Enhanced Thermal Stability of Gold and Silver Nanorods by Thin Surface Layers

Using in situ transmission electron microscopy, we find that a carbon shell governs the morphological transitions of gold and silver nanorods upon heating. Encapsulated Ag nanorods show a surprising nonuniform sublimation behavior starting from one side and leaving behind the shell. Uncovered gold nanorods transform their shape to spheres well below the bulk melting temperature through surface diffusion, which is prevented by a thin carbon shell. The thermodynamic properties of materials are known to differ from their bulk values when particles have nanometer dimensions.1,2 Responsible for this size effect is the large surface-to-volume ratio of nanoparticles, the sometimes unique crystal structure of nanoparticles, and the chemical nature of their surface. Gold nanoparticles were among the first artificially created inorganic nanostructures,3,4 and their response to heating has been extensively studied.5,6 The thermal stability of noble metal particles is of interest both for fundamental reasons and because gold and silver nanoparticles show a unique optica