Study On Preparation,Microstructure And Properties Of One-dimensional Single-Crystal PbTiO₃,TiO₂ Nanostructure

One-dimensional single-crystal oxide functional materials with fascinating optical,electrical and magnetic properties have been the focus of materials science.As a typical ABO₃-type perovskite oxide functional material,one-dimensional single-crystal PbTiO₃ nanomaterials exhibits modulable ferroelectric,piezoelectric,negative thermal expansion and photocatalytic properties,therefore showing promising application prospects in the field of energy,information and environment.However,the microstructure and ferroelectric polarization screening mechanism of PbTiO₃ ferroelectric polar surface have not been investigated clearly.On the other hand,TiO₂ is a traditional high-efficiency photocatalytic material with efficient UV light absorption,high photogenerated carrier density and mobility,therefore showing promising prospect as efficient photoanodes in photoelectrochemical system.Nevertheless,the photoconversion efficiency of one-dimensional single-crystal TiO₂ based photoanodes is still subjected to its wide band gap,low separation and injection efficiency of photo-generated carriers,and the influence of the heterostructure on photoanodes properties also needs to be further investigated.Based on the background mentioned above,it would be highly beneficial to the understanding and application of one-dimensional single-crystal PbTiO₃ and TiO₂ materials by modulating their morphology,microstructure,properties and exploring the relevant mechanism.This dissertation reviews the researches about the crystal structure and the current status of ABO₃-type perovskite oxide,PbTiO₃ and TiO₂,respectively,and summarizes the relationship between their microstructure and properties including ferroelectricity,thermal expansion and photocatalysis.However,the screening mechanism of PbTiO₃ ferroelectric polar surfaces and its influence on properties need to be further investigated.The relationship between microstructure,interface and properties in TiO₂ based single-crystal heterostructured photoanodes needs to be further clarified.Accordingly,we propose a design to create mesopores based on the density differences in the structural phase transformation process,and synthesized one-dimensional single-crystal mesoporous ferroelectric PbTiO₃ fibers.The ferroelectric polarization screening mechanisms in closed and open mesopores are studied,respectively,and the relationship between ferroelectric polarization screening,microstructure and properties is also explored.On the other hand,we propose a design of a single-crystal core-shell heterojunction with high-quality and large-area interface via a self-template process,and synthesized one-dimensional single-crystal core-shell heterostructured TiO₂/STO?SrTiO₃?nanorod arrays.The influences of interface and shell thickness on photoelectrochemical performance are then discussed in detail.The main contents and results are listed as follows:1.One-dimensional single-crystal mesoporous PbTiO₃ fibers were prepared by utilizing the 13%density differeneces in the structural phase transformation process between pre-perovskite phase and tetragonal perovskite phase.The as-prepared tetragonal perovskite fibers exhibited single-crystal and single-domain microstructure with regular facets.The diameter was characterized to be 100-400 nm.The closed mesopores with truncated tetrahedron morphology and a size of 5-10 nm were embedded in the fibers.Each mesopore exposed a pair of?001?surfaces,corresponding to positive/negative ferroelectric polar surfaces,respectively.The bound charges on the surfaces need to be screened.HRTEM and EELS characterization have demonstrated the accumulation of oxygen on positive polar surface and the appearance of oxygen vacancies on negative polar surface,realizing the ferroelectric polarization screening of the polar surfaces in mesopores.2.The Rietveld refinement of in-situ XRD pattern indicated that the thermal expansion coefficient?TEC?of one-dimensional single-crystal mesoporous PbTiO₃ fibers was calculated to be-2.00 ×10-6 K-1 and-3.85 × 10-6 K-1 at 293?623 K and 15?300 K,respectively,which was lower than that of the porous-free bulk PbTiO₃ particles?-2.72×10-5 K-1,293?623 K?and could be referred as zero thermal expansion.According to the results of in-situ electron microscopy,the micro-area near the mesopores demonstrated a positive thermal expansion from 293 K to 573 K,which was related to the disappearance of ferroelectricity caused by the oxygen-based ferroelectric polarization screening.Therefore,the overall zero thermal expansion of one-dimensional single-crystal mesoporous PbTiO₃ fibers were originated from a synergetic effect of the positive thermal expansion near the micro-area of the mesopores and the negative thermal expansion of intrinsic PbTiO₃.3.One-dimensional single-crystal PbTiO₃ fibers with pores exposed on the surface were synthesized via a nitric acid etching process by removing the porous-free layer of one-dimensional single-crystal mesoporous PbTiO₃ fibers.The as-prepared fibers were characterized to be 100-400 nm in diameter and a large amount of open mesopores with a size of 5-10 nm could be observed on the surface.It was found that the exposure of surface mesopores lead to the increase of c/a at room temperature from?1.056 to?1.064 and the shift of Raman vibration peak of A1?1TO?phonon from 150.33 cm-1 to 152.60 cm-1,which indicated the enhancement of tetragonality and ferroelectricity in one-dimensional single-crystal PbTiO₃ fibers with pores exposed on the surface.A possible mechanism was proposed that the oxygen-based spontaneous ferroelectric polarization screening in enclosed mesopores has changed after the exposure of the mesopores,which was supported to the change of oxygen chemical position and the reduction of defect photoluminescence derived from XPS and PL characterizations.Furthermore,the results of CO₂ photocatalytic reduction measurement indicated that the exposure of mesopores could efficiently enhance the CO₂ photocatalytic reduction property of the one-dimensional single-crystal PbTiO₃ fibers?CO generation from pristine 0.48 ?mol·g-1·h-1 to 1.21 ?mol·g-1·h-1?.4.One-dimensional single-crystal rutile TiO₂ nanorod arrays were prepared via a hydrothermal process,and further variation of the reaction conditions could successfully control its morphology.The as-prepared TiO₂ nanorods synthesized at 150?for 8 h was characterized to be 1-1.5 ?m in length and 100-200 nm in diameter with regular facets.Subsequently,one-dimensional single-crystal core-shell heterostructured TiO₂/STO nanorods arrays were successfully prepared via a second hydrothermal method maintaining the original arrays morphology.Microstructure characterization demonstrated the large-area and high-quality interface between TiO₂ and STO.The epitaxial relationship of the TiO₂/STO heterojunction was determined to be TiO₂?001?//STO?110?.The thickness of SrTiO₃ could be controlled from 5-10 nm to 20-40 nm by altering the concentration of reactant.5.According to the photoelectrochemical measurement,the as-prepared sample with a SrTiO₃ shell thickness of 5-10 nm demonstrated the highest photocurrent density of 0.76 mA/cm2?1.23 V vs.RHE?,which was two times of that in pristine TiO₂?0.38 mA/cm2,1.23 V vs.RHE?nanorod arrays.Further studies showed that the sample with the STO shell thickness of 5-10 nm exhibited the separation efficiency and injection efficiency of?59%and?100%at 1.23 V?vs.RHE?,respectively,which is much higher than that of prestine TiO₂ of?37%and?83%.However,the sample with thicker SrTiO₃ shell?20-40 nm?showed lower separation efficiency and injection efficiency of?52%and?84%,respectively.The results indicated that the introduction of SrTiO₃ shell could efficiently improve the separation efficiency and injection efficiency of the photo-induced charge carriers,therefore leading to improved photoelectrochemical performance.Moreover,the improvement of the photoelectrochemical performance first increased and then decreased with the increase of SrTiO₃ shell thickness.