Investigation Of The Dynamic Process Of Two Dimensional Nanomaterials By In-Situ Liquid Cell TEM

Two-dimensional(2D)materials with their large specific surface ratio and a large number of unsaturated atoms have important applications in physical and chemical material science.At present,due to its strong controllability,wet chemical synthesis is considered to be a convenient and repeatable method for the preparation of 2 D nanomaterials with controllable size and thickness,and has a good prospect for industrial application.However,most of them are based on non-in-situ studies,and the formation of the mechanism is often unknown due to lack of direct observation.Therefore,it is of great significance for understanding the growth mechanism and dynamic transformation of 2 D materials to obtain information by means of direct characterization in the field of controlled synthesis of 2 D materials.In this paper,we used in-situ liquid environment transmission electron microscopy(TEM)to study the dynamic growth and the phase transformation process of 2 D nanomaterials in the liquid cell by adjusting the ligands,the concentration of the precursor and the proportion of different precursors.In combination with in-situ TEM data analysis and non-in-situ synthesis,and theoretical calculation,the following research progress is preliminarily obtained.(?)The dynamic growth and morphology transformation process of 2D platinum nanosheets was observed at atomic level.The results revealed that the atomic growth paths of platinum nanosheets from 3D to 2 D and the reversible transformation between 2 D and 3 D in a chloride ion-rich precursor solution,which results from the preferential adsorption and desorption of Cl with modifying the surface energy and etching certain crystal facets,respectively.(?)The shape induced evolution of 2D superlattices from hexagonal phase to cubic phase was observed at the atomic level.When the morphology of nanocrystal changed from octahedron to cube,the ordered phase transition from hexagonal phase to cubic phase was realized by sliding and rotating of collective particles.Therefore,the strong van der Waals interaction between the vertical {100} faces of the cube is the main driving force of the phase transition and the reason for the stability of the 2 D superlattice.(?)The formation process of PtNi-Ni(OH)2 core-shell structure at atomic level was observed.The nucleation growth of Ni(OH)2 layer material in PtNi-Ni(OH)2 coreshell structure at the gas-liquid interface can be observed in the N,NDimethylformamide(DMF)with Pt:Ni molar ratio of 1;2.5.In addition,by adjusting the molar ratio of Pt:Ni,the core-shell structure of PtNi-NiO is formed when Pt:Ni molar ratio is 1:1.25;nanoparticles is formed when Pt:Ni molar ratio is 2.5:1.(?)We observed the nanodroplet-mediated assembly of PtNi nanoparticle into threedimensional nanostructures on the gas-solid-liquid interface.The self-assembly of DMF nanoparticles driven by DMF nanodroplet on the gas-solid-liquid interface was observed in the DMF precursor with a Pt:Ni molar ratio of 2.5:1.Because of the interaction of van der Waals force between the droplets,they attract each other,polymerize in contact.Finally,they form spherical three-dimensional nanostructures under the action of surface tension.In this paper,the dynamic growth and phase transformation of two-dimensional nanomaterials were observed by in situ liquid environment transmission electron microscopy.This will enrich the connotation of the ligand induction and growth control of two-dimensional crystals and deepen the understanding of the growth law of twodimensional crystals.The research results have certain guiding significance for the controllable preparation of two-dimensional metal materials and the self-assembly of nano-structures.