Research On Construction And Strategy Of Controlling Photogenerated Carrier Separation Efficiency Of ZnFe₂O₄ Photoelectrode

With the rapid development of the industrial revolution,the global energy crisis and environmental problems have been serious increasingly.Exploring the clean and renewable energy is of great strategic significance for solving the current energy shortage,environmental pollution and achieving sustainable development for human.It is a promising technology to obtain hydrogen energy by solar photoelectrochemical（PEC）water splitting.The key of this technology is to develop suitable semiconductor materials as photoelectrodes.Among many semiconductors,zinc ferrite（ZnFe₂O₄）has become one of the candidate materials for PEC water splitting due to its suitable optical band gap,band structure and stable chemical properties.However,the pure ZnFe₂O₄photoelectrode has some shortcomings such as poor conductivity and slow surface oxygen evolution kinetics,resulting in low photocarrier separation efficiency,and its actual solar hydrogen production efficiency is far lower than its theoretical value.In this work,ZnFe₂O₄ thin films with two different morphologies were prepared for PEC water splitting by controlling the iron source in the precursor solution through hydrothermal method.The mechanism of the influence of iron source on the morphology and PEC performance of ZnFe₂O₄ was investigated.Three different methods containing dual metal ions gradient doping,constructing heterojunctions and supporting cocatalysts were used to control the photocarrier separation efficiency of ZnFe₂O₄ and explore their respective mechanisms.The results show that when the iron sources are FeCl₃·6H₂O and Fe₂（SO₄）₃,ZnFe₂O₄（Nanorods,NRs）and ZnFe₂O₄（Nanopetals,NPs）are fabricated with photocurrent densities of 0.11 mA/cm² and 0.07 mA/cm²（1.23 V vs.RHE）,respectively.On the basis of ZnFe₂O₄ NRs,Cu-Sn dual ions gradient doped ZnFe₂O₄ NRs are prepared by dip-coating and high temperature annealing.The photocurrent density of Cu.Sn-ZnFe₂O₄ NRs is 0.46 mA/cm²（1.23 V vs.RHE）,because Cu-Sn dual ions gradient doping could expand the band bending of ZnFe₂O₄,increasing the carrier density and conductivity of ZnFe₂O₄ and improving the carriers separation efficiency of ZnFe₂O₄.Additionally,the ZnFe₂O₄ NRs are modified by constructing heterojunctions and supporting cocatalysts.The photocurrent densities of ZnFe₂O₄/Ag₂S and ZnFe₂O₄/CoAl-LDH are 0.34 mA/cm² and 0.60 mA/cm²（1.23 V vs.RHE）,respectively.The ZnFe₂O₄/Ag₂S heterojunction could absorb and use more photons,and the gradient energy level of the heterojunction is beneficial to accelerate the separation of photogenerated carriers.CoAl-LDH（Nanosheets,NSs）on ZnFe₂O₄NRs could improve the surface oxygen evolution kinetics of ZnFe₂O₄ NRs and promote the separation of photogenerated carriers,therefore,the PEC performance of ZnFe₂O₄NRs is improved effectively.