In the present study, we attempted to prepare a photocatalytic titanium dioxide (TiO 2 ) layer on nickel-titanium (NiTi) alloy surface through a simple thermal oxidation process in air. At 723 K, an amorphous TiO 2 layer including a slight amount of Ni was formed on the surface. Above 873 K, the TiO 2 layer crystallized into a rutile phase. At 1023 K, a complex oxide of NiTiO 3 was also formed. The methylene blue degradation test for evaluating photocatalytic activity showed that the formed TiO 2 layers act as photocatalyst under ultraviolet (UV) light illumination, and its activity is superior to that of the surface layer formed by oxidizing a pure Ti substrate. The adhesion strength of the surface layers formed at 723 K on NiTi alloy was higher than that of a commercially available TiO 2 -coating. We conclude that thermal oxidation as a surface modification technique is expected to make NiTi alloys give photocatalytic activity.
Anodic oxidation of titanium substrate in an electrolyte comprising ammonium nitrate in ethylene glycol is an innovative technique for fabricating a visible-light-responsive photocatalytic titanium dioxide (TiO 2) layer. In this study, the effects of the duration of anodization on photocatalytic activity were investigated to improve its performance. Surface morphologies of the fabricated layer were comparatively flat for small durations of anodization, but small cracks were generated when anodizing for over 120 min. The thickness of the layer increased with treatment time up to 60 min, beyond which time no further increase in thickness was observed. The predominant structure of the oxide layer was TiO 2 with a rutile structure, and its amount increased with longer treatment duration. Nitrogen doping was observed in the layer regardless of the duration although its chemical state and photocatalytic activity varied depending on the duration. Under ultraviolet light irradiation, the change in activity was small, whereas the activity was drastically enhanced with longer treatment duration in the case of visible light irradiation. Based on these results, we concluded that the optimum duration of anodization is 60 min in the present setting because a TiO 2 layer showing good photocatalytic activity without any surface cracks could be formed.
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