Preparation And Properties Of Bi₂Sn₂O₇-based Heterojunction Composite Photocatalysts

Semiconductor photocatalysis has emerged as a promising technology to alleviate environmental pressure and resource constraints.As a novel type of pyrochlore semiconductor material,bismuth stannate(Bi2Sn2O7)has wide applications in solving alleviate environmental pressure and resource constraints as a result of its steady structure and excellent photocatalytic activity,and it is considered as a new trend in the field of photocatalysis.However,the poor light absorption ability and the easy recombination of photogenerated electrons and holes have become the main obstacles for its practical application.In this paper,the Bi2Sn2O7 photocatalysts were modified by constructing a heterojunction structure.The mechanism of photocatalytic degradation was discussed by investigating the morphology and structure of the catalyst,the performance of photocatalytic degradation of dyes,heavy metal ions,antibiotics and spirulina,which provides new ideas and clues for the development of more efficient photocatalysts for environmental pollution management.The specific research content as follows:(1)The novel Z-scheme heterojunction Bi2Sn2O2/Ag/Ag3PO4 photocatalysts were designed and constructed by a hydrothermal combined precipitation method.The Ag3PO4 microspheres were simply precipitated and supported the nanoparticles Bi2Sn2O7,and then Ag+ on the surface were photoreduced to Ag0 nanoparticles anchored on the surface of Ag3PO4.The photocatalytic effects of Bi2Sn2O7/Ag/Ag3PO4 composite photocatalysts were evaluated with Rhodamine B(RhB),Heavy cadmium ion Cr(Ⅵ),and xylenol orange(XO)as the target pollutants.The structure of the photocatalysts was characterized by X-ray diffraction(XRD),scanning electron microscopy(SEM),X-ray photoelectron spectroscopy(XPS)and ultraviolet visible diffuse reflectance(UV-vis DRS).Combined with ultraviolet photoelectron spectroscopy(UPS)analysis and active species trapping experiments,the degradation mechanism of Bi2Sn2O7/Ag/Ag3PO4 was analyzed.The results showed that when the m(Bi2Sn2O7):m(Ag3PO4)=0.4:1.0,the degradation efficiency of RhB has the highest degradation efficiency of 98.2%in 80 min under visible light irradiation,which is 8.3 folds and 4.2 folds higher than that of Bi2Sn2O7 and Ag3PO4,respectively.Besides,Bi2Sn2O7/Ag/Ag3PO4 possessed the effectively degradation of XO and Cr(VI).The results of the active species trapping experiments showed that h+ and ·O2-act as the main photocatalytic reactive species.The photocatalytic mechanism of the Z-scheme Bi2Sn2O7/Ag/Ag3PO4 has been proposed based on the experimental results together with the UPS analysis.(2)In order to overcome the adverse effects caused by the photocorrosion effect of Ag3PO4-based materials,Bi2Sn2O7 was modified the BiOBr,and Bi2Sn2O7 was attached to the surface of BiOBr nanosheets by a hydrothermal method to obtain x-BiOBr/Bi2Sn2O7(x=0.01～0.05)composite photocatalysts.The morphology,structure,photochemical properties and photocatalytic performance of the prepared BiOBr/Bi2Sn2O7 composite catalyst were investigated.The results showed that when the n(Bi2Sn207):n(BiOBr)=0.02:1,the degradation efficiency of tetracycline(TC)was reached 73.6%within 80 min under visible light irradiation,which were much higher than is 5.21 and 1.38 folds that of Bi2Sn2O7 and BiOBr catalysts,respectively.It is verified that BiOBr/Bi2Sn2O7 conforms to type Ⅱ heterojunction by the determination of energy band positions and active species.(3)In order to maximize the retention of the highest oxidation and reduction ability of the binary semiconductor photocatalysts,the BiOBr in the composite semiconductor BiOBr/Bi2Sn2O7 was modified using thiourea as a sulfur source.The S-BiOBr nano-flowers were synthesized by the isomorphous replacement combined with a hydrothermal method,and then Bi2Sn2O7 nanoparticles was deposited in-situ on the surface to construct a 3D/0D Z-scheme heterojunction S-BiOBr/Bi2Sn2O7 composite photocatalysts,which was used for the adsorption and degradation of organic dyes,antibiotics,and algae.XRD,SEM,XPS and other characterization methods were used to investigate the morphology and structure,chemical valence states of elements,and carrier paths of photocatalysts before and after the introduction of S element,and further investigate the mechanism of photocatalytic degradation of pollutants.The results showed that when the n(Bi2Sn2O7):n(S-BiOBr)=0.01:1,under visible light source,the composite photocatalyst degrades RhB to 99.6%in 80 min;TC removal rate reaches 56.3%;Algae cells can be inactivated within 20 min.3D excitation emission matrix(3D EEM)fluorescence spectroscopy tracks the degradation intermediates of pollutants.After 5 cycles of degradation,0.01S-BiOBr/Bi2Sn2O7 still has high photocatalytic degradation efficiency,indicating that the photocatalyst has high stability.Electron paramagnetic resonance(EPR)analysis combined with XPS valence band testing verified that S-BiOBr/Bi2Sn2O7 conforms to a Z-scheme heterojunction.