Rapid Synthesis of Pure and Doped ZnS Quantum Dots for Photocatalytic Degradation of Biological Dye Pollutants

Pure and doped (Fe³⁺ and Mn²⁺) ZnS quantum dots, aided with the capping agent 2-mercaptoethanol, were synthesized using chemical precipitation technique at room temperature. The novel photocatalyst was characterized by X-Ray Diffraction (XRD), Transmission Electron Miocroscopy (TEM), Fourier Transform Infrared (FTIR), UV-Visible and Atomic Absorption Spectroscopy (AAS). FTIR confirmed the capping agent and suggested a possible structure for capped quantum dot. XRD revealed the zinc blend FCC structure and UV-Vis suggested the particle size decreased on doping due to blue shift of absorption maxima. Band gap calculations suggested that doped particles were having higher band gap than undoped, thus can be treated as potential nanophotocatalyst. TEM, UV-Visible and XRD data revealed the particle size in rage of 2–3 nm, less than Bohr radius confirming the quantum confinement of the synthesized particles. Malachite Green and Methyl Orange were chosen as sample industrial dyes for degradation studies and effect of quantum dot, pH and initial dye concentration were studied. More than 2–3 cycles were not suggested for the reuse of the particles. Langmuir-Hinshelwood kinetic model was fit to the optimised data and the pseudo first order kinetic rate constant was obtained. Possible explanation for higher photocatalytic activity of doped quantum dots can be due to the entrapment of electrons and holes by these dopants so that they form oxidative radicals, rather than recombining in radiative or non-radiative way.