Temperature Driven Morphological Changes of Chemically Precipitated Hydroxyapatite Nanoparticles

Hydroxyapatite (HA) is synthesized by a wet chemical route using calcium hydroxide and ortho-phosphoric acid at various temperatures (40, 80, and 100 °C). X-ray diffraction of the precipitate particles revealed HA as the predominant phase (>99%) with a small amount of β-tricalcium phosphate. Fourier transform infrared spectroscopy indicated the presence of carbonate substitution, which decreased with increasing temperature. Transmission electron microscopy observations revealed needle-shaped particles with a high aspect ratio at 40 °C, which changed to spheroidal when the precipitation temperature was increased to 100 °C. The changes in the morphology with temperature were analyzed taking into account the driving force for the HA precipitation and the supersaturation level of Ca2+ and PO4 3- ions with respect to HA. The analysis indicated that the supersaturation level of the reactants, especially the concentration of Ca2+ ions, played a predominant role on the precipitate morphology for this classical acid−base reaction.