Surface structure and photocatalytic performance of an anodic oxide layer fabricated on titanium in a nitrate/ethylene glycol electrolyte with different treatment durations

• TiO2 layers were fabricated by anodizing in a nitrate/ethylene glycol electrolyte.
• The nitrogen doping in the TiO2 layer was confirmed regardless of the duration.
• Photocatalytic activity under UV light was gradually increased in the duration.
• Visible light activity was abruptly enhanced when the duration over 60 min.
• The rutile structure had an advantage with respect to the visible-light 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 (TiO2) 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 TiO2 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 TiO2 layer showing good photocatalytic activity without any surface cracks could be formed.