Synthesis And Visible Light-driven Photocatalytic Mechanism Of Rare-earth Ions Modified BiVO₄

Photocatalytic technology based on semiconductor materials is an advanced oxidation process, which can produce active species with strong oxidizing by semiconductors via photoexcited electrons and holes. Compared to the well invesitigated materials TiO2 photocatalyst, BiV04 possess more posotive response to visible light so that it has attracted more attention in recent decade. However, the main drawback of the low photoexcited carrier’s separation efficiency in BiVO4 crystal severely restricts its visible light photocatalytic activity. In order to improve the visible-light driven photocatalytic activity of BiVO4, two paths to enhance the photoexcited carrier’s separation efficiency in BiV04 crystal were applied in this work. One is to fabricate heterojunction on BiV04 surface by loaded another oxide semiconductor. The contact electric field in heteroj unction can suppress the recombination of electron-holes; the other is to compose solid solution semiconductors based on BiVO4 via rare earth-doped. The lattice defects in these materials can capture photoexcited electrons and holes to improve the catalytic efficiency under visible light irradiation.1. Composite photocatalysts CeOx/BiV04 with a heteroj unction structure were synthesized using hydrothermal and ion-impregnation method. The composites were characterized by X-ray diffraction, BET, HRTEM, SAED and XPS. UV-vis diffuse reflectance spectroscopy was used to investigate the absorption range and band gap of photocatalysts. The photocatalytic activities of the prepared samples were also examined by studying the degradation of methylene blue under visible-light irradiation. Results showed that the composite with 5.7 wt.% CeOx/BiVO4 exhibited the highest photocatalytic activity. The separation of the photogenerated charge and the effect of heterojunction structure on photocatalytic activities could be well interpreted based on the heterojunction structure with consideration of the Fermi level. The charge-separation process and the important role of deposited cerium were proven by photoluminescence spectroscopy and electron paramagnetic resonance spectra.2. Based on the above results, Ln2O3/BiVO4 (Ln:Sm, Eu, Tb) photocatalysts were synthesized via hydrothermal and ion-impregnation method to investigate the capture effect on electron-hole pairs of rare earth element redox reaction. The composites were characterized by X-ray diffraction、 SEM、 TEM and XPS. The effect of different rare earth oxides, different loaded content on the photocatalytic activity under visible light irradiation were discussed. The differences between the different oxides of rare earth ions in the catalytic mechanism was demonstrated by designing active species quenching experiments. And the the mechanism of redox heterojunction enhancing electron-hole separation was proposed.3. Solid solution photocatalysts Bi1-xCexVO4+δ (0≤ x≤ 0.3) were synthesized by Ce substituted for Bi in BiVO4 lattice using a hydrothermal method. X-ray diffraction, Raman spectra and HRTEM revealed that the crystal phase transformed from monoclinic to tetragonal phase, probably due to the substitution of cerium ions in Bi3+ positions. UV-vis diffuse reflectance spectroscopy was used to investigate the absorption range and band gap of the photocatalysts. The photocatalytic activities of the prepared samples were examined by studying the degradation of MB and phenol under visible light irradiation and the best performance was attained for the sample of Bi0.8Ce0.2VO4+δ. The results of photoluminescence spectra and photocurrent demonstrated that the recombination of photogenerated charges was greatly depressed and the photocatalytic activity was improved by the substitution of Ce for Bi in BiVO4. Furthermore, the proposed mechanism of the enhanced photocatalytic activity was discussed.4. Based on the above results, solid solution photocatalysts Bi1-xLnxVO4+δ (Ln: Sm，Tb) were synthesized by Ln substituted for Bi in BiVO4 lattice using a hydrothermal method to study the effect of different types of lattice defects caused by different lanthanides. The structure of solid solution photocatalysts were characterized by X-ray diffraction and Raman spectra. The photocatalytic activity under visible light irradiation of different samples were estimated by degradation efficiency of rhodamine B.