【作者】 李颖；

【导师】 高学平；

【作者基本信息】 南开大学 ， 无机化学， 2009， 博士

【摘要】 碱金属和碱土金属钛酸盐是一类重要的无机非金属材料,具有优异的物理化学性能,作为新能源材料具有潜在和重要的应用前景。其中尖晶石结构的碱金属钛酸盐Li₄Ti₅O₁₂是一种新型能源材料,其结构稳定,作为锂离子电极材料在充放电过程中体积几乎不发生明显变化,具有非常好的循环性能,因此被广泛用于锂离子二次电池的负极材料。钙钛矿结构的碱土金属钛酸盐MTiO₃（M=Ca、Sr和Ba）是具有优良介电、压电、铁电和电光转换性能的功能材料,广泛用于电容器、传感器和随即存取存储器等电子器件。在无机材料领域中,探索合成具有纳米结构无机材料的方法,建立其相应的生长机理模型,并研究材料的结构与性能的关系在理论研究与实际应用方面具有重要的意义。本论文基于一维纳米结构钛酸盐的反应活性,以纳米管和纳米纤维作为结构单元构筑了具有新颖形貌的碱金属和碱土金属钛酸盐,采用SEM、TEM、HRTEM和XRD等分析手段对其结构和生长机理进行了表征,并研究其电化学和光电化学性能。论文采用质子钛酸盐纳米棒为先驱体,在LiOH碱性溶液中经过100℃水热离子交换得到具有一维纳米结构的中间体,该中间体产物经过800℃高温烧结后得到长几个微米,直径在100-200 am之间的Li₄Ti₅O₁₂纳米棒。结构分析表明采用低温水热有利于中间体及最终烧结产物保持一维棒状纳米结构。电化学测试显示,Li₄Ti₅O₁₂纳米棒因其具有一维纳米结构使Li⁺具有较短的传输距离而表现出较好的循环稳定性和高倍率性能。特别是在1600 mA/g（～10C）充放电电流密度下,Li₄Ti₅O₁₂纳米棒仍具有较高的比容量133.8 mAh/g,同时在1.4 V（vs.Li⁺/Li）具有平坦的放电平台。论文采用钛酸钠纳米管为先驱体,在LiOH碱性溶液中经过超声离子交换首先得到表面光滑,长为几百个纳米,外直径在10-15 nm的钛酸锂纳米管中间体。该钛酸锂纳米管经过不同温度烧结后得到形貌和组分均不同的Li-Ti-O化合物。研究表明,400℃所得产物为外直径为10-15 nm的层状Li-Ti-O纳米管;500℃烧结产物为直径20-50nm的纳米纤维,组成为Li₄Ti₅O₁₂和贫锂的锐钛矿相Li_xTiO₂。600℃产物是粒径大约50nm左右的纳米棒,组分为Li₄Ti₅O₁₂和贫锂的锐钛矿相Li_xTiO₂。电化学测试表明,400℃烧结所得管状产物具有较高的比容量和高倍率性能。循环伏安测试结果显示其峰电流与扫速成线性关系,表明该材料具有赝电容特征,这与其具有开放通道的管状一维纳米结构密切相关。论文采用质子钛酸盐纳米纤维为先驱体,在NaOH碱性条件下与MCl₂（M=Ca、Sr和Ba）进行水热反应24h,制备得到具有不同形貌的碱土金属钛酸盐。首次制备出具有方形开口的CaTiO₃微米管。同时制备出由纳米颗粒原位生长形成的SrTiO₃和BaTiO₃类棒状微米结构。研究表明碱土金属离子的浓度、水热反应温度和NaOH的浓度等是影响产物形貌、结构和碱土金属钛酸盐生长机理的重要因素。本文根据质子钛酸盐纳米纤维的反应活性和与不同碱土金属的反应特点,首次讨论提出两种可能的生长机制来解释CaTiO₃微米管以及SrTiO₃和BaTiO₃棒状结构的生长过程。其中,由纳米纤维组成的纤维束是形成CaTiO₃微米管的中间产物,纤维束中间产物通过“Ostwald ripening process”长大,再经过不断重结晶过程形成具有开口端的CaTiO₃微米管。而SrTiO₃和BaTiO₃具有和质子钛酸盐纳米纤维相同的TiO₆结构单元,Sr²⁺和Ba²⁺能够与H⁺发生离子交换形成具有活性点的钛酸盐纳米纤维,SrTiO₃和BaTiO₃纳米颗粒在纳米纤维母体上原位生长并发生相变最终形成棒状微米结构。三种不同形貌的碱土金属钛酸盐可以作为染料敏化太阳能电池的光电极材料,交流阻抗（EIS）表明三种材料的光电极反应受钛酸盐/染料/电解液界面的电荷转移过程控制。论文采用质子钛酸盐纳米管为先驱体,在NaOH碱性条件下和MCl₂（M=Ca、Sr和Ba）进行水热反应24h,在不同水热温度下制备出具有不同形貌的碱土金属钛酸盐。在150℃水热温度下制备得到形状不规则、表面粗糙的微米管状CaTiO₃和较短的棒状BaTiO₃。在80℃水热条件下可以制备得到由纳米颗粒组成的100-200 nm的类花状SrTiO₃团聚体。在强碱水热条件下,质子钛酸盐纳米管之间的范德华力使其倾向于团聚形成具有相同取向的纳米管束,随着Ca²⁺离子不断被消耗逐通过“Ostwald ripening process”和进一步溶解重结晶渐形成了两端具有锥形开口端的CaTiO₃微米管。而SrTiO₃类花状团聚体和BaTiO₃短棒的形成机理与纳米纤维制备的SrWiO₃和BaTiO₃类棒状结构相似,包括离子交换及原位相转变。更多还原

