Synthesis of silica/Au core-shell nanostructures by galvanic replacement of silica/Ag in aqueous and alkaline medium

We present here a facile one-step method for the synthesis of silica/Au core-shell nanostructures by exploiting the potential difference of AuCl4− and Ag in aqueous as well as alkaline media. Initially, silica/Ag core-shell nanostructures were synthesised by coating Ag nanoparticles on silica core (size ∼150 nm) in a two-step process (seeding and growth) and were characterised for their morphological, structural and optical behaviours. A complete coverage of silica core with Ag nanoparticles was seen from scanning electron microscope and transmission electron microscope images. The presence of resonance peaks in the optical spectrum manifests the nature of the shell (thin shell ∼413 and 650 nm, thick shell ∼434 nm). Galvanic replacement of silica/Ag core-shell nanostructures in chloroauric acid solution (HAuCl4) was studied in both the aqueous and alkaline medium, where an aqueous environment results into fast and effective replacement as compared to an alkaline medium, which has been confirmed from optical absorption studies. The optical studies showed that in an alkaline environment, on galvanic replacement of Ag with Au, the individual absorption peak of Ag (∼414 nm) and Au (∼520 nm) disappeared, whereas new absorption wavelengths in higher region (600–800 nm) of electromagnetic spectrum were observed. A detailed mechanism is proposed for the same to explain this behaviour. A range of novel new plasmonic core-shell nanomaterials can be synthesised as an intermediate of this facile one-step reaction.