Study On Preparation And Properties Of Bismuth Titanate With Pyrochlore Structure

The bismuth titanate（Bi₂Ti₂O₇）with fcc cubic pyrochlore structure is a metastable compound.Since it is easy to be decomposed into Bi₄Ti₃O₁₂2 and Bi₂Ti₄O₁₁above 670°C,it is difficult to prepare pure phase Bi₂Ti₂O₇ by conventional high temperature method.Owing to its good visible light photocatalytic activity and the long term inconsensus about its doubtful ferroelectrcity,Bi₂Ti₂O₇ has great importance in scientific research and potential for future applications as well.In recent years,low-temperature wet chemical methods such as sol-gel method and co-precipitation have been used to prepare Bi₂Ti₂O₇ powder.However,there are few reports about hydrothermal preparation of Bi₂Ti₂O₇.In this thesis,the low-temperature wet chemical synthesis,stoichiometry of composition,photocatalytic activity,and dielectric properties of Bi₂Ti₂O₇ were studied.First of all,the growth conditions and growth mechanism for the preparation of Bi₂Ti₂O₇ by hydrothermal method were studied.Bismuth citrate and tetrabutyl titanate as raw materials were used.By adjusting a series of parameters affecting the hydrothermal reaction,a pure phase Bi₂Ti₂O₇ with an octahedral morphology on the scale of micrometer in particle size was prepared for the first time under the optimal condition of 220°C,solution pH=12,a filling ratio of 80%and reaction time of 72hours.The growth process is accompanied by a change in particle morphology,from the initial amorphous aggregates to be crystallized into a thin rod with the radial dimensions on the nanometer scale,which then grow into micron-sized octahedrons.The hydrothermal growth mechanism of Bi₂Ti₂O₇ was analyzed by the layer growth model,with the fastest growing（100）planes become the six vertices,and the lowest growing（111）planes become the eight faces of the octahedron.By XRD and EDS analysis,it was found that the product composition is more consistent with Bi_1.74Ti₂O_6.624（JCPDS:01-89-4732）.The results prove that all the pure phases prepared in this thesis and in the literatures with the nominal composition Bi₂Ti₂O₇are actually non-stoichiometric with bismuth vacancies and oxygen vacancies.We interprete this as a result of atomic size effect on the structure stability of the cubic pyrochlore phase.Secondly,pure phase Bi₂Ti₂O₇ with particle size of about 50 nm was prepared by co-precipitation method using bismuth nitrate and tetrabutyl titanate as starting materials at 550°C for 16 hours.And Y-substituted Bi_2（1-x）Y_2xTi₂O₇（x=0.02,0.04,0.06,0.08,0.10）were also synthesized by co-precipitation method.The absorption edges of hydrothermal synthesized octahedral Bi₂Ti₂O₇ and psudo-spherical Bi₂Ti₂O₇ prepared by co-precipitation were located at 457nm and425nm,respectively,and the band gaps were 2.75eV and 2.918eV,respectively,which indicate that Bi₂Ti₂O₇ particles prepared by both methods possess visible light catalytic properties.The visible light catalytic efficiency of Bi₂Ti₂O₇ was tested by degradation of rhodamine B.Because of the nano-size effect（reduced probability of electron-hole recombination,large specific surface area）,hythermal synthesized powders with nanometric rod-shaped particles show higher visible light photocatalytic efficiency than the one with only micron-sized octahedral particles.And the same applies to the nano powders prepared by the co-precipitation method comparing to the hydrothermal synthesized powders with octahedral morphology.With small amount of Y substitution of Bi,the band gap of the catalyst slightly decreases,and the visible light photocatalytic activity increases.Finally,in order to address the long term puzzle of that whether Bi₂Ti₂O₇ is ferroelectric or not,we prepared successfully pure phase Bi₂Ti₂O₇ ceramics by sintering the above wet-chemistry synthesized pure phase Bi₂Ti₂O₇ powders at1200℃for 45min.Dielectric measurements however showed no ferroelectric phase transition in the measured temperature range from room temperature to 750°C.The dielectric constant at room temperature is about 127,and the dielectric loss is low（0.005⁰.015）.The dielectric constant value shows little variance below 300°C.