【作者】 李铭；

【导师】 韩高荣； 任召辉；

【作者基本信息】 浙江大学 ， 材料学， 2017， 博士

【摘要】 钙钛矿氧化物及其异质结因多层次的丰富的物理及化学性质,如压电性、铁电性、铁磁性以及超导性等,已成为当今凝聚态物理和材料科学研究的热点课题之一。其中,钙钛矿铁电氧化物因其独特的铁电极化性能和极化屏蔽机制已被广泛地研究及应用。然而,铁电氧化物极化表面对异质结生长的调控作用以及界面微结构、性能的关联尚不清楚。因此,基于铁电极化设计并构建钙钛矿氧化物的异质结、系统研究异质结生长与调控、界面屏蔽机制及其功能性,将为新型光电、磁性、催化、传感等方面的拓展应用提供有效的科学理论支持。本文首先概述了钙钛矿氧化物的结构、制备方法、热学性能及表面特性,同时总结和讨论了氧化物异质结的发展现状及新奇的界面性能。这些界面的新奇性能多是由于异质结外延界面处极化不连续所导致的,因此铁电极化由于其较高的极化电场对外延界面的结构及性能影响很大。但需要指出的是,铁电极化表面静电力对异质结外延晶体生长的影响及调控尚未深入研究,主要问题在于之前大部分外延生长研究所使用的基板为多铁电薄膜,这种铁电薄膜中的应力难以忽略并且需要外电场来调控其极化方向。基于此,我们提出了以铁电极化表面静电力来驱动氧化物外延生长,从而制备铁电氧化物异质结的思路。本文首先系统研究了两种PbTi03(PTO)纳米材料的铁电极化与微结构之间的关联,重点分析讨论了PTO的铁电极化以及极化屏蔽;创新性地采用单晶单畴的PTO纳米片作为基底材料,调控Ti02/PTO异质结材料的外延生长,结合微结构表征及性能研究,重点研究了铁电极化表面对异质结生长的调控作用;进一步采用原子级高分辨HAADF-STEM对生长的TiO2/PTO异质结界面进行分析,探究其界面微结构与铁电极化屏蔽,揭示了极化依赖的异质结光催化性能;制备了超薄锐钛矿Ti02纳米片,探索了其光催化解水性能。本文主要研究内容及结果如下:(1)采用水热法成功制备了介孔PTO纳米纤维和单晶PTO纳米片,系统地研究了两种PTO纳米材料的结构特性和铁电极化。研究结果表明,经固态相变得到的钙钛矿四方相PTO纳米纤维内部存在大量的具有规则形貌的封闭介孔,介孔尺寸在4-10 nm之间;该介孔PTO纳米纤维热膨胀性能可随着退火时间变化而变化,退火5min的样品表现出近零膨胀的性能,其热膨胀系数为-0.15×10-5/℃;零膨胀性能的产生是由于极化屏蔽导致的介孔正膨胀性能与PTO本身负膨胀性能叠加产生的,介孔PTO纳米纤维铁电极化在介孔处发生屏蔽,可能的屏蔽机制为介孔负极化面上产生氧空位来屏蔽负电荷,同时正极化面上聚集电子产生大量的Ti3+屏蔽正电荷。证实单晶PTO纳米片具有原子级平整的极化表面以及单畴的特征;通过ABF-STEM测定离子位移确定了其铁电极化强度为67±4 μC/cm2,且在近表面处仍旧维持较高的铁电极化强度。(2)创新性地采用单晶单畴PTO纳米片为基底,利用PTO极化表面静电力调控锐钛矿TiO2晶体的外延生长。研究结果表明,TiO2晶体在PTO纳米片极化表面的生长具有选择性,在酸性条件下,TiO2选择性生长在PTO纳米片的正极化面上,在中性条件下,TiO2生长在PTO纳米片的两个极化面上,而在碱性条件下,TiO2选择性生长在PTO纳米片的负极化面上;其选择性生长与不同体系中PTO铁电极化表面电荷屏蔽密切相关。同时,TiO2/PTO异质结中TiO2薄膜存在～13 nm的饱和厚度,并且在饱和厚度以下可以通过调控反应物浓度控制薄膜厚度(3-13 nm),第一性原理计算发现TiO2薄膜饱和厚度取决于异质结中PTO铁电极化表面的静电场有效距离(15 nm)。(3)进一步将铁电极化表面静电力调控的外延生长拓展到其他氧化物体系,在PTO铁电表面设计外延生长不同组分、结构以及应变的氧化物(STO/PTO及BFO/PTO),结果表明,可以全部成功制备出具有原子级平整界面的氧化物异质结,且其晶体外延生长均选择性地发生在PTO纳米片的正极化面上,证实了利用铁电极化表面静电力调控氧化物材料外延生长的普适性。(4)系统研究了 TiO2/PTO异质结中外延TiO2晶体的形貌与暴露面生长调控及其机理。研究结果表明,不同矿化剂下制备的TiO2/PTO异质结呈现不同的形貌,HF(pH=2)条件下TiO2呈现单面生长的(001)面暴露的薄膜形貌,H2O(pH=7)条件下TiO2呈现双面生长的(101)面暴露的八面体形貌,NH3-H2O(pH=10)条件下TiO2呈现单面生长的(101)面暴露的八面体形貌,KOH(pH=12)条件下TiO2呈现单面生长的(101)面暴露的纳米纤维形貌,且所有异质结的外延关系均为TiO2(100)//PTO(100);理论计算揭示了 TiO2晶体的生长形貌取决于两种晶体的晶格匹配度、表面能和晶面能。(5)系统探讨了 TiO2/PTO异质结界面微结构与铁电极化屏蔽机制,研究了极化依赖的异质结催化性能。研究结果表明,TiO2/PTO异质结正极化界面具备原子级平整的界面结构;在PTO临近界面处的三个单胞中,Ti位移明显降低,说明异质结中PTO的铁电极化在界面处被屏蔽;通过高分辨HAADF-STEM证实TiO2/PTO异质结界面的正极化屏蔽机制既包含原子的重构(Pb原子位移),也包含了电子的重构(Ti3+的出现);同时TiO2/PTO异质结具有极化依赖的可见光降解性能,正极化面生长的TiO2/PTO异质结在可见光下几乎无光降解性能,而负极化面生长的TiO2/PTO异质结在可见光下表现出很高的光降解活性(动力学参数0.057min-1);第一性原理计算揭示了正负极化界面上不同的能带排列,在可见光照射下,PTO内产生的光生空穴只能在负极化界面流向TiO2,而正极化面上空II穴只能从TiO2流向PTO。(6)首次使用乙醇溶剂热法合成锐钛矿超薄Ti02纳米片,系统研究了其生长机理与光解水性能。研究表明,该纳米片在横向的尺寸较大(～200 nm),同时厚度减小到～2.511 nm,其高活性(001)晶面的暴露率达到了～97%。其生长机理可能为乙醇溶剂的引入使得TiO2纳米片表面吸附大量有机基团,尤其是醇氧基(CH3CH2O-),而这些表面有机基团的存在可以增加表面F-的稳定性,同时进一步提高表面F-的含量。该纳米片紫外光产氢速率达到17.86mmolh-g-1 365 nm波长下对应的量子效率为34.2%,为当前TiO2相关催化剂中最高。更多还原

【Abstract】 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 μC/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.更多还原