Strategies for the formation of oxygen vacancies in zinc oxide nanoparticles used for photocatalytic degradation of phenol under visible light irradiation

Pollution is a long term environmental issue that is becoming a great concern faced by the world. The biggest source that leads to pollution is the increasing industrialisation due to the leakage of industrial waste into the environment, which can affect soil and ground water quality. Phenol is one of the hazardous wastes generated in which its level in water must be controlled. In this study, zinc oxide nanoparticles (ZN) were successfully synthesised by a facile microwave (MW)-assisted method under various power densities. It was found that a higher power density increased the crystallinity and surface area of the ZN but reduced the band gap energy of the catalyst. MW heating acted as a medium in the formation of oxygen vacancies (OV) where OV played an important role in the photocatalytic activity especially for an application under visible light irradiation. The particle size, surface area and different morphologies of the catalysts were also affected by the changes of power density and indirectly influenced the photoactivity. The ZN prepared with 0.12 (ZN1), 0.37 (ZN2) and 0.56 (ZN3) W g-1 power density were successfully degraded phenol with 64%, 70% and 87%, respectively under visible light irradiation. During the photocatalytic process, it was found that OV not only acted as an electron acceptor that inhibited the electron-hole recombination and eased the charge carrier migration but also narrowed the band gap energy. The ability of ZN3 in the degradation of other phenol derivatives such as 2-chlorophenol (2-CP), 4-chlorophenol (4-CP) and 2,4-dichlorophenol (2,4-diCP) also showed a positive response whereby ZN3 was successfully degraded 58%, 50% and 63% of 2-CP, 4-CP and 2,4-diCP, respectively