Study On The Enhanced Photovoltaic And Photocatalytic Effects By Ferroelectric Polarization In Perovskite Oxides

Perovskite ferroelectric oxides possess both ferroelectric and semiconductor properties.The polarized electric field inside ferroelectrics can effectively promote the separation of photogenerated carriers,which has attracted extensive attention in photovoltaic and photocatalytic fields.The ferroelectric polarization in ferroelectric extends throughout the bulk,thus the focus of ferroelectric photovoltaic and photocatalytic effects is on the separation of photogenerated electron-hole pairs by polarized electric field.However,the multi-domain structure within ferroelectric materials and the shielding effect of polarization severely hinder the separation of photogenerated carriers by polarized electric fields.This work aims to enhance the photovoltaic and photocatalytic performance of ferroelectric semiconductors by preparing spontaneously polarized ferroelectric materials,using poling process by external electric field and piezoelectric effect.These methods can enhance the polarization electric field,thereby improving the separation efficiency of photogenerated electron-hole pairs.The details are presented as follows:（1）Study on the photovoltaic performance of self-polarized flexible BiFeO3 thin film.The（111）SrRuO₃/（111）BiFeO₃/Au ferroelectric thin film photovoltaic device was fabricated on mica,and the self-polarized BiFeO₃ film can effectively separate the photogenerated carriers.Researchers have prepared ferroelectric thin films with spontaneous polarization on single-crystal SrTiO₃ substrates.However,there are relatively few studies on flexible substrates.In this paper,a self-polarized BiFeO₃ film was successfully prepared on a mica substrate by pulsed laser deposition,and Schottky junctions were constructed by growing semi-transparent Au electrodes.Finally,the thickness of the mica substrate was reduced to less than 10μm to obtain a flexible photovoltaic device.The open-circuit voltage is 0.52 V and short-circuit photocurrent density is 1.02 m A/cm² when the laser with 405 nm wavelength illuminates the device.The device shows no significant degradation in photovoltaic performance after 10,000 bends with a bending radius of 3 mm.The photovoltaic efficiency increases with temperature over a certain range and reaches a maximum at 150°C,which is caused by multiple factors such as ferroelectric polarization,oxygen vacancies and interfacial charge shielding.In addition,the device has significant photo-capacitance and photo-impedance effects.The photogenerated carriers increase the interface charge polarization,dipole polarization,ion polarization and conductance,significantly increasing the photo-capacitance and reducing the photo-impedance.（2）Study on photoelectrochemical performance of Bi2MoO6 photoelectrode enhanced by ferroelectric polarization.The polarized electric field in ferroelectric thin films can separate photogenerated electron-hole pairs,which can be used to enhance photoelectrochemical performance.Bi₂MoO₆ has excellent photocatalytic and photoelectrocatalytic performance and is theoretically ferroelectric.However,there are no studies that manipulate its own ferroelectricity to modulate the photoelectrocatalytic performance.We fabricated Bi₂MoO₆-nanopillars photoelectrodes on conductive ITO glass substrates,and verified the flip-flop ferroelectric polarization by macroscopic hysteresis loops and microscopic evolution of ferroelectric domains.The photoelectrode exhibits a photocurrent density of 195μA/cm² at 1.23 V vs.RHE（vs.reversible hydrogen electrode）,as well as a fast photoresponse.We further polarized the Bi₂MoO₆ film so that the polarization is towards ITO.The ferroelectric electric field increases the magnitude of upward（downward）band bending toward the Bi₂MoO₆/electrolyte（Bi₂MoO₆/ITO）interface,producing a wider space charge region at the Bi₂MoO₆-electrolyte and Bi₂MoO₆-ITO interfaces,thus increasing the separation efficiency of photogenerated carriers and enhancing the photoelectrochemical performance.As a result,the photocurrent density of the Bi₂MoO₆ photoelectrode with polarization orientation towards ITO reaches 250μA/cm²（at 1.23 V vs.RHE）,which is 28%higher than that of as-prepared sample.（3）Study on piezo-photocatalytic performance of Bi4Ti3O12 nanoplates.Theoretically,the polarization electric field in ferroelectric nanoparticles exists in the whole bulk,which can effectively separate the photogenerated carriers and make them reach the surface of the nanoparticles to participate in the catalytic reaction.However,the static ferroelectric polarization is easily shielded by the charges in the external environment,which weakens the ability of the ferroelectric polarization to separate photogenerated carriers.In this paper,we synthesized well-defined,monodisperse and highly crystalline Bi₄Ti₃O₁₂ nanoplates（MS-BiTO）by the molten salt method,while the sample（SSR-BiTO）prepared by typical solid-state sintering method was used for comparison.Due to the smaller size and larger specific surface area,the photocatalytic degradation rate of Rhodamine B（RhB）by MS-BiTO nanoplates reaches 0.0665 min^-1,which is 4.4 times of SSR-BiTO(0.0151 min^-1).Under the co-excitation of light and ultrasound,the shielding effect of ferroelectric polarization is broken and the polarized electric field can separate photogenerated carriers efficiently.The piezo-photocatalytic rate of MS-BiTO nanoplates reaches 0.1414 min^-1,which is 5.6 times of its piezocatalytic rate and 2.1 times of its photocatalytic rate.In addition,MS-BiTO also has a relatively high piezo-photocatalytic degradation rate for methyl orange dye.In summary,the MS-BiTO nanoplates synthesized by molten-salt method can decompose dyes with high efficiency.（4）Study on piezo-photocatalytic performance of NiO@PbTiO3 pn heterojunction.Most of the piezo-photocatalytic nanomaterials reported so far have multi-domain structure,which leads to small ferroelectric polarization and piezoelectricity within the ferroelectric material,limiting the piezo-photocatalytic performance.In this paper,we synthesized single-crystal and single-domain PbTiO₃ nanoplates with spontaneous polarization by hydrothermal method and loaded p-type NiO nanoparticles on their surface to construct NiO@PbTiO₃ pn heterojunction.Due to the presence of the built-in electric field of pn junction,NiO@PbTiO₃is able to separate photogenerated carriers more efficiently than PbTiO₃.The photocatalytic degradation rate of RhB over NiO@PbTiO₃ composite is 4.71 times of pure PbTiO₃.Under the synergistic effect of light and ultrasound,the shielding effect of ferroelectric polarization is broken,which improves the separation efficiency of photogenerated carriers by polarized electric field.The ferroelectric electric field is further coupled with the built-in electric field of pn junction,which greatly improves the photocatalytic performance of the piezoelectric composite catalyst.The piezo-photocatalytic reaction rate of the NiO@PbTiO₃ composite is1.82 times of its photocatalytic reaction rate,and 3.34 times of pure PbTiO₃.In summary,the synergetic photocatalysis and piezocatalysis provide a new strategy for designing high-performance catalytic hybrid.