Properation Of BiFeO₃Photocatalyst And Their Modification

In this dissertation, we investigated the effect of precursor Bi3+/Fe3+ion concentration on the hydrothermal synthesis of BiFeO3crystallites. The optical absorption features and magnetic properties of products were investigated; The cylinder-like BiFeO3particles were successfully prepared by a hydrothermal method. It is found that the height of the resulted BiFeO3microcylinders has a dependence on the hydrothermal reaction time. The BiFeO3particles with different morphologies and sizes were synthesized via a hydrothermal process, where the morphology and size was tailored by using different KOH concentrations in precursor solution. The photocatalytic properties of the samples were evaluated, and the photocatalytic activities of the BiFeO3samples were enhanced by the decoration of Ag nanoparticles on their surface. The main points obtained are summarized as follows:(1) It is demonstrated that the phase-purity and morphology of the products is highly dependent on the metal ion concentration. Phase-pure BiFeO3crystals can be prepared at the Bi3+/Fe3+ion concentration ranging from0.025to0.0625M. The samples prepared at n(Bi3+/Fe3+)=0.025M,0.0375M,0.05M, and0.0625M, are composed, respectively, of cuboid-like particles (100-200nm), regular spherical agglomerates (30-40μm) made up of irregular grains with size about several hundred nanometers, irregular flower-like clusters formed from irregular grains of several hundred nanometers in size, and octahedron-shaped particles (500-600nm). These samples have a similar bandgap energy of2.20eV and exhibit a typical antiferromagnetic behavior at room temperature.(2) BiFeO3microcylinders were synthesized via a hydrothermal condition. SEM observation reveals that with increasing the hydrothermal reaction time from6to15h, the microcylinders grow from-0.7to～4.1μm in height, whereas their diameter remains to be3.7-3.8μm with a minor change. The microcylinders are mainly made up of sphere-like grains of100-150nm in size. A possible growth mechanism of the BiFeO3microcylinders is proposed. The photocatalytic activity of the as-prepared BiFeO3samples were evaluated by the degradation of acid orange7under simulated sunlight irradiation, revealing that they possess an appreciable photocatalytic activity. Magnetic hysteresis loop measurement shows that the BiFeO3microcylinders exhibit a typical antiferromagnetic behavior at room temperature.(3) Based on a hydrothermal route, BiFeO3particles with different morphologies and sizes were synthesized by varying the KOH concentration in precursor solution. The samples prepared at n(KOH)=3M,4.5M,6M, and7.5M are composed, respectively, of octahedron-shaped particles (500-600nm), cube-like particles (200-500nm), irregular spherical agglomerates (9-16μm) formed from disk-like grains with diameter of1.4-2.8μm and thickness of0.2μm, and cuboid-shaped particles with length-to-width ratio of1.4:1-3.5:1 and width size ranging from80to280nm. Ag nanoparticles were deposited on the surface of BiFeO3particles by a chemical reduction method to produce Ag@BiFeO3nanocomposites. The photocatalytic activity of prepared samples was evaluated by degrading rhodamine B under simulated sunlight irradiation. It is demonstrated that Ag-decorated BiFeO3particles exhibit an enhanced photocatalytic activity compared to bare BiFeO3particles. This can be explained by the effective transfer of photogenerated electrons from the conduction band of BiFeO3to Ag nanoparticles and hence increased availability of holes for the photocatalytic reaction. Hydroxyl radicals were detected by the photoluminescence technique using terephthalic acid as a probe molecule and are found to be produced over the irradiated BiFeO3and Ag@BiFeO3photocatalysts; especially, an enhanced yield is observed for the latter.