Ion doping behavior of stöber silica-nanosynthesis and application
Date of Issue2016
School of Physical and Mathematical Sciences
A*STAR Singapore Institute of Manufacturing Technology
The research work presented in this thesis focused on the fundamental study of nanostructure synthesis, such as the colloidal synthesis control, the hollowing behavior mechanism study, the fabrication control, the silica or polymer shell coating control, polymerization control, as well as primary application based on the mechanism study, such as the transition metal doping control, the porosity control, as well as the drug loading control in core-shell nanostructures. We systematically study the Stöber silica nucleation and deposition process under kinetics and thermodynamics theory. On one hand, the ion doping behavior is critical to silica nucleation and deposition, which leads to the chemical composition difference between the inner layer and the outmost layer of silica nanoparticles. On the other hand, the ion-doping control can help us to design novel nanostructures, and it is possible to be applied to other metal oxides. The whole research open new windows to the future design and application in other areas, such as biosensors, drug delivery, energy conversion, and so on. The Stöber silica etching behavior attracted our attention, and we started to study the reason of selective etching behavior for silica nanoparticles (Chapter 2). It is not because of etching but because of ion doping behavior during the synthesis which leads to the composition difference between the inner layer and the outmost layer. After many control experiments the surface redeposition mechanism, and the Ostwald ripening mechanism have been ruled out, which were usually applied to explain metal oxide hollowing behavior. The new breakthrough helps us to view silica as counter ions trapped inside which is critical to porosity. The elemental analysis, EDX line scan and TEM provide evidence to above hypothesis. Based on the ion pairing mechanism during silica nucleation and deposition we designed many experiments to get novel silica nanostructures (Chapter 3). By tuning the solvent ratio, the ions concentration, and the acid incubation time length we can control the ion doping level inside silica nanoparticles, which will give many different kinds of novel structures, such as hard-soft-hard silica, lower porous silica, and transition metal doped silica. Besides, we can dope positive charged organic molecules inside silica nanoparticle, such as dyes, drugs, and so on. The above synthesis control is important to silica application in biosensors, drug delivery, catalysts, and battery. The deeper study was conducted for the silica coating metal nanoparticles under the ion pairing mechanism of silica formation. We found that the metal nanoparticles encapsulation can be adjusted by tuning solvent ratio (Chapter 4), which means that ion doping difference can affect the silica nucleation and deposition on the nanoparticle surface. We can tune the hydrophilic nanoparticle coating from full encapsulation to partially encapsulation by solvent ratio control in Stöber system/reverse emulsion system. Interestingly, we can also control the hydrophobic nanoparticle encapsulation from concentric to eccentric, and Janus nanoparticle by the similar solvent ratio control in reverse emulsion system. It is a simple, general method to synthesize Janus nanoparticles by ion doping control during the metal nanoparticle coating process. In Chapter 5, another type of core-shell structure, metal nanoparticle coated by poly(styrene)-block-poly(acrylic acid) (PSPAA) shell was studied. Firstly, we studied the polymer nucleation and deposition on the nanoparticle surface, and then control the solvent ratio to synthesize different kinds of core-shell nanoparticles, such as concentric nanoparticles, multi island core-shell nanoparticles, as well as Janus nanoparticles. We also study the mobility of the polymer shell by tuning the solvent ratio, the temperature, as well as the other parameters. All these studies provide a simple way to synthesis core-shell (Metal NP@PSPAA) nanostructures. Finally, the Metal NP@PSPAA core-shell nanoparticles were used to fabricate novel nanostructures (Chapter 6). We have successfully controlled the nanoparticle polymerization according to change the charge repulsion of the Metal NP@PSPAA monomer. Co-, homo-, and block polymerization of the nanoparticles have been studied while tuning different parameters, such as the solvent ratio (water/DMF, or THF), the seed ratio, the polymer chain length, and so on. This method is easy to control and very simple which opens new window for novel nanostructure design.