The Investigation On Surface/Interface Dynamic Behavior Of Nanomaterials

Nanomaterials have wide applications in the fields of chemistry,materials science,energy conversion and life science.By tailoring surface/interface or structure of nanomaterials,catalyst performance and energy device efficiency can be improved.As the scale or dimension of materials decreases,the surface/interface effect become more significant.A better understanding of surface/interface chemistry of nanomaterials can help us design new functional materials.In this thesis,we investigated the shape-controlled synthesis,interfacial dynamics and electrochemical performance of nanomaterial to illustrate the relation between surface/interface chemistry,structure and performance.Five related research projects were conducted:(i)in situ TEM study of PbSe nanocrystal degradation,(ii)in situ observation of the superlattice phase transition induced by ligand displacement,(iii)imaging of solid-liquid(metal-water)interface,(iv)controlled synthesis of concave nanomaterials by surface modification,(v)tailoring the structure of carbon for high performance Li-S batteries.The details of the research results are shown below:1.In situ TEM study of PbSe nanocrystal degradation.PbSe nanocrystals have attracted widespread attention due to a variety of potential applications.However,the practical utility of these nanocrystals has been hindered by their poor air stability,which induces undesired changes in the optical and electronic properties.An understanding of the degradation of PbSe nanocrystals when they are exposed to air is critical for improving the stability and enhancing their applications.Here,we use in situ transmission electron microscopy(TEM)with an environmental cell connected to air to study PbSe nanocrystal degradation triggered by air exposure.We have also conducted a series of complementary studies,including in situ environmental TEM study of PbSe nanocrystals exposed to pure oxygen,PbSe nanocrystals in H2O using a liquid cell,ex situ experiments,such as O2 plasma treatment and thermal heating of PbSe nanocrystals under different air exposure.Our in situ observations reveal that when PbSe nanocrystals are exposed to air(or oxygen)under electron beam irradiation,they experience a series of changes,including shape evolution of individual nanocrystals with the cuboid intermediates,coalescence between nanocrystals,and formation of PbSe thin films through drastic solid-state fusion.Further studies show that the PbSe thin films transform into an amorphous Pb rich phase or eventually pure Pb,which suggest that Se reacts with oxygen and can be evaporated under electron beam illumination.These various in situ and ex situ experimental results indicate that PbSe nanocrystal degradation in air is initiated by the dissociation and removal of ligands from the PbSe nanocrystal surface.2.In situ observation of the superlattice phase transition induced by ligand displacement.The behavior of individual nanocrystals during superlattice phase transitions can profoundly impact the structural perfection and electronic properties of the resulting superlattices.However,details of nanocrystal morphological changes during superlattice phase transitions are largely unknown due to the lack of direct observation.Here,we report the dynamic deformability of PbSe semiconductor nanocrystals during superlattice phase transitions that are driven by ligand displacement.Real-time high-resolution imaging with liquid phase transmission electron microscopy reveals that following ligand removal the individual PbSe nanocrystals experience dramatic directional shape deformation when the spacing between nanocrystals reaches 2-4 nm.The deformation can be completely recovered when two nanocrystals move apart or it can be retained when they attach.The large deformation may arise from inter-nanocrystal dipole-dipole interactions and it is strongly related to the structural defects in the connected nanocrystal network.3.Imaging of solid-liquid interface.The interface between states of matter have been of intense interest to scientists for many years,and a better understanding of solid-liquid interface is critical for science and industry.However,the nature of solid-liquid interface is mostly unknown because of the restricted characterization techniques.Here,we show the direct observation of a dynamic interphase between indium nanoparticle surface and bulk water in a liquid-cell transmission electron microscope.Observation reveals that the dynamic interphase between the bulk metal and liquid,which separate the solid and liquid.The interlayer has a distinguished phase,composition and physical properties from the bulk metal and liquid.Our results indicated that the interfacial phase is amorous Indium rich liquid phase.These new discoveries will shed light on the understanding of interfacial chemistry.4.Controlled synthesis of concave nanomaterials by surface modification.We have developed a facile procedure that can create asymmetrical building blocks by uniformly deforming nanospheres into C?v symmetry at low cost and high quality.Concave polystyrene@carbon(PS@C)core-shell nanospheres were produced by a very simple microwave-assisted alcohol thermal treatment of spherical PS@C nanoparticles.The dimensions and ratio of the concave part can be precisely controlled by temperature and solvents.The concavity is created by varying the alcohol-thermal treatment to tune the swelling properties that lead to the mechanical deformation of the PS@C core-shell structure.The driving force is attributed to the significant volume increase that occurs upon polystyrene core swelling with the incorporation of solvent.We propose a mechanism adapted from published models for the depression of soft capsules.An extrapolation from this model predicts that the rigid shell is used to generate a cavity in the unbuckled shell,which is experimentally confirmed.This swelling and deformation route is flexible and should be applicable to other polymeric nanoparticles to produce asymmetrical nanoparticles.5.Tailoring the structure of carbon for high performance Li-S batteries.Lithium-sulfur(Li-S)batteries are next generation of chemical power sources for energy storage and electrical vehicles,because of its high theoretical capacity and high energy density with cheap nontoxicity sulfur cathode.However,for the large-scale applications it is still a major challenge to produce Li-S batteries with remarkable capacity and long stability.Herein,a graphitized carbon/sulfur composites cathode was fabricated with an ultrahigh sulfur percentage of 90 wt%,which could deliver a high initial overall discharge capacity of 1070 mAh/g and a discharge capacity of 804 mAh/g after 50 cycles.Even with a sulfur loading as high as 4 mg/cm2,the graphitized C/S composites can still deliver a high initial overall discharge capacity of 908 mAh/g and a discharge capacity of 739 mAh/g after 100 cycles.The graphitized carbon with high electrical conductivity,adjustable pore size,pore volume and surface area were synthesized by using commercialized nano-CaCO3 as template and graphitization catalyst.Density functional theory calculation revealed the graphitized structure exhibited stronger adhesion strength with polysulfide.Moreover,the porosity of graphitized carbon enhances the adsorption between carbon and polysulfide.In this thesis,the effects of surface/interface and structure on the chemical reaction process were studied.By using advanced in-situ TEM characterization techniques,we studied the failure mechanism of air-sensitive materials,the process of superlattice phase transition,and the interface of metal-aqueous solutions at molecular or atomic level.This thesis enriches the content of surface/interface chemistry,which provides theoretical guidance for the design of new functional materials and devices.