Metal-organic Frameworks Derived Composite Photocatalysts And Their Applications In Photocatalysis

The conversion of solar energy into chemical energy in the presence of photocatalyst has attracted extensive attention to relieve future energy shortage and environmental issues.Generally,a complete photocatalytic process includes light absorption,e-h separation and redox reactions.Thus,the efficient photocatalytic performance required photocatalysts with a broadly visible light adsorption,excellent charge separation efficiency,long-term stability and strong redox ability.Recently,many reports have confirmed that metal-organic frameworks（MOFs）can be readily employed as photocatalysts or templates to fabricate heterojunction photocatalysts.Herein,a MOF-templated strategy has been developed in our work to prepare a series of heterojunction photocatalysts with superior photocatalytic activity.The main research contents are as follows:In chapter two,as everyone knows,loading cocatalysts can effectively enhance the surface hydrogen reduction in photocatalytic water splitting by introducing a positive Schottky barrier.NiS has been regarded as a promising cocatalyst to replace the noble metals due to its low cost and equivalent or even better performance.However,there has been a huge controversy over whether the NiS cocatalyst is used to trap electrons or holes in photocatalytic process.In this chapter,a new type of NiS decorated ZnO/ZnS nanorod heterostructure photocatalysts is first designed from the corresponding bimetallic–Zn Ni–MOFs.The Zn species in the bimetallic–MOFs can spatially separate the Ni species to restrain their aggregation,which is beneficial for the formation of NiS with a small enough size.The optimal heterostructures photocatalyst exhibits excellent hydrogen production rate of 27 mmol g^-1 h^-1,which is about 7 times higher than that of ZnO/ZnS heterostructure.XPS and OCP characterizations disclose that NiS can effectively facilitate the migration of the electrons.DFT calculations,including differential charge density,Mulliken population analyses and d–band center,intuitively reveal that the real role of NiS in photocatalytic process is to capture the electrons rather than the holes.In chapter three,we first combine the p–type Co₃O₄ cocatalysts with n–type ZnO@ZnS core–shell heterojunction to create a Co₃O₄–decorated ZnO@ZnS p-n junction by using bimetallic ZnCo–MOFs as template.In this hybrid,both the heterojunction and p–n junction can accelerate the bulk charge separation,while the highly dispersed Co₃O₄ cocatalysts can enhance the surface oxygen evolution reaction kinetics.As a result,the outstanding H₂ and O₂ production rates of 3853 and 1927μmol g^-1 h^-1 were obtained for the optimal sample.Such excellent performance can be evidenced by charge separation efficiency calculation.XAFS,XPS and DFT calculation indicated that the electrons and holes can be effectively assembled on ZnO and Co₃O₄,respectively.This work provides a new strategy for combining the advantages of cocatalyst and p–n junction as well as the core–shell heterojunction to enhance the photocatalytic water splitting without sacrificial agent.In chapter four,two types of Z-scheme photocatalysts,namely In₂S₃-Bi₂S₃（IBS）and In₂O₃-Bi₂O₃（IBO）have been successfully prepared from the same bimetallic metal-organic frameworks（InBi-MOFs）.The IBS was constructed with a full hollow structure owing to the different diffusion velocity of the metal ions and S^2-ions,while the IBO was formed with a yolk-shell structure due to the existence of the temperature gradient from the exterior shell to the inner yolk in the pyrolysis process.The IBS and IBO showed superior H₂ production rate of 22.73 and 3.6 mmol g^-1 h^-1,which were approximately 13.2 and 4.1 times higher than that of single In₂S₃ and In₂O₃ due to the effects of hierarchical hollow structures and Z-scheme heterojunction.The unique full hollow and yolk-shell structures could be clearly revealed by transmission electron microscopy,while the Z-scheme electron migrate pathway was thoroughly confirmed by various experimental studies and density functional theory calculation.In chapter five,using MIL-68（In）as template,indium vacancy(V_In)defects are successfully introduced into In₂S₃ via adding excess sulfur source during the hydrothermal reaction.Then an ion exchange reaction is employed to integrate defective In_2-xS₃ and Cd_1+xIn_2-xS₄ into hierarchical tubular hybrids with ultrathin and loose two-dimensional（2D）nanosheet subunits.Photoluminescence（PL）emission spectra,EPR,TEM and XPS results verify that existence of abundant V_In on In_2-xS₃and In_2-xS₃/Cd_1+xIn_2-xS₄ composites.The ultrathin and loose 2D nanosheets on the composites surface are conducive to adsorb more reactants and expose more active sites,while the In defects in In_2-xS₃ sample induced a defect energy level above the VB in In₂S₃,which acted as an acceptor level for the photoexcited electrons and shorten the migration path of the electrons.By integrating the efficient adsorption capacity and charge separation efficiency,the In_2-xS₃/Cd_1+xIn_2-xS₄ composite shows superior photocatalytic removal rate of dyes and heavy metal ion Cr（Ⅵ）.