We have studied the structure of nanometer-sized silica particles called fumed silica, which is a synthetic amorphous silicon dioxide produced by burning silicon tetrachloride in an oxygen-hydrogen flame, using infrared and Raman spectroscopies and a high-energy x-ray diffraction method. It has been demonstrated that the structure of fumed silica is not identical to that of the normal bulk silica glass in terms especially of the distribution of the size of silica rings. Three-and four-membered rings are more frequent in fumed silica than in the bulk silica glass. It has also been shown that the network structure of fumed silica is more flexible than that of the bulk one, probably explaining the reason why fumed silica can accommodate a large number of three-and four-membered rings in the structure.

We have studied the photoluminescence (PL) characteristics of nanometer-sized amorphous silica particles after appropriate annealing in air and in vacuum. A broad visible PL band peaking at ∼450 nm, which is characterized by a nonexponential decay in the order of nanoseconds, has been found to develop when the samples are heat-treated in air. On the other hand, the vacuum-heated samples do not show such an increase in PL in the visible region but instead show a development of PL in the near-ultraviolet region. The different PL characteristics observed between the air- and vacuum-heated samples are discussed in terms of the different dehydroxylation reactions in air and in vacuum. Possible models of the respective emission centers are presented on the basis of the density functional theory calculations.

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