Fabrication parameter-dependent morphologies of self-organized ZrO2 nanotubes during anodization

Zirconia (ZrO2) nanotubes have been synthesized using a facile anodizing process in organic electrolyte systems containing a low content of fluoride. The nanotube architecture evolution was recorded at different anodization periods (1–24 h) by scanning electron microscopy. A compact layer was found between the Zr substrate and its upper tubular layer after 1 h of anodization, whereas after further anodization for 3 h the compact layer disappeared. Meanwhile, ZrO2 nanotubes turned to a uniform structure from top to bottom. However, after 18–24-h-long anodization, the uniform tubular layer was replaced by a random layer composed of various structural defects. Since the compact layer was not completely dissolved, the retained compact layer yielded O-rings with double walls on the outer surface of the nanotubes.