In Situ Investigations Of Metal Nanocrvstal Growth And Dissolution Dynamics With Liquid Cell TEM

In materials science research,the researchers have presented a vast variety of methods to precisely understand and manipulate the growth processes of nanomaterials.Nonetheless,there still exists some unsolved problems in nanocrystals nucleation and growth.One of the main technical hindrance is that we cannot see the dynamic growth of nanocrystals in liquid at real time and at very high spatial resolution.Transmission electron microscopy(TEM)is a very powerful tool to characterize morphologies,structures,elemental compositions and electronic states of materials.However,conventional TEM equipments can only be used for thin,solid samples.For liquid state materials,as they are difficult to maintain thin,stable layers in high vacuum TEM chambers,are nearly impossible to be observed in TEM.The development of liquid cell TEM enables the in situ observation and analysis of liquid samples in TEM.The main research in this dissertation includes the observation of dynamic growth and dissolution of nanocrystals with liquid cell TEM.The detailed research contents and achievements are summarized as following:1.The oxidative etching process of palladium(Pd)nanocubes in ferric chloride(FeCl3)solution has been observed using liquid cell TEM.The relationship of dissolution rate(dr/dt)and nanocrystal size(r)matches well with modified Kelvin equation.The critical size(Rcritical)of dissolving nanocrystals has been estimated with experimental results,which is around 5 nm.The dissolution rate(dr/dt)is nearly a constant when the radius of the dissolving nanocrystal is above Rcritical.But when the radius of nanocrystal is smaller than Rcritical,the dissolution rate(dr/dt)will increase dramatically during the decrease of nanocrystal size,in which circumstance the equilibrium solubility has to be taken into consideration in dissolution process.By comparing the dissolution rates of monodispersed and aggregated nanocrystals,it is found that the dissolution process for nanocrystals are influenced by their surroundings.The reason is that the diffusion of atoms and ions is inhibited in the confined area between neighbouring nanocrystals.In addition,the aggregation kinetics have been analyzed through observing the nanocrystal dimers dynamics which are influenced by van der Waals force and electrostatic force.The influences of Fe3+ ion and electron beam have been analyzed here.We found that the Fe3+ ion is the dominant oxidative etchant,and electron beam accelerates the etching process of Pd nanocrystals.The dissolution process of Pd nanocrystals has also been conducted in scanning transmission electron microscopy(STEM)mode,the distribution of dissolving agents has been observed and the diffusion coefficient has been estimated from in situ STEM results,however,it is much smaller than the standard diffusion coefficient of the corresponding agents.2.The heterogeneous growth of silver(Ag)atoms on Pd nanocrystals and the homogeneous growth of Pd atoms on Pd nanocrystals have be conducted and analyzed in liquid cell TEM.The homogeneous growth of Pd atoms on Pd nanocrystals is layered growth process,which is named F-M growth mode.During growth,the morphologies of Pd nanocrysyals transform from spherical shapes to polyhedral shapes with distinct facets.Different with the homogeneous growth here,the growth mode of Ag atoms on Pd nanocrystals is a combination of layered growth and island growth,which is named S-K growth mode.With ex situ characterizations of Ag-Pd nanostructures,we found that the layered grown Ag lattice matches the substrate Pd nanocrystal.However,the dendritic Ag structures from island growth are polycrystalline structure.The morphologies of Ag grown nanostructures on Pd nanoscystals are different with different precursor concentrations of silver nitrate(AgNO3)in aqueous solution.With higher concentration of AgNO3,the grown Ag layers are thicker during the first step of layered growth and the Ag dendrites are larger during island growth step.The intrinsic reason with the different growth dynamics of Ag growth and Pd growth might be the lattice mismatch between Ag layers and Pd nanocrystals.The strain raised from lattice mismatch cannot be released.In comparison,the overgrowth of Pd layer on Pd nanocrystals would not generate lattice strain between overlayers and substrates.