Study On The Polarization Effect To Enhance The Photocatalytic Performance Of BiOBrO₃ And AgFeO₂

Semiconductor photocatalysis technology has great application potential in developing renewable energy and controlling environmental pollution.The solar energy conversion performance of photocatalytic materials is mainly determined by the generation,separation,transport and transfer of photo-generated carriers.Due to the limited migration and the easy recombination of photogenerated electron-hole pairs during the transfer from body to surface,the quantum conversion efficiency is difficult to meet the requirements of practical application.In recent years,polarized electric field engineering has been proved to be an effective modification method to improve quantum efficiency.Different types of polarization effects（e.g.,macroscopic polarization,piezoelectric polarization,ferroelectric polarization,and surface polarization）can generate built-in electric fields that facilitate the separation and transmission of photogenerated electron-hole pairs.As the deepening of the application of polarization effect in photocatalytic technology,the exploration of the generation,regulation and coupling of polarization effect has become a key technical link to be broken through.Here,two layered compounds,PbFCl type BiOXO₃（X=Br,I）and delafossite type AgFeO₂,were selected as research objects to systematically research the generation,regulation and coupling mechanism of macroscopic polarization,surface polarization and spin polarization effects,as well as the mechanism of enhancing photocatalytic performance.The main research contents and results are as follows:（1）By exploring the action mechanism of different types of built-in electric fields in four layered compounds BiOX and BiOXO₃（X=Br,I）photocatalytic materials,it is confirmed that the intensity and direction of surface polarization and macroscopic polarization effects can be controlled by adjusting the microstructure units and their composition.In BiOBrO₃,macroscopic polarization generates the largest built-in electric field and becomes a key factor in inhibiting photogenerated electron-hole pairs recombination.However,the directions of surface polarization and macroscopic polarization in BiOXO₃ are perpendicular to each other,and the coupling between the two has a negative counteracting effect,making the photocatalytic performance of BiOIO₃ lower than that of BiOI with only surface polarization effect.Thus,when different polarization effects are coupled with each other,the consistency of the direction of the built-in electric field generated by the polarization effect is the decisive factor to improve the photocatalytic performance.（2）Delafossite AgFeO₂ samples with different crystalline phases were prepared by hydrothermal co-precipitation method.They are very similar in their basic physical and chemical properties except for the significantly different ordered distribution of magnetic spin.The magnetic spin order distribution is the most obvious in 3R-AgFeO₂ sample,and it shows the best photocatalytic performance.Under the action of external magnetic field,the photoelectric chemical properties of 3R-AgFeO₂ are further enhanced.The difference and adjustability of spin polarization effect in different crystal phases and electronic states（from 2H phase to 3R phase and from antiferromagnetic state to ferromagnetic state）of delafossite AgFeO₂ are attributed to the different spatial arrangement of the spintronic states of Fe atoms.（3）The introduction of oxygen defect causes the centrosymmetric3R-AgFeO₂ to be transformed into non-centrosymmetric AgFeO_2+δ,thus showing macroscopic polarization effect.The prepared AgFeO_2+δ（δ=0.87）samples show the best photocatalytic performance under the action of the macroscopic polarization built-in electric field.Under the action of external electric field,the maximum increase degree of photocurrent density indicates that the further adjustment of macroscopic polarization effect can enhance the photocatalytic performance.After the application of external magnetic field,the coupling between macroscopic polarization and spin polarization shows a positive promoting effect,which can significantly enhance the photocatalytic performance of delafossite AgFeO₂.（4）A series of AgFeO₂ nanosheets with different thicknesses and widths were prepared by adjusting the hydrothermal reaction conditions.The results show that（001）crystal surface is the polar surface of delafossite AgFeO₂.For the nanosheets with the same thickness,the photocatalytic activity of AgFeO₂increases with the exposed area of（001）crystal plane.For the same exposed area,the smaller the thickness of AgFeO₂ nanosheets,the stronger the photocatalytic activity.The characterization of surface microstructure and properties proved that the phenomenon was caused by the surface polarization effect of AgFeO₂.Furthermore,the photocatalytic performance of the AgFeO₂nanosheets was significantly enhanced by the introduction of excess oxygen by annealing and the combined action of external magnetic field and external electric field,indicating that the coupling between surface polarization,macroscopic polarization and spin polarization in the AgFeO₂ nanosheets can promote each other.In this thesis,the polarization effect of two layered compounds PbFCl BiOXO₃（X=Br,I）and delafossite AgFeO₂ was systematically studied,and it was found that different types of polarization internal electric fields could be obtained through reasonable design,regulation and modification of the microstructure and components of layered compounds.These polarization effects can be coupled with each other to significantly promote the separation and transmission of photogenerated electron-hole pairs,improve the quantum conversion efficiency,and enhance the photocatalytic performance under the action of external electric field and magnetic field.The above research findings will provide new research ideas and cases for the polarization effect in the design,synthesis and application of highly efficient photocatalytic materials.