Solid State Synthesis、Photocatalytic Properties And Mechanism Of SnO₂ Based Bi₂O₃ Series N-p Heterojunctions/Homojunctions

Photocatalytic technology is recognized as one of the most important means to solve the energy and environmental problems,especially in the treatment of organic wastewater,which makes up for the shortage of biological treatment and physical chemistry treatment.Since 1972,titanium-based photocatalytic materials have been widely studied and many important research results have been obtained,while Sn O₂（Eg=3.6 e V）has a lower valence band than that of Ti O₂（Eg=3.2 e V）,therefore,the holes of valence band excited by the suitable light source have stronger oxidation,besides,Sn O₂is harmless,cheap and easy to get,which makes it an ideal catalyst to degrade many kinds of organic compounds.However,the application of Sn O₂in the field of photocatalysis is limited.On the one hand,Sn O₂has a broad band gap larger than 3.6 e V,it can only harvest the ultraviolet part of the solar spectrum,but most of the visible light can not be used effectively,on the other hand,the traditional synthesis methods,such as hydrothermal method and solvothermal method,which are energy and time-consuming,also limit the practical application of Sn O₂materials.In view of this,the method of constructing Sn O₂-based n-p heterojunction/homojunction is adopted in this paper,and the n-type wide band-gap Sn O₂and the narrow band-gapα～Bi₂O₃andβ～Bi₂O₃which can be adjusted by the phase are selected as the research objects,and the low temperature solid-phase synthesis method was adopted,the n-p type Sn O₂/Bi₂O₃composites with adjustable phase were synthesized,and the built-in electric field formed at the interface by them was used to promote the migration of photocarriers at the interface,the range of light absorption was continuously broadened by modulating the phases of Bi₂O₃.At the same time,in order to prevent the second pollution of the environment during the synthesis of the template,the content of Bi OCl in the intermediate state of the composite was controlled by controlling the synthesis conditions of the material,that is,by changing the bismuth source and calcination temperature under the non-template conditions,a continuously rising valence band is constructed through the theory of energy band engineering,and the sample synthesized in this way shows an efficient photocatalytic behavior under visible light.In this study,the material with the best photocatalytic activity was selected by Rh.B simulated pollutant.Then the all optical absorption material was used in the co-existence of tetracycline wastewater and Cr⁶⁺,the structure-activity relationship between microstructure and photocatalytic activity was investigated systematically,and the mechanism of photocatalytic reaction was discussed by means of energy band structure analysis.The main contents and conclusions of this thesis are as follows:（1）Sn O₂-based n-p Sn O₂/Bi₂O₃composites with different ratios were prepared by solid-state synthesis.The results show that the relatively pure n-p Sn O₂/Bi₂O₃composite is obtained when the ratio of n:（Sn:Bi）is 2/1 and 3/1.With the increase of Sn ratio,the visible light response of R31 is wider than that of R21,reaching up to 540 nm.However,its photocarrier separation effect is not as good as that of R21.The phase transformation of n-p Sn O₂/Bi₂O₃at different calcination temperature was studied when the ratio of n Sn:Bi was 2/1.The absorption boundary of S500 calcined at 500℃was extended to 525 nm.S500has good photocatalytic activity because of its relatively good crystallinity,high separation efficiency of photo electron-hole pairs and uniform distribution of Sn O₂and Bi₂O₃,which can remove 70%Rh.B in 180 min under visible light irradiation.The photocatalysis efficiency of Rh.B(initial concentration 5 mg?L^-1)is much higher than that of Sn O₂and Bi₂O₃in pure phase.The free radical quenching experiments showed that·O₂^–and holes were the main active species.This study confirms that the n type Sn O₂and p type Bi₂O₃are band matching composite semiconductor materials.