Facile Fabrication of Oxygen-Defective ZnO Nanoplates for Enhanced Photocatalytic Degradation of Methylene Blue and In Vitro Antibacterial Activity

In this study, we examined whether catalysts with many defects have excellent photoactivity. We prepared ZnO nanoplates with varying degrees of defects in a short time of 4 h by varying the crystal growth temperature at 50, 100, 150, and 200 °C under a strong alkali NaOH atmosphere of 4.0 M. During high-temperature preparation of ZnO, crystal defects were reduced and crystallinity was further increased. In crystallized systems over 100 °C, rhombic nanoplates were used to control particle shape and induce growth in only two axes. The PL, Raman, and XPS analyses confirmed the presence of strong oxygen vacancies in all ZnO nanoplates, and the vacancies decreased with increasing crystallization temperatures. Methylene blue (MB) dye was initially fixed at 50 mg/L with a peak decrease in absorption at 600–700 nm, confirming its decomposition over time. For the 5 h reaction, the MB removal concentration follows the following order: ZnO-50 < ZnO-100 < ZnO-150 < ZnO-200. The study confirms that ZnO-200 nanoplates with fewer oxygen vacancies decompose MB more quickly. ZnO-200 nanoplates synthesized at 200 °C provided the best sterilization performance when tested against gram-positives and gram-negatives, Escherichia coli and Staphylococcus aureus, respectively. ZnO-200 nanoplates after 3 h showed a high sterilization performance of 96.95% (86.67% in a dark room) for staphylococcus aureus and 95.82% (74.66% in a dark room) for Escherichia coli when irradiated with light. Particularly noteworthy in this study is that ·OH and ·O2− radicals are generated more strongly in ZnO-200 than in ZnO-50 nanoplates. These results show that too-strong oxygen vacancies rather inhibit the antibacterial performance, and that the virtue of moderation also exists in the catalytic activity.