Pressure Engineering through Silica Nanoconfinement for Melting Point Elevation of Gold Nanoparticles

Au nanoparticles (NPs) are inherently thermally unstable at elevated temperatures because their large surface-to-volume ratio promotes melting, migration, and coalescence. Here, we demonstrate that silica (SiO2) nanoconfinement thermodynamically elevates the melting point (Tm) of Au NPs through confinement-induced compressive pressure. Atomic-resolution in situ heating scanning transmission electron microscopy (STEM) combined with local Fourier crystallinity mapping reveals that bare Au NPs lose crystallinity at 1,000 °C, whereas SiO2-encapsulated Au cores (Au@SiO2) remain crystalline. Accordingly, in situ heating TEM shows bare Au NPs progressively merge while Au@SiO2 remain structurally isolated. Temperature-dependent lattice-parameter measurements reveal significantly suppressed thermal expansion of confined Au cores, corresponding to an estimated compressive stress of 4-5 GPa. Finite-element simulations support this estimate, showing that SiO2 shell shrinkage generates several-GPa core pressure, with thicker shells producing higher pressure and greater Tm elevation. These findings establish nanoconfinement-induced pressure as a general thermomechanical strategy for stabilizing metallic NPs under extreme thermal environments.