTs. Influence of H₃PO₄ content on anodizing voltage and morphology of ATNTs were compared in detail. The XRD pattern illustrated that the crystallinity decreases with increasing H₃PO₄ concentration, and the anion-contaminated layer thickens with the increase of H₃PO₄ concentration. Meanwhile, the existence of the anion-contaminated layer also proved the limitations of the filed-assisted dissolution theory, while the double-walled structure can be explained by oxygen bubble model and plastic flow model. (paper)