Fabrication and characterization of TiO2-ZrO2-ZrTiO4 nanotubes on TiZr alloy manufactured via anodization

Titanium and its alloys are able to grow a stable oxide layer on their surfaces and have been used frequently as substrates for anodization in an electrochemical surface treatment. A nanotubular oxide layer is formed in the presence of fluorine anion (F-) via anodization due to the competition between oxide formation and solvatization. In this study, a highly ordered titania-zirconia-zirconium titanate (TiO2-ZrO2-ZrTiO4) nanotubular layer was formed on the surface of Ti50Zr alloy via anodic oxidation in an F-containing electrolyte. The sizes of the nanotubes (i.e., the inner and outer diameters, and wall thicknesses), morphology, crystal structure, hydrophilic properties and components of the TiO2-ZrO2-ZrTiO4 nanotubular layer before and after annealing were examined by scanning electron microscopy (SEM), thin film X-ray diffraction, X-ray photoelectron spectroscopy (XPS) analysis and water contact angle measurements. The results indicated that the inner diameter, outer diameter and wall thickness of the as-formed TiO2-ZrO2-ZrTiO4 nanotubes were distributed in the ranges of 3-120 nm, 12-165 nm and 3-32 nm, respectively, and depended on the F- concentration of the electrolyte and the applied potential during anodization. The number of smaller nanotubes increased with increasing F- concentration and the mean nanotube inner and outer diameters increased with increasing applied potential. The as-formed TiO2 and ZrTiO4 nanotubes exhibited an amorphous structure and the as-formed ZrO2 nanotubes displayed an orthorhombic structure. These phases transformed into anatase TiO2 and orthorhombic ZrO2 and ZrTiO4 after annealing. The hydrophilic properties of the TiO2-ZrO2ZrTiO4 nanotubular layer were affected by the size distribution of the nanotubes. The surface roughnesses and the nanotubular character transformed the nanotubes to exhibit superhydrophilic properties after annealing. The TiO2-ZrO2-ZrTiO4 nanotubular surface on Ti50Zr alloy exhibited higher surface energy than that of the TiO2 nanotubular surface on commercially pure (CP) titanium.