Date of Award

Spring 1-1-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry & Biochemistry

First Advisor

Steven M. George

Second Advisor

Will J. Medlin

Third Advisor

Cortland G. Pierpont

Fourth Advisor

Niels H. Damrauer

Fifth Advisor

David M. Jonas

Abstract

Atomic Layer Deposition (ALD) is a an excellent technique for depositing conformal thin films on complex geometries in layer by layer fashion. The mechanisms of depositing TiO2, platinum, and ethoxysilane molecules were probed with in situ Fourier transform infrared (FTIR) in order to better understand and improve the process. Each of these studies involves TiO2.

There are many uses for thin films of titanium dioxide, a semiconductor and high dielectric material. Current Atomic Layer Deposition (ALD) of TiO2 generally involves water or ozone, which can oxidize and corrode some substrates of interest. Ritala et al. successfully deposited an assortment of metal oxides using no water, but instead, metal alkoxides and metal halides as precursors. Presented is a study of ALD of titanium dioxide using titanium tetrachloride (TiCl4) and titanium tetraisopropoxide (TTIP). In situ Fourier transform infrared (FTIR) studies revealed that the mechanism for TiO2 ALD using titanium tetrachloride and titanium tetraisopropoxide changed with temperature. At temperatures between 250 and 300°, the isopropoxide species after TTIP exposures quickly underwent β-hydride elimination to produce TiOH species on the surface. The observation of propene by quadrupole mass spectrometry supported the β-hydride elimination reaction pathway. Deposition was investigated between 150 and 300° on substrates including zirconia, alumina, and silica. Quartz crystal microbalance results and X-ray reflectivity showed that the system grew 0.5-0.6 Å/cycle at 250° X-Ray photoelectron studies also confirmed TiO2 film growth.

In another aspect of ALD use, self-limiting chemistry assisted with terminating a surface with alkoxysilanes. Tire rubber contains additives such as carbon black or silica particles to provide strength. Although in theory Kevlar fibers would provide strength while lowering the density and increasing car fuel efficiency, in practice Kevlar fibers disperse only very poorly in the rubber, leading to inhomogeneity. In order the increase the mixing likelihood between rubber and Kevlar, the reactions of some sulfurous siloxanes were examined on both aluminum oxide and titanium oxide. The titanium oxide adhesion layer allowed the deposition of molecules on the surface that looked promising for improving mixing with rubber and decreasing the weight of tires.

Atomic layer deposition offers the possibility of more precision in platinum deposition. In a platinum deposition study, the nucleation and growth of non-conformal platinum on TiO2 and WOx powder using Pt(hfac)2 and formalin was examined with in-situ FTIR and transmission electron microscopy (TEM). Interest in substitution of Pt/C as the oxidation reduction reaction catalyst in polymer electrolyte membrane fuel cells (PEMFCs) led to the ALD synthesis of Pt/WOx and Pt/TiO2. A nucleation period on the order of 100 cycles was observed, after which, platinum loading and particle size measurably increased with increasing cycle number. The adsorption of the hfac ligand on the metal oxide substrate effectively inhibits nanoparticle coalescence during the growth phase, which led to further investigation of its use as a site-blocking agent. The results showed that Pt particle distance could be increased with the use of hfacH.

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Chemistry Commons

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