In Situ Liquid Cell TEM Study Of The Growth Mechanism Of Zinc Oxide And Zinc Nanostructures

Nanomaterials have been the research focus in the field of materials science due to their unique and excellent properties,as well as their wide applications in many fields,such as energy,biomedicine,catalysis and so on.Compared to other synthetic methods of nanomaterials,the solution growth method has attracted numerious researchers because of its advantages in the preparation process,cost and scalability.As a fundamental issue towards the controlled synthesis of nanostructures,a deep understanding of the nucleation and growth mechanism of nanostructures in liquid phase environment is of great significance for controlling the size,structure,and morphology of nanomaterials,thereby tuning their physical and chemical properties.Recently,as a new breakthrough in nanofabrication and electron microscopy,the emergence of liquid cell transmission electron microscopy（TEM）has made it possible to directly investigate the dynamics of material transformations in liquid environments.And it is a powerful tool to study the fundamental theory with respect to the nucleation and growth mechanism and morphology control mechanism of nanostructures.In this dissertation,we have directly observed the dynamic growth of II-VI semiconductor ZnO nanostructures and reactive metal Zn nanocrystals in aqueous solution by in situ liquid cell TEM,revealing the correlative growth mechanism and growth kinetics of the nanostructures.The main research contents and achievements are summarized as following:（1）Combining with the in situ experiments,we have discussed the electron beam effect including the irradiation decomposition of water,the temperature rise in the liquid cell and radiation damage of Zn nanocrystals.The solvated electrons and hydrogen H₂from the decomposition of water are the direct reaction basis for the in-situ synthesis of Zn nanocrystals and the immediate cause of the formation of bubbles in the aqueous solution,respectively.We found the nucleation and growth of bubbles on the liquid-solid interface between the aqueous solution and silicon nitride membrane is common.And we have observed the diffusion of gas molecules and coalescence between two adjacent bubbles.We have estimated the temperature rise induced by electron beam according to two-dimensional heat conduction equation.And we concluded that the temperature rise is proportional to the electron beam current.We found the sputtering of Zn atoms caused by high-angle elastic scattering of electron beam is responsible for the formation of nanohole in the Zn nanocrystals and the material separation of Zn nanocrystals.（2）We have directly observed the precipitation and subsequent etching of ZnO nanostructures in aqueous solution using the in situ liquid cell STEM with low electron dose.ZnO Nanostructures with different sizes and morphologies were synthesized in situ,by adjusting the concentration ratio between the two precursors（Zn（NO）₃?6H₂O and HMTA）.We found that the nanostructures grow mainly by means of monomer addition.With increasing HMTA concentration,the diffusion-limited growth kinetics will switch to reaction-limited kinetics.This change in kinetics may explain the different morphology and size of the synthesized ZnO nanostructures.We have observed the dissolution of ZnO nanoparticles when the concentration of HMTA is 50 mM.And we proposed the surface dissolution mechanism based on the hydrolysis reaction of ZnO.（3）We have directly observed the growth of Zn nanocrystals in aqueous solution using in situ liquid cell TEM for the first time.Real-time observation of growth trajectories of typical Zn nanoparticles reveals the coexistence of classical（monomer addition and Ostwald ripening）and non-classical crystallization mechanisms（coalescence）in different growth stages.Quantitative analysis of the interparticle coalescence suggests that the surface diffusion and grain boundary migration are responsible for the shape evolution of coalesced nanoparticles.And we have directly observed the grain boundary migration during the relaxation of the coalesced nanoparticle.Analysis of the growth/dissolution kinetics during the Ostwald ripening process implies that a depletion zone（diffusion layer）around the nanocrystals is present.