Surface Reconstruction And Lattice Distortion Of Ag And Au Nanoparticles Under Annealing Conditions

Nanoparticles will always undergo a variety of microstructure evolution under annealing conditions,which will affect the macroscopic properties of the material.Understanding the precise structure of nanoparticles is very useful for determining their stability and related properties.However,due to the limitations of equipment and technology,there are still many key phenomena that have not been observed on the atomic scale.At present,the high temporal and spatial resolution of in-situ transmission electron microscopes can solve this problem.In this paper,the in-situ heating transmission electron microscope was used to study the structural evolution of Ag and Au nanoparticles under annealing conditions.Because the amorphous carbon covers the surface of the nanoparticles,and Au has a high catalytic activity for the C element.Amorphous carbon is catalyzed into graphene and wrapped on the surface of Au nanoparticles at high temperature,while Ag does not cause the amorphous carbon to generate graphene.Therefore,we studied the surface reconstruction process of Ag nanoparticles and the lattice distortion of Au nanoparticles bound by graphene at the atomic scale.The specific research content and results are as follows:(1)Ag nanoparticles are restructured through surface free atom migration during high temperature annealing conditions.The surface reconstruction process can be divided into two main steps.First,the atoms on the arc-shaped surface migrate,resulting in the gradual disappearance of the surface steps,and the curvature surface is reconstructed into a flat(111)facet.Second,atoms migrate and fill the defect positions on the adjacent surface steps,leading to the gradual formation of a flat(11-1)facet,which then grows in a layer-by-layer manner.These two processes occur simultaneously and eventually form Ag NPs,the surfaces of which are surrounded by {111} facets.(2)The surface atoms of Au nanoparticles are bound by graphene and cannot migrate,the stress of graphene causes the lattice distortion of Au nanoparticles,a polycrystalline structure composed of different types of grains is formed.The interplanar spacing of different crystal grains has an expansion of 3.0% to 6.8% compared with standard FCC Au,and the angle between the {111} crystal planes also changes about-2° to 35°.And formed crystal defects such as twins,stacking faults,and dislocations.