Abstract
General factors to characterize a starting solution for wet-chemical synthesis are solvent species, solute species (reagent), and their ratio (= concentration). No other factors than these three are scarcely noticed. Here we focus our attention to the forth factor, the "age" (standing time after preparation) of it. It is well-known that monodispersed spherical particles can be prepared by controlled hydrolysis of metal alkoxides (what is called Stober method). Although the precise control can be made by adjusting temperature, concentration, and pH, there may be an unexpected change of the particle size or an undesirable degradation of monodispersibility due to an unknown reason. In this research we introduce the new parameter, the age of starting solution, into an optimized synthesis of spherical particles. In addition, soft sonication (weak irradiation of ultrasonic vibration) is proposed for more efficient aging. In the synthesis of spherical silica with starting solutions of different age, we demonstrate the following effects of aging on the precipitation behavior; (1) increase of the incubation period for precipitation, (2) increase of the final particle size, and (3) improvement of the monodispersibility. The increment of the final particle size as a function of aging time is twice larger in the case of soft sonication than in the case of normal aging. We assume from these results that a starting solution, especially that including an organic solvent with low dielectric constant, may be inhomogeneous at microscopic level just after the preparation. In the present case, the as-prepared solution of tetraethylorthosilicate (TEOS) and ethanol may involve some clusters, which may cause unfavorable nucleation to degrade the monodispersibility. It is inferred that those "clusters" can be broken or dispersed by aging for days or weeks. Reduction of the aging period by soft sonication would be due to high frequency (〜tens of kHz) and small amplitude (〜nm) of the vibration.
