Study On Modification Of Bismuth Ferrite-based Composite Nanofibers

As a material with broad application prospects,multiferroics materials with simultaneously displaying the superior properties of ferroelectric and ferromagnetic ordering,and are widely used in ferroelectric integrated devices,energy conversion memories,smart sensors,multi-state memories,high density random and so on.As a typical multiferroic materials,BiFeO₃exhibits simultaneous ferroelectric and ferromagnetic properties with suitable band gap and good photocatalytic activity at room temperature.It has received extensive attention for many researchers in recent years.However,BiFeO₃has a long-period spin-modulated helical antiferromagnetic ordered structure,showing weak antiferro-magnetic properties.Second,because of the volatilization of Bi ions and the fluctuation of the valence state of Fe ions during the calcination process,it is easy to result in a strong leakage current,and it is difficult to obtain strong ferroelectric properties,which prevents the potential application of BiFeO₃in the field of magnetoelectricity fields.Third,the photocatalytic activity of BiFeO₃is still lower than those of ideal values due to the high photogenerated electron-hole recombination rate.Fourth,there is a lack of joint application research based on multi-functionality,and insufficient research on the mechanism of BiFeO₃multi-performance systemic improvement.Based on the above discussion,this paper focuses on the construction of high-performance magneto-electric sensors and magnetic-photocatalytic dilute magnetic semiconductor.Based on the multiple properties of BiFeO₃at room temperature,from the perspective of improving the performance of BiFeO₃multiferroic ferrites,BiFeO₃-based composite nanofibers were prepared by self-assembly coaxial electrospinning technology throug a composite or doped ways,the structure,morphology and element valence variations of the composite nanofibers were analyzed in detail.A systematic characterization was carried out to study the magnetoelectric coupling properties,wave absorption properties and magneto-photocatalytic properties.Meanwhile,the influences of low temperature on the exchange bias effect and exchange coupling effect of composite nanofibers were discussed.In addition,the growth mechanism,structure and the transformation mechanism of BiFeO₃-based composite nanofibers were studied.The main research results are as follows:1.BiFeO₃nanofibers and nanoparticles with different morphologies were successfully prepared by electrospinning and hydrothermal methods.The surface of BFO nanofibers is formed by the adhesion of irregular tiny crystallites,and the average diameter of 87 nm.The surface of BFO nanoparticles is consist of agglomerated particles,and the crystal size ranged from 86 nm to 400 nm.Compared with BFO nanoparticles,the surface of BFO nanofibers with irregular microcrystalline enables to higher interfacial area and lower grain size.The finite size effect constrains the helical spin-modulated structure,which resulting in the strong size dependence and enhancing the magnetization.This special fibrous structure makes the dye molecules more accessible to the catalyst surface,promoting the efficient diffusion of hydroxyl radicals,and has a greater photocatalytic degradation efficiency for methyl orange.2.Single BiFeO₃nanofibers and mixed-phase BiFeO₃-Co（Ni）Fe₂O₄（BFO-C/NFO）composite nanofibers were prepared by coaxial electrospinning technology,respectively.The crystal structure,morphology changes,Raman property,element valence evolution and electromagnetic property of spinel-phase composite nanofibers was studied.The results show that the secondary phases could not be ignored in the prepared process.Meanwhile,the proportion of each phase was quantified by Rietveld fitting results.The composite structure is distorted,and the Fe O₆octahedron cooperates with the unique spatially modulated spin cycloid structure of BiFeO₃materials.An uncompensated spin magnetic moment appears on the surface of the fiber,which enhances the magnetic properties of the composite nanofiber.In addition,the antiferromagnetic deposition caused the horizontal eccentric displacement of the M-H curve,which lead to the exchange bias effect.The existence of Bi₂Fe₄O₉impurity phase affects the ferroelectric hysteresis characteristic,the charge conduction mechanism is changed due to the structural distortion,which results in a larger leakage current density.3.（1-x）BiFeO₃-x CoFe₂O₄（x=0.15,0.35,0.55,0.75）composite nanofibers were prepared by coaxial electrospinning technology.The crystal structure,microscopic morphology,ferroelectric properties and microwave absorption of composite nanofibers were studied.By changing the content ratio of BFO and CFO,the electro-magnetic properties in the structure evolution process are studied.Structural analysis confirmed that the intensity of some diffraction peaks display a regular trend with the increase of CFO content.Four phases exist in composite nanofibers.The increase of CoFe₂O₄makes the magnetic properties such as saturation magnetization,remanent magnetization,coercive force and magnetic moment increased regularly.The super-exchange interaction in BC14 enhances the ferromagnetic properties.The research of electro-magnetic parameters indicates that the CFO composite can realize the precise control of the electromagnetic properties of composite nanofibers.4.Pure-phase BiFeO₃nanotubes and BiFeO₃-Ce O₂-Bi₂Fe₄O₉（BFO-CE-BF）ternary composite nanofibers were investigated.There are four space groups of R3c,Pm-3m,Pbma and I23 in the BFO-CE-BF ternary composite nanofibers.BFO-CE-BF has a smaller grain size,and the presence of size effect and surface effect increase the magnetic and photocatalytic properties.The low temperature（10 K）suppressed the magnetic reversal,weakened the exchange coupling between the soft and hard magnetic phases,and enhanced the remanent magnetization and coercivity of the composite nanofibers.In addition,compared with pure BFO nanofibers,the composite nanofibers have higher electron-hole separation efficiency,which can be a suitable candidates for photocatalytic degra dation of organic pollutants.5.The low temperature exchange bias effect and magnetic and photocatalytic degradation properties of Bi_0.8Re_0.2Fe O₃（Re=Nd/Sm）（BNFO/BSFO）composite nanofibers were systematically studied.The crystal structure changes from the rhombohedral phase of R3c space group to the orthorhombic phase of the Pbam and Pnma space groups after substitution.The doping causes the BiFeO₃octahedron to collapse,which affects the interaction between Fe³⁺ions and the magnetic moment.The remanent magnetization and coercivity of the composite nanofibers changed significantly at 10 K and 300 K,and the lower temperature suppressed the magneto caloric and valence fluctuations,which had an important effect on the magnetic properties.The exchange bias effect（374.32 Oe）of BNFO nanofibers at 10 K is much higher than that of BSFO nanofibers（22.96 Oe）.Meanwhile,the absorption edge of BSFO nanofibers undergoes a slight red shift,which indicated larger photocatalytic degradation.The highly magnetic nanocatalysts can realize the secondary separation of magnetic adsorption and used to be potential candidates in the fields of photocatalysis and semiconductor industry.