This dissertation concerns the synthesis of ultra-fine titania particles starting with titanium ethoxide as a precursor. The particle size and morphology was controlled using polymeric stabilizers which imparted in-situ steric stability during the particle growth. An attempt was made to elucidate the factors which govern the particle size and morphology by studying the solvent-polymer-particle surface interactions. For this purpose, the effects of polymer concentration, molecular weight, structure, and the solvent system were studied.
Titanium dioxide particles were synthesized via the hydrolysis of tetraethylorthotitanate in ethanol in the presence of hydroxypropylcellulose, HPC, as an in-situ steric stabilizer. The effects of HPC concentration, molecular weight, and water concentration on TiO₂ particle size and morphology were determined along with the effect of HPC on the particle growth rate. The particle size decreased by ten-fold upon the addition of HPC at high water concentrations due to the combined effects of increased HPC adsorption and increased nucleation rates. Water was shown to segregate at the TiO₂ surface and to promote HPC adsorption through enhanced hydrogen bonding. Mean particle diameters as small as 70 nm were obtained. The particles grown with HPC had surface morphologies that suggest the particles aggregated by flocculating initially into a secondary energy minimum followed by condensation reactions that welded the aggregates together. Particle growth rate studies show that HPC can undergo an alcoholysis reaction with tetraethylorthotitanate when mixed in the absence of water. Electrophoresis measurements show the absence of any significant electrostatic stabilization effects under conditions giving the smallest particle sizes.
In another related study, titanium dioxide particles were synthesized via the hydrolysis of tetraethylorthotitanate, TEOT, in mixtures of ethanol and tetrahydrofuran, THF, in the presence of hydroxypropylcellulose, HPC, as an in-situ steric stabilizer. The effects of THF concentration on the particle size and growth rate were studied at fixed concentrations of water, TEOT, and HPC. Particles grown in the absence of HPC were highly aggregated while the size of the primary particles comprising the aggregates decreased with THF concentration. The particle growth rate increased significantly with THF concentration due to the decrease in the hydrogen bonding capability of the solvent system which increased the activity of the water. In the presence of HPC, both the growth rate and the particle size decreased dramatically with increasing THF concentration. The decrease in the solubility of HPC with THF concentration presumably enhanced HPC adsorption at the particle surface which led to increased colloid stabilization. Particles as small as 50 nm in diameter were obtained when the particles were grown in 1:1 THF:ethanol volume mixtures.
In an attempt to study the effect of polymer structure and structure on particle size and morphology, a low molecular weight (Mw = 2000) polymer with poly(propylene oxide) back-bone was end functionalized to make it a suitable in-situ steric stabilizer. Effects of polymer concentration, functionality at chain-ends, and molecular weight on the particle size were studied. The polymer functionalized at both ends imparted in-situ steric stability at C<sub>p</sub> > 8 g/l. Particles having mean size of 0.2 microns were obtained at C<sub>p</sub> = 16 g/l. This work demonstrated that end-functionalized low molecular weight polymers offer a novel way to synthesize in-situ steric stabilizers.
An application of the synthesized ultra-fine titania particles was studied. For this purpose, novel thin films of ultra-fine titanium dioxide particles dispersed in a matrix of hydroxypropylcellulose polymer were made on quartz and silicon substrates. The TiO₂/HPC composite films were transparent in the visible region and completely blocked ultraviolet radiation at 300 nm. These films were crack-free and uniform in composition and thickness. Transparent films of amorphous TiO₂ were made by burning out the HPC at 500°C. These films were highly uniform and had no macroscopic cracks. X-ray diffraction revealed a transition to the anatase form upon sintering at 600°C. A film sintered at 700°C had a porosity of 38%. The crystalline films remained transparent until they densified at 800°C. / Ph. D.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/38925 |
| Date | 28 July 2008 |
| Creators | Nagpal, Vidhu JaiKishen |
| Contributors | Materials Engineering Science, Davis, Richey M., Desu, Seshu B., Wilkes, Garth L., Riffle, Judy, Ward, Thomas C. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation, Text |
| Format | xx, 240 leaves, BTD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
| Relation | OCLC# 70549243, LD5655.V856_1993.N347.pdf |
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