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Electron Microscopy of Nanoporous Crystals
Accounts of Materials Research ( IF 14.6 ) Pub Date : 2021-12-15 , DOI: 10.1021/accountsmr.1c00216
Yi Zhou 1 , Zhuoya Dong 1 , Osamu Terasaki 1 , Yanhang Ma 1
Affiliation  

Nanoporous crystals, such as silica mesoporous crystals (SMCs), zeolites, metal–organic frameworks (MOFs) and covalent organic frameworks (COFs), usually have unique geometrical features of periodically arranged pores from micro-, meso- to macro-scale. They have great potential to be structurally designed toward novel materials for various applications, such as storage, separation, and catalysis. In addition to the synthesis and applications of these materials, the structural analysis of nanoporous crystals is also critical for obtaining novel materials and unravelling their structure–property relationships. The crystal structure determines the pore size and connectivity of cages/channels, which are closely related to the performance of these materials in adsorption and catalysis. However, there are significant challenges in structural analysis due to small crystal size, electron-beam sensitivity, and local defects. Due to the strong interaction between electrons and matter, electron microscopy (EM) can provide rich structural information, which at the same time can also cause structural damage to samples. As early as 1960s, EM had been used to directly observe the lattice fringes of zeolites. Subsequently, the structures of many nanoporous crystals were solved and structural details such as defects, intergrowths, and surface structures were revealed by exploiting EM during the past decades. With the development of three-dimensional electron diffraction (3D ED) and low-dose high-resolution electron microscopy imaging, these approaches become more routine for solving the structure of nanocrystals and for revealing the local structural details. In this Account, we present our previous work on EM studies of several typical nanoporous materials, including SMCs, zeolites, MOFs, the TiO2@MIL-101 composite, and COFs. Various methods and strategies were developed and applied to different materials based on the characteristics of their structures. For example, 3D electrostatic potential maps of SMCs can be reconstructed by Fourier synthesis of crystal structure factors obtained from high-resolution TEM images, leading to the discovery of many 3D mesostructures. New zeolites/MOFs/COFs structures, which were not previously solved due to their structural complexity and small crystal size, have been determined using 3D ED data from nanosized crystals or using a combination of electron diffraction and high-resolution imaging. In addition, EM methods for determining the handedness of nanocrystals have also been developed and successfully applied to SMCs and STW zeolite based on high-resolution EM imaging or dynamical electron diffraction. Recently, a new strategy involving a combination of 3D ED and cryogenic protocol was proposed to study the structure, dynamics, and the host–guest interactions in a COF material. The direct-space strategy for structure solution, implemented using a genetic algorithm, was also demonstrated to be a successful approach for solving structures from low-resolution 3D ED data. By now, EM has become one of the most widely used methods in the study of nanoporous materials. We hope this Account will promote further developments of EM and stimulate the design and synthesis of new functional nanoporous materials.

中文翻译:

纳米多孔晶体的电子显微镜

纳米多孔晶体,如二氧化硅介孔晶体(SMCs)、沸石、金属有机骨架(MOFs)和共价有机骨架(COFs),通常具有从微观、中观到宏观尺度周期性排列的孔隙的独特几何特征。它们具有巨大的潜力,可以在结构上设计成用于各种应用的新型材料,例如存储、分离和催化。除了这些材料的合成和应用外,纳米多孔晶体的结构分析对于获得新材料和揭示它们的结构-性质关系也至关重要。晶体结构决定了笼/通道的孔径和连通性,这与这些材料在吸附和催化方面的性能密切相关。然而,由于晶体尺寸小、电子束敏感性和局部缺陷,结构分析面临重大挑战。由于电子与物质之间的强相互作用,电子显微镜(EM)可以提供丰富的结构信息,同时也会对样品造成结构损伤。早在 1960 年代,EM 就已被用于直接观察沸石的晶格条纹。随后,在过去的几十年中,通过利用 EM,解决了许多纳米多孔晶体的结构,并揭示了诸如缺陷、共生和表面结构等结构细节。随着三维电子衍射(3D ED)和低剂量高分辨率电子显微镜成像的发展,这些方法对于解决纳米晶体的结构和揭示局部结构细节变得更加常规。在这个帐户中,我们介绍了我们之前对几种典型纳米多孔材料的电磁研究工作,包括 SMC、沸石、MOF、TiO2 @MIL-101 复合材料和 COF。根据材料的结构特征,开发了各种方法和策略并将其应用于不同的材料。例如,SMC 的 3D 静电势图可以通过对从高分辨率 TEM 图像获得的晶体结构因子进行傅里叶合成来重建,从而发现了许多 3D 细观结构。新的沸石/MOFs/COFs 结构由于其结构复杂性和晶体尺寸小而以前未被解决,现已使用来自纳米尺寸晶体的 3D ED 数据或使用电子衍射和高分辨率成像的组合来确定。此外,还开发了用于确定纳米晶体旋向性的 EM 方法,并成功应用于 SMC 和STW基于高分辨率 EM 成像或动态电子衍射的沸石。最近,提出了一种结合 3D ED 和低温协议的新策略来研究 COF 材料中的结构、动力学和主客体相互作用。使用遗传算法实现的结构求解的直接空间策略也被证明是从低分辨率 3D ED 数据求解结构的成功方法。到目前为止,EM已成为研究纳米多孔材料最广泛使用的方法之一。我们希望这个帐户能够促进 EM 的进一步发展,并促进新功能纳米多孔材料的设计和合成。
更新日期:2022-01-28
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