Study On The Controlled Synthesis And Formation Mechanism Of Silver Nanoparticles

As an advanced functional material, silver nanoparticles have extensive applications in civil and industrial areas. For example, they can be used as catalytic materials, cryo-genic superconducting materials, biosensor materials, microelectronic materials and bacte-riostatic materials, etc. The capability of silver nanoparticles is dependent on their struc-ture, shape, size, size distribution and the chemical-physical environment. Generally, the shape, size and size distribution of silver particles can be controlled by adjusting the reac-tion conditions such as reducing agent, stabilizer and so on or employing different syn-thetic techniques. Therefore, it is necessary to study the nucleation and growth mechanism of silver nanomaterials. In the past few years, nanoparticle production by a size-controlled or shaped-controlled procedure has become a new and interesting research focus. Based on the large number published literatures, the dissertation has firstly made a brief intro-duction on the synthetic methods as well as the Ultraviolet-visible (UV-Vis) spectral prop-erties of silver nanoparticles, and then it has evaluated the influencing factors of silver nanoparticles prepared in microemulsion method by a controlled preparation procedure. The research progresses in silver anisotropic nanomaterials synthesized by chemical re-duction are discussed. The shortages in the relevant research fields are also analyzed. Subsequently, the studies on the controlled synthesis and formation mechanism of spheri-cal and rod-like silver nanoparticles have been performed in detail.Sodium di(2-ethylhexyl) sulfosuccinate (AOT) has been synthesized by purging with N2 and catalyzing with p-toluenesulfonic acid. Effects of reaction temperature, vacuum and molar ratio of 2-ethyl hexanol to maleic anhydride on esterification are studied. The esterification product is colorless and transparent and the esterification conversion is up to 99.6% at the condition of 130℃, 29.33 KPa, n(2-ethyl hexanol):n(maleic anhydride) = 1.82:1 and reaction time 2h. Optimal sulfonation conditions are 130℃, molar ratio of NaHSO3 to di(2-ethylhexyl)maleate) = 1.10:1, mass fraction of AOT = 10% and reaction time 3.5h. After purification by methanol dissolution, activated carbon adsorption, hexane extraction and rotatory evaporation, the obtained product is a kind of white waxy solid. Fourier transform-infrared spectrometry (FT-IR) analysis shows that the product has a higher quality and can substitute for imported product.Spherical silver nanoparticles have been prepared in AOT-dodecane-H2O ternary mi-croemulsion system. The UV-Vis absorption spectra and transmission electron micros-copy (TEM) have been used to trace the growth process and elucidate the structure of the silver nanoparticles. UV-Vis spectra show that the reaction is incomplete at low N2H4 concentration and the Ag4+intermediates form at early stages of the reaction and then the clusters grow or aggregate to larger nanoparticles. The higher the AgNO3 concentration is, the faster the growth rate of the silver particles is. The higher molar ratios of water to AOT (W) value has been found to give larger particle size and broader size distribution. TEM micrographs confirm that the silver nanoparticles are all spherical. The resulting particles have a very narrow size distribution. Meanwhile, the diameter size of the particles is so small that the smallest mean diameter is only 1.6 nm. FT-IR results show that the surfac-tant molecules are strongly adsorbed on the surface of silver particles through a coordina-tion bond between the silver atom and the sulfonic group of AOT molecules, which en-dowed the particles with a good stability in oil solvents. Thus, the silver colloidal has a high stabilization and can be preserved for a long time without precipitation. As dodecane is used as oil solvent to prepare silver nanoparticles, the formed nano-silver sol is almost nontoxic. As a result, the silver nanoparticles need not be separated from the reaction solu-tion and the silver sol may be directly used in antibacterial fields.Spherical silver nanoparticles are also synthesized in AOT microemulsion system using short carbon chain alkanes as oil phase. Effects of various factors on the formation of silver nanoparticles have been studied by UV-Vis spectra technology. As the increase of the molar ratio of water to surfactant, the absorption intensity of UV-Vis spectra increase and the half band width decrease obviously. It means that the diameter of microemulsion as well as the mean diameter of the formed particles increases with the water content in the system. Hydrazine hydrate can be reduced the silver nitrate solubilized in the water core of another microemulsion completely because of its higher solubility in AOT mi-croemulsions. In some degree, the amount of silver nanoparticles obviously increases with the concentration of AgNO3 and hydrazine hydrate. However, the stability of silver col-loidal solution would be destroyed, if the concentration of hydrazine hydrate were too high. The plasma resonance intensity of silver colloid increase and the most absorption position happen to blue-shift clearly. The results mean that the quantity of silver nanopar-ticles has increased and the mean diameter of the particles has also decreased.Size and shape controlled silver nanoparticles have been synthesized in sodium do-decyl sulfate (SDS) quaternary microemulsion system. The effects of reaction time, AgNO3 concentration and the molar ratios of water to SDS (W) on the particle diameter and size distribution are investigated. TEM micrographs confirm that the mean diameters of spherical silver nanoparticles obtained at two typical values of W=5, 20 are 6.5nm, 12.1nm, respectively, and the size distribution of the particles increases obviously with the W values. Flake-like silver powders can be obtained by centrifugation of the particles. The result indicates that SDS surfactant molecules not only can form a protection layer on the surface to prevent the particles from aggregation, but also may act as a template to orient the particle growth to form flake-like particles, which may be important in the fabrication of flake-like silver powders.In the capping action of citrate, silver nanocrystals have been prepared by chemical reduction. The effects of reaction temperature, charging mode of reductant and the aged time of silver colloid on the formation of silver nanocrystals are mainly investigated. The effects of reaction conditions can be further confirmed by synthesizing silver nanorods under the induction of different silver nanocrystals. The silver colloid is prepared under three temperatures, the half-band width of resonance peaks are clearly different. The re-sults show that the diameter of nanocrystals prepared at 0℃is smallest and the particle diameter increase with the temperature, but has a maximum. The mean diameter of resul-tant nanocrystals is about 2 nm in one-time mode and the mean diameter of them will at-tain 10 nm in continuous mode, when NaBH4 is added into the solution. Appropriate aged treatment is favorable to perfect the crystalline form. The aged time has obvious influence on the smaller nanocrystals and has little influence on the larger nanoparticles. However, the nanoparticles will aggregate into larger nanoparticles if the solution is aged for a long time.Silver nanorods and nanowires of controllable aspect ratio have been synthesized in the mixed solution composed of cetyltrimethylammoniun bromide (CTAB), ascorbic acid (Vc), AgNO3 and silver seeds by adding NaOH. The longer reaction time, the greater the aspect ratio of the formed nanorods is. The aspect ratio of silver nanorods decrease with the increase of reaction temperature, meanwhile, the monodispersity of size and shape of the nanorods turn better. On the contrary, it is advantageous to the formation of silver nanorods with higher aspect ratio. As fewer seeds are easily distributed into rod-like mi-celles and its nucleation and growth can be controlled by the micelles, the silver rods in-crease in average aspect ratio as the seed concentration decrease. The micelle concentra-tion is not the main factor to influence the formation of silver nanorods. The relative amount of NaOH in solution has an important role in the fabrication process of silver nanorods and nanowires. Ascorbic acid will ionize to form the ascorbate dianion when the amount of NaOH is higher. By contraries, ascorbic acid ionizes to form the monoanion of ascorbic acid at a low concentration of NaOH. Two different forms of ascorbic acid have different complexation capabilities with CTAB and silver seed in solution, which is im-portant in nanorod and nanowire formation.