The Solid State Reaction Process Of Nanopaticles Investigated By In-situ Transmission Electron Microscopy

In-situ transmission electron microscopy (TEM) observation has been used as a tool to investigate solid state reaction dynamic processes of nanoparticles and interface structure at atom scale. For example, during incident electron beam interaction with the surface of Au and ZrO₂ nanoparticles with a short time, it causes solid state reaction and formed alloy trough interface between Au and ZrO₂; The interaction of electron beam with metal oxide nanoparticles (ZrO₂) leaded the shape and phase deformation of the nanoparticle; metal nanoparticle inlaying amorphous carbon produces a composite structure e.g. the metal nanoparticle encapsulation in carbon fullerene. All of above dynamic interaction processes can be investigated by TEM with time. The conclusions are as follow:Firstly, the solid reaction of Au with ZrO₂ nanoparicles has been observed by in-situ TEM technology under electron beam irradiation. The factors such as irradiation time and the size of Au nanopaticle can be related to the sold reaction. For Au nanoparticles smaller than certain critical size and a semi-coherent interface between Au and ZrO₂ nanoparticles, the whole nanoparticle occur alloy and form the Au4Zr alloy. While for larger Au nanoparticles, a core (Au)-shell (Au4Zr) structure would form. The interface between Au and ZrO₂ became straight (no wetting) than before (wetting). While for larger Au nanoparticles, a core-shell structure was formed with Au as the core and Au4Zr as the shell. The irradiation leads change in the wetting behavior during all alloy processes. And the same time, the shape change of Au nanoparticle because of their coalescent cause change in the capability of the catalyst.Secondly, through electron beam irradiating on diverse sizes platinum nanoparticles, we found for small platinum catalyst particles (with diameters less than 5 nm), carbon shells formed on the surfaces of the platinum catalyst particles. It was found that the formation process was different from the previous studies and belonged to a typical catalyze growth mechanism. The formation process could be simplified to four stages such as the decomposition of amorphous carbon, the carbon shells starting from the nucleation of amorphous carbon on the preferred (111) planes of platinum nanoparticles, the first graphitic shell in which small platinum nanoparticles encapsulation closure, the second graphitic shell closure, which was accord with Nanol's theory. At the moment, a composite structure e.g. Pt nanoparticles encapsulation in graphite shell was formed. They happen coalesce and their coalescence was hindered by the surrounding shells under continued irradiation. After the smaller platinum catalyst particles ultimately coalesced, they interacted to form a larger compact platinum particle after breaking the encapsulating shells. For larger platinum nanoparticles (with diameters larger than 5 nm), no encapsulation of platinum nanoparticles was observed and there occured only the coalescence of platinum nanoparticles.Finally, the result of nano-sized ZrO₂ with electron irradiation was found that the transformation of structure and phase. The experimental results show that the size of nanoparticle was affected on transformation. For the smaller size ZrO₂ nanoparticles, occured phase transformation of tetragonal- orthorhombic and before transformation, the smaller particles undergo the process of coalesce and growth until the particle size was sufficient large (13.6 nm). For the larger ones (¹3nm), beside the phase transformation of tetragonal - monoclinic, the shape of the particle also changed before the shape transformation, the particle occurred rotation. The critical grain size in the experiment was accord with dates in Baldinozzi's theory model.