Studies On Preparation And Properties Of Photocatalytic Multi-doped BiFeO₃

Semiconductor-based photocatalytic technology is one of an effective ways to solove environmental pollution.The ferroelectric materials have a special advantage in the photocatalytic field since their internal polarization can suppress the recombination of electron-hole pairs.Among them,the BiFeO₃ has both multiferroic properties at room temperature and narrow energy band gap,so it has attracted much attention in this field.However,the low degradation efficiency caused by narrow light absorption range and low photocatalytic rate limits their practical application.Therefore,in this thesis based on BiFeO₃ the following researches have been carried out:The effects of Sr and Co doping on structural,multiferroic and optical properties were investigated in the Bi_1-xSr_xFe_1-yCo_yO₃（x,y=0,0.05）powders prepared by the hydrothermal method.With Sr and Co doping,the crystal structure of powders changes from R3c structure for BiFeO₃ to P4mm structure for Bi_0.95Sr_0.05Fe_0.95Co_0.05O₃,the particle size decreases,and the surface morphology becomes homogeneous.Besides,the doping of Sr and Co can significantly enhance the multiferroic properties of Bi_1-xSr_xFe_1-yCo_yO₃ samples.The maximum magnetization（5.66 emu/g）and remnant polarization（0.416μC/cm²）are observed in Bi_0.95Sr_0.05Fe_0.95Co_0.05O₃.Meanwhile,the energy band gap of Bi_1-xSr_xFe_1-yCo_yO₃ has decreased from 2.06 e V to 1.24 e V.The highest degradation efficiency of methyl orange～76%is obtained in Bi_0.95Sr_0.05Fe_0.95Co_0.05O₃ within 3 hours under the visible light.The improved photocatalytic performance can be attributed to the reduction in band gap,the improvement of ferroelectric properties and the decrease of particle size induced by Sr and Co doping.The single phase Bi_0.95Sm_0.05Fe_1-xNb_xO₃（0≤x≤0.1）nanoparticles were synthesized by the sol-gel route,and the effect of Nb doping on their magnetic,ferroelectric and photocatalytic properties were studied.With the increase in Nb contents,the phase structure of nanoparticles transformes from R3c phase to Pnma phase.Meanwhile,the particle size decreases significantly,and the morphology becomes homogeneous.The maximum remnant magnetization（0.014 emu/g）and maximum polarization（0.592μC/cm²）are obtained in Bi_0.95Sm_0.05Fe_0.9Nb_0.1O₃.Besides,it has been observed that the energy band gap has been slightly reduced from 2.14 to 2.03 e V with Nb doping,indicating an improvement of photocatalytic activity.The highest degradation efficiency（～74%）of methylene blue is obtained in Bi_0.95Sm_0.05Fe_0.93Nb_0.07O₃nanoparticles,which can be attributed to the optimum particle size and the smallest energy band gap.The magnetic,ferroelectric,and photocatalytic properties of（1-x）BiFeO₃-x BaTiO₃（0≤x≤0.4）powders synthesized by sol-gel method have been investigated.With the increase in BaTiO₃ contents,the phase structure of samples changes from R3c phase to P4mm phase.The particle size decreases and the morphology becomes homogeneous with the introduction of BaTiO₃.Besides,the doping of BaTiO₃can significantly enhance the multiferroic properties of samples and the maximum remnant magnetization（0.261 emu/g）and remnant polarization（20μC/cm²）have been acquired in 0.8BiFeO₃-0.2BaTiO₃ and 0.7BiFeO₃-0.3BaTiO₃ samples,respectively.The absorbance in ultraviolet and visible light regions is improved obviously in 0.7BiFeO₃-0.3BaTiO₃.The energy band gap of the samples decreases from 2.06 e V to 1.57 e V with the introduction of BaTiO₃,indicating that the excitation rate of photogenerated electron-hole pairs is improved.Among them,the highest methylene blue degradation efficiency of～62%within 3 hours under the visible light is achieved in the 0.7BiFeO₃-0.3BaTiO₃ which can be attributed to its narrow energy band gap and large remnant polarization.