Hydrothermal temperature dependence of CaWO₄ nanoparticles: structural, optical, morphology and photocatalytic activity

In this paper, CaWO₄ nanoparticles were prepared by the hydrothermal method using the temperature of 100 (CaWO₄-100), 120 (CaWO₄-120), 140 (CaWO₄-140), and 160 °C (CaWO₄-160) for 1 h. These nanoparticles have a tetragonal structure (I4₁/a) exhibiting a high crystallinity degree and average crystallite size from 24.61(0.12) nm to 40.13(0.18) nm. The main active modes were found from vibrational Raman and FTIR spectroscopies, agreeing with the structural characterization obtained by Rietveld refinement and Williamson-Hall plot results. The optical bandgap for these materials ascertained by diffuse reflectance spectroscopy is 3.9(05) eV, 3.9(9) eV, 3.9(6) eV, and 4.01(02) eV for CaWO₄-100, CaWO₄-120, CaWO₄-140, and CaWO₄-160, respectively. Irregular polyhedral nanoparticles were observed from FE-SEM images for all synthesized samples, wherein their EDX spectrum shows all characteristic atomic peaks of calcium tungstate. Finally, these samples yield a remarkable photocatalytic reaction in Rhodamine B dye (RhB) solution. However, the CaWO₄-100 nanoparticles exhibit a more effective performance than in the other synthesized samples, mainly associated with lower particle size and high dislocation density. Moreover, the pH effect yields the best performance for acid medium (pH = 1), promoted by direct oxidation of RhB dye molecules through both holes and superoxide radicals.