Hydrothermal Growth and Photocatalysis of Layered Oxide Parametric Functional Nano-crystals

Author:Li Zhi Ang

Supervisor:lu ya lin liu min


Degree Year:2019





Nowadays,quantum functional materials,which have multiple physical properties coexistence,are very important for the new quantum technology.As new quantum functional materials,layer-structured Bi-containing Aurivillius oxides have drawn much attention because of their "natural superlattice" structure,which can realize the coupling of different ordering parameters.Through the coupling of parametric orderings(such as electrons and orbital spins,charges,defects,lattices,etc.)in the structure,these materials can response to various external fields(such as electric field,magnetic field,light and stress,etc.).Therefore,they can be applied to the quantum storage,information transmission,energy conversion and many other future technologies.The preparation of single crystal materials is a very challenging but valuable work for studying the Aurivillius oxides.They are too difficult to be prepared because of their complicated composition.The similar thermodynamic ground-state energies between these Bi-layer materials are another disadvantage for their preparation.But the single crystal materials can make us understand the mechanism of physical performance more intuitively and accurately,especially the coupling mechanism between various ordering parameters.Because of no single crystal materials,some researchers have still questioned the room temperature ferromagnetism in some Aurivillius phase compounds.Furthermore,the preparation of single crystal materials is very important to the application in the future.For these reasons,the growth of Aurivillius phase single crystal material is the main purpose of our study.We choose hydrothermal method as the crystal growth method and study the morphology control,formation mechanism of the nano crystals.We also try to prepare the larger single crystal in our work.Furthermore,we study the special physical and chemical properties of Aurivillius phase nanomaterials with different morphologies and compositions.The main content of Chapter 1 is a review.The first part introduces the physical properties of quantum functional materials and Aurivillius phase materials,including their main preparation methods and research progress of ceramics,nanomaterials,and epitaxial thin films.The second part mainly focuses on some synthetic principles of hydrothermal method.At the end of this chapter,the advances on the study of hydrothermal growth of Aurivillius phase materials has been introduced.In Chapter 2,we synthesized Bi4Ti3O12 nanosheets with different thickness by surfactant-assisted hydrothermal method.The synthesis mechanism of nanosheets with different thicknesses have been explained.We found that the crystal orientation is influenced by the surfactant template.The photocatalytic performance of hydrogen production has been examined.With various factors,the photocatalytic performance of hydrogen production was significantly improved when the thickness was reduced to 2-3 unit cells(6-10 nm).In Chapter 3,we found a new way to dope nitrogen into bismuth based Aurivillius phase oxides Bi4Ti3O12,which avoided the reduction of bismuth ions.We confirmed the most possible positions of the substitutive nitrogen atoms by experiment and computation.The experiment indicated that the process of crystallization combines the in-situ reaction and oriented attachment.After doping N,the photocatalytic activity is enhanced because of oxygen vacancies generation and better capacity of charge separation for photo generated electron-hole pairs.This work provides a universal method for nitrogen doping in Bi-based Aurivillius compounds,which may pave the way for doping nitrogen into bismuth based oxides.In Chapter 4,the morphology control of Aurivillius Bi11Ti6Fe2.8Co0.2O18 microcrystals was investigated in details in the hydrothermal process,where NaOH concentration and citrate acid play a critical role.We obtained three different morphology of Bi11Ti6Fe2.8Co0.2O18 microcrystals.The truncated-tetragonal-bipyramid-like crystals,with sizes of 5-10 μm and good spontaneous crystallinity,has been obtained.The size variation can be explained by a nanophase Gibbs free energy.Besides,the dependence of magnetic properties on the morphologies was also investigated.The research on the morphology control would serve as the guidance to synthesize larger Aurivillius phase single crystals.In Chapter 5,we use the supercritical hydrothermal method to grow Bi4Ti3O12 and BisFeTi3O15 single crystals.The experiments indicated the feasibility of growing large Bi4Ti3012/BisFeTi3O15 single crystals through the supercritical hydrothermal method.The basic conditions of crystal growth with supercritical hydrothermal method were confirmed.In Chapter 6,we designed the CoFe2O4-SiO2 core-shell structured nanoparticles to achieve magnetism and good adsorption ability of methylene blue.The CoFe2O4 nanoparticles are the one of secondary product in synthesizing Co-doped Aurivillius phase compounds.Heat treatment can further burn out the adsorbed dyes and the good adsorption performance is sustained after several heat-treating loops.This work provides an idea for designing recyclable adsorbents.In Chapter 7,we briefly summarize the entire thesis and outlook the prospect of future work.