The absorption mechanism of titanium dioxide for ultraviolet light may be: the electronic structure of nanometer titanium dioxide is composed of a conduction band formed by a valence electron band and an empty orbit. When it is irradiated by ultraviolet light, it has a larger energy than its band gap (about 3.2eV). The light is absorbed and the electrons in the valence band are excited to the conduction band. As a result, the valence band lacks electrons and holes are generated, forming electron-hole pairs that are easy to move and have strong activity. On the one hand, such electron-hole pairs can recombine with each other during various redox reactions, releasing energy in the form of heat or generating fluorescence, on the other hand, they can be dissociated into the crystal lattice and migrate freely to the surface of the crystal lattice. Or the free holes and free electrons in other reaction sites are immediately captured by the surface groups. Under normal circumstances, titanium dioxide will activate surface water to generate surface hydroxyl groups to capture free holes and form hydroxyl radicals. Free free electrons will quickly combine with absorbed oxygen to generate superoxide radicals, which will also kill surrounding bacteria and viruses. die. It can be seen that ultraviolet radiation, surface water activation degree and oxygen absorption rate are the three basic conditions for the photoactivity of titanium dioxide. It is precisely because nano-titanium dioxide will generate free radicals after absorbing ultraviolet rays, which will accelerate the aging of the skin and cause harm to the skin. Therefore, when using nano-titanium dioxide as a sunscreen, the three basic conditions should be reduced or eliminated to reduce or eliminate its photoactivity and reduce its harmfulness.
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