Morphological comparison and growth mechanism of TiO2 nanotubes in HBF4 and NH4F electrolytes

At present, the growth mechanism of TiO2 nanotubes in different electrolytes is still controversial. Field assisted dissolution (FAD) theory believes that fluoride ion is the main factor affecting the growth of TiO2 nanotubes. In this paper, we conducted a comparative experiment in HBF4 and NH4F electrolytes which containing the same fluoride ion molarity. By comparing the growth morphology of TiO2 nanotubes in two electrolytes, orderly nanotube arrays were obtained in NH4F electrolyte. However, only dense films and nanotube embryos were formed in HBF4 electrolyte. Obviously, FAD theory cannot explain these phenomena. Ionic current and electronic current theories and oxygen bubble model are reasonable for these phenomena. In the NH4F electrolyte, there are high electronic current and intense oxygen evolution, so the nanotube embryos cannot be seen. The low electronic current in the HBF4 electrolyte and the slow evolution of oxygen gas lead to the discovery of the nanotube embryos. This phenomenon also confirms that the formation process of TiO2 nanotubes is that anodic oxides grow upward around the oxygen bubble to form the nanotube walls.