Production of Reactive Oxygen Species and Electrons from Photoexcited ZnO and ZnS Nanoparticles: A Comparative Study for Unraveling their Distinct Photocatalytic Activities

The photoactivity of semiconductor nanostructures makes them potentially useful for environmental remediation and antibacterial applications. Understanding the mechanism underlying the photochemical and photobiological activities of photoexcited semiconductors is of great importance for developing applications and assessing associated risks. In the current work, using electron spin resonance spectroscopy coupled with spin trapping and spin labeling techniques, we comparatively and systematically investigate the abilities of ZnO and ZnS to generate hydroxyl radical, superoxide, singlet oxygen, photoinduced electrons, and oxygen consumption during irradiation. It was found that although ZnO and ZnS, when photoexcited, can produce hydroxyl radical, superoxide, and singlet oxygen, ZnO is more effective than ZnS in producing hydroxyl radical and singlet oxygen while ZnS is more effective than ZnO in generating superoxide. The characterization with ESR spin labeling and oximetry indicates ZnS is about 4 times more active than ZnO in production of photoinduced electrons and consumption of oxygen. We compared the photocatalytic and antibacterial activities of ZnO and ZnS and found that ZnO exhibits efficient and broad photocatalytic and antibacterial activity, conversely, ZnS is only effective in photodegradation of RhB and killing Staphylococcus aureus. The distinct photocatalytic activities of ZnO and ZnS nanoparticles were attributable to their unique capability to facilitate the generation of reactive oxygen species and charge carriers during photoirradiation. These results provide valuable information for understanding the photocatalytic mechanism of metal oxide and metal sulfides and for predicting their photocatalytic activities.