Preparation, Structural Characterization And Photocatalytic Property Of Doped Bismuth Oxide Photocatalyst

Photocatalysis, as one of the advanced oxidation technologys, has a wide applicationprospect in water controlling field because of its moderate and clean process. In therealm of semiconducting photocatalysts, the band gap of bismuth oxide is about2.85eV,which can absorb a part of the visible-light. Based on this, bismuth oxide has beenrecommended as one of the novel visible-light-responsive photocatalysts. To expand therange and intensity of visible-light absorbance and prevent the recombination betweenphotoelectrons and holes of bismuth oxide, doping is an effective method. Based on thedoping researches of bismuth oxide, this dissertation mainly focuses on the followingissues.1. Tin-doped bismuth oxide photocatalysts were prepared by an impregnationmethod. The crystal structures and optical properties of the as-synthesized samples wereanalysed and tested by X-ray diffraction (XRD), X-ray photoelectron spectroscopy(XPS), X-ray fluorescence spectrometry (XRF), scanning electron microscopy (SEM),Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible spectrophotometry(UV-Vis), and photoluminescence spectroscopy (PL). And the visible-light activities ofthese samples were tested by the photocatalytic degradation of2,4-dichlorophenolsolution. Results show that the visible-light photocatalytic activities of the tin-dopedbismuth oxides were improved significantly. Meanwhile, the doping amounts andaclcining temperatures influence their photocatalytic activities a lot. The best calciningtemperature is500℃, and the best tin doping amount is2%(in mole ratio). Using thetin-doped bismuth oxide obtained under optimum conditions as photocatalyst, thedegradation rate of2,4-dichlorophenol reaches80.8%in320minutes. What’s more, thephase transformation of bismuth oxide from tetragonal to monoclinic can be effectivelyinhibited due to appropriate tin doping, which improves the stability of tetragonalbismuth oxide at room temperature. At the same time, tin doping could narrow the bandgap energy, expand the range and intensity of visible-light absorbance, and effectivelydecrease the recombination of photoelectron-hole pairs. 2. Lanthanum-doped bismuth oxide photocatalysts were also prepared byimpregnation method. In this chapter, lanthanum doping amounts and aclciningtemperatures as the major factors to photocatalytic activities were studied. Thephotocatalytic degradation experiment shows that the3%(in mole ratio)lanthanum-doped bismuth oxide sample calcined under500℃has the superiorphotocatalytic activity. Using this sample as photocatalyst, the degradation rate of2,4-dichlorophenol reaches84.6%in320minutes under visible-light irradiation. Thedoped lanthanum partially substitutes bismuth atom in the lattice of bismuth oxide,which exists in the form of Bi–O–La chemical bond. And a new complex metal oxidecompound (La0.176Bi0.824O1.5) appears in the catalysts, which could effectively preventthe phase transformation of bismuth oxide from tetragonal to monoclinic and inhibit therecombination between photoelectrons and holes. Based on those factors, thephotocatalytic performances of lanthanum-doped bismuth oxide were enhancedevidently compared with the un-doped sample.3. Nitrogen-doped bismuth oxide photocatalysts were synthesized by a precipitationmethod. The analytic results indicate that the un-doped bismuth oxide displays a singlemonoclinic phase. While the nitrogen-doped samples are mixed crystals composed oftetragonal bismuth oxide and Bi5O7NO3phase. The oxygen in the lattice of bismuthoxide is partially substituted by nitrogen atom, existed stably in the form of Bi–Nchemical bond. Compared with un-doped bismuth oxide, the light absorption propertiesof nitrogen-doped samples apparently extend into visible-light region. At the same time,the recombination between photoelectron-hole pairs and band gap decrease evidently.Furthermore, the N/Bi=0.5sample shows an excellent visible-light photocatalyticactivity, and the degradation rate of2,4-dichlorophenol reaches75.3%in320minutes.In the photocatalytic degradation experiment, the relationship between initialconcentrations and degradation rates of2,4-dichlorophenol was examined. Based on this,the kinetics characteristic of photocatalytic degradation of2,4-dichlorophenol wasdiscussed. The research result shows that it follows the kinetic equation ofLangmuir-Hinshelwood model. At the same time, the mineralization and dechlorinationof2,4-dichlorophenol can be well completed.