Synthesis,Modification And Photoactivity Of Metal-organic Frameworks(MOFs)

Metal-organic frameworks（MOFs）are a kind of porous polymers composed by metal cations coordinating with organic ligands.They have been widely used in some fields such as gas storage and separation,fluorescence sensing,drug delivery and catalysis due to their characteristics of high specific surface area,porosity,structure tailoring and function tunable.Since the semiconductor-like behavior of MOFs was first reported in2007,MOFs as a new type of photocatalyst has been developed rapidly,especially in the fields of photodegradation of dyes,photoreduction of carbon dioxide,photolysis of water for hydrogen production and etc.However,compared with traditional semiconductor photocatalysts,the efficiency of photogenerated electron-holes recombination of MOFs is still relatively higher,which leads to the lower photocatalytic activity.Therefore,exploring rational strategy to improve the efficiency of electron-hole pairs separation and enhance the photocatalytic activity of MOFs has become one of the hot topics.Herein,with the understanding of energy band position,crystal structure and basic photocatalytic mechanism of Zr-MOFs,we proposed three structural optimization strategies to improve the efficiency of electron-hole separation for improving their photoactivities.The feasibility of the three methods was proved by systematic optical characterization and photocatalytic activity evaluation.Accordingly,the relevant mechanism was explained and demonstrated in detail.The main strategies and results are listed as follows:（1）Construction of electron transfer channels between Zr-MOFs and semiconductors:Non-toxic,low-cost semiconductor of Ti O₂with energy band position matching with Zr-MOFs was selected to synthesize and optimize Zr-MOFs/Ti O₂heterojunctions for falicitating electrons transfer.The photoactivity evaluation results of photodegradation dyes showed that when the molar ratio of Ti:Zr was 49,the degradation efficiency of Zr MOFs/Ti O₂（UT49）presented significant improvments compared with pure Zr-MOFs and Ti O₂,and the dye degradation efficiency still exceeded 80%after four cycles.The active species annihilation experiments revealed that O₂^·-and h⁺directly involved in the oxidation of dye molecules instead of·OH.Interestingly,it was also found that these heterojunction materials exhibited bifunctionality.For example,when the molar ratio of Ti:Zr is 0.05,Zr-MOFs/Ti O₂（UT0.05）not only showed enhanced photocatalytic ability,but also has a certain potential in the separation of carbon dioxide and methane.（2）Construction of multi-directional electrons transfer channels of Zr-MOFs:By introducing graphene,an electronic conductor,we precisely manipulated the interaction mode between graphene and the outer surface of Zr-MOFs to synthesize an effective wrapping structure（RGOWU6N）through electrostatic interaction method,realizing the multi-directional transportation of photoelectrons along the multiple contact surfaces,and improving the intrinsic photogenerated electron-hole separation efficiency of Zr-MOFs.The results showed that compared with the original Zr-MOFs,the physical mixing of graphene and Zr-MOFs（RDGO/U6N）and the single-spot or single-face contact（RCGO/U6N）of graphene and Zr-MOFs,the formation of the wrapping structure can significantly increase the fluorescence lifetime of Zr-MOFs and significantly reduce the excitons recombination efficiency.In the hydrogen production tests of water splitting,the hydrogen generation efficiency was increased by about 2times and maintained stable after 4 cycles.（3）Construction of inside-outside co-decoration structure of Zr-MOFs for electron transfer:On the basis of the above-mentioned surface interaction study of graphene coated MOFs,the cocatalyst of Pt was further encapsulated into Zr-MOFs crystals to form the internal and external comodified structure.The results showed that the outer modified structure was formed by the interaction of C=O of graphene oxide and-NH₂of Zr-MOFs,while the inner modified structure is formed by the nucleation growth of Zr-MOFs with Pt particles as the nucleus.The optical characterization results showed that the inner and outer comodified structure can realize the multi-directional transportation of photoelectrons along the surface of Zr-MOFs,and simultaneously accelerate the electron transfer rate from the inner of Zr-MOFs crystals to the Pt.The synergistic effect effectively improves the electron hole separation efficiency in the whole Zr-MOFs framework.The experimental results of photocatalytic hydrogen production showed that the efficiency of hydrogen production from water was increased by 21 times and 32 times respectively compared with the introduction of only graphene and Pt.Moreover,the external modification structure can reduce the loss of cocatalyst to a certain extent,which is conducive to maintaining the high stability of the materials.