Synthesis and catalytic activity of M@SiO₂ (M = Ag, Au, and Pt) nanostructures via "core to shell" and "shell then core" approaches

M@SiO₂ (M = Ag, Au, and Pt) core-shell nanostructures were prepared by the "core to shell" and "shell then core" approaches. In the former, the metal core size could be controlled in the 6-9 nm range with a narrow size distribution, and the shell porosity was tunable. The preparation was straightforward and efficient, without requiring specialized high-speed centrifugation. Au@SiO₂ containing mesoporous SiO₂ shells (Au@meso-SiO₂) exhibited good thermal stability and high CO oxidation activity (T₁₀₀ = 235°C) even after being subjected to calcination in air at 550°C. In the latter approach, the core size could be controlled at < 10 nm with a narrow size distribution, and the shell porosity was tunable to a fine degree. 4-Nitrophenol was readily reduced at room temperature in the presence of Au@meso-SiO₂ obtained through the "shell then core" approach. The SiO₂ shell mesoporosity minimized the diffusion limitation of 4-nitrophenol. The core-shell structures from both approaches were uniformly dispersed. Employing Si sources with differing functionality allowed the SiO₂ shell and metal core properties to be modified in these approaches, which is beneficial for application.