Synthesis,Charge Separation And Hydrogen Production By Water Splitting In MOF-Based Photocatalysts

As a novel class of crystalline porous solid materials,metal-organic frameworks(MOFs),constructed by metal ions or clusters together with multifunctional organic ligands,featuring high BET surface area,tailorability,functionality,abundant unsaturated sites,have shown wide applications in gas absorption and storage,molecule separation,fluorescent sensors,catalysis,supercapitors,drug delivery and porous templates,etc.The secondary building units of MOFs are metal-oxo clusters that show semiconductor-like behavior;therefore,MOFs have been frequently used in the region of photocatalysis in recent years.Besides,owing to the highly porous and ordered structures,MOFs can be utilized as soft or hard template to gain porous carbon and metal oxide materials with high surface area which have potential applications in energy storage and conversion.However,as conventional inorganic semiconductor materials suffer from difficulties in further-enhanced photocatalytic activity,rational design of MOFs and their composites for photocatalysis will be very promising.The synthesis of MOFs and their composites not only enables a deeper understanding of the electron-transfer mechanism for photocatalysis,but also provides a unique perspective for the development of other porous material-based photocatalysts.Based on the continued research in design and synthesis of MOF materials with applications in heterogeneous catalysis in our lab,the present dissertation will focus on the enhanced photocatalytic hydrogen production from water splitting by metal-MOFs composites and MOFs-derived porous metal oxides,for their improved charge utilization.Here,metal-MOF composites are rationally synthesized with high efficiency of charge transfer and e-h separation,which is significant for the high photocatalytic hydrogen production efficiency.Furthermore,we have discussed the structure influences of MOF-based porous inorganic semiconductors for photocatalytic activity.The main details can be summarized as follows:1.Noble-metal nanoparticles(NPs),especially Pt NPs,are frequently-used as electron acceptors for enhanced photocatalytic hydrogen production from water splitting.Thus metal-Pt composites with Schottky junction often show higher photocatalytic hydrogen production activity than the pure MOF materials.Therefore,Pt nanoparticles of?3 nm are supported on or incorporated inside a MOF,UiO-66-NH2,denoted as Pt/UiO-66-NH2 and Pt@UiO-66-NH2,respectively,for photocatalytic hydrogen production by water splitting.Both Pt-decorated MOF composites exhibit significantly improved yet distinctly different photocatalytic hydrogen production activities compared with the pristine MOF,highlighting the Pt location relative to the MOF that influences the photocatalytic efficiency differently.The Pt@UiO-66-NH2 greatly shortens the electron-transport distance and suppresses the electron-hole recombination,thereby yielding much higher photocatalytic hydrogen production activity than Pt/UiO-66-NH2.The involved mechanism has been further unveiled by photoluminescence spectroscopy and ultrafast transient absorption tests,which reveal the charge transfer processes of each photocatalyst.2.A novel photocatalytic system has been developed by building two metal-MOF interfaces for the futher enhancement of the photocatalytic hydrogen production from water splitting.The activity enhancement of photocatalytic hydrogen production based on semiconductor catalysts requires efficient separation of photogenerated electrons from holes,and the extension of light absorption from UV to visible region to improve the light harvesting.The construction of Pt-MOF Schottky junction is an effective strategy to boost charge separation and suppress the electrons-holes recombination.And the introduction of Au or Ag nanoparticles with surface plasmon resonace into MOF photocatalysts would be one of the most effective approaches to extending the light absorption to visible or even near-IR region,while remaining the high redox ability of the wide-bandgap MOF.Thereby integrating plasmonic effect and Schottky junction into one MOF structure would be expected to greatly promote the electrons-holes transfer and separation,and result in very high photocatalytic efficiency.3.The most promising thermally stable MOF,MIL-53(Al),is selected and used as hard template.By introducing metal nitrates into the MOF pores,porous metal oxides and subsequently metal sulfides can be successfully obtained by a nanocasting method.The resultant metal oxides show high BET surface areas,by partially inheriting the pore character of the MOF template.Besides,the metal oxides/MOF can be further transformed into metal sulfides/MOF,and finally hierarchically porous metal sulfides can be gained by MOF removal for enhanced photocatalysis application.As a proof of concept,preliminary investigation of water splitting on the hierarchically porous CdS(HP-CdS)has been carefully conducted.Results show that HP-CdS possess much higher activity than both corresponding bulk and nanosized counterparts,under visible light irradiation,highlighting the nanosize effect and porous structure of HP-CdS photocatalyst.