Synthesis of Luminescent Silica Crystals via a Sonochemical Reduction Route

Reduction of silica (SiO2) in the bulk form is quite challenging because the bulk silica in both the amorphous and crystalline states is a highly stoichiometric material. In this work, a solution-based synthesis route is developed to produce off-stoichiometric amorphous silica (SiOx with x < 2) by using sonochemical reduction of silica-based materials in the presence of sodium. The resulting amorphous SiOx materials are found to crystallize after annealing at temperatures higher than similar to 800 degrees C under Ar or vacuum environment, resulting in off-stoichiometric silica crystals. The main crystal phase is cristobalite, and a small amount of tridymite was also included. The thus obtained silica crystals exhibit a broad photoluminescence (PL) emission peaking at similar to 500 nm, characterized by the PL decay constant on the time scale of microseconds. The spin forbidden emission of oxygen vacancies in a silica crystal lattice is likely to be responsible for the observed broad visible PL emission. The present sonochemical reduction process provides a simple but useful method to fabricate luminescent off-stoichiometric silica crystals with no luminescent activator ions.