Preparation、Characterization And Photo Catalytic Activities Of Metal Ferrites Modified Composites

Photocatalysis, one of the most attractive techniques for environmental-organic-pollution control, has been widely investigated. Among semiconductor photocatalysts, TiO2has been extensively used due to its outstanding photocatalytic activity, such as nontoxicity, chemical stability, high catalytic activity and low price. However, there are two main disadvantages in the application of TiO2. One limitation is the rapid recombination of photo generated electron-hole pairs. The other disadvantage is the limitation of utilizing visible light because TiO2is only sensitive to UV light due to its large bandgap (3.2eV for the anatase phase and3.0eV for the rutile phase). Therefore, numerous methods were proposed to enhance the photocatalytic activity of TiO2such as the incorporation of metal cations or non-metal anions into the TiO2matrix, the introduction of oxygen vacancies into the TiO2lattices, and the combination of two different semiconductors. Moreover, in order to make full use of the solar light, a visible light active photocatalyst is still desired.Metal ferrite is a type of semiconductor with a spinel structure, which can absorb visible light due to its small bandgap (about2.0eV) and is not sensitive to photoanodie corrosion. Hence, this material is considered as a potential solar energy material for photoelectric conversion. In addition, Ag3VO4, whose band structure is very special, has been recognized as a photocatalyst for water splitting and pollutant degradation under visible light irradiation.In this thesis, we mainly investigate the preparation, characterization and photocatalytic activities of three types of composites semiconductors (MgFe2O4/TiO2、MgFe2O4/Ag3VO4and ZnFe2O4/Ag3VO4). 1、MgFe2O4/TiO2composite photocatalysts were successfully synthesized using a mixing-annealing method. The synthesized composites exhibited significantly higher photocatalytic activity than a naked semiconductor in the photodegradation of Rhodamine B. Under UV and visible light (λ≥420nm) irradiation, the optimal percentages of doped MgFe2O4were2wt.%and3wt.%, respectively. The effect of calcination temperature on photocatalytic activity was also investigated. The origin of the high level of activity was discussed based on the results of X-ray diffraction, UV-visible diffuse reflection spectroscopy, scanning electron microscopy, transmission electron microscopy, and nitrogen physical adsorption. The enhanced activity of the catalysts was mainly attributed to the synergetic effect between the two semiconductors, whose band potential matched suitably.2、The heterojunction structure composite was formed between the as-prepared monoclinic scheelite Ag3VO4with an average diameter of about50nm and spinel ZnFe2O4with an average particle size of about500nm by calcining the mixed semiconductors. The samples were characterized by X-ray diffraction, UV-vis absorption spectra, scanning electron microscopy and transmission electron microscopy. The prepared ZnFe2O4/Ag3VO4coupled at a nanoscale demonstrated greatly enhanced activity in photodegradation of Rhodamine B under visible light irradiation (λ≥420nm). Its degradation constant (k) was about11times that of the typical N-doped TiO2. The remarkable photocatalytic activity of ZnFe2O4/Ag3VO4could be explained by the intersemiconductor electron-hole transfer mechanism caused by the matched band positions of these two semiconductors. Morever, the involved active species, the role of oxygen and the effect of pH values on photocatalytic activity were also presented.3、MgFe2O4/Ag3VO4was successfully prepared by a milling-calcining method. The photodegradation of RhB under visible light irradiation was used to investigate their photocatalytic performance. The test result exhibits that the photocatalytic activity of the composite is deeply affected by the MgFe2O4contents and the calcination temperature. When the MgFe2O4content is0.2wt.%with the calcination temperature of573K, the photocatalyst shows the highest photocatalytic performance. The RhB solution can be decomposed completely in30min under visible light irradiation. Based on the results of the XRD, SEM and UV-Vis experiments, we believe that there exist coupling effect between MgFe2O4and Ag3VO4. The photoexcited electrons can be transferred from the conduction band of MgFe2O4to that of Ag3VO4, while the holes in the valence band of Ag3VO4can be transferred to the valence band of MgFe2O4. The charge transfer between the two semiconductors can retard the recombination of electron-hole pairs and promote the photocatalytic activity of composites.