ZnWO.sub.4 nanocrystals prepared by thermal plasma processing

In this paper, we propose a new way of making pure zinc tungstate (ZnWO.sub.4) nanocrystals through thermal plasma processing. Initially, we prepared amorphous powders by the coprecipitation method at room temperature and then, these powders were processed at different temperatures between 400 and 600 °C for 1 h by the thermal plasma technique. Structural analyses with characterizations by X-ray diffraction, Rietveld refinement data, micro-Raman, and Fourier transform-infrared spectroscopies indicate a transition from the amorphous phase (coprecipitated) to intermediate (W.sub.18O.sub.49 and ZnO phases) and desired ZnWO.sub.4 phase between 400 and 550 °C for 1 h. These characterizations confirmed the best effect of thermal plasma at 600 °C for 1 h to obtain single-phase ZnWO.sub.4 pristine with a wolframite-type monoclinic structure and typical vibrational modes. The ultravioleta-visible diffuse reflectance spectroscopy revealed a decrease in optical band energy values from 3.97 to 3.20 eV. Field emission scanning electron microscopy showed clear images of a considerable morphological modification of small spherical and irregular particles referring to the W.sub.18O.sub.49 and ZnO phases to a growth more homogeneous of rod-like ZnWO.sub.4 nanocrystals related to pristine phase with size from 132 to 155 nm. The sonocatalytic, photocatalytic, and sonophotocatalytic (SPC) properties for degradation of Rhodamine 6G (Rh6G) were investigated. The better SPC degradation rate at Rh6G dye in 80 min was performed with ZnWO.sub.4 nanocrystals processed at 600 °C. Finally, the radical scavengers assays indicated that hydroxyl (·OH) and superoxide ( [Formula omitted]) radicals are the main ones responsible for the chemical reactions to Rh6G dye degradation.