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. Author manuscript; available in PMC: 2019 Mar 15.
Published in final edited form as: Chem Eng J. 2018 May 15;340:155–172. doi: 10.1016/j.cej.2018.01.060

Fig. 4.

Fig. 4

Scheme of the possible mechanisms in metal oxide nanoparticles (i.e. titanium dioxide, TiO2, or zinc oxide, ZnO, nanoparticles NPs) to generate reactive oxygen species (ROS) in water-based media. Here both ultraviolet (UV) and ultrasound radiations are depicted as a source to generate ROS at the surface of semiconductor metal oxide NPs in water. In particular, the ultrasound produces cavitation bubbles that, during their collapse, emit the sonoluminescent light. As a result, the semiconductor NPs are photo- or sono-excited, injecting electrons (e−) from valence to the conduction band, leaving the holes (h+) in the valence band. The separated e− and h+ react with water and gas molecules adsorbed on the semiconductor surface, generating the ROS (.O2−, .OH, H2O2). Moreover, the radiative recombination of electron-hole pair can lead to a photon emission able to generate singlet oxygen (1O2) from the oxygen molecules (O2). Reprinted under a Creative Commons License. Copyright 2017 The Author(s) from Ref. [91].