Dissertations, Theses, and Capstone Projects

Date of Degree

2000

Document Type

Dissertation

Degree Name

Ph.D.

Program

Physics

Advisor

Micha Tomkiewicz

Committee Members

Pedro Montano

Fred Pollak

Fred Smith

Ken Miyano

Subject Categories

Physics

Abstract

Pollution of our environment has become one of the most serious problems of this century. Titanium dioxide aerogels is composed of nanocrystallites of TiO2 close-packed in roughly spherical mesospheres, which are agglomerated to form a highly-porous sponge. Aerogel morphology produces porosities of around 80% and surface area that can be as large as 1000 m2/g. The aerogels efficiently adsorb organic materials from the environment and when illuminated with UV light, decompose or oxidize the organic pollutants, ultimately to safe by-products such as CO2 and H 2O.

In this work we modify the synthetic conditions to produce aerogels with different morphologies and measure the photocatalytic activity for photodecomposition of salicylic acid, as a function of size of the nanocrystallites. The results show that the quantum efficiency increases monotonically with the surface area and that the quantum efficiency per unit surface area increases linearly with the size of the nanocrystallites. These results are interpreted in terms of a "cracking model" of fixed size mesospheres and limited penetration of the salicylic acid into the mesospheres.

We use non-invasive, non-destructive techniques such as Raman scattering and EXAFS to follow the evolution of the TiO2 structure as it is generated in the sol-gel, as well as changes in the structure after the aerogel is made and modified by annealing. Both techniques reveal structural information in the liquid and solid phases and enable us to monitor the crystallinity in the gelation process in a way that might lead to an ability to control the synthetic process and achieve a desired morphology.

Comments

Digital reproduction from the UMI microform.

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