Synthesis and chemical transformation of Ni nanoparticles embedded in silica

Ni nanoparticles (NPs) catalyze many chemical reactions, in which they can become contaminated or agglomerate, resulting in poorer performance. We report deposition of silica (SiO₂) onto Ni NPs from tetraethyl orthysilicate (TEOS) through a reverse microemulsion approach, which is accompanied by an unexpected etching process. Ni NPs with an average initial diameter of 27 nm were embedded in composite SiO₂-overcoated Ni NPs (SiO₂-Ni NPs) with an average diameter of 30 nm. Each SiO₂-Ni NP contained a ∼7 nm oxidized Ni core and numerous smaller oxidized Ni NPs with diameters of ∼2 nm distributed throughout the SiO₂ shell. Etching of the Ni NPs is attributed to use of ammonium hydroxide as a catalyst for deposition of SiO₂. Aliquots acquired during the deposition and etching process reveal agglomeration of SiO₂ and Ni NPs, followed by dissociation into highly uniform SiO₂-Ni NPs. This etching and embedding process may also be extended to other core materials. The stability of SiO₂-Ni NPs was also investigated under high-temperature oxidizing and reducing environments. The structure of the SiO₂-Ni NPs remained significantly unchanged after both oxidation and reduction, which suggests structural durability when used for catalysis.