Influence of crystallographic facet orientations of sol-gel ZnO on the photocatalytic degradation of p-nitrophenol in water

Three zinc oxide catalysts with different morphologies are synthesized by the sol-gel method. Zinc acetate and hexamethylenetetramine (HMTA) are used to produce nanorods (NR) and nanodiscs (ND). ZnO nanoflowers (NF) are produced from different reactants, namely zinc nitrate and sodium hydroxide. The photocatalysts are efficient for degrading p-nitrophenol under halogen lamp illumination (300–800 nm). X-ray diffraction confirms the presence of the wurtzite structure, and scanning electron microscopy confirms the desired morphologies. In order to understand the differences in the kinetic rate of degradation between the three catalysts, surface defects are investigated using photoluminescence and Raman spectroscopy. Moreover, colloidal stability and specific surface area are determined by zeta potential and nitrogen sorption measurements, respectively, and allow the impact of the different parameters on the photocatalytic performance of the samples to be clearly understood. Although they do not have the highest number of defects nor the largest specific surface area, ND shows the best degradation results by reaching 75% of degradation after 8 h. This result can be attributed to the morphology of this catalyst, where the polar facets are exposed to the medium and play a crucial role in the photocatalytic performance by enhancing the lifetime of the electron/hole pairs generated upon illumination. The polar nature of both catalyst and pollutant increases the contact between them and, consequently, the degradation efficiency. Graphical Abstract Controlling the morphology of ZnO allows better degradation of p-nitrophenol under UV-vis. Three morphologies were compared, and the nanodiscs showed the best photocatalytic activity.