Synthesis of stable SiO ₂@Au-nanoring colloids as recyclable catalysts: Galvanic replacement taking place on the surface

A surface galvanic replacement process was developed to prepare SiO ₂@Au-nanoring composite colloids with high activity and good recyclability for liquid-phase catalysis. The growing process can be monitored by the plasmonic absorption of composite colloids, and the formation of the ring structure can be explained by galvanic replacement confined on the colloid surface. The in situ growth strategy developed in this work provides a useful method to prepare reactive and stable nanostrucutres with the desired shape and exposed crystal facets on the supporting materials. The in situ grown SiO ₂@Au-NR colloid has a thin, amorphous, but permeable, silica layer extending from the colloid surface and wrapping around the Au nanoring, which firmly fixes the nanostructure without blocking the path for reactant molecules to reach the nanocrystal inside, leading to an improved stability in various chemical environments. Compared with the later-absorbed SiO ₂@Au- nanoparticle colloid, the SiO ₂@Au-NR colloids have comparable high activity and enhanced recyclability in catalysis.