Dopant Effects on the Photocatalytic Activity of Colloidal Zinc Sulfide Semiconductor Nanocrystals for the Oxidation of 2-Chlorophenol

摘要

In this work, colloidal zinc sulfide nanocrystals, prepared with selected concentrations of manganese(II) ions in the range of 0-2% and stabilized with a polymeric capping agent, were used in the photocatalytic degradation of 2-chlorophenol in aqueous solutions. Particle sizes for the synthesized nanocrystals were determined using UV-vis absorbance spectroscopy and X-ray diffraction (XRD). UV-vis spectra for all of the synthesized nanocrystals display an excitonic peak at around 290 nm, which corresponds to a particle size of approximately 3 nm, in agreement with the XRD analysis. Photoluminescence emission spectra recorded for undoped ZnS nanocrystals exhibit an emission peak at 460 nm in the spectrum, whereas peaks corresponding to the dopant ion are red-shifted to 590 nm. With increasing addition of the dopant ion, the area of the peak at 460 nm first increases and then decreases for higher dopant concentrations, whereas the area of the peak at 590 nm increases. This finding clearly indicates that the presence of the dopant ion alters the electronic states of the nanocrystals, which, in turn, affects the photocatalytic properties of the colloidal nanocrystals. Apparent rate constants for the photodegradation of 2-chlorophenol in aqueous solution utilizing undoped and doped colloidal ZnS nanocrystals were calculated using pseudo-first-order kinetic analysis. As the dopant concentration used during synthesis was increased, the apparent rate constants increased initially and then decreased. Results from kinetic experiments performed after nitrogen purging showed a decrease in the apparent rate constant, suggesting that the reaction mechanism involves a reactive oxygen species such as superoxide. Taken together, these results provide further insight into the effects of dopants on the activity of colloidal semiconductor nanocrystals employed as photocatalysts in environmental remediation applications.