Direct imaging of the pores and cages of three-dimensional mesoporous materials

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Abstract

Mesostructured composite materials, with features ranging from 20 to 500 Å in size, are obtained by the kinetically controlled competitive assembly of organic and inorganic species into nanostructured domains. Short-range order is limited, and long-range order is determined by weak forces such as van der Waals or hydrogen-bonding. Three-dimensional mesoporous materials obtained by removing the organic phase are of particular interest for applications such as catalysis and chemical sensing or separation, for which structural features such as cavity shape, connectivity and ordered bimodal porosity are critical. But atomic-scale structural characterization by the usual diffraction techniques is challenging for these partially ordered materials because of the difficulty in obtaining large (> 10 µm) single crystals, and because large repeat spacings cause diffraction intensities to fall off rapidly with scattering angle so that only limited small-angle data are available. Here we present a general approach for the direct determination of three-dimensional mesoporous structures by electron microscopy. The structure solutions are obtained uniquely without pre-assumed models or parametrization. We report high-resolution details of cage and pore structures of periodically ordered mesoporous materials1,2, which reveal a highly ordered dual micro- and mesoscale pore structure.

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Figure 1: SEM images and X-ray powder diffraction patterns of as-synthesized SBA-1 (a) and SBA-6 (b) .
Figure 2: HREM images of SBA-6 (calcined) and their Fourier diffractograms.
Figure 3: Direct image of 3D cages and bimodal pore structure in SBA-6 and SBA-1.
Figure 4: HREM images of SBA-16 (calcined) and their Fourier diffractograms.
Figure 5: Direct image of 3D pore structure of SBA-16.

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Acknowledgements

This work was supported in part by CREST, Japan Science and Technology Corporation (O.T.), by the National Research Laboratory Program of Korea (R.R.), and by the National Science Foundation (G.D.S.) and the Army Research Office (G.D.S.). O.T. thanks S. Andersson for encouragement and support. Y.S. thanks the Japan Society for the Promotion of Science.

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Correspondence to Galen Stucky.

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