Electron Microscopy of Nanoporous Crystals

Abstract

Conspectus 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 h...

Improving Data Quality in Traditional Low‐Dose Scanning Transmission Electron Microscopy Imaging

“…Therefore, the change of the probe can be estimated by analyzing the Gaussian image of the light source which is inversely proportional to the value of α. Assuming the intensity distribution of the probe is a normal distribution, its magnitude g (full width at half maximum [FWHM]) can be estimated from Equation (1), where the brightness of the electron source is B.…”

“…Electron microscopy studies of beam sensitive materials, [ 1–3 ] such as zeolites, [ 4–6 ] metal–organic frameworks (MOFs), [ 7–9 ] covalent organic frameworks (COFs), [ 10–12 ] and organic–inorganic hybrid perovskite materials, [ 13–15 ] have recently received a wealth of interests in their elaborate structures and versatilities in granting new functions. However, these delicate structures are easily damaged under electron beam (e‐beam) radiation, which introduces great difficulties to their electron microscopy characterization.…”

“…Electron microscopy studies of beam sensitive materials, [1][2][3] such as zeolites, [4][5][6] metal-organic frameworks (MOFs), [7][8][9] covalent organic frameworks (COFs), [10][11][12] and organic-the beam damage was reduced largely and building blocks of MIL-101 crystal was revealed clearly.…”

In Situ Three-Dimensional Electron Diffraction for Probing Structural Transformations of Single Nanocrystals

“…During the past decade, the emergence and development of three-dimensional electron diffraction (3D ED) methods opens new opportunities for the structure determination of crystals. ,− With the strong interactions between electrons and matter, single crystal diffraction data can be obtained from a submicrometer or nanometer sized crystal. 3D ED has made large contributions to the structural characterizations of many different types of crystals, such as minerals, metal oxides, porous crystals, pharmaceutical molecules, proteins, etc . − However, the application of this method has not been achieved yet in the in situ studies of structural phase transitions of single nanocrystals.…”

Atomic-Level Imaging of Zeolite Local Structures Using Electron Ptychography

“…Atomic-level imaging of zeolites using electron microscopy, especially imaging of the active sites, is essential to deeply understanding their chemical behavior and becomes indispensable in order to rationally design materials with targeted properties. − Zeolites, as one of the most important kinds of porous materials, have been widely used in catalysis, selective separation, and so on, due to their particular structures. − High-resolution imaging of zeolites has been very challenging, as they are extremely sensitive to electron beams. Therefore, exploration of new electron microscopy imaging techniques and methodology that would enable the direct imaging of zeolites at the atomic level is highly desirable. − The implementation of aberration correctors and low-dose imaging techniques has allowed the direct visualization of zeolite framework T (T = Si, Al, Ge, etc.) atoms. ,, However, the image resolution is diminished due to the low signal-to-noise (SNR) ratio, suffering from the low collection efficiency of the detectors and the low number of electrons used to form the images.…”