Study Of Synthesis And Formation Mechanism Of Silver Nanoparticles With Synchrotron Radiation Based Techniques

Metallic nanoparticles have receieved great attention due to their unique properties,which are highly dependent on the particle size and shape.As it known,the nucleation and growth mechanis of metallic nanoparticls is crucial to control the size and shape of the particles.So to obtain nanomateirals with different properties,the synthesies process should be designed and controlled precisely,which means understanding of the growth mechanism of nanoparticles is cruail.However,great challenges were faced for researches on particle growth mechanisms.One of these challenges is to obtain the dynamic imforation of the initial stage of formation process of nanoparticles.Synchrontron radiation light source has many advantages such as high energy,high penetrability,good collimation and high temporal-spatial resolution,which are very suitable for nanoscience,especially on dynamic and time resolution experiments.In resent years,both theory and technology of synchrontron radiation technique have achieved to new height which brings efficient methods to uncover the mysterious veil of the nanoworld.In this thesis,silver nanoparticles are studied for easy synthesis and low cost.With the combination of normal nano material characterize methods and synchrontron radiation based methods,the synthesis process of pure Ag and Ag-Ni nanoparticles were studied.Also,size,structure and morphology changes of Ag nanopowder from room temperature to 700 ℃ were derived.Since aqoeous solution has serious absorbtion of X-ary,to obtain better time resolution data,a continuous-flow device was designed and incorporated in the SAXS instrument.The different positions of the continuous-flow device are corresponding to the different reaction time of Ag+ reduction into Ag.With this method,the time resolution can be achieved to 100 ms.Four kind in-situ SAXS samples of initial Ag+ concentration of 2,4,6 and 8 mM were studied.When the initial Ag+ concentration rises from 2 mM to 6 mM,critical radius of Ag nuclei also rises,while the Ag+ concentrations rises to 8 mM,critical radius declines.Change of particles size vurses Ag+ concentration has the same trend with critical radius.According to the changes of critical radius and particle size,a diffuse-coalescence model is proposed,which can fit the particle growth curve quite well.With different initial Ag+ concentrations,differen critical radius of Ag nuclei were detected.That are the critical radius and the growth rate would decided the final size of Ag nanoparticles.Temperature driven directional coalescence mechanism at high temperature is proposed based on the changes of morphology,size and structure of Ag nanoparticles during the heating process.For morphology of particles,it was quasi-spherical from room temperature to 600 ℃,and the particles are peanut-like shaped at 700 ℃;For particle size,the growth is slow from room temperature to 400 ℃,particle size grows much quicker from 400 ℃ to 700 ℃;For crystal structure,there is only one crystallite in one particle from room temperature to 500 ℃,there are two or more crystallites in one particle from 500 ℃ to 600 ℃,there is one crystallite in one peanut-like particle from 600 ℃ to 700 ℃.The whole heating process can be devided into four stages,the first stage from 25 to 400 ℃ behaves as a slower growth Ostward ripening process;the second stage from 400 to 500 ℃ behaves as a faster Ostward ripening process;the third stage from 500 to 600 ℃ behaves as a random coalescence process;the fourth stage from 600 to 700 ℃ behaves as a directional coalescence process.Ag-Ni alloy nanoparticles were prepared with a chemical reduction method.It is found that the two kinds of metallic atoms in particles are placed as small sized Ni atoms in Ag matrix.There are two kinds of crystal structures for the alloy nanoparticles which are single crystal structure and multi-twined structure.Size-dependent free energy of particles in both crystal structures with different Ag/Ni ratios are calculated.Free energies are nearly the same for two kinds of particle structures which means these two kinds of particle can be coexistent.In dynamic aspect,it is the original structure of the nuclei that decides the final structure of Ag-Ni alloy nanoparticles.These researches supply deeper understandings of formation mechanism of Ag/Ag-Ni,provide theoretical basis for controllable of nanoparticle synthesis;temperature driven directional coalescence of silver nanoparticles provides experimental and theoretical basis for the application at high temperature.It is hoped that these results could be used for studies of other kinds of nanoparticles.