Interfacial Modulation Of Hematite-Based Heterostructure Photoanodes For Efficient Photoelectrochemical Water Oxidation

Photoelectrochemical（PEC）water splitting,converting solar energy into high energy density and clean hydrogen energy,is an ideal solution to solve environmental pollution and alleviate energy crisis at the same time.Given the high overpotential and slow kinetics of the water oxidation reaction,the key to the application of PEC water splitting for hydrogen production lies in the development of efficient semiconductor photoanodes to improve the water oxidation reaction.The water oxidation reaction on photoanodes consists of three basic processes:（1）the semiconductor absorbs photons with energy greater than or equal to its forbidden band width to generate electron-hole pairs;（2）the carriers are separated and transported in opposite directions under the action of the applied bias and the built-in electric field,and the holes migrate toward the semiconductor-electrolyte interface;and（3）the holes cross the semiconductor-electrolyte interface to oxidize water to release oxygen.Hematite（α-Fe₂O₃）is an ideal candidate for photoanode due to its excellent stability,large natural abundance,and suitable band position.However,its poor electrical conductivity,low carrier separation efficiency,and large overpotential still limit its practical water oxidation performance.To address these problems,several generations of researchers have developed a series of feasible measures,such as doping for significantly improving the conductivity ofα-Fe₂O₃,heterojunction engineering for improving the charge separation efficiency,and co-catalysts for improving the surface hole injection efficiency and reducing the overpotential.However,the improved water oxidation performance by these classical modifications is still far from the requirements of the application.To further achieve more efficient PEC water splitting,this paper further improves the charge separation and transfer between heterogeneous interfaces by coupling chemical bonds or modifying multifunctional interlayers between the interfaces of semiconductor photoanodes and heterojunctions/co-catalysts.A deep understanding of the mechanism of interfacial chemical bonding/functional layers on the composite photoanode,the mechanism of photogenerated charge transport and migration between the layers,and the complex reaction kinetics at the multiphase interface is essential to achieve efficient photoelectrochemical performance.It is essential for efficient PEC water oxidation to understand deeply the mechanism of interfacial chemical bonding/functional layers on the composite photoanode,the mechanism of photogenerated charge transport and migration between the layers,and the complex reaction kinetics at the multiphase interface.In this paper,we used F^-and Gd³⁺-dopedα-Fe₂O₃（F-Fe₂O₃,Gd-Fe₂O₃）photoanodes as models,respectively,to investigate in depth the interfacial modulation of hetero-structured photoanodes by interfacial chemical bonding or interfacial functional layers,which in turn affects the charge transport behavior between heterogeneous interfaces,and also focused on the passivation of surface trap states and the improvement of PEC water oxidation kinetics by these modification measures.The details of the study are as follows.（1）Type-II heterojunctions bonded with interfacial S-O bonds are used for enhanced charge separation and transport.In this section,type-II heterojunctions were constructed by anchoring In₂S₃nanoparticles on the surface of F-Fe₂O₃ photoanodes.The F doping increased the donor density and decreased the charge transfer resistance,which in turn improved the electrical conductivity ofα-Fe₂O₃.The formation of interfacial S-O bonds was demonstrated by comparing In₂S₃/F-Fe₂O₃ type-II heterojunctions prepared by both physisorption and chemical bonding,and revealed that interfacial S-O bonds significantly improved the two-phase lattice mismatch as well as significantly reduced the surface defect states caused by suspended bonds on the crystal surface.The In₂S₃/F-Fe₂O₃ type-II heterojunction bonded by interfacial S-O chemical bonding exhibits a photocurrent density of 2.21 m A·cm^-2 at 1.23 V vs.RHE,which is about 3.45 times that of the pristineα-Fe₂O₃.This is attributed to the type-II band alignment between F-Fe₂O₃ and In₂S₃ that provides a strong driving force for charge separation and transport,reduced charge transfer resistance,and interfacial chemical bonding that reduces the surface states and thus improves carrier recombination.（2）Surface reconstruction of the co-catalysts and interfacial conjugated functional layers regulate the kinetics of water oxidation.In this section,we first uniformly coated an ultrα-thin layer of cobalt silicate（Co-Sil）co-catalyst on the surface of F-Fe₂O₃ photoanode by photo-assisted electrophoretic deposition（PEPD）,and systematically investigated the surface reconstruction of the co-catalyst as well as the morphological modulation and electronic states of cobalt ions under different experimental preparation conditions.The cobalt species underwent surface reconstruction,resulting in a significant increase in their average oxidation state,which is more favorable for the water oxidation reaction;the presence of silicate groups could also reduce the adsorption energy of OOH·.This synergistically reduces the onset potential and overpotential,enhances the charge separation efficiency and significantly improves the surface water oxidation kinetics.Further,we used the Gd-Fe₂O₃ photoanode as a model,and introduced a conjugated covalent triazine framework（CTF-BTh）interfacial functional layer by in situ electro-polymerization between the Gd-Fe₂O₃ and Co-Sil co-catalyst.Detailed characterization and PEC tests revealed the modulation of the chemical microenvironment and catalytic activity center at the interface between the Gd-Fe₂O₃ photoanode and the Co-Sil co-catalyst by CTF-BTh.In particular,the coordination of N and S in CTF-BTh with Co ions in Co-Sil provides a channel for charge flow and also enhances the average oxidation state of Co species,thus accelerating the water oxidation reaction.At the same time,the conjugated system of CTF-BTh is easy for electron delocalization,which effectively suppresses the recombination of electron-hole pairs between heterogeneous interfaces.In addition,Gd doping increased the donor density,reduced the charge transfer resistance,and improved the electrical conductivity ofα-Fe₂O₃,and the proton-coupled electron transfer（PCET）process during Co-Sil-catalyzed water oxidation significantly accelerated the hole trapping and utilization.These modifications synergistically promoted surface charge transfer and improved hole utilization for more efficient water oxidation.（3）Multifunctional interfacial non-conjugated polymer ultrathin intercalation improves the water oxidation performance of p-n junction photoanodes.In this section,an insulating non-conjugated polymer ultrα-thin layer（poly diallyldimethylammonium chloride,PDDA）was constructed by self-assembly between n-type Gd-Fe₂O₃ photoanode and Co₃O₄ nanoparticles with p-n junction and cocatalyst dual functions,forming a unique p-type semiconductor/insulating layer/n-type semiconductor（p-i-n）hybrid photoanode（Co₃O₄/PDDA/Gd-Fe₂O₃）,and the modulation effect of the insulating thin layer on the p-n junction interface was deeply studied.First,PDDA modulates the band structure of the composite photoanode so that the p-i-n junction has a larger band bending than the p-n junction,which further facilitates the carrier separation.Further,PDDA also passivates the surface traps,and provided more active sites for water oxidation in concert with Co₃O₄.Finally,PDDA also modulates the water oxidation kinetics,specifically,PDDA can both pump out holes and adsorb water oxidation reactants,thus effectively transferring photogenerated holes to Co₃O₄ and promoting the positive water oxidation reaction.Under these positive modulations,the"p-i-n"junction has better photoelectrochemical water oxidation performance than the p-n junction.