Titanium dioxide is a highly photoactive catalyst used extensively to abate and detoxify polluted waters, air, and soils. It is also commonly employed in commercial sunscreen lotions as a physical filter to block UVA/UVB sunlight radiation, designed to protect people from sunburns and skin cancers. The thesis is two-dimensional. Part A deals with fundamental studies of photocatalysis. Part B addresses more practical considerations of the use of titanium dioxide in suncreams. The thesis addresses the fundamentals of Heterogeneous Photocatalysis to provide additional understanding of the activity of TiO 2 , and of the (primary) events occurring during the photocatalytic process. A protocol is developed for measuring photochemical quantum yields, Z, in heterogeneous media. Turnover quantities and the nature of photocatalysis are treated mathematically. The spectral dependence of quantum yields and wavelength-dependent selectivities of photocatalysts are examined for the first time. Of import, reactions occurring at the TiO 2 particle surface involve hot charge carriers that do not relax down the conduction and valence band electronic manifolds as fast as conventional Wisdom had predicted. Further, the thesis focuses on the genotoxic effects of TiO 2 on DNA. The spectroscopic features of TiO 2 , make it an excellent UVA/UVB sunblock. We demonstrate that, when commercial TiO 2 specimens are illuminated with UV radiation, hydroxyl radicals are generated, which damage supercoiled DNA plasmids and human skin cells as evidenced by single- and double-strand breaks, and kill yeast cells. Passivation of the titania particle surface reduces the photoactivity, of titanium dioxide with profound impact on the photooxidation of phenol. Also, damage to DNA., human skin cells and yeast cells is dramatically attenuated by modified titania samples, thus making them safer for use in sunscreen lotions. Moreover, the modified specimens completely retain their UVB/UVA absorption characteristics. Contrary to manufacturers claims, commercial sunscreen lotions exposed to simulated sunlight are not photostable. Preliminary spectroscopic and photochemical studies on active ingredients in various solvents are reported. Results indicate that the organic chemical UV filters photodegrade on exposure to simulated sunlight in relatively short time. The nature of the solvent and the presence of oxygen influence the photodegradation. Formation of singlet oxygen is suspecte
