Diffraction and imaging are two important types of characterization methods for atomically resolving material structure in reciprocal space and real space, respectively. The characterization methods based on X-ray diffraction are usually considered to be efficient for determining periodic crystal structures and even resolving the accurate position of each atom. Although the real-space imaging methods have played an irreplaceable role in characterizing local structures, such as surfaces, interfaces, and defects, they have not demonstrated any superiority over the diffraction methods in analyzing periodic bulk lattices. However, for complex porous and reticular structures, such as zeolites and metal organic frameworks (MOFs), accurate structural analysis by X-ray diffraction is becoming increasingly limited. On the one hand, the intrinsic local flexibility in framework enables various non-periodic restructuring under different conditions, while the diffraction methods only provide averaged information over bulk materials instead of these local changes. On the other hand, some of these porous materials exist in the form of nanocrystals, which are difficult to use single-crystal X-ray diffraction to determine their fine structures.
To address the above challenges, recently, Prof. Boyuan Shen and Prof. Tao Cheng from the Institute of Functional Nano and Soft Materials (FUNSOM) of Soochow University, in collaboration with Xiao Chen and Prof. Fei Wei from the Department of Chemical Engineering of Tsinghua University, have recently demonstrated the effectiveness of iDPC-STEM for beam-sensitive materials and the advantages of real-space imaging over diffraction method for studying porous materials through the imaging and analysis of MIL-125 lattice structures. The iDPC-STEM imaging helped us reveal the unexpected rotated node feature of MIL-125 lattice caused by the rotation of Ti-O nodes, which has not been realized by the previous structural analysis combining the PXRD and simulation. Then, the first-principle calculations explained why such rotated node structure exists and it cannot be identified in the PXRD patterns. From the edge to the center of MIL-125 particle, the rotation angles of Ti-O nodes show a clear trend of progressive increase, while the defects and surface terminations in MIL-125 are also related to such distribution of Ti-O node rotation. The discovery of rotated node structure in MIL-125 provides insights on local flexibility and node-linker coordination in reticular chemistry, which may guide further study of the structure-property relation in MOFs at the molecular and atomic scales. Moreover, in this case, the real-space imaging methods represented by electron microscopy can contribute to a more accurate structural analysis on these complex porous materials than the diffraction methods, which may help confirm the bulk lattice structures in these materials. It will encourage more researchers to reasonably question the previous diffraction analysis of complex structures and reveal more local structural changes in these materials through real-space imaging.
Fig 1. High resolution iDPC-STEM imaging of MIL-125.
Fig 2. Discovering rotated node structure in MIL-125.
Link to paper: https://doi.org/10.1038/s41467-024-51384-9
Title: Real-space imaging for discovering a rotated node structure in metal-organic framework
Authors: Jiale Feng, Zhipeng Feng, Liang Xu, Haibing Meng, Xiao Chen*, Mengmeng Ma, Lei Wang, Bin Song, Xuan Tang, Sheng Dai, Fei Wei*, Tao Cheng* & Boyuan Shen*
Editor: Danting Xiang, Xin Du