Aqueous Solution Synthesis of CaF2Hollow Microspheres via the Ostwald Ripening Process at Room Temperature

Nearly monodispersive CaF2hollow microspheres were synthesized by a facile aqueous solution route from the mixed aqueous solutions of CaCl2, Na2WO4, and NaF at room temperature. The as-prepared products were characterized by X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy (TEM), high-resolution transmission electron microscopy, and N2adsorption-desorption techniques. The CaF2hollow microspheres have an average diameter of about 1.5 µm and a hollow interior of 0.5 µm. The shell is composed of numerous single-crystalline nanoparticles with diameter of about 20 nm. The morphologies and diameters of the CaF2products are strongly dependent on the experimental parameters, such as the concentration of the aqueous NaF solution and the reaction temperature. The synthetic experiments indicate that the growth process of CaF2hollow microspheres involves first the formation of CaWO4solid microspheres and then the formation of CaF2solid microspheres through the reaction between CaWO4and F-ions controlled by the difference of the solubility product for CaWO4and CaF2. Phenomenological elucidation based on TEM observations and XRD patterns of intermediate products at different precipitation stages indicates that the formation mechanism for the CaF2hollow microspheres is related to the Ostwald ripening mechanism. N2adsorption-desorption measurement shows that the CaF2hollow microspheres possess a high Brunauer-Emmett-Teller surface area and porosity properties. The synthetic procedure is straightforward and represents a new example of the Ostwald ripening mechanism for the formation of inorganic hollow structures in an aqueous solution at room temperature.