| As a relatively new class of crystalline porous materials of inorganic-organic hybrid,metal-organic frameworks(MOFs),consisting of metal ions and organiclligands by coordination bonds,exhibit designed various structure types,adjustable chemical multifunction and high specific surface area with the diversity of metal ions,organic ligands and coordination modes.They have great potential in the fields of gas storage and separation,catalysis,sensing,building new materials as carriers or templates,as well as drug transport,photocatalysis is one of the most promising applications of MOFs materials.Photocatalytic water splitting is a promising solution to alleviating the shortage of fossil fuels and the environmental pollution caused by fossil fuel combustion(as H2 is a kind of high energy and clean fuel).To actualize H2 evolution from water splitting,it is necessary to develop a cost-effective catalyst system which not only integrates visible-light harvesting,charge transfer,and catalysis together but also demonstrates effectiveness and durability under the photocatalytic conditions.As the LMCT and ?-?*excitation of ligand of MOFs fall into UV and the blue end of visible regions,MOFs exhibit similar photocatalytic activities to semiconductors.Furthermore,the high porosity of MOFs would shorten the transport distance of charge carriers to the pore surface throughout the structure for reactions,which greatly suppresses the classical volume recombination of electron-hole;the crystalline nature also avoids the defects that might produce recombination centers,which still remains a challenge in bulk semiconductor materials.Currently,the study of MOFs in the field of photocatalysis has focused on the study of zirconium,aluminum or titanium MOFs,which have visible light responsive ligands(such as BDC-NH2 ligands or porphyrin ligands).In order to enhance the photocatalytic efficiency and reduce the cost of catalysts simultaneously,we start from the following three aspects to consider the related work:(1)Some of the visible light responsive ligands of MOFs,which quickly accept the visible light excitation then generate photo electrons,can act as photosensitizers;(2)The base metal complex with photocatalytic activity introduced into the MOF cage as an electron acceptor can realize the spatial charge separation and further enhancement of the water splitting efficiency;(3)The domain/fixation of MOFs can realize the heterogenization of homogeneous Co complex photocatalyst,and the synergistic effect of the two component can achieve highly efficient hydrogen production and catalytic cycle.Based on the above-mentioned ideas,we rationally design the composite catalysts in this way:the photocatalytic active[Co?(TPA)Cl][Cl]complex is restricted in the cage of visible light responsive MOF:MIL-125-NH2,considering the molecular size of[Co?(TPA)Cl][Cl]is smaller than the diameter of the cage of MIL-125-NH2,whereas lager than the window of MIL-125-NH2,which assures the stable existence of[Co?(TPA)Cl][Cl]in the cages of MIL-125-NH2 and efficient charge transfer from the BDC-NH2 to[Co?(TPA)Cl][Cl].Besides,the composite catalyst with different loading capacity of Co complex was synthesized by a simple synthesis method and the experimental results show that the photocatalytic activity of the composite catalyst is improved with the increase of Co complex loading.In addition,the efficiency of the composite catalysts is much higher than[Co?(TPA)Cl][Cl],MIL-125-NH2 and their physical mixture,which proves the high synergistic effect between[Co?(TPA)Cl][Cl]and MIL-125-NH2.We explore the mechanism by the ESR measurement,which confirms that the high efficiency of the composite catalyst is due to the rapid electron transfer process:electron transfers from the ligand BDC-NH2 to the Co(II)complex directly basically without passing through Ti-oxo cluster.Our research work provides a useful reference for the study of the synthesis of MOF-based-composite photocatalysts and photocatalytic properties. |
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