Single-step coating of mesoporous SiO(2) onto nanoparticles: growth of yolk–shell structures from core–shell structures

Yolk–shell nanoparticles based on mesoporous SiO(2) (mSiO(2)) coating of Au nanoparticles (Au NPs) hold great promise for many applications in e.g., catalysis, biomedicine, and sensing. Here, we present a single-step coating approach for synthesizing Au NP@mSiO(2) yolk–shell particles with tunable size and tunable hollow space between yolk and shell. The Au NP–mSiO(2) structure can be manipulated from core–shell to yolk–shell by varying the concentration of cetyltrimethylammonium chloride (CTAC), tetraethyl orthosilicate (TEOS), Au NPs, and NaOH. The growth mechanism of the yolk–shell particles was investigated in detail and consists of a concurrent process of growth, condensation, and internal etching through an outer shell. We also show by means of liquid-cell transmission electron microscopy (LC-TEM) that Au nanotriangle cores (Au NTs) in yolk–shell particles that are stuck on the mSiO(2) shell, can be released by mild etching thereby making them mobile and tumbling in a liquid-filled volume. Due to the systematical investigation of the reaction parameters and understanding of the formation mechanism, the method can be scaled-up by at least an order of magnitude. This route can be generally used for the synthesis of yolk–shell structures with different Au nanoparticle shapes, e.g., nanoplatelets, nanorods, nanocubes, for yolk–shell structures with other metals at the core (Ag, Pd, and Pt), and additionally, using ligand exchange with other nanoparticles as cores and for synthesizing hollow mSiO(2) spheres as well.