Covalent organic frameworks (COFs) are crystalline, highly nanoporous materials, consisting of extended periodic ligand network nanostructures, which are extremely amenable to synthetic design by appropriately selecting the monomers used in synthesis. Though progress had been made in the development of novel COFs, largely by trial and error approaches, little is understood about the fundamental nucleation/growth mechanisms that lead to formation of the different COF nanostructures and morphologies, greatly limiting our ability to rationally develop next-generation COF materials.<br/>Advances in liquid-cell transmission electron microscopy (LCTEM) instrumentation now enable direct variable-temperature (VT)-LCTEM observations of fully hydrated/solvated systems while uniformly heating the liquid. Here, we study the nanoscale nucleation and growth during the formation of COF-5 nanoparticles (NPs) using VT-LCTEM. The Dichtel lab has developed a new COF-5 synthesis route for producing size-stabilized NPs (diameters 20-200 nm), whereby the diameter of the population of discrete colloidal COF-5 NPs can be readily modulated by tuning the relative solvent concentrations in the precursor solution. In our VT-LCTEM studies, COF nucleation/growth does not occur at room temperature, even when irradiated by the e- beam at low dose rates. However, when the same COF-5 growth precursor solution is heated to, and held at 80 C, discrete COF NPs nucleation and growth, seeded on the SiNx window surface, is observed (Figure 1A), which could be consistently reproduced in multiple separate VT-LCTEM experiments. We employ the MOTA-method for particle tracking and quantitative size-measurement image analysis to accurately determine the particle size of the population as the particles grow and size-stabilize over time (Figure 1B). We use these quantitative VT-LCTEM measurements as both a complementary characterization tool to bulk-scattering techniques for accurately determining the true size and morphology of nanomaterials in solution, and more importantly as a window into the underlying mechanisms of solvothermal COF formation, with the aim of applying this knowledge to intelligently modify synthesis conditions to promote the formation of more desirable COFs.

<i>In situ </i>Variable-Temperature Liquid Cell Transmission Electron Microscopy (VT-LCTEM) study of size-stabilized Covalent Organic Framework (COF) nanoparticle nucleation and growth during solvothermal synthesis

253rd National Meeting (2017)

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In situ Variable-Temperature Liquid Cell Transmission Electron Microscopy (VT-LCTEM) study of size-stabilized Covalent Organic Framework (COF) nanoparticle nucleation and growth during solvothermal synthesis