Templated Synthesis and Chemical Behavior of Nickel Nanoparticles within High Aspect Ratio Silica Capsules

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One-dimensional transition metal nanostructures are of interest in many magnetic and catalytic applications. Using a combination of wet chemical synthesis, optical (infrared), and structural characterization methods (powder X-ray diffraction, scanning and transmission electron microscopy), we have investigated four paths to access 1D nickel nanostructures: (1) direct chemical reduction of a self-assembled nickel–hydrazine coordination complex, (2) thermal decomposition of the silica encapsulated nickel–hydrazine complex, (3) treatment of the silica encapsulated nickel–hydrazine complex with sodium borohydride followed by thermal annealing, and (4) electroless nickel plating using silica encapsulated nickel seed particles. We find that only route 1, which does not require a silica template, results in the formation of nickel nanorods, albeit some particle aggregation is observed. Routes 2 and 3 result in the formation of isotropic nickel structures under a reducing atmosphere. Route 4 results in heterogeneous nucleation and growth of existing particles only when partial etching of the silica capsule occurs. Detailed examination of the encapsulated nickel particles allows studying the effect of silica surface silanols on the oxidation of encapsulated nickel particles, the presence of nanoparticle–silica support interactions, the sintering mechanism of nickel and nickel oxide particles, and the fate of boride impurities. Nickel/silica nanostructures are strongly magnetic at room temperature.