The Construction Of BiVO₄ Based S-Scheme Heterojunction And Its Photoelectrochemical Properties

Photoelectrochemical（PEC）water splitting technology for hydrogen production is an ideal way to obtain hydrogen energy.However,the solar-to-hydrogen（STH）conversion efficiency of PEC water splitting technology for hydrogen production is still low,which limits its wide application.Construction of the high-performance photoanode materials is the key to achieve efficient PEC technology for hydrogen production.Due to the ability of S-scheme heterojunctions to achieve efficient separation of photogenerated electron-hole pairs with strong oxidation and reduction ability,the construction of S-scheme heterojunction powder photocatalysts has attracted extensive attention in the field of photocatalysis in recent years.However,on the one hand,the application of S-scheme heterojunctions as photoanodes for photoelectrochemical water splitting for hydrogen production has rarely been reported;on the other hand,the effect of the energy band structure of S-scheme heterojunctions on the separation and transport of photogenerated electron-hole pairs has not been studied.Therefore,in this paper,we have studied the construction of high-efficiency S-scheme heterojunction photoanodes.BiVO₄thin film is used as the base material and compounding with TiO₂,WO₃andα-Fe₂O₃with different band gaps to prepare S-scheme heterojunction photoanodes with different energy band structures.Then the photoelectrochemical properties were investigated separately to probe the effect of energy band structure on the separation and transport of photogenerated electron-hole pairs in S-scheme heterojunctions,which provides a new theoretical basis and approach for the design and preparation of highly efficient photoanodes.The main studies are as follows:（1）The BiVO₄/TiO₂heterojunction films were prepared by depositing TiO₂with wide band gap on the surface of BiVO₄films using a simple soaking method.The characterization results show that anatase TiO₂nanocrystal particles are wrapped on the surface of BiVO₄particles,forming BiVO₄/TiO₂heterojunctions.The photochemical performance tests show that the photocurrent density of BiVO₄/TiO₂thin films can reach 2.2 m A/cm²（1.23 V vs.RHE）without co-catalyst,which is 1.6 times higher than that of pure BiVO₄thin films.The photoelectrochemical analysis shows that the constructed BiVO₄/TiO₂heterojunction can significantly improve the separation and transfer efficiency of photogenerated charges compared with the pure BiVO₄film,which is due to the formation of S-scheme BiVO₄/TiO₂heterojunction that not only promotes the separation of photogenerated charges and preserves the high redox ability of photogenerated electrons and holes,but also improving charge transport of the BiVO₄/TiO₂photoanode/electrolyte interface.（2）S-scheme BiVO₄/WO₃heterojunctions were prepared by depositing WO₃with a narrow band gap on the surface of BiVO₄films using a surface drop coating method.The characterization results show that WO₃nanocrystals are deposited on the surface of BiVO₄particles to form BiVO₄/WO₃heterojunctions.The photocurrent density of BiVO₄/WO₃films reaches 3.2 m A/cm²（1.23 V vs.RHE）,which is 2.4 times higher than that of pure BiVO₄films;the photocurrent density is further increased to 4.6 m A/cm²with co-catalyst.The photoelectrochemical analysis shows that the coupling with the narrower bandgap WO₃semiconductor can further increase the charge separation and transfer efficiency of the BiVO₄photoanode.This may be attributed to the S-scheme BiVO₄/WO₃heterojunction formed by coupling with the narrower bandgap WO₃,which can both increase the concentration and improve the separation efficiency of photogenerated electrons and holes,and retain more photogenerated electrons and holes with high redox capability.（3）S-scheme BiVO₄/α-Fe₂O₃heterojunctions were prepared by coupling narrower bandgapα-Fe₂O₃with BiVO₄using a simple immersion method.The characterization results show thatα-Fe₂O₃nanoparticles are coated on the surface of BiVO₄particles to form S-scheme BiVO₄/α-Fe₂O₃heterojunctions.The photocurrent density of BiVO₄/α-Fe₂O₃films reaches 2.2 m A/cm²（1.23 V vs.RHE）,which is 1.6 times higher than that of pure BiVO₄films.The photochemical analysis shows that the transfer efficiency of photogenerated electrons and holes for BiVO₄/α-Fe₂O₃is significantly higher than pure BiVO₄films,while the separation efficiency is not significantly improved.This may be due to the fact that the surface deposition ofα-Fe₂O₃can increase the rate of water oxidation reaction on the photoanode surface,whereas the lower conductivity and light absorption coefficient ofα-Fe₂O₃are not favorable for the separation of photogenerated electrons and holes.Further comparisons of the three systems reveal that the PEC performance of S-scheme heterojunction photoanodes is closely related to the energy band structure,conductivity and light absorption properties of materials.