Preparation And Photocatalytic Properties Of Bismuth Based Z-scheme Photocatalyst

Global environmental pollution and energy crisis have become two major challenges for social development in the 21^st century.Semiconductor photocatalysis can directly convert solar energy to degrade pollutants in wastewater,and can also convert greenhouse gas CO₂ into high value-added fossil fuel,which is considered to be an ideal way to solve the above problems.However,there are still some problems in the development of highly efficient photocatalysts,such as high carrier recombination rate and insufficient redox ability.Therefore,how to improve the transport and separation of photogenerated carriers and enhance the redox capability of photocatalysts is the key to the development of highly efficient photocatalysts.In recent years,from the perspective of bionics,on the basis of inheriting the advantages of conventional heterojunction,Z-scheme heterojunction which simulates photosynthesis has changed the transport mode of photogenerated carriers,effectively improved the redox ability of catalysts,and made a breakthrough in the development of efficient catalysts.However,up to now,there are still some key problems in the development of Z-scheme heterojunctions,including the energy band structure matching between semiconductors,the regulation of charge carrier transmission pathway,and how to further enhance the redox capability of Z-scheme photocatalysts.In order to solve the above problems,bismuth-based semiconductor materials（Bi VO₄ and Bi OBr）were used as the research objects in this paper to prepare visible-light responsive direct Z-scheme photocatalysts.The photocatalytic performance and mechanism were studied.The main innovative research results are as follows:From the perspective of band structure matching,Z-scheme heterojunction Bi VO₄{040}/Ag₆Si₂O₇ was prepared by selective deposition of Ag₆Si₂O₇nanoparticles on the highly active crystal surface of Bi VO₄.Through fine-tuning the crystal structure of Bi VO₄,surface heterojunctions were formed between different crystal planes of Bi VO₄,which effectively inhibited the surface self-recombination of photogenerated charge carriers.The Z-scheme heterojunction showed excellent photodegradation performance.Compared with pure Bi VO₄,the degradation rate of Rh B by Bi VO₄{040}/Ag₆Si₂O₇ was 28.2 times higher.The main reasons for the improvement of photocatalytic performance were as follows:On the one hand,the construction of Z-scheme photocatalyst could not only facilitate the transport and separation of photoinduced carriers at the heterojunction and improve the utilization of visible light,but also enhance the redox ability of photocatalysts;On the other hand,the surface heterojunction formed between different crystal faces of Bi VO₄possessed the"charge pre-separation"effect,which greatly inhibited the surface self-recombination of photogenerated electrons and holes,and promoted the improvement of photocatalytic performance.For the semiconductor materials which tended to form conventional heterojunctions,it could be converted into Z-scheme heterojunction through the strategy of surface modification by selecting the third semiconductor with well-matched band structure,so as to improve the photocatalytic performance.In this study,bismuth stannate（Bi₂Sn₂O₇）nanoparticles were modified on the surface of the conventional heterojunction Bi VO₄@Zn In₂S₄ to transform the photogenerated electron transport path between Bi VO₄ and Zn In₂S₄ into a Z-scheme,and the“symmetrical”ternary dual Z-scheme heterojunction Bi VO₄@Zn In₂S₄/Bi₂Sn₂O₇ was successfully obtained.The fabrication of ternary dual Z-scheme photocatalyst not only expedited the separation of photoinduced carriers,enhanced the redox capacity of the photocatalysts,and promoted the absorption of visible light,but also increased the surface of photoreduction reaction,and effectively improved the photocatalytic performance.The degradation rate of Rh B by the as-prepared photocatalyst Bi VO₄@Zn In₂S₄/Bi₂Sn₂O₇ is 63,12,2.7 and 5.1 times that of Bi VO₄,Bi₂Sn₂O₇,Bi VO₄@Zn In₂S₄,and Znin₂S₄/Bi₂Sn₂O₇,respectively.Z-scheme heterojunction was successfully constructed on the basis of the above two research,and oxygen vacancy was introduced into the Z-scheme heterojunction through defect engineering strategy.The synergistic effect of oxygen vacancy and Z-scheme could further improve the performance of the photocatalysts.In this paper,bismuth bromide nanoflowers with oxygen vacancies（Vo-Bi OBr）were prepared by a surfactant assisted solvothermal method,and formed the Z-scheme heterojunction with Bi₂Sn₂O₇ nanoparticles.Compared with Bi OBr/Bi₂Sn₂O₇,Vo-Bi OBr/Bi₂Sn₂O₇ possessed better photodegradation activity,which were largely owing to the following aspects:Oxygen vacancies leaded to the upward shift of the band structure of Bi OBr,which further enhanced the reduction ability of Z-scheme heterojunction,benefiting for the photodegradation;The introduction of oxygen vacancies could lead to the generation of defect energy levels in the band gap of Bi OBr,which could capture the photogenerated electrons from its own band gap and further promote the separation of photogenerated electrons and holes.And the existence of defect energy levels also increased the surface where reduction reaction could occur.In addition,the existence of defect energy level also leaded to the formation of new electron transition path,which was conducive to the absorption of visible light.At the same time,the enhanced reducibility was also conducive to the photocatalytic reduction of CO₂ which required higher thermodynamics.In this paper,the problems in the construction of highly efficient Z-scheme photocatalysts were analyzed.From the perspective of energy band structure matching and redox ability enhancement,the development of efficient bismuth-based Z-scheme photocatalysts was realized through the strategies of crystal facet control,surface modification and oxygen vacancy introduction.