Enhanced removal of methylene blue under simulated sunlight over oxygen vacancy-mediated ZnO photocatalysts

The role of oxygen vacancies in enhancing photocatalytic activity has attracted increasing attention. In this work, ZnO nanorods with enriched surface oxygen vacancies were successfully synthesized via a facile hydrothermal process combined with calcination in the presence of urea, and the content of the oxygen vacancies could be tuned by adjusting the calcination temperature and time. The characterization results proved that the content of oxygen vacancies reached 45.47% with a calcination temperature and time of 500 °C and 4 h, respectively. Additionally, the increased oxygen vacancy content was conducive to not only narrowing the ZnO bandgap, but also accelerating the separation and transfer of photoproduced electron–hole pairs, thus enhancing the methylene blue (MB) removal. The maximum removal efficiency of MB reached 97.65% within 120 min under simulated sunlight irradiation, and the catalyst exhibited stable performance after five consecutive cycles. The degradation intermediates of MB determined by liquid chromatograph–mass spectrometer (LC–MS) were the aromatic and ring-opening products of demethylation, desulfurization and hydroxylation. The toxicities of these compounds decreased significantly based on the germination and growth of Vigna radiata. This study provides a controllable and simple strategy for the design of ZnO with abundant oxygen vacancies and high activity in a photocatalytic system under simulated sunlight.