Abstract
The main purpose of this study was to develop the techniques of the preparation of nanocrystals. In this dissertation, three techniques including (I) the deaggregation of silver powders assisted by supercritical CO2, (II) the synthesis of silver nanoparticles in CO2-expanded liquids, and (III) the synthesis of metal sulfide nanocrystals using wet-chemical method had been studied.System I. The deaggregation of silver powders assisted by supercritical CO2The mixture of silver particles/organic solvent/dispersing agent in the reactor was pressurized with CO2 ranging from 800 to 2000 psi for a period of time, followed by the depressurization through a nozzle rapidly. The organic solvents of toluene, hexane and ethyl acetate and the dispersing agents of isostearic acid and dodecanethiol were used. The process temperature was ranged from 25 to 50°C. After the process of depressurization, the silver particles solution was investigated by dynamic laser scattering (DLS).It was found that the operation with the pressurized CO2, especially in the supercritical condition, could help the deaggregation of silver powders and the size of deaggregated silver particles was less than 1000 nm. However, part of the deaggregated silver particles tended to assemble into thin films on the surface of solution and the wall of receiver. The anti-solvent effect induced by adding CO2 or insufficient amount of dispersing agent to cap the surface of silver particles might be the reasons. In addition, during depressurization through nozzle, the volume of gas expanded greatly leading to the nebulization of organic solvent. Thus, the huge receiver to collect the nebulizing solvent droplets was required.System II. The synthesis of silver nanoparticles in CO2-expanded liquidsA soluble form of silver carboxylate, silver isostearate (AgISt), was synthesized and characterized. The results of ATR-FTIR, 1H-NMR, XRD, DSC and TGA indicated that the methylated branched alky chains in AgISt exhibited a steric hindrance to impede the growth of layered structure of AgISt molecules, which led to the high solubility of AgISt in non-polar solvents. A novel technique to synthesize silver nanoparticles (AgNPs) using CO2-expanded liquids as the processing medium was proposed. AgISt and hydrogen (H2) were utilized as silver precursor and reducing agent, respectively. The operative pressure of H2 and CO2 were ranged from 14 to 800 psi and from 200 to 800 psi, respectively.At 40°C, the averaged size of synthesized AgNPs was ranged from 2 to 7 nm. While the applied pressures of H2 and CO2 were increased, the size distribution of AgNPs was narrower and the formation rate of AgNPs was increased. The investigations of HRTEM, SAED, ATR-FTIR showed that AgNPs were grown in face-centered cubic phase and capped with isostearic acid, which was derived from the reduction of AgISt with H2. Further increase the reaction temperature to 60 or 80°C, the formation rate of AgNPs was reduced and the size distribution of AgNPs became broader. The reason might be that the resistance of mass transfer of H2 in CO2-expanded liquids limited the reduction reaction of AgISt and H2 as temperature was increased.System III. The synthesis of metal sulfide nanocrystals using wet-chemical methodMetal isostearates including zinc isostearate (ZnISt2), cadmium isostearate (CdISt2), and copper isostearate (CuISt2) were synthesized by the cation exchange reaction of sodium isostearate with the corresponding metal ions. The results of XRD and DSC indicated that no layered structure was form in metal isostearate, which led to their high solubility in non-polar solvents. Metal isostearates were employed as precursors to react with H2S to synthesis metal sulfide nanocrystals in wet-chemical method. By using ZnISt2 as precursor, ZnS nanowires were formed at 40~120°C, whereas nanorods were formed at 160°C. By using CdISt2 as precursor, rod, bipod, tripod, and tetrapod shapes of CdS nanocrystals were formed at 40~120°C. The investigation of HRTEM indicated that the arms and cores of multipod-shaped CdS were grown in wurtzite phase and zinc blende phase, respectively. Further increased the temperature to 160°C, spherical, rod-like and warm-like CdS nanocrystals were formed. By using CuISt2 as precursor, irregular aggregated CuS were form at 40°C, whereas circular, triangular, and hexagonal CuS nanocrystals were form at 80~160°C. The XRD pattern indicated that CuS nanocrystals were grown in covellite phase.