| The use of photocatalysis to achieve energy conversion and pollutant degradation is considered to be a very promising technology for solving the current global energy crisis and environmental pollution problems.The most important problem to be solved in the photocatalytic reaction process is how to improve the separation efficiency of photoelectron-hole pairs,thereby increasing the photocatalytic efficiency.The built-in electric field based on spontaneous polarization inside the ferroelectric material is very favorable for the separation of the photoelectron-hole pair,so the ferroelectric material is considered as a very promising photocatalytic material.At present,researches on ferroelectric photocatalysts mainly focus on ABO3 type perovskite oxides(BaTiO3,PbTiO3,LiNbO3,etc.).However,these oxides are difficult to prepare nanoparticles having a specific active surface exposed and a high specific surface area due to their intrinsic structure limitations,which greatly restricts their photocatalytic activity.In addition,the large band gap of the vast majority of ABO3 type perovskite oxides is detrimental to the absorption of visible light.Therefore,the photocatalytic activity of the simple ABO3 type perovskite ferroelectric oxides is still limited.In recent years,a new type of layered-perovskite ferroelectric oxide has attracted the attention of scientific researchers due to its excellent photocatalytic activity.This layered-perovskite oxide consists of a perovskite layer and a bismuth oxide layer alternately arranged along c-direction.The main representatives are Bi2WO6,Bi3TiNbO9 and Bi5FeTi3O15 and so on.On the one hand,based on the relatively specific layered-structure of such ferroelectric oxides,it is generally easier to prepare nanoparticles having a specific active exposed surface and a high specific surface.On the other hand,many of these oxides have a certain visible light response or a ferroelectric-effect-based contaminant sensitized degradation/water splitting(visible-light).These are important reasons for its excellent photocatalytic activity.This dissertation firstly explores the controllable preparation of nano-morphologies of these ferroelectric oxides,and then characterizes their photodegradation and water splitting hydrogen-oxygen evolution activity.Finally,we focus on how the built-in electric field in these perovskite-like ferroelectric oxides affects its photocatalytic activity.This paper focuses on the above three aspects and is divided into six chapters:The chapter 1 is a literature review.The first part introduces the development history and research background of photocatalysis,followed by a brief description of the basic principles of photocatalytic technology.Finally,the most studied classic photocatalysts and the current development bottlenecks are listed and analyzed.The second part mainly introduces the basic overview of ferroelectric oxides which have received increasing attention in recent years,and reviews the research progress of perovskite/layered-perovskite ferroelectric oxides in the field of photocatalysis.At the same time,we focused on the influence of spontaneous polarization of ferroelectric oxides on their photocatalytic activity.The third part mainly analyzes the future development trend of perovskite/layered-perovskite ferroelectric oxides as photocatalysts.The fourth part briefly outlines the main content and research significance of this paper.The chapter 2 mainly describes the hydrothermal synthesis and photocatalytic activity of a layered perovskite oxide Bi3Fe0.5Nb1.5O9.The hydrothermally synthesized Bi3Fe0.5Nb1.5O9 nanosheets(BFNO-H)exhibited much higher visible-lightphotodegradation of RhB and SA activity than the conventional solid-state synthesized sample(BFNO-S).In the case of similar visible-light absorption,BFNO-H normalized by specific surface area still shows higher catalytic activity than BFNO-S,indicating that the increase of specific surface area is not the only factor for inproving the photodegradation activity.Considering that the spontaneous polarization of the Bi3Fe0.5Nb1.5O9 ferroelectric has both components in the ab plane and c direction,which matches well with the crystallographic orientation of the BFNO-H nanosheet.We believe that this good match is another important influencing factor for improving the photocatalytic activity of BFNO-H.This research provides a new idea for the design and preparation of high-efficiency ferroelectric nanophotocatalysts.The chapter 3 mainly introduces the ferroelectric polarization-related photodegradation activity of Bi3TiNbO9 which has the same structure as that of Bi3Fe0.5Nb1.5O9.The Bi3TiNbO9 nanosheet(BTNO-M)synthesized by the molten salt method exhibits more excellent catalytic degradation activity than the classical commercial catalyst P25.Further studies have found that the excellent catalytic activity of BTNO-M derives from its strong pollutant adsorption capacity.Although BTNO-M has a much smaller specific surface area than P25,it exhibits several times higher pollutant adsorption capacity than P25.The corona-poling experiment proves that the supercontaminant adsorption capacity of BTNO-M originates from its ferroelectric external screening effect.This discovery provides a new idea for the design and preparation of high-efficiency photocatalysts!Based on the chapter 3,the chapter 4 further studies the water splitting hydrogen-oxygen evolution activity of BTNO-M synthesized by a series molten-salt temperature.Interestingly,BTNO-M with a higher percentage of {001} exposed facet has higher hydrogen-evolution activity,while BTNO-M with a higher {110} exposed facet shows higher oxygen-evolution activity.This means that we can selectively achieve hydrogen-evolution optimization or oxygen-evolution optimization by simply adjusting the relative proportions between the {001} exposed facet and the {110} exposed facet.We believe that the internal ferroelectric polarization of Bi3TiNbO9 is the intrinsic cause for the formation of different active exposed facets.In order to further explore the important role of ferroelectric polarization in the photocatalytic reaction,in Chapter 5,we prepared a ferroelectric heterojunction Bi1.65Fe1.16Nb1.12O7(Bi2FeNbO7)/g-C3N4 and focused on its polarization-related photodegradation performance.The heterojunctions with optimal ratio shows much higher photodegradation activity than their constituent monomers.More importantly,the g-C3N4/Bi2FeNbO7 heterojunction exhibits higher degradation activity than the g-C3N4/Bi1.65Fe1.16Nb1.12O7 heterojunction,although the latter has a stronger light absorption capability.The ferroelectric test results show that Bi2FeNbO7 has a stronger ferroelectric response,suggesting that the ferroelectric built-in electric field plays an important role in the heterojunction photocatalytic reaction.This work demonstrates that the built-in electric field in the ferroelectric is indeed very favorable for the separation of electron-hole pairs.The chapter 6 is the summary of the full-text content and innovation points.It analyzes some of the deficiencies in the research work and forecasts the further research work in the future. |
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