Photocatalytic Performance of an Anodic TiO₂ Layer Fabricated in a NH₄NO₃/Ethylene Glycol Electrolyte with Various Crystallographic Phases

Abstract

Anodizing Ti substrates in an ammonium nitrate/ethylene glycol electrolyte is an innovative process capable of fabricating nitrogen-doped photocatalytic titanium oxide (TiO₂) layer in one step. This fabricated layer comprises both rutile and anatase TiO₂ phases; however, a major phase contributing to its excellent visible-light responsive photocatalytic performance is still unknown. In the present work, the crystallographic phase of an anodic layer was controlled by exploiting a post-thermal treatment and relationship between the phase variation and photocatalytic performance was then investigated to determine the major phase contributing to this performance. Post-thermal treatment to the anodic TiO₂ layer had little influence on the surface morphology and nitrogen doping, but the crystallographic phase, more specifically the ratio of anatase to rutile phases, changed with the heating temperature. The photocatalytic activity, evaluated by methylene blue decolorization and ethylene decomposition, increased with an increase in the ratio of anatase phase, while the correlation with the rutile phase was not observed. X-ray diffraction (XRD) analysis using a grazing incidence geometry showed that the anatase phase was concentrated in the topmost surface region when compared with the rutile phase. In conclusion, the variation of the photocatalytic performance was related to the growth of the anatase TiO₂ phase in the layer, with the treatment temperature of 723 K showing the highest photocatalytic activity.

Fig. 3 N 1s XPS spectra obtained from the sample surfaces treated at 523 K, 723 K, and 923 K.