Flexoelectric Effect In Epitaxial Heterostructures And Its Electrocatalytic Oxygen Evolution Reactio

Flexoelectric effect is an electro-mechanical coupling which widely exist in solid dielectric materials,regardless of the structural symmetry.It has shown many interesting physical phenomena and application prospects in various electronic functional materials.However,there are few reports on the surface characteristics of flexoelectric polarization.Forced to solve the energy crisis and environmental problems,researchers have made continuous exploration in electrocatalytic water splitting.Built-in electric field provides an important way to regulate the catalytic reaction,which can not only optimize the adsorption/desorption toward reactants and key intermediates,but also promote the charge transfer process at interface.In previous studies,the built-in electric fields were always designed by heterogeneous structures or defect engineering.However,the regulation mechanism of the electro-polarized field which induced by flexoelectric effect on electrocatalysis remains unclear.Therefore,we performed below research on flexoelectric effect in epitaxial film heterostructures.Firstly,the LaFeO₃（LFO）epitaxial heterostructures with and without flexoelectric effect were prepared on La Al O₃（LAO）and Sr Ti O₃（STO）substrates,respectively.The lattice structure and strain gradient were characterized by high resolution scanning transmission electron microscopy.Scanning Kelvin probe microscope and X-ray photoelectron spectroscopy were used for comparative analysis.There is an upward flexoelectric polarization in the LFO film grown on LAO substrate,and the surface potential of LFO layer is significantly improved due to the incomplete screening of the positive polarization bound charges.In addition,the flexoelectric polarization also can downward bend the band direction in LFO layer,which further modulates the potential barrier and transport characteristic of Pt/LFO interface in electronic devices.On this basis,LFO dielectric thin-films of different thicknesses were grown on STO and LAO substrates,respectively.The flexoelectric polarization existed in all the LFO/La Ni O₃/LAO epitaxial heterogeneous.Electrochemical measurement,electrical characterization test and COMSOL Multiphysics were performed to characterize and analyze the electrocatalytic properties and kinetic process mechanism of oxygen evolution reaction for LFO thin films.Due to the competition between electron transport and dielectric polarization in LFO layer,the performance of oxygen evolution reaction shows an obvious thick-dependent behavior.Flexoelectric polarization can not only improve the concentration of OH^-reactant at the solid-liquid interface and optimize the reaction microenvironment,but also promote the adsorption of OH^-and electron transfer,enhancing the kinetic process of oxygen evolution reaction as a whole.Finally,the enhancement mechanism of flexoelectric polarization on oxygen evolution of LFO dielectric films was verified by electrochemical impedance spectroscopy,and the interference of lattice strain factor was eliminated by density functional theory calculation.