Investigation Of The Photovoltaic Property And Photocatalytic Performance Of KNN-based Ceramics

Based on the consideration of environmental issues and human sustainable development,lots of environmental friendly lead-free piezoelectric materials have been widely investigated to replace（Pb（Zr,Ti）O₃（PZT））piezoelectric ceramics.Among them,（K,Na）Nb O₃（KNN）-based ceramics are considered as the most promising materials to replace lead-based materials owing to their excellent piezoelectricity and electromechanical property,and thus receiving wide attention.In the field of energy harvesting,the stability and power level provided by the single-source energy harvester are usually insufficient for practical application,which inspires researchers to employ perovskite oxides as hybrid energy harvester.However,it is still quite difficult to realize narrow bandgap and high piezoelectric property in lead-free ceramics simultaneously,for the highest piezoelectric constant(d₃₃)reported now is just 150 p C/N.Therefore,it is of great interests to design lead-free materials with both giant piezoelectricity and excellent photovoltaic property.Besides,KNN-based ceramics have the advantages of good semiconductor property,great ferroelectricity,low-cost etc.,which inspired us to study their application potential in the field of photodegradation.In the first part of the work,we prepared（1-x）(K_0.48,Na_0.52)Nb O₃-x(Bi_0.5Na_0.5)(Zr_0.5Ni_0.5)O_3-δ（（1-x）KNN-x BNZN）ceramics using modified two-step sintering method.XRD,UV-vis,and Impedance Analyzer,etc.,were used to characterize the structure and bandgap of（1-x）KNN-x BNZN.The influence by the content of BNZN to the KNN-based ceramics has been systematically studied.Also,we propose a plausible mechanism to explain the multiple sub-bandgap induced by Ni²⁺.By realizing morphotropic phase boundary（MPB）through adjusting the content of BNZN,optimum piezoelectricity,dielectricity,ferroelectricity,and photovoltaic property have been achieved:d₃₃～318±10 p C N^-1,P_r～30.7μC cm^-2,T_c～360℃,V_oc×J_sc～697.6 n A V cm^-2.This part of work would give a new paradigm for designing multifunctional ceramics with great piezoelectricity,ferroelectricity,and photoresponse property.In the second part of the work,we prepared Ag nanoparticles loaded KNN-BNZ powders by photo-reduction method.XPS and UV-vis were used to determine the CB/VB position and bandgap of KNN-BNZ.Results show that the bandgap of KNN-BNZ is～3.33 e V,and the CB position is～-0.93 e V,the VB position is～2.40 e V.The KNN-BNZ ceramic powder can exhibit cuboid morphology by just simple solid-reaction process,which can expose lots of{100}facets with high active energy,thus contribute to the separation of charge carriers and promote the photocatalytic performance.Owing to the surface plasmon resonance effect of the Ag nanoparticles,over 90%of Rhodamine B（Rh B）was degraded within 10 min by using Ag-loaded KNN-BNZ as photocatalyst under UV-vis light irradiation.Besides,the Ag-KNN-BNZ had no detectable loss of photocatalytic performance after five continuous cycles both in aqueous system,indicating the good catalytic stability.Moreover,the active species in the process of Rh B photocatalytic degradation were investigated by designing the experiments of in situ capture of active species and using PL,ESR technique.Research showed that the photogenerated holes on the VB could react with H₂O or OH^-to produce·OH radicals,which is the dominant active species for the degradation of Rh B dye.In the three part of the work,the photocatalytic oxidative desulfurization performance of KNN-BNZ ceramic powders was studied.This is the first time to applying ceramic powders in the field of desulfurization.Good photocatalytic oxidative desulfurization performance of DBT was observed in the presence of unpoled KNN-BNZ powders,which can remove～87±1.12%of DBT after being irradiated for2h.When combining the effect of light irradiation and polarization,the degradation rate of DBT was further enhanced,as the poled KNN-BNZ powders could remove～92.25±1.40%of DBT.Nearly 127%enhancement in the desulfurization rate was achieved using poled KNN-BNZ powders as compared to the usage of unpoled KNN-BNZ powders under UV-vis light irradiation.Obviously,this improvement could be attributed to the polarization induced built-in electric field inside the KNN-BNZ,which not only led to more effective separation of photogenerated electrons and holes but also facilitated their migration to the surface of KNN-BNZ.