Preparation Of ?-Fe₂O₃ Based Photoanode And Its Study On Photoelectrochemical Water Oxidation Performace

With the development of human society,the energy crisis and environmental pollution problems are increasing.The development of clean and sustainable energy has important strategic significance.Inexhaustible solar energy is one of the most promising clean and sustainable energy sources.However,the fluctuant and intermittent nature of insolation make such applications unsuitable for the storage and dispatch of solar energy for consumption.Therefore,converting of solar energy into the form of chemical bonds is an attractive approach for efficient,economical,and convenient utilization of solar energy.Among these methods,photoelectrochemical(PEC)water splitting for hydrogen production has long been regarded as one of the important ways to achieve clean and sustainable energy.In the PEC water splitting device,the semiconductor photoelectrode is the core component in the PEC water splitting,which has a decisive influence on the solar energy conversion of the equipment to produce hydrogen.Among the various candidates evaluated,hematite(?-Fe₂O₃)is one of the most promising photoanodes for water-splitting PEC cells due to its:(1)better absorption of visible light due to its narrow band gap(1.9².2 eV),(2)the valence band position is positive than the oxidation potential of water,(3)the theoretical conversion efficiency of solar hydrogen production can reach 15.4%,(4)the maximum photocurrent density of photolysis water can reach 12.6 mA cm^-2,(5)the strong photoelectrochemical stability,low cost,and abundant storage in nature.However,its shortcomings such as low absorption coefficient,poor conductivity,short carrier life,low carrier mobility,easy charge recombination and slow water oxidation kinetics have seriously affected the PEC performance of?-Fe₂O₃.So far,all reports on the PEC activity of?-Fe₂O₃ are far below its theoretical value,which hinders its practical application.This thesis takes?-Fe₂O₃ as the research object,in view of the above shortcomings,we adopt a series of methods including passivation layer modification,foreign atom doping,p-n heterostructure construction and co-catalyst loading to optimize and modify?-Fe₂O₃,so as to improve its PEC water oxidation performance.The research content is as follows:1.MIL-125(Ti)-derived TiO₂ layer and ultra-thin carbon layer as double passivation layer to reduce the surface state of?-Fe₂O₃,the?-FeOOH as co-catalyst to reduces the onset potential and improve the water oxidation kinetics of?-Fe₂O₃.The surface state is the recombination center of photo-generated charges,its presence is not conducive to the separation of charges,and directly affects the photocatalytic performance of?-Fe₂O₃.In this paper,?-Fe₂O₃/TiO₂/C/FeOOH photoanode is prepared in which TiO₂ layer derived from MIL-125(Ti)and an ultra-thin carbon layer as double passivation,and?-FeOOH as co-catalyst.This photoanode has efficient water oxidation performance.The reasons are as follows:the MIL-125(Ti)-derived TiO₂ layer with a large specific surface area is used as the first passivation layer,which largely passivated the?-Fe₂O₃ surface state and inhibited the recombination of photogenerated electrons and holes,what's more,it improves PEC performance,the photocurrent density of?-Fe₂O₃ water splitting increased from the initial 0.6 mAcm^-2 to 1.26 mAcm^-2;and more importantly,using an ultra-thin carbon layer with electron transport characteristics as the second passivation layer can further improve the passivation of the surface state and transfer electrons rapidly,thus effectively separating photogenerated electrons and holes,and the photocurrent density increased to 1.93 mA cm^-2;when further loaded with?-FeOOH co-catalyst,the kinetics of water oxidation was improved.Under 1.23V_RHE bias,the photocurrent density of?-Fe₂O₃/TiO₂/C/?-FeOOH photoanode reached 2.95 mA cm^-2,what's more,the onset potential is reduced and it shows a significant negative shift.2.Through doping Ce³⁺,constructing a MOF-derived p-Cu₂O/n-Ce-Fe₂O₃ p-n junction,and loading a co-catalyst to improve the conductivity,charge separation efficiency,and reduce the onset potential,further improving the PEC water oxidation performance of?-Fe₂O₃.The rare earth element Ce has rich energy levels and 4f electronic transition characteristics.The experimental results show that doping Ce can improve the conductivity and photocurrent density of?-Fe₂O₃.In this paper,the p-Cu₂O/n-Ce-Fe₂O₃energy level matching type II p-n heterojunction photoanode is constructed by coupling p-Cu₂O derived from HKUST-1 with n-type Ce-Fe₂O₃.The p-Cu₂O/n-Ce-Fe₂O₃photoanode has excellent charge separation efficiency and efficient photocatalytic water oxidation performance,and the photocurrent density can reach 3.2 mA cm^-2 under1.23 V_RHE bias.The analysis of the results shows that the photogenerated charges are effectively separated by the built-in electric field of the p-n junction formed by p-Cu₂O/n-Ce-Fe₂O₃.At the same time,MOF-derived Cu₂O has a large specific surface area and a pore-rich structure,which can provide more active sites for water oxidation.After further loading with FeOOH cocatalyst,the photocurrent density can reach 4.2mA cm^-2,which is 7.24 times that of unmodified?-Fe₂O₃,and the onset potential shifted negatively by 80 mV.3.Coupling ZIF-67 with Ce-Fe₂O₃ to improve the PEC water splitting activity of Ce-Fe₂O₃.The Ce-Fe₂O₃/ZIF-67 photoanode is prepared by hydrothermal and immersion deposition methods.The experimental results show that ZIF-67 improves the Ce-Fe₂O₃light absorption intensity,charge separation efficiency and photocurrent density,and also reduces the surface charge transport resistance.This is because ZIF-67 has a large specific surface area and a rich pore structure to passivate the Ce-Fe₂O₃ surface state and improve the separation efficiency of photogenerated charges,and what is imortant is that ZIF-67 and Ce-Fe₂O₃ form a band-matched II heterojunction can effectively inhibit the recombination of photogenerated electrons and holes.At a bias voltage of1.23 V_RHE,the photocurrent density reached 2.07 mA cm^-22 and the initial potential shifted negatively by 61 mV.