Preparation of CaF₂ Microspheres with Nanopetals for Water Vapor Adsorption

The hierarchically structured flower-like CaF₂ microspheres with nanopetals, named FL-CaF₂, were synthesized via a hydrothermal method using calcium acetate Ca(Ac)₂ and NaBF₄ as calcium and fluorine sources, respectively, assisted by the chelating reagent trisodium citrate (Na₃Cit) with the optimal pH of the synthesis solution. Meanwhile, a reference sample, named FL-CaF₂-R, was reproduced using ethylenediaminetetraacetic acid disodium salt (Na₂EDTA) as the chelating reagent, based on the recipe and synthesis procedure from the literature. Various techniques such as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform-infrared spectroscopy, and N₂ adsorption-desorption at 77 K were then used to characterize the synthesized samples. The results show that FL-CaF₂ with a larger diameter has a much higher thermal stability than FL-CaF₂-R because the larger the nanocrystallite size, the higher the thermal stability. The adsorption of water vapor on CaF₂ is irreversible because CaF₂ can interact with the adsorbed water molecules strongly. The dual-site Langmuir model was used to describe the measured adsorption isotherms of water vapor on FL-CaF₂ at low water vapor pressures and 298, 308, and 318 K. FL-CaF₂ has a much higher water-adsorption capacity than those reported in the literature. Furthermore, the isosteric heat of adsorption as a function of loading, derived from the measured isotherms, varies from ca. 46 to 43 kJ mol^-1 in the whole loading range investigated. Finally, the applications of FL-CaF₂ are anticipated in the dehydration of hydrogen fluoride gas as well as in catalysis.