Design Of Heterojunction Structure Based On Metal Oxides/Sulfides And Photoelectrochemical Performance

Photoelectrochemical water splitting technology based on semiconductor materials can directly convert solar energy into high chemical energy density,which is one of the effective ways to achieve environmental pollution control and clean energy production.Among them,developing the efficient and stable semiconductors with desired heterojunction structure and catalytic activity are the key steps to realize this process.Here,we aim to the semiconductor heterojunctions and completed three works:1)Developed a new method to construct a multilayer heterojunction based on the successive ionic layer adsorption and reaction method(SILAR);2)Studied the relationship between low-dimensional van der Waals heterojunction and its interface charge characteristics;3)Discussed the physical mechanism of the enhancement of low energy light absorption by the heterojunctions.We use a variety of control mechanisms and theoretical models to analyze the above-mentioned semiconductor heterojunctions from the perspectives of band structure,interface charge interaction,charge separation and transport at the interface,and oxidation kinetics at the solid/liquid interface.The physical mechanism of photoelectrochemical water splitting was investigated and the research contents are as follows:1.The loose and porous Bi₂Mo O₆@Bi₂Mo₂O₉ heterojunction was prepared on FTO substrate by the one-step SILAR method.By adjusting the precursor anion and cation concentration ratio,two bismuth molybdate crystals were controllably constructed.The photoelectrochemical results showed that the photocurrent density of Bi₂Mo O₆@Bi₂Mo₂O₉ photoanode under AM 1.5G and 1.23 V vs.RHE was 3.6 and 8.0times higher than that of Bi₂Mo O₆ and Bi₂Mo₂O₉,respectively.The first-principle calculation of the semiconductors'band structure proved the integrated photoanode was a Type II heterojunction,which was conducive to charge separation and transfer.Experimental results inddicaed that the charge separation efficiency and oxidation kinetics efficiency of the heterojunction at 1.23 V vs.RHE reached 20.5%and 54.6%,suggesting a great improvement compared with bare semiconductor.In addition,Bi₂Mo O₆@Bi₂Mo₂O₉ heterojunction exhibited excellent stability in neutral solution,and potentially in commercial application.2.Based on Bi₂Mo₂O₉ and Bi₂S₃ semiconductors,we designed the pn/Z-Scheme/pn four-layer heterojunction photoanode(Bi₂S₃/Bi₂Mo₂O₉/i₂S₃/Bi₂Mo₂O₉)for the first time through the SILAR method.By constructing a third Bi₂Mo₂O₉ thin layer,the four-layer heterojunction could achieve a photocurrent density of 3.1 m A cm^-2at 1.23 V vs.RHE under one sunlight,which was much higher than the double-layer heterojunction Bi₂S₃/Bi₂Mo₂O₉(1.7 m A cm^-2).Phoetoelectrocatalytic kinetics studies implied that the charge separation efficiency and oxidation kinetic efficiency of the multilayer were 41%and 81%at 1.23 V vs.RHE.Mechanism analysis indicated the two Bi₂S₃ layers in the multilayer enhanced the photo absorption;the third Bi₂Mo₂O₉layer formed double p-n junctions at the interfaces through interfacial charge interaction.Hence,the heterojunction promotes the transfer of holes to the surface;the Z-Scheme structure at the second interface realizes the rapid carriers'recombination to accelerate the charge separation between the bilayers of Bi₂S₃.3.A low-dimensional van der Waals heterojunction In₂S₃@Bi₂S₃ was constructed by 1D Bi₂S₃ and 2D In₂S₃ semiconductors.Theoretical calculations showed that 2D In₂S₃ and 1D Bi₂S₃ was integrated through van der Waals interaction.Both experimental and theoretical calculation results identified the surface potential of Bi₂S₃ was higher than that of In₂S₃,proving a certain potential difference was existed at the interface,inducing the built-in electric field from Bi₂S₃ to In₂S₃ by carriers'redistribution.Thereby,promoting the transfer of photogenerated holes to In₂S₃ and the transfer of photogenerated electrons to Bi₂S₃.Under AM 1.5G illumination and 1.23 V vs.RHE,the water oxidation photocurrent of In₂S₃@Bi₂S₃ reached 2.0 m A cm^-2,which was 5.0times higher than Bi₂S₃.This work reveals the physical mechanism of charge interaction at interface,and provides a reference for studying the relationship between the built-in electric field and the enhanced charge transfer.4.A fluorescent-semiconductor heterojunction was constructed for photoelectrochemical water splitting.Here,fluorescent materials were employed to absorb low energy photons,and release high energy photons through an up-conversion process,thereby promoting the light absorption performance of semiconductors.We choose NaYF₄:Yb,Er as up-conversion fluorescent materials,and mixing Ti O₂,Ta₂O₅,In₂O₃ and Fe₂O₃ to construct heterojunctions.The results showed that the fluorescence-semiconductor heterojunction based on Fe₂O₃ could increase the photocurrent density by 70.1%.The 980 nm excitation fluorescence spectrum proved the intensity of upconversion fluorescence emission peak of Fe₂O₃/NaYF₄:Yb,Er was weaker than that of NaYF₄:Yb,Er,revealing NaYF₄:Yb,Er and Fe₂O₃ had energy transfer process by light absorption.The fluorescence upconversion emission spectrum of NaYF₄:Yb,Er and the ultraviolet-visible absorption spectrum of Fe₂O₃ were partially overlaped,which supported the mechanism of energy radiation-reabsorption within heterojunctions.This study provides new ideas for expanding the absorption of low-energy photons by fluorescent-semiconductor heterojunction.In summary,a variety of novel heterojunctions,such as Type II,pn/Z-Scheme/pn heterojunction,van der Waals heterojunction,and fluorescence-semiconductor heterojunction were successfully constructed.We investigated the photoelectrochemical activity at photoelectrode's surfaces and interfaces,and developed a new preparation method for multilayer heterojunctions.Our results reveal the influence mechanism of charge redistribution at the interface of low-dimensional heterojunctions,and established a fluorescence-semiconductor heterojunction towards to radiation-eabsorption as well.The water splitting performances of these heterojunctions are significantly improved,and therefore provide a reference for solar energy harvesting and other new optoelectronic micro-nano devices.