Fabrication of Highly Ordered TiO₂ Nanotube Arrays via Anodization of Ti–6Al–4V Alloy Sheet

Uniform and highly ordered TiO₂ nanotube arrays were fabricated by the electrochemical anodic oxidation on Ti-6Al-4V surface, using graphite plate as cathode and ethylene glycol (EG) with addition of a certain amount of H₂O and NH₄F as electrolyte, and the anodization voltage went up to a presetting voltage by stepwise increment. The morphology, structure and composition of TiO₂ nanotube arrays were characterized by SEM, EDS, XRD and XPS. The formation process of TiO₂ nanotubes was introduced in brief. The experiments were arranged by an orthogonal experiment method and the experimental results showed that the formation of TiO₂ nanotube arrays was influenced by not only each factor (F⁻ content, H₂O content, external voltage and duration), but also cross correlation among the four factors. The optimal condition was F⁻ content 0.2 wt%, H₂O content 4 vol%, external voltage 40 V and duration 1 h in the studied electrochemical system, and the length of obtained nanotubes was 1.5 m, the outer diameter was ∼100 nm and the aspect ratio was 15. As-formed nanotube arrays were amorphous and changed to anatase TiO₂ after annealed at 500 °C for 2 h in air ambience. XPS survey spectra revealed the surface of as-formed nanotube arrays containing Ti, O, C, F and N. The nanotube arrays on Ti-6Al-4V surface with better thermo-stability and crystallinity would have great potential in biomaterials.