ZnWO4 nanocrystals prepared by thermal plasma processing.

In this paper, we propose a new way of making pure zinc tungstate (ZnWO4) 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 (W18O49 and ZnO phases) and desired ZnWO4 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 ZnWO4 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 W18O49 and ZnO phases to a growth more homogeneous of rod-like ZnWO4 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 ZnWO4 nanocrystals processed at 600 °C. Finally, the radical scavengers assays indicated that hydroxyl (·OH) and superoxide ( O 2 · - ) radicals are the main ones responsible for the chemical reactions to Rh6G dye degradation. A new method of processing using thermal plasma was employed to make pristine ZnWO4 nanocrystals at 600 ºC for 1 h and their structural evolution, shape, photocatalytic, sonocatalytic, and sonophotocatalytic properties were investigated in detail. [ABSTRACT FROM AUTHOR]