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

Abstract

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 O₂ or H₂), while the TEM measurements were carried out at 1 Torr of H₂ and 0.5 Torr of O₂. Surprisingly, the NPs were found to be stable against sintering under both gaseous atmospheres up to 650 °C. Nevertheless, drastic sintering via diffusion–coalescence was observed upon annealing in H₂ at 800 °C. In contrast, an identically prepared sample demonstrated lack of agglomeration at the same temperature under O₂. The latter observation is ascribed to a strengthened chemical bond at the NP/support interface due to the formation of PtO_x species at low temperature. Subsequently, oxidative NP redispersion – associated with some loss of Pt due to the formation of volatile PtO_x species – is inferred from the behavior in O₂ at/above 650 °C. In contrast, SiO₂ reduction catalyzed by the presence of the Pt NPs and Pt silicide formation was found in H₂ at 800 °C, which might play a role 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 role played by their initial size, size distribution and dispersion on their stability against sintering.