Template-free synthesis of ZnWO4 powders via hydrothermal process in a wide pH range

ZnWO₄ powders with different morphologies were fabricated through a template-free hydrothermal method at 180 °C for 8 h in a wide pH range. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-visible and luminescence spectrophotometers were applied to study the effects of pH values on crystallinity, morphology, optical and luminescence properties. The XRD results showed that the WO₃ + ZnWO₄, ZnWO₄, and ZnO phases could form after hydrothermal processing at 180 °C for 8 h with the pH values of 1, 3-11, and 13, respectively. The SEM and TEM observation revealed that the morphological transformation of ZnWO₄ powders occurred with an increase in pH values as follows: star anise-, peony-, and desert rose-like microstructures and soya bean- and rod-like nanostructures. The highest luminescence intensity was found to be in sample consisting of star anise-like crystallites among all the samples due to the presence of larger particles with high crystallinity resulted from the favorable pH under the current hydrothermal conditions.