Sunlight-Driven Photocatalytic Degradation of Ciprofloxacin by Carbon Dots Embedded in ZnO Nanostructures

Surface modification of ZnO nanoparticles (NPs) is considered as a key for enhanced sustained generation of reactive oxygen species (ROS) in the reaction medium for improving the photocatalytic degradation phenomenon. Here, we present a simple chemical method of synthesizing carbon dots (CDs) embedded on the surface of ZnO NPs denoted as ZnO/CD nanocomposites (NCs) as a photocatalyst for efficient sunlight-driven degradation of ciprofloxacin (CIP). The X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies confirmed the formation of needle-shaped clusters of ZnO/CD NCs. The band gaps of ZnO/CD NCs and pristine ZnO are determined from Tauc's plot. Under optimized conditions, the batch of CIP at pH 6.3 +/- 0.1 treated with 0.6 g L-1 of ZnO/CD NCs revealed 98% photocatalytic degradation of CIP in 110 min by harnessing longer UV and the visible components of sunlight. The rate of photocatalytic degradation was 0.30 min(-1), which was 3 times higher than that of the pristine ZnO NPs. The enhanced rate of degradation by ZnO/CD NCs is attributed to the improved mobility of charge carriers and the generation of enhanced ROS. This is confirmed by the scavenging studies of hydroxyl radicals and superoxide radicals. The higher ROS generation is attributed to the inhibition of charge carrier recombination facilitated by the CDs embedded on the surface of ZnO NPs. The applicability of ZnO/CD NCs as a photocatalyst is demonstrated from five cycles of repeatability studies. After five cycles of operation, the structure and surface morphology of the used ZnO/CD NCs were stable, confirmed by XRD, TEM, and Fourier transform infrared studies. It is concluded that ZnO/CD NCs are efficient sunlight-driven photocatalysts for improved photocatalytic degradation of CIP, a model emerging contaminant.