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In Situ Imaging of Silicalite-1 Surface Growth Reveals the Mechanism of Crystallization

Science  16 May 2014:
Vol. 344, Issue 6185, pp. 729-732
DOI: 10.1126/science.1250984

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Crystal Growth

Two main routes for the growth of crystalline species are either via molecule-by-molecule attachment to existing nuclei or via the addition of preformed metastable precursors, but do these mechanisms need to be mutually exclusive? Lupulescu and Rimer (p. 729; see the Perspective by Dandekar and Doherty) developed an in situ atomic force microscopy (AFM) technique to study the crystallization of materials under extreme conditions of temperature (25° to 300°C) and alkalinity (up to a pH of 13). The growth of the zeolite silicalite-1 involved both the attachment of metastable precursors and of individual molecules.

Abstract

The growth mechanism of silicalite-1 (MFI zeolite) is juxtaposed between classical models that postulate silica molecules as primary growth units and nonclassical pathways based on the aggregation of metastable silica nanoparticle precursors. Although experimental evidence gathered over the past two decades suggests that precursor attachment is the dominant pathway, direct validation of this hypothesis and the relative roles of molecular and precursor species has remained elusive. We present an in situ study of silicalite-1 crystallization at characteristic synthesis conditions. Using time-resolved atomic force microscopy images, we observed silica precursor attachment to crystal surfaces, followed by concomitant structural rearrangement and three-dimensional growth by accretion of silica molecules. We confirm that silicalite-1 growth occurs via the addition of both silica molecules and precursors, bridging classical and nonclassical mechanisms.

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