Dynamic Crystallization And Regulation Of Silver Nanoparticles

The diverse morphology of silver nanoparticles determines their unique properties and widespread application prospects.Although a variety of methods have been used to synthesize and exchange the morphology of silver nanoparticles,the formation mechanism of different morphologies is still unclear due to the lack of understanding of the formation process.The controllable synthesis of silver nanoparticles is still a black box operation,which relies heavily on experience and lacks effective theoretical guidance.In recent years,the development of in-situ liquid electron microscopy has given us the opportunity to reveal the black box process,so that we can directly observe the crystallization process of silver particles.Based on the previous work in our group,silver nanoparticles are prepared by off-line and in-situ methods in this thesis,and their morphology and formation process are effectively regulated.Silver nanoparticles were prepared from aqueous silver nitrate solution by off-line electrochemical synthesis.The morphology of the synthesized silver nanoparticles was regulated by changing the current density(reaction rate)and the viscosity of solvents(diffusion rate).The results show that the formation of silver particles is controlled by the reaction rate at low current,and the slow nucleation and growth rate leading the formation of large polyhedral particles.When the current increased,the silver ion consumption rate near the crystal increased,and the diffusion limited condition was formed,which resulted in the formation of silver dendrite.In this situation,the diffusion dominates the morphology evolution of silver particles,forming silver nanoparticles with dendritic morphology.Besides,agar is added into the electrolyte solution to change the viscosity of the solution and the diffusion rate.The low diffusivity at high viscosity brings about the spatio-temporal evolution of the growth interface near the electrode;when there is a high concentration gradient near the electrode,dendritic structures are formed;while in the region away from the electrode,the low concentration gradient leads to a nucleation-dominated process that produces polycrystalline spheres.The change of growth pattern caused by the change of concentration gradient is the main reason for the change of the morphology of silver particles.In order to reveal the microscopic crystallization process of silver nanomaterials,in-situ liquid chips were introduced into the scanning electron microscope to observe the dynamic crystallization process of silver nanoparticles formed by silver nitrate aqueous solution under electron beam irradiation.Irradiation of water molecules with an electron beam produces transient reducing substances and oxidizing substances.The strong reducing agent(hydrated electron)generated by the electron beam reacts with silver ions in the solution to form sliver particles,and the strong oxidant(hydroxyl radical)oxidizes the generated silver atom to cause the dissolution of silver particles.When the quantified electron dose rate is 7.33×1011 Gy/s,the concentration distribution of hydrated electrons and hydroxyl radicals change with time and space,and the peaks of the ratio of hydrated electrons and hydroxyl radicals appear at different positions,which reveals the reason for this dynamic crystallization process.By simulating the concentration distribution of irradiated products under different electron dose rates,the results showed that the ratio of hydrated electron and hydroxyl radical concentration would increase with the increase of electron dose rate,and the experimental regulation was carried out accordingly.The simulation results show that the large electron dose rate leads to the reduction of the experimental process,and the small electron dose rate is dominated by oxidation.Later experiments verified the simulation results.The small dose rate(1010 Gy/s)would cause the oxidation etching of silver particles,while the large dose rate(1012 Gy/s)would cause the reduction of silver ions.If the electronic dose rate continued to increase(1013 Gy/s),the dendritic structure would be generated,which was consistent with the diffusion restriction system in the off-line electrochemical reaction.When the electron dose rate reaches 1015 Gy/s,silver nanoparticles distributed in concentric circles are formed.This is caused by the H2 bubbles generated at a high dose rate,which reduces the diffusion rate of the oxidizing substance.