Hydrothermal Synthesis And Multiferroic Properties Of Bismuth Layered Perovskite Oxide Single-crystalline Nanoplates

Quantum functional materials with multiple parameter regulation and wide spectral response are the important carrier of new theories,effects,devices and applications in the field of quantum science.The design and preparation of new quantum functional materials is the key to the development of quantum information technology.As a member of quantum functional materials,single-phase multiferroic materials based on electric and magnetic order is a typical material system in which many order parameters coexist and are closely coupled with each other.The coexistence of ferroelectricity(FE)and ferromagnetism(FM)and unique magnetoelectric coupling effect in the single-phase multiferroic oxides makes it possible to regulate magnetization using electric field and inversely regulate polarization utilizing magnetic field in a single material.It is also an idea host to investigate the interaction between lattice,charge,orbit and spin etc.However,it is still a big challenge to find one new single phase multiferroic material,especially at room temperature,due to the competing electronic requirements for FE and FM.Empty d-orbits are the requirement for FE in order to allow cations off-center displacement while FM necessitates unpaired electrons,i.e.an incomplete electronic occupation of d-orbital.Recently,Aurivillius phase compounds with a formula of Bi4Bin-3Fen-3-xCoxTi3O3n+3(BFCTO)have been proved to be one potential single phase multiferroic material system.The research about Aurivillius-phase multiferroic materials had made some achievements so far,but there are still some problems needed to be solved and improved.For example,the current research focuses on ceramics and thin films,and the multiferroic properties in Aurivillius phase nanomaterials are rarely reported.In addition,some researchers have doubted the intrinsic ferromagnetism of Aurivillius-phase compounds,and they think that the ferromagnetism comes from the small amount of magnetic second phase in the material rather than the intrinsic properties.Single crystal materials are required to clarify these problems,therefore,the Co doped four-layer Aurivillius-phase single crystal nanosheets were fabricated and studied in this thesis.Firstly,the Bi5FeTi3O15 and Bi5Fe0.9Co0.1Ti3O15 nanosheets were synthesized by hydrothermal method and the corresponding growth mechanism was also studied in detail.Based on this,Bi5Fe0.9Co0.1Ti3O15 single-crystalline nanoplates were further synthesized and its intrinsic multiferroic properties were verified.In addition,the new multiferroic materials SmBi4FeTi3O15 and SmBisFe2Ti3O18 single-crystalline nanosheets were synthesized as well.This research could serve as guidance to realize a controllable synthesis of Aurivllius nanomaterials,on the other hand,the dispute on the intrinsic ferromagnetism in the Auri-villius phase compounds was answered.The details are as follows:In chapter 1,firstly,the basic concept of ferroelectric,ferromagnetic,multiferroic and magnetoelectric coupling effect are introduced.Secondly,the physical mechanism of ferroelectric and magnetic coexistence in single-phase multiferroics and two typical representatives(BiFeO3 and TbMnO3)are described.Finally,the research progress and existing problems in Aurivillius-phase multiferroics are also reviewed.In chapter 2,the advanced characterization technologies for quntuan functional materials are introduced,and magnetic structure imaging technology based on TEM and Piezoreponse Force Microscopy are especially emphasized.In chapter 3,synthesis and magnetic properties of four-layer Aurivillius-phase BisFeTi3O15(BFTO)and BisFe0.9Co0.1Ti3O15(BFCTO)nanosheets is introduced.We successfully synthesized BisFeTi3O15 and BisFe0.9Co0.1Ti3O15 nanosheets by hydrothermal method.Their lateral length and thickness are～500 nm and～100 nm Besides,the magnetic properties of the two nanosheets were also investigated.At room temperature,BFTO is antiferromagnetic,while the obvious hysteresis loop with 2Mr=0.55 emu/g and 2Hc=2600 Oe at 300 K are observed in BFCTO nanosheets.And the dielectric permittivity and dielectric loss decrease with increased frequency at room temperature.Besides,the formation mechanism of BFCTO nanosheets is investigated in detail.The formation of BFCTO naonsheets experiences the following process:the aggregation of precursors → oriented attachment of Bi7Ti5x-yFexCoyO21-δ nanorods into nanosheets—the formation of BFCTO nanosheets by Ostwald ripening process.In chapter 4,synthesis and intrinsic multiferroic properties of four-layer Aurivillius-phase Bi5Fe0.9Co0.1Ti3O15 single-crystalline nanoplates is introduced.Firstly,the BFCTO single-crystalline nanoplates were successfully synthesized by the hydrothermal method.To verify the intrinsic multiferroic properties,the electron holography and PFM were conducted in an individual single-crystalline nanoplate.By the electron holography,the denser phase contours are detected inside the nanoplate,and the closed magnetic flux lines make a significant magnetic interaction between the neighboring nanoplates,strongly proving the ferromagnetic nature of BFCTO compounds.Furthermore,in an individual single-crystalline nanoplate,the ferroelectric domains are also observed by PFM.Besides,the magnetic properties of the BFCTO plates were also investigated.At room temperature,2Mr,2Hc and Tc reach 0.43μB/f.u.,1.5 kOe and 730.2 K,respectively.In chapter 5,synthesis and magnetic properties of four-layer Aurivillius-phase SmBi4FeTi3O15(SBFTOO15)and five-layer Aurivillius-phase SmBi5Fe2Ti3O18(SBTO18)single-crystalline nanosheets is introduced.Through the selection of the concentration of NaOH,we synthesized SBFTOl 5 and SBFTO18 single-crystalline nanosheets.They both display the traditional nanosheet morphologies of Aurivillius oxides.Besides,the magnetic properties of SBFTO15 and SBFTO18 were also investigated.At room temperature,SBFTO15 is paramagnetic while SBFTO18 is ferromagnetic with 2M.,2M,and 2Hc reach 0.045 emu/g、0.016 emu/and 668 Oe.Furthermore,during the decreasing of temperature,the 2Ms and 2M.of SBFTO18 are monotone increasing,while the 2H,is increasing at begining and then decreasing when temperature blows 100 K.In chapter 6 synthesis and photocatalytic water-splitting properties of Bi2WO6 ultrathin nanosheets is introduced.It exhibits a good H2 gas and O2 evolution activity under UV-vis illumination with the total amount of H2 gas and O2 evolution reaches 320.08 μmol g-1 and 466.1 6 μmol·g-1 respectively within 4 hours of illumination,while its bulk counterpart does not have H2 gas evolution activity and a weak O2 gas evolution activity with lower amount of O2 evolution of 186.37 μmol·g-1 during the same pried time.Further study indicates that the excellent photocatalytic water splitting performance of Bi2WO6 ultrathin nanosheets derives from ⅰ)increasing photo-absorption in the UV region due to N-vvv extremely large surface area(31.68 m2/g)that allows absorption to happen quickly and efficiently,ⅱ)large drive force from more negative conduction band bottom potential and more positive valence band top potential,ⅲ)plentiful adsorption and reaction sites on surface originated from oxygen vacancies and ⅳ)the large internal electric field provides large driving force to separate photogenerated carriers and then accelerate them to transfer from the interior onto the surface with low bulk recombination loss.In chapter 7,we summarize the whole work in this thesis and present an outlook for the future work.