|Author:||Tan, Noel Peter Bengzon|
|Title:||Novel metal/core-shell polymer composite particles : synthesis, characterization and potential applications|
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Applied Biology and Chemical Technology|
|Pages:||xI, 268 leaves : illustrations (some color) ; 30 cm|
|Abstract:||This thesis aims to develop metal/polymer composite particles through a simple, inexpensive and greener synthetic route using polymeric nanoparticles as both nanoreactors and templates, and explore potential application of the composite particles as smart catalysts in organic synthesis. Specifically, amphiphilic polymeric particle that is composed of poly(N-isopropylacrylamide)/polyethyleneimine) (PNIPAm/PEI) with a core-shell nanostructure, was used as the nanoreactor which is capable of self-reducing metal salt to generate metal nanoparticle without the aid of additional reducing agent. It also serves as the template which can control the growth of metal nanoparticle, as well as encapsulate and stabilize the resulting metal nanoparticle. Since the PNIPAm/PEI microgel is sensitive to both temperature and solution pH, and can be stably dispersed in water, the accessibility of the encapsulated metal nanoparticles can be controlled through turning solution pH and temperature. Thus this type of metal/polymer composite particles are promising smart catalyst for aqueous-based organic synthesis. The thesis begins with the introduction of metal/polymer composite particles. It reviews current approaches in synthesizing this type of composite materials such as chemical methods, biological synthesis and polymer template mediated synthesis. Drawbacks of these methods on the synthesis of composite particles are also pointed out in details. To address these problems, a novel type of polymer material, namely amphiphilic core-shell particles, is proposed as a green nanoreactor to generate composite particles without the need to use conventional reducing agent. The background of the amphiphilic core-shell particles including their synthesis, unique features and potential applications are subsequently discussed. Chapter two discusses the rationale of using amphiphilic core-shell particles and the challenges for their usage. Specific objectives of the research are then described. Chapter three provides detailed research methodologies implemented in this work including materials used, synthetic methods, measurements and characterization with various analytical tools such as particle size and surface charge measurements; FTIR; NMR; UV-Vis spectroscopy; field emission scanning electron microscopy (FESEM); transmission electron microscopy (TEM); X-ray diffraction spectroscopy (XRD);X-ray photoelectron spectroscopy (XPS); and atomic force microscopy (AFM). Chapter four describes gold nanoparticle formation using a PNIPAm/PEI core-shell microgel to generate smart metal/polymer composite particles. The microgel template acts not only as the nanoreactor for gold salt reduction, but also as the stabilizer and immobilizer for the encapsulation of resultant gold nanoparticles. The kinetics of gold salt reduction and gold nanoparticle formation were carefully investigated. Results indicated that the reduction rate of PEI/PNIPAm microgels is 625 times faster than the native PEI. Factors affecting the formation of Au/PNIPAm/PEI composite particles such as solution pH, temperature, amino to gold salt ratio and temperature of the composite particles have been systematically investigated. Physical properties of the resulting composite particles such as particle size, surface charge, colloidal stability, morphology, nanostructure and gold nanoparticle property were carefully examined. The mechanism of gold nanoparticle formation on the PEI shell was also proposed.|
Chapter five extends the established system from gold nanoparticles to bimetallic nanoparticles (i.e. Au@Ag) using a successive reduction method through utilizing the pre-synthesized gold nanoparticles in microgel template as seeds for the silver nanoparticles to grow and develop. The favorable formation of the second metal nanoparticles was attributed to the under-potential difference between gold and silver ions. Chapter six demonstrates the catalytic activities of both the gold (Au) and gold with silver (Au@Ag) in microgel template through the reduction of p-nitrophenol to p-aminophenol as a catalytic model. Results from this study showed that the use of gold nanoparticles increased the reduction rate by ten folds. Furthermore, when bimetallic nanoparticles (Au@Ag) were used, the reduction rate further increased by a factor of ten. Investigation of the effects of pH and temperature on the catalytic activity of the gold nanoparticles in microgel template was also systematically carried out. Results showed that in a pH range of 3 11, their corresponding catalytic activities decreased as pH increased. At pH 3, catalytic activity was calculated at 7.50 x 10⁻³ s⁻¹But increasing solution pH to 7 resulted in ten times slower in reduction rate (7.4 x 10⁻⁴ s⁻¹ ), and it eventually ceased at pH 11. When temperature increased, catalytic activity decreased. At 25°C, the catalytic activity was at peak of 7.4 x 10⁻⁴ s⁻¹ . But when temperature was increased to 29°C, catalytic activity decreased to 3.16 x 10⁻⁴ s⁻¹ , and further down to 1.33 x 10⁻⁴ when temperature reached 33°C. This behavior demonstrates that tuning the solution pH and temperature enable us to control the catalytic activity. Chapter seven sums up the overall conclusions on the syntheses of the composite particles and the effect studies to be conducted in the future. It further discusses the significance and implications made in the work, and provide recommendations for future studies.
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