【Abstract】 Alkaline titanate(Li₄Ti₅O₁₂) and alkaline earth titanate（MTiO₃,M = Ca,Sr,and Ba） have attracted much attention as a result of their novel properties and technical applications.These materials have the advantage in response to the increasing demands for cleaner and more efficient energy conversion and storage systems.Along with the intensive development of multifunctional nanomaterials,it is necessary to fabricate alkaline and alkaline earth titanate nanomaterials with novel morphology and investigate on the size-dependent properties of these titanate compounds.In particular,as a promising candidate for anode materials in rechargeable lithium-ion batteries,spinel Li₄Ti₅O₁₂ was reported as zero-strain insertion material,which can undergo two-phase reaction and accommodate the structure changes during lithium insertion and extraction processes.Alkaline earth titanates（MTiO₃,M = Ca,Sr,and Ba） with a perovskite structure have been investigated intensively due to their unique dielectric,piezoelectric,and ferroelectric properties,which are of great interest in the technological applications such as capacitors,transducers,actuators,and nonvolatile random-access memorgy devices.In this work,we focused on the chemical reactivity of one-dimensional titanate nanostructures in alkaline solution.Based on nanostructured titanate reactivity, protonated titanate nanotube or nanofiber can serve as nanometer-sized building block to fabricate alkaline titanate and alkaline earth titatane materials with more complex morphology.The microstructure,morphology,and composition of the obtained titanate compounds are characterized by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,and high resolution transmission electron microscopy（HRTEM）.Firstly,Li₄Ti₅O₁₂ nanorods are fabricated after calcination of the hydrated lithium titanate,which is prepared from hydrothermal treatment of titanate nanorods in aqueous LiOH.The hydrothermal temperature has an impact on the chemical reactivity of titanate nanorods in LiOH solution and the relatively low temperature is beneficial for the retention of rodlike morphology.The galvanostatic charge-discharge tests were conducted to measure the electrochemical performance of the Li₄Ti₅O₁₂ nanorods.It is demonstrated that the Li₄Ti₅O₁₂ nanorods calcined at 800℃have excellent high rate discharge capability and good cycle stability during insertion and extraction processes,owing to the good crystallinity,unique structure, and the short diffusion distances originated from one-dimensional morphology.Secondly,the layered nanotubes of Li-Ti-O compound are prepared by ultrasonic treatment of sodium titanate nanotubes in LiOH solution,which is involved in the ion-exchange process.It is found that Li-Ti-O compound maintain layered structure below 400℃and undergo phase transition to a mixture of Li-poor anatase Li_xTiO₂ and spinel Li₄Ti₅O₁₂ as the main phases at 500 and 600℃.The lithium titanate nanotubes calcined at 400℃exhibit the large capacity and good high rate capability. Typical CV curves of the samples at various scan rates demonstrate that the faradaic pseudocapacitive process is involved in electrochemical lithium intercalation of the sample calcined at 400℃,in agreement with the linear relationship between the anodic peak currents and the scan rates.Based on the reactivity of titanate nanofibers,ternary perovskite oxides MTiO₃ （M = Ca,Sr,and Ba） with specific morphologies have been successfully prepared at low temperature for 24 h in NaOH solution.The resulting CaTiO₃ products possess a novel microtubular structure with rectangular cross-section,while SrTiO₃ and BaTiO₃ show the assemblies consisting of aggregated nanoparticles in a compact fashion.On the basis of the experimental results,we have proposed two types of growth mechanisms to elucidate the formation processes of CaTiO₃ and MTiO₃（M = Sr,and Ba） microstructures,respectively.The fabrication of microtubular CaTiO₃ undergoes the initial dissolution of titanate nanofibers by Ostwald ripening process,which results in the conversion into micrometer-sized fiber-bundles,and the recrystallization occurs simultaneously until tubular microstrucures are obtained. Completely different from the formation of CaTiO₃ microtubes,formation of MTiO₃ （M = Sr and Ba） microstructures involves ion-exchange reaction and in situ growth process.In addition,the photoelectrochemical properties of the as-obtained products were investigated,indicating that the charge transfer process across the MnTiO₃/dye/electrolyte interfaces is the dominant reaction in the MTiO₃ electrodes. Finally,series of alkaline earth titanate with specific morphology have also been synthesized through hydrothermal treatment using titanate nanotubes as a precursor. Namely,CaTiO₃ microtubes with rectangular open end and rough surface,and short BaTiO₃ nanorods are obtained in NaOH solution at 150℃for 24 h,whereas the flowerlike SrTiO₃ assemblies composed of nanoparticles are prepared at the lower temperature of 80℃.The experimental results indicate that the parent titanate nanotubes involved in the reaction exhibit high chemical reactivity as a precursor. The ion concentration,reaction temperature,and alkaline concentration play crucial roles in the phase transition and shape evolution.Moreover,the growth processes of alkaline earth titanate based on the titanate nanotubes share the similar mechanisms to those obtained from titanate nanofibers.更多还原