Study On The Synthesis And Properties Of BiFeO₃ Thin Films

Multiferroic materials have become a research hotspot in the field of new smart devices in today's society.The so-called multiferroic material is a material that causes ferroelectric?antiferroelectric?,ferromagnetism?antiferromagnetism?and ferroelasticity because of the order of structural parameters.Bismuth ferrite?BiFeO₃,BFO?thin film has attracted more and more attention as a single-phase multiferroic material.BFO is a perovskite structure.Its oxygen eight face rotates at a certain angle around the diagonal axis,forming an oblique six party structure which deviates from the ideal perovskite structure.As a kind of ferroelectric and ferromagnetic material at the same time,BFO also has high ferroelectric Curie temperature?T_C=1103K?,antiferromagnetic Neel temperature?T_N=643K?and large ferroelectric polarization?Pr=100C/cm²?.These special properties make it applied in high density,low power,non volatilization nobel sensors,polymorphic magnetoelectric power sensors,memory,spintronics and optoelectronic devices.Because of its small optical band gap?2.2-2.9eV?and good chemical stability,BFO is used as a kind of efficient photocatalyst in visible light area.In this thesis,BFO thin films on the Si/SiO₂ substrate used to sol-gel method were synthesized and the crystal structure,magnetic properties and photocatalytic properties of the films were studied.The effects of annealing time and annealing temperature on the crystal structure,magnetic properties and photocatalytic properties of BFO thin films were discussed.The main conclusions are as follows:?1?Firstly,the influence of annealing temperature on the structure and morphology of BFO thin films was discussed.It is concluded that the crystallization of BFO thin films is best when the annealing temperature is 500?.With the increase of annealing temperature,impurity phase and surface pore appear in BFO film,the result indicates that too high temperature leads to volatilization of Bi elements,resulting in heterogeneous phase.Raman spectroscopy also showed that the quality of BFO film at 500?is the best.?2?Secondly,the effects of annealing time?2min,4min,8min,16min and 32min?on the crystal structure and surface morphology of BFO films were discussed.The experimental results showed that the samples with better morphology were obtained at the annealing time of 16min.When the annealing time is short,the crystallinity of the film is not enough to cause the appearance of pores and impurity phase,while the annealing time is too long,the intensity of the diffraction peak and the grain size decreases,resulting in the decrease of the crystallinity of the film.?3?VSM results show that the magnetization of BFO thin films decreases with the increase of annealing temperature.The saturation magnetization is the largest and the coercive force is the smallest at 500?.Because of the temperature rises,the heat movement of the atom is more intense,and the spin orientation of the electronics tends to be disordered,which leads to the decrease of the saturation magnetization?4?The optical properties test shows that the optical band gap of BFO film increases with the increase of annealing temperature?500?,550?,600??,and the narrow optical band gap of BFO film is obtained at 500?.Because annealing temperature will directly affect the crystallinity,purity,surface roughness and interface of the film,which will indirectly affect its optical band gap.With the increase of annealing temperature,the impurity phase and air holes of the BFO films decrease the valence band electrons excited by the visible light,which may lead to the widen of the optical band gap.?5?Photocatalytic test shows that the photocatalytic efficiency of BFO film on methylene blue solution decreases with the increase of annealing temperature?500?,550?,600??.The BFO photocatalyst with better photocatalytic efficiency was obtained when annealing temperature is 500?.The high annealing temperature reduces the optical absorption of the thin film,so its photocatalytic efficiency in the visible region is also decreasing.BFO film has the best degradation effect on methylene blue solution of 20mg/L.