Aqueous Solution Synthesis of CaF₂ Hollow Microspheres via the Ostwald Ripening Process at Room Temperature

Abstract

Nearly monodispersive CaF2 hollow 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 N2 adsorption-desorption techniques. The CaF2 hollow microspheres have an average diameter of about 1.5 microm and a hollow interior of 0.5 microm. The shell is composed of numerous single-crystalline nanoparticles with diameter of about 20 nm. The morphologies and diameters of the CaF2 products 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 CaF2 hollow microspheres involves first the formation of CaWO4 solid microspheres and then the formation of CaF2 solid microspheres through the reaction between CaWO4 and F(-) ions controlled by the difference of the solubility product for CaWO4 and CaF2. Phenomenological elucidation based on TEM observations and XRD patterns of intermediate products at different precipitation stages indicates that the formation mechanism for the CaF2 hollow microspheres is related to the Ostwald ripening mechanism. N2 adsorption-desorption measurement shows that the CaF2 hollow 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.

Facile and Controllable Synthesis of Monodisperse CaF2 and CaF2:Ce3+/Tb3+ Hollow Spheres as Efficient Luminescent Materials and Smart Drug Carriers

“…As F À ions can be released from BF 4 À in aqueous solution, the [ [35][36][37] With further reaction, CaF 2 hollow spheres are obtained by evacuation of the intermediate solid spheres, which is believed to be the result of the Ostwald ripening mechanism induced by a chemical transformation. [29,[38][39][40][41][42] During the ripening process, the inner crystallites would dissolve and migrate outwards, leaving channels connecting the inner and outer spaces in the CaF 2 shells. [29] The solid cores could be excavated with enough ripening time due to the higher surface energy.…”

“…[29,[38][39][40][41][42] During the ripening process, the inner crystallites would dissolve and migrate outwards, leaving channels connecting the inner and outer spaces in the CaF 2 shells. [29] The solid cores could be excavated with enough ripening time due to the higher surface energy. Once the cores in the center of the microspheres are consumed completely, hollow microspheres are formed.…”

“…[25][26][27] These properties lead to a wide range of potential optical applications in optics, biological labels, and lenses, [28,29] as well as components of insula- samples still show green luminescence of Tb 3 + under UV irradiation, and the emission intensity of Tb 3 + in the drugcarrier system varies with the released amount of ibuprofen, so that drug release can be easily tracked and monitored by means of the change in luminescence intensity. The formation mechanism and luminescent and drugrelease properties were studied in detail.…”

“…[31,32] Therefore, CaF 2 is widely used in UV lithography, UV-transparent optical lenses, surface conditioning of glass, promoting agents for bone/tooth reconstruction, and biocompatible luminescent markers. [29,32,33] Moreover, CaF 2 is an attractive host for phosphors activated with rare earth ions (RE 3 + ), which display unique up/down-conversion luminescence properties arising from their 4f electron configuration. [26,33] Furthermore, RE 3 + ions incorporated in the CaF 2 host lattice increase the refractive index with respect to pure CaF 2 , which makes it very attractive as an active waveguide for optical integrated devices.…”

Hydrothermal Synthesis and Luminescence Properties of Monodisperse Spherical CaF<sub>2</sub> and CaF<sub>2</sub>:Ln<sup>3+</sup> (Ln = Eu, Tb, Ce/Tb) Microcrystals

“…What's more, CaF 2 has a large amount of excellent properties, such as low refractive index, wide band gap (Eg = 12.1 eV) and optically transparent in a wide range of wavelength from mid-infrared to vacuum ultraviolet [13,14]. As a result, with the development of inorganic nano/micro-scale fluorides materials, CaF 2 is going to play an essential role in UV lithography, UV-transparent optical lenses, glass surface modification, bio-mimetic materials processing for bone/tooth building and biological fluorescent labels [12,14,15].…”