Studies On Synthesis And Properties Of Bismuth Ferrite Materials

As an important class of materials,Bismuth ferrite based compounds have attracted a lot of attention because of their special structures,properties and wide application prospects.BiFeO3 with perovskite structure has good magnetoelectricity and multiferroic properties,and it has great application prospects in the field of magnetoelectric sensors and information storage.Bi2Fe4O9 with mullite structure is an important material because of its potential applications as gas sensor and good catalyst.In addition.BiFeO3 and Bi2Fe4O9 are multiferroic,and their band gaps are narrower than other semiconductor materials,so they have great potential in the field of ferroelectric photovoltaic.In consideration of application in information storage,solar cell and silicon technologies,we have prepared BiFeO3,Bi2Fe4O9 and their derivatives thin films by sol-gel method.Firstly,we successfully prepared the stable precursor of BiFeO3 and Bi2Fe4O9,then explored the optimal preparation condition of BiFeO3 and Bi2Fe4O9.Based on this,BiFeO3 and Bi2Fe4O9 thin films were doped with chromium(Cr)and cobalt(Co)respectively.BiFe1-xCrxO3(x = 0,0.03,0.06,0.09)and Bi2Fe4(1-y)Co4yO9(y=0,0.03,0.06,0.09)thin films were successfully prepared.Now we mainly analyze the influence of Cr doping on BiFeO3 thin films and Co doping on the Bi2Fe4O9 thin films from the aspects of microstructure,stress,micro-morphology,optics and magnetism with a variety of test methods.The main work results are showed as follows:1.The BiFeO3 thin films were grown on quartz and FTO(fluorine-doped SnO2 conductive glass)substrates by sol-gel method.The optimal preparation conditions were investigated and Cr doping was performed.Bi2Fe4O9 precursor was successfully prepared.Bi2Fe4O9 thin film was prepared on FTO substrate,and the process parameters were optimized.Then Bi2Fe4(1-y)Co4yO9 thin films were successfully prepared on FTO substrates by sol-gel method.2?The structure and properties of BiFeO3 films and the effect of Cr doping on the structure and morphology of BiFeO3 films are deeply analyzed by X-ray diffraction pattern,microscopic image and stress calculation.XRD patterns show that the prepared films have a rhombic structure.The peak positions of the diffraction peaks(012)and(110)shift toward higher 2? value,indicating that the interplanar spacing becomes smaller,which is due to the fact that Cr3+ radius(0.615 A)is smaller than Fe3+ radius(0.645 A).The increase of Cr makes the internal stress of BiFe1-xCrxO3 on the FTO substrates decrease gradually from tensile stress to compressive stress.SEM indicates that the proper proportion of Cr can effectively improve the crystal quality of BiFeO3 and reduce the grain size of the films.The transmission spectrum and photoluminescence spectrum(PL spectrum)of BiFe1-xCrxO3 films have a red shift,which suggests the optical band gap of BiFeO3 decreases with Cr doping.This is because that Cr doping introduces a new energy level and increases the DOS of the band and new localized state in the forbidden band.In the process of films growth,oxygen vacancies are unavoidable.They can capture electrons to form F center(indicated by Vo)which has magnetism.The electron orbit of the F center is coupled with the 3d3 electron orbit of adjacent Cr3+.The electron magnetic moment of F center induces the 3d3 electron spin equilibrium adjustment of Cr3+.Finally,the electron spin of the F center is antiparallel to the electron spins of the two adjacent Cr3+ ions,and the electron spins of the two Cr3+ ions are parallel.The Cr3+-Vo-Cr3+groups introduce magnetic order within local lattice.The super-exchange angle between Fe3+ and Cr3+ is less than 180°,so it is not a complete anti-ferromagnetic coupling.The magnetic moments of the two ions are also different,and the lattice distortion caused by Cr doping breaks the original anti-ferromagnetic order of Fe3+ in BiFeO3.These reasons lead to the enhancement of ferromagnetism in BiFe1-xCrO3 thin films.3.Study on the properties of Bi2Fe4(1-y)Co4yO9 thin films.The XRD pattern shows that the undoped Bi2Fe4O9 film has orthorhombic mullite structure.With the increasing of Co dopant,a,b,and c vary in size,indicating that Co doping has caused lattice distortion.Co doping reduces the pores on the surface of the films.The crystal grains are uniformly distributed.Some Raman peaks of the Bi2Fe4(1-y)Co4yO9 films shift to higher wavenumer with Co doping,because the radius of Co2+ is smaller than that of Fe3+,which is consistent with the XRD analysis result.Co doping introduces a new energy level,and the outermost 3d7 of Co2+ becomes the new highest occupied orbit,which makes the valence band to extend towards the forbidden band.As a result,the band gap of Bi2Fe4O9 decreases.With the substitution of magnetic Co ions on the Fe position,the films exhibit enhanced ferromagnetism with clear hysteresis loops,because Co doping can destroy the antiferromagnetic sequence formed by the Fe in original Bi2Fe4O9.In addition,Co has good magnetic property,and the magnetic moment of Co2+ is different from the magnetic moment of Fe3+.So the combination of Co2+ and Fe3+ can generate net magnetic moment.As a result,the ferromagnetism of the Bi2Fe4(1-y)Co4yO9 thin films is significantly improved.