Fabrication And Catalytic Properties Of Novel S-scheme-based Heterojunction Photocatalysts

Semiconductor-based photocatalysis is one of the most promising strategies that have been recently developed for environmental remediation and energy production.Photocatalyst materials could convert the free solar irradiation to value-added energy sources as well as decompose the various types of organic and inorganic contaminations to more harmless products.Generally,single photocatalysts have low photocatalytic efficiency which is primarily due to poor visible light absorption and weak redox ability.To enhance the visible light absorption,a semiconductor has to own narrow band gap.On the other hand,to improve the redox ability,a semiconductor should have a more negative conduction band and more positive valance band,meaning a wide band gap energy.These two requirements are in conflict,which reflects that a single semiconductor could not be able to concurrently meet these conditions.In addition,fast recombination rate of charge carriers is the most common obstacle that substantially reduce the photocatalytic efficiency of semiconductors.For these reasons,single semiconductors are required to be modified using an efficient and cost-effective method.To overcome these limitations,various strategies such as dye-sensitization,metal and nonmetal doping,morphology control,combination with carbon-based materials and coupling with an appropriate semiconductor have been developed.Among them,semiconductor-semiconductor heterojunctions have become a hot research topic which attracted massive researchers’attention,opening new opportunities to design more efficient hybrid photocatalysts and broaden the application of photocatalyst materials in environmental purification and energy production.These types of heterojunctions could tackle both thermodynamic（poor visible light absorption and weak redox ability）and kinetic（rapid recombination of charge carriers）drawbacks of single semiconductors,aiming to the construction of a hybrid photocatalyst with improved efficiency.In this regard,Type-II and Z-scheme family heterojunctions have been explored,but they have revealed some fundamental thermodynamic limitations.In this regard,S-scheme-based heterojunctions are the most recently developed semiconductor-based photocatalysts,which could address the shortcomings of the previously proposed heterojunction systems.S-scheme heterojunctions are consisted of at least two semiconductors,including one oxidation semiconductor and one reduction semiconductor.In this new type of heterojunctions,photoexcited electrons and holes with weak redox potential are recombined and those with the highest redox ability are reserved,which could be participated in the photocatalytic oxidation and reduction reactions.Interestingly,two narrow band gap semiconductors could be coupled,and this way an S-scheme hybrid photocatalyst with improved visible light harvesting could be obtained.In addition,the electrons and holes are separated on different semiconductors and in this way the recombination rate of charge carriers are efficiently inhibited.It should be noted that some structural defects such as oxygen vacancy sites are usually created at the interfacial contacts,which could create new band levels below the conduction band,resulting in the band gap narrowing and thus increasing the visible light absorption.This dissertation is focused on the fabrication of S-scheme heterojunctions with high-performance photocatalytic activity upon the visible light irradiation for purification of aqueous solution as well as production of hydrogen energy.It is worth noting that Malachite Green（MG）and Bisphenol A（BPA）was selected as colored and colorless model pollutants.Briefly,we selected Bi₂WO₆,Zn O and WO₃ as the base single semiconductors and modified their electronic structure by coupling with Zn Bi₂O₄or Ce O₂ cocatalysts,aiming to the fabrication of efficient visible-responsive S-scheme heterojunction photocatalysts.The charge transfer mechanism of the proposed Bi₂WO₆-Zn Bi₂O₄,Zn O-Zn Bi₂O₄and Ce O₂-WO₃heterojunctions has not been previously studied.These heterojunctions are firstly designed,characterized and thoroughly investigated in this dissertation.The first chapter of the present dissertation is general discussion on the fundamentals and challenges of heterojunction photocatalysts.The second chapter provides the details on the starting materials,preparation methods,characterization technique,and photocatalytic experiments.In chapters 3,4 and 5,we prepared Bi₂WO₆-Zn Bi₂O₄,Zn O-Zn Bi₂O₄and Ce O₂-WO₃ S-scheme heterojunctions,respectively,and applied them for photocatalytic water purification and hydrogen production under visible light/simulated sunlight irradiation.These chapters cover the following abstract:In chapter 3,novel 2D/2D S-scheme Bi₂WO₆-Zn Bi₂O₄ heterojunction was synthesized through combination of sonochemical and hydrothermal procedures.The results revealed that sonochemical pretreatment could alter the morphology of the obtained heterojunction from spherical to flake-like during the hydrothermal procedure.Bi₂WO₆-20Zn Bi₂O₄ demonstrated the highest photocatalytic performance compared to the pure samples.This improvement is associated with the efficient separation of charge carriers through 2D/2D interfacial contacts as well as enhanced visible light absorption.However,further Zn Bi₂O₄ loading had no significant effect on the enhancement of photocatalytic activity owing to the shielding effect of Zn Bi₂O₄ against incident light.Based on the obtained results from active species trapping experiments,electron spin resonance and Mott-Schottky calculations,S-scheme charge transfer pathway was proposed as the predominant mechanism for the photocatalytic reactions.In chapter 4,0D/1D Zn Bi₂O₄/Zn O heterojunction was successfully synthesized though tha facile hydrothermal procedure.The highest H₂ production(3.44 mmol g^-1 h^-1)was obtained on Zn O-20Zn Bi₂O₄ sample,which is around 12.7 times greater than pure Zn O.According to the HRTEM result,the intimate interfacial connections are formed between Zn O and Zn Bi₂O₄ which could act as trapping centers for charge carriers and results in boosted photocatalytic activity.Further,a high aspect ratio of 1D Zn O nanorods and small size of 0D Zn Bi₂O₄nanoparticles（～10 nm）increases the number of interfacial contacts and thus the charge carriers’recombination was suppressed more efficiently.Based on the trapping experiments,ESR and Mott-Schottky analysis,Zn Bi₂O₄-Zn O hybrid photocatalyst followed the S-scheme charge transfer mechanism.In chapter 5,hierarchical 3D WO₃-Ce O₂ hollow sphere heterojunction was fabricated via hydrothermal followed by precipitation method.The incorporation of Ce O₂ into the WO₃ microspheres was confirmed by XRD,BET,HRTEM,and XPS analysis.The results revealed that the photocatalytic activity of WO₃-Ce O₂heterojunction was greatly enhanced compared to pure Ce O₂ and WO₃ samples,and the best photocatalytic activity was achieved on the WO₃-30Ce O₂ heterojunction.This could be ascribed to the efficient separation of charge carriers which was facilitated by oxygen vacancies formed at the interfaces of two coupled semiconductors.EIS analysis verified that the charge transfer resistance of WO₃-30Ce O₂ heterojunction was decreased,which is due to the heterojunction effect.Moreover,based on the Mott-Schottky calculations,radical trapping experiments and ESR analysis,hydroxide radicals were identified as the main active species,and an S-scheme charge transfer mechanism was suggested to explain the enhanced photocatalytic activity.