The thermal stability of noble metal particles is of interest both for fundamental reasons and because gold and silver nanoparticles show a unique optical response (plasmons) making them useful for a range of optical applications.

Using in-situ TEM, we find that a carbon shell governs the morphological transitions of gold and silver nanorods upon heating.

Performing the in-situ heating experiment with silver nanorods leads to a surprising observation: the silver nanorods are stable against heating to temperatures close to the bulk melting point, show no or little shape transformation, but eventually a strongly anisotropic sublimation. A thin cage-like structure with the original particle dimension is left behind.

To study the role of the material, we also investigated samples of gold nanorods. Covered rods slowly change their shape only close to the bulk melting temperature, i.e. at considerably higher temperatures than pristine gold nanorods. However, gold nanorods never show the strongly anisotropic sublimation behavior as observed with silver nanorods.

The fast and anisotropic sublimation behavior of silver particles may be explained by the different vapor pressures of the two materials. The observed difference between gold and silver nanorods is a strong hint how slightly different surface stability and surface chemistry affects the nanocrystal stability of otherwise similar materials to an unexpected degree. The thermal stabilization of gold rods by a thin surface coating may help in high power applications (e.g. enhancement of nonlinear optical effects) by pushing the thermal destruction threshold considerably.

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