Photodegradation kinetics of stearic acid on UV-irradiated titania thin film separately followed by optical microscopy and Fourier transform infrared spectroscopy

We use optical microscopy for observing the photocatalytical decomposition of SA on titania film during exposure to a simulated solar illumination. The titania films were prepared via sol-gel and deposited on a borosilicate glass substrate by dip-coating. A low amount of SA was deposited by dipping the titania film in a methanolic SA solution. Optical micrographs showed that the deposited SA forms islands with variable heights and sizes and that upon UV-irradiation, these latter decomposed gradually from the edges towards the center. The SA photodecomposition as evaluated by measuring the integrated IR absorbance of SA in the 2700-3000cm region, proceeded according to a pseudo first-order kinetics with respect to SA. By contrast, it was of the zero-order type when monitoring the decomposition of an individual SA island by microscopy. From these results, we showed that the pseudo first-order kinetics is the result of a collective effect of many independent and parallel zero-order reactions with variable durations. This scenario describes well both the experimental data and those reported in literature.