In-situ transmission electron microscopy investigation on the evolution of Pt nanocrystals in oxidizing and reduicng atmospheres

Metal nanocrystals exhibit unique properties due to their high surface-to-volume ratio and have great potential for applications in the fields of electronics, magnetics, optics and catalysis. However, their high specific surface area leads to easy coarsening in operation, which may greatly degrade their performances, especially when they are exposed to various chemical environments or at high temperatures. Therefore, the direct visualization of nanocrystals' structural evolution when they are coarsening is crucial to gain insight into the mechanism and develop more effective means to improve the size stability of nanocrystals. In this work, we investigated the structural evolution of Pt nanocrystals with sizes of ~ 4 nm on SiN x film in both oxidizing and reducing atmospheres at a moderate temperature (300 ) in the aberration-corrected environmental transmission electron microscopy (ETEM). The sizes of nanocrystals remain almost unchanged when annealed in the oxygen atmosphere with volatile PtO x formation on the surface, hindering nanocrystals sintering and leading to Pt loss. On the other hand, obvious coarsening of nanocrystals resulting from Ostwald-ripening and nanocrystal migration and coalescence was observed in the reducing atmosphere. Our findings reveal the dynamic structural evolution of nanocrystals in different atmospheres and provide possible ways to improve the size stability of nanocrystals.