Preparation And Photocatalytic Activity Research Of ZnFe₂O₄–based Hollow Nanospheres

Nowadays, functional nanomaterials with controlled shapes and desired morphology are of great importance in the application of the photocatalyst, because of their unique chemical and physical properties with conspicuous enhanced photocatalytic property compared to their solid counterparts. Hollow nanomaterials with remarkable interior space have been intensively investigated because of their low density, larger specific surface area and nanostructured wall. Nanomaterials are effective to enhance the photocatalytic activities by minimizing the particle size, so as to achieve higher surface area and more active catalytic sites, but it brings another negative effect. When the particle size is decreased to nanoscale, the separation and the recycling of the photocatalysts from the treated water are practical obstacles which hinder their application in industrial use, even though they have high photocatalytic activity. Thus, some researchers are working on magnetic photocatalysts that can be separated from the treating system by applying an external magnetic field. Spinel-structured Zn Fe2O4 with strong magnetism, outstanding photochemical stability and low cost, as well as a narrow bandgap about 1.9 e V, which gives rise to the visible-light response, have become increasingly attractive in the area of photocatalysis. However, as single phase photocatalysts, their activity is low, because of its fast recombination of charge carriers. Combining two or more semiconductors with appropriate band positions to improve the photocatalytic performance of the semiconductors is an established idea, because the composite of semiconductors with different band gaps and positions possess a built-in potential gradient at the interface, which facilitates the separation of electron and hole pairs and reduces the chance of recombination.In this thesis, to address the scientific issues of improving the photocatalytic efficiency of Zn Fe2O4, several magnetically separable Zn Fe2O4-based composite hollow nanospheres were synthesized via construction of Zn Fe2O4 systems, choosing of composite base and controlling of morphology. The visible light induced photocatalytic activities of composite photocatalysts were evaluated via the photocatalytic degradation of Rh B. In detail, the following investigations were carried out in this dissertation:(1) Phenolic formaldehyde microspheres were synthesized by St?ber method in mixed resorcinol and formaldehyde. Phenolic formaldehyde microspheres, with monodisperse, narrow size distributions and strong adsorption, and the particle size of the phenolic formaldehyde microspheres can be tuned from 370 to 1050 nm, are good for the next step synthesizing Zn Fe2O4-based hollow nanospheres.(2) Magnetically separable Zn Fe2O4-based composite hollow nanospheres with dimension of 230 nm were successfully synthesized via impregnating-calcination process using phenolic formaldehyde nanospheres(PFS) as template. Zn Fe2O4, Zn Fe2O4-Fe2O3 and Zn Fe2O4-Zn O hollow nanospheres were prepared by tuning the rate of Zn2+ to Fe2+. The results of photodegradation under visible light irradiation exhibited the order: Zn O/Zn Fe2O4 > Fe2O3/Zn Fe2O4 > Zn Fe2O4 > bulk Zn Fe2O4, and close investigation revealed that the high surface area, thin shell thicknesses, and matching heterostructure of the as-prepared Zn O/Zn Fe2O4 and Fe2O3/Zn Fe2O4 composites could dramatically improve the photocatalytic activities, which facilitates the efficient separation of photoinduced electron-hole pairs.(3) Magnetically separable ternary hybrid ZnFe2O4-Fe2O3-Bi2WO6 hollow nanospheres were synthesized by an effective three-step approach. Specifically, using phenolic formaldehyde microspheres(PFS) as template direct the sequential coating of α-Fe2O3/Zn Fe2O4 layer and subsequent Bi2WO6 layer via impregnating-calcination process. The photocatalytic activity under visible light irradiation in the order of Zn Fe2O4-Fe2O3-Bi2WO6 > Zn Fe2O4-Bi2WO6 > Bi2WO6 > Zn Fe2O4-Fe2O3 > Zn Fe2O4. The enhanced activity could be attributed to the cascade electron transfer from Zn Fe2O4 to α-Fe2O3 then to Bi2WO6 through the interfacial potential gradient in the ternary hybrid conduction bands, which facilitates the charge separation and retarded the charge pair recombination.(4) Magnetically separable Zn Fe2O4-Zn O-Ag3PO4 hollow nanospheres was fabricated via a coprecipitation method by depositing Ag3PO4 nanoparticles onto the surface of Zn Fe2O4-Zn O hollow nanospheres. The photocatalytic activity under visible light irradiation increases in the order of Zn Fe2O4-Zn O-Ag3PO4 > Zn Fe2O4-Ag3PO4 > Ag3PO4 > Zn Fe2O4-Zn O > Zn Fe2O4. The enhanced activity could be attributed to the cascade electron transfer from Zn Fe2O4 to Zn O then to Ag3PO4 through the interfacial potential gradient in the ternary hybrid conduction bands, which facilitates the charge separation and retarded the charge pair recombination.In summary, a series of new visible light response magnetically separable Zn Fe2O4-based hollow nanospheres were constructed in this thesis. Not only the efficiencies of visible light utilization and catalytic degration of Rh B were improved, but also the mechanism of photocatalytic degradation was revealed. These investigations are expected to help promote the photocatalytic technology towards practical application and are of positive significance in the situation of the currently increasing environmental pollution.