An in situ transmission electron microscopy study of sintering and redispersion phenomena over size-selected metal nanoparticles: environmental effects

The thermal and chemical stability of micelle-synthesized size-selected Pt nanoparticles (NPs) supported on thin SiO2 (20 nm) films was monitored in situ via transmission electron microscopy (TEM) under pure hydrogen and pure oxygen environments. The coarsening treatment was performed for 30 min at each temperature (1 Torr of either O-2 or H-2), while the TEM measurements were carried out at 1 Torr of H-2 and 0.5 Torr of O-2. Surprisingly, the NPs were found to be stable against sintering under both gaseous atmospheres up to 650 degrees C. Nevertheless, drastic sintering via diffusion coalescence was observed upon annealing in H-2 at 800 degrees C. In contrast, an identically prepared sample demonstrated lack of agglomeration at the same temperature under O-2. The latter observation is ascribed to a strengthened chemical bond at the NP/support interface due to the formation of PtOx species at low temperature. Subsequently, oxidative NP redispersion - associated with some loss of Pt due to the formation of volatile PtOx species - is inferred from the behavior in O-2 at/above 650 degrees C. In contrast, SiO2 reduction catalyzed by the presence of the Pt NPs and Pt silicide formation was found in H-2 at 800 degrees C, which might play a rote in the enhanced coarsening observed. Subsequent exposure of the PtSi NPs to oxygen led to the formation of Pt-SiO2 core-shell structures. Our findings highlight the dynamic structural transformations that nanoscale materials experience under different environments and the important rote played by their initial size, size distribution and dispersion on their stability against sintering.