Synthesis And Characteristics Of Alkaline Titanate And Alkaline Earth Titanate

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.