Structure Evolution Of Metal Nanocrystals Revealed By In-situ Liquid Cell Transmission Electron Microscopy

Metal nanocrystals are widely used in areas ranging from electronics,catalysis,optics,energy,bioscience,and other fields due to their unique properties.The physicochemical properties of metal nanocrystals are determined by their morphology,structure,and size.How to control the morphology and size of metal nanocrystals to achieve specific functions is important in nanoscience.Unveiling of the etching and growth mechanisms and morphological evolution of single nanoparticle in liquid is critical.In this thesis,we study the mechanisms of etching of gold(Au)nanorods and growth of silver chloride nanoplate on Au@Ag nanorods by taking advantages of the recent advances in in-situ transmission electron microscopy(TEM).The main research contents are as follows.1.We used in-situ liquid cell TEM to systematically study the structure and morphology changes of an individual Au nanorod by changing the electron beam dose rate.Three different etching mechanisms of gold nanorods are operative.When the dose rate is low(3.5×10⁹ Gy/s),the nanorod is slightly etched and reshaped into an equilibrium structure with low-energy facets,since the concentration of oxidative species produced by electron beam radiolysis of water remains low.With increase in dose rate,the nanorod is driven out of its equilibrium state and transiently adopts an ellipsoidal shape,to subsequently dissolve rapidly.When the dose rate reaches 4.5×10¹⁰Gy/s,unusual self-accelerated etching of nanorods resulting in intermediates with different aspect ratios is seen,which is believed to be critical for nanostructure modification.Besides,it is found that the etching rate increases significantly as the size of the nanorods decreases especially when the size is below the critical size.It should be noted that the critical size may vary with the degree to which the system deviates from equilibrium.These results enrich the understanding of the oxidation and etching mechanism,and shed light on controllable synthesis and rational design of nanostructures.2.The etching of gold nanorod by oxygen nanobubble in hydrogen bromide aqueous solution has been studied.We experimentally confirm that the location of gas bubbles in the liquid is crucial to increasing the speed of the solid-liquid-gas reaction.The local reaction is dramatically accelerated by up to 20 times when the distance between the gas bubbles and the solid(nanorods)is less than 1 nm.Combing nanorod and nanobubble tracking with molecular dynamics simulations,we provide a rationale for the increased reaction rate.The strong attractive van der Waals interaction between the solid(Au nanorod)and gas(O₂ molecules)at distances in the 1 nm range enable the rapid adsorption of gas molecules on solid surfaces rather than the concentration gradient-dominated diffusion,thus increasing the reaction rate.We propose a thorough solid-liquid-gas reaction pathway,which includes gas transport through diffusion(>1 nm),adsorption(<1 nm)and surface reaction,and highlights the critical role of the thin liquid layer(less than 1 nm)between the solid and gas in significantly enhancing the reaction rate at triple phase regions.3.The epitaxial growth of silver chloride(Ag Cl)hexagonal nanoplate on Au@Ag nanorod in HCl solution has been conducted and analyzed through in-situ liquid cell TEM.The Au@Ag nanorods are etched to provide Ag⁺ions for the subsequent growth of Ag Cl nanoplates.In this experiment,different concentrations of Ag⁺ions in the solution can be achieved by controlling the electron beam dose rate to change the etching rate of Au@Ag nanorods.The nanoplates show diffusion-limited growth when the dose rate is low.The growth rate of the edges of the nanoplate depends on the surrounding Ag⁺ions.However,when high electron dose rate is used,reaction-limited growth is dominant.The growth rate is only related to the surface energy of each facet.The six edges of nanoplates belonging to the{200}family grow at similar rates.This work shows the initial growth process of Ag Cl and reveals the relationship between the final morphology and reaction conditions,which enriches the understanding of the growth mechanism of nanoplate.