Modulation in band gap and efficient charge separation in Cd substituted ZnO quantum dots with enhanced photocatalytic and antibacterial activity

An efficient UV and visible light sensitive Cd substituted ZnO quantum dots (Cd-ZnO QDs) are synthesized through a facile soft-chemical approach. Cd-ZnO QDs are well characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet-visible (UV-vis) and x-ray photoelectron spectroscopy (XPS). TEM micrographs of Cd-ZnO QDs indicates well-spherical particles of size similar to 5-6 nm. The optical analysis of Cd-ZnO QDs confirms that the band gap and defect emission are strongly dependent on the Cd concentration. Moreover, the intensity of defect emission increases with increase of Cd substition from 0 to 3% which indicates that Cd substitution enhances defect emission of ZnO. Further, increase in the Cd concentration (5 and 10%) suppresses the defect emission. Such type of variation in Cd-ZnO QDs may be ascribed due to the non-radiative energy transfer. Under UV light, Cd-3-ZnO QDs shows an excellent photocatalytic activity, whereas under solar light Cd-10-ZnO QDs shows enhanced photocatalytic activity as compared to pristine ZnO. The enhanced photocatalytic activity of Cd-ZnO QDs is ascribed to the reduction in recombination rate, higher defect concentration and absorption in visible region. Additionally, Cd-3-ZnO QDs exhibits a good antibacterial activity. Higher defect concentration and accumulation of QDs on the cell membrane may trigger the ROS generation and cause the lipid peroxidation. This seems to be possible mechansim for the antibacterial activity of Cd-3-ZnO QDs.