Preparation And Performance Of Metal-n Codoped Nano TiO₂ Visible Light Photocatalysts Via Coprecipitation Route

With the rapid development of industry, energy crisis and environment pollution become more and more serious. Consequently, how to save energy and how to control and deal with the enviroment pollution have been very important topic nowadays. Among all the nano photocatalytic materials, TiO2 become the most promising photocatalyst for its strong oxidation activity, stability, nontoxicity and cheapness. However, pure TiO2 has a large band gap energy (3.2eV) and can only take advantage of ultraviolet light, a small portion of solar energy, and the light induced electron-hole pairs have the tendency of recombination, both of which result in the low quantum yield and low photocatalysitic activity, thus prohibiting dreadfully its application. Therefore, to extend the photoresponse to visible light range and suppress the recombination of light induced electron-hole pairs to increase photocatalytic activity is the key to further promote the practical application of TiO2 photocatalysts. On the basis of summarizing the development of the modification of nano TiO2 photocatalyst, the main aspects of the research work of this paper have been listed as following:1. Nonmetal N doping TiO2 has been studied in order to extend its range of photoresponse spectrum and increase its photocatalytic activity. The crystal phase, morphology, element and absorbency were characterized by X-ray diffraction (XRD), Transmittance electronic microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and UV-vis spectrophotometry (UV-vis). Compared with pure TiO2, the grain size of the doping samples is smaller. The absorbency increases greatly in the visible region because of the narrowing of the band gap and the formation of doping level. The visible-light photocatalytic activity of N doped TiO2 was investigated to be high by degrading the model compound, methylene blue (MB) under the irradiation of incandescent lamp.2. La-N co-doped TiO2 nanocrystals were synthesized using coprecipitation method. The effect of ion doping concentration on photocatalytic activity was investigated. The results show that the highest photodegradation namely 45.2% under the irradiation of incandence light was obtained at the optimum molar concentration of 2% N and 0.3% La. The doping of La3+ with bigger ion radius could easily result in lattice deformation, and the entrance of La3+ into interstice of Ti4+ crystal lattice could form structure defect which is favorable for hydroxylation of the surface grains to obviously heighten the photocatalytic activity of samples; at the same time La and N co-doping could narrow the bang gap energy,thus result in the red-shift of absorption edge and increase the photocatalytic activity responding to visible light. Moreover, La doping suppresses effectively the recombination of light induced electron-hole pairs.3. Mo-N co-doped nano TiO2 photocatalysts were synthesized using co- precipitation method. XRD analysis shows that the obtained photocatalysts are anatase, Mo-N co-doping can inhibit the transformation of TiO2 from anatase to rutile; UV-vis results show that the Mo-N codoping enhances the absorption of visible light and induces the obvious red shift of the absorption band edge; XPS measurements indicate that, Mo atoms exist in the form of +6 price, and largely replace Ti4+ to enter into the TiO2 lattice. The optimum conditions for preparing Mo-N-TiO2 are as follows: molar of Mo:N:Ti= 0.0125:0.02:1, calcining temperature = 500℃and the decoloration degree of MB solution = 55.7 % after 2 h of irradiation. The replacement of Ti4+ in the lattice of TiO2 by Mo+6 causes increase of the surface lewis acid, which is beneficial to the migration of photo-generated electron in conduction band to the surface, thus promoting the separation of photo-generated hole-electron pairs. The synergy effect of surface lewis acid increasing and red shift of band edge enhances the visible-light catalytic activity of Mo-N doped nano TiO2.This paper has adopted coprecipitation route to prepare the metal-N codoped TiO2 visible light photocatalysts, and several results of great significance concerning the TiO2 photocatalytic mechanism and the effect of the doped ions, doped concentration and calcining temperature on photocatalytic activities were obtained. The research work has good originality, which will promote the development of the theoretical and applied studies of TiO2 photocatalysts in the effective ulilization of solar energy.