Atomic-resolved In-situ Investigation Of Nanocrystals Nucleation And Growth Of Transition Metals And Its Compounds

Transition metal and its compounds nanocrystals play a significant role in materials science and industrial applications.Designing and fabricating industrialgrade nanocrystals with rational structures and properties is undoubtedly one of the most critical challenges in energy,catalysis,information,and biomedicine fields.This requires a deep understanding of the nucleation and growth mechanisms of nanocrystals at the atomic scale during the fabrication process.In-situ transmission electron microscopy(TEM),especially the rapid development of environmental transmission electron microscopy,provides the possibility to reveal the nucleation and growth mechanisms of nanocrystals.Based on the research progress of in-situ environmental TEM,this article will focus on the nucleation and growth,structural evolution,and atomic dynamics of transition metal nanocrystals and its compounds under reaction conditions.The design and investigation of growth of Pt nanocrystal particles,PtOx nanocrystal particles,MoS2 nanocrystals,etc.,within environmental TEM will be explored.Using the in-situ grown nanocrystals as model materials,we will discuss the nucleation and growth dynamics mechanisms of nanocrystals and the impact of environmental atmosphere,interfacial interactions,chemical reaction pathways,and other factors on nucleation and growth.This thesis mainly carried out three aspects of in-situ experiments,with the growth environment gradually transitioning from simple to complex coupling,to achieve the growth of various materials.In the first part,Pt nanocrystals were insitu prepared by irradiating and decomposing electron beam-sensitive H2PtCl6 material.The kinetically controlled nucleation process of Pt nanocrystals was insitu observed,and the two-step mechanism of Pt nanocrystal nucleation was determined by real-time analysis of the dynamic evolution process.The critical size of～2.0 nm for Pt nanocrystals in the two-step nucleation was determined through statistical analysis combined with high-resolution time-sequential images.These results defined the crystallization process of Pt nanocrystals and determined the stable structure construction process of the Pt nanocrystals transitioning from disordered to ordered,with the {111} facets as the priority.This part of the research provided a deep understanding of the nucleation mechanism of nanocrystals.In the second part of this thesis,the focus is on the influence of solid-solid interfaces and gas-solid interfaces on the nucleation and growth process during the nucleation and growth of nanocrystals.Pt nanocrystals were prepared in different atmospheric environments using thermal decomposition.The regulatory role of reducing and oxidizing atmospheres on nanocrystal size was determined,and the formation of the PtOx oxide layer on the Pt surface under low-temperature conditions was discovered,as well as the resulting particle stability enhancement.At the same time,under CO environmental conditions which are sensitively of Pt nanocrystals,the formation of the C coating layer and its influence on the size stability of Pt nanocrystals were also determined.This part studied the growth and structural evolution of Pt nanocrystals under different atmospheric conditions,providing a deep understanding of the interfacial mechanisms in the structural evolution of nanocrystals.In the third part of this thesis,a chemical vapor deposition(CVD)reaction environment was designed within TEM,and the growth mechanism of MoS2 under CVD conditions was explored,including precursor phase transitions,defects,nucleation,and disordered/ordered structural transformations.In the part of nucleation kinetics under reaction conditions,this thesis determined the two-step mechanism of MoS2 nucleation by slowing the reaction rate and determined the process of amorphous clusters transforming into ordered monolayer MoS2.In the growth part,the defect transmission mechanism between MoS2 layers and defectguided epitaxial growth,as well as the formation of exfoliated layers,were discovered.These results summarized the nucleation and growth mechanisms of nanocrystalline materials in CVD condition.This thesis has developed a variety of general methods for in-situ growth of nanocrystals within TEM,and used them to prepare Pt nanocrystals,PtOx nanocrystals,MoS2 nanocrystals,etc.The nucleation and growth kinetics processes at the atomic scale were studied,and the influences of interfaces,defects,phase transitions,and epitaxy on the nucleation and growth processes were discussed.This provides a deep understanding and experimental evidence for the rational design and precise construction of nanocrystal materials.