Elucidating the Formation of 2D Conjugated Metal Organic Frameworks via In-Situ Nanofluidic-Liquid-Cell Transmission Electron Microscopy

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Metal-organic frameworks (MOFs) represent an emerging class of crystalline porous materials constructed from metal containing nodes and organic ligands. The vast chemical and structural diversity of MOFs opens up exciting possibility of crystal engineering tailored for next generation applications. Recently, two-dimensional conjugated MOFs (2D c-MOFs), featuring in-plane π-delocalization and weak out-of-plane π-π stacking, have attract considerable attention in the grand family of 2D materials. 2D c-MOFs offers simultaneously high electrical conductivity, abundant active sites and tunable redox states, rendering them promising candidates for future applications in electrochemical energy storage/conversion. However, despite the intriguing physical and chemical properties of 2D c-MOFs, their formation mechanisms and structure-property correlations remain poorly understood. Particularly, the fundamental processes governing 2D c-MOFs nucleation and growth, and the relationship between reaction parameters and synthetic outcome, remain largely unexplored. Here, we aim at direct observation of 2D c-MOF formation via in-situ liquid-cell transmission electron microscopy (LC-TEM) down to molecular/atomic scale. The main challenges of reaching this goal are: (1) radiolysis of the water molecules within the LC, leading to highly reactive radicals, which directly etch the reaction products and LC windows, (2) the conventional LC design consists of large fluid volume, the achievable resolution is significantly inferior to the instrumental resolution, and (3) the extremely low electron resilience of MOFs, particularly due to the presence of C-H bonds. To address these issues, we will utilize a newly designed nanofluidic LC for high-resolution LC-TEM. The microfabricated nanochannels allow well-controlled liquid thickness down to a few nanometers. And the microfluid systems is able to flush out radiolytic products during imaging, thus reducing the detrimental effects of water radiolysis. As an additional strategy, deuterated water will be used to increase the lifetime of reaction products in the liquid environment. Apart from the liquid cell design, the intrinsic stability of MOFs presents another pivotal factor to the success of the proposed work. In our preliminary study, we found that a hydrogen-free benzenehexathiol (BHT) Cu MOF can withstand electron doses comparable to those of inorganic materials. Therefore, to substantially enhance the electron resilience, hydrogen-free and halogenated 2D c-MOFs will be synthesized and utilized for LC-TEM studies.