Effect of Fluoride Content on Morphology and Phase Composition of ZrO2 Nanotubes

The present research demonstrates the significant influence of fluoride content on the morphology, phase structure and chemical composition of ZrO2 nanotube arrays which were prepared via a direct electrochemical anodization of zirconium foil in glycerol―based electrolytes composed of 5vol% H2O and different contents of NH4F from 0.1 mol/L to 1.1 mol/L. The as―prepared nanotube arrays with external diameter of 90―130 nm and length of 4.8―9.1 μm were characterized by scanning electron microscope (SEM), X―ray diffraction (XRD) and X―ray photoelectron spectrum (XPS). The results show that the highly ordered and vertically aligned tetragonal ZrO2 nanotube arrays can be obtained in the electrolyte containing 0.7 mol/L NH4F, and they become amorphous at lower NH4F concentration and collapse at higher NH4F concentration. Moreover, the F/O atomic ratio near the top surface of nanotubes is also dependent on the F― concentration in the electrolyte.