（2）Taking advantage of the narrower band gap ofβ～Bi₂O₃than that ofα～Bi₂O₃,the shape and phase of n-p Sn O₂/Bi₂O₃were controlled by the solid phase synthesis.n-p Sn O₂/β～Bi₂O₃composites with narrow band gap were synthesized through different dosage of CTAB.The results show that when the proportion of CTAB is 5%,the phase transition of n-p Sn O₂/Bi₂O₃composite takes place obviously,and a n-p Sn O₂/β～Bi₂O₃with narrower band gap ofβ～Bi₂O₃is obtained.The two-phase distribution of Sn O₂/Bi₂O₃in 5CTAB was observed by FESEM.The absorption boundary of Sn O₂/β～Bi₂O₃is widened to610 nm,but that of Sn O₂/Bi₂O₃without modulated by CTAB is only 525 nm.Under the irradiation of 500 W xenon lamp,100%Rh.B can be obtained by Sn O₂/β～Bi₂O₃within 180 min.The removal rate of TOC Rh.B solution is70%,which is higher than that of Sn O₂/α～Bi₂O₃.The free radical quenching test showed that the hole was the main active species in the photocatalytic reaction,and·O₂^-something played an auxiliary role.The results show that theβ～Bi₂O₃with narrower band gap plays an important role in broadening the optical absorption boundary of the composite,and the intrinsic defects such as the new energy level formed by oxygen vacancy in the sample are favorable for the excitation of Sn O₂under the irradiation of visible light.（3）A kind of black ultra-long visible light-responsive n-p（Sn O₂）_1.3/（α～Bi₂O₃）_x/（β～Bi₂O₃）_1-xwas synthesized by solid state synthesis with PVA-350 as template and reductant.The black material is more favorable to absorb visible light.Modulated with 15%dosage of PVA,the light absorption boundary of is widened to 725 nm.Under the irradiation of simulated sunlight（400w metal halide lamp）,99%Rh.B(with the initial concentration 5mg?L^-1)is completely decolorized.When the initial concentration of Rh.B is raised up to10 mg?L^-1,the dosage of catalyst is kept at 1 mg?L^-1,the decolorization rates of15PVA,Sn O₂/β～Bi₂O₃and Sn O₂/α～Bi₂O₃are 80%,75%and 65%respectively with visible light irradiation for 300 min.The results of XRD showed that it just because of the increase of the crystallinity of the 15 PVA that can still keep more than 90%decolorization rate even after the 10th recycling,So its stability was better than that of the Sn O₂/Bi₂O₃sample which synthesized without the regulation of PVA.This study proves that PVA,as a template and reductant,plays a key role in the heterogeneous formation of materials,and in the synthesis of nanomaterials,the surfactant acts not only as a template but also as a reductant to control the surface structure,electronic structure and light absorption properties of the composite.（4）Sn O₂/Bi OCl/α（β）～Bi₂O₃with heterogeneous and homogeneous response to visible light was synthesized through adjusting the calcination temperature to increase the content of intermediate Bi OCl and by introducing another Bi source which is rich in chloride ions.XRD,HRTEM,XPS and FT-IR confirmed the formation of the ternary structure Sn O₂/Bi OCl/α（β）～Bi₂O₃.FESEM observed that the composite structure of Sn O₂/Bi OCl was firstly formed in the process of calcination.With the calcination temperature increasing to 500℃,part of Bi OCl was transformed intoα～Bi₂O₃.The effect of calcination temperature on the phase transformation of the composites was studied.It was found that the content of Bi OCl in Sn O₂/Bi OCl/α（β）～Bi₂O₃increased at 300℃andα～Bi₂O₃andβ～Bi₂O₃coexisted simultaneously in the composites.The absorption boundary of SB500-2 is widened to 476 nm when calcined at 500℃for 2hours,while it was widened to 514 nm for SB300-2 when calcined at 300℃for 2h.The highest decolorization rate of SB300-2 was obtained in 240 min under the simulated visible light irradiation of 500 W xenon lamp.The reaction rate constant is 5.8 times of that of Sn O₂/α～Bi₂O₃.The free radical quenching experiments showed that h⁺was the main active species in the photocatalysis,followed by·OH and·O₂^-.The results show that adjusting the calcination temperature makes the material produce new transition phase and intrinsic defects such as oxygen vacancy,which is beneficial to form new energy levels and increase the absorption of visible light.（5）The photocatalytic degradation of TC and Cr⁶⁺was studied by using super long visible light response（Sn O₂）_1.3/（α～Bi₂O₃）_x/（β～Bi₂O₃）_1-x.It was found that under the optimal conditions of initial p H=3 and initial concentration of 20mg?L^-1,the degradation rates of TC and Cr⁶⁺were 92%and 80%respectively within 40 min.While the degradation rates decreased to 60%and 60%respectively in the co-existence system of TC and Cr⁶⁺.That is,the co-existence of TC and Cr⁶⁺inhibits the photocatalysis process of each other.The free radical scavenging experiments showed that·O₂^-is the main active species in the photocatalytic process.The electrons competition between Cr⁶⁺and O₂to the CB of the composite lead to the decrease of the photocatalytic efficiency of both TC and Cr⁶⁺.The composite showed good photocatalytic stability.After five cycles,the photocatalytic effect of TC and Cr⁶⁺was still kept at 90%and 72%in a single system.