Electron Microscopy Study On Structure-activity Relationship Of Two-dimensional Metal Chalcogenide Materials

The two-dimensional layered semiconductor materials are attracted widely attentions for their potential application on ultra-thin（atomic layer thickness）FETs,heterojunctions,bendable transparent electronic and optoelectronic devices.However,the properties of this kind material are affected significantly by thickness,phase,stacking and defect.Transmission electron microscopy（TEM）is a powerful method for characterizing microscopic atomic structures in the field of materials science research.Based on this,transmission electron microscopy is used to study the structural evolution of SnS₂as anode materials upon lithiation/delithiation in lithium ion battery;the thermal stability of SnS₂and the strain field of polycrystalline Pt Se₂two-dimensional materials at grain boundaries and their effects of the distribution of charge density.1.Layered SnS₂（1T or 1H phase）is currently drawing great attentions as anode of lithium-ion batteries（LIBs）both in academic and industry due to its high theoretical capacity and diverse structure feature.However,the exact structure-dependent evolution process upon lithiation/delithiation is still blind to us due to the difficulties in fabrication of pure phase SnS₂and atomic structure characterizations.Herein,We report the preparation of flake-like 1T SnS₂and their structure evolutions as anode of LIBs in battery cycling.The atomic image demonstrates a clear 1T SnS₂phase for the first time.Its morphology and structure evolution accompanied lithiation/delithiation was observed by TEM and demonstrate a clear insertion-conversion-desertion process corresponding to the discharge/charge plateau.This provided guidance for designing and fabrication of pure phase metal sulfide materials in the future.2.Heating and electron beam irradiation are effective methods to induce structural phase transitions in two-dimensional（2D）metal dihalides.The phase transitions have important effects on electrical conductivity,thermal conductivity and catalytic activity.Here,we observed the SnS₂phase transition between the 1T and 1H phases under heating was observed on the atomic scale by in-situ scanning transmission electron microscopy（STEM）and energy dispersive spectroscopy（EDS）.The phase of 1T and 1H mixed was formed under in-situ heating and electron beam irradiation.First-principles calculations show that the phase transition between 1T and1H is performed by multiple layers of phase transitions rather than layer by layer.Furthermore,the band gap of the mixed 1T-1H phase SnS₂was found to be much smaller than the reported pure 1T SnS₂.Our results provide microscopic insights into the transformation mechanism of two-dimensional SnS₂and highlight how heat and radiation can be used to modulate the photocatalytic and electrocatalytic activity of layered chalcogenides.3.The strain effect can change the charge density of the material,thereby affecting the electrical characteristics of the material and the adsorption or catalysis of the surface.Here,we applied the method of high-angle annular dark field image（HAADF）with atomic resolution and differential phase contrast imaging（DPC）to directly observe the atomic arrangement and the effect of strain in the polycrystalline Pt Se₂two-dimensional material under strain Interatomic electric field.Our results reveal the effect of strain on the charge density distribution of materials,opening a new path for directly examining the local chemical properties of strains in atom-scale materials.This is of great significance for revealing the electrochemical mechanism and then designing materials with better performance.The research work in this paper provides more ideas for the study of two-dimensional materials in electron microscopy,namely the preparation of pure phase two-dimensional materials.Its reaction mechanism in lithium batteries,the phase transition path under in-situ thermal effect and electron beam irradiation and the effect of some two-dimensional material strain on charge density provide guidance.