(95e) Silver/Silica (Ag/SiO2) Core-Shell Particles: The Effect of Alcoholic Solvent

Reliable methods for synthesis of core-shell metal-dielectric particles are sought due to the desirable properties of such particles in a variety of applications ranging from catalysts to sensors. Silica shell provides noble metal particles with chemical and colloidal stability while retaining their catalytic properties. For silver nanoparticles that exhibit surface plasmon resonance (SPR) absorption in the visible part of the spectrum, this valuable optical characteristic is maintained: the SPR absorption peak is red-shifted, but controllably so in the stable silica refractive index environment. We describe a simple aqueous synthesis of silver nanoparticles ranging 40-100 nm in average size with < 20% size variation, and their coating with a uniform silica shell. Saccharides (glucose or maltose) are utilized simultaneously as the reducing agent and stabilizer in the silver nanoparticle synthesis, and Tollens reagent is used as the source of silver ions (Ag⁺). Prompt and adequate mixing of the reagents promotes rapid generation of multiple silver (Ag⁰) nuclei which results in smaller silver nanoparticle sizes, while slower mixing yields larger particles due to less nuclei generated at the initial stage of the reaction. The as-synthesized silver nanoparticles can be easily coated with silica shells by using alkoxysilanes (e.g. tetraethoxy- or tetrabutoxysilane) as the silica precursor and dimethylamine as the catalyst for the alkoxysilane hydrolysis. Unlike the more commonly used ammonium hydroxide catalyst, dimethylamine does not form a soluble complex with silver within the timeframe of silica shell growth, therefore no change in silver nanoparticle size or concentration occurs. Concentrating the as-synthesized silver nanoparticle colloids by centrifugation allows using different concentrations of silver nanoparticles without changing the water-alcohol ratio in the silica shell synthesis. A common issue in core-shell particle synthesis by solution methods is heterogeneous nucleation whereupon two populations are formed: the desired core-shell particles and undesired coreless particles of the shell material. We show that the choice of alcoholic solvent has an enormous influence on the product, mainly by affecting the silica precursor hydrolysis rate, but also the silver particle aggregation, which stands responsible for the formation of multi-core Ag/SiO₂ particles. In methanol, where tetraalkoxysilane hydrolysis rate is the fastest, small stable silica nanoparticles are formed in the bulk of the solution and silver particles remain uncoated. In ethanol, it depends on the silver surface area available whether homogeneous nucleation of silica on silver is achieved. At lower silver particle concentration, also coreless silica particles are formed. In case of 2-propanol, only core-shell Ag/SiO₂ particles are formed, but due to faster aggregation of silver in 2-propanol, also multi-core particles are noted where several silver particles are encapsulated within the same silica shell. In conclusion, we present reliable synthesis of silver nanoparticles and their coating with silica shells where by choosing the right alcoholic solvent and silver concentration, no byproduct of secondary silica particles occurs.