Study On Growth Control,Microstructures And Properties Of PbTiO₃,TiO₂ And Their Heterostructures

Owing to their multi-level fascinating physical and chemical properties,such as piezoelectricity,ferroelectricity,ferromagnetism and superconductivity,perovskite oxides and their heterostructures have become the focus of condensed matter physics and materials science.In particular,perovskite ferroelectric oxides have been widely investigated and applied due to their ferroelectric property and the ferroelectric screening mechanisms.However,the effect of the ferroelectric oxide surfaces on the growth of their heterostructures,the relationship between the interface microstructures and the properties,has not been revealed.Therefore,on the basis of ferroelectric polarization,designning and constructing the perovskite oxide heterostructures,systematically investigating the growth and control of heterostructures growth and thus the interface polarization screening and their functionality,will provide scientific references for applications in the field of photoelectric,magnetic,catalytic,sensors.In this dissertation,we firstly reviewed the crystal structure,preparation,properties and surface characteristics of ferroelectric oxides,summarized the development and novel interfacial properties of perovskite oxide heterostructures.These properties of the heterostrutrue were mainly induced by a polar-discontunity at the interface,and therefore the ferroelectric polarization would have a strong effect on the structures and properties of the heterostructure interface due to the high polarization electric field.However,it is still unfulfilled and highly expected for controlling the epitaxial growth with the spontaneous polarization surface electric field.This is due to in part the fact that most previous epitaxial growth has been characterized on multi-domain ferroelectric films,in which the substrate stress should be considered and an external electric field is needed.Accodingly,we proposed that an electrostatic force derived from ferroelectric surface could be employed to drive the oxide epitaxial growth with to prepare ferroelectric oxide heterostructures.In this dissertation,we firstly explored the relationship between ferroelectric properties and microstructures for two typical PTO nanostructures,focusing on the PTO ferroelectricity and the polaization screening.Innovatively,we used the single-crystal and single-domain PTO nanoplates to direct the TiO2 epitaxial growth on the PTO polar surfaces.Combining with their microstructures and properties,we focus on the control effect and mechanism of polarization surface on the heterostructure growth.We further performed the atomic-level HAADF-STEM images to characterize the interfaces of TiO2/PTO heterostructures to investigate the effect of ferroelectric polarization on the interfaces as well as the ferroelectric polarization-dependent photodegradation.In the end,we prepared ultra-thin anatase TiO2 nanosheets for water splitting to evaluate the photocatalytic performance.The main contents and results are listed as follows:(1)Mesoporous PTO nanofibers and single-crystal PTO nanoplates were prepared and the structures and the ferroelectric polarization properties were investigated for both of them.It was found that the mesoporous PTO nanofibers formed by a solid-solid transformation consisits of a large number of closed mesopores with regular morphology,and the size of mesopores was determined to be 4-10 nm.The coefficient of thermal expansion of such mesoporous PTO nanofibers could be tuned by the annealing time,and the mesoporous PTO nanofibers annealed at 5min showed a near-zero thermal expansion(ZTE)with the coefficient of thermal expansion to be-0.15×10-5/?.The ZTE property is argued to arise from a synergye effect for positive thermal expansion of the mesopores and the negative thermal expansion of PTO.The polarization screening occurred around the mesopores in the PTO nanofibers could be achieved by the oxygen vacancy formed at the negative side of the mesopores and electrons accumulated on the positive side which generates numerous Ti3+ instead.The single-crystal PTO nanoplates were confirmed to be single-domain and possess an atomic smooth polar surface.With the ion shifts determined in the images of ABF-STEM and HAADF-STEM,the ferroelectric polarization value of PTO nanoplates was calculated to be 67±4 ?C/cm2 and a high polarization is maintained near the surface.(2)Innovatively,we used the single-crystal and single-domain PTO nanoplates as substrate to control the TiO2 epitaxial growth by surface electrostatic force.It was foxxnd that TiO2 selectively grown on the positive side of PTO under acidic conditions,on both polar sides under neutral conditions and selectively grown on the negative side of PTO under alkali conditions.The selective growth was strongly related with the polarization-induced surface charge adsorption.Meanwhile,there exsited a saturation thickness(?13 nm)of the TiO2 films on PTO nanoplates,which allowed us to control the TiO2 growth for smaller thickness by reducing the initial reactant concentration.First-principle calculations suggest that the growth thickness of TiO2 on PTO nanoplates was determined by the effective distance of the electrostatic potential(?15 nm)originating from the PTO ferroelectric polarization.(3)On the basis of the previous study of TiO2 growth,we further expanded the concept of polarization surface to control crystal growth of other oxides.Three oxides with different composition,structure,and strain were designed to grow on the surface of PTO nanoplates,including STO/PTO and BFO/PTO heterostructures with atomic smooth interfaces.Interestingly,all the oxides could selectively grow on the positive side of PTO nanoplates.(4)We further investigated the TiO2 mophology,exposed facet and control mechanism of TiO2/PTO heterostructures systematacially.It was found that the TiO2/PTO heterostructures had different morphology by using different mineralizer,TiO2 has a film mophology with(001)exposed selectively grown on positive side of PTO with HF(pH=2),octahedron mophology with(101)exposed grown on both side of PTO with H2O(pH=7),octahedron mophology with(101)exposed selectively grown on negative side of PTO with NH3·H2O(pH= 10),and nanowire mophology with(101)exposed selectively grown on negative side of PTO with KOH(pH=12).For these heterostructures and the epitaxial relationship was determined to be TiO2(100)//PTO(100).Furthermore,the theoretical calculations revealed that the morphology of TiO2 was determined by the lattice strain,surface energy as well as the crystal facet energy.(5)The microstrutures and polarization-screening mechanism of Ti02/PTO heterostructures were discussed systematacially,and the interface-induced properties of the TiO2/PTO heterostructures were investigated.The positive interfaces of TiO2/PTO heterostructures were characterized to be atomic smooth;the displacement of Ti atoms within the first three PTO unit cells near the interface was very low,implying that the ferroelectric polarization of PTO was screened near the interface.Based on the atomic level HAADF-STEM images,the polarization screening at the positive interface of TiO2/PTO heterostructures is possibly attributed to both the atomic reconstruction(Pb atomic displacement)and the electronic reconstruction(Ti3+ formation).TiO2/PTO heterostructures demonstrated a polarization-dependent photodegradation property for MB solution,where the heterostructures on the positive side showed nearly no photodegradation acticity.In constrast,a high photodegradation activity was observed on the negaitive side(Km=0.057 min-1).First-principle calculations confirm a different band alignment on the two sides,implying that under visibe light irradiation,the positive side interface favored the transfer of holes from TiO2 to PTO,and the negative side interface favored the transfer of holes from PTO to TiO2.(6)For the first time,we prepared the ultrathin anatase TiO2 nanosheets by a facile ethanol solvothermal route,and investigated the growth mechanism and water splitting properties.Such nanosheets were determined to be with a thickness of?2.5 nm and a side length of-200 nm,giving rise to-97%reactive {001} facets exposed.The growth mechanism was revealed to be related with the introduction of ethanol solvent,which increased the surface adsorbed organic groups especially the CH3CH2O-,and further stabilized and enhanced the content of surface chemisorbed F-on the TiO2 nanosheet.Such nanosheets exhibited a H2 evolution rate as high as 17.86 mmolh-1g-1,and the corresponding apparent quantum efficiency had been determined to be 34.2%at 365 nm,the highest value of other TiO2-based photocatalysts to the best of our knowledge.