The Different Behavior of Rutile and Anatase Nanoparticles in Forming Oxy Radicals Upon Illumination with Visible Light: An EPR Study

Photoexcited TiO2 has been found to generate reactive oxygen species, yet the precise mechanism and chemical nature of the generated oxy species especially regarding the different crystal phases remain to be elucidated. Visible light‐induced reactions of a suspension of titanium dioxide (TiO2) in water were investigated using electron paramagnetic resonance (EPR) coupled with the spin‐trapping technique. Increased levels of both hydroxyl (˙OH) and superoxide anion (˙O2−) radicals were detected in TiO2 rutile and anatase nanoparticles (50 nm). The intensity of signals assigned to the ˙OH and ˙O2− radicals was larger for the anatase phase than that originating from rutile. Moreover, illumination with visible (nonUV) light enhanced ˙O2− formation in the rutile phase. Singlet oxygen was not detected in water suspension of TiO2 neither in rutile nor in anatase nanoparticles, but irradiation of the rutile phase with visible light revealed a signal, which could be attributed to singlet oxygen formation. The blue part of visible spectrum (400–500 nm) was found to be responsible for the light‐induced ROS in TiO2 nanoparticles. The characterization of the mechanism of visible light‐induced oxy radicals formation by TiO2 nanoparticles could contribute to its use as a sterilization agent. In view of the wide use of TiO2 for biologic applications, it is important to study the reactivity of TiO2 nanoparticles in water suspensions and under visible light illumination. The rutile and anatase TiO2 nanoparticles phases were found to generate reactive oxygen species in aqueous suspensions. Both superoxide anions and hydroxyl radicals were detected in NP suspensions. TiO2‐rutile also responded to visible light irradiation, and elevated levels of superoxide and singlet oxygen were observed.