Real time in situ spectroscopic ellipsometry studies of the photocatalytic oxidation of stearic acid on titania films

We used in situ spectroscopic ellipsometry to measure the photocatalytic activity of titania films on fused silica and glass substrates. Amorphous and anatase TiO2 films with a variety of microstructures were prepared by reactive sputtering and pyrolytic deposition. The titania films were coated with thin, spin-cast films of stearic acid [CH3(CH2)(16)COOH] to represent an organic contaminant. Photooxidation rates were determined from ellipsometric measurements of the reduction in stearic acid film thickness during exposure to UV irradiation at 313 or 365 nm. The photooxidation rate was found to be proportional to I-alpha, where I is the irradiance. The exponent a correlated with the TiO2 crystallinity, having values of approximately 0.7 and 0.8 for amorphous and anatase films, respectively. The largest photooxidation rate was observed for the pyrolytically deposited anatase sample on which X-ray reflectometry and spectroscopic ellipsometry measurements detected the presence of a low-density TiO2 surface layer. To assess the performance of these films in practical applications, the specimens were exposed to wavelength and irradiance conditions that simulated a solar UV spectrum. The most photocatalytically active sample had a stearic acid film removal rate of 22 nm/h, which would be suitable for self-cleaning window applications.