Hydrothermal Synthesis And Photocatalytic Properties Of Bismuth Ferrite Compounds

In recent years, with the increase of population and the development of industry, the energy and environmental problems have become more and more serious. Since semiconductor catalysts can be applied for the degradation of organic pollutants by using sunlight, photocatalytic technology has been considered as one of the promising solutions to address the increasing energy crisis and environmental pollution, which has attracted considerable attention. Titanium dioxide (TiO2) has been widely studied and used as a photocatalyst for the degradation of organic pollutant, water splitting, air purification and sterilization. However, the application of pure TiO2is limited, because of its wide band gap (3.2eV). TiO2can only be excited with ultraviolet light (λ<380nm), which makes up only a small fraction (4%) of the total solar spectrum. Over the past years, many efforts have been devoted to the extension of the photoresponse of TiO2to the visible region by doping with metal ions or non-metals, and sensitizing with organic dyes, organic-inorganic hybrid dyes, and narrow band gap semiconductors. However, the corresponding photocatalytic efficiency is not ideal for practical applications. Therefore, it is of great interest to develop efficient visible-light photocatalysts for the degradation of organic pollutants.On the basis of these, bismuth ferrite compounds were prepared by hydrothermal process and their visible-light photocatalytic activities were studied in this thesis. The main contents were outlined below:(1) BiFeO3powders with various morphologies were successfully prepared via combining the sol-gel technique and hydrothermal process for the first time. Microspheres and microcubes were obtained as sol was used as a precursor, and submicrometer-sized particles could be prepared by using gel. Possible formation mechanism of BiFeO3microcrystals was the Ostwald ripening process. It was supposed that the difference of the morphology between BiFeO3microcrystals should be attributed to the changed crystal growth behavior caused by the varying OH" concentration. Moreover, the results of UV-vis diffuse reflectance spectroscopy and photodegradation experiments reveal that BFO powders can be used as an effective photocatalyst under visible light. (2) BiFeO3@carbon core/shell nanofibers (BFO@C NFs) with different thickness of carbon layers were successfully prepared by combining the electrospining technique and hydrothermal process for the first time. The photodegradation experiments show that BFO@C NFs samples exhibit higher degradation efficiency for methyl orange than the pure BFO NFs. It was proposed that the introduction of carbon should not only enhance the light absorption of BFO NFs and the adsorption capacity for methyl orange, but also facilitate the separation of photogenerated electron-hole pairs, both of which result in the enhanced photocatalytic activity of BFO NFs. In addition, BFO@C NFs could be easily recycled without decreasing the photocatalytic activity, which will promote their application for removal of the organic pollutants in waste water.(3) Bi2Fe4O9crystals with various morphologies were successfully prepared by introducing inorganic salt (KNO3) during the hydrothermal process for the first time. Submicron-sized Bi2Fe4O9crystals were successfully in the absence of mineralizer KNO3, while micron-sized Bi2Fe4O9crystals with rod-like morphology were obtained in the presence of KNO3. Transmission electron microscopy was employed to characterize the products and the results demonstrate that the rod-like crystals grow preferentially along the [001] direction. The Ostwald ripening process was the possible formation mechanism for the microrods according to a series of time-dependent experiments. It was suggested that the block effect of NO3-should result in the rod-like powders. Moreover, the optical absorption of the as-prepared Bi2Fe4O9crystals and the results of photodegradation experiments suggest their promising applications as photocatalysts in the visible range.(4) Pure Bi2Fe4O9powders with different morphologies and particle size have been prepared via surfactant-free solvothermal route in water and water-ethanol mixed solvents for the first time. The effect of the solvent on the morphology of Bi2Fe4O9crystals and the formation process of rod-like microrods and defective crystals were investigated. Rod-like and cubic shape Bi2Fe4O9crystals could be easily obtained by changing the volume ratios of H2O/EtOH. The results indicate that the rod-like Bi2Fe4O9crystals could be formed in the more ionic solvent, and may be related to the anisotropic nature of the crystal structure. The lower photocatalytic activity of Bi2Fe4O9microrods could be attributed to their larger particle sizes, smaller specific surface areas and smaller band gap.(5) Bi25Fe04o powders were successfully synthesized via hydrothermal method in the presence of mineralizer LiNO3. The effect of the hydrothermal condition, i.e., the concentration of mineralizer, the stoichiometry of reagents, and the amount of LiNO3, on the phase structure and morphology of the as-prepared samples were investigated. The results demonstrate that the introduction of Li+not only promotes the formation of Bi25FeO40, but also induces the decrease of particle size. Moreover, the absorption feature of the Bi25FeO40powders and the results of photodegradation experiments suggest their promising applications as photocatalysts activated by visible light.