Interfacial Regulation Of Hematite Based Heterojunction And Its Effect On Photoelectrochemical Performance

Environmental issues and energy crisis are increasingly serious,and it is urgentto develop renewable energy sources.Solar energy has the superiority of being universal,clean,renewable and powerful,which has been widely used in photovoltaic,photothermal and other aspects.However,the intermittent and fluctuating nature of solar energy greatly limits its application.Hydrogen energy has many advantages such as high energy density,cleanness,non-pollution,and diverse utilization modes,which is considered as an ideal form of energy storage.Photoelectrochemical（PEC）water splitting can convert solar energy into hydrogen and store it,which is a promising energy technology.Water splitting includes hydrogen evolution and oxygen evolution reactions.The oxygen evolution reaction is a highly complex proton coupled multi-electron transfer process,which is the main obstacle restricting the efficiency of water splitting.Therefore,the development of highly efficient and stable photoanodes is of importance for researchers to improve the photoelectrochemical efficiency.Hematite（α-Fe₂O₃）has become one of the most popular photoanode materials because of its suitable band-gap,good chemical stability,non-toxicity and abundant reserves.Although hematite has the aforementioned advantages,the inherent drawbacks significantly limit its application in water splitting including band location mismatch,slow oxygen evolution kinetics,serious charge recombination and low carrier mobility.Therefore,it is necessary to modify hematite by various means to improve its photoelectrochemical efficiency,including morphology engineering,elemental doping,loading cocatalysts and construction of heterojunctions.It has been demonstrated an effective way to fabricate efficient composite photoanodes by constructing heterojunctions with suitable materials.Recently,metal chalcogenides have attracted extensive attention due to their characteristic properties such as suitable band-gap and band position,high carrier mobility and large specific surface area as well as good photo/electrocatalytic activity.Based on this,we combined hematite and chalcogenides to construct efficient heterojunction photoelectrodes by adjusting the material morphology,structure and interface,and studied its photoelectrochemical mechanism.The main research contents and results are as follows:（1）Covalent S-O bonding enables enhanced photoelectrochemical performance of Cu₂S/Fe₂O₃ heterojunction for water splitting:The Cu₂S/Fe₂O₃ heterojunction was constructed by deposition of Cu₂S nanoparticles on the surface of hematite nanorods by successive ion layer adsorption and reaction.The results show that S-O covalent bond can be formed between Cu₂S and Fe₂O₃,which can be strengthened by annealing.Compared with bare Fe₂O₃ photoanode,Cu₂S/Fe₂O₃ heterojunction can improve the charge separation and transfer efficiency,broaden the light absorption range,and inhibit the carrier recombination.More importantly,the S-O bond formed at the interface between Cu₂S and Fe₂O₃ makes the contact closer,reduces the interfacial contact resistance,and promotes the charge transfer.In addition,due to the photothermal properties of Cu₂S,the heterostructure shows a higher photothermal effect,which increases the temperature of the local reaction environment and improves the rate of oxygen evolution reaction.The results show that the photocurrent density of Cu₂S/Fe₂O₃ heterojunction photoanode increases by 177%to 1.19 mA cm^-2 at 1.23 V vs.RHE,and the formation of S-O covalent bond significantly enhances the photoelectrochemical activity and stability.（2）Design and synthesis of NiSe₂/Fe₂O₃ heterojunction and its photoelectrochemical performance for water splitting:We integrated the NiSe₂ nanoparticles on the surface of Ti:Fe₂O₃ films for the photoelectrochemical water oxidation.The resulting heterostructure photoanode exhibited significantly enhanced PEC performance with a current density of 1.14 mA cm^-2 at 1.23 V vs.RHE.The investigations on mechanism reveals that a build-in electric field was formed to accelerate the charge transfer and separation,the charge carrier concentration was significantly enhanced by the formation of oxygen vacancies induced by NiSe₂,the lifetime of photogenerated electrons and holes were substantially prolongated due to the suppression of recombination by the surface passivation and promoted surface kinetics for water oxidation by of Se-O species.This work brings inspiration to integrate multifunctional modifiers on photoelectrodes that allows for addressing several issues simultaneously and consequently improving the PEC performance for hydrogen production via water splitting.