Electrical,Magnetic,and Photocatalytic Properties Of BiFeO₃-based Ceramics And Heterojunctions

In recent years,the ABO3 type perovskite oxide has attracted wide attention and been greatly investigated because of its multifunction.Its rich physical properties are related to the unique structure.For example,the hybridization between B cation 3d and oxygen 2p orbitals is essential for ferroelectricity.The ferromagnetism is decided by the partially occupied d orbital of B cation.The electronic structure of simple perovskite-type oxides near the Fermi level is formed mainly by mixing d-orbitals of B cation and 2p-orbitals of oxygen.Thus,these always result the properties as ferroelectricity,ferromagnetism,piezoelectricity and photocatalysis(photoelectric response)in the perovskite simultaneously.Besides,due to the great structure adjustability,the A site or B site cation can be doped by the elements with different ionic radius and different electronic structure,so that features such as negative and positive charger center,crystal structure and electron spin can be changed to regulate the above-mentioned properties.Therefore,perovskite material not only has rich physical properties but also can be manually adjusted to achieve ideal performance.Multiferroic is one important physical property of perovskite oxides,multiferroic materials generally has ferroelectricity and(anti)ferromagnetic simultaneously.The single-phase multiferroic materials are rare at room temperature,while BiFeO3 is a room temperature single-phase multiferroic material that has been widely investigated,the antiferromagnetism of BiFeO3 is decided by the partially occupied d-orbitals of Fe3+ ion,the distorted structure caused by the lone 6s2 electron pair of Bi3+ produces the ferroelectricity,and the empty electron orbitals of 6s2 makes the visible light photocatalytic activity of Bi-based compounds possible.However,there are also some defects of BiFeO3.Firstly,the ferromagnetism of BiFeO3 is very weak because of its G-type antiferromagnetic structure.Secondly,the leakage current of BiFeO3 is so large that the saturated ferroelectric polarization cannot be obtained.Finally,although the Bi-based materials could obtain visible light response easily,their band structure may not be suitable for photocatalytic reaction.Based on the above discussion/in order to these problems,in this work,we investigate the effect of substitution,solid solution and heterojunction formation on the electrical,magnetic and photocatalytic properties of BiFeO3 based materials.Specifically,we first fabricated the B-site Cr ion substituted 0.675BiFei-xCrxO3-0.325PbTiO3 multiferroic ceramic,we studied the changes and mechanism of its multiferroic.Secondly,we prepared the g-C3N4/xBiFeO3 heterojunction by mechanical milling and investigated the change of its photocatalytic activity.Finally,considering that the conduction band potential of BiFeO3 is too positive to have a higher photocatalytic activity,while another Bi-based perovskite,Bi4Ti3O12,has more suitable valance band and conduction band potential,so we investigate the photocatalytic activity of Bi4Ti3O12 and further research the effect of carbon quantum dots coating on the photocatalytic properties of Bi4Ti3O12.The main achievements and conclusions of this thesis are:1.The multiferroic properties of 0.675BiFei-xCrxO3-0.325PbTiO3 ceramic is enhanced after Fe ion is replaced by Cr ion.(1)The ferroelectricity of 0.675BiFeO3-0.325PbTiO3 is improved by Cr ion substituting,which is related with reduced defect dipole due to the substitution of Cr.(2)The piezoelectricity is improved because of the reduced oxygen vacancy and the enhanced ferroelectric polarization.(3)The resistance of ceramic is increased due to the reduction of oxygen vacancy,which results in the decrease of dielectric loss.(4)The ferromagnetism of ceramic is improved with the increase of Cr ion content,because Fe ions and Cr ions in the lattice can form ferromagnetic coupling.2.The g-C3N4/xBiFeO3 heterojunction has excellent visible light photocatalytic activity.Milling process and the BiFeO3 content affect the photocatalytic activity of heterojunction.The material with 10wt%BiFeO3 prepared with 20:1 ball/powder ration has the highest photocatalytic degradation rate of RhB.Moreover,the photocatalytic efficiency can be improved by the mechanical pressing and the electoral poling.Mechanisms responsible for such enhanced photocatalytic efficiency are:(1)Uniform heterojunction microstructure provides more photocatalytic reaction active sites.(2)The transfer mechanism of photogenerated carriers of the heterojunction is Z transfer model,the separation of photogenerated carriers is improved by band alignment of g-C3N4 and BiFeO3.(3)Due to the ferroelectricity and the piezoelectricity of BiFeO3,the mechanical pressing and the electoral poling can promote the orientation arrangement of ferroelectric domains,forming a local electric field,which promotes the separation of photogenerated carriers,thus,the photocatalytic efficiency is improved.3.The Bi4Ti3O12 does not have visible light photocatalytic activity,but the load of carbon quantum dots(CQDs)can significantly improve the visible light photocatalytic activity of Bi4Ti3O12,so that the Bi4Ti3O12 can be used to photocatalytic degradation of RhB or photocatalytic hydrogen evolution under visible light.The reason for this is given from two aspects:(1)the up-conversion of CQDs promotes the absorption of visible light of Bi4Ti3O12.(2)CQDs can act as electron acceptor,facilitating the separation of photogenerated carriers.(3)the superior conductivity of CQDs suppress the recombination of photogenerated carriers.