Characterization of metal-organic frameworks by transmission electron microscopy

https://doi.org/10.1080/23746149.2022.2046157

“To address the increasing energy consumption and serious environmental problems, it is critical to develop efficient and clean energy conversion and storage devices. Among various categories of materials, metal-organic frameworks (MOFs) are one of the promising candidates that can realize the practical application of these devices. Therefore, it has been recognized that revealing the composition-structure-property relationship of MOFs by transmission electron microscopy (TEM) can offer some guidelines for designing novel materials with desirable properties. Nevertheless, owing to their organic parts, MOFs are too beam sensitive to be characterized by high energy electrons. To resolve this problem, various advanced techniques have been developed to accomplish the static and dynamic MOFs characterizations. Herein, we made a brief summary of the updated progress on characterization of MOFs by TEM until now, and revealed the key issues associated with static and dynamic TEM characterization of MOFs.”

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TEM imaging uses a parallel beam, while STEM uses a converged beam, which is scanned across the specimen. This generates a range of signals, which can be collected by various detectors to generate image and spectroscopic information. The bright field (BF) detector intercepts the directly transmitted electrons and produces bright field diffraction contrast images analogous to those produced in TEM imaging [Citation72]. Annular detectors, permit the transmitted electrons to pass through a central hole and intercept electrons scattered out of the beam at various angles. Electrons incoherently scattered by the atomic nuclei in the specimen through high angles, can by intercepted to form a high angle annular dark field image (HAADF). The contrast in such high HAADF images, is sensitive to both specimen thickness and the atomic number of the material under study [Citation73]. The atomic number dependence is proportional (approximately) to Z^1.7 [Citation74,Citation75]. The incoherent nature of the HAADF signal makes HAADF images directly interpretable (unlike HRTEM phase contrast image, which involve coherent scattering). HAADF images produce very high contrast between high and low Z elements. The corollary of this is that light atoms, such as oxygen, produce very little signal in such images and can be difficult to image. The annular bright field-STEM (ABF-STEM) technique can help visualize low and high Z atoms in the same material by collecting directly transmitted electrons from an annular region around the periphery of the transmitted beam-the centre of which is masked [Citation76]. More recently, integrated differential phase-contrast scanning transmission electron microscopy (iDPC-STEM) technique was developed, which is useful in characterizing sensitive materials, especially those containing light elements. The technique allows both low and high Z elements to be visualized and the contrast of iDPC-STEM images is easy to interpret as it is an averaged electronic potential proportional to atomic number. The signal-to-noise ratio, even under ultra-low dose mode, is relatively high as it excludes non-integrable vector field [Citation77–81].

In addition to crystal structure and imaging, chemical composition, electronic structure and bonding information are also accessible through STEM-based methods. In the last several decades, many powerful spectroscopic techniques have been developed including energy-dispersive X-ray analysis (EDX) for chemical analysis and electron energy loss spectroscopy (EELS)-based spectroscopy and imaging/mapping (energy filtered TEM (EFTEM)), which can provide both compositional and bonding information [Citation68,Citation69,Citation77–89].

It should be noted when imaging MOFs at the same electron dose in TEM and STEM modes separately, the local electron flux in STEM (highly focused beam) is higher than that of TEM (broad beam) and can cause destructive damage. Therefore, despite the many benefits of STEM, the majority of effort has been devoted to characterizing MOFs in TEM mode. The development of direct-detection electron-counting (DDEC) cameras has greatly enhanced low-dose imaging in TEM, as these can produce usable images formed with just a few electrons per pixel. Many MOFs have been studied using DDEC-equipped TEMs in TEM mode [Citation68,Citation69,Citation82,Citation86,Citation96,Citation97]. For example, a HRTEM image of intact ZIF-8 has been obtained (Figure 2f–i) using an ultralow electron dose (4.1 e⁻Å⁻² in total) [Citation82]. These HRTEM images resolved its local structure and provided hints of its self-assemble mechanism and surface termination modes. However, the zone-axis images were obtained through a time-consuming trial and error process.

The life time of MOFs under beam irradiation is short and on-axis HRTEM images are required to reconstruct a MOF structure, so it is important to reduce the time needed to acquire on-axis HRTEM images [Citation68]. The time needed to acquire an interpretable HRTEM image is the sum of the time needed for locating a specific crystal zone axis, aligning short-exposure images, and determining the defocus value [Citation68]. Hence, to reduce the total beam exposure time, new approaches have been developed: (1) To tilt the sample to the nearest zone axis directly, software has been developed to calculate the required goniometer tilting angles from the Laue circle of an electron diffraction pattern taken at the current (off-axis) orientation. (2) To achieve reliable alignment of successive short-exposure noisy images, a ‘Amplitude filter’ was developed to process such images. (3) To calculate the precise defocus value of an intact HRTEM image, the specimen was deliberately destroyed after the capture to obtain a series of images with various defocus value. The high efficiency and universality of these methods has been confirmed by their successful application in acquiring atomic-resolution TEM images of several beam-sensitive MOFs (UiO-66, ZIF-8, HKUST-1), a zeolite, and a perovskite (CH₃NH₃PbBr₃) (Figure 3a–g) [Citation68]. Following this work, low-dose HRTEM was also used to study the ‘missing linker’ and ‘missing cluster’ in UiO-66 (Zr) (Figure 3h–o), and the effect of various additives on the surface structure and thermal stability of MIL-101 (Cr) (Figure 3p–r) [Citation69,Citation96]. Moreover, DDEC-equipped low dose Cs-corrected HRTEM successfully revealed the atomic/molecular configurations of MIL-101 {111} sublayer surfaces and inorganic Cr₃(m₃-O) trimer-regulated surface transition [Citation65].

Recently, iDPC-STEM has been widely applied to the characterization of various materials, especially those containing light elements. In the early stages of the technique’s development, iDPC-STEM characterization of MOFs resulted in high image contrast but the resolution was lower than that of corresponding low dose HRTEM images [Citation96]. The image resolution of iDPC-STEM was improved recently through tuning of imaging conditions [Citation77,Citation80,Citation98]. For instance, the Cr/BDC super tetrahedrons details of MIL-101 have been clearly resolved (Figure 4a–c) and match the structural model of MIL-101 from the <110> projection perfectly [Citation80]. The information limit of its corresponding FFT pattern (Figure 4d) is ~1.8 Å, which is smaller than that of the ADF-STEM (16 Å) and HRTEM (2.5 Å) images acquired by a DDEC camera, respectively. Moreover, two types of surface termination and step-edge sites were disclosed by the iDPC-STEM images (Figure 4e–j). iDPC-STEM was also used in combination with HAADF-STEM to study the dynamic reaction of MIL-101-Cr towards constant beam irradiation, which results in dynamic pore change and visible crystal bending [Citation98]. The iDPC-STEM and HAADF-STEM results show both the lattice plane and the specific position of the crystal affect its dynamic local variation towards beam irradiation.

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