Study On The Preparation Of Nonmetal-doped TiO₂ Nanotubes And Their Photoelectrocatalytic Performance

Nanosized TiO₂ was extensively studied due to its excellent photocatalytic activity. The most important focus has been on the improvement of photocatalytic activity and the efficiency of utilizing solar energy.Many researches have proved that photoelectrocatalysis is an efficient process to improve photocatalytic activity of TiO₂, and nonmetal-doping can activate its visible-light activity.In this paper,TiO₂ nanotubes electrode was prepared by anodizing Ti in oxalic acid+NH₄F electrolyte.The results indicated that the length of TiO₂ nanotubes was determined by the balance between the electrochemical reaction rate and chemical dissolution rate.When the anodic voltage was 20 V and pH=4.9,the diameter was in the range of 35-55 nm and the length was approx.900 nm at 120 min.In order to improve the visible-light photocatalytic activity,the novel method was developed for doping nonmetals into TiO₂ nanotubes based on CVD.TiO₂ nanotubes were doped with F and/or B by CVD.The results were summarized as follows:(1) F-doping could extend the absorption band edge of TiO₂ to 419 nm.The photoelectrocatalytic synergetic factor was 2.09.(2)B-doping could extend the absorption band edge of TiO₂ to 444 nm,and the photoelectrocatalytic synergetic factor was 1.45.B-doped TiO₂ nanotubes possessed good stability and could be repeatedly used.(3)The intensity of adsorption peaks for F-B-TiO₂ greatly increased, moreover,it was interesting to find that some adsorption peaks appeared in the visible light range of 400-600 nm.Actually,F-doping into TiO₂ produced several beneficial effects,including the creation of surface oxygen vacancies,the enhancement of surface acidity and the Ti³⁺ions,and B-doping resulted in the creation of surface oxygen vacancies and the enhancement of surface acidity.Therefore,F-B-codoping of TiO₂ presented an obvious synergetic effect,which could promote the separation of photogenerated carriers,and thus the photocatalytic reaction under visible irradiation could be accelerated.The electronic structure of anatase TiO₂ doped with F and B was analyzed by ab initio band calculations based on the density functional theory.It was found that the conduction band consisted of the electrons of F 2p,B 2p and Ti 3d orbital.Moreover, a strong interaction occurred between the electrons of F 2p and Ti 3d orbital,resulting in the movement of electrons of Ti 3d orbital towards that of B 2p,and thus the whole conduction band moved near the Fermi level.This action narrowed the band gap and as a result,the red-shift phenomenon occurred. The intermediates during the photocatalysis of MO were determined by GC-MS analysis.The possible pathway of the degradation of MO was deduced as follows: firstly the azo bond was attacked by·OH and broken,then the break of C-N bond and the substitution of benzene ring by·OH simultaneously occurred,resulting in the generation of hydroxybenzene and o-benzoquinone,finally o-benzoquinone was opened the ring to generate carboxylic acids.Moreover,the kinetic model of photoelectrocatalytic reactor was studied and the theoretical curve was closed to the experimental values,indicating that the model was fitted with the reactor in this work.