Understanding The Nucleation And Growth Mechanisms Of Nanocrystals By Liquid-cell Transmission Electron Microscopy

Nanocrystals are widely used in nano-chemistry,materials science and medical fields due to their unique physicochemical properties.Recently,more and more studies have shown that the understanding of nucleation and growth mechanisms is helpf'ul for controlled synthesis of nanocrystals.Although nucleation and growth theories have been developed and enriched to some extent,it is still challenged to understand the pre-nucleation stage and growth kinetics of nanocrystals.These challenges force us to deeply explore the nucleation and growth mechanisms of nanocrystals.Liquid-cell transmission electron microscopy with a high spatial and temporal resolution can provide nanocrystals morphology,size and structure as well as growth kinetic information,which greatly promote the rapid development of theoretical models of crystal nucleation and growth.But considering the complex chemical and physical processes in solution as wc.ll as the richness and complexity of crystallization pathway,it is very important to further probe their nucleation and growth mechanisms.In this work,we use liquid-cell transmission electron microscopy to study the nucleation and growth process of metal nanocrystals in solution and achieve three key breakthroughs.In Chapter 2,the investigations of pre-nucleation,nucleation and crystallization of Pd nanocrystals in aqueous solution reveal the existence of a new transition phase-cluster cloud structure.The eluster eloud structLire not only partieipates in the nucleation process of Pd nanocrystals,but also involves into the nanocrystal rearrangement to single crystal structure.In combining with the computational simulation results,it is suggested that cluster clOLid-mediated nanocrystal formation and rearrangement process involves the phase separation of the ordered-disorder phase,which is more energy-supported than the internal rearrangement of nanocrystals.This new discovery deepens the understanding of non-classical nucleation and crystallization theoretical models.In Chapter 3,the in situ observations of coalescence process of Au nanocrystals in aqueous solution unravel that the atomic nanobridge structure forms prior to contact when nanocrystals approach each other.Such nanobridge structure initiates the contact and promotes subsequent fusion.In combination with calculation,it is suggested that water molecules and auric ions play a key role during the formation of nanoloridges.It is the first time that we in situ reveal the formation mechanism of nanobridge and its role,which greatly deepens our understanding of the coalescence mechanism of nanocrystals in solution.In Chapter 4,the observations of growth process of Au penta-twinned nanostructures in aqueous solution indicate that only a truncated decahedron forms in the absence of cetyl-trimethyl ammonium bromide(CTAB),whereas in the presence of CTAB,a penta-twinned nanorod forms by stress-relief induced groove reentrant and selective atoms adsorption on the {111} crystal planes.On the basis of calculations,it is believed that the synergy of strain and surface energy promotes the anisotropic growth of nanocrystals.It is the first time that we observe the in situ growth process and reveal the detailed growth kinetics of penta-twinned nanorods.This work provides a new insight into the anisotropic growth of nanorods and lays a foundation for the controlled synthesis of nanomaterials.In summary,this paper reveals the nucleation and growth mechanism of metal nanocrystals in solution at nanoscale by in situ liquid-cell transmission electron microscopy from these three aspects:nanocrystal nucleation,coalescence growth and surfactant-regulated morphology evolution.The in-depth understandings of these mechanisms will help to control the norphology and size of the nanocrystals,which will promote the applications of nanocrystals in various scientific fields.