Controllable Synthesis And Mechanism Analysis Of ZIF-based Metal-organic Framework Nanocrystals

MOFs are new porous materials periodically constructed by metal ions or clusters with organic ligands under the action of coordination bonds,and nanoscale 2D MOFs exhibit good conductivity,high surface areas,and easy to process into film and other characteristics.However,the synthesis of two-dimensional monodisperse MOF nanocrystals is still in the development stage due to the difficulty in controlling the morphology of MOFs.The types of MOFs reported are limited,classical sonication exfoliation or chemical exfoliation technique have been developed to get nanosheets of MOFs possessing a layered structure by overcoming the weak interactions among the layers.The categories of two-dimensional non-layered MOFs reported so far are limited,but lots of MOFs are of non-layered structure,and until now the lack of effective 2D synthetic methods for non-layered MOFs greatly limits application of non-layered MOF,such as chemical sensors,catalysis and micro-devices.This topic proposes a chemical solution method based on small molecule sodium dodecyl sulfate(SDS)negatively charged electrostatic pre-assembled MOF precursors,and successfully prepared monodisperse square non-layered ZIF-8 and ZIF-67 MOF two-dimensional nanocrystals.The specific research work is as follows:(1)Developing an electrostatic shearing method based on SDS,mixed zinc acetate(Zn Ac₂·2H₂O),SDS and 2-methylimidazole(2-MIM)aqueous solution.Then the mixed solution was allowed to stand for reaction overnight at room temperature to obtain a square,good monodispersity and flat 2D zinc-based zeolite imidazole framework(ZIF-8)nanosheet.The 2D ZIF-8 nanosheets exhibit a uniform square-like shape with a lateral size of about 600 nm and a thickness of 18.2 nm.When the SDS concentration is relatively low,the obtained ZIF-8morphology is cubic nanocrystal;As the SDS concentration gradually increased,ZIF-8 gradually showed a uniform monodisperse nanosheet morphology.ZIF-8 nanosheets tend to be formed when the SDS concentration is higher,and the higher the reactant concentration,the smaller the size of the collected nanosheets.(2)Using the same method as above,the cobalt acetate,SDS and 2-MIM aqueous solution were mixed and allowed to stand overnight at room temperature for reaction to obtain a square,monodisperse and flat two-dimensional cobalt-based zeolite imidazole framework(ZIF-67)nanosheet.The ZIF-67 nanosheets exhibit a uniform square-like shape with a lateral size of about500 nm and a thickness of 20 nm.When the SDS concentration is in the lower range,the obtained ZIF-67 morphology is cubic nanocrystal.With the increase of SDS concentration,ZIF-67 exhibited uniform and monodisperse morphology.When the concentration of SDS is constant,the size of the nanosheets decreases with the increase of the reactant concentration.(3)Taking the growth process of 2D ZIF-8 nanosheets as an example,the interaction between Zn²⁺and SDS was further studied by dynamic light scattering(DLS).The results show that the particle size of SDS micelles is mainly in the two particle size ranges of 200–300 nm and 1300–1600 nm.When the zinc acetate solution and SDS are mixed,Zn²⁺ions are pre-assembled with SDS,and the micelle structure is changed to the lamellar structure.After transformation,the particle size of the mixed micelles increases to 2500–3000 nm.After adding 2-MIM ligand,ZIF-8nanocrystalline nuclei are formed in the solution,and the particle size becomes 290–300 nm.SDS molecules should pre-assemble with metal ions via electrostatic interaction,adsorb on the crystal surface(rich in metal ions),thereby limiting the direction of crystal growth,and forming 2D sheet structure.