Formation of self-ordered TiO₂ nanotubes by electrochemical anodization of titanium in 2-propanol/ NH₄ F

TiO₂ nanotubes (TiNTs) were formed by the electrochemical anodization of titanium in 2-propanol/water containing 0.14 M NH₄ F as the supporting electrolyte. The effects of the water content, time of anodization, and potential on the growth behavior of TiNT were studied. At the optimum solvent composition of 16 vol % water/84 vol % 2-propanol, the reaction was kinetically controlled. The TiNTs obtained under optimum conditions had heights of up to 1800 nm and inner diameters of about 90 nm. Increasing the potential beyond 20 V did not improve the quality of the nanotubes because of the introduction of disordering. The scanning electron microscopy and X-ray diffraction analysis revealed a preferred growth direction of the TiNTs relative to the substrate surface. A thin barrier layer existed at the nanotubes/metal interface, which was also confirmed by ellipsometry. This barrier layer was enriched with the rutile phase, whereas the nanotubes were enriched with the anatase phase.