THERMAL STABILITY OF POROUS GOLD NANOPARTICLES COVERED WITH ALUMINA

Nanoporous metals have wide practical application as chemical sensors, catalysts, and plasmonic waveguides. The problem is their low thermal stability and propensity to coarsening. Thus, the stabilization of morphology of nanoporous metals can expand the range of their practical applications at high temperatures. Here we report the thermal stability of porous gold nanoparticles (PGN) covered with alumina at the temperatures up to 1000 ºC.

The porous gold nanoparticles on SiO₂/Si substrate were obtained by solid state dewetting of Au/Ag bi-layers (8 nm Au/20 nm Ag) followed by selective dealloying of Ag. Afterwards, a 5-nm thick Al₂O₃ was deposited on PGN by atomic layer deposition (ALD). As-received samples were annealed for 60 min at the temperatures from 350 °C up to 1000 °C in air, and characterized with the aid of high-resolution scanning electron microscopy (HRSEM). We also performed the in-situ SEM heating and in-situ XRD annealing experiments.

No morphology changes were revealed up to the temperature of 500 ºC indicating that ALD coating effectively suppresses coarsening of porous nanostructure. The following heating results in a slow diffusion of gold along the alumina layer, which appears in SEM-images as the contrast change of individual ligaments. The diffusion of gold promotes the formation of well-faceted gold nanoparticles at the temperature of 900 ºC. Gold emerges from the alumina coating leaving behind the hollowed shell, which is confirmed by EDS spectra. In the case of interconnected particles surface diffusion transports the material over a long distance to a seed that later develops into a micrometer-sized Au particle. We performed a simulation of the growth of faceted gold particles and estimated the critical radius of a seed.