Research On Charge Separation Management And Photoelectrochemical Properties Of ?-Fe₂O₃-based Photoelectrodes

In the field of solar-to-hydrogen photoelectrochemical?PEC?water splitting,it is the key to broaden the light response range,promote the charge separation and utilization efficiency for improving solar energy conversion efficiency.As a kind of n-type visible light responsive semiconductor material,hematite??-Fe₂O₃?has received extensive attention due to its narrow band gap,high theoretical value of solar-to-hydrogen conversion efficiency,good stability,low cost and non-toxicity.However,because of the poor conductivity and high photogenerated carriers recombination efficiency,the practical solar-to-hydrogen conversion efficiency can not reach the theoretical value.Therefore,learning the modification of?-Fe₂O₃ to improve its carrier separation efficiency is the key to improve the PEC performance of?-Fe₂O₃.In this paper,?-Fe₂O₃ films with different morphology were synthesized by hydrothermal method,the PEC property of?-Fe₂O₃ films with different morphology were compared and the chemical reaction process and film formation mechanism during the synthesis were analyzed.Dual axial gradient-doped?Zr and Sn?Fe₂O₃nanorod film and Ca-doped Fe₂O₃/Fe₂O₃ branched nanorod homojunction film were prepared by elemental doping in?-Fe₂O₃ and applied to photoelectrochemical water splitting.The effects and mechanisms of different doping elements and doping modes on the PEC performance of?-Fe₂O₃ were investigated.Finally,modifying the homojunction by cobalt phosphates?Co-Pi?cocatalyst,and then the PEC performance of the tandem photoelectrochemical cell was tested by combining the homojunction photoanode with Cu₂O/Pt photocathode.The results show that the dual axial gradient-doped?Zr and Sn?Fe₂O₃ nanorod film is successfully prepared by two-step hydrothermal method,and the photocurrent density reaches 1.64 mA/cm² at 1.23 V vs.RHE,which is 27.3 times than that of bare Fe₂O₃.By comparing single doping,uniform doping and dual axial gradient doping,it is proved that the dual axial gradient-doping not only increases the carrier concentration but reduces the surface trap state by Y-axial gradient Sn doping and widens the band bending and increases the electric field in band bending region by X-axial gradient Zr doping,thereby greatly facilitating the separation and transfer of carriers.Then,Ca doping leads?-Fe₂O₃ into a p-type conductivity,and the photocurrent density is 2.14 mA/cm² at 1.23 V vs.RHE of the branched nanorod homojunction formed by the combination of n-type?-Fe₂O₃ with Ca-Fe₂O₃.The enhanced photocurrent can be attributed to that the special 3D branched structure could effectively avoid the formation of electron trapping sites at the surface of Fe₂O₃ NRs,resulting in the accelerated charge separation,additionally,the desired lattice matching p-n homojunction leads to appropriate band alignment and further facilitating the charge separation.After modified with Co-Pi cocatalyst,the photocurrent density of the homojunction reaches 2.42 mA/cm² at 1.23 V vs.RHE,and the water oxidation onset potential shifts to 0.64 V from 0.72 V.The tandem photoelectrochemical cell combined by Co-Pi/Ca-Fe₂O₃/Fe₂O₃ homojunction photoanode with Cu₂O/Pt photocathode reaches a photocurrent of about 0.04mA/cm² after 3000 s of illumination without an external bias.The present work preliminarily confirms the viewpoint that the photoelectrode can realize solar-to-hydrogen directly without external bias,and provides a promising method of the application of photoelectrochemical water splitting.