Photocatalytic performance and surface characterization of plant-mediated ZnO nanoparticles for methylene blue degradation

This study evaluated the photocatalytic performance of green-synthesized ZnO materials through the degradation of methylene blue (MB) under simulated and natural solar irradiation. Owing to its enhanced visible-light absorption compared with TiO 2 , ZnO is a promising material for solar-driven environmental remediation. ZnO photocatalysts were synthesized using extracts from rosemary, comfrey, olive leaves, bitter orange and banana, resulting in materials with distinct morphologies and textural properties. The relationship between physicochemical characteristics and photocatalytic performance was investigated through structural, morphological, and surface analyses, including X-ray diffraction (XRD), FTIR spectroscopy, scanning electron microscopy (SEM), surface area, point of zero charge, adsorption studies, pore volume and reactive oxygen species (ROS) quantification. Photocatalytic activity was strongly influenced by the morphology and surface properties of the ZnO materials. Notably, MB degradation didn’t require significant prior adsorption of the pollutant on the catalyst surface. Among all materials, rosemary-derived ZnO exhibited the highest photocatalytic activity regardless of the synthesis route employed (hot-plate or hydrothermal). This superior performance (kinetic rate k = 0.023 min −1 at only 33W irradiation) was associated with its distinctive morphology, consisting of nanorods (50 to 200 nm), together with the largest specific surface area (~ 14 m 2 g⁻ 1 ) and the widest average pore diameter (~ 39 nm). Scavenger experiments identified superoxide radicals (·O 2 ⁻) as the predominant reactive species involved, with their quenching causing a 17-fold decrease in the apparent kinetic constant. These data highlight the importance of morphology and surface characteristics in governing the activity of plant-mediated ZnO photocatalysts and demonstrate their potential as sustainable materials for wastewater treatment.