| The photogenerated electrons and holes can be effectively separated and migrate in opposite directions because of the spontaneous polarization in the ferroelectric oxide materials,which suppresses the recombination of carriers and prolongs their lifetime.These special properties make ferroelectric oxide a candidate for good photocatalysts.Perovskite ferroelectric oxides such as LiNbO3,BaTiO3 and PbTiO3 have already been used in semiconductor photocatalysis.Such ferroelectric materials are generally considered to be polarized semiconductors with wide band gaps,which are not good for the absorption of visible light.Besides,due to the limitations of their special ABO3 type crystal structure,the 1D or 2D nanostructure ferroelectric materials with high specific surface area are often difficult to prepare,which will greatly limit their efficiency of photocatalysis.In addition,for ferroelectric oxides,the preparation of single domain nanoparticles is also the key factor to improve the ferroelectric polarization effect on photocatalysis.Therefore,we hope to find new,efficient and structurally tunable ferroelectric oxides.In recent years,research on photocatalysis applying layered perovskites has received extensive attention.Among them,Aurivillius phase oxide containing Bi element has attracted interests due to its excellent visible light photocatalytic activity.The decrease in the band gap of Bi containing compounds is due to the orbital hybridization of O 2p and Bi 6s,which can improve the valence band edge.Further studies indicate that the high dispersion of the Bi 6s-O 2p hybridized valence band facilitates the efficient migration of photogenerated holes and promotes the process of photocatalytic reaction.The Aurivillius phase oxide with a regular layered symbiotic structure is a variant of layered perovskites.It is constructed with n layers of perovskite(An-1BnO3n+i)2-slabs sandwiched by fluorite(Bi2O2)2+ blocks.This special structure makes it usually a ferroelectric material and belongs to the class of ferroelectric oxides.At present,due to its excellent activity as a single-phase ferroelectric oxide photocatalyst,research interest in these Aurivillius phase semiconductors has greatly increased.Common researches include Bi2WO6 and Bi2MoO6 with one layer structure,2-layer structure of Bi3TiNbO9 with 2 layer structure a,Bi4Ti3O12 with 3 layer structure and BaBi4Ti4O15 with 4 layer structure and so on.However,research on more complex multi-layer system is still rare.Recently,the five-layer Aurivillius phase oxide Bi6Ti3Fe2O18(BFTO)has attracted attention due to its ferroelectric,magnetic and optical properties.Whether or not the nano-sized BFTO material has ferroelectricity remains to be studied.Directly observing the ferroelectric domain of an individual nano-sheet or micron-sheet is still a technical difficulty.It is also an interesting direction to manipulate the structure and morphology of BFTO nanomaterials and to study the influence of ferroelectricity on their photocatalytic properties.According to the above questions,the specific work of this paper is as follows:The first chapter is a literature review.The first part introduces the basic theories of ferroelectric materials,and then outlines the effects and principles of ferroelectric polarization on photocatalysis.Finally,discusses the problems and difficulties of ferroelectric oxides as photocatalysts.The second part firstly introduces the structural properties of Aurivillius phase oxides.Then describes their basic performance characteristics and research status of the multiferroic properties,and finally discusses its development in the field of photocatalysis.The second chapter mainly introduces the hydrothermal synthesis of Aurivillius phase ferroelectric oxide BFTO and its magnetic and ferroelectric properties.The BFTO nanosheets with different sizes are synthesized by hydrothermal method.The effect of sample size on their magnetic properties is studied.It is found that they exhibit paramagnetic and antiferromagnetic superposition at room temperature.Besides,they also show weak ferromagnetism.Small-sized BFTO nanosheets present stronger magnetic properties and possess larger effective magnetic moment and more eg electrons after detailed calculation.X-ray absorption spectroscopy(XAS)has been applied to test the L-edge of magnetic ion(Fe3+)in the sample,and it also proves that the eg electron number has increases in small-sized BFTO nanosheets.Therefore it can be concluded that the magnetic enhancement of the sample may be ascribed to the stronger distortion in the octahedral lattice.In addition,by adjusting the appropriate mineralizer concentration and reaction time,an individual larger-sized micro-sheet has been obtained,and the ferroelectric loop of the sample is obtained.Moreover its microscopic ferroelectric domains are also characterized by piezoresponse force microscopy(PFM),verifying the existence of ferroelectricity in the material.This has prepared for our subsequent study on the effect of ferroelectricity in BFTO materials on photocatalysis.In the third chapter,a Bi6Fe2Ti3O18-BiOBr(BFTO/BOB)ferroelectric heterostructure has been designed and synthesized,and its photocatalytic oxygen evolution performance is explored.BFTO/BOB heterostructure with intimate 2D/2D interfaces are successfully grown by converting part of ferroelectric BFTO nanosheets to BOB via an ion-exchange chemical reaction.For the first time,it is discovered that the conversion of BFTO to BOB is a self-limiting process,i.e.,the reaction auto-stops at a certain ratio of BFTO/BOB even high concentration of reactants(hydrobromic acid)are used.This phenomenon is ascribed to the precipitation-dissolution equilibrium due to the higher solubility product constant of BiOBr.The fine 2D/2D structure of ferroelectric BFTO/BOB with good energy band alignment leads to two times and seven times higher oxygen evolution rates than non-ferroelectric BFTO/BOB and pure BFTO under full-spectrum irradiation,respectively.The promoted separation and transfer of photogenerated electron-hole pairs at the interface is clarified by the soft X-ray absorption spectroscopies,Mott-Schottky curves and photocurrent tests.What’s more,the ferroelectric spontaneous polarization at the interface between the ferroelectric BFTO and the BiOBr also shows a synergetic effect for the oxygen evolution enhancement.Our finds may open up new windows to prepare 2D/2D heterostructres with improved photocatalytic activities.The fourth chapter mainly introduces the synthesis of BFTO nanowires and their performance for pollutants photodegradation.Mesoporous ferroelectric BFTO nanowires with a diameter of 100-150 nm are synthesized through electrospinning following by optimal calcination.The effect of sintering temperature on the morphology has been studied.The surface of the nanowire is modified with Au nanoparticles with a grain size of less than 10 nm.The sample exhibits high specific surface area and good visible light response photocatalytic ability,which could realize the efficient degradation of RhB.In addition,after additional corona poling experiments,the photocatalytic efficiency of the sample has been further improved,suggesting that the spontaneous polarization of ferroelectrics in BFTO nanowires plays an important role in promoting the separation and transfer of photogenerated electron-hole pairs.In order to clarify the mechanism of the effect of the magnetism in the multiferroic photocatalyst on the photocatalytic process,it is necessary to systematically study its magnetism first.Therefore,the work in chapter 5 studies the relationship between the magnetocrystalline anisotropy and the layer number in Co/Fe co-doped Bi7Fe3-xCoxTi3O2,(BFCTO)oriented ceramics.Aurivillius oxide ceramics with highly[00l]-oriented grains were prepared by a facile pressureless sintering method,and the perovskite layer number was modified by varying the cobalt content.Afterwards,the unique perpendicular magnetic anisotropy and the intriguing anisotropic ferroelectric properties have been observed.The magnetic anisotropy in the oriented ceramics is demonstrated to be caused by the magnetocrystalline anisotropy with the easy magnetization direction along the c-axis.Co-0.65,which is dominated with the higher-numbered five-layer structure,has possessed the highest magnetic anisotropy.While with the decrease of the layer number,the magnetocrystalline anisotropy becomes weaker in the ceramics.The special magnetocrystalline anisotropy possibly arises from the unquenched 3d orbitals combined with the special layered crystal structures.The sixth chapter is the summary of the thesis and the prospects of the future research. |
