Growth Mechanism Of Nanocrystals Revealed By In-situ Liquid Cell Transmission Electron Microscopy

Nanocrystals are widely used in electronics,energy,bioscience,catalysis,and other fields due to their unique physicochemical properties.It is very important to control the structure of nanocrystals,so as to adjust their physicochemical properties to match with specific applications,especially for the nanodevices.Understanding the structural evolution and growth mechanism of nanocrystals during liquid-phase synthesis will be beneficial in designing and applying many functional nanodevices.However,some mechanisms in the growth of nanocrystals remain largely unknown due to the lack of direct observation.In this thesis,we study the growth mechanisms of metal oxide nanocrystals and core-shell nanoparticles by in-situ liquid cell transmission electron microscopy（TEM）.The main research contents are as follows.1.The growth of metal oxide nanocrystals is observed by in-situ liquid cell TEM,including Pb₃O₄and Cu₂O nanocrystals.We study the growth mechanism and structural evolution of nanocrystals in the liquid phase.It is the first observation that Pb₃O₄ nanocrystals with a regular quadrilateral shape are formed,in which nanocrystal facets preferentially grow along the[002]direction.Theoretical calculations confirm that the surface energies of nanocrystal facets have played a key role in controlling the preferred direction.Specifically,the nanocrystal facet with higher surface energy grows at a faster rate.However,Cu₂O nanocrystals is a diffusion-limited growth in solution.The nanocrystals grow layer-by-layer along[200]and[111]and form spherical Cu₂O nanocrystals.The process of coalescence growth is researched through observing nanocrystals.Importantly,this study would provide a new understanding of the role of various basic physical forces in the interactions between nanocrystals during the coalescence growth.The main driving forces received at each stage are closely related to their separation distance.Such understanding of the growth pathways and quantification of formation kinetics are important for the design of hierarchical nanomaterials and the control of nanocrystal self-assembly for functional devices.2.The growth of Au-Ag core-shell nanocrystal has been studied.We experimentally confirm that the addition of PVP plays a crucial role in changing the growth mode of Ag shell.With the absence of PVP,Ag atoms diffuse in the solution and absorb onto the Au nanobipyramid to form a homogeneous Ag shell.The Ag shell is formed on the side surfaces of Au nanobipyramid.This preferential deposition of Ag atoms is because the side surfaces of the Au nanobipyramids are high-index-faceted.The deposition of Ag atoms on these facets can lower their surface energies.PVP can promote the formation of Ag nanocrystals,and also,excess PVP can adsorb on two ends of Au NBP to induce the coalescence of small Ag nanocrystals.With the addition of PVP,growth mode of the Ag shell switches from atom deposition to nuclei coalescence.Finally,the Au nanobipyramid can be encapsulated completely by the formed Ag shell.Atom deposition is the most studied growth mode at present.However,our study finds a new PVP-mediated growth mode of Au-Ag core-shell nanocrystals.The exposed facets of Au nanoparticles can influence the facet selection for the deposition of Ag atoms.But Ag shell growth mode is independent of the exposed facets of Au core.This work affords valuable guidance to design core-shell nanocrystals with a specific composition and shape for various potential applications.3.This study is the first to observe the growth of Au-AuCl₃ core-shell nanostructure and the corresponding anticorrosion behaviors of AuCl₃ deposited shell.The presence of CTAB can substantially influence the growth mode and structure of AuCl₃ shell,by a direct or indirect way,intervene the dissolution of Au nanobipyramids.This growth mode of AuCl₃ shell is Volmer-Weber（V-W）,AuCl₃ forms 3D-nanoclusters on the surface of the Au seed.When AuCl₃ shell grows to a certain thickness（18 nm）,it can protect Au nanobipyramid from dissolving;before this,Au nanobipyramid has been dissolved into ellipsoidal nanoparticle.This phenomenon is caused by the presence of residual CTAB.The Au nanobipyramid samples are multiple washed and centrifugated to remove residual CTAB molecules and then do experiments under the same experimental conditions.This growth mode of AuCl₃ shell is Frank-van der Merwe（F-M）,AuCl₃ shell grows in a layer-by-layer manner on the Au nanobipyramid surface.A thin dense layer（a thickness of 3 nm）can protect the Au seed.The morphology of Au nanobipyramid is almost unchanged throughout the experiment.It provides valuable guidance to design highly efficient and stable catalysts.