Photoluminescent Silica Nanotubes and Nanodisks Prepared by the Reverse Micelle Sol-Gel Method

The reverse micelle sol-gel method was used earlier to prepare silica nanotubes, in aerosol OT/ n-heptane/water microemulsions containing FeCl3. The present communication reports the remarkable effect of the amount of water in the microemulsions on the shape, size, and spectral properties of the silica nanostructures formed. Nanotubes are formed, as expected, at lower water contents. However, for higher water contents, nanodisks form in predominance. This rather surprising observation indicates the formation of flat, disklike water pools in this medium. Notably, a phase separation occurs at higher water contents, and this appears to be essential for the formation of the disklike nanostructures. Hence, we propose that flat water pools form at the interface of the two liquid phases. The nanotubes and nanodisks exhibit blue photoluminescence. The photoluminescence or the nanotubes is more susceptible to quenching by moisture than that of the nanodisks. Luminescence is restored by heating or purging nitrogen or oxygen. Time-resolved photoluminescence studies conform to a model in which the luminescence is ascribed to a particular kind of defect center, with some contribution from surface-associated defects.