Effects Of Doped Ba And Fe Ions On Optical Band Gap,magnetic Properties And Visible Light Photocatalytic Performance Of Potassium Niobate

The perovskite ferroelectric materials with ABO₃ type have their wide application as the photocatalysts in photocatalytic degradation of the organic pollution pigments,which have the advantages of ferrous spontaneous polarization,strong structural stability and adjustable band gap of light absorption.Potassium Niobate?KNbO₃?is a typical ABO₃ type perovskite ferroelectric material.However,KNbO₃ with its wide band gap only absorbs the ultraviolet part of sunlight,which hinders its applications in visible light catalysis.In this thesie,Ba ions and Fe ions were separately introduced into the A and B sites of KNbO₃ by doping method,including Fe ions doping,Ba ions doping,and Ba-Fe codoping.We have studied the effects of these three types doping on the optical band gap,magnetic properties and visible light activity of KNbO₃,respectively.The results and analysis are as follows:1.KNb_1-xFe_xO_3-??K_1-xBa_xNbO_3-? and K_1-xBa_xNb_1-xFe_xO_3-? samples were prepared by the solid-state reaction method.2.Consistent with the parent KNbO₃,KNb_1-xFe_xO_3-? maintains orthorhombic perovskite structure with Amm2 phase and ferroelectric spontaneous polarization.By doping,the impurity bands of Fe²⁺,Fe³⁺ and oxygen vacancies are introduced,which greatly reduces the band gap of the parent KNbO₃,regulating the light absorption of the samples into the visible light.Hence,the photocatalytic activity is improved from the root of the reaction.Besides,the coexistence of oxygen vacancy,Fe²⁺ and Fe³⁺ are favorable for the migration of photogenerated electron hole carriers,reducing the carrier recombination rate and enhancing the photocatalytic activity.The introduction of magnetic Fe ions make the dopant samples show the weak room temperature ferromagnetism.The remanent magnetization increase firstly,and then decrease in the small amounts of doping,which results from the action of the bound magnetic polaron?BMP?in the doped materials.While in the case of high concentration doping,the enhancement of the magnetic properties is related to the increase of the Fe ion concentration.3.With the doping of Ba ions in A site,the structure of K_1-xBa_xNbO_3-? did not change.The particle sizes of the samples decrease with the concentration of dopant.Since the A-siteion is not directly involved the valence band and the conduction band of the KNbO₃ crystal,the band gap reduction caused by doping is pretty small,and the improvement of the visible light photocatalytic activity is not obvious.4.For Ba and Fe ions codoping system,with the increase of doping concentration,the crystalline structure of K_1-xBa_xNb_1-xFe_xO_3-? transform from the orthorhombic perovskite structure with Amm2 phase to the cubic ideal perovskite structure with Pm3 m phase,and simultaneously,the ferroelectric spontaneous polarization weakened.However,Ba and Fe ions codoping introduced Fe ions donor band,acceptor band and oxygen vacancy donor band between the valence band and conduction band,which significantly reduced the optical band gap of the crystal.What's more,the particle sizes of the samples decrease with the increase of doping.Therefore the visible light photocatalytic activity of the doped samples is obviously improved.The size effect of the grains enhances the remanent magnetization in the doped materials.