| Summary: | 博士 === 國立中興大學 === 化學工程學系所 === 96 === Silver has been great interest in traditional industries, microelectronics, electronic and photonics technology, medicine and health care. The smaller size of silver structures have much surface atoms, higher chemical property and activity, low sintering temperature, and higher electrically conductivity. The surface plasmon resonance, light and microwave absorbency, biochemical activity, and bactericidal activity can be enhanced with decreasing diameter of silver. The dispersity and shape of silver nanostructures also affect thermal, electrically, and optical characteristic. The high aspect nanostructures which lower surface plasmon resonance frequency would influence light absorbency and scatter. In addition, the superior fluidity, diffusivity, mass transfer, and high density of supercritical fluids can increase solubility and reaction rate. The supercritical fluids which have changing the salvation of solvent and surfactant tails also can use to manipulate nano-structures and enhance the dispersive ability of nano-particles in liquid.
In the references, it is never been found the supercritical fluids-assisted synthesis of nanostructure in polyol process. Accordingly, this research will synthesize silver nanostructures by supercritical fluid assisted with polyol process to make reactive solution reaching supersaturation speedily to increase nucleation rate of silver, and promote mass transfer and reaction rate. Polyvinyl pyrrolidone (PVP) can protect silver to stable dispersions against agglomeration. Besides, the terminated ends with hydroxyl group would help reduce in the presence of aqueous solution. This study proves it is able to synthesize silver in de-ionic water without the addition of any reducing agent. Furthermore, PVP is a facet-selective capping agent to exhibit silver (100) plane. The different shape silver nanostructures can be synthesized by using the absorbability of PVP molecules and manipulating supercritical fluids density.
Silver nanostructures have been synthesized by the ethylene glycol with the assistance of supercritical carbon dioxide, with silver nitrate used as the base material, PVP as the stabilizer for the silver clusters, and ethylene glycol as the reducing agent and solvent. In this research, the effects of the temperature, pressure, molar ratio of PVP/AgNO3, molecular weight of PVP, and the concentration of AgNO3 on as-synthesized silver nano-structures in dispersion solution are characterized by UV-Visible spectroscopy, XRD, TEM, FESEM, and AFM. As shown in the results, the significant conclusions are summarized as below:
（1） PVP has reducing power in de-ionic water without the addition of any reducing agent, but the ability is weaker than ethylene glycol.
（2） The face-centered cubic structure and uniform dispersion silver nano-particles have been successfully prepared with the diameters between 5-25nm by manipulation temperature and pressure in most suitable condition.
（3） The size of silver nano-particles can be controlled by adjusting supercritical carbon dioxide density. Increasing temperature would raise growth rate to enlarge the particle size of silver, but increasing pressure promote nucleation rate to fabricate smaller particles.
（4） The two inlets process can synthesize high aspect ratio of silver nano-wires; and the size, aspect ratio, and yield can be controlled by varying density of supercritical carbon dioxide.
Increasing temperature would help growth to enlarge the size and aspect ratio of silver nano-wires, while increasing pressure can synthesize smaller size and higher aspect ratio and yield of silver nanowires.
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