Structure Control And Property Study Of ZIFs Thin Films

As a new porous material,metal-organic framework(MOF)formed by the connection of organic ligands to metal ions shows many advantages: high specific surface area and tunable pore size.Zeolitic imidazolate framework materials(ZIFs)are a class of MOF materials and formed by using Zn or Co as a metal source and imidazole or imidazole derivatives as organic ligands.In ZIFs materials,ZIF-8 attracts much attention and belongs to a three-dimensional sodalite(SOD)topology structure formed by the coordination of zinc ions with 2-methylimidazole.Another one kind of ZIFs is ZIF-L,whose composition is the same as ZIF-8 and structure is similar to ZIF-8,and the deprotonated 2-methylimidazole molecules take part in the coordination to form layered structures.At present,there are many works on the synthesis and properties of ZIFs powders,but they are difficult to be applied widely.ZIFs thin films supported by the substrate are expected to solve this problem.However,ZIFs thin films have attracted little attention due to their difficult preparation and structural regulation.To solve this problem,the investigation on ZIF-8 and ZIF-L thin films was carried out.ZIF-8 and ZIF-L thin films with good uniformity and stability were successfully prepared on glass substrates using 2-methylimidazole and zinc acetate dihydrate as raw materials,methanol as excellent solvent by a facile spin coating method.The main results are presented as follows.(1)The effects of reactant concentration and ratio on product morphology were studied systematically.When the concentration of 2-methylimidazole is constant,the concentration of zinc acetate dihydrate can be adjusted to control the relative intensity of exposed crystal planes and morphology of ZIF-8 films;When the concentration of zinc acetate dihydrate is kept constant,the particle morphology of ZIF-8 film varies from the truncated rhombohedral dodecahedron to the cubic phase with the increasing of 2-methylimidazole concentration;And when the molar ratio of reactants is 6:1,as the concentration of zinc source and ligand decreases to a certain extent,the particles of ZIF-8 film can be changed from rhombic dodecahedron to truncated rhombic dodecahedron obviously.This is probably because that the concentration of both zinc ions involved in the coordination formation of ZIF-8 and acetate ions affects the nucleation and growth of ZIF-8.(2)The effect of the molar ratio of the reactants on the phase transition of ZIFs was studied.When the molar ratio of reactants slowly decreases from 6:1 to 1:1,the transition from three-dimensional(3D)ZIF-8 to two-dimensional(2D)ZIF-L occur.This is mainly attributed to the degree of deprotonation of 2-methylimidazole ligand related to pH values in the reaction system,that is,ZIF-8 is formed when the pH value is about 9,and ZIF-L is formed when the pH value is about 5.In this sense,the pH value puts important effects on the stability of the crystal planes of ZIF-L,resulting in different orientation of the films.(3)The optical properties of ZIF-L thin films were investigated in detail.The synthesized ZIF-L films exhibit crystal orientation and photoluminescence property.As increasing the annealing temperature of ZIF-L thin films,the color of ZIF-L thin films caused by photoluminescence can be changed from blue to orange red.This indicates that the annealing temperature can control the structure to a certain extent to achieve tunable emission wavelength.