Preparation Of Two-nonmetal-codoped TiO₂ Nanotubes Using Anodization And Their Photocatalytic Performance Under Visible Light Irradiation

Nanosized TiO₂ has excellent photocatalytic activity to degrade most of pollutants and has developed to be the novel treatment technology. The most important focus has been on the improvement of photocatalytic activity and the efficiency of utilizing solar energy. The researches has proved that nonmetal-doping can activate its visible-light activity.TiO₂ nanotubes were prepared by anodizing Ti in different electrolytes and the influence of operation parameters on their morphology was discussed. The results indicated that the morphology of TiO₂ nanotubes was different when preparing in the different electrolytes and the electrolyte composition controlled the growth of them. The length of TiO₂ nanotubes was determined by the balance between the electrochemical reaction rate and chemical dissolution rate. When the anodic voltage was 25 V and pH = 3.5, the morphology was clear at 120 min.In order to improve the visible-light-driven photocatalytic activity, S-F-codoped and P-F-codoped TiO₂ nanotubes were obtained by tailoring the electrolytes in anodization. The facile method integrating the preparation with nonmetal-doping was developed. The results were summarized as follows:(1) F-doping produced several beneficial effects, including the creation of surface oxygen vacancies, the enhancement of surface acidity and the Ti³⁺ ions, which inhibited the recombination of photogenerated carriers. The photoelectrocatalytic synergetic factor was 1.33. The density functional theory calculations indicated that the valence and conduction band of F-TiO₂ consisted of the electrons of O 2p and Ti 3d, while the small amount of electrons of F 2p did not change the band gap of TiO₂.(2) S-F-codoping extended the absorption band edge of TiO₂ to visible range and a strong absorption peak appeared in the range of 400-600 nm. The appropriate increase in the concentration of S can promote the absorption of visible light. The removal efficiency reached 98.1 % of AO7 at 180 min and the photoelectrocatalytic synergetic factor was 4.75. F-doping produced several beneficial effects, including the creation of surface oxygen vacancies, the enhancement of surface acidity and the Ti³⁺ ions, and S-doping also produced the two former effects. As a result, S-F-codoping produced a synergetic effect. The density functional theory calculations indicated that the upper valence appeared doping states due to S-doping, and the transition of S 3p to the conduction band was the reason for the redshift of S-F-TiO₂ compared with undoped TiO₂.(3) P-F-codoping also extended the absorption band edge of TiO₂ to visible range. The removal efficiency of AO7 by the sample annealed at 973 K reached 63.5 %. The photoelectrocatalytic synergetic factor was 2.37. Based on the density functional theory calculations, we found that a strong interaction occurred between the electrons of P 3p orbital and the valence bottom, resulting in the whole conduction band became wider. This actionnarrowed the band gap and as a result, the red-shift phenomenon occurred.(4) The intermediates during the photocatalysis of AO7 were determined by UV-Vis analysis. The possible pathway of the degradation of AO7 was started at the brokenness of the benzene ring and naphthalene ring by·OH.