Preparation And Application Of Metal-Organic Framework Nanosheet Microcapsules And Of Their Composites

Using two-dimensional metal-organic framework?MOF?nanosheets as new building blocks to create more complex architectures at the mesoscopic/macroscopic scale has attracted extensive interest in recent years.Nevertheless,it remains a great challenge to assemble MOF nanosheets into hierarchical hollow structures so far.In this paper,a continuous and ultrafast droplet microfluidic strategy was presented,which can produce hierarchical MOF nanosheet microcapsules with precisely controlled sizes on large scale.First of all,the reaction of metal ions and organic ligands at two immiscible liquid phase interfaces was proved and TCPP-M(TCPP=tetrakis?4-carboxy-phenyl?porphyrin;M=Cu²⁺,Zn²⁺,Co²⁺)nanosheet memabranes were prepared by in-situ interfacial reaction.The obtained TCPP-M nanosheet membranes displayed a rough surface,which were distinctly different with layered MOF counterparts.Then,the above reaction was transferred into the microfluidic system for the preparation of hierarchical MOF nanosheet microcapsules?HMNMs?with controllable sizes and the results showed that the size of the microcapsules was related to the flow rate of the continuous and dispersive phase in the microfluidic system.Secondly,FESEM,TEM,and AFM images showed the hollow structure of HMNMs?Cu?and their shell with a continuous smooth inner layer and a porous outer layer.By observing the growth process of TCPP-Cu nanosheet membrane via in-situ interfacical reaction,we proposed that the growth mechanism of the shell follows the reaction/diffusion process.Furthermore,the crystal structure of TCPP-Cu nanosheet was characterized by XRD,FTIR,UV-vis,and XPS.The results showed that TCPP-Cu are consistent with a 2D "checkerboard" structure comprising no-metal-centered TCPP units connected by binuclear Cu₂?COO?₄ paddlewheels.TCPP-Zn and TCPP-Co nanosheet also showed similar results.Finally,the nanoparticles?NPs?were introduced into the HMNMs?Cu?to prepare multifunctional hybrid microcapsules.FESEM and TEM images confirmed that the magnetite NPs and Au NPs were successfully encapsulated and loaded both inside and outside the microcapsules.Subsequently,the hybrid microcapsules were used to catalyze the reduction of 4-nitrophenol,which exihibited higher catalytic activity than pure Au NPs and can be fully recycled.Furthermore,we found that the enhanced catalytic activity of hybrid microcapsules can be ascribed to the strong ?-? interaction between TCPP-Cu nanosheet and 4-nitrophenol.