2, Universal Technology Corporation, Dayton, Ohio, United States
3, Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio, United States
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 initial average 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.