Unique Core-shell MOF-based Heterojunction And Their Photoassisted Pollutant Removal Performance

With the rapid development of economy and technology,the water pollution as a serious hazard to human health has received more and more attention.As well known,the traditional water treatment technology is difficult to remove the organic pollutants completely in the water.However,the advanced oxidation technology processes（AOPs）were reported to remove many organic pollutants effectively.With the aids of light,electricity and other condition,the reactive oxygen species produced in the AOPs can effectively degrade pollutants into CO₂ and H₂O.Among them,AOPs assisted by peroxymonosulfate（PMS）has received widely concerned owing to wide application ranges and high removal capability.There are many kinds of catalysts that can activate PMS.However,it is still a great challenge for researchers to design new catalysts with excellent catalytic performance.Literature has shown that MOFs,especially Fe-MOFs and Ti-MOFs,were widely applied in AOPs fields due to the excellent photoresponse,non-toxicity and other advantages.However,there still exist many drawbacks that limit their wide applications,e.g.the fast recombination of photo excited electron-hole in MOFs materials.Therefore,the rational design of MOFs composites and their derivatives has been paid more and more attention by scholars.In this article,a new strategy was designed to prepare different core-shell MOF hetero-junctions or derivatives,using amorphous Ti-MOFs and Fe-2MI as precursor.The prepared materials show high specific surface area and excellent visible light absorption ability.When applied in the SR-AOPs systems,they showed enhanced degradation efficiency of organic pollutants.The main contents are summarized as follows:1.NS@MIL-101（Fe）heterostructure was synthesized by using amorphous Ti-MOF as the precursor in hydrothermal conditions.The nanocomposites（NCs）were composed of ultrathin nanosheets（NS）as the shell and the octahedron MIL-101（Fe）microcrystal as the core.The formation of heterojunctions is a dynamically controlled nucleation process.The formation of the architectures depends on the participation of appropriate amount of the Ti-MOF precursor and organic linkers.The organic titanium source or single/mix ligand cannot promote the formation of ultrathin NS.Their formation relates closely with the amorphous Ti-MOF precursors with moderate reactivity.The approach breakthroughs the traditional lattice matching rules,and can expand to synthesize other MOF@MOF materials.The NCs show higher specific surface area and photocatalyst ability.About 96%of orange II could be degraded within60 min at p H=7.And it showed good catalytic degradation ability in p H=3-9.Mechanism research showed that the high catalytic efficiency had close relationship with the enhanced light adsorption ability of the unique structure,more exposed active sites and transition metal Fe and Ti.Under the illumination process,the heterojunction showed higher rate of electron separation,transfer and low recombination,which are favorable for the production of more ROS.This synthetic strategy of the heterogeneous MOF catalyst provides a new sight to design new catalysts in the future.2.Using Fe-2MI@NH₂-MIL-125（Ti）core-shell structures as the precursor,Fe Ti O₃@Ti O₂/C ternary materials were prepared by thermal carbonization in N₂atmosphere at different temperature.This endows the carbonized material with abundant graphene defects and excellent absorption ability to different organic pollutants.The effective coupling of Fe Ti O₃ and Ti O₂ on the carbon framework can significantly improve its catalytic activity.Catalytic experiments have revealed that the catalysts can not only activate PMS,but also breakthrough the drawbacks of Fe Ti O₃and Ti O₂,such as poor light absorption capacity and difficult activation by the visible light.Within 15 minutes,the degradation rate of Rhodamine B（Rh B）reached more than 95%.Further,the degradation rate of Rh B could reach more than 90%in a wide p H range（3-9）in the Catalyst/PMS/Light system.