High-resolution transmission electron microscopy of small amorphous silica particles

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Abstract
High-resolution transmission electron micrographs of two specimens of amorphous silica prepared under radically different conditions-slow polymerization from aqueous Si(OH)4 solution and by vapour-phase hydrolysis of Si(Cl)4 in a hydrogen -oxygen flame -have been obtained. Both processes provide particulate silica with a mean diameter of around 150 Å. High-resolution images of solution-precipitated silica have a more regular, ordered appearance than those of the flame-hydrolysed material. The presence of apparently ordered regions in solution-precipitated silica particles is surprising, since the diameter of 150 Å is larger, by a factor of 3, than the computed upper limit for observation of lattice fringes of random arrangements of 15 Å crystallites. The significance of this result is discussed in terms of the symmetry of packing of ordered units in an ‘amorphous cluster’ model of the structure.

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