Preparation Of ZnS Nano-films And Their Photocatalytic Activity

Semiconductor photocatalytic technology has wide application prospects in thedegradation of organic pollutants and poisonous water pollutants. Among the varioussemiconductor materials, ZnS has been attracting much attention due to the stablechemical properties, non-toxic, corrosion resistant, and low cost. Pure ZnS with wideband gap can only absorb ultraviolet part in sunlight, resulting in the inefficient use ofsolar energy and low photocatalytic activity. Thus, improving the catalytic quantumefficiency of ZnS in the visible light range has been becoming one of the hottesttopics in semiconductor photocatalytic technology.In this paper, the nearly spherical ZnS and La-ZnS thin films, using zinc sulfatefor zinc source, ammonium thiosulfate for sulfur source, the mixed citric acid sodiumand tartaric acid as complexing agent, were successfully synthesized via chemicalbath deposition on conductive glass substrates. Annealing and doping La has beenapplied to modify the ZnS films. The surface morphology of films was characterizedusing a scanning electron microscope （SEM, Hitachi S-4800）. The energy dispersivespectroscopy （EDS） attached to SEM was used to analyze the composition of thefilms. The phase formation and crystalline of films were determined using an X-raydiffraction （D8）. The room temperature optical absorption spectra of the films weredetermined using a double beam UV-VIS spectrophotometer （TU-1901） in thewavelength range of300–800nm. Fluorescence spectra （FL） were measured usingF-2500fluorescence spectrophotometer with an excitation wavelength of380nm laserline.1. The synthesized thin films are compact and composed of nanoparticles varyingfrom3nm to10nm in diameter. Both undoped and La-doped ZnS samples exhibitface-centered cubic （fcc） structure. The La doping reduces the composite ionmobility, which decreases the nucleation rate and film rate, thus increases the particlesize of film.2. With the increasing of annealing temperature and heating rate，the grain size ofZnS increases. The peak intensity of fluorescence decreases and the adsorption bandwavelength of ZnS thin films exhibit a remarkable red-shift. Increasing annealingtemperature and the heating rate can greatly enhance the photocatalytic activities ofZnS films under visible light irradiation.3. The grain particle size of ZnS tends to increase and the diffraction angle of La-doped ZnS films slightly shifts to lower angle with La doping. The emission peakintensity of fluorescence decreases with increasing addition of the dopant. Theadsorption band wavelength of ZnS thin films red-shifts with the increasing doping Laratio and reaches the maximum value at molar ratio of La/Zn equal to0.3, indicatingthat the presence of appropriate amount of La doping can promot the electron/holeseparation and enhance the absorption of ZnS thin films on the visible region, which,in turn, affects the photocatalytic properties of the films under visible light irradiation.The photocatalytic performances of undoped and doped ZnS films were evaluated byphotodegrading methyl orange under both ultraviolet light and visible light irradiation.The results demonstrate that La doping can significantly enhance the visible lightphotocatalytic activity of ZnS films. The present study provides a simple method fordesigning the highly efficient semiconductor photocatalysts that can effectivelyutilize sunlight.