Synthesis,Optical-electrical Properties And Applications Study Of Low Dimensional Niobium-based Oxides

The innovation of technological products promotes the progress of our society.However,it brings the conflict between device,energy and environment.Researching novel,multi-functional and environment-friendly materials can solve the problem.Among them,niobium-based materials get special attention.As some typical cases,the niobium-based ferroelectric materials,the niobium-based layered materials and the niobium-based energy storage materials,all of them have the widespread applications.The niobium-based ferroelectric materials can be used to prepare faster ferroelectric memory.It has been recognized as one of the most promising alternatives to Pb-based piezoelectric materials.Owing to the unique interlayer regions,niobium-based layered materials can be as the high-efficiency photocatalyst for solving environmental pollution problems.The niobium-based energy storage materials has structural merit for the application in lithium ion batteries with faster charge-discharge rate and longer lifespan.However,the current research on niobium-based material is relatively less.There still remain some pivotal problems need to be clarified for future applications.Our main research works are listed as follows.?1?High-quality lead-free piezoelectric K0.5Na0.5Nb1-_xMn_xO3-_??KNNMx?films have been successfully prepared by a modified sol-gel method.The effects of Mn substitution on the microstructure,morphology,lattice vibrations,and optical and ferroelectric properties of the KNNMx films have been investigated in detail.The research results indicate that the increased valence of Mn²⁺,which is substituted at the Nb⁵⁺site as Mn³⁺,plays an important role in reducing the amount of both oxygen vacancies and holes.As a result,the KNNMx films shows enhanced ferroelectricity.The optimal ferroelectric properties were obtained in the film doped with x=0.06,whose remnant polarization?2P_r?and coercive field?2E_c?values at the applied electric field of 1000 kV cm^-1 are 51?C cm^-22 and 265 kV cm^-1,respectively.In addition,the Adachi dielectric function model and a four-phase layered model were successfully applied to fit the measured ellipsometric spectra and reasonably described the optical dispersion behavior of the KNNMx films.The optical band gap slightly decreases,while the high-frequency dielectric constant linearly increases with increasing Mn concentration.The analysis of optical band gap and the extinction coefficient reveals the correlation between optical properties and structural phase transition.The KNNM0.06 film mainly shows the distinct in-plane?180^o?polar nano-domains.?2?Layered K₄Nb₆O₁₇?K4N?films and nano powders have been successfully synthesized using modified sol-gel and hydrothermal methods.The optical and electrical properties of K4N films have been investigated in detail,as well as the photocatalysis and energy storage applications of K4N nano powders.The research results indicate that K4N film was preferred orientation along the?040?peak to grow.It was believed that K4N film is with weak ferroelectricity.A relatively smaller 2P_r of 1.9?C cm^-2 was obtained in the K4N film at an applied electric field of 630 k V cm^-1.The dielectric functions of K4N film have been extracted by fitting ellipsometric spectra with the Adachi dielectric function model and a four-phase layered model.Raman scattering,transmittance and infrared reflectance spectra,and first-principles calculation were conjointly performed to further reveal the intrinsic optoelectronic features and relevant mechanisms of K4N.The initial pH of a dye solution has been proven to play a vital role in affecting the adsorption and photocatalytic performances of K4N.Due to the effects of dye sensitization,K4N shows superior photodegradation performance under both ultraviolet and visible light.K4N nanolaminas prepared in our work has a relatively higher H₂ production rate of about 1460?mol h^-1 g^-1.In addition,K4N as a potential anode material for lithium ion batteries has been also investigated.K4N nanolaminas reveals a considerable discharge specific capacity of about 513 mA h g^-1.?3?Interlayer-controlled KNb₃O₈ nanoflakes have been prepared based on alkaline hydrothermal conditions in the aid of adding different amount of thiourea.Related optoelectronic properties,photocatalysis and energy storage applications have been investigated in detail.The research results indicate that the initial pH of dye solution and discrepant preferential orientation of interlayer peak have been proved to deeply influence the adsorption and photocatalysis performances of KNb₃O₈.5 g of thiourea derived KNb₃O₈ with preferential?020?orientation has a large BET specific surface areas of⁶3.5 m² g^-1.It shows much better photocatalytic activities than commercial Degussa P25.As for the application in lithium-ion batteries,pure KNb₃O₈ nanoflakes reveals a discharge specific capacity of about 550 mA h g^-1,but with a poor rate feature.The discrepant preferential orientation of interlayer peak has a little effect on the electrochemical properties of KNb₃O₈ nanoflakes.Polydopamine derived carbon as a conductive coating shell was applied to modify KNb₃O₈ for enhancing its lithium insertion ability.It still possesses a high specific capacity of about 110 m A h g^-1 at the current density of 10 A g^-1,as compared with the 0.2 A g^-1 rate(about 310 mA h g^-1).It also exhibits outstanding cyclic stability with a capacity retention of?95%after3000 cycles.?4?T-Nb₂O₅@graphene composites have been successfully prepared by one-step and two-steps hydrothermal reactions,and modified graphene hydrogel method.Related electrochemical properties have been investigated in detail.The research results indicate that the free-anchored T-Nb₂O₅@graphene three-dimensional networks?NG-3D?can be successfully prepared based on graphene hydrogel method.NG-3D has a large BET specific surface areas of⁸0 m² g^-1.As for the application in lithium-ion batteries,it performs outstanding rate character[129 m A h g^-1?25C rate?,110 mA h g^-1?50C rate?and 90 mA h g^-1?100C rate?].It also shows ultra-stable cycling performance of over 20000 cycles with a high capacity retention of?70%at the current density of 10 A g^-1.Moreover,when the copper foil was substituted with cheaper and lighter aluminum foil as the anode current collector,it still has a high specific capacity of about 164 and 101 mA h g^-11 at 0.5C and 25C rate,respectively.It also shows ultra-stable cycling performance of over 20000 cycles with a high capacity retention of?75%.In addition,our assembled nonaqueous hybrid supercapacitors device based NG-3D electrode can reach a high energy density and power density.