Rare Earth Doped Bifeo < Sub > 3 < / Sub > The Preparation Of Nanoparticles And Visible Light Catalytic Properties

BiFeO₃ is known to be one of the several single-phase room temperature multiferroic materials, which has simultaneous ferroelectric （T_C 830℃） and G-type antiferromagnetic （T_N 370℃） properties above room temperature . In addition to the magnetoelectric applications at information storage, spintronics, and sensors, BiFeO₃ is also considered as a kind of visible-light responsive photocatalysts for water splitting and degradation of organic pollutants due to its suitable band gap. In spite of some studies have been made on the photocatalytic properties, the performance of BiFeO₃ needs to be furthered studied from standpoint of widely practical application and commercial benefit. Moreover, the existence of the morphotropic phase boundary （from the rhombohedral phase to orthorhombic phase transition） were found in the rare earth doped BiFeO₃ epitaxial films, whererase the dielectric and piezoelectric properties were greatly enhanced in the morphotropic phase boundary. Is there also exisit morphotropic phase boundary in rare earth doped BiFeO₃ nanoparticles, and how about the photocatalytic properties at this boundary?In this thesis, pure, multiphases, and rare-earth (Gd³⁺,Nd³⁺,Eu³⁺,Dy³⁺) doped BiFeO₃ was prepared by a sol-gel method. X-ray diffraction （XRD）, Raman spectroscopy and Transimisson Electron Microscopy were used to characterize the microstructure of the particles. The optical and visible-light photocatalytic properties were also investigated in detail. The main obtained results are shown in as follows:1) Pure BiFeO₃ nanoparticles with visible-light photocatalytic properties were prepared by sol-gel method, however, the photocatalytic activity is is extremely low. Only 25% rhodamine B was degraded by BiFeO₃ nanoparticles within 15 hours. Then the pH value of the rhodamine B was modified, and the photocatalytic activity of the BiFeO₃ nanoparticles was observed to be the best at pH = 2 for the decolorization of rhodamine B. Low pH value of the solution is favourable for degading rhodamine B by the BiFeO₃ nanoparticles. However, through the analysis of the XRD pattern of the BiFeO₃ nanoparticles before and after the photocatalysis, we can see a new impure phase appeared when the pH value is too low, leading to the reduction of the catalytic activity. We also synthesized several BiFeO₃ nanoparticles via a sol-gel method with different gel-drying temperatures. BiFeO₃/γ-Fe₂O₃ multiphase nanoparticles could be gradually formed with enhancing the gel-drying temperature. Our results showed that at an optimal ratio between the two phases of BiFeO₃ andγ-Fe₂O₃, the photocatalytic activity of multiphase BiFeO₃ nanoparticles were better than that of pure BiFeO₃ nanoparticles. In addition, much enhanced photocatalytic activity of multiphase BiFeO₃ nanoparticles was observed when adding a small amount of H₂O₂ during the photocatalysis, indicating the samples have photo-Fenton-like catalytic activity. Moreover, multiphase BiFeO₃ nanoparticles were also demonstrated to have an excellent photocatalytic stability, which is useful for the efficient recovery of photocatalyst.2) Effects of the Gd³⁺ dopant on the microstructure and photocatalytic activity of the BiFeO₃ nanoparticles were studied. X-ray diffraction and Raman spectra results of Bi_1-xGd_xFeO₃ （BGFO_x, x=0, 0.05, 0.1 and 0.15） reflect that the crystal structure of the samples remain stable for x<0.1, while compositional-driven phase transition from rhombohedral to orthorhombic and an impurity phase of GdFeO₃ is observed at x=0.1 and x=0.15. The photocatalytic activity to decompose Rhodamine-B under visible-light illumination is found to be increased in BGFO_x as x increases from zero to 0.1, and then decreases for x=0.15. Detailed explanation was given in the thesis. This part of the thsis has been published on Journal of Physical Chemistry C.3) Other rare-earth (Nd³⁺, Eu³⁺ and Dy³⁺) doped BiFeO₃ nanoparticles were also prepared repectively to invsetigate more deeply on the relationship between the microstructure and photocatalytic activity. The photocatalytic activity was found to be the maxima around the morphotropic phase boundary from the rhombohedral to orthorhombic phase for all the three rare-earth doped BiFeO₃ nanoparticles. The doped concentration for the appearance of morphotropic phase boundary are different with different rare-earth elements since their ionic radii are different. The maximum in photocatalytic activity around the morphotropic phase boundary is explained by an anomalously high dielectric constant at the rhombohedral-orthorhombic phase boundary that enlarges the space-charge region on the interface of the particle and solution.