Photocatalytic degradation of methylene blue with ZnO@C nanocomposites: Kinetics, mechanism, and the inhibition effect on monoamine oxidase A and B

To improve the photocatalytic activity of hexagonal wurtzite ZnO nanoparticles, ZnO@C nanocomposites were synthesized at low temperature via two-step hydrothermal method. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA) and UV-visible diffuse reflectance spectroscopy (UV-Vis DRS) were carried out to characterize the ZnO and ZnO@C photocatalysts, indicating that there was a carbon layer on the surface of the ZnO@C and that the presence of a carbon layer enhanced the response under visible light. The photocatalytic degradation of methylene blue (MB) was chosen as a model reaction to evaluate the photocatalytic activity and stability of the ZnO and ZnO@C photocatalysts. The results indicated that the photocatalytic performance of the ZnO@C07 was approximately 1.78 times higher than that of ZnO. Additionally, the ZnO@C07 photocatalyst showed excellent photocatalytic stability, which was dependent on the stability of the structure of the ZnO@C07 photocatalyst. The possible photocatalytic mechanisms of the ZnO and ZnO@C photocatalysts were proposed based on electron spin resonance (ESR) measurements and the calculation of the synergetic factor. The synergetic effect between ZnO and carbon-layer was on account of the higher concentration of surface OH- species (ratio of OH-/O2- was 0.18 for ZnO and 1.48 for ZnO@C07), and the more surface OH- resulting in higher intensity of center dot OH and center dot O-2(-) under sunlight irradiation. The inhibition effect between MB and monoamine oxidase A (MAO A) was higher than that of MAO B on the basis of IC50 values with IC50) (MB/MAO A) = 0.2 mu M and IC50 (MB/MAO B) = 5.6 mu M, which was attributed to the stronger interactions between MB and MAO A (Van Der Waals' force) than that between MB and MAO B (Van Der Waals' force, pi-pi interaction, pi-cation interaction). Our findings provide valuable information on the photocatalytic degradation of organic pollutants by modified metal oxide photocatalysts and offer a synthetic evaluation method to predict the molecular mechanism of the inhibition effect between MB and protein.