Self-powered Photodetection And Water Splitting Properties Of Self-polarized BiFeO₃-based Films

Bismuth ferrite（BiFeO₃）is a direct bandgap ferroelectric semiconductor with a band gap of 2.7 eV.The research on the photovoltaic effect,photodetection and photocatalysis of BiFeO₃ films has always been paid attention by researchers.The photoelectric effect of films depends on the polarization state and stability.It is significant for the design and application of photoelectric effect of thin films to introduce the built-in electric field of vertical thin films through self-polarization.In this paper,the self-polarized BiFeO₃ films are prepared with sol-gel method.On this basis,BiFeO₃-based double-layer films and Zn²⁺-Ti⁴⁺co-doped BiFeO₃ films are prepared.The formation of built-in electric field and the stabilization mechanism of BiFeO₃-based films are discussed.The photovoltaic effect,self-powered photodetection and water splitting properties of BiFeO₃-based thin films are studied.The BiFeO₃ film with strong（010）orientation（～200 nm）is obtained by introducing LaNiO₃ buffer layer on Pt substrate.During the repeated spin coating crystallization annealing process,the Bi³⁺volatilization introducing defects such as oxygen vacancies,forming ion concentration distribution and causing self-polarization of the film.It is found that the relatively low oxygen vacancy concentration and self-polarization characteristics are obtained in BiFeO₃ films prepared by precursor Bi excess of 7 mol%.The built-in electric field formed by the self-polarization of the film effectively separates the photogenerated carriers and thus exhibits an obvious photovoltaic effect.Under the condition of 0.2 mW/cm² green light illumination,the maximum open circuit voltage is0.46 V and the short circuit current is 0.6μA/cm².At the same time,the photoresponse at zero bias has good repeatability and sensitivity.The study on the photovoltaic and UV-visible photodetection performance of self-polarized BiFeO₃/XTiO₃（X=Sr,Zn,Pb）double-layer films shows that the photovoltaic effect and photodetection performance can be improved when the heterojunction electric field is in the same direction as the self-polarized built-in electric field of BiFeO₃ film.Among them,the significant performance is obtained in BiFeO₃/ZnTiO₃ double-layer film,the photocurrent response time at zero bias is around 20 ms,and the responsivity and detectivity under 365 nm illumination reach 7.4 mA/W and 7.5×10¹¹ Jones,respectively.The performance is not only due to the combined action of the built-in electric field in BiFeO₃ and the heterojunction electric field to improve the separation efficiency of photogenerated carriers,but also related to the high UV absorption rate of ZnTiO₃ layer.In addition,the bending deformation further improves the self-powered photodetection performance of BiFeO₃/PbTiO₃ double-layer film.The self-polarization behavior and photoelectric properties of BiFeO₃ films after co-doping of Zn²⁺-Ti⁴⁺at AB sites are studied.The microstructure of co-doped BiFeO₃ is analyzed and characterized by first principles calculation,X-ray diffraction and high-resolution electron microscopy.When Zn²⁺-Ti⁴⁺ions form ionic pairs,the oxygen octahedron tilts and produces obvious lattice distortion.The domain distribution and photovoltaic properties show that Zn²⁺-Ti⁴⁺doping enhances the self-polarization of the films.It is found that the significant photodetection performance is obtained in 2 mol%Zn²⁺-Ti⁴⁺co-doped films,and the responsivity and detectivity are 10 mA/W and 2×10¹¹Jones.After aging at 393 K for 30 minutes,the responsivity and detectivity of the films decreased by less than 18%,indicating that Zn²⁺-Ti⁴⁺co-doping can effectively improve the self-polarization stability of the films.The oxygen evolution reaction performance of Zn²⁺-Ti⁴⁺co-doped BiFeO₃ film anode（Pt as cathode）in 1 mol/L KOH solution is studied.It is found that the overpotential of 2 mol%co-doped film is 350 mV at the current density of 5 mA/cm² under dark condition.The analysis shows that the bound charges at the interface between the self-polarized co-doped BiFeO₃ films and the electrolyte attracts a large number of free charges.The free charges first participate in the adsorption and oxidation process of OH^-at low potential,reducing the driving force required for the oxygen evolution reaction.Under the simulated sunlight illumination condition,the photogenerated carriers are separated under the action of the built-in electric field of the film,which further enhances the oxygen evolution reaction performance of the co-doped BiFeO₃ films.Under the synergism of photocatalysis and photovoltaic effect,the underpotential（-20 mV）state is appear at the current density of 5 mA/cm².