Revealing Structure Properties of ZIF-8 Particles Prepared by Wet Chemical Etching via 3D Electron Tomography

Wet chemical etching has been one crucial method in the design strategy of functional materials, from microfabrication to nanoparticle synthesis. It has been suggested that shapes of building blocks could change the structures that particles assemble into. While much effort has been devoted to ZIF-8 colloids to create particles with different shapes, such as convex polyhedral particles, cubic particles, and hollowing microboxes, micro or nanoporous ZIF-8 colloids with high quality and high yield remain largely underexplored because of limited fundamental understanding of the etching mechanism. Here we achieved the preparation of porous metal–organic framework (MOF) particles in colloids by wet chemical etching of as-synthesized parent particles. To understand the etching mechanisms, the pH value of the etching solution was modified as a controllable parameter. Different types of porous ZIF-8 particles were observed as the etching solution pH was varied. Quantitatively, the 3D electron tomography technique was applied to analyze porous ZIF-8 particles giving many structure properties such as the surface area, pore size and distribution, and skeletonization. This 3D quantification reveals the advantage of the porous structure, which will benefit its potential application after atomic doping treatment. The porous ZIF-8 colloids can also self-assemble into different superstructures facilitated by its novel morphology, and a linear polymerization was observed in one type of porous ZIF-8 colloid. The assembled superstructure depends on particles’ orientation in solution, and the self-assembly kinetics was captured by in situ microscopy. We anticipate that the wet chemical etching method and 3D tomography technique reported here opens a new door to understand and engineer functional porous materials.