The Shape Evolution And Coalescencegrowth Of Au NPs Under In Situ Conditions

In practical applications,the surface effect of gold nanoparticles（Au NPs）leads to coalescence growth and shape changes of NPs widely.However,so far,relevant research has rarely provided information on the movement of atoms at the atomic level during the coalescence process,and the understanding of the evolution of the microstructure of Au NPs on the atomic scale is still unclear.This will not be conducive to the structural optimization and performance adjustment of Au NPs,so it is necessary to use in situ technology to track the dynamic changes of NPs under relevant conditions.In this paper,using high-temporal resolution in situ transmission electron microscopy technology,the dynamic evolution process of Au NPs under in situ gas phase conditions and the coalescence growth process of Au NPs under in situ heating conditions are studied as follows:（1）During the CO₂hydrogenation reaction catalyzed by Au NPs,Au NPs with a particle size≤5 nm will quickly agglomerate to form 5-10 nm Au NPs.As the reaction progresses,the surface of the coalesced Au NPs undergoes differential reconstruction while the shape of the Au NPs constantly oscillates.First-principles calculations show that the adsorption of the reactant gas CO₂induces the coalescence of Au NPs,while the adsorption of CO and/or H₂O generated by the reaction causes the shape of Au NPs to oscillate.That is,the adsorption of various gas molecules leads to continuous changes in the most stable structure of the active center of the catalyst.（2）Under the conditions of electron beam irradiation and heating,the movement of Au atoms in different stages of Au NPs coalescence was observed.Au NPs move through the center of mass until the critical distance between NPs is～1 nm.At this critical distance,the Au atoms on the surface of the NP diffuse to form thin Au chains or bridges between the NPs,that is,the atomic material transport channel.Au atoms continue to diffuse between Au NPs and are arranged in an orderly manner to gradually fill the Au bridges.With the growth of the Au bridge,the local internal stress generated after the connection of Au NPs drives the regional structure reconstruction of Au NPs through the coordination of NP rotation or/and grain boundary migration.