Design, Synthesis And Photocatalytic Mechanism Of Lanthanum Tungstate-based Visible Light Catalysts

In recent years, Bi2WO6 has attracted much attentions for its important applications in various areas such as energy, wastewater treatment and organic synthesis. However, the photocatalytic activities of Bi2WO6 photocatalysts are still not satisfying because of the limited absorption range of the visible light region and higher electron-hole recombination efficiency. To facilitate the applications of photocatalysts in the treatment of dye-containing wastewater, a series of visible-light-driven photocatalyst based on Bi2WO6 have been synthesized by morphology control or construction of heterostructures. The photocatalytic performances on the degradation of rhodamine B was systematically investigated, and the corresponding photocatalytic mechanisms were also proposed. The main contents of this thesis are summarized as follows:(1) Kinetically controlled synthesis of Bi2WO6 with different structures by a NH4 F assisted hydrothermal method: Using NH4 F as the morphology control agent, hierarchical Bi2WO6 nanostructures with different morphologies, such as flower-like and pancake-like morphologies were selectively prepared using a simple hydrothermal process. Based on the time-dependent experiments, a formation mechanism based on kinetic control was proposed. The photocatalytic activities of the as-synthesized Bi2WO6 nanostructures were investigated upon the visible light irradiation. The result indicates that the as-obtained hierarchical Bi2WO6 particles exhibit morphology-dependent photocatalytic activities for the degradation of Rhodamine-B(Rh B). Further research shows that the differences in photocatalytic activities among the as-obtained samples are associated with the surface adsorption properties, which may result from the synthetic conditions.(2) To broaden the light absorption range and inhibit the electron-hole recombination, a series of Bi2WO6/MS(M=Cd, Zn, Sn, Ag, etc) heterostructures were designed and synthesized using 3-mercaptopropionic acid(MPA) as the surface functionalizing agent. The diffuse reflection spectra of these samples showed that the band gap of the prepared Bi2WO6/MS heterostructures were narrowed, and the light absorption range were broadened. The result of the PL spectrum and photocurrent measurement of the samples indicate that the electron-hole recombination are dramatically weakened as compared to bare Bi2WO6 or the corresponding metal sulfides. The as-prepared photocatalyst compsites exhibit enhanced photocatalytic performances for the degradation of Rh B as compared to bare Bi2WO6. Free radicals trapping experiment suggest that the photo-generated holes play vital roles in the degradation of Rh B.(3) To elevate the oxidation potential of photocatalyst, two ternary Z-scheme composite photocatalysts Bi2WO6/Ag X/Ag(X = Br, I) with high oxidation potential were designed and synthesized. The photodegradation experiments indicate that the photocatalytic activity of these composite photocatalyst were improved tremendously compared with bare Bi2WO6. The enhanced photocatalytic activities of the as-formed heterostructures could be attributed to the synergistic effects of surface plasmon resonance of Ag nanoparticles and the formation of Z-scheme photocatalyst. All of these two factors could broaden the light absorption range as well as inhibit the recombination of electron-hole pairs. Ammonium oxalate, benzoquinone, Ag NO3 and t-Bu OH were used as scavengers to investigate the underlying photocatalytic mechanism, the result indicates that ?O2- played comparatively major impact during the photodegradation process. The as-prepared Z-scheme composite photocatalyst exhibit higher oxidation power than the ordinary dual composite photocatalyst, which inevitably lead to the better photodegradation efficiency for Rh B as compared to the ordinary dual composite photocatalyst.