Design And Simulated Sunlight Photocatalytic Performances Of New Bismuth-based Oxides With The Layered Structure

Photocatalytic oxidation technology developed in recent decades, and it is expected to become the ideal of environmental pollution control and treatment technology in the 21st century. As an environmental purification technology, photocatalytic process is the mineralization of organic pollutants by chemical oxidation techniques, which is decomposed into water, carbon dioxide and non-toxic inorganic acid. The sunlight can be used by photocatalyst to drive the oxidation-reduction reaction as the excitation source. The feature makes photocatalytic technology be more potential value to apply in energy. Due to the limitation of its electronic structure, TiO2 photocatalyst is difficult to use sunlight effectively. Therefore, we focus on design and development of new and efficient non-TiO2-based photocatalyst with visible-response by hydrothermal technique. A series of Bi-based oxides with layered structure are synthesized and characterized. We investigate the relationship between prepared conditions and the crystal phase and morphology of products, examine the photocatalytic activity under the visible light irradiation, and explain the degradation mechanism.1. Controllable fabrication of bismuth vanadates and their photocatalytic activityBismuth vanadates (BiVO4) with various crystal structures (tetragonal scheelite, tetragonal zircon and monoclinic scheelite) and morphologies (sheet-like, dendritic-like and flower-like) were controllably fabricated by using a mild additive-free hydrothermal treatment process by controlling the hydrothermal time, pH value and molar ratio of Bi to V in the starting materials. The crystal structures, morphologies, and photophysical properties of the products were well-characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Raman scattering spectra (Raman), UV-Vis diffuse reflectance spectra (UV-Vis/DRS) and X-ray photoelectron spectroscopy (XPS). Subsequently, their UV- as well as visible-light photocatalytic performances was evaluated via dyes rhodamine B (RB) and methylene blue (MB) degradation. The results show that compared with tetragonal scheelite and tetragonal zircon BiVO4, monoclinic scheelite BiVO4 has the highest UV- as well as visible-light photocatalytic activity toward dyes RB and MB degradation among all tested BiVO4 powders, confirming that the crystal structure of the BiVO4 predominantly affects its photocatalytic activity. In addition, several monoclinic scheelite BiVO4 products with different morphologies exhibit similar photocatalytic activities under UV light irradiation (? > 254 nm). However, the photocatalytic activities of them under visible light irradiation (? > 420 nm) are different. Monoclinic scheelite BiVO4 with a flower-like morphology shows the highest UV- as well as visible-light photocatalytic activity toward dyes RB and MB degradation, comfirming the morphology also play an important role to its photocatalytic activity.2. Preparation of Bi5Nb3O15 and their photocatalytic activityWe successfully synthesize orthorhombic Bi5Nb3O15 by the mild hydrothermal method at the first time, which is difficult to obtain by the traditional high temperature solid state reaction. The products were characterized by XRD, FESEM, TEM, UV-Vis/DRS and XPS. The crystal structure, particle size and optical absorption properties of Bi5Nb3O15 influenced by hydrothermal time, pH value, and molar ratio of initial materials were investigated. The results show that Bi5Nb3O15 with perfect crystal structure was obtained at pH = 9, nBi : nNb = 1 : 0.7 and hydrothermal treatment at 200 ?C for 24 h. High crystallinity and little particle size are favorable to improve photocatalytic activity. Dyes methyl orange (MO) are totally decomposed afte 2 h irradiation under simulated sunlight (? > 320 nm). To further prove Bi5Nb3O15 is excellent visible light photocatalytic activity, rather than from the photosensitive dye, the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D), which has no absorption in visible range, is selected as model contaminant. The degradation rate can reach 90% afte 30 min irradiation.3. Preparation of Ag/Bi5Nb3O15 and their photocatalytic activityAg/Bi5Nb3O15 with different loading (1~20%) was successfully prepared by photo-deposition method. The visible-light photocatalytic activity is examined by the degradation of tetrabromobisphenol-A (TBBPA) at different range of wavelengths and pH values, and the results were compared with Degussa P25. The appropriate reaction system is neutral. The best loading of Ag is 10%. Ag/Bi5Nb3O15 (10%) exhibits higher photocatalytic activity under simulated sunlight irradiation (? > 320 nm). Especially ? > 420 nm, the activity of Ag/Bi5Nb3O15 (10%) is significantly higher than Bi5Nb3O15 and Degussa P25. On the one hand, this enhanced photocatalytic activity can mainly attribute to the layered structure of Bi5Nb3O15. The [Bi2O2]2+ layer in middle has positive charge, which can strongly attracted NbO6 octahedron. The distortion of NbO6 octahedral structure in layered structure can effectively suppress the recombination of electronics and holes, and improve photocatalytic efficiency. On the other hand, compared with other methods of doping noble metal, the Ag atoms can be well-proportioned on the catalyst surface and layer, and avoid aggregate. It is favorable to enhance photocatalytic activity. The noble metal Ag has strong ability to capture electronic and surface plasmon resonance (SPR), which can improve the visible-light photocatalytic activity. In addition, the method of catalyst recycling is also studied. Based on the detection of intermediate products, we deduce the degradation mechanism of TBBPA. It is significant to study the transformation of TBBPA in the environment.