Shape control synthesis of fluorapatite structures based on supersaturation: prismatic nanowires, ellipsoids, star, and aggregate formation

Fluorapatite nanostructures of various shapes (prismatic, ellipsoidal, star, and aggregate) were synthesized and their structures correlated with the supersaturation of the system. Reagent concentration and pH were adjusted and the change in supersaturation was simulated by the Geochemist's Workbench® software and the MINTEQ database. A higher pH caused changes to the FAP surface charge and was shown to be the dominant force behind aggregate formation. This led to nanorod aggregates and when combined with an increase in reagent concentration, FAP stars were generated. Increasing reaction temperature (room temperature to 100 °C) allowed release of calcium by the chelating agent, EDTA, which steadily increased the supersaturation as demonstrated by simulation. This condition led to ellipsoidal nanorods. As the crystal growth continued with an increasing reaction temperature of up to 150 °C, ellipsoidal nanorods transformed to prismatic nanowires. This transformation was explained by the decreasing supersaturation of the system as the growth nutrients were consumed. Microwave irradiation, the role of fluorite, and control of monodispersity for the FAP synthesis are also discussed.