Construction And Properties Investigations Of Nanosheets Of Two-Dimensional Metal Organic Frameworks

With the developments of nanoscience,two-dimensional materials with a lateral size ranging from tens of nanometers to micrometers but a few nanometers in thickness have been widely used in photovoltaics,catalysis,biomedicine,materials chemistry,and engineering for their high specific surface area,rich surface ? electrons,catalytic active site and excellent photoelectric response.Among the two-dimensional materials,two-dimensional metal organic frameworks,as a class of emerging nanomaterials with great applications,have received great attention due to their unique characteristics.Two-dimensional MOFs constructed by organic ligands and metal nodes usually have adjustable nanometer thickness,high specific surface area,exposed active sites,high toughness,and optical transparency.These characteristics endow MOFs nanosheets excellent properties in gas separation,catalysis,sensing,and energy storage.However,the following aspects restrict the further development of two-dimensional MOFs:(?)the thickness of the MOFs nanosheets is not uniform and the preparation of single-layer two-dimensional MOFs is still difficult;(?)The area of most MOFs nanosheets is small,generally within 100 × 100 nm2;(?)The difference in properties caused by different pores on the surface of MOFs nanosheets is still indistinct.Here,we took two-dimensional metal organic framework nanosheets as the main research object,and assembled a series of two-dimensional materials with nanostructures by bottom-up or top-down methods.Focusing on the two-dimensional materials,the effects of the structures on the properties are investaged.Four contents are mainly studied in this thesis:(1)The effects of pore size and layer number of metal-porphyrin coordination nanosheets on sensing DNA.We designed and synthesized two novel porphyrin molecules 5,10,15,20-tetrakis[4'-(terpyridinyl)phenyl]porphyrin and 5,10,15,20-tetrakis[4'-(terpyridinyl)-1,1'-biphenyl]porphyrin.With these two molecules,a facile method was introduced to fabricate large-area,porous,and uniform substrate-supported MOFs nanosheets with precisely controllable layer numbers using the Langmuir-Blodgett technique.As a prototype sensor device,a series of substrate-supported Co-TTPP and Co-TTBPP nanosheets were further investigated for the fluorescence detection of DNA towards rapid clinical diagnosis.On the basis of precise layer numbers and structure of the nanosheets,we studied the difference of their properties by changing the types of monomer and the numbers of layer.The results revealed the synergistic contributions of suitable pore size and optimal layer number of Co-TTPP nanosheet to the high-efficiency DNA detection.In addition,this investigation helps us understand the structure-activity relationship.(2)Photoactive porphyrin-based metal-organic framework nanosheets.5,10,15,20-Tetrakis[4'-(terpyridinyl)phenyl]porphyrin and 5,10,15,20-tetrakis[4'-(terpyridinyl)-1,1'-biphenyl]porphyrin were used as building blocks for the construction of metal-organic framework(MOF)nanosheets utilizing Langmuir-Blodgett technique.The MOF-nanosheet-modified ITO electrodes were established into liquid junction solar cells,and the photoconversion efficiencies of cells have been evaluated upon the visible light irradiation.As dye molecule,porphyrin can perform photoelectric conversion effectively and the two-dimensional material assembled with porphyrin retains the properties.The results show that the formation of ordered structures in two-dimensional materials could reduce the aggregation of dye molecules,thereby promote the photoelectric conversion ability.On the other hand,the formation of nanosheets introduced holes in the structure and guest molecules C60 could be introduced into the nanosheets to limit electron-hole recombination and promote charge transfer,and then improve the photoelectric conversion efficiency.(3)Transformation from MOFs nanosheets to COFs nanosheets based on Ni3(HITP)2.In this content,we first proposed the concept from MOFs to COFs.Two-dimensional N13(HITP)2 was selected as the initial material,and it was converted into COF material by post synthesis.O-phenylenediamine,used as the research object,coordinated with nickel ions to form a two-dimensional conjugated M-(NH)4 type MOF Ni(DIB)2.1,1,3,3-Tetramethoxypropane undergoes a Schiff base ring formation reaction at different times and solvents to explore the optimal reaction conditions.DMF and DMAE were found as suitable solvents for this type of reaction.By investing the crystalline state and morphology change of the reaction products from Ni3(HITP)2 and 1,1,3,3-tetramethoxypropane,the conversion of MOFs to COFs was confirmed and the reaction time greatly affected the integrity of the transition.By applying Ni3(HITP)2 and COF-1 nanosheets to photocatalytic carbon dioxide reduction,it was confirmed that COF-1 had the same catalytic performance as Ni3(HITP)2.(4)Two-dimensional porphyrin cage and its electrocatalytic properties.A new type of cage Porphyrin-NDI was designed and synthesized by using 5,10,15,20-tetra(4-formylphenyl)porphyrin and NDI diamine,and the cobalt ion was introduced into the N4 hole in the center of the cage porphyrin.Then,the cage was assembled into a two-dimensional material which was used for electrocatalytic CO2 reduction.By optimizing the pH value,water content,carbon doping ratio,and the electrode potential,carbon dioxide can be reduced to carbon monoxide selectively,and the ratio of carbon monoxide to hydrogen could achieve 9:1.The compound still maintains more than 80%of its activity after 10 hours catalytic reaction.Compared with Co-TPP,the organic cage which is connected by covalent bond forms an inner cavity and presents a larger surface area,which can increase the connection between carbon dioxide molecule and the catalytic center and achieve more effective reduction.