Study On The Preparation And Photocatalytic Properties Of Novel Bi-based Oxyhalides

With the rapid development of economy and the improvement of people’s living standard,environmental pollution caused by energy consumption is becoming more and more serious.Photocatalysis technology,driven by abundant solar energy,can be applied to water splitting for hydrogen and oxygen evolution,degrade dyes,antibiotics,pesticides,heavy metal ions in water to improve environmental pollution problems.However,there are some drawbacks to utilizing mainstream photocatalysts,such as Ti O₂,as the large bandgap（3.2 e V）can only excited by UV light in solar light.Zn S,Cd S and Bi OX limited by the poor photostability.In recent years,Bi-based semiconductors have been used as potential visible light response photocatalysts due to their suitable band gap,unique band structure and layered structure.Simultaneously,the VBM of Sillén-Aurivillius phase Bi-based oxyhalides is mainly occupied by O 2p orbital,avoiding photocorrosion phenomenon,and its perovskite layer[ABO₃]has anisotropic charge transfer and cation exchange characteristics,which can provide more active sites for the redox reaction in the photocatalytic process and optimize the photocatalytic reaction.Based on this,this paper designs the relationship between B-position transition metal elements Ti and Nb,and carries out the following researches:A new type of Ti Sillén-Aurivillius phase Bi₅Ti₂O₁₁Cl was synthesized by molten salt method.The effects of different calcination processes on the crystal structure,surface morphology and photocatalytic performance of the halide were studied.It was found that Bi₅Ti₂O₁₁Cl synthesized by molten salt method showed a nanosheet morphology,and the layered nanosheet structure of Bi₅Ti₂O₁₁Cl could form a self-established internal electric field,promote the photogenerated carrier separation,and enhance the reoxidation reaction on the surface of the material.Therefore,the oxygen production rate under the full spectrum was as high as 42.3μmol/g/h.In the process of photocatalytic degradation of organic pollutants Rh B,adjusting the p H of the reaction solution can optimize the surface reaction potential of oxyhalides,strengthen the free radical action and enhance the photocatalytic activity.Subsequently,Sillén-Aurivillius Bi₄Nb O₈X（X=Cl,Br）was synthesized by transforming B-site transition metal from Ti to Nb.However,the photocatalytic performance of its intrinsic material was much lower than that of Bi₅Ti₂O₁₁Cl.In order to improve its photocatalytic performance,a heterojunction photocatalyst was constructed by the combination of graphite phase carbon nitride（g-C₃N₄）and Bi₄Nb O₈X,which promoted the separation ability of photogenerated electron-hole pairs and inhibited the recombination of carrier,thus enhancing its photocatalytic activity.The conduction potential ratio of O₂/·O₂^-of Bi₄Nb O₈X（X=Cl,Br）was corrected,and photogenerated electrons reduced O₂ to superoxide radical·O₂^-,which degraded and mineralized Rh B.On the other hand,the valence band potential of g-C₃N₄ is more negative than that of H₂O/·OH,which can produce hydroxyl radicals·OH,under the influence of·OH and·O₂^-,the heterojunction processed higher photocatalytic activity.Sillén-Aurivillius phase halogen oxide Ba Bi₄Ti Nb O₁₁Cl of Ti and Nb double transition metals was synthesized by solid phase method and molten salt method.Due to the low melting point of molten salt,which promotes the complete diffusion of precursor ions,Ba Bi₄Ti Nb O₁₁Cl nanosheets synthesized by molten salt method have high crystallinity,less impurity phase,higher photo-generated carrier transport efficiency and higher relative catalytic activity.In addition,lattice distortion of the[Ti O₆]and[Nb O₆]octahedrons in the perovskite block leads to ferroelectric polarization of Ba Bi₄Ti Nb O₁₁Cl material.The external stress can generate free positive charge on the compressive surface and negative charge on the tensile surface to form an internal electric field,which provides power for the efficient separation of photogenerated e^--h⁺pairs.When the piezoelectric photocatalytic system acts on the Ba Bi₄Ti Nb O₁₁Cl nanosheets under visible light irradiation,the internal electric field generated by the mechanical force on the ferroelectric material results in the effective separation of the photogenerated e^--h⁺pairs,which significantly improves the catalytic activity of the catalyst.This work focuses on revealing the differences of photocatalysis,piezoelectric catalysis and piezoelectric photocatalysis mechanisms of novel Bi-based nanosheets,and provides references for the development of other high-efficiency piezoelectric photocatalysts for Sillén-Aurivillius layered oxide halide materials.