Enhanced bioactivity of self-organized ZrO₂ nanotube layer by annealing and UV irradiation

Superhydrophilic ZrO₂ nanotube layer was prepared by anodic oxidation of commercial pure Zr in aqueous solutions containing 1 M (NH ₄)₂SO₄ and 0.15 M NH₄F. The effect of annealing and ultraviolet (UV) irradiation treatment on the microstructure, water contact angle and bioactivity of the ZrO₂ nanotube layer was investigated. The as-anodized nanotube layer consists of cubic and amorphous ZrO₂, no apatite crystals are deposited on its surface even after immersion in simulated body fluids (SBF) for 30 days, exhibiting weak apatite-inducing ability. After annealing at 450 °C for 3 h, the nanotube layer is composed of cubic and monoclinic ZrO₂, and its apatite-forming ability is significantly enhanced because of its lattice structure matching that of apatite, apatite can be induced after immersion in SBF for 15 days. UV irradiation of the ZrO₂ nanotube layers does not alter their surface morphologies and phase components, however, can improve the bioactivity only when the ZrO₂ nanotube layer is well crystallized. The enhanced bioactivity by UV irradiation is thought to result from the abundant basic ZrOH groups on the crystallized ZrO₂ nanotube layer. Annealing and UV irradiation treatment do not alter the superhydrophilic nature of the ZrO₂ nanotubes.