Synthesis And Photocatalysis Of Metal Decorated Metal-Organic Frameworks(MOFs)

Metal-organic frameworks（MOFs）,with unique porous structures composed of metal ions and organic bridging ligands,have received widespread attention in the research field of photocatalysis.Their well-ordered porous structure allows the fast transfer of electrons and can expose multiple active sites.However,two significant issues remain challenging on MOF-based photocatalysts:（i）the high electron-hole recombination efficiency and（ii）the insufficient light harvesting ability.The combination of MOFs and other guest species such as metal nanoparticles（MNPs）is one of the effective strategies to solve the above issues.Therefore,in order to expanded their applications in the field of photocatalysis,it is necessary to investigate the roles of MNPs and how they affect the electron-hole separation efficiency and light absorption capacity of MNPs/MOFs.Herein,to gain a deeper understanding of the influence of size and morphology of MNPs on photocatalytic hydrogen production performance of MNPs/MOFs.We selected Pd,Pt,Au to decorated Ti-MOFs to improve the separation efficiency of electron-hole or light absorption capacity of Ti-MOFs.The mechanism was also discussed in detail.The main results are as follows:（1）Utilizing Ti-MOFs（MIL-125-NH₂）defects to embed more metal nanocrystals:A unit mass of defects-abundant of DA-MIL-125-NH₂were prepared by adding different amounts of dodecanoic acid in the preparation process of MIL-125-NH₂.We investigated the effect of defects in MIL-125-NH₂on the performance of photocatalytic H₂production.The results showed that with the increasing of defects,DA-MIL-125-NH₂exhibited a lower H₂evolution rate because the defects can act as the recombination centers of excitons.However,using the same dosage of Pt precursors,when the defects created properly,it is possible to embed more Pt（～4-fold）into the defects of DA-Ti-MOFs,compared to pristine MIL-125-NH₂,correspondingly lead to a significant enhancement of H₂generation abilities.（2）Construction of multi-directional electrons transport channels between Ti-MOFs（MIL-125-NH₂）and Pt and Pd:By encapsulating Pt nanoparticles into Ti-MOFs and decorating Pd on the surface of Ti-MOFs（Pt@Ti-MOFs/Pd）,realizing multi-directional transportation of photogenerated electrons and improving the electron-hole separation efficiency of MIL-125-NH₂.The results showed that,compared with Pt@Ti-MOFs and Pd/Ti-MOFs,Pt@Ti-MOFs/Pd exhibited the fastest charge separation efficiency and the photocatalytic H₂production rate increased by 1.6 and 4.3times,respectively.This composites also maintain stable after 4 cycles.（3）Size and shape-controlled Pd nanoparticles on Ti-MOFs（MIL-125-NH₂）:To investigate the size and shape effects of Pd particles on photocatalytic H₂production activities of Pd/MIL-125-NH₂.Pd nanocubes with an average size of 15 nm,12 nm,7nm and Pd truncated octahedrons with an average size of 7 nm were prepared,and decorate them on the surface of MIL-125-NH₂.The results showed that the electron-hole separation efficiency can be increased by reducing the size of Pd nanocubes.In addition,Pd truncated octahedrons with{111}facets promote electron-hole separation more effectively than Pd nanocubes with{100}facets.Thereby MIL-125-NH₂decorated with 7 nm Pd truncated octahedrons showed the highest photocatalytic hydrogen production efficiency,which is over 1.5,3.3 and 4.7 times than that of 7 nm,12nm and 15 nm Pd cubes.（4）Constructing Pt@Ti-MOFs/Au（Pt@MIL-125/Au）Schottky junction to enhance the light absorption and electron transfer capabilities:To investigate the morphology effects of Au particles on the light absorption capacity and electron-hole separation efficiency of Pt@Ti-MOFs/Au.Au rods/spheres（Length:67 nm,52 nm,38nm,18nm.Width:18 nm）were prepared and construct Pt@MIL-125/Au Schottky junction.The results showed that with the introduction of Au,all the composites show significantly enhanced light absorption abilities.In addition,as the Au nanorods gradually shorten to spherical,the transverse polarization will be increased which can lead to a significant enhancement on electron-hole separation efficiency and photocatalytic H₂production efficiency.The enhancement can be attributed to the accelerated electrons transfer rate induced by transverse polarization and evidenced by ESR analysis.