The use of titanium dioxide (TiO2) in cosmetics is extensive; in addition to makeup such as pressed powder cakes, liquid foundations, lipsticks and eye makeup, it is used in sunscreen creams. However, its rutile form for cosmetics may produce small amounts of hydroxyl radicals upon exposure to sunlight. Therefore, in the present article, the authors use salicylic acid (SA) as a hydroxyl radical capture agent to examine the development of free radicals in solutions or creams containing TiO2 to further examine its safety upon UV exposure.
TiO2 is a nontoxic, fine white powder consisting of uniform particles that has good dispersion and narrow particle size distribution. Its coloration and covering power are higher than zinc oxide or lead carbonate, and for particle sizes in the range of 0.2–0.5 μm, TiO2 has the largest reflective index (n = 2.7) among all white pigments. It also shows stable chemical properties such as being: insoluble in water and weak acids, slightly soluble in alkaline solutions, and soluble in hot sulfuric and hydrochloric acid. Crystal forms of TiO2 include anatase, rutile and brookite.
The anatase and rutile forms are the most often used for pigment and photocatalytic reactions. These forms are tetragonal structures and their molecule bonding type and curvatures are expressed as different densities and band gap energies. When the anatase form is heated to approximately 600°C, it transforms into the rutile form. Typical TiO2 on the market is a mixture of the anatase and rutile forms doped with other metal ions.
Photocatalytic Oxidation via TiO2 Catalyst
In 1972, the photocatalytic splitting of water on TiO2 electrodes marked the beginning of a new era in heterogeneous photocatalysis. Since then, research efforts have been under way to understand the fundamental processes and to enhance the photocatalytic efficiency of TiO2. Such studies often relate to energy renewal and energy storage.
In a heterogeneous photocatalysis system, photo-induced molecular transformations or reactions take place at the surface of a catalyst. Depending where the initial excitation occurs, photocatalysis can generally be divided into two classes of processes. When initial photo-excitation occurs in an adsorbated molecule, which then interacts with the ground state catalyst substrate, the process is referred to as catalyzed photoreaction. If initial photo-excitation instead takes place in the catalyst substrate and the photo-excited catalyst then transfers an electron or energy into a ground state molecule, the process is called sensitized photoreaction.