Synthesis, Characterizations, Properties And Applications Of Nano BiFeO₃

BiFeO₃（BFO） withtriangle distortedperovskitestructure,oneofthetypicalfew single-phasemultiferroics with a simultaneous coexistence of ferroelectric and G-type antiferromagnet orderparameters at room temperature, has a wide range of applications in new memory devices,electrically tunable microwave devices and spintronics devices. However, nano BFO is capturingthe public’s attention. Nano BFO possesses potential applications in various nanoscale devices,such as high-density magnetic storage, magnetic-logic gate, color monitor with degaussing systemand automatic temperature controller, because it exhibits distinctive nanoscale coupling, sizeeffects and atomic-scale interface coupling mechanism affecting unique physical and chemicalproperties. Though BFO nanostructure materials have been synthesize by some procedures,producing nano BFO with efficient catalytic activity by simple synthesis routeis not an easy task.Hence, the work of this dissertation aims to synthesize, characterize and investigatephysical-chemical properties and applications of nano BFO. The primary contents include thefollowing aspects:（1） Micro-nano crystalline wafer-like BFO was successfully fabricated via a hydrothermalmethod. Micro-nano crystalline wafer-like BFO with a rhombohedral perovskite structure wassynthesized within a lower alkaline mediumat a relative lowtemperature. Ahigh crystallinity anda mono-dispersion products were obtained, without any dispersant or mineralizer. And theproducts exhibited a typical antiferromagnetic order at ambient temperatures. The elementalcomposition of BFO was given the formula Bi0.99Fe0.96O₃.06by the XPS and EDS spectra. In theFTIR spectrum of the wafer-like BFO, two strong absorptive peaks around548.38cm^-1and439.65cm^-1were attributed to the Fe-O stretching and O-Fe-O bending vibrations, beingcharacteristicsoftheoctahedralFeO6groups intheperovskitecompounds.Therewerethreepeaksassigned to A1modes, and eight peaks assigned the E modes in the Raman spectrum of thewafer-like BFO. Some signal peaks shift to lower frequency because of anharmonic effects ofthelattice or possible strain existing in sub-microparticles.The results showed that the BFO with2.05eV narrow band gap was beneft for the effcient utilization of visible-light. The visible-lightphotocatalytic activityofwafer-like BFOwasevaluated by degradationofRhodamine B （RhB） in the system of BFO-H₂O₂. Within3h, about90.5%of RhB (10mg L^-1) was degraded.But in thesame conditions, the degradation of RhB was only27.3%with nano TiO2as catalyst. Theelectrocatalytic properties were investigated by cyclic voltammograms.The modifed electrode ofsynthesized wafer-like BFO-chitosan-glassy carbon （BFO-CS-GCE） was characterized in thesupporting electrolyte. Obvious enhancement of the peaks currents can be observed byBFO-CS-GCE electrode, which indicated that the synthesized BFO possessed a highelectrocatalytic activity.（2） A novel microwave-assisted sol-gel approach was applied to the synthesis of bismuthferrite nanoparticles （BFO NPs）. A highly crystalline and pure-phase BFO with a rhombohedralperovskite structure was prepared with a low concentration of NH3·H2O and an appropriateamount of dispersing agents PEG （6000）. The preferential orientation of the growth direction ofBFONPscrystalwasalongthe（104） plane andthemean diameterwas73.7nm. Theproductshada weak ferromagnetism response at room temperature and the saturation magnetization （Ms） wasabout45.77emu g^-1. The UV-visible spectrum of BFO NPs demonstrated that the narrowband-gap energy was2.18eV, and there were two absorption peaks: the peak around450nm wasdue to metal-to-metal transition and the one around650nm was due to crystal feld transition,respectively. Anovel sensor based onBFO NPs-CS-GCE had been developed for the detection ofenvironmental pollutants. The results suggested that BFO NPs possessed a high electrocatalyticactivity for p-Nitrophenol （p-NP）. Besides, the oxidation catalytic activity of BFO NPs wasevaluated by degradation of methylene blue （MB） solution under ultrasound-andmicrowave-combined-assisted irradiation. The degradation efficiency of MB (10mg L^-1) was90.9%with BFO NPs as catalyst and H₂O₂as oxidant under the optimized conditions for15min.Thepossible mechanismoftheoxidationcatalyticdegradationwasalso proposed.（3） The optical, magnetic and electric behaviors of nano BFO with different morphologieswere compared, respectively. The results showed that micro-nano crystalline wafer-like BFOsynthesized by hydrothermal technique exhibited antiferromagnetic behavior. BFO NPs andirregularly shaped BFO aggregation, which both exhibited weak magnetism, were fabricated bymicrowave-assisted sol-gel method within citric acid and ammonia aqueous （NH3·H2O） solution,respectively. The BFO NPs has the potential applications in electrochemical analysis due to its quite high electrocatalytic activity. Irregularly shaped BFO aggregation revealed relatively narrowband gapenergy. The results implied that product morphology hada greater impact ontheoptical,magnetic and electric properties of nano BFO, because different morphologies led to differentmicrostructures.