Synthesis And Photocatalytic Activity Of Upconversion/metal Organic Framework Composite Photocatalyst

Metal-organic frameworks(MOFs),known as porous coordination polymers(PCPs),are porous crystalline materials consisting of metal ions or clusters coordinated with multidentate organic ligands to form two-or three-dimensional infinite structures.Over the past several years,MOFs have become one of the most popular research areas in the chemical and materials sciences due to their large surface areas and tunable pore sizes.Compared to traditional inorganic materials,MOFs materials have numerous interior catalytic sites,and these sites are also available to reagents via their open pore channels.Since iron is an earth-abundant element,iron-based MOFs show a promising feature in the photocatalytic applications.MIL-53(Fe),a typical Fe-based MOF constituted by Fe(Ⅲ)octahedra and 1,4-benzenedicarboxylic acid,is regarded as one ideal photocatalyst due to its high stability,low cost,nontoxic nature and visible light response.However,MIL-53(Fe)has a narrow absorption band,which severely limited its wide utilization of the solar energy.In this dissertation,we integrate MIL-53(Fe)photocatalyst with upconversion nanocrystals and prepare nanocomposites with a support-type or core-shell morphology.Such kinds of nanocomposites can extent the absorption of MOF materials to the near-infrared region,thus significantly improving its photocatalytic performance.The detailed work was summarized as follows.First,we developed a microwave-irradiation route to rapidly synthesize MIL-53(Fe)octahedrons with a unique structure and high yield.The prepared MOF octahedrons possess a novel concave surface with a uniform size and regular shape.Such special morphology feature benefits the deposition of upconversion nanoparticle(UCNPs)on theMOF surface with high stability.Tn the MOF/UCNPs particles,the MIL-53(Fe)could effectively absorb the UV and visible light while the UCNPs can convert NIR light to be UV/vis emissions to be utilized by the MOF.As such,the light utilization range of MOF was extended to NIR region.The content of supported UCNPs on the composites could be facilely controlled by the addition amount of the UCNPs during assembly,and its effect on their photocatalytic activities was explored and elucidated.Furthermore,we synthesized NH2-MIL-53(Fe)/UCNP composites by changing the TPA ligand with 2-NH2-TPA.The results suggested that the introduction of amino group could further extend the absorption of MOF and improve their photocatalytic activities.Second,we developed a facile route to prepare uniform NaYF4:Yb,Tm@MIL-53(Fe)nanocomposites.Compared to their support-type counterparts,these nanocomposites could greatly improve the stability of photocatalysts and the utilization of their surface catalytic sites due to the uncovered surface.Through the layer-by-layer technique,we have achieved the NaYF4:Yb,Tm@MIL-53(Fe)particles with tunable shell thickness and explore their photocatalytic activities.In a similar way,we modified the organic likers of MOF with amino groups and significantly improved their photocatalytic activities.The photocatalytic mechanism of the nanocomposites was revealed with various techniques such as absorption spectra,photoluminescence spectra,photocurrents,and determination of reactive species.Particularly,the luminescence decay curves implied that energy transfer between the core-shell NaYF4:Yb,Tm@MIL-53(Fe)was through the fluorescence resonance energy transfer(FRET)route.Such FRET route is more effective than the radiation-reabsorption route,which could significantly promote the utilization of upconversion emissions in the composites.