Modulation of particle size and morphology of zinc oxide in graphitic carbon nitride/zinc oxide composites for enhanced photocatalytic degradation of methylene blue

Particle size and morphology of photocatalysts play an important role in enhancing their photocatalytic activity. In this study, graphitic carbon nitride/zinc oxide (g-C3N4/ZnO) composites with different ZnO particle sizes and morphologies were successfully prepared by obtaining g-C3N4 from melamine followed by in-situ formation of ZnO on g-C3N4 using the hydrothermal method for the methylene blue (MB) degradation. The particle size and morphology of ZnO were modulated by varying the pH of the hydrothermal solution with pH values of 8.0, 9.0, 10.0, and 11.0. A higher pH led to a smaller particle size and a cone-like structure on the edge of the ZnO particles. The optimal pH was shown to be 10.0 based on the phase purity, homogeneity, and particle size of the resulting ZnO. The g-C3N4/ZnO composite prepared with a pH of 10.0 achieved the highest MB degradation efficiency among the prepared photocatalysts, with degradation efficiency value of 97.7% after 150 min. This high degradation performance can be attributed to the larger surface area and the suppression of charge carrier recombination, which were induced by the formation of a heterojunction between g-C3N4 and ZnO, the small ZnO particle size, and the cone-like edge ZnO morphology. A larger surface area increases the contact area with the reactant, while the formed heterojunction, small ZnO particle size, and the cone-like edge ZnO morphology facilitate the separation of electron-hole pairs.