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

金属-有机骨架（M0 F）代表了一类新兴的由含金属的节点和有机配体构造的结晶多孔材料。MOFs巨大的化学和结构多样性为下一代应用定制的晶体工程开辟了令人兴奋的可能性。近年来，二维共轭MOFs（2D c-MOFs）由于具有面内π离域和弱面外π-π堆积的特性，在二维材料大家族中引起了广泛的关注。二维c-MOFs同时提供高电导率，丰富的活性位点和可调的氧化还原状态，使它们成为未来电化学储能/转换应用的有希望的候选者。然而，尽管二维c-MOFs的物理和化学性质令人感兴趣，但其形成机制和结构-性质相关性仍然知之甚少。特别是，2D c-MOFs成核和生长的基本过程，以及反应参数和合成结果之间的关系，仍然在很大程度上未被探索。在这里，我们的目标是通过原位液池透射电子显微镜（LC-TEM）直接观察到分子/原子尺度的2D c-MOF形成。实现这一目标的主要挑战是：（1）LC内的水分子的辐解，导致高反应性自由基，其直接蚀刻反应产物和LC窗口，（2）常规LC设计由大的流体体积组成，可实现的分辨率显著低于仪器分辨率，以及（3）MOF的极低的电子弹性，特别是由于C-H键的存在。为了解决这些问题，我们将利用新设计的纳米流体LC进行高分辨率LC-TEM。微加工的纳米通道允许良好控制的液体厚度下降到几个纳米。微流体系统能够在成像过程中冲洗出辐解产物，从而减少水辐解的有害影响。作为一种额外的策略，氘化水将用于增加反应产物在液体环境中的寿命。除了液体电池设计之外，MOFs的固有稳定性是所提出的工作成功的另一个关键因素。在我们的初步研究中，我们发现，无氢苯六硫醇（BHT）铜MOF可以承受电子剂量的无机材料。因此，为了显著增强电子弹性，将合成无氢和卤化的2D c-MOF并用于LC-TEM研究。

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.