Preparation Of Hematite-based Photoanodes And Photocatalytic Performance Study

Photoelectrochemical（PEC）decomposition of water is an efficient,environmentally friendly and sustainable technology for decomposing water to produce hydrogen.Photoelectrode materials absorb sunlight to produce photogenerated electrons and holes,which are separated by an external circuit to oxidize and reduce water to oxygen and hydrogen at the working electrode and counter electrode respectively,which can effectively solve global problems such as fossil energy shortage and greenhouse effect.Hematite can effectively utilize visible light and has excellent optical properties and chemical stability,while the element Fe is widely sourced,environmentally friendly and inexpensive.α-Fe₂O₃ is widely considered to be the most promising photoanode material.However,the conductivity ofα-Fe₂O₃ needs to be further improved to effectively facilitate photogenerated carrier transport.In addition,the surface ofα-Fe₂O₃ lacks an effective four-electron water oxidation reaction active site.As a result,the actual photocurrent density that can be generated byα-Fe₂O₃ is much lower than the theoretical calculated value.To address these problems,this thesis will start from both doping and water oxidation co-catalyst construction to enhance the photochemical properties ofα-Fe₂O₃,as follows.（1）Using hydrothermal method to prepare Ti-dopedα-Fe₂O₃ nanorods and optimize doping concentration and annealing process parameters,with a Ti doping amount of 30μL,annealing temperature of 750℃,annealing time of 5 min,and an external bias of 1.23 V vs.RHE,the sample TF3 exhibited the highest photocurrent density(J_ph)of 0.38 m A·cm^-2,which is 5.4 times that of the undoped sample.XPS analysis shows that the high-valent Ti doping can increase the electron density ofα-Fe₂O₃,and Mott-Schottky analysis shows that the carrier concentration of Ti:Fe₂O₃(5.03×10¹⁷cm^-3)is three times that of the undoped sample,which is the reason for the improvement ofα-Fe₂O₃ conductivity.At the same time,the introduction of Ti element increases the surface hydroxyl density ofα-Fe₂O₃,which is beneficial for the interaction and charge transfer betweenα-Fe₂O₃ and water.The results of the photoinduced charge injection efficiency(η_inj)show that Ti-dopedα-Fe₂O₃ makes it easier for photo-generated holes（h⁺）to transfer to the liquid phase,thereby exhibiting higher photoinduced charge separation efficiency(η_sep),with TF3 sample’sη_inj andη_sepbeing 1.5 times and 3.7 times those of the undoped sample,respectively.Finally,TF3sample’s IPEC and ABPE are 2.4 and 1.75 times those of the undoped sample.（2）To further increase the injection and separation of photo-generated charges,an amorphous CoFeO_x co-catalyst layer was constructed on the Ti:Fe₂O₃ photoanode surface via a liquid-phase method,and the Co/Fe ratio and deposition time of the co-catalyst layer were optimized.When the Co/Fe molar ratio was 3:1 and the deposition time was 60 min,TF3/CF1-60 exhibited the highest J_ph(0.87 m A·cm^-2)at an external bias of 1.23 V vs.RHE,which is 2.3 times that of Ti:Fe₂O₃.The reasons for the improvement of the photoelectrochemical water oxidation performance by the amorphous CoFeO_x catalyst layer were analyzed using tests such as OCP,light-dark transient current curves,andη_inj curves.After covering the CoFeO_x amorphous layer,the OCP increased to 1.95 times that of the Ti:Fe₂O₃ photoanode,and the ratio of transient/steady-state photocurrent decreased from 1.11 to 1.03,indicating that the CoFeO_x amorphous layer can passivate surface states and increase the band bending,which can serve as a water oxidation co-catalyst to accelerate the water oxidation process.In addition,theη_inj of the TF3/CF1-60 sample increased by 1.7 times,and the peak of the chopping current curve of the photoelectric current-time curve decreased,indicating that the holes were no longer concentrated on the photoanode surface but injected into the electrolyte to participate in the water oxidation reaction.The CoFeO_xamorphous layer promotes charge transfer at the semiconductor/electrolyte interface.