Researchers at the University of Minnesota have used high-resolution TEM microscopy to image the atomic structure of ultrathin zeolite nanosheets, which are used by industries as specialized molecular filters. The researchers observed one-dimensional defects in a two-dimensional structure of porous zeolite called MFI. By imaging the atomic structure of MFI nanosheets at such an extraordinary level of detail, they were able to discern that these one-dimensional defects resulted in a reinforced nanosheet structure that changed the filtration properties of the nanosheet significantly. The minute differences were detected by researcher Prashant Kumar. “After staring at noisy images in the TEM for countless hours, I finally saw the symmetry breaking in the TEM images of MFI nanosheets — I knew this was unusual,” Kumar said. Although the differences were subtle, they had pronounced consequences on the ability of the nanosheets to recognize and selectively transport molecules, enabling selective separations and catalysis. This image shows atomic-scale details from transmission electron microscopy that reveal the porous structure of an MFI nanosheet, with MEL intergrown in it. Courtesy of Prashant Kumar et al., University of Minnesota. When the researchers performed simulations to test the material’s pattern and performance, they found that the knitted materials were less responsive to stress and more selective in separating molecules based on size and shape. Membranes made from the enhanced nanosheets for the simulations were tested under industrial conditions and demonstrated record-strong filtration performance, the researchers said. “TEM imaging of thin zeolite crystal at the atomic-scale has been a long-standing challenge as these crystals are readily damaged under the high-energy electrons, which are needed for atomic-scale imaging,” professor K. Andre Mkhoyan said. “It requires a deep understanding of the mechanisms of beam damage for zeolite crystals and the doses of electron beam that the zeolite can take. This work pushed the limits of our electron microscopes, where we can reliably produce atomic-resolution images of such extremely thin — just 3 nanometers thick— zeolite nanosheets with identifiable one-dimensional intergrowths.” The discovery could be applied to gasoline, plastics, and biofuel production to improve efficiency. “The discovery by TEM of one-dimensional intergrowths in two-dimensional nanosheets and the practical implications suggested by modeling bring the potential of this concept to a new level and suggest new opportunities for targeted synthesis that we have not imagined possible,” professor Michael Tsapatsis said. The research was published in Nature Materials (www.doi.org/10.1038/s41563-019-0581-3). This video by University of Minnesota researchers shows one-dimensional intergrowths in two-dimensional zeolite nanosheets and their effect on ultraselective transport. According to the researchers, this breakthrough discovery pushes the limits of microscopy to improve efficiency of fuel and plastic production. Courtesy of Prashant Kumar et al., University of Minnesota